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L A N D M A N A G E M E N T H A N D B O O K

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The Best Place on Earth

Ministry of Forests and Range Forest Science Program

Managing Forested Watersheds for Hydrogeomorphic Risks on Fans

Managing Forested Watersheds for Hydrogeomorphic Risks on Fans

D.J. Wilford, M.E. Sakals, W.W. Grainger, T.H. Millard, and T.R. Giles

The Best Place on Earth

Ministry of Forests and Range Forest Science Program

CitationWilford, D.J., M.E. Sakals, W.W. Grainger, T.H. Millard, and T.R. Giles. 2009. Managing forested watersheds for hydrogeomorphic risks on fans. B.C. Min. For. Range, For. Sci. Prog., Victoria, B.C. Land Manag. Handb. 61. www.for.gov.bc.ca/hfd/pubs/Docs/Lmh/Lmh61.htm

The use of trade, �rm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented as information only. Funding assistance does not imply endorsement of any statements or information con-tained herein by the Government of British Columbia. Uniform Resource Locators (URLs), addresses, and contact information contained in this document are current at the time of printing unless otherwise noted.

Prepared byD.J. WilfordMinistry of Forests and Range Smithers, BC

M.E. SakalsMinistry of Forests and Range Smithers, BC

W.W. GrangerGrainger and Associates Consulting Ltd.Salmon Arm, BC

Prepared forB.C. Ministry of Forests and RangeResearch BranchVictoria, BC

© 2009 Province of British Columbia

Copies of this report can be obtained from:Crown Publications, Queen’s Printer PO Box 9452 Stn Prov Govt 563 Superior Street, 2nd Flr Victoria, BC V8W 9V7 1 800 663-6105 www.crownpub.bc.ca

For more information on Forest Science Program publications, visit: www.for.gov.bc.ca/scripts/hfd/pubs/hfdcatalog/index.asp

Library and Archives Canada Cataloguing in Publication DataManaging forested watersheds for hydrogeomorphic risks on fans / D.J.

Wilford ... [et al.].

Includes bibliographical references.ISBN 978-0-7726-6119-7

1. Mass-wasting--British Columbia--Forecasting. 2. Landslide hazard analysis--British Columbia. 3. Forests and forestry--Environmental aspects --British Columbia. 4. Forest management--British Columbia--Planning. 5. Forest hydrology--British Columbia. 6. Alluvial fans--British Columbia. 7. Colluvium--British Columbia. I. Wilford, D. J. (David J.), 1950- II. British Columbia. Ministry of Forests and Range III. British Columbia. Forest Science Program

SD387.E58M36 2009 634.961 C2009-909966-7

T.H. MillardMinistry of Forests and Range Nanaimo, BC

T.R. GilesMinistry of Forests and RangeKamloops, BC

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ABSTRACT

Fans are linked to their watersheds by hydrogeo-morphic processes—�oods, debris �oods, and debris �ows. These processes move water, sediment, and debris from the hillslopes of a watershed through channels to the fan. Fans in British Columbia are often the site of residential developments, and trans-portation and utility corridors, as well as high-value habitat for �sh and high-productivity growing sites for forests. Collectively, these features are termed “elements-at-risk” because they may be vulnerable to watershed-generated hydrogeomorphic processes that issue onto the fan. These processes may be natu-ral or result from land use activities, and can cause the partial or total loss of some or all of the elements on the fan.

In British Columbia, forest harvesting and road building is associated with increased hydrogeomor-phic hazards. The downstream effects of these for-estry activities in source areas may be far-reaching and extend beyond the scope of conventional site-oriented planning. A �ve-step approach is presented to assist land managers undertake risk analyses and assessments that place their proposed developments within the watershed-fan system. The �ve steps are: 1) identify fans and delineate watersheds; 2) identify elements-at-risk on fans; 3) investigate fan processes; 4) investigate watershed processes; and 5) analyze risks and develop plans. This scheme is applicable to forested watersheds throughout British Columbia.

ACKNOWLEDGEMENTS

The need for this handbook was raised by the For-estry Committee of the Council of Forest Industries and represents 3 years of collaborative research involving many people throughout British Co-lumbia. The concept for the �ve-step approach was created by Bill Grainger and we are indebted to him for his foresight. This handbook has gone through many revisions over the past 3 years, incorporating suggestions from workshop participants; this ver-

sion has had the bene�t of very thorough reviews by Todd Redding, Robin Pike, Ian Smith, Steve Webb, Rita Winkler, and David Maloney. This handbook has bene�ted from a meticulous editorial review by Steve Smith. We are indebted to the Forest Invest-ment Account–Forest Science Program, BC Timber Sales, and the B.C. Ministry of Forests and Range for �nancial support.

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CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1The Five-step Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Step 1 Identify Fans And Delineate Watersheds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Step 1.1 The Fan-watershed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Step 1.2 Fan Identi�cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Step 1.3 Watershed Delineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Step 2 Identify Elements-at-risk on Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Step 2.1 Human Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Step 2.2 Anthropogenic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Step 2.3 Natural Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Step 3 Investigate Fan Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Step 3.1 Hydrogeomorphic Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Step 3.2 Event Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Step 3.3 Event Magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Step 4 Investigate Watershed Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Step 4.1 Watershed-fan Process Linkages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Step 4.2 Office Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Step 4.3 Field Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Step 4.4 Synthesis of Watershed Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Step 5 Analyze Risks and Develop Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Step 5.1 Understanding Risk Analysis and Risk Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Step 5.2 Consequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Step 5.3 Hazard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Step 5.4 Risk Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Step 5.5 Assessing Risk and Making Management Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Step 5.6 Document, Monitor, Evaluate, and Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

appendices1 Wathl Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 Eagle Summit Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Shale Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Hummingbird Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

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Tables1 Characteristics of hydrogeomorphic process deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Forest management focus for different hydrogeomorphic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Predictive models for dominant hydrogeomorphic processes using the relative

relief number and watershed length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Example of long-term probabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Qualitative frequency de�nitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 Example of qualitative hazard analysis matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Qualitative risk analysis matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23A1.1 A matrix combining hazard and consequence to determine risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31A1.2 Consequences, hazards, and risks of all the identi�ed hazards and elements-at-risk, except

the coarse sediment loading hazard and the consequence of potential impacts to residences and human safety, which are dealt with separately . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

A1.3 Coarse bedload sediment risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32A2.1 Qualitative frequency de�nitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39A2.2 A qualitative hazard matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40A2.3 A qualitative risk matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40A4.1 Relative relief numbers for the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51A4.2 Forest management focus for different hydrogeomorphic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55A4.3 A matrix combining hazard and consequence to determine risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Figures1 An aerial oblique of a fan—the clearcut harvested portion of the landscape . . . . . . . . . . . . . . . . . . . . . . . . 32 An aerial photograph with notation indicating fan areas in red and potential �ow pathways in blue . . . . 33 A landform map with alluvial and colluvial fans highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 An example of an actively growing fan resulting from high sediment transport from the watershed

to the fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 An example of a dissected fan resulting from abundant water but limited sediment transport

from the watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 A topographically de�ned watershed above a fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 A water intake structure at the apex of a fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 This building was damaged by a debris �ow caused by a drainage diversion related to a forest road . . . . 99 This drainage structure is not designed to accommodate the water, sediment, and debris from

naturally occurring hydrogeomorphic events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1010 A debris �ow initiated as a result of hydrophobic soils in the Kuskanook watershed,

damaging homes and the highway on the fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18A1.1 The Wathl Creek watershed and fan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27A1.2 A view of the Wathl Creek fan and Kitamaat Village looking east into the watershed. . . . . . . . . . . . . . 28A2.1 Eagle Summit Creek watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34A2.2 Gully and fan reaches of Eagle Summit Creek, looking south . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35A2.3 A view of the drainage structure on the Trans-Canada Highway at Eagle Summit Creek . . . . . . . . . . 36

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A2.4 A view of the upper steep reach in Upper Eagle Summit Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37A2.5 A view looking upstream in the lower reach of Eagle Summit Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38A3.1 Location map for Block SKI 101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42A3.2 Shale Creek watershed and fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43A3.3 Channel network on Shale Creek fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44A3.4 Stream 2 �owing in an older and larger channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44A4.1 Map of Hummingbird and Mara Creek watersheds, and the Swansea Point Fan . . . . . . . . . . . . . . . . . . 47A4.2 Distribution of new debris �ow sediment on the Swansea Point fan, mapped on July 14, 1997 . . . . . . 48A4.3 Looking downstream from the con�ned valley reach of Hummingbird Creek to the apex

of the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49A4.4 The “Hinkelstein” at the apex of the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49A4.5 Aerial photograph BC2616: 76 of lower Hummingbird Creek, 1959, showing a recent landslide

to the north on similar steep, northwest-aspect terrain, a possible relic debris slide colonized by a younger forest stand, and the location of the 1997 debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . 51

A4.6 Pre- and post-development runoff-contributing areas to the culvert located above the debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

A4.7 Aerial photograph of lower Hummingbird Creek watershed with the debris avalanche below the road in right centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

1

INTRODUCTION

British Columbia’s mountainous landscape con-tains numerous forested watersheds connected to fans that have human development or high natural resource values. Fans are the cone-shaped deposits of sediment formed where stream channels leave the con�nes of mountain valleys (Bull 1977). Fans are desirable sites for development because of their gentle gradients and workable materials. Unfor-tunately, fans are often dynamic landforms. They are run-out areas for hydrogeomorphic events (i.e., debris �ows, debris �oods, and �oods) originating in the watersheds. Fans commonly have uncon�ned stream channels, resulting in the broadcasting of water and sediment, and channel avulsions. Risks from hydrogeomorphic events in�uencing a fan must be carefully considered when natural changes occur in a watershed (e.g., wild�re or mountain pine beetle) or prior to watershed development (e.g., forest harvesting or other developments). This is because of the potential to change water and sediment regimes that can lead to changes in the timing, magnitude, and frequency of hydrogeomorphic events affect-ing fans. Even without land use changes in water-sheds, natural hydrogeomorphic events on fans have resulted in the loss of life and high �nancial costs throughout British Columbia (Septer and Schwab 1995) and world-wide (Sidle et al. 1985). It is therefore important that resource development in watersheds

above fans be planned and undertaken with an un-derstanding of the fan-watershed system and a criti-cal consideration of the risks to downstream features on fans (Jakob et al. 2000).

This manuscript describes �ve steps used to recognize and analyze risk in fan-watershed systems. The information presented builds upon a body of knowledge regarding fan-watershed systems from British Columbia and beyond. Notable sources include a multi-year investigation of forested fans across British Columbia (Wilford 2003; Wilford et al. 2002, 2003, 2004a, 2004b, 2005a, 2005b, 2006), Land Management Handbook 56: Landslide Risk Case Studies in Forest Development Planning and Operations (Wise et al. 2004), and Debris Flow Hazards and Related Phenomena (Jakob and Hungr 2005a). This handbook provides direction to those who, during the course of their work, encounter fans and need to analyze associated risks (e.g., forest practitioners and planners, engineers and geoscien-tists, regional planners). This manuscript attempts to provide an understanding of watershed-fan pro-cesses and the planning and assessment of watershed activities to help better manage potential environ-mental, social, and economic risks. It also helps to identify when it is prudent to engage hydrogeomor-phic specialists (i.e., hydrologists, terrain specialists, and others) in the process.

THE FIVE-STEP METHOD

Forest-land management in British Columbia has a largely site-level approach to planning. While many strong programs exist to address the importance of site-level management issues, some situations call for a wider perspective. For example, landslides and peak �ows in a watershed can lead to serious hy-drogeomorphic impacts on a fan located many kilo-metres away, destabilizing the fan surface, affecting infrastructure, and altering aquatic habitat. These effects are a result of connections in the fan-water-shed system, and risk analyses on fans should match the scale of these interactions between the fan and the upstream watershed.

The Gully Assessment Procedure (British Co-lumbia Ministry of Forests and British Columbia

Ministry of Environment 2001) presents an approach to assessing fan-watershed systems, but it is ap-plicable only to coastal areas of British Columbia. The Watershed Assessment Procedure Guidebooks (BCMOF and BCMOE 1999a) indirectly address risks on fans at a watershed scale. While the Mapping and Assessing Terrain Stability Guidebook (BCMOF and BCMOE 1999b) states that off-site downslope/down-stream elements-at-risk and possible consequences should be considered, assessments have been gener-ally conducted at the site level and often stop short of a risk assessment by focussing primarily on the hazards (e.g., initiation zones) (Schwab and Geertse-ma 2008). In 2006, the Province of British Columbia formalized landslide risk assessments for residential

2

developments (Association of Professional Engineers and Geoscientists of British Columbia 2006), which should result in better management of new activities on fans, given existing hydrogeomorphic risks. How-ever, these formal guidelines do not address incre-mental risks to existing elements (e.g., homes, roads, drainage structures) on fans due to natural processes or land use developments in the watershed.

This handbook provides the �rst comprehensive, provincial-level framework for assessing risk on fans from upstream watershed activities. The following �ve steps for analyzing risk in fan-watershed systems are used as a framework for this handbook:

Step 1: Identify Fans and Delineate Watersheds• Identify the physical inter-connections that exist

between areas of potential activities (watersheds) and areas of potential impacts (fans).

Step 2: Identify Elements-at-risk on Fans• Recognize and inventory values on the fan that

may be affected by the hydrogeomorphic process-

es in the watershed (i.e., either natural processes or those related to proposed management ac-tions).

Step 3: Investigate Fan Processes• Identify the nature of hazardous hydrogeomor-

phic processes (type, frequency, and disturbance extent).

Step 4: Investigate Watershed Processes• Identify watershed features controlling hydrogeo-

morphic processes (e.g., watershed hydrology, geomorphology, and the role of vegetation cover).

• Identify the potential for incremental hazards as-sociated with management activities.

Step 5: Analyze Risks and Develop Plans• Develop planning options for the watershed and

assess the associated risks.• Document the process and establish a plan to

monitor, evaluate, and report.

Step 1.1 The Fan-watershed System

A fan is a cone-shaped deposit of sediment formed where a stream channel leaves the con�nes of a mountain valley (Bull 1977) (Figure 1). It is an ex-pression of its watershed; the fan is created by and represents a summary of the hydrologic and geo-morphic processes in the watershed. The watershed, or catchment, is the source area for water, sediment, and woody debris, and the stream channels are the transport zone. The local climate provides the water and solar energy; geology and biota provide the transportable material and control the watershed morphology. Water and organic and inorganic mate-rials are moved from hillslopes, into stream chan-nels through various pathways. Materials eventually move out of the watershed and onto the fan. This strong linkage between the watershed, channel,

STEP 1 IDENTIFY FANS AND DELINEATE WATERSHEDS

and fan requires a watershed-level perspective.Watersheds can be of various scales. For example,

a small cutblock may comprise most of the run-off-contributing area of a culvert along a mainline logging road. The same cutblock may be only a very small portion of the watershed contributing water, sediment, and debris to a fan containing a highway bridge-crossing located several kilometres from the block. Medium-sized watersheds (on the order of �fty to hundreds of square kilometres) may include numerous tributary junction fans—each with its own watershed. Water and sediment leaving the tributary junction fans subsequently in�uence main-stem channel conditions and transport regimes. In other situations, mid-slope fans may cause the domi-nant �ow pathway to change from one downstream watershed to another as the stream naturally moves about on the fan surface (Figure 2).

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Figure 1 An aerial oblique of a fan—the clearcut harvested portion of the landscape. This is a “classic” fan, radiating as a cone-shaped landform where the stream leaves the confines of the hillslope.

Figure 2 An aerial photograph with notation indicating fan areas in red and potential flow pathways in blue. Depending on the dominant flow pathway on the mid-slope fan, water and sediment can be delivered to any of the valley- bottom fans.

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Step 1.2 Fan Identification

A “classic” fan has a cone shape; however, the devel-opment of a fan is affected by the space into which it builds and the hydrogeomorphic processes that occur there. Many fans are bounded on one or both sides by hillslopes, often resulting in an asymmetric shape to the fan. Some fans build into a broad valley that is not affected by any other hydrogeomorphic process while some fans build into a con�ned valley with a valley-bottom river. The valley-bottom river may remove sediment from the toe of the fan, thus limiting the size of the fan and potentially the spatial extent of sediment and debris deposits on the surface of the fan. Some fans build into lakes or the ocean (fan deltas), affecting both the spatial extent of the fan and the internal structure of the fan.

Identifying fans that may be affected by manage-ment activities in and beyond the planning area is a critical component in developing appropriate management strategies and prescriptions. A given planning unit may encompass no fans but may belong in the contributing areas of many separate fan-watershed systems. Conversely, a planning area may include little of the contributing areas, but many fans. Also, multiple scales of fan-watershed systems may be present, so it is important to identify fans, including their apexes and their boundaries. Various methods are available, and any method that identi�es the fans, particularly the fan apexes, is ap-propriate.

A common method is to use stereoscopic aerial photographs. A range of aerial photograph scales (e.g., 1:30 000 and 1:60 000) allows different vertical exaggerations and different levels of visible detail to be reviewed. A degree of aerial photographic in-terpretation skill is required, but can be developed through practice and by interpreting areas where one is familiar with ground conditions.

On topographic maps the typical “fan-shaped” morphology is often apparent where stream con�ne-ment ends at valley-bottom �oodplains or lakes. Us-ing stereo aerial photographs and topographic maps together will usually result in the most accurate fan delineation. Using either or both of these methods, the size of fans that can be identi�ed will vary de-pending upon the scale of available photographs or topographic maps.

