EVALUATION OF NEW MODELS FOR SIMULATING EMBANKMENT DAM BREACH

Tony L. Wahl

Bureau of Reclamation – Denver, CO

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What is CEATI International?What is CEATI International?

» Since 1891, the Canadian Electrical Association (CEA) has been the forum for electrical business in Canada

» In 1974, CEA initiated its R&D Program to serve the research needs of Canadian electric utilities

» In 1998, CEA’s R&D Program opened its doors to international participation

» In 2001, CEA Technologies Inc. (CEATI) was separated from the Canadian Electrical Association

» CEATI International is now the “Centre for Energy Advancement through Technological Innovation”

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Interest GroupsInterest Groups» 14 Interest Groups in the areas of electrical energy…

– Generation

– Transmission

– Distribution

– Utilization

» Dam Safety Interest Group– About 40 dam owners

– Jointly sponsors research & development projects

– Participants from Canada, the United States, Europe, Australia, and New Zealand

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Dam Safety Interest Group (DSIG)Dam Safety Interest Group (DSIG)

» Areas of Interest:Areas of Interest:– Risk assessment for dam safety– The use of geophysical methods in the diagnostics

and monitoring of embankment dams– Erosion and piping in dams– Reliability of discharge facilities– Ice loadings– Probability (frequency) of extreme

floods– Emergency preparedness– Testing of embedded dam anchors

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Dam-Break Modeling: Recent HistoryDam-Break Modeling: Recent History

» Lethal Dam Failures in 1970s– Canyon Lake

– Kelly Barnes

– Laurel Run

– Buffalo Creek

– Teton

» 1977 DAMBRK model developed

– Could route peak breach outflows to determine inundation depths, flood consequences

– Could determine peak breach outflow, given a description of how a breach would develop

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Modeling Breach DevelopmentModeling Breach Development

» Concrete dam failure modes (sliding, overturning, structural) are usually instantaneous and complete

» Embankment dam failures usually involve erosion, which takes time and depends on many factors– Regression equations to relate breach

parameters to dam and reservoir characteristics• Many developed in 1980s and refined in 1990s

– Adequate for cases in which the area of interest was in the “far-field”

– Too crude for the “near-field”

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Physically-Based Breach ModelingPhysically-Based Breach Modeling

» Dr. Danny Fread recognizedneed for modeling erosionprocesses to obtain betterresults in near field

» May 18, 1980 eruption ofMt. St. Helens createdlandslide dam on Toutle River

» Dr. Fread developed NWS-BREACH model to analyze possible breach of this dam

» NWS-BREACH released to public in 1988

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Modeling Developments in 1990sModeling Developments in 1990s

» Flood routing capabilities much improved– 2D modeling

– Integration with GIS to improve consequence analysis

» Little change in breach modeling during this time

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CEATI Dam Erosion and Breach ProjectCEATI Dam Erosion and Breach Project

» Since 2001 the DSIG has had an interest in improving the tools used to model embankment dam erosion and breaching

» Key Questions– Will a dam breach?

– What is the outflow hydrograph?

– What is the warning time?

» Available methods mostly unchanged since late 1980s1. Regression models for predicting peak outflow

2. Regression models for predicting breach parameters

3. Breach erosion models, such as NWS-BREACH

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Shortcomings of Available MethodsShortcomings of Available Methods

» Regression models for peak outflow– No aid in determining whether breach occurs

– Little detail about hydrograph shape or warning time

» Regression models for predicting breach parameters– Uncertainties are large, especially for time parameters

• Breach initiation time• Breach formation time

» Breach erosion models (e.g. NWS-BREACH)– Used sediment transport equations, not true erosion

models

– Poor modeling of erosion of cohesive materials

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Large-Scale Physical TestsLarge-Scale Physical Tests

» Since 2000, many organizations have been performing small-scale and some large-scale embankment breach tests– European IMPACT Project (22 lab tests and sponsorship

of Norwegian field tests)

– Norwegian tests (23 lab tests, 5 field tests of 6-m-high dams)

– Agricultural Research Service (7 overtopping tests and 4 piping tests of 2-m-high dams)

» New breach erosion models under development– Physically-based simulation of erosion processes– Better modeling of the erosion of cohesive soils

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Project ObjectivesProject Objectives

» Dam breach erosion project was initiated in 2004, with a focus on erosion and breach processes and prediction of breach outflow hydrographs at the dam

» We want to develop physically-based models for overtopping erosion and internal erosion leading to dam breach and facilitate the integration of those technologies into existing flood routing tools like HEC-RAS, MIKE11, Telemac, InfoWorks, etc.

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ParticipantsParticipants» Electricité de France

– Case studies…erodimeter and piping erosion research» Hydro Québec / Ecolé Polytechnique Montréal

– Numerical modeling of dam breach, development of Firebird breach model» Bureau of Reclamation

– Laboratory testing…investigate erodimeters» Agricultural Research Service

– Large-scale laboratory testing and development of SIMBA/WinDAM models» HR Wallingford

– Large-scale testing (IMPACT project), developers of HR-BREACH model» US Army Corps of Engineers

– Integration of breach modeling technology into HEC-RAS suite» Elforsk AB

– Model evaluation

» Other interested parties and sponsors– BC Hydro, Churchill Falls, Elforsk AB, EoN Vasserkraft, Great Lakes Power,

Manitoba Hydro, New York Power Authority, Ontario Power Generation, Seattle City Light, Scottish & Southern Energy, National Weather Service

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Project OverviewProject Overview

» Phase 1: Information Gathering– Reviewed and assembled case-study and large-scale

laboratory test data

– Reviewed and identified numerical models under development

» Phase 2: Model Development and Implementation

» Phase 3: Model Enhancement

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Tasks in Phase 2Tasks in Phase 2

