Post on 17-Jan-2016
description
Evaluation of Pool-Riffle Maintenance Processes
in an Incised Urban Channel
Using a 3D Hydrodynamic Model:
Implications for Stream Restoration
John S. Schwartz, PhD, PE
Keil J. Neff, PhD Candidate
Department of Civil & Environmental Engineering
University of Tennessee – Knoxville
Frank E. Dworak
GEI Consultants, Inc., Centennial, Colorado
2008 NCSU Stream Restoration Conference
Biological Significance of the Pool-Riffle Structure
pool
riffle
Ordination results from:Schwartz JS & EE Herricks. 2008.Fish use of ecohydraulic-based mesohabitat units in a low-gradient Illinois stream: implications for stream restoration. Aquat Conserv: Marine & Freshw Ecosystems 18: 852-866.
Fish species and other
aquatic organisms have
evolved to exploit
resources differentially
between pool and riffle
mesohabitats
fish species grouped
Embarras River, Champaign County, Illinois, August 2001
Degradation of Pool-Riffle Structure
in Incised Channels
Longitudinal Profile, Water Surface Elevations, and Habitat Type Delineations
WF of NB of the Chicago River (Northbrook, IL) - June 1999
190
191
192
193
194
195
0 100 200 300 400 500 600 700 800
Longitudinal Stream Distance (m)
Bed
Ele
vati
on
(m
)
Bed Elevation WSL POOL RIFFLE GLIDE
• riffle-glide sequence dominated pre-restoration
Schwartz JS & EE Herricks. 2007. Evaluation of pool-riffle naturalization
structures on habitat complexity and the fish community in an urban Illinois stream. River Research and Applications 23: 451-466.
MacRae (1996)
identifies pool-
riffle sequence
degradation in
urban channels.
NorthbrookRestorationProject
Pre-projectmonitoring
Stream Restoration:
A Focus on Maintenance of Pool-Riffle Structure
191.0
192.0
193.0
194.0
195.0
0 100 200 300 400 500 600 700 800 900
Stream Distance (m)
Be
d E
lev
ati
on
(m
)
RR Bridge Business
DistrictShermer
Road
City
Park Walters
Road
0
2
4
6
8
0 10 20 30 40 50
x (m)
y (
m)
1.0
1.0
0.8
0.8
0.60.40.2
Plan view of unit Single unit: 35 m length and 7.5 m width
1:3 transverse slope in pool area
1:10 longitudinal slopes
Stream Restoration:
A Focus on Maintenance of Pool-Riffle Structure
Schwartz JS & EE Herricks. 2007. Evaluation of pool-riffle
naturalization structures on habitat complexity and the fish community in an urban Illinois stream. River Research and Applications 23: 451-466.
0
2
4
6
8
10
12
RR Bridge Business
Dist.
Shermer
Rd.
City Park Walters
Rd.
Elm Street Oak Street
Fis
h B
iom
ass (
g/m
2)
1999 2000 2001 2002 2003
DownstreamWithin Project Reach
** ** *** * *
0
5
10
15
20
25
30
RR Bridge Business
Dist.
Shermer
Rd.
City Park Walters
Rd.
Elm Street Oak Street
Fis
h D
en
sit
y (
#/1
00m
2)
DownstreamWithin Project Reach
* ** ** *** * ***
***
a)
b)
black = pre-construction
gray = post-construction
Biodiversity increased from 0.49 to 1.29
Species Richness increased from 4 to 11
Pool-Riffle Maintenance:
Geomorphological Principles and Theory
• Pool-riffle sequences observed in both sinuous and straight channels, associated with alternating bar morphology
• Pool spacing occurs on average 5 to 7 channel widths;
however pool spacing range is from 1.5 to 23 (Keller and Melhorn 1978)
• Hydraulic patterns of flow acceleration-deceleration.
Rhoads and Welford. (1991). Morphology of single-row alternate bars in straight channels. Shaded areas are below reach-averaged bed elevation, arrows indicate patterns of flow, and heavy arrows (red line) mark position of the thalweg.
Pool-Riffle Maintenance:
Geomorphological Principles and Theory
Riffle-Pool Sequence:Theoretical models of flow
structure in a straight channel for:
A) twin periodically reversing surface-convergent helical flow cells (Einstein & Shen 1964); and
B) surface-convergent flow produced by interactions between the flow and a mobile bed, creating riffle-pool sequences (Thompson 1986).
Figure from Knighton (1998)
Helical flow patterns observed
in meanders by Dietrich (1987).
Pool-Riffle Maintenance:
Geomorphological Principles and Theory
Yalin (1992) defines burst cycles as alternating regions of high-speed and low-speed, associated with bed erosion and deposition (pool-riffle maintenance).
Coherent macro-turbulent structures scale to
channel depth, approximately 6 flow depths
Reach-scale Morphological Adjustments:
Channel Evolution Model
Figure from Simon and Hupp (1986); Simon (2004).
Research Questions
• Do helical flow patterns occur in incised channels with woody vegetated banks?
• How are hydraulic patterns impacted by increased large roughness elements on the banks?
• Can flow acceleration-deceleration patterns be induced by modifying bank vegetation to promote pool-riffle
development, theoretically?
Planform Constrained Channel due to Urban Development
Beaver Creek, Knox County, Tennessee
Cohesive soils bed and bank; mixed bed substrates gravels to fines; channel width approximately 6.5 m, bankfull depth 2.1 m; channel slope 0.0001 m/m.
Study Site:Beaver Creek, Knox County, Tennessee
near bankfull flows
Ridge and Valley Geology
3D Hydrodynamic Modeling:FLOW3D® Model used for 3 Channel Structures
Tecplot Image
2. Channel with the
Trees Absent
-- used as for experimental
comparison with trees
1. Channel with Trees
-- present state of the
research site.
3. Channel: Proposed
Restoration Design
Modeled discharge near bankfull depth , Q = 1.68 m3/s; Reach Length = 105 m
Flow3D® Model utilized full expression of Navier-Stokes Equation.
Model Results: Velocity Vectors at y-z Cross-sections
90m
93m
97m
90m
93m
97m
90m
93m
97m
Maximum Velocity Vector for each cross-section
Restoration DesignWith No Trees Channel With Trees
(m/s)
55m
79m
82m
55m
79m
82m
55m
79m
82m
Restoration DesignWith No TreesChannel With Trees
Model Results: Velocity Vectors
(m/s)
25m
42m
45m
25m
42m
45m
25m
42m
45m
Restoration DesignWith No TreesChannel With Trees
Model Results: Velocity Vectors
(m/s)
• High turbulent kinetic energy around trees
• Kinetic energy dissipated near boundary
roughness, bank morphologic heterogeneity
• Résistance from bank morphology and trees
influence macro-turbulent structures.TKE
(m2/s2)
Channel with Trees Present
Channel with no Trees Present
Channel with Restoration Design
Model Results: Near-surface Macro-Turbulence
Summary Points
• Helical Flow Reach-scale Patterns:
1.) Some evidence of helical flow in channel with no tress, however difficult to detect from cross-sectional figures, in part due to influences from bank morphological heterogeneity; and
2.) helical flow patterns not evident in channel with banks trees.
• Additional analysis planned:
1.) FLOW3D model verification of ADV measurements taken during an
April 2007 flood; and
2) model