Soil, sur�cial geology, and landform mapping has

been completed for many areas of British Colum-bia at 1:50 000 scale and generally identi�es larger fans in mapped areas (e.g., Runka 1972). Due to the relatively small map scale, many smaller fans will be amalgamated into larger polygons and thus may not be represented. More recently, landform mapping has been done in association with terrain stability mapping, which is usually completed at 1:20 000 scale and therefore is much better at identifying smaller fans (Figure 3). Much of this work is also available in digital format (available at: www.em.gov.bc.ca/mining/geolsurv/terrain&soils/frbcguid.htm). However, in some areas, low-gradient terrain units are combined into large polygons (e.g., colluvial and morainal blankets with fans) and the location of in-dividual fans may not be indicated. When using any mapped information, it is important to understand what the purpose of the work was, what the methods were, and at what scale the original mapping was completed.

Attempts have been made to automatically iden-tify fans using geographic information systems (GIS); success was limited due to challenges in identifying the distal (outer) boundaries of fans. The boundaries frequently transition into �oodplains with very little change in gradient.

Ground-truthing is validating remotely sensed information through �eld checks. It is recommended to ground-truth potential problem areas or to ran-domly validate mapping. Once in the �eld, fans can be difficult to identify because: they can be large, making it difficult to conceive of the entire landform without seeing it; distal portions may grade into wet-land areas, making �eldwork difficult or impossible; and fan boundaries may grade into other low-gradi-ent surfaces such as �oodplains, making fan bound-ary delineation difficult. Not withstanding these challenges, �eldwork is essential to complete Steps 2 and 3 of a fan-watershed assessment.

There is considerable discussion in the literature regarding the temporal origins of fans. In some situations, �eld evidence (e.g., elevated fan surfaces) supports the position that fan origin is linked to late-glacial sedimentation episodes unrelated to modern conditions (Ryder 1971a, 1971b; Ritter et al. 1993). During these periods, considerable volumes of sedi-ment were deposited as fans. As watersheds reveg-etated, sediment supply declined and streams from the watersheds eroded into the fan surface, leaving

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Figure 3 A landform map with alluvial (Ff in yellow) and colluvial (Cf in orange) fans highlighted. Of note: one fan was missed by the mapper but identified during fieldwork—it was mapped as a complex of morainal blanket and morainal veneer (Mbv) and is located just below the centre of the map and highlighted in orange (a portion of polygon 100).

elevated fan surfaces and lower surfaces that are in�uenced by contemporary hydrogeomorphic pro-cesses. Inactive elevated or “paraglacial” fan surfaces can be found on many fans in British Columbia, but other types of fans are also found. Watersheds with abundant sediment transported to their fans may have fans that are actively growing (Figure 4) (Beaty

1970). Watersheds that have abundant water but limited sediment transport may have dissected fans (Figure 5) (Hunt and Mabey 1966). Some fans are considered to be in a steady-state, dynamic equilib-rium with sediment supply and water (Denny 1965, 1967; Hooke 1968). The �eldwork described in Steps 2 and 3 will identify the nature of speci�c fans.

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Figure 4 An example of an actively growing fan resulting from high sediment transport from the watershed to the fan.

Figure 5 An example of a dissected fan (A) resulting from abundant water but limited sediment transport from the watershed. The stream channel (B) is incised into the fan surface 6–10 m.

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Step 1.3 Watershed Delineation

It is necessary to identify the watershed upstream of each fan in the planning area, and the fans outside of the planning area if activities are planned in the watersheds of these fans. A watershed, catchment, or drainage basin is de�ned as the area where the water arriving at the surface will drain to a point of inter-est (Strahler and Strahler 1987). Properly delineat-ing a watershed is one of the most important steps toward understanding the fan-watershed system. Watershed boundaries can be determined though a variety of methods, ranging from a pencil and a topographic map to advanced GIS.

For individual or a small number of watersheds, the best method is manually drawing watershed

boundaries on appropriately scaled paper or digital topographic maps, as described below and illustrated in Figure 6.

1. Determine the point of interest. In the case of fan studies, the point of interest is usually the apex, or top, of the fan.

2. Draw the boundary moving upslope from the point of interest, crossing contour lines perpen-dicularly. An exception is commonly required in the immediate vicinity of the apex where the scale of the map is generally insufficient to properly characterize the surface topography.

3. Continue drawing until a closed polygon has been created de�ning the catchment area above the fan.

Figure 6 A topographically defined watershed above a fan.

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In this way, topographic watershed boundaries are readily de�ned and reasonably accurate and thus are the standard for hydrology, geomorphology, and engineering studies. There may be errors, such as in areas of low relief where surface water �ow path-ways may not follow the mapped surface expression. Where groundwater �ow paths do not conform to topographically de�ned watershed boundaries, er-rors can also be introduced.

Manual delineation can also be completed with GIS. The advantage of using GIS is that not only can the watershed boundary be easily altered but water-shed area and other relevant watershed metrics can be readily and accurately attained. A planimeter or digitizer can be used to determine watershed area if the topographic information is in paper format.

STEP 2 IDENTIFY ELEMENTS-AT-RISK ON FANS

A key part of risk analysis is to identify what is at risk on fans, and these are referred to as “elements-at-risk”—human safety, public and private property (including building, structure, land, resources, recreational site, and cultural heritage features), transportation system/corridor, utility and utility corridor, domestic water supply, �sh habitat, wildlife (non-�sh) habitat and migration, visual resource, and timber (BCMOF and BCMOE 2002).

Once all the involved watersheds and fans to be included in the analysis have been identi�ed, the elements-at-risk on the fans need to be inventoried. Both natural and human-made features should be included. This step involves office work (aerial pho-tographs and maps) and �eldwork. Include all ele-ments that are on the fan surface as de�ned in Step 1 and as re�ned in the �eld during this step.

The information collected for each element should include: description, location, value (qualitative or quantitative), a description of fan features in the vicinity of the element (e.g., immediately beside an old channel), and a description of the exposure (e.g., summer residences only, or office building occupied 10 hrs/day). Identifying elements-at-risk early in the risk analysis is important so that the intensity of the remainder of the investigation can be adjusted ac-cordingly. If the elements-at-risk are high value (e.g., human safety), more effort should be applied than if the elements-at-risk are of lower value (e.g., forest stand values).

Step 2.1 Human Safety

Potential injury or loss of human life is generally considered the highest-value element in risk analyses (Wise et al. 2004). Therefore, the presence of humans on the identi�ed fans creates a strong potential for

legal liability. A positive note is that humans are mobile features that: 1) may not always be present in that location, and thus have limited temporal expo-sure to fan hazards; and 2) can vacate the area if given enough notice.

Step 2.2 Anthropogenic Features

While humans can be mobile, their improvements frequently are not, and any damage may result in litigation. Thus anthropogenic features must be included as elements-at-risk. This refers to any human-made features on the identi�ed fan(s). Elements-at-risk on the higher surfaces should be described and their speci�c exposure to hydrogeo-morphic events should be evaluated. Assessment of human-made features requires an awareness not only of direct costs (e.g., repairing the railway fol-lowing a debris �ow) but also the indirect costs (e.g., loss of business revenues due to delays). Indirect costs can be very signi�cant, as in the case of trans-continental rail lines, where one day of traffic closure may cost $10 million (Jacob and Hungr 2005b). Common anthropogenic features are water intakes (Figure 7), roads (including bridges), railways, houses (Figure 8), institutional buildings, power transmission lines, and buried �bre optic lines. Some features such as drainage structures on the fan may not just be elements-at-risk, but—if poorly designed to accommodate the water, sediment, and debris from naturally occurring hydrogeomorphic events—may increase the likelihood of negative impacts on the fan (Figure 9). If there is a history of elements affected by �oodwater or debris from hydrogeomorphic events, subsequent activities in the watershed may be blamed for any future damage. Thus, documenting the fan history prior to proposed

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Figure 7 A water intake structure at the apex of a fan.

Figure 8 This building was damaged by a debris flow caused by a drainage diversion related to a forest road.

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or existing watershed activities is critical, not only to identify the hydrogeomorphic issues that need to be addressed, but to provide a defence should impacts occur that are unrelated to those watershed activi-ties.

Step 2.3 Natural Features

Fish habitat and forest sites are common natural features on forested fans. Special habitats or rare fea-

tures may also exist and should be researched. With any feature, an understanding of why it exists on the fan is important to correctly managing upslope areas to preserve the conditions. A common example is where high-quality spawning gravels and �sh habitat are present in the stream channel on the fan. To pre-serve the natural conditions, management within the watershed should not lead to degradation of water quality, changes in bedload movement, or signi�cant changes to peak stream�ows.

Figure 9 This drainage structure is not designed to accommodate the water, sediment, and debris from naturally occurring hydrogeomorphic events. This is an additional risk factor for forest manage-ment in the watershed.

STEP 3 INVESTIGATE FAN PROCESSES

The fan surface is investigated from the office and in the �eld to identify the nature of hydrogeomorphic processes—their type, frequency, and disturbance extent. This step can be undertaken while investigat-ing the fan for elements-at-risk. Further information on the topics covered in this section can be found in Wilford et al. (2005b) and Jakob and Hungr (2005a) (particularly Chapters 2, 8, and 17).

Step 3.1 Hydrogeomorphic Processes

The three principal hydrogeomorphic processes in�uencing fans—debris �ows, debris �oods, and �oods—leave characteristic deposits on fan surfaces (Table 1). Evidence of all three processes can be found on some fans. For example, it is common for debris �ow fans to also experience debris �oods and

Table 1 Characteristics of hydrogeomorphic process deposits (after VanDine 1985; Smith 1986; Pierson and Costa 1987; Wells and Harvey 1987; Costa 1988)

Characteristics Flood Debris �ood Debris �ow

Mode of deposition Grain-by-grain, dominated Rapid grain-by-grain En masse by traction processes aggradation from both (bouncing or rolling along suspension and traction the streambed)

Strati�cation Massive or horizontal None or horizontal None strati�cation (with cross strati�cation strati�cation)

Grading Variable: as a result of Frequently normal-graded None; reverse; reverse to sequential processes rather (coarse on bottom, �ne normal than a single process on top)

Sediment characteristics Clast-supported with an Clast-supported, with Matrix-supported; rarelyand texture open framework or predominantly coarse sand, clast-supported; very poor to distinctly �ner grained moderate to poorly sorted, extremely poor sorting; matrix of in�ltrated sand; bmax typically <80 cm but extreme range of particle rounded clasts; wide range may be larger sizes; bmax 60–230 cm, may of particle sizes; sorting contain megaclasts up to from front to tail; bmax 400 cm <10 cm to >20 cm

Orientation of clast Always perpendicular to Large cobbles to boulders are Variable, based on locationlong-axisa; �ow; usually well usually perpendicular to �ow; within �ow; parallel to �owimbricationb imbricated pebbles to small cobbles are is most prominent; weak to usually parallel to �ow; weak no imbrication imbrication and collapse packing

Landforms and deposits Bars, fans, sheets, and Similar to water �ood but Marginal levees, terminal splays; channels have large deeper deposits lobes, trapezoidal to width-to-depth ratio U-shaped channel

a A clast (e.g., pebble, gravel, cobble) has three axes: “a” is the longest, “b” is the second longest, and “c” is the shortest. The “b” axis determines the sieve size that the clast will pass through. The term “bmax” refers to the “b” axis of the largest clast that can be found in a deposit.

b Imbrication is the shingling or overlapping of clasts with the upper edge of each clast inclined downstream, similar to a deck of tilting cards. Clasts are moved into this position by stream�ow.

�oods, and while �oods and debris �oods can occur more frequently than debris �ows, debris �ows can be far more damaging. Identifying hydrogeomor-phic processes is a key element of the fan-watershed assessment because each process is the result of watershed factors that need to be addressed when planning activities in the watersheds.

Debris �ows are homogeneous (single phase) sediment–water mixtures similar to wet concrete, with sediment concentrations between 70 and 90% by weight (or 47–77% by volume), and result in the formation of marginal levees and terminal lobes

(Costa 1984, 1988; VanDine 1985; Smith 1986; Pierson and Costa 1987; Wells and Harvey 1987; Hungr et al. 2001). Debris �ows in British Columbia commonly have peak discharges 2–50 times larger than the 200-year �ood (Jakob and Jordan 2001). Debris �ow levees are steep-sided features that are often in paral-lel pairs, marking the path of the debris �ow. Levees are formed as a debris �ow traverses a fan—the core continues downslope while the slower-moving edges of the �ow come to rest as deposits of large-diameter sediment. Levee height is proportional to event �ow depth, and is typically 1–2 m high.

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Debris �oods, also referred to as hyperconcen-trated �ows, occur when a greater volume of stream-bed materials is mobilized than in a �ood. As with �oods, the mixing of sediment is not complete and there is a rapid increase in solids concentration toward the bed. Debris �oods have sediment concen-trations between 40 and 70% by weight (or 20–47% by volume); sediment deposits are bars, fans, sheets, and splays (Costa 1988; Hungr et al. 2001). While debris �oods move more and larger sediment than �oods, peak discharges are at most 2–3 times that of major �oods (e.g., 100-year �ood) (Hungr 2005). Debris �oods commonly have a maximum particle size of the same order as the peak depth of major �oods—for example, up to several tens of centime-tres (< 1.0 m) in the case of typical small mountain streams (Hungr et al. 2001).

Floods have sediment concentrations between 1 and 40% by weight (or less than 20% by volume); sediment deposits are bars, fans, sheets, and splays (Wells and Harvey 1987; Costa 1988; Hungr et al. 2001). Characteristically the bars, sheets, and splay deposits from �ood events are not as extensive as debris �ood deposits, are better sorted, and have a sediment size that is not as large. For example, a �ood in a 2 m wide stream channel may deposit sediment several metres each side of the channel, while a debris �ood may deposit sediment 10 metres each side of the same-sized channel.

Fans can be formed from one or a combination of these three processes, any of which may no longer be actively forming the fan. For example, the major portions of many fans in British Columbia were formed as the glacial ice was melting approximately 10 000 years ago, and there was abundant meltwater and exposed, unconsolidated sediment. These are referred to as “paraglacial fans” (Ryder 1971a, 1971b), and sediment deposits on the fan surface may not re�ect contemporary processes. For example, if a fan does not have recent evidence of debris �ow activity, but debris �ow levees are present on a fan surface with 500-year-old trees and well developed soil horizons, it is likely that the deposits re�ect processes that are not currently active in the fan’s watershed. However, caution must be used when deciding whether evidence is no longer re�ective of contemporary hydrogeomorphic processes. Jakob and Jordan (2001) identify return periods of up to 500 years for some debris �ows, and, historically, the

return period of events considered too dangerous for residential construction in British Columbia was 1 in 475 years or less (British Columbia Ministry of Transportation and Highways 1996).

Step 3.2 Event Frequency

Investigations of hydrogeomorphic activity on fans clearly indicate that events are neither rare nor extreme (Innes 1985; Jakob and Jordan 2001). To illustrate this point, evidence of recent hydrogeo-morphic activity was investigated with a random sample of 51 fans in the Bulkley Timber Supply Area, an area with a mix of plateaus and mountainous terrain. There had been little or no human activ-ity in the watersheds of the sample fans. While this area would not be characterized as overly unstable, hydrogeomorphic activity was observed on 41 fans, indicating that 82% of the fans had disturbance occurring on at least a portion of the fan surface in the last 50 years. A similar study was undertaken on the southern British Columbia coast, and recent hydrogeomorphic activity was found on 49 of 55 fans (89%) (Millard et al. 2006). On the other hand, in the drier southern interior of the province there are wa-tershed-fan systems that have recently experienced hydrogeomorphic activity on their fans outside of stream channels, after several hundred years of inac-tivity (Giles et al. 2005). These longer return period systems, with events often related to widespread forest health issues or severe forest �res, can result in underestimation of the hydrogeomorphic hazards.

Different methods can be used to determine the frequency of events. In populated areas it is common to �nd records of events in local archives (Septer and Schwab 1995). Dendroecology—using scars, dating cohorts of trees, and analyzing tree ring widths for abrupt, long-lasting growth changes—is a useful method of establishing a year, and in some cases a season, of events (Strunk 1997). However, if recent events have disturbed more of the fan surface, it is common to lose some or all of the “tree record” of older events. This method requires a degree of prepa-ration of increment cores from trees—sanding and microscopic identi�cation of growth rings to identify growth changes. For a more detailed description of dendroecology techniques refer to Dendroecology—A Guide for Using Trees to Date Geomorphic and Hy-drologic Events (Wilford et al. 2005a).

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Dendroecology can be as simple as determining a possible date for a hydrogeomorphic-event deposit on a fan by determining the stand age on top of the deposit. In �re-dominated ecosystems, the lack of evidence of a stand-establishing �re on a fan sur-face (e.g., burnt stumps and logs and ash in the soil) suggests that the stand was established following a hydrogeomorphic event, and the stand age will cor-respond closely to the age of the event (Grainger and Wilford 2004). Where there is evidence of stand- establishing �re on a fan surface, it is difficult to determine how many stand rotations may have occurred on the deposit, and the deposit may be hundreds (if not thousands) of years old.

Trees can be partially buried by sediment in the hydrogeomorphic riparian zone, and will not die if the roots have sufficient oxygen and water. Charac-teristically these trees do not have a basal or “butt” �are, but the �are can re-establish over a period of time. While lack of butt �are and recent deposits of sediment are indications of hydrogeomorphic activ-ity, it is possible for trees to recover after a period of time (as little as 25 years) and re-establish basal �are. Dendroecology is a useful means of establish-ing whether trees have in fact been buried and have recovered a “normal” form (e.g., there will be an abrupt reduction in radial growth rings immediately following the event, and continuing reduced growth for a period of 10 or more years, then a gradual re-lease as the tree establishes a new root system just below the soil surface).