1. Evaluation of three numerical breach models– SIMBA (ARS)

– HR-BREACH (HR Wallingford)

– FIREBIRD BREACH (Montreal Polytechnic)

2. Evaluation of methods for quantifying erodibility of cohesive embankment materials

leading to…

3. Integration of breach modeling technologies into HEC-RAS dynamic routing model

4. Potential efforts to facilitate integration with commercial flood routing models

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The Models: Common CharacteristicsThe Models: Common Characteristics

» Models are all physically-based» Models utilize quantitative input parameters

describing erodibility of cohesive materials» Models are intended to perform well without

specific calibration to a particular case» Models are not computationally intensive

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The ModelsThe Models

» SIMBA – Simplified Breach Analysis (USDA-ARS)– Simulates breach by overtopping of homogeneous earth

embankments with negligible protection on the downstream face

– Four stage failure process1. surface erosion leading to development of a headcut on the

downstream face of the embankment

2. headcut advance through the crest to initiate the breach

3. breach formation as the headcut advances into the reservoir

4. breach expansion during reservoir drawdown

– Erosion formulas are fixed and most calibration factors have been determined from lab testing. Complete model is not calibrated to any specific data set.

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The ModelsThe Models

» HR BREACH (HR Wallingford)– Overtopping or piping-induced breach of cohesive, non

cohesive and simple composite (i.e. zoned) structures.

– Simulated processes:• Initial erosion of embankment surface protection (grass or

rock cover)• Headcut erosion through embankment • Potential failure of breach side slopes by shear or bending• Potential for sliding or overturning of core section

– Limited selection of erosion formulas

– Not calibrated to any specific data set

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The ModelsThe Models

» FIREBIRD BREACH (Montreal Polytechnic)– Overtopping-induced breach of homogeneous earthfill or

rockfill dams

– One dimensional unsteady flow, St. Venant equations coupled with sediment continuity

– Able to handle transcritical flows

– Side slopes are evaluated for ability to resist sliding along a simple inclined face

– Choice of erosion formulas

– Can be more computationally intensive

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Model EvaluationModel Evaluation

» Evaluate model performance against large-scale laboratory tests and case-study data– 2 ARS outdoor laboratory tests

2.3-m high homogeneous dams, overtopping1 breach, 1 non-breach

– 3 overtopping breach tests performed in Norway during the IMPACT project (5- to 6-m high dams)

• homogeneous clay• homogeneous gravel• zoned embankment

– 2 real dam failures• Oros (Brazil)• Banqiao (China)

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ARS TestsARS Tests» Two overtopped embankments, 2.3 m high

– SM Silty Sand, complete breach in 51 minutes– CL Lean Clay, headcut damage, but no breach after 20

hours» 2.5 orders of magnitude

difference in erodibility ofmaterials

» Constant inflow, smallreservoir

Hanson, G.J., Cook, K.R., Hunt, S. 2005. Physical modeling of overtopping erosion and breach formation of cohesive embankments. Transactions of the ASAE, 48(5):1783-1794.

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Norwegian Tests - Part of IMPACTNorwegian Tests - Part of IMPACT» Three overtopped embankments, 5 to 6 m high

– Homogeneous clay, placed very wet– Homogeneous gravel, surface frozen– Zoned rockfill with moraine core

» Inflow regulated at upstream reservoir– Clay dam: Peak inflow arrived

shortly after initial breach…reservoir level went back up…peak outflow driven by peakinflow

– Flow regulation not attempted forgravel dam test

– Inflow was too little, too latefor zoned test

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Oros Dam (Brazil, 1960)Oros Dam (Brazil, 1960)

» 35-m high dam, failed by overtopping during construction

» Core material probably a Sandy Lean Clay, with PI=10» Well-compacted, except maybe last lifts

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Oros Dam - SummaryOros Dam - Summary

» Thick, erosion-resistant embankment, large reservoir» Slow erosion

– 12 hrs to initiate breach

– 6.5 to 12 hrs to form breach and drain reservoir

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Banqiao Dam (China, 1975)Banqiao Dam (China, 1975)

» Hand-built dam with homogeneous earth shells and clay core wall of “arenaceous shale”

» Assumed to be poorly compacted and highly erodible» 1 hr breach initiation» 2 to 2.5 hrs to fully form breach

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Evaluation CriteriaEvaluation Criteria

» Evaluate performance using– initial inputs (best available information and judgment)– optimized inputs

» Objective criteria– Time to initiate breach (erode through crest)– Time to form breach (reach full width)– Final breach width– Breach widening rate– Peak outflow

» Subjective criteria– Do models exhibit appropriate sensitivity?– Ease of determining input data and selecting parameters– Ease of operation

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Current StatusCurrent Status

» Team met at last year’s USSD meeting in Portland» Members have been working this summer to perform the

evaluation runs» Group will meet again later this week to compare results

and try to reach consensus on:– Which models and model components are working well?

– What technologies are presently ready to be integrated into state-of-the-art models?

– Where is more work needed?

» SIMBA and HR-BREACH models are being integrated into USDA WinDAM and Wallingford Software InfoWorks products

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ChallengesChallenges» TIME: Too many models, cases, scenarios» Each case study presents unique evaluation challenges

– Real failures have questions about dam materials and erodibility, and about observed breach and outflow characteristics

– Lab tests have “real-world” logistical complications and limitations related to reservoir size

– Failure to accurately model breach initiation phase can require judgment to evaluate how well the model reproduced later stages of the breach process

» Evaluation process has already been extremely valuable

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