On �ood fans, individual events may be less im-portant than the overall �ood and sediment regime. The spatial extent and complexity of the channel network on the fan can be used to assess factors such as sediment supply or the type of vegetation on the fan (Millard et al., in press)

The fan surfaces identi�ed in Step 1 may include elevated surfaces that have not been in�uenced by contemporary hydrogeomorphic processes. On for-ested fans this can be evident in lower site produc-tivity on the elevated surfaces of some fans (Wilford et al. 2006); however, site productivity may not vary in some climatic regions of British Columbia. In those situations, differences in stand age and soil pro�le development may indicate a lack of contem-porary hydrogeomorphic disturbance.

The output from this part of the fan investigation should be the identi�cation of how often a particu-lar type of event has occurred in the past. In some

situations the best one can do is determine the time since the last event—because evidence of previous events has been removed by subsequent events. In these situations, the time estimate is an estimate of the event return period (T), or the number of years between event occurrences.

Step 3.3 Event Magnitude

The disturbance extent of hydrogeomorphic events is an indication of the power and volume of water and/or material that issues from a watershed. The distur-bance extent of hydrogeomorphic events on forested fans can be determined from aerial photographs and �eldwork. Evidence on aerial photographs can range from strips of bare sediment to strips of century-old trees (cohorts). However, some evidence may not be apparent on aerial photographs due to the nar-row swath cut through the forest, and some events may not be powerful enough to clear forests—sedi-ment is spread on the fan surface beneath trees, and �eldwork is necessary to identify these disturbances. A zone of potential disturbance outside the stream channel on a fan is de�ned as the hydrogeomorphic riparian zone. This zone can range from a few metres wide to hundreds of metres wide. For a more de-tailed discussion of identifying hazardous zones on fans refer to Forest Management on Fans—Hydrogeo-morphic Hazards and General Prescriptions (Wilford et al. 2005b).

Incised stream channels on fans may give the impression that hydrogeomorphic events will not in�uence the fan surface. However, depending upon the con�guration of the channel and banks, debris �ows can deposit up to 6 m of material in a single event, resulting in signi�cant disturbance beyond an incised channel (Osterkamp and Hupp 1987). Evidence of over-bank disturbance from hydrogeo-morphic events and channel conditions should be explored prior to assuming that elements are not at risk. Channels with a high likelihood of debris jam-ming may require an extra degree of caution when developing forest management recommendations.

The natural disturbance extent of hydrogeomor-phic events can also be modi�ed by anthropogenic features and practices (e.g., riparian harvesting). Some features such as dikes tend to convey the disturbance downstream to lower positions on the fan; however, if features are constructed of local streambed materials without armouring, they are

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more likely to be eroded and fail. On fans, both inadequate drainage structures and roads that climb in elevation toward stream channels can increase the extent of disturbance (Wilford et al. 2003). Removal of the hydrogeomorphic riparian zone through forest harvesting, or through agricultural or resi-dential clearing, increases the potential for greater disturbance on the fan surface (Wilford et al. 2003). The fan investigation should document the poten-tial hydrogeomorphic implications of any existing anthropogenic in�uence.

The outcome from this part of the investigation should be an estimate of the probable magnitude, runout zone, power, and potential damaging nature of the event to pair with the frequency of occurrence determined in Step 3.2. It is not uncommon that more than one kind of event has occurred on the fan, or on different parts of the fan, and that these sepa-rate types have different frequencies, and different implications for management in the watershed (see the Eagle Summit Creek case study in Appendix 2).

STEP 4 INVESTIGATE WATERSHED PROCESSES

The fan investigations provide an understanding of past hydrogeomorphic processes that formed the fan, and thus identify the processes produced by the watershed. Investigating the watershed above the fan is necessary to understand watershed characteris-tics, how they are linked to the processes observed on the fan, and how that linkage creates the exist-ing hydrogeomorphic hazard. Only then one can determine the degree to which planned development activities in the watershed could in�uence those processes and potentially alter the existing hazard, creating additional or incremental hydrogeomorphic hazards.

A series of avenues are pursued when investigat-ing a watershed—including both office work and �eldwork. The watershed investigation is similar to that undertaken using the Forest Practices Code Wa-tershed Assessment Procedures (BCMOF and BCMOE 1999a) with a focus on geomorphic processes issuing from the watershed, particularly debris �ows and debris �oods.

Step 4.1 Watershed-fan Process Linkages

This section discusses how watershed-initiated hydrogeomorphic processes are connected to the fan and which watershed processes could lead to increased or incremental hydrogeomorphic hazards. Subsequently, this identi�es an appropriate manage-ment focus (presented in Table 2 and in italics in the descriptions of the hydrogeomorphic processes).

FloodThere are two basic types of �oods: hydrologic and dam-release. Rainfall and/or snowmelt generate

hydrologic �oods, which are the most common type of �ood in British Columbia. Forests can play an important role in hydrologic �oods, particularly in snowmelt-dominated watersheds where forest har-vesting or natural deforestation (e.g., mountain pine beetle or wild�re) can alter snow accumulation and melt rate processes (Winkler et al. 2005), potentially resulting in increased peak �ows. Roads, trails, and soil compaction (e.g., landings and other compacted areas) can also in�uence �ood generation (Harr et al. 1979; Wemple and Jones 2003). Equivalent clearcut area (ECA) and percent of the watershed in roads and compacted areas are two important watershed indica-tors where �ood impacts to fans are possible (BCMOF and BCMOE 1999a). However, attention must be paid to mass wasting because land use–related landslides can increase sediment delivery to �ood fans, result-ing in channel avulsions and fan destabilization (see Shale Creek Case Study presented in Appendix 3).

Dam-release �oods are caused by a rapid release of large volumes of stored water such as moraine, snow avalanche, or debris �ow dam ruptures, glacial lake outbursts, beaver dam failures, and downstream dilution of debris �ows and debris �oods. Dam-re-lease �oods are relatively rare, but they do occur in British Columbia. There is a record of glacial out-burst �oods occurring in northern British Columbia (e.g., Alsek River, Tulsequah Lake, Salmon, and Bear Rivers near Stewart, and the Kitimat River) (Septer and Schwab1995); however, with the melt-ing of alpine glaciers, the occurrence could increase (Geertsema and Clague 2005). Landslides that block stream channels and subsequently release rapidly have been documented throughout British Columbia (Jakob and Jordan 2001). Beaver dams may fail as the

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result of a variety of processes, including high-inten-sity precipitation, rapid snowmelt, animals burrow-ing through the dam, human destruction of portions of dams, lack of maintenance by beaver, and collapse of upstream dams (Hillman 1998; Butler and Malan-son 2005). A dam-release �ood can exceed a design stream�ow �ood (e.g., 100-year return period) by a factor of up to 100 (Jakob and Jordan 2001). The potential for dam-release �oods should be explored, particularly where landslide-prone terrain has previ-ously, or could potentially, produce a landslide dam. In addition to local knowledge regarding past dam-release �oods, forest cover on a fan may contain dendroecological evidence of past events—cohorts of trees and damage to older trees (Wilford et al. 2005a).

Debris �oodThere is a continuum from water �oods through debris �ows, with debris �oods being an intermedi-ary process. Debris �oods should be viewed as an extraordinary form of �ood in which an extremely large volume of sediment with a wide range in grain size is moved (essentially the whole streambed) and deposited in a short period of time (Smith 1986). Where debris �ows have been observed closely, some evolve into debris �oods as they proceed down-stream due to addition of water from tributaries (Pierson and Scott 1985). Thus the factors that lead to both debris �ows and �oods should be explored in de-bris �ood watersheds—increase in sediment delivery to streams and the potential to increase peak stream-�ows, and sediment and debris loading in stream channels and gullies.

Table 2 Forest management focus for different hydrogeomorphic processes. Key issues are bolded.

Process Initiation Management focus

Flood Runoff ECA – harvest, �re, forest health Road density Fire – reduced soil in�ltration Landslides – increasing sediment load to channels on the fan (in�lling channels)

Dam rupture Dam rupture – beavers, road drainage structures Landslide dams – harvest and roads (coast), gentle-over-steep (interior)

Debris �ood Runoff ECA – harvest, �re, forest health Road density Fire – reduced soil in�ltration

Landslide Harvest and roads on unstable terrain and gentle-over-steep in the interior

Dam rupture Dam rupture – beavers, road drainage structures Landslide dams – harvest and roads (coast), gentle-over-steep (interior)

Debris �ow Landslide Harvest and roads on unstable terrain and gentle-over-steep in the interior

Runoff—in-channel ECA – harvest, �re, forest health initiation Road density Fire – reduced soil in�ltration

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Debris �owValley-con�ned debris �ows in humid temperate forests characteristically have three forms of initia-tion (Takahashi 1981). The most common in British Columbia is by impulse loading of channels, where an open-slope failure (debris slide or debris ava-lanche) that enters a steep stream channel destabi-lizes sediment in the channel and proceeds down channel by the force of gravity as a debris �ow (Sidle et al. 1985; Jordan 1994; Millard 1999). This highlights the need to identify landslide-prone terrain that is tributary to steep gullies and stream channels and to manage the hazards appropriately. The two other forms of initiation occur during a major runoff event with the mobilization, or bulking, of sediment and debris stored in the stream channel, or the rupture of a debris jam. Particularly in snow-dominated wa-tersheds, in-channel mobilization of sediment can be affected by changes in peak �ow runoff due to changes to the forest canopy—as a result of wild�res, insect infestations, or forest management activities includ-ing road density.

All three forms of initiation may occur following extreme climatic inputs, which are assumed to occur randomly. While climatic events are essential (Septer and Schwab 1995), other conditions must be present for either an open-slope failure or mobilization of channel sediments (Miles and Kellerhals 1981; Dagg 1987). For example, if there is little or no sediment and debris load in a channel, an in-channel debris �ow is less likely to occur. A recent debris �ow may have removed sediment and debris from a channel, and the time to the next signi�cant event generally depends on how rapidly the sediment load in the channel is recharged. The exception is where very large open-slope landslides enter stream channels and have enough material to develop into signi�-cant debris �ows regardless of the rate of sediment entrainment along the pathway. The rate of sedi-ment delivery to channels varies widely between watersheds, depending on geology and vegetative cover. Some channels experience rapid sediment recharge and debris �ows can occur more than once a year, while others require centuries or millennia after an event to recharge channel sediment enough to experience another debris �ow. A key factor in determining debris �ow potential is to examine the sediment and debris loading in stream channels and gullies that is available for entrainment by an event, and to understand the hillslope-channel conditions in a watershed.

Step 4.2 Office Investigations

The �rst phase of office investigations is to compile information related to watershed processes, to gain an overview of the watershed, and to focus subse-quent �eldwork.

MorphometricsIn mountainous or “graded” watersheds, topo-graphic maps or GIS analyses can be used to generate several basic topographic measurements that can be used to provide a �rst approximation of the hy-drogeomorphic processes. The relative relief number is watershed relief divided by the square root of wa-tershed area (Melton 1965). The relative relief number has been used in various locations to differentiate debris �ow, debris �ood, and �ood watersheds (Table 3). There are some cautions when using this ap-proach. First, it is only an approximation and should not be used to guide management decisions in the watershed unless it is supported by �eld observations and the detailed fan investigation undertaken in Step 3. Second, research shows regional variation within British Columbia, and so this approach should be used cautiously, particularly in areas not previously studied. Third, the method is designed for “graded” mountain watersheds and does not apply to water-sheds in plateau terrain. For example, the Yard Creek watershed near Sicamous is 100 km2 with a large low-gradient upland plateau. The relative relief num-ber predicts that the dominant hydrogeomorphic process will be �ooding. However, there are exten-sive debris �ow deposits at the fan apex, originating from a small steep-gradient tributary just above the fan, which cannot be identi�ed except through aerial photograph interpretation and �eldwork. While the large fan is formed predominantly by �oods, forest management planners must be aware that debris �ows can originate from a particular part of the watershed and that these hazards must be managed accordingly.

Sediment production and movement: surface erosion, landslides, and stream channelsThe general location and nature of sediment sources in a watershed can be determined from aerial photo-graphs, topographic maps, and interpretative maps. Speci�c details on sediment production are explored during �eldwork (e.g., sediment textures, connectiv-ity to stream channels, causes of landslide initiation).

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Aerial photographs provide a perspective on both where landslides have occurred and the connectivity of the landslide-prone terrain to stream channels. Comparison of recent and historic aerial photo-graphs is useful, particularly in situations where forest growth obscures the evidence of landslides.

Topographic maps are used to identify speci�c steep slopes and gentle-over-steep situations, and to provide indications of connectivity of hillslopes to stream channels. GIS analysis can be used to identify steep slopes that could be prone to landslide activity; however, this approach is strengthened when other factors such as slope water movement and landslide history are included in the modelling analysis (Pack et al. 1998).

Sur�cial geology and terrain maps provide information on the nature of sur�cial materials in the watershed. This is particularly useful in areas with lacustrine deposits that have a high potential to degrade water quality—either in the natural setting or as a result of land use activities. Terrain stability maps identify landslide hazards, but are generally restricted to either operating areas or the timber harvesting land base (i.e., the maps do not identify slope stability issues in alpine areas, which in some cases can be signi�cant, as described by Geertsema et al. 2006).

Forest �re severity maps of burned areas are valu-able in identifying areas with reduced soil in�ltra-tion—water repellency, removal of surface cover, and soil sealing (Larsen et al., in press). These soil condi-tions have resulted in signi�cant increases in rainfall

runoff (Scott and Van Wyk 1990; Moody and Martin 2001) and debris �ows (Jordan et al. 2004) (Figure 10). The implications of hydrogeomorphic impacts to fans can be signi�cant.

Naturally unstable areas in the lower portion of a watershed have been found to more strongly in�u-ence the nature of hydrogeomorphic processes (e.g., more sediment can be delivered to fans during a hydrogeomorphic event) than unstable areas that are located further from the fan in the headwaters of the watershed (Wilford et al. 2005b).

Aerial photographs are used to identify stream reaches. The Channel Assessment Procedure Guide-book (BCMOF and BCMOE 1996a) is a useful refer-ence. If the channels are wide enough it is possible to identify the general nature of the bed materials and channel form (e.g., Hogan 2008 notes 30 m wide channels with 1:50 000 aerial photographs down to 10 m wide channels with 1:5 000 aerial photographs). Aerial photographs are also used to identify gullied terrain and determine the degree of connectivity between the gullies and stream channels. The ability of stream channels to transport sediment is a key factor in determining the downstream consequences of sediment that enters a stream channel (Hogan and Wilford 1989).

Flood generation: forest cover and road densityForest cover maps are used to identify the extent and location of natural and anthropogenic disturbance, including wild�res, forest health issues, and past for-est harvesting. These maps are also used to identify

Table 3 Predictive models for dominant hydrogeomorphic processes using the relative relief number (RRN) and watershed length (the distance from the fan apex to the most distant point on the watershed boundary)

Hydrogeomorphic process Class limits Source

Flood RRN < 0.30 Jackson et al. 1987 Wilford et al. 2004b Millard et al. 2006Debris �ood and debris �ow RRN > 0.30 Jackson et al. 1987Debris �ood RRN > 0.30 and Length > 2.7 km Wilford et al. 2004bDebris �ood RRN 0.30–0.60 Millard et al. 2006Debris �ow RRN > 0.60 and Length < 2.7 km Wilford et al. 2004bDebris �ow RRN > 0.52 Bovis and Jakob 1999Debris �ow RRN > 0.60 Millard et al. 2006

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potential changes to forest cover, such as the spread of mountain pine beetle. Speci�c issues relating to forest health should be explored, as the implications to forest cover can be signi�cant in both immature and mature stands (Woods et al. 2005). The Veg-etation Resource Inventory (VRI) is the source of information on forest cover; however, data are gener-ally not entered on the heights of regeneration until the openings have been declared free-to-grow—a period of up to 20 years. Data on the extent of for-est disturbances and the subsequent regeneration are used to calculate the equivalent clearcut area (ECA)—a measure of hydrologic disturbance to the forest cover (BCMOF and BCMOE 1999a). The location of hydrologically recovering stands is important in snowmelt-dominated watersheds due to the more rapid snowmelt in openings and immature stands (Winkler and Roach 2005). Snowmelt from higher-elevation disturbed areas can synchronize with lower-elevation melt, increasing peak stream�ows. Conversely, accelerated low-elevation snowmelt could desynchronize runoff, tending to decrease peak stream�ows.

Road density can be determined using GIS. Ad-ditional areas should be included in the calculation of compacted areas in the watershed (e.g., landings

and skid trails). Research has identi�ed that as little as 5% of a watershed in roads and compacted areas can in�uence peak �ows (Harr et al. 1979; King and Tennyson 1984). The value determined in the office may be an overestimate that can be re�ned during �eldwork. For example, deactivated roads and spe-ci�c road segments may not be in�uencing hillslope hydrology and peak �ow generation (Wemple and Jones 2003).

Step 4.3 Field Investigations

Fieldwork in the watershed is undertaken to explore key factors identi�ed in the office work, particularly to verify data and collect information that cannot be generated in the office. Fieldwork is undertaken at both the watershed scale and the proposed develop-ment scale. The watershed scale explores the key processes identi�ed in the previous �eld and office work. A �eld review of the area under plan (pro-posed roads, harvesting, and other developments) should focus on areas identi�ed during the office work as potentially problematic—surface erosion and landslide-prone terrain in particular. This will verify whether the situations are problematic and lead to the identi�cation of management options.

Figure 10 A debris flow initiated as a result of hydrophobic soils in the Kuskanook watershed, damaging homes and the highway on the fan. (Source: D. Boyer, B.C. Ministry of Environment.)

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Sediment production and movement: stream channels, gullies, landslides, drainage structures, and surface erosionAn overview of the channel network is undertaken to describe key features in the reaches identi-�ed from aerial photographs. Descriptions should include bed and bank materials, riparian condition, and an assessment of sediment and debris loading. A useful aid is the Channel Assessment Procedure Field Guidebook (BCMOF and BCMOE 1996b). The ripar-ian zone can be explored during this �eldwork for evidence of hydrogeomorphic activity (e.g., dendro-ecology).

Gullies identi�ed during the office work should be examined to identify sediment and debris loading, evidence of debris �ow activity, and connectivity to the stream system. While the Gully Assessment Pro-cedure (BCMOF and BCMOE 2001) was developed for coastal British Columbia gully systems, the approach is worthy of consideration for assessing gullies in other areas of the province.

An overview of the road system, including non-status roads, is needed to address linkages to all three hydrogeomorphic processes. The age of the road system could provide useful information, as construction standards changed signi�cantly with the introduction of the Forest Practices Code in 1995 (Horel 2006). It is necessary to identify past and potential landslide risks from road systems, and the potential linkage to stream channels.

Drainage structures should be examined across stream channels or gullies where hydrogeomorphic hazards have been identi�ed. The issue to assess is whether the structures will pass or block potential events, and determine the consequences.

If an element-at-risk in a watershed is domes-tic water production, �eldwork should explore for evidence of surface erosion issues. Selective sampling for surface erosion may be advisable (e.g., Carson et al. 2007). A challenge to be aware of is that evidence of past surface erosion can be masked by revegeta-tion, road reconstruction, or grading and surface maintenance. The Wathl Creek Case Study (Ap-pendix 1) also explored other issues associated with drinking water—contamination from wood leachate, hazardous materials, and wildlife.

Flood generation: forest cover and roadsThe status of regeneration should be examined, par-ticularly in watersheds where peak �ows have been

identi�ed as a factor for hydrogeomorphic processes. It is likely that the VRI information is not current, and this can have signi�cant implications for the calculation of equivalent clearcut area.

Forest health issues identi�ed in the office should be examined in the �eld from two perspectives. It is important to integrate the current hydrogeomorphic implications of forest health issues as well as the potential future issues. This is particularly relevant in watersheds that have a high potential impact to the forest cover (e.g., mountain pine beetle). In wa-tersheds subject to signi�cant forest health issues, it is likely that there will be hydrogeomorphic conse-quences, regardless of human activities.

Areas that have had intense forest �res in the past 5 years should be examined to determine the degree of reduced soil in�ltration and explored for evidence of erosion or overland �ow. The issue of hydrophobic soils has only recently become apparent in British Columbia, but the hydrogeomorphic consequences have been signi�cant (Jordan et al. 2004).

A �eld examination of the road system identi�es the degree to which roads are in�uencing hillslope hydrology and peak �ow generation. Emphasis in the �eld should be on identifying the degree of deactiva-tion of non-active roads (i.e., determine if drainage structures are still in place, and whether roads have compacted surfaces that are in�uencing water move-ment) and the quality of road cross-drainage (i.e., identifying if ditchlines are collecting subsurface �ows and delivering the water to streams rather than back onto the forest �oor). The office determination of percentage of the watershed in roads and com-pacted areas should be re�ned with this �eldwork.

Step 4.4 Synthesis of Watershed Processes

Compiling the �eld and office information will give an overview of the current and projected hydrogeo-morphic situation in a watershed that could affect the watershed-fan system and potentially increase the watershed hazard.

The next step is to develop or review plans in light of the potential for proposed activities to in�uence those watershed processes, and assess the potential risks from those activities to elements on the fan.

This would include updating ECA calculations, reviewing harvest locations for the potential to synchronize or desynchronize snowmelt runoff, identifying road and harvesting location relative to

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landslide-prone terrain, projecting road density, and exploring any other activities that could increase identi�ed hazards in the watershed. If forest health or other natural issues are in�uencing the forest

cover in a watershed, it is important to develop a forecast of background changes that can be antici-pated regardless of any new planning initiatives.

STEP 5 ANALYZE RISKS AND DEVELOP PLANS

In Step 5, all the previously gathered information is integrated in an analysis of existing and potential incremental risks in the watershed-fan system. This understanding is then used to guide management decisions concerning natural impacts to the forest cover (�re, forest health) or proposed (harvesting, road) activities in the watershed. Risk de�nitions and procedures in this section are consistent with Land Management Handbook 56, Landslide Risk Case Studies in Forest Development Planning and Operation (Wise et al. 2004), which can be refer-enced for further discussion of these topics.

Step 5.1 Understanding Risk Analysis and Risk Assessment

Risk is a measure of the probability of a speci�c event occurring, and the consequence, or adverse effects, of that event on speci�c elements, such as human health, property, or the environment. Simply stated, risk is the product of hazard and conse-quence.

Risk analysis includes determining hazards and consequences. Hazards are the potentially damaging events and the hazard analysis includes information such as the process, magnitude (including spatial extent), and likelihood of occurrence. Consequence refers to the likelihood of damage or losses to an element-at-risk in the event of a speci�c hazard; analysis of the consequence includes the spatial and temporal exposure and the vulnerability of the elements-at-risk. Consequence can also include the value of losses or damage to elements. The concepts of risk, hazard, and consequence are useful tools for land managers to employ, but completing hydrogeo-morphic risk analyses is the realm of quali�ed professionals.

Risk analysis includes using some process to com-bine the hazards and consequences to estimate the risk to elements. In the forest management context

this means understanding both the existing risks and the incremental risks as a result of proposed or imposed forest management activities in a water-shed.

Risk assessment includes evaluating the results of the risk analysis and determining whether that risk is acceptable. Generally this entails compar-ing the risk analysis results with public, corporate, and/or legislative standards of acceptable risk. Risk assessment can include hydrogeomorphic hazard specialists and other experts, but the �nal decision regarding risk acceptability is generally the respon-sibility of land managers and regulatory decision makers. Under the British Columbia Forest and Range Practices Act (2006), the determination of acceptable risk, and the decision to proceed with any forest management activities on the basis of that risk assessment, is the responsibility of forest licensees and BC Timber Sales.

If the risk assessment determines that the estimat-ed risks are unacceptable or intolerable, two options are available: cancel further plans, or identify risk control and mitigation measures to reduce the haz-ards and/or reduce the consequences. Identi�cation of measures involves hydrogeomorphic hazard spe-cialists, who can determine the effectiveness of risk control and mitigation measures. With this informa-tion, land managers can revisit the risk assessment in light of the control and mitigation measures and their associated costs.

The �ve-step procedure presented in this hand-book addresses risk analysis and risk control meas-ures. For a further discussion of risk assessment and acceptable risk, the reader is referred to Cave (1992), Fell (1994), Wise et al. (2004), and APEGBC (2006).

Step 5.2 Consequence

In risk analysis it is common to analyze hazards prior to analyzing consequences. However, within

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the forestry context it is common for managers to want to know what the potential consequences are �rst, because the level of consequence frequently identi�es the level of effort needed in all stages of the risk analysis. This approach also ensures that the consequence, which can be far away from the area managed, is not overlooked or paid less attention than it deserves. The development area is generally investigated for hazards, but the consequence area, if left until a later planning stage, can mean that work in the development area is either wasted or misdi-rected. Thus, in keeping with the approach taken in the forest sector, consequences are discussed prior to hazards.

Consequence considers the effects of the speci�c hazardous hydrogeomorphic event on the elements-at-risk on the fan (probability between 0 and 1, or qualitatively low, moderate, high). This can include an analysis of the likelihood the element will be at the site at the time the event occurs, which is sometimes called the temporal exposure. For �xed elements, such as houses, highways, or forests, this probability is always equal to 1 and does not need to be analyzed further. For highway and railway traffic, or human occupation of residences or institutions, there is gen-erally a temporal exposure probability of < 1.

Vulnerability of a given element will vary de-pending on the severity of the hydrogeomorphic event and the sensitivity of the element. Damage can be minor, partial, or total, and this can be expressed quantitatively (between 0 and 1) or qualitatively (low, moderate, high). This determination is gener-ally a joint effort involving hydrogeomorphic hazard specialists and other appropriate specialists such as �sheries biologists, foresters, and highway or railway transportation experts, depending on the elements potentially at risk.

Element exposure and vulnerability can be com-bined to further re�ne the consequence, as described in more detail by Wise et al. (2004). The information gathered in Steps 2 and 3 of this process should allow such further re�nements, if necessary.

Risk analysis can include estimating the worth of individual elements to produce a monetary conse-quence value. This can be expressed quantitatively as a numerical dollar value, or qualitatively as a low, moderate, or high dollar value. A qualitative ap-proach may be more appropriate for some elements, such as species at risk. Human lives should be as-signed the highest possible value in any consequence rating scheme.

For the risk analysis process described in this handbook, the consequence value should be de-scribed qualitatively (low, moderate, high). Other more quantitative procedures are possible, as de-scribed by Wise et al. (2004).

Step 5.3 Hazard

In this analysis, hazard is de�ned as the probability or likelihood of occurrence of a hydrogeomorphic event of a speci�c process and magnitude, and with a speci�c power and spatial disturbance extent. It can be expressed as a quantitative probability with a value between 0 and 1, or qualitatively a value be-tween very low and very high.

The investigations described in Steps 3 and 4 should determine the type, magnitude (size, power), and runout or impact area of an expected event, and the frequency (how often the event is likely to occur over time). This describes the existing hydrogeomor-phic hazard, which is the result of natural climatic, geologic, vegetation, and hydrologic watershed conditions, as well as any existing human land-use changes in the watershed. Its likelihood of occur-rence can be expressed as the return period T, or the frequency (annual probability) Pa, which is simply 1/T.

Section 5 describes the watershed processes con-tributing to the various hydrogeomorphic hazards. An understanding of the existing hazards is the �rst step in analyzing the potential hazards that could result from proposed or naturally imposed activities in the watershed, as also described in Section 5 and summarized in Table 2.

Following the procedures described there, one should arrive at a description of the development-related, or incremental hydrogeomorphic hazard, which should include event type, magnitude, and frequency, as a result of a particular forest activity. It is important to note, however, that the probability of occurrence of forest development-related hazards will be different than the existing natural hazard, because forest development effects usually have a relatively limited time frame compared to natural hazards. For example, the hydrological effects of harvesting, �res, or insect-related mortality decrease over several decades as the stand re-grows (BCMOF and BCMOE 1999a); roads can have a speci�ed use period after which they are deactivated, and poten-tial hazards decrease relative to the level and efficacy of the deactivation; and hazards associated with

22

hydrophobic soils in severe burns generally decrease signi�cantly after several years. A recent study of landslides on northern Vancouver Island found that the greatest frequency of landslides from harvested cutblocks occurred in the �rst 5 years following har-vesting (Horel 2006).

The long-term probability of a hydrogeomorphic event (Px) resulting from a particular forest develop-ment can be calculated if the frequency, or annual probability of occurrence (Pa) and the duration of effect of the forest development (x) are known, from:

Px = 1-(1-(Pa))x Equation 1

For example, if previous experience suggests that a road in a particular landscape position is expected to result in one landslide every 50 years, and that landslide could initiate a debris �ow in the channel it affects, the forest road–related debris �ow hazard has a return period (T) of 50 years and an annual frequency (Pa) of 1/50= 0.02. If the road will be deac-tivated to remove the landslide hazard after 20 years (x), the long-term probability of the forest road– related debris �ow (Px) is:

P20 = 1-(1-(0.02))20 = 0.33 Equation 2

Therefore, there is a probability of 0.33 that a debris �ow will be initiated during the 20-year life of the road.

Table 4 shows the long-term probabilities calculated using Equation 1, for a range of annual probabilities of events and a range of life spans of the watershed activities contributing to those events.

Table 5 summarizes the ranges of quantitative probabilities from Table 4, described by each qualita-tive hazard term, and de�nes those qualitative terms.

Continuing the previous example, there is a prob-ability of 0.33, or a high likelihood, that a particular 20-year road would initiate a debris �ow of a par-ticular magnitude and runout extent within that 20-year period. This is the incremental (watershed activity–related) hazard at that site.

The �nal step in the hazard analysis is to assign a qualitative value to the event magnitude, based on its power, which is a measure of its likelihood of causing damage, and its extent, which is a measure of how large an area and what risk elements it could affect. Table 6 is an example of a hazard analysis matrix.

Assuming the debris �ow in our example would reach risk elements on the fan, one could conclude that, since debris �ows are generally high-power destructive events and their extent includes risk elements, the event magnitude would be considered high. Therefore, with a high frequency and a high magnitude, the debris �ow hazard in this case would be very high.

Table 4 Example of long-term probabilities (adapted from Wise et al. 2004, Table A4.3)

Pa = Px, long-term probability of occurrence: Px = 1-(1-Pa)x annual x = life of watershed activity (years)prob 1 2 5 10 20 25 50 100 200 250 500 1000 2000 25001/1 1 1 1 1 1 1 1 1 1 1 1 1 1 11/2 0.50 0.75 0.97 1 1 1 1 1 1 1 1 1 1 11/5 0.20 0.36 0.67 0.89 0.99 1 1 1 1 1 1 1 1 11/10 0.10 0.19 0.41 0.65 0.88 0.88 0.93 0.99 1 1 1 1 1 11/20 0.05 0.10 0.23 0.40 0.64 0.72 0.92 0.99 1 1 1 1 1 11/50 0.02 0.04 0.10 0.18 0.33 0.40 0.64 0.87 0.98 0.99 1 1 1 11/100 0.01 0.02 0.05 0.10 0.18 0.22 0.39 0.63 0.87 0.92 0.99 1 1 1 VH1/200 0.01 0.01 0.02 0.05 0.10 0.12 0.22 0.39 0.63 0.71 0.92 0.99 1 11/250 0 0.01 0.02 0.04 0.08 0.10 0.18 0.33 0.55 0.63 0.87 0.98 1 11/500 0 0 0.01 0.02 0.04 0.05 0.10 0.18 0.33 0.39 0.63 0.86 0.98 0.991/1000 0 0 0 0.01 0.02 0.02 0.05 0.10 0.18 0.22 0.39 0.63 0.86 0.921/2000 0 0 0 0 0.01 0.01 0.02 0.05 0.10 0.12 0.22 0.39 0.63 0.711/2500 0 0 0 0 0.01 0.01 0.02 0.04 0.08 0.10 0.18 0.33 0.55 0.631/5000 0 0 0 0 0 0 0.01 0.02 0.04 0.05 0.10 0.18 0.33 0.39 VL L M H

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Step 5.4 Risk Analysis

The �nal step in the risk analysis procedure is to combine the hazard and consequence to arrive at an estimate of the hydrogeomorphic risks from speci�c watershed activities to speci�c elements on the fan. Table 7 is an example of a qualitative risk matrix; other matrices are possible. In this way, the qualita-tive consequence and risk values determined in the previous sections are combined to yield a qualitative risk value.

Depending on whether or not consequence ex-posure, vulnerability, or worth have been taken into account, the result could be an analysis of partial risk, speci�c risk, or speci�c value of risk (Wise et al. 2004). In any case, this last step completes the input from technical specialists—the risk analysis.

Table 5 Qualitative frequency definitions (adapted from BCMOF and BCMOE 2002, Table A10.2)

Quantitative Probability Qualitative Hazard Qualitative Description

Px>0.64 Very high Event is imminent after the watershed activity (i.e., any phase of forestry operations)

0.18<Px ≤0.64 High Event is probable during the lifetime of the watershed activity

0.04<Px ≤0.18 Moderate Event is possible, but not likely during the lifetime of the watershed activity

0.02<Px ≤0.04 Low Remote likelihood of event during the lifetime of the watershed activity

Px≤0.02 Very low Very remote likelihood of event during the lifetime of the watershed activity

Table 6 Example of qualitative hazard analysis matrix (adapted from Wise et al. 2004)

MagnitudeFrequency High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

Table 7 Qualitative risk analysis matrix (adapted from Wise et al. 2004)

ConsequenceHazard High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

Step 5.5 Assessing Risk and Making Management Decisions

With the results of the risk analysis and technical support where necessary, it is the role of land manag-ers to complete the risk assessment and determine if the estimated risks are acceptable or tolerable. Three options are usually available:

1. the risks are considered to be unacceptable and the project is cancelled,

2. the risks are considered to be acceptable with the planned forest development or if forest manage-ment strategies are applied to reduce the potential incremental hazards, or

3. the risks are considered to be acceptable if the consequences can be reduced by designing, con-structing, and maintaining protective structures on the fan such as dikes or berms (VanDine 1996).

24

In some cases where the risks are considered to be acceptable, options 2 and 3 are both implemented.

The hydrogeomorphic hazards in a speci�c water-shed are identi�ed in Steps 3 and 4, and summarized in Table 2. This knowledge leads directly to manage-ment strategies to reduce the potential incremental hazards. In debris �ow and debris �ood watersheds it is essential to avoid initiating landslides, particu-larly those that could directly affect the channel. A key step in this direction is to complete terrain stability mapping and develop plans that recog-nize and adequately manage unstable terrain. Field investigation may indicate that avoidance of unstable terrain is a preferred management option, but it may also determine that reasonable options are available. In coastal British Columbia, approximately one-half of all landslides occur in clearcuts and are not road-related (Jakob 2000); proposed harvesting must be assessed and managed accordingly.

It is widely acknowledged that forest roads are a major factor in landslide initiation, due both to poorly constructed roads and to their in�uence on hillslope hydrology. In a coastal logging–landslide study, no difference in soil disturbance was found between helicopter and conventional yarding, but there was a higher incidence of open-slope landslides on the conventionally yarded areas—due primar-ily to the subtle but signi�cant in�uence of roads on hillslope hydrology (Roberts et al. 2004). In the interior of British Columbia, most signi�cant land-slides associated with forestry operations are related to drainage diversion by forest roads and trails located on gentle to moderately sloping terrain, some distance upslope from steeper, more landslide-prone terrain (gentle-over-steep) (Grainger 2002). Speci�c terrain stability �eld assessment techniques and strategies to manage these potential hazards have been described by Grainger (2002).

Mitigation opportunities for landslides may exist in some watersheds. Addressing current or potential erosion from non-status roads may be an appropri-ate strategy. Alternative yarding systems should be explored, although unconventional systems such as helicopters may not result in a reduction in land-slides, particularly in gullied terrain (Roberts et al. 2004).

Maintaining an appropriate ECA is important in �ood and debris �ood watersheds, and also in watersheds where there is a potential for in-channel debris �ow initiation. There is no speci�c ECA that

guarantees that no incremental hazard will occur in all situations. However, a series of factors should be considered when establishing a conservative ECA, in-cluding watershed-speci�c processes and character-istics, past impacts to elements on the fan, the nature of the elements-at-risk, and location of past and pro-posed forest harvesting or natural disturbance to the forest cover. Watershed-speci�c mitigative opportu-nities may exist through desynchronizing snowmelt runoff; for example, by locating forest harvesting on mid- to low-elevation or southern-aspect slopes

There is ongoing research on cumulative forest harvesting and ECA effects on stream�ows. Differ-ences have been highlighted between watersheds dominated by rainfall, rain-on-snow, and spring snowmelt. Huggard (2006) describes the differ-ent hydrological effects over time of dead standing timber and salvage harvesting at the stand level, and the Forest Practices Board (2007) explored the issue at the watershed level. The effect of speci�c watershed characteristics on the harvesting-related stream�ow impacts were examined by Whitaker et al. (2002) and Schnorbus and Alila (2004). Land managers should involve experts with an appropriate knowledge of watershed processes, linkages between cumulative impacts and stream�ows, and local conditions.

Road density can be a peak-�ow hazard factor where inadequate cross drains allow long sections of road to direct intercepted �ows into stream channels. Strategies to limit road density include total-chance watershed-level planning to avoid constructing unnecessary roads, to deactivate roads, and to provide frequent cross drainage to limit the potential for ditch lines becoming a part of the stream channel network (Wemple et al. 1996). A mitigative option would be to assess and deactivate non-status roads in the watershed.

Non-licensee stakeholders often have great res-ervations regarding forest harvesting in watersheds. Often they will refer to the protective services of those forests to various ecological and watershed functions. It is poorly understood that forests must be managed to maintain high levels of protective services because forests are not static entities (Sakals et al. 2006). Forest licensees may gain the support of watershed stakeholders if they commit to a long-term management program to enhance the protec-tive services of forests.

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Both forest harvesting and lack of forest manage-ment (e.g., not undertaking post-wild�re rehabilita-tion as described by Curran et al. 2006) within a watershed can have a high likelihood of affecting the downstream fan. With an awareness of the total risk situation and appropriate accommodations for re-ducing those risks, forest harvesting and forest man-agement will be able to continue in many watersheds with minimal, and potentially positive, effects.

Step 5.6 Document, Monitor, Evaluate, and Report

A key element in watershed management is to evalu-ate the effectiveness of plans and practices. The ob-jective in applying the �ve-step approach is to limit the hydrogeomorphic in�uence on a fan as a result of forest management in the associated watershed. Documenting information collected and decisions

made will allow for retrospective analysis as time unfolds. Documentation is critical because it could be a long time before climatic events occur that trigger hydrogeomorphic responses from the wa-tershed. When such events occur it is essential that monitoring be undertaken to determine whether responses on the fan were “beyond the natural range of variability.” Information collected on the fan in Step 3 will prove valuable for this assessment. The collection of additional watershed information may be required to update Step 4. Results of the evalua-tion should feed into an update of Step 5, and should be applied where appropriate to future fan-watershed risk analyses. Reporting results to interested parties (e.g., the public or government agencies) is critical to maintain support for the forest development, and may be necessary in situations where hazards are identi�ed.

CASE STUDIES

Four case studies from across British Columbia are presented in the Appendices to illustrate the applica-tion of the �ve-step approach for analyzing risk in fan-watershed systems in different geographic set-tings and with different management issues. Wathl Creek watershed is a community watershed with the village of Kitamaat located on the fan. The watershed has had no forest development to date, and the case study demonstrates the application of the �ve-step approach in providing guidance to forest develop-ment planning. Since there was not a development plan to analyze, some aspects of the risk analy-sis could not be completed. Eagle Summit Creek watershed has had past forest development, and the Trans-Canada Highway crosses the lower fan. The watershed has produced debris �ows and debris �oods in the past, and the highway culverts are not

able to pass all the material—impacts have occurred to the highway. The �ve-step approach was applied to provide forest development direction that will limit incremental hydrogeomorphic hazards. In the Shale Creek case study, the �ve-step approach was used in analyzing impacts from historical logging on the fan and in the watershed. This information was then used to provide guidance for harvesting second-growth stands on the fan. The Hummingbird Creek case study uses the �rst four steps of the �ve-step approach to describe the circumstances leading to a debris �ow and the resulting impacts to infrastruc-ture on the fan. The �ve-step method is then used to provide guidance—based on what could have been determined prior to the debris �ow, along with knowledge that we now have regarding debris �ows in that terrain setting.

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SUMMARY

The �ve-step approach for analyzing risk in fan- watershed systems is designed to move forest man-agement from a site-level focus to a watershed-level perspective. This broader perspective allows for the recognition of the linkages between fans and their watersheds, the hydrogeomorphic processes in�u-encing the fans, and an assessment of incremental risks associated with watershed development plans or natural factors in�uencing the forest cover.

Changes to the forest cover in a watershed, either through forest harvesting or natural factors, can have signi�cant effects on downstream fans. For this reason, when management decisions are to be made, we encourage the use of the fan-watershed system as a conceptual framework to be applied across a range of spatial scales.

The �ve steps for analyzing risk in fan-watershed systems are:

Step 1: Identify Fans and Delineate Watersheds• Characterize the physical inter-connections that

exist between areas of potential activities (water-sheds) and areas of potential impacts (fans).

Step 2: Identify Elements-at-risk on Fans• Recognize and inventory values on the fan that

may be affected by the hydrogeomorphic pro-cesses in the watershed (e.g., either natural pro-cesses or those related to proposed management actions).

Step 3: Investigate Fan Processes• Identify the nature of hazardous hydrogeomor-

phic processes (e.g., type, frequency, and distur-bance extent).

Step 4: Investigate Watershed Processes• Identify watershed features controlling hy-

drogeomorphic processes (e.g., includes water-shed hydrology, geomorphology and the role of vegetation cover).

• Identify the potential for incremental hazards associated with management activities.

Step 5: Analyze Risks and Develop Plans• Develop planning options for the watershed and

assess the associated risks.• Document the process and establish a plan to

monitor, evaluate, and report.

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APPENDIX 1 Wathl Creek case study

Wathl Creek is located near the head of Kitimat Arm of Douglas Channel, and is a designated community watershed (Figure A1.1). The watershed-fan system was the subject of a hydrogeomorphic assessment (Grainger and Wilford 2008), undertaken prior to any signi�cant forest development planning for the watershed (one block had been harvested via a low watershed divide). This case study demonstrates the application of the �ve-step approach to guide forest development planning in an undeveloped watershed.

Step 1 Identify fans and delineate watersheds

The boundaries of Wathl Creek watershed and fan were identi�ed manually using topographic maps and aerial photographs, and mapped at a scale of 1:20 000. A series of small fans is present through-out the watershed—primarily in the mid and upper reaches. Individual tributary fan watersheds were not delineated, but the fans were identi�ed on a land-form map (Denny Maynard and Associates 2001).

Figure A1.1 The Wathl Creek watershed and fan.

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Step 2 Identify elements-at-risk on fans

Natural features – Salmon habitat is present on the fan; a waterfall in the lower reach of the watershed is an impassable barrier.Anthropogenic features – Kitamaat Village is located on the Wathl Creek Fan (Figure A1.2). Structures include a school, administrative buildings, and many houses. Two groundwater wells on the fan provide domestic water to the village. Low �ows in the creek and groundwater have been an issue during the summer, and in July 2003 one well temporarily ran dry. A bridge crosses Wathl Creek, providing access to the west side of the fan where most of the village is located. A long dike constructed of primarily local channel material is present along the full channel length on the fan, providing protection for most of the village.Human safety – Approximately 800 people live in Kitamaat Village on the Wathl Creek Fan.

Step 3 Investigate fan processes

Hydrogeomorphic process – Much of the fan surface has been modi�ed by human activity (forest re-moval, construction of roads and dike). A dike has

been constructed along the western stream channel bank on the fan in response to signi�cant �ooding between 1964 and the late 1970s. The dike has cut off several old channels that �owed across the western portion of the fan. The eastern portion of the fan is elevated, well above any river �ood level. Given the evidence on the fan, we concluded that the dominant hydrogeomorphic process in�uencing the fan was �ood. The highest instantaneous peak �ows are gen-erated by rain-on-snow or rain events in September, October, and November. The largest monthly stream discharges occur in June and July during the snow-melt freshet.Event frequency – The largest storms for this area probably occurred in 1891 and 1917 and it is likely that the area has not experienced storms of this mag-nitude since (Schwab 2000). Local residents reported large �oods and bridge washouts on the fan in 1974, 1978, and 1988, and these events were con�rmed by Septer and Schwab (1995). The impacts on the Wathl Creek fan could have been related to inadequate con-struction of the dike and bridge. Some of the largest �oods in regional stream discharge records occurred in 1991 and 1992. It may be that these �ows were not as great in Wathl Creek, or if they were as large, they were less memorable because the new bridge was

Figure A1.2 A view of the Wathl Creek fan and Kitamaat Village looking east into the watershed.

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designed to handle large storm �ow and no bridge washout occurred.Event magnitude – Peak �ows are currently being con�ned by the dike, although it is possible for the dike to be over-topped—surveys show that the 200-year �ood level is higher than the existing berm near the fan apex.

Step 4 Investigate watershed processes

Office investigationsMorphometrics – The watershed has an area of 127 km2 and relief of 1.6 km, and is graded (i.e., moun-tainous). The relative relief number is 0.14, suggest-ing that Wathl Creek is a �ood watershed (Wilford et al. 2004b).Sediment production and movement – Landform, terrain stability, and sediment transport maps were available for the watershed. These maps and aerial photographs were used to identify nine stream reaches and divide the watershed into four sections based on different levels of sediment production (e.g., frequency of gullies, presence of stability class IV and V, and general slope angles). Flood generation – Aside from one small block, the watershed has natural forest cover and no roads. The potentially operable forest land is below 900 m and is approximately 30% of the watershed.

Field investigationsThe watershed was inspected by helicopter with sev-eral stops to explore �oodplains, channels, and wood quality in the forest.Sediment production and movement – Connectivity between unstable terrain and stream channels was inspected in the �eld. Most gullies had a direct con-nection to the stream channel. Zones of the water-shed with unstable slopes have a very limited valley �at. Streambed, streambank, and sediment storage characteristics were described for each of the nine reaches. Eight reaches were identi�ed as non-alluvial transport zones (primarily bedrock), and only one reach had an alluvial channel.Flood generation – The logged area had very low relief and represented only a small portion of the overall watershed. A signi�cant portion of the water-shed is steep, with shallow soils, and limited forest cover. No forest health issues were identi�ed.Synthesis of watershed processes – Office work and �eldwork identi�ed signi�cant areas with slope

stability concerns, and this terrain is generally di-rectly connected to the stream channel. The terrain stability maps accurately describe unstable slopes in the watershed. The stream channel in the watershed has very limited storage capability and efficiently conducts materials through to the fan.

Step 5 Analyze risks and develop plans

Consequence – elements of concernWater quality – The groundwater wells are located less than 20 m from Wathl Creek and are 18–20 m deep. The nature of the subsurface materials is not known. A conservative position is to assume that the aquifer has coarse sediment and that there is a direct connection to the stream channel. Suspended sediment would most likely be trapped; however, any dissolved chemical or biological contaminants may or may not be removed. Currently, the stream rarely runs turbid, and since the wells have been installed there has not been an issue with suspended sediment in the water supply.

The clean water in Wathl Creek has provided a good environment for �sh and other organisms. Fish species present include chum, coho, pink, and sockeye salmon.Water supply – The oral record suggests that there have been periods where groundwater levels are so low that supply is compromised.Residences, infrastructure, and human safety – Most of Kitamaat Village is located on the southwest side of Wathl Creek, on the lower-elevation portion of the fan. The village is protected by the dike; however, the dike may not be adequate to protect the village from a 200-year �ood. If the dike is breached, �ow in the channel will be reduced, resulting in the deposition of bedload, channel in-�lling, and subsequently more �oodwater issuing forth across the fan surface.

Hazards Potential forest development – It was suggested that it would be prudent to use the Community Watershed Guidebook (BCMOF and BCMOE 1996c) as a mini-mum set of standards to be followed for any forest operations within the watershed. It was noted that the hydrologic and geomorphic impact of any forest development, regardless of the level of planning, is dependent on how operations are carried out in the watershed. It was suggested that the licensee, Haisla

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Forestry Limited, which is owned by the residents of Kitamaat Village, should keep the community informed of any forestry activities in the watershed.Water quality – Several water quality issues were raised by band elders and councillors:

Biological contamination by wild animals We are not aware of any cases where forest develop-ment in a coastal setting has resulted in contami-nation of previous pristine waters supplies as a result of wildlife feces contamination. Based on this, it is our opinion that there is a very low like-lihood that forest development would result in an introduction of contamination of water supplies by wild animals. However, there are some uncer-tainties and we recommend monitoring wild animal populations in the watershed, particularly in the north and south Plateau areas.

Contamination by wood leachate We are not aware of any cases where forest harvesting has caused noticeable contamination in downstream domestic water supplies by wood leachate. There are cases of contamination from wood sorts and log dumps where there is a large amount of wood concentrated in a small area. These activities should not be undertaken in the watershed. We conclude that there is a very low likelihood that forest development would cause chemical contam-ination by wood leachate.

Contamination by hazardous material spills The record of this type of contamination in for-estry operations suggest that the likelihood of a signi�cant spill is low. Steps can be taken to con-siderably reduce this hazard. Following appropri-ate procedures, there is a very low likelihood that forestry operations will result in contamination of Wathl Creek by a hazardous materials spill.

Sediment loading Given the granitic bedrock in the watershed, the sur�cial materials have a relatively low amount of �ne-grained soils. The landform maps identify the location of several �ned-grained glacio-lacustrine sediment depos-its, and these could become a �ne-grained sedi-ment source if not managed properly. Overall, there is a low likelihood of �ne-grained sediment impacts to surface water quality and �sh habitat. There is a very low likelihood of �ne-grained sedi-ment impacts to groundwater-sourced domestic water supplies.

The additional or incremental coarse bedload sediment hazard due to forestry activities is a function of two things: the likelihood of erosion or landslides, and the likelihood of a signi�cant amount of the mobilized sediment reaching Wathl Creek. A signi�cant amount of sediment means that it will make a noticeable difference in sediment loads, compared to the relatively high natural background sediment levels already in Wathl Creek. Given the terrain stability map-ping, we are able to conclude that: there is a very high incremental sediment hazard from both forest harvesting and road construction in the Upper Mountain Valley area and in the gullies of the larger tributary streams in the Mid Mountain Valley and South Plateau areas. Given the limited amount of unstable terrain and lack of connec-tivity to the stream channel in the lower chan-nel reaches, there is a low incremental sediment hazard from harvesting and road construction on the open slopes of the Mid Mountain Valley, in the North Plateau area, and in the South Plateau area.

Water quantityPeak �ows – Given that the operable forest land base occupies only 30% of the watershed, and that harvesting within this area will be constrained due to terrain, riparian protection, old-growth manage-ment, and other harvesting restrictions, the actual area harvested will be less than 30% of the water-shed. We conclude that the incremental peak �ow hazard is very low.Low �ows – Research has shown that, except in cases where there has been signi�cant soil compaction or there is signi�cant fog drip, harvesting generally increases or has no measurable effect on low �ows. Given the limited operational area in the Wathl Creek watershed there is a very low likelihood of development-related impacts to low �ows.

Except for the coarse sediment loading hazard in some portions of the watershed, all other identi�ed incremental forestry-related hazards were judged to be low to very low.

Risk analysis and recommendations Risk is the product of hazard (or likelihood an event occurring and affecting a value) and the conse-quence (or the exposure, vulnerability, and worth

31

of the element-at-risk; for example, the high-quality domestic water supply). While we have suggested consequence values in order to carry out the follow-ing risk analyses, it is worth repeating that the �nal determination of the consequence values and the subsequent risk analysis are the responsibility of the licensee, regulatory authorities, and stakeholders, as is the determination of acceptable risk and the deci-sion to proceed with any development based on that analysis. To manage all potential hazards and associ-ated risks, two general recommendations are made:

– The Community Watershed Guidebook (BCMOF and BCMOE 1996c) should be used as a minimum set of standards.

– Water users should be kept informed of any forestry activities planned or happening in the watershed, and of how potential hazards and risks are being managed.

For the risk analysis we use Table A1.1, and the consequences, hazards, and risks for a series of the elements-at-risk are presented in Table A1.2.

The coarse bedload sediment risk from proposed development in various portions of the watershed to residences, other buildings, and human safety on the fan is presented in Table A1.3.

Table A1.1 A matrix combining hazard and consequence to determine risk

Hydrologic Consequencehazard High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

Table A1.2 Consequences, hazards, and risks of all the identified hazards and elements-at-risk, except the coarse sediment loading hazard and the con-sequence of potential impacts to residences and human safety, which are dealt with separately

Incremental hazard from forestry – Issue Consequence likelihood Risk

Domestic Low to Very low Very lowwater – suspended Moderatesediment

Domestic High Very low Lowwater – industrialchemicals

Domestic High Very low Lowwater – biological

Domestic High Very low Lowwater – woodleachates

Fish – suspended Moderate Low Lowsediment

Fish – low �ows High Very low Low

Peak �ows High Very low Low

Low �ows High Very low Low

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Development planning and mitigative strategiesRecommendations include:

• No harvesting or road building in the unstable Upper Mountain Valley portion of the watershed.

• No harvesting in the large tributary gullies of the mid-watershed.

• Road building with caution through the large tributary gullies. Carry out detailed on-site assessments to determine where there may be acceptable gully crossing sites, use experienced road layout personnel, thorough geotechni-cal assessment and engineering, and cautious construction and maintenance practices. All this increases costs and there will be locations where gully crossings with an acceptable level of risk will not be feasible.

• On Mid Mountain Valley open slopes, with good forestry practices through all planning and implementation phases (including total chance planning based on thorough geotechnical and hydrological assessments), much of the area can

be accessed and harvested with an acceptable level of risk to elements-at-risk on the fan.

A series of forest management planning scenarios may be developed using these recommendations along with other considerations (e.g., timber qual-ity and equipment availability). These scenarios, along with mitigative strategies, should be evaluated against the risk analysis. In the case of Wathl Creek, it is likely that the residents of Kitamaat Village will have input to any �nal forest management plans.

We noted in the report that there is a potential risk to residences, infrastructure, and human safety in Kitamaat Village because of apparent problems with the dike along the southwest side of Wathl Creek. These problems exist whether any forest development takes place in the watershed or not. It was recommended that the Band Council undertake further investigation of the level of safety the dike is providing the village, with respect to the cur-rent 200-year design �ood level, and with respect to expected future storm magnitudes and sea-level rise resulting from ongoing global climate change.

Table A1.3 Coarse bedload sediment risk

Consequence – impacts to Forest development Coarse sedimentArea buildings and human safety sediment hazard risk

Upper Mountain Valley High Very high Very high

Large tributary gullies – High Very high Very highMid Mountain Valley,South Plateau

Mid Mountain Valley – High Low Moderateexcept large tributaries

South Plateau – except High Very low Lowlarge tributaries

North Plateau – all High Very low Low

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APPENDIX 2 Eagle Summit Creek Case Study

Eagle Summit Creek is located in the Monashee Mountains near the headwaters of the South Thomp-son River, upstream of Shuswap Lake and just east of the Thompson River–Columbia River drainage divide, about 30 km west of Revelstoke, B.C. The watershed-fan system was the subject of a hydrogeo-morphic assessment (Grainger 2007). The report is summarized using the �ve-step approach.

This case study demonstrates that there can be different processes in a watershed producing differ-ent hydrogeomorphic events with different occur-rence intervals and magnitudes. Knowledge of the different watershed processes and the different ways those processes can be affected by various forest management activities allows land managers to focus their decisions where they will be most effective in managing risks.

Step 1 Identify fans and delineate watersheds

The boundaries of Eagle Summit Creek watershed and fan were identi�ed manually using topographic maps and aerial photographs and �eld reviews, and mapped at a scale of 1:20 000. The fan is rela-tively small, about 240 m wide and 170 m long, but it blocks the narrow Eagle River valley-bottom, forming Clanwilliam Lake just upstream of the fan (Figure A2.1). The watershed has some steep alpine mountains at its upper extent and a broad, gently to moderately sloping plateau forming the mid wa-tershed. The stream is in a steep, 1600 m long gully dissecting the plateau (Figure A2.2).

Step 2 Identify elements-at-risk on fans

Natural features – Timber on the fan. There are no recognized �sh values in Eagle Summit Creek or Clanwilliam Lake.Anthropogenic features – An approximately 200-m section of the Trans-Canada Highway (TCH) crosses the lower fan. The highway crossing of Eagle Sum-mit Creek consists of two 1.0 m high oval culverts (Figure A2.3). Hydrogeomorphic impacts could disrupt automobile and truck traffic on the main highway connection between Alberta and southern British Columbia. The CPR mainline runs along the

north side of the Eagle River, adjacent to the toe of the Eagle Summit Creek fan.Human safety – Hydrogeomorphic impacts to the Trans-Canada Highway could compromise human safety and life. A small debris slide from the lower, steep, Eagle River valley walls caused a fatality in the spring of 2002 when a machine operator was swept into Clanwilliam Lake and drowned, while clearing debris from a previous natural landslide. In Decem-ber 1967 a debris �ow in Camp Creek, on the north side of the Eagle River Valley, and approximately 15 km west of Eagle Summit Creek destroyed the Camp Creek Bridge over the TCH, resulting in two fatalities to passengers in a car that entered the debris �ow path due to loss of the bridge.

Step 3 Investigate fan processes

Debris �ow – There are well developed levees on the steep concave fan adjacent to the channel, with boulders up to 2.0 m in diameter. Fan gradients are 50% near the apex, 30% through the mid-fan, and decrease to 10% near the toe, downslope of the TCH. The fan surface is irregular, with abandoned chan-nels and large boulders and blocks underlying the vegetated surface.

There is a mature western redcedar, Douglas-�r, and western hemlock stand with trees from 0.45 to 0.55 m diameter, estimated to be 90 years old, growing on the fan surface, including the boulder levee. Except for a couple of trees growing within a few metres of the channel, no scarring was noted on mature trees, even those immediately adjacent to the channel at the fan apex. There are also burned stumps throughout the fan, thought to be from the stand-establishing �re for the current stand. If the stand had been established by a hydrogeomorphic event any evidence of the previous stand would have been obliterated. This indicates that there has been almost no hydrogeomorphic disturbance outside the channel in at least 250 years, and possibly for much longer. It was concluded that there have been large debris �ows in Eagle Summit Creek that have extended down the fan to the present position of the TCH, but that none have occurred in at least 250 years.

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Figure A2.1 Eagle Summit Creek watershed (2001 airphoto). Proposed cutblocks are shaded.

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Figure A2.2 Gully and fan reaches of Eagle Summit Creek, looking south.

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Figure A2.3 A view of the drainage structure on the Trans-Canada Highway at Eagle Summit Creek.

Debris �ood – In the Eagle Summit Creek chan-nel on the fan, recently moved sediment is mainly cobbles and small boulders < 0.75 m diameter. Some larger blocks have been introduced to the channel from high failing cliffs immediately above it.

A small group (cohort) of young western red- cedar trees occupies a small gravel sediment wedge adjacent to the channel. Dendroecology analysis (Wilford et al. 2005a) of the trees indicated that they were established in the early 1950s and show impact scars dating from 1967 and 1982. The fact that these young trees were not destroyed indicates that it was not a high-powered debris �ow that affected them. Three hydrogeomorphic events are reported to have affected the TCH in the last 35 years: in 1967, in the early 1980s, and one at an unspeci�ed date (Thurber 1987), which corresponds well with the dendroecol-ogy �ndings. The early 1950s cohort establishment suggests that at least three and possibly four events have occurred in the channel in the last 55 years. These events appear to be largely contained in the channel and extend to the TCH. It is noted that the capacity of the existing TCH crossing is very likely too small to pass a debris �ood. At least some of

the recent debris �oods have blocked the culverts and deposited sediment on the highway, disrupt-ing traffic. No injury or other damage was reported (Thurber 1987)

As discussed in Step 3.1, debris �oods have peak discharges that are, at most, two to three times that of major �oods (Hungr 2005), and have maximum particle size of the same order as the peak depth of major �oods—up to several tens of centimetres (< 1.0 m) in the case of typical small mountain streams (Hungr et al. 2001), which describes the �ow magnitude and sediment size observed in Eagle Summit Creek on the fan. It was concluded that there is an existing debris �ood hazard that extends to at least the TCH and has a return period (T) of approximately T = 14 years (i.e., a 55-year period of record with four debris �ood events).

Step 4 Investigate watershed processes

Office investigationsMorphometrics – The watershed has an area of 6.4 km2 and relief of 1.68 km. The relative relief num-ber is 0.66, suggesting that Eagle Summit Creek is a

37

debris �ow watershed (Jackson et al. 1987; Wilford et al. 2004b).

However, relative relief number is designed for “graded” or concave mountainous watersheds, and there is a wide, moderately to gently sloping plateau in the middle of the watershed. We have observed that morphometrics designed for graded watersheds often do not give the correct results for watersheds with signi�cant upland plateaus.Sediment production and movement – Terrain stability mapping and air photos show that Class IV (potentially unstable) and Class V (unstable) ter-rain is ubiquitous along the steep Eagle River valley walls, with debris slides, rock falls, and debris �ows. On the upland plateau, terrain was largely Class I to Class III (stable). Little sediment was being deliv-ered to the channel on the plateau, or transported through it to the steep reach above the fan. Flood generation – Stream�ow records show that peak �ows occur during the spring freshet snow-melt in late May and most commonly in early June. Detailed analysis of damaging �oods in Eagle Summit Creek and debris �ow in a smaller tributary stream in the region in 1967 and 1968 show that rain on melting snow caused those large runoff events (Thurber 1987).

There had been extensive harvesting on the pla-teau area of the watershed throughout the 1980s. The

existing equivalent clearcut area (ECA) at the time of the assessment was approximately 21%.

Field investigationsAreas proposed for harvest and road building were investigated on the ground, as was Eagle Summit Creek channel, from the plateau down to its con�u-ence with the Eagle River. The channel and sur-rounding areas were also observed from the air in a helicopter �y-over.Sediment production and movement – Most sedi-ment production and delivery to the channel oc-curred in the steeper stream reaches along the south Eagle River valley wall. This section was subdivided into two reaches based on sediment regime charac-teristics (Figure A2.2).

The upper steep reach is about 1.0 km long with 35–55% channel gradients. Gully sideslopes range from moderate (30–50% gradient) to very steep (< 90% gradient) with bedrock slab failures and talus slopes of large blocks. There were also lesser till and colluvium slopes, with some soil slumping observed above the channel. The channel is generally 6–8 m wide, with 2–3 m deep substrate of large blocks and boulders from 1.0 to 4.0 m diameter (Figure A2.4).

The lower reach has similar channel gradients, with mixed gully sideslopes of bedrock slab failures and thick till and colluvium with numerous failing

Figure A2.4 A view of the upper steep reach in Upper Eagle Summit Creek.

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gullies delivering sediment directly to the Eagle Summit Creek mainstem from small tributary debris �ows. The channel has a mixed substrate of blocks and boulders, cobbles and gravel, with some sections scoured to bedrock. Immediately upstream of the fan there is a 50% gradient reach with mainly grav-els, cobbles, and small blocks (< 1.0 m diameter) and abundant woody debris, partially derived from pre-1970s harvesting on steep gully sidewalls. In spite of the reported recent events there was a high sediment load in the channel immediately above the fan, and also in other sections of the channel, particularly near the con�uence of tributary debris �ows that are actively delivering sediment to the main Eagle Sum-mit Creek channel (Figure A2.5).

Synthesis of watershed processes – It was concluded that the upper steep reach could be the source of a debris �ow if a sizeable landslide should affect the channel. In-channel debris �ow or �ood initiation of the 1.0–4.0 m diameter sediment by high stream �ows is considered unlikely. From the channel load-ing observed, it was estimated that 15–25 m3 of blocky sediment could be mobilized in a debris �ow from each metre-long section of channel. Depending on where along the channel a debris �ow originated, from 25 000 to 40 000 m3 of blocky sediment could be mobilized, which would reach the fan and deposit anywhere along the 200 m of the TCH crossing the fan. This is an order of magnitude greater than events observed in the last 55 years. Debris �ows have not been observed in Eagle Summit Creek for at least 250 years. Thus there is an existing debris �ow hazard with a very low frequency and a very large magnitude (power and extent). Landslide risk is the watershed process that needs to be focussed on when considering forest activities that could have an im-pact on the upper steep reach of Eagle Summit Creek on the Eagle River valley wall.

It was also concluded that the frequent debris �oods observed in the last 55 years all originate in the steep lower reach, where there is more abundant �ner-grained sediment (gravel to small boulders and blocks) in the channel. There appears to be a high rate of sediment recharge to the lower steep reach. It was concluded that, in the lower steep reach, debris �oods can be initiated by small tributary debris �ow damming of the channel or by in-channel initiation in the mainstem by high stream �ows. Because of the sediment loading and the low return period between events of approximately 14 years, the lower steep reach is near its debris �ood failure threshold most of the time. Therefore, there is an existing debris �ood hazard with a high frequency and a much smaller magnitude (power and extent within the channel), which does however affect the TCH. It is considered very unlikely that development on the plateau could initiate landslides that could affect the lower steep reach above the fan. Therefore, the watershed process that needs to be addressed when considering forest activities is an increase in peak �ows from harvest-ing on the plateau.

Figure A2.5 A view looking upstream in the lower reach of Eagle Summit Creek. The small boulders and fine sediment originate from sidewall debris flows. This material is draped over the large block substrate of Eagle Summit Creek.

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Step 5 Analyze risks and develop plans

The debris �ow return period in Eagle Summit Creek is at least 250 years, and possibly more. Therefore, the frequency, or annual debris �ow probability, is esti-mated to be at least 1/250 or 0.004. Debris �oods in Eagle Summit Creek have a return period of approxi-mately 14 years, a frequency of 1/14 or 0.07. These are the existing natural hazards at the site.

Incremental landslide and debris �ow hazard and riskProposed developments included some harvesting at the edge of the upland plateau, with some cable har-vesting proposed on the upper slopes of the steeper Eagle River valley walls. Also, an old road along the edge of the plateau would be upgraded to access this block. Extensive harvesting on the plateau located up to 2.0 km from the steep Eagle Creek valley walls could increase the existing watershed ECA by about 50%, to nearly 30% of the watershed.

The harvesting and road were a concern, par-ticularly from possible disturbance and diversion of poorly incised ephemeral streams in the steeper cable harvest area, and from the potential for the road to intercept and divert hillslope drainage onto steeper downslope areas.

The road was proposed to be long term (20 years). Given this time frame and the sensitivity of the gentle-over-steep terrain in this area to drainage concentration by roads, it is appropriate from a risk management perspective to include a margin of error with respect to road drainage. It is possible that dur-ing the life of the road, a drainage diversion could occur that would elevate the likelihood of a landslide reaching Eagle Summit Creek. An assumption was made that the landslide frequency would increase from the historical value of 0.004 to 0.05 (1 in 20 years). From Table A2.1 this would be considered a high incremental frequency.

The power and destructive force of a 25 000–40 000 m3 debris �ow comprised in large part of large boulders and blocks would be very high, and this very high-powered event would extend to the TCH anywhere along the 200 m where it is on the Eagle Summit Creek fan. This was considered a large to very large magnitude event. From Table A2.2, with a high incremental frequency and a high to very high magnitude, there is a very high incremental debris �ow hazard.

This large, powerful debris �ow would likely bury some of the TCH, disrupting traffic along one of the three main car and truck routes between British Columbia and the rest of Canada. The Eagle Summit

Table A2.1 Qualitative frequency definitions (adapted from B.C. Ministry of Forests 2002, Table A10.2)

Quantitativelong-term frequency (annual Qualitative Qualitativeprobability) frequency description

> 0.64 Very high Event is imminent after the watershed activity

> 0.18, < 0.65 High Event is probable during the lifetime of the water- shed activity

> 0.04, < 0.19 Moderate Event is possible, but not likely during the lifetime of the watershed activity

> 0.01, < 0.05 Low Remote likelihood of event during the lifetime of the watershed activity

< 0.02 Very low Very remote likelihood of event during the lifetime of the watershed activity

40

Creek crossing would likely be seriously damaged and would require rebuilding or replacement. If a vehicle and its occupants were hit by the debris �ow it would probably result in a fatality or fatalities. If a vehicle ran into the deposit, injury and possibly a fatality could result.

The managing foresters considered the possibil-ity of more detailed consequence analysis, including exposure and vulnerability of traffic and human safety, but decided that it would not add value to the risk analysis. Their decision was that any serious forestry-related impact to the TCH was a high to very high consequence.

From Table A2.3 it was concluded that with a high to very high incremental debris �ow hazard, and high to very high consequences on the fan, there was a very high incremental risk from the harvesting and road upgrading being considered near the Eagle Summit Creek gully.

To reduce the debris �ow hazard and thus the risk, risk control measures were adopted as part of the planning process. Regarding the road above the steep Eagle Summit Creek gully walls, and the pro-posed cable harvesting on upper gully wall slopes, a Terrain Stability Assessment (TSA) recommended the following:

• On > 60% gradient gully wall slopes, the outer 2 m of the existing road prism �ll should not be disturbed. Any widening of the road should take place by removing cutslope material, which was located on the moderate to gently sloping plateau. Excavated material should not be placed on the outer 2 m of the existing prism, but used to raise the grade or end-hauled.

• A drainage plan was prepared after a detailed assessment of hillslope drainage was undertaken by a geotechnical professional during a spring freshet. Drainage control structures (culverts and ditch blocks) were sized, located, �agged, and painted in the �eld, and their location mapped and documented.

• Deactivate the road by removing all culverts and installing cross ditches at all culvert locations after harvesting and replanting.

• The geotechnical professional should conduct �eld reviews during road construction and following deactivation, or at any time during operations if any unexpected sub-surface �ows were encountered that would require any change to the drainage plan.

• The lower block area with easily disturbed, poor-ly incised ephemeral streams should be deleted from the harvesting plan.

• Any bladed skid and backspar trails in the remaining block should be deactivated by full pullback and slope recontouring immediately following yarding and prior to the next season’s freshet.

It was concluded that these measures would reduce the incremental landslide and debris �ow hazard to low.

Incremental peak �ow and debris �ood hazard and riskThe watershed and fan analyses concluded that existing peak �ows are likely close to the natural threshold stream �ow values required to initiate debris �ooding in the steep lower Eagle Summit Creek reach—a reach with abundant gravel to small boulder sediment where debris �oods could be initi-ated. Fieldwork and reports of events indicate that this threshold is exceeded on average every 14 years. Therefore, any noticeable peak �ow increase due to upslope forest development could push the system over that threshold and initiate debris �oods more frequently.

Table A2.2 A qualitative hazard matrix (adapted from Wise et al. 2004)

MagnitudeFrequency High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

Table A2.3 A qualitative risk matrix (adapted from Wise et al. 2004)

ConsequenceHazard High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

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Proposed clearcut harvesting on the gentle- gradient plateau extended up to 2 km upslope from the steep Eagle Summit Creek gully on the Eagle River valley walls. Recent research has shown that in the snowmelt-dominated watersheds of southern interior British Columbia the hydrological effects of clearcut harvesting can lead to increases in all sizes of peak �ows (Schnorbus and Alila 2004). Therefore, there was the potential for proposed clearcut harvesting to increase the existing debris �ood frequency of 0.07 to some unknown higher frequency. This increased frequency would exist for at least 20–30 years, when hydrological green-up would signi�cantly decrease the clearcut debris �ood hazard. Although the speci�c effect that upslope harvesting could have on peak �ows was not known to any degree of certainty, it was concluded by the risk analysis team that a noticeable increase in peak �ows could entail at least a moderate, and possibly a high, incremental debris �ood frequency.

Based on past occurrences, a debris �ood would be largely contained in the channel, at least until it reached the TCH crossing, which it would affect. This was considered a moderate magnitude event. Therefore, from Table A2.2, with a potential high incremental �ood frequency and moderate magni-tude, there was potentially a moderate, and possibly a high, debris �ood hazard.

A debris �ood would affect the highway crossing, most likely by blocking the culverts and overtopping the highway and disrupting traffic. The event would probably not cause a fatality directly, but could cause a vehicle pile-up that could result in damage to vehicles, and possibly human injury. Again, the land managers decided that any impact to the TCH that would disrupt traffic and potentially result in human injury was a high consequence. Therefore, from

Table A2.3 there was a potential high to very high debris �ood risk, which was considered unacceptable by the land managers.

Because of the potential for harvesting-related elevated peak �ows, the Terrain Stability Analysis recommended that a forest hydrology professional review the proposed harvesting further for potential increased peak �ow effects. That review concluded that the proposed clearcut harvesting above the H60 line (the line above which contains 60% of watershed area, see Figure A2.1) could lead to stream�ow effects that could increase the frequency of �ood and/or de-bris �ood events on the fan. The hydrologists report recommended deleting the proposed harvest area above the H60 line until either:

• the B.C. Ministry of Transportation and High-ways are noti�ed that previous events have shown that the existing culverts at the TCH crossing are inadequate to pass the frequent debris �oods that occur in Eagle Summit Creek, and that will occur at some interval in the future whether additional forest harvesting occurs in the watershed or not, and a suitably designed span is installed over Eagle Summit Creek, or

• hydrologic green-up of existing harvested areas has achieved a stand height and canopy closure such that upper watershed ECA is reduced to an acceptable level.

Harvesting of proposed areas below the H60 elevation was not considered an increased peak �ow hazard. In fact, lower-elevation harvesting could advance snowmelt in those areas early in the spring freshet, desynchronizing melt from the lower-eleva-tion area from peak �ow–generating snowmelt in the upper watershed area. This could result in sustained periods of moderately high, but not extreme, �ows.

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APPENDIX 3 Shale Creek Case Study

British Columbia Timber Sales (BCTS) is develop-ing harvesting plans for a cutblock located on the alluvial fan of Shale Creek, Moresby Island, Haida Gwaii Forest District (Cutblock SKI 101, Figure A3.1). The fan was previously harvested in 1951, and harvesting within the watershed of Shale Creek com-menced shortly after the fan was logged. Evaluation of the watershed and fan processes, and the effects of the initial harvesting of the Shale Creek fan and its watershed, are important for planning appropriate cutblock layout and fan management.

The evaluation used aerial photographs from 1964, 1977, and 1994, as well as recent satellite im-agery (circa 2007) to examine watershed and fan conditions. Maps provided additional information. Fieldwork on the fan examined the channel network and other fan features.

Step 1 Identify fans and delineate watersheds

The Shale Creek watershed-fan system is located on the Skidegate Plateau area of Moresby Island. Figure A3.2 shows the watershed boundary of Shale Creek and the fan boundary. The watershed is approxi-mately 10.6 km2 in area, with an elevation range of 80–620 m (Figure A3.2). Shale Creek fan builds into a 1 km wide valley that trends west–east, which includes Skidegate Lake. To the immediate west of the fan, the valley is con�ned by hillslopes project-ing into the valley from the northwest and southeast, and the valley narrows to about 0.5 km in width. The fan is limited in its ability to expand to the southwest by the hillslope con�nement, but extensive fan ex-pansion to the east is possible. The fan toe transitions

Figure A3.1 Location map for Block SKI 101.

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into a �oodplain that extends east to Skidegate Lake, a distance of about 1.5 km (Figure A3.1). The fan toe is partially de�ned by wetter soils and a change in timber type.

Step 2 Identify elements-at-risk on fans

Located in a relatively remote area, there is no hu-man habitation on the fan. A mainline logging road and power line cross the fan. Streams on the fan and their high-value �sheries are the main element-at-risk.

Step 3 Investigate fan processes

The fan is low gradient (< 2%) and shows evidence of site-level, high-power, water �ood events (Wilford

Figure A3.2 Shale Creek watershed and fan. Mapped stream channels shown on the fan are not correct, as is often the case with fan channel networks.

et al. 2005). Two channels currently exist on the fan in the central and western portions (Figure A3.3). Stream 1 is the main channel that carries water �ows and sediment from the watershed and ranges from 10 to 25 m wide, with the greater widths near the apex of the fan. Upstream of about 0 + 700 m on Stream 1 there are abandoned or slightly active channels that avulse from, and return to, Stream 1. Stream 2 is smaller, with channel widths typically less than 5 m. This stream is inset into an older channel that is 15–20 m wide, similar in width to the presently active Stream 1 (Figure A3.4). Upstream of 0 + 349 m on Stream 2 there is a broad area of recent gravel deposits (less than about 50 years in age) that have buried the well de�ned older channel edges visible downstream of 0 + 349 m. This broad gravel area extends upstream to the left bank of Stream 1 near

44

Figure A3.3 Channel network on Shale Creek fan. Preliminary proposed harvesting areas are shown in pink.

Figure A3.4 Stream 2 flowing in an older and larger channel. Old channel banks are outlined in yellow. Groundwater re-emergence results in limited flow variability, little sediment transport, and previously active gravel bars becoming vegetated.

45

0 + 724 m (Stream 1 measurement). The blockage of Stream 2 has isolated it from Shale Creek �ows; only a tributary source and groundwater re-emergence results in channel �ow.

Step 4 Investigate watershed processes

The relative relief number (Melton 1965) for Shale Creek watershed is 0.17, indicating that debris �ows or debris �oods are not likely delivered to the fan (Millard et al. 2006). Although most of the water-shed has low likelihood of landslides, there are some slopes that are subject to landslides. In particular, the channel reach immediately upstream of the fan apex is narrowly incised into steep unstable slopes, where most landslides deposit directly into Shale Creek. The channel gradient of Shale Creek imme-diately upstream of the fan for a distance of almost 1 km is 2–4%, indicating that landslides that initiate in this area and deposit sediment into the creek will not continue to the fan as debris �ows. Field investiga-tion of the fan did not �nd any evidence for debris �oods or debris �ows, which con�rms that Shale Creek is a �ood-only fan.

History of forest management and geomorphic responseThe fan area was harvested in 1951. The 1964 air pho-tos show Stream 2 as the active channel that carries water and sediment from the watershed. The 1964 air photos also show that much of the lower portion of Shale Creek watershed was harvested by 1964, including the steeply incised slopes in the channel reach directly upstream from the fan apex. By 1964, at least 14 logging-related landslides had occurred on these valley slopes immediately upstream of the fan, with additional slide areas visible in the 1977 photos. Most of these landslides deposited sediment directly into Shale Creek. Although individual landslide events were not transported directly to the fan as debris �ows or debris �oods, the channel would have transported much of the landslide sediment to the fan through subsequent �ood events. It is likely that the sediment from these landslides resulted in the extensive gravel deposits near 0 + 724 m on Stream 1. Although Stream 2 appears to be the only active channel in the 1964 photos, by 1977 it appears to have been blocked.

Stream 1 parallels the Moresby Mainline road for approximately 200 m (Figure A3.3), which is built

above the surface of the fan and acts as a dike. No channels are evident on the downstream side of the mainline road along the section where Stream 1 parallels the road. The alignment of Stream 1 with the road, and the lack of channels below the road in the area of the stream alignment, indicates that the road was present prior to the existence of Stream 1. This evidence, in combination with the large gravel deposits blocking the entrance of Stream 2 and the presently small channel size of Stream 2 inset into an older, larger channel, all support the air photo evidence that Stream 2 was the only active chan-nel on the fan prior to watershed logging, and that the blockage of Stream 2 resulted in the creation of Stream 1 below 0 + 730 m.

With the exceptions of Stream 1, Stream 2, and partially active channels near the apex of the fan, only a few very faint remnants of channels were found on the Shale Creek fan. All other surfaces appeared old, with no �uvial activity for at least cen-turies, and possibly millennia. The lack of channels across the surface of the fan indicates high channel stability. Increased sediment deposition on the fan, such as that which occurred from numerous land-slides after the 1950s and 1960s logging of the slopes upstream of the fan, is likely the main cause of chan-nel instability.

Step 5 Analyze risks and develop plans

The primary element-at-risk on the fan is �sheries habitat. The �sh habitat is likely best protected by avoiding avulsions, which appear to be rare on this fan, and by avoiding introducing sediment into the channels. Harvesting of almost all trees on the fan in 1951 did not appear to have caused the avulsion on the fan, although streamside logging likely resulted in damage to stream banks. Logging-related land-slides within the watershed likely led to an avulsion. The history of the watershed-fan system indicates that to avoid management-caused avulsions on the fan, watershed management is at least as important as fan management.

BCTS has agreed to follow the Haida Gwaii Strategic Land Use Agreement for Block SKI 101. This agreement has the objective of retaining active �uvial units by maintaining forests within 1.5 tree lengths of the outer edge of the active �uvial unit. For fans, the active �uvial unit is de�ned as the hy-drogeomorphic riparian zone.

46

The hydrogeomorphic riparian zone on Shale Creek fan extends from the left (or northeast) chan-nel bank of Stream 1 to the right (or southwest) channel bank of Stream 2. Upstream of about 0 + 700 m on Stream 1, the active �uvial unit in-cludes some abandoned or slightly active channels to the northeast of Stream 1. This zone encompasses both the currently active channel (Stream 1) and Stream 2, which has become isolated from sedi-ment and water �ows of Shale Creek. Since this fan appears to exist primarily as a single-thread channel network and not a complex of channels across the surface of the fan, this likely represents an oversized active �uvial unit. All of the SKI 101 block areas are at least 2 tree lengths from the active �uvial unit. This exceeds the stated objective in the Haida Gwaii Strategic Land Use Agreement and should provide protection to bank stability.

The avulsion of the fan channel sometime between 1964 and 1977 indicates that the channel

network on Shale Creek fan is sensitive to watershed processes. The avulsion was likely caused by logging-related landslides that led to extensive sediment de-posits on the fan, blocking the then-active channel. These events demonstrate the sensitivity of the Shale Creek fan to forestry practices within the watershed. Future harvesting within the Shale Creek watershed should consider the potential for fan destabilization as a result of landslides that could lead to increased sediment deposition on the fan.

ConclusionThe forest management history of the Shale Creek watershed-fan system demonstrates that effective fan management must include consideration of watershed management. Planning for the SKI 101 block incorporated fan and watershed assessments to understand how the channel network on the fan responds to fan and watershed disturbances.

APPENDIX 4 HUMMINGBIRD CREEK CASE STUDY

Hummingbird Creek is located in the Hunters Range on the east side of Mara Lake near Sicamous in the southern interior of British Columbia (Figure A4.1). Mara Creek joins Hummingbird Creek just below the apex of the fan, and the joint fan is locally known as Swansea Point. On July 11, 1997, a debris avalanche initiated in the watershed, entered Hummingbird Creek, and initiated a debris �ow. The debris �ow travelled down the channel to the apex of Swansea Point fan where sediment deposition began. The volume of the debris �ow was estimated at 92 000 m3, which resulted in the destruction of two houses and damage to several other houses, as well as to High-way 97A and local roads.

This case study uses the �rst four steps of the �ve-step method to describe the Swansea Point fan and Hummingbird watershed, the elements-at-risk, and the hydrogeomorphic processes that occur, including the 1997 debris �ow event. A retrospective application of step �ve describes how the use of the method might have prevented the 1997 debris �ow, highlighting the fact that relatively subtle changes in a watershed can have very signi�cant implications on the fan. Knowledge of the potential hydrogeo-morphic processes that can occur in a watershed allows land managers to focus their decisions where

they will be most effective in managing risks for a downstream fan.

Step 1 Identify fans and delineate watersheds

The Swansea Point fan is built out into the waters of Mara Lake. The watershed boundaries of Humming-bird and Mara Creeks and the Swansea Point fan are illustrated in Figure A4.1. Hummingbird watershed is approximately 16.4 km2 in area. The lower half of Hummingbird Creek follows a well de�ned, south-west-trending faultline and then turns abruptly west as it reaches the fan. The fan extends out into Mara Lake; the sub-aerial portion of the fan is approxi-mately 0.6 km2. The distance from the fan apex to the lakeshore is approximately 1.3 km. The fan gradient ranges from approximately 18% at the apex to 5% along the lakeshore.

Step 2 Identify elements-at-risk on fans

Human safety – Approximately 250 people live permanently in the Swansea Point area (Statistics Canada 2007), and during the summer more come to stay at resorts on the fan.

47

Figure A4.1 Map of Hummingbird and Mara Creek watersheds, and the Swansea Point Fan. The dashed line follows the 1500-m contour, which is the break be-tween the gentle upland plateau and the steeper escarpment.

Anthropogenic features – Non-indigenous settle-ment began on Swansea Point fan in the early 1900s and there are presently approximately 277 private dwellings, of which 122 are occupied year-round (Statistics Canada 2007). A large condominium complex developed in 2007 was reviewed by the Columbia-Shuswap Regional District but the con-cerns of residents on the fan were primarily the sew-age disposal system, upgrading of the water system, increased boat use on Mara Lake, highway access, population density, beach access, and noise—very little attention was paid to the hydrogeomorphic hazards.

Highway 97A, connecting Sicamous to the Okan-agan Valley, climbs the fan towards the apex and there are numerous local roads on the fan, a poten-

tially problematic situation (Wilford et al. 2005). In 1997, Hummingbird Creek passed under the highway through a 3-m elliptical multi-plate culvert. A trash rack was in place to remove boulders immediately upstream of the culvert. During the debris �ow event in 1997, the trash rack rapidly blocked with debris and diverted most material from the channel. The culvert was relatively undamaged during the event and remains in use today. In the aftermath of the debris �ow, the upper channel was widened and a levee was constructed and armoured with the largest boulders available in the vicinity. The culvert outlet and downstream banks were armoured extensively to promote channel stability.

Swansea Road, the main access road on Swansea Point running west from the highway down to Mara

48

Lake, was heavily damaged by the �ner-grained fraction of the debris �ow and required rebuilding.Natural features – As a result of the passage of the 1997 debris �ow and subsequent mitigation works, there is little or no complexity to the fan channel (i.e., the channel is quite straight with predominant-ly bare sand, gravel, and boulders along the margins; there are no vegetated banks and very little wood or organic material). Mara Lake is home to a wide variety of �sh, including several types of salmon and trout. Fish are not present in the Hummingbird wa-tershed: the steady gradient of the lower channel pre-cludes �sh passage, and upper portions have no lakes or wetlands for �sh habitat. Rainbow trout are noted in the Mara Creek watershed, likely in the lakes on the upper plateau. Fish passage into Mara Creek is obstructed at the con�uence with Hummingbird Creek by a very steep section of channel.

Step 3 Investigate fan processes

Hydrogeomorphic processes – From �eld observa-tions, dendroecology, anecdotal reports, and re-view of various sets of aerial photographs, there is abundant evidence that debris �ow and debris �ood events have occurred on the fan in the past, with several lines of evidence pointing to a large event in 1935 (Jakob et al. 2000). Human activities on the fan prior to 1997 have probably altered morphological

evidence of previous events, such as boulder lobes and levees. However, there are exposures of debris �ow deposits overlying �uvial sediments, indicating a previous history of debris �ows on the fan (Inter-agency Report 1997). The 1997 debris �ow – The Swansea Point fan can be divided into upper and lower sections by Highway 97A (Figure A4.2): the upper fan section is approxi-mately 600 m long and the lower section is approxi-mately 700 m long.

The upper fan is con�ned by bedrock ridges, has moderate gradients between 10 and 18%, and has coarse sedimentary deposits. The active hydrogeo-morphic riparian zone (Wilford et al. 2005) remains relatively con�ned, about 50 m wide, for the �rst 400 m below the apex, then widens to over 100 m wide at the highway crossing. The typical particle size near the fan apex is up to 1 m (b-axis). One exceptionally large boulder (the “Hinkelstein,” 9 m tall, 8 m wide, and 6 m long) can be seen in Figures A4.3 and A4.4.

The lower fan has gentler gradients and �ner sedimentary deposits. Most of the coarse material (particle size > 0.4 m) was deposited on the upper fan, but �ner debris �ow material and water crossed both north and south of the highway culvert. To the south, the debris �ow carved a new channel across the road and then returned most of the �ow back into the lower channel. To the north, much of the

Figure A4.2 Distribution of new debris flow sediment on the Swansea Point fan, mapped on July 14, 1997 (from Inter-agency Report 1997). The shaded area incor-rectly follows Mara and not Hummingbird Creek at the lower right.

49

Figure A4.3 Looking downstream from the confined valley reach of Hummingbird Creek to the apex of the Swansea Point fan. Note presence of bedrock on channel margins, large boulders in the channel, and the “Hinkelstein” boulder in the centre.

Figure A4.4 The “Hinkelstein” at the apex of the Swansea Point fan. This huge boulder was transported during the 1997 debris flow.

50

�ow was carried straight down Swansea Road or uti-lized other old channel segments on the fan surface, before reaching Mara Lake in several locations.

Above the apex of the fan, the creek �ows in an incised, steep-sided, bedrock-cored channel 8–10 m wide, with a gradient between 18 and 21% (Figure A4.3). The average gradient between where the debris avalanche entered the creek and the fan apex is 27%. Event frequency – Two methods are used to recon-struct debris �ow occurrences on the Swansea Point fan: a collection of anecdotal information and an aerial photograph review. Anecdotal information – One account, presumed from the 1930s, indicates that the fan was covered by a layer of boulders 1.2 m in diameter, with the largest boulder having dimensions of 2.4 by 3 m (Jakob et al. 2000). The highway bridge across Hummingbird Creek was destroyed in this event. Aerial photograph review – Aerial photographs show that the main channel has migrated in recent history. At the beginning of the 20th century, the main channel was located on Railway Belt surveys as heading more westerly across the fan (Fuller 2001). It remained in this position until after 1928 (as seen on aerial photograph A360: 28). In the 1928 aerial photograph there are two road alignments, the newer of the two being roughly the present align-ment of Highway 97A. On the 1951 aerial photo-graphs (BC1292: 32 and 33) the creek �ows west until it passes under Highway 97A, then turns southwest and �ows into Mara Lake. This is likely the result of a debris �ood or debris �ow event reaching the fan and blocking the channel, and water being diverted onto the old road alignment. This remains the pres-ent channel location.

Synthesis of fan processesCompilation of all evidence suggests that there was a relatively large event that occurred between 1928 and 1951, most likely in 1935. The classi�cation of the event as either a debris �ow or debris �ood is unclear, but it apparently had sufficient material to block the existing channel and cause diversion onto an old roadbed, which has subsequently become the present-day channel. While there is some evidence of more frequent smaller events, there have likely been two large potentially damaging events in the last 75 years.

Step 4 Investigate watershed processes

Office investigationsWhile none of the aerial photographs dating back to 1928 shows evidence of recent landslides on the steep valley sidewalls above the lower channel, there is evidence of older vegetated linear features and possibly a large relic landslide. There is also a recent large debris slide on similar terrain just north of Hummingbird Creek (Figure A4.5).Morphometrics – The relative relief number (RRN) (Melton 1965) has been found useful for determining the hydrogeomorphic event that will emanate from a watershed (Wilford et al. 2004b). It has been found that for RRN < 0.3 the dominant hydrogeomorphic process is �ooding; for 0.3–0.6 and > 0.6 with water-shed length > 2.7 km the process is debris �ood; for > 0.6 with watershed length < 2.7 km debris �ows can occur (Wilford et al. 2004b). However, the RRN is designed for “graded watersheds”—watersheds that have steep terrain in the headwaters and gentle terrain in the lower reaches. Caution must be used when applying the RRN to reversely graded water-sheds and in de�nition of the area of a watershed.

There are two distinct watersheds feeding onto the Swansea Point fan, Hummingbird and Mara. The individual watersheds indicate that Humming-bird (RRN = 0.44) and Mara (RRN = 0.39) are debris �ood watersheds, but the combined area suggests it is more likely a �ood watershed (RRN = 0.29) (Table A4.1).

Hummingbird and Mara watersheds are clearly not graded; the headwaters have gentle slopes and the lower portions are much steeper—a situation that is referred to as “gentle-over-steep.” To com-pensate for this, subdivision of the watershed to isolate the steeper sections was completed (Figure A4.1). For Hummingbird Creek, subdivision of the watershed into the steeper section below the 1500 m contour gives an RRN of 0.51. This ratio indicates that the steeper portion of the watershed, which is more susceptible to landslides, is likely to be characterized by debris �ood events moving through the channels and reaching the fan. Given that the 1997 event was a debris �ow, a watershed RRN between 0.29 and 0.51 indicates �ood or debris �ood, which underestimates the actual hydrogeomorphic hazard, and also sug-gests that the two watersheds should be treated as separate for the RRN analysis.

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Figure A4.5 Aerial photograph BC2616: 76 of lower Hummingbird Creek, 1959, showing a recent landslide to the north on similar steep, northwest-aspect terrain, a possible relic debris slide colonized by a younger forest stand, and the location of the 1997 debris avalanche.

Table A4.1 Relative relief numbers for the Swansea Point fan

Elevation Range Area km2 (km) RRN

Hummingbird 16.4 1.79 0.44

Mara 21.2 1.79 0.39

Combined 37.4 1.79 0.29

Hummingbird 4.61 1.10 0.51< 1500m

Sediment production and movement – Terrain stability mapping of the Hunters Range completed after the Hummingbird debris �ow shows that Class IV (potentially unstable) and Class V (unstable) ter-

rain is common along the steeper face above Mara Lake. On the upland plateau, the terrain was gener-ally Class I to Class III (stable) with shallow bedrock covered by a thin veneer of sediment. Channels are moderately incised into the plateau surface, and sedi-ment production and delivery to streams is limited to the stream escarpment slopes. The steeper slopes on the Mara Lake face show more signs of sediment delivery to the channels with thin linear strips visible in the tree cover on aerial photographs (Figures A4.5, A4.6, and A4.7).

Several other creeks along the Hunters Range above Mara Lake, including Mara, Rodgers, and Johnson Creeks, have widened channels and exposed sediments in the 1951 aerial photographs (BC1292: 31–35 and BC1285: 34–37). There is no evidence on the

52

aerial photographs of logging within the upper wa-tersheds, and recent landslide scars are not apparent, suggesting that in-channel �oods or debris �oods might have been the cause. Sparse or young timber types across the top of the Hunters Range hint at the occurrence of widespread wild�re. Flood Generation – Snow packs of 2–3 m thickness are typical in this area. Limited stream�ow records on smaller watersheds in the southern interior offer only partial information on timing and volumes of �ows from the Hunters Range. Chase Creek water-shed (approximately 40 km west of Mara Lake) is the closest gauged watershed that drains directly from an upland area. Chase Creek experiences peak �ows generated from snowmelt between mid-April to mid-June, but maximum elevations are 500 m lower. Coldstream Creek watershed (about 50 km south of Mara Lake) is the next closest station and has peak �ows generated between April and mid-June, but maximum elevations are 600 m lower. From these records, peak �ows in Hummingbird and Mara Creeks are estimated to occur between mid-April through late June.

Field investigationsOn July 11th, 1997, a debris avalanche initiated on an open slope directly below a culvert on the Skyline Forest Service Road (Inter-agency Report 1997). A skid trail in a 1994 cutblock diverted water, which increased the drainage area leading to the culvert by approximately a factor of 3 (Figure A4.6). This inter-ception and concentration of hillslope runoff to the culvert is considered a “major contributing factor in initiating the debris avalanche” (Inter-agency Report 1997).

Snowpacks across the North Okanagan-Shuswap were 25% above normal in 1997, and followed record levels of precipitation in the area over the preceding 9 months (October 1996 through June 1997), result-ing in antecedent soil moisture conditions that were substantially higher than normal. Heavy precipita-tion recorded at local climate stations between July 5 and 12, (60- to 85-year return period) contributed more water to the already wet soils, and late on July 11 the debris avalanche was triggered.

The slope gradient below the culvert was up to 70% and had a cover of sur�cial sediment < 1 m thick over glacially smoothed, benchy, granitic-gneiss bedrock. The debris avalanche widened gradually downslope over a total slope distance of 560 m, from 5 m at the headscarp to 110 m where it entered

Hummingbird Creek (Figures A4.6 and A4.7). The debris avalanche then rapidly transformed into a debris �ow, which travelled 2.4 km down Humming-bird Creek onto the fan (Figure A4.7). An estimated 25 000 m3 of wood and sediment entered the creek from the debris avalanche. A further 67 000 m3 of debris, or 28 m3 of debris per linear m of channel, was estimated to be scoured from the creek channel. The total volume of the debris �ow was calculated at over 92 000 m3 (Jakob et al. 2000). Deposition was estimated as 49 000 m3 on the upper fan and 36 000 m3 on the lower fan, and 7 000 m3 reached Mara Lake. Following the debris �ow, the channel has a reduced amount of stored sediment, and a landslide entering the channel now would have less sediment to entrain.

Synthesis of watershed processesThe upper portion of Hummingbird Creek water-shed is unlikely to generate debris �ows or debris �oods. The relatively low channel gradients are less able transport signi�cant sediment loads to the lower half of the watershed. Floods may be generated from snowmelt in the upper watershed, but the timing of known events in late June and early July suggests that snowmelt has less of an effect than rainfall. Lower sections of the watershed have higher prob-abilities of generating events because there is direct connectivity of steep, potentially unstable slopes adjacent to the channel, and the channel gradient is relatively steep (Millard 1999). The lower channel has little or no �oodplain to arrest landslide debris, and consequently landslides have the potential to transform into a debris �ood or debris �ow. The 1997 debris �ow occurred as the result of intense long- duration rainfall on top of unusually high ante-cedent moisture conditions and thus there was an increased amount of stream�ow to propagate the landslide sediments through the creek to the Swan-sea Point fan.

Step 5 Analyze risks and develop plans

It is useful to review the 1997 debris �ow from the perspective of the �ve-step method for managing risks on fans from upstream watershed activities. In particular, to address the question “could the method have in�uenced how forest development in the watershed took place and possibly reduced the likelihood that a debris �ow would have occurred?” The following analysis uses the risk analysis pro-

53

Figure A4.6 Pre- and post-development runoff-contributing areas to the culvert located above the debris avalanche. (Source: R. Winkler, B.C. Ministry of Forests and Range and D. Anderson, B.C. Ministry of Environment.)

Figure A4.7 Aerial photograph of lower Hummingbird Creek watershed with the debris avalanche below the road in right centre (15BCC04021: 202, taken in 2004). The debris avalanche developed into a debris flow that proceeded down the channel to the Swansea Point fan.

54

cess to review what was known, or could have been known, prior to the logging and road building in the watershed that preceded the 1997 debris �ow. It should be noted that, while hindsight is useful, this analysis is not meant as a criticism of licensee or Ministry of Forests practices in Hummingbird Creek watershed prior to the 1997 debris �ow, which in our understanding were to the standards of the day (For-est Practices Board 2001).

SummaryStep 1 Identify fans and delineate watersheds This would bring to the attention of forest plan-

ners the connection between the upper plateau areas of the watershed where forest activities were planned, and the fan several kilometres away from those activities.

Step 2 Identify elements-at-risk on fans The presence of widespread risk elements (high

density of residences, major provincial highway and other infrastructure) that could potentially suffer losses would be identi�ed. Damage to resi-dences, infrastructure, and the highway would be considered a high consequence. Potential human injury or loss of life by residents or highway users would be considered a high to very high conse-quence, depending on the risk matrix employed. These high consequences would require that the remainder of the investigation and risk analysis be carried out very thoroughly.

Step 3 Investigate fan processes This would reveal the physical presence of debris

�ow deposits on the fan. With the potential for high consequences, a thorough review of past events, by various methods, would be appropri-ate. A review of historical aerial photographs would show that a major channel avulsion had occurred—further evidence of a signi�cant hy-drogeomorphic event. A review of published or oral history in the area, as was done subsequent to the 1997 event, would con�rm the historic occurrence of at least one large damaging event over half a century ago. From this it could be concluded that Hummingbird Creek could pro-duce a large damaging event, and that it had been some time since one occurred.

Step 4 Investigate watershed processes This would include a traverse along the channel

above the fan to assess sediment and woody de-bris loads available to be mobilized. High levels

of entrainable material would have been found prior to the 1997 event. This would have informed investigators of the potential for a large event. The texture of the material could give an indica-tion of whether mobilization of the channel bed would require a landslide impact or if in-channel initiation could occur from peak �ows.

Investigation of fan and watershed processes would lead to the conclusion that a large, poten-tially damaging hydrogeomorphic event had oc-curred on the fan circa 1935, and that a debris �ow had also occurred some time in the past. It was not clear whether the 1935 event was a debris �ow or debris �ood. Therefore the linkages to be considered between watershed processes and forest development would include the production and delivery of both sediment and runoff (Table A4.2).

The potential for landslides that could initiate debris �ows or debris �oods exists along the lower stream reach. Given the state of knowledge prior to 1997 (Hungr et al. 1984; Van Dine 1985, 1996), it would have been considered possible, but unlikely, that a landslide affecting the relatively low-gradi-ent (average 27%) channel would have initiated a debris �ow. Subsequent to the Hummingbird event, and several other events in southern interior Brit-ish Columbia, there is an increased awareness of the potential of large debris �ows in relatively low-gradi-ent channels.

There remains the possibility that a large landslide could dam the relatively narrow V-shaped channel, causing stream�ow to back up until the dam was breached, initiating a large debris �ood. Manage-ment of incremental landslide hazards on the lower steep slopes would be identi�ed as a concern.

Landslide hazards could be from operations on the lower steep slopes, or from operations on the moderate to gently sloping plateau some distance above those slopes. Prior to the Hummingbird event, management of road and trail drainage on “gentle-over-steep” plateau terrain was not widely recognized as a serious hazard. Subsequent to the 1997 debris �ow, and partially because of it, man-agement of forest development in gentle-over-steep terrain has been addressed through research and the development of best practices (Grainger 2002; Jordan 2002).

Investigation of channel-sediment texture in the steeper reach could give some indication of whether

55

Table a4.2 Forest management focus for different hydrogeomorphic processes. Key issues are bolded.

Process Initiation Management focus

Flood Runoff ECA – harvest, �re, forest health Road density Fire – reduced soil in�ltration Landslides – increasing sediment load to channels on the fan (in�lling channels)

Dam rupture Dam rupture – beavers, road drainage structures Landslide dams – harvest and roads (coast), gentle-over-steep (interior)

Debris �ood Runoff ECA – harvest, �re, forest health Road density Fire – reduced soil in�ltration

Landslide Harvest and roads on unstable terrain and gentle-over-steep in the interior

Dam rupture Dam rupture – beavers, road drainage structures Landslide dams – harvest and roads (coast), gentle-over-steep (interior)

Debris �ow Landslide Harvest and roads on unstable terrain and gentle-over-steep in the interior

Runoff—in-channel ECA – harvest, �re, forest health initiation Road density Fire – reduced soil in�ltration

in-channel debris �ow or debris �ood initiation due to peak �ows could occur, and whether ECA and other peak �ow watershed linkages would need management attention.

Step 5 Analyze risks and develop plans If one were to complete this step today for a simi-

lar set of conditions as existed at Hummingbird Creek prior to 1997, and with the knowledge of forest development and watershed processes we now have, the available evidence would indicate that there was a moderate to high likelihood of a landslide affecting the channel and resulting in a hydrogeomorphic event that would reach the fan. Given the uncertainties in the analysis, this would be considered a moderate to high hazard.

With the high to very high consequences existing on the fan, obtained from standard risk matrices (Table A4.3), there would be a high to very high incremental risk due to proposed forest develop-ment on the upland plateau, and further risk management would be appropriate in any sub-sequent plans. From a forestry perspective, the most feasible course of action would be to reduce the incremental development-related hazards.

With the hindsight we now have regarding man-agement of gentle-over-steep terrain and resulting landslide hazards (Grainger 2002), it would be recog-nized as appropriate to fully deactivate the roads and trails within the cutblock immediately follow-ing harvesting to re-establish or maintain natural

56

drainage paths. This deactivation would likely have prevented enlargement of the runoff-contributing area to the culvert above the slide. Other actions to reduce hillslope runoff due to harvesting include:

• Road construction methods should minimize subsurface �ow interception in high cut slopes, such as decreasing the road width or using a wider �ll to reduce cut slope height.

• Drainage redirection should be avoided by designing roads with no ditches or cross-drain culverts. Road cross-drainage can be maintained by out-sloping the road surface or by road con-struction with engineered continuous sub-grade drainage.

• In high-risk situations, it is professionally responsible for the geoscientist or engineer undertaking terrain assessment or road engi-neering design to prescribe site visits during and

Table A4.3 A matrix combining hazard and consequence to determine risk

ConsequenceHazard High Moderate Low

Very high Very high Very high HighHigh Very high High ModerateModerate High Moderate LowLow Moderate Low Very lowVery low Low Very low Very low

after road building and harvesting, to assess site conditions during operations and recommend changes to prescriptions or designs if necessary, and to ensure that operations are or were carried out in conformance with recommendations or designs (APEGBC Quality Management Bylaw 14(b)(4)).

If a steep area downslope of a potential develop-ment is judged to have a moderate or higher natural landslide potential, the prudent decision may be that the development should not occur within a speci-�ed distance of the marginally stable slope. This was probably not the case at Hummingbird Creek prior to 1997 (Inter-agency Report 1997). Although we can never know for sure what the outcome of various management options would have been, it is likely that if some or all of the above risk-mitigation steps had been taken, harvesting and road building on the upland plateau above Hummingbird Creek could have occurred without causing a major debris �ow.

Gentle-over-steep terrain similar to the Hum-mingbird case study is not unique. This case study presents a sobering example of what can happen when apparently subtle changes in a watershed result in signi�cant impacts to the downstream fan. Ap-plication of the �ve-step method may not always pre-vent such impacts, but it does provide a framework to rigorously explore hydrogeomorphic hazards, consequences, and risks.

57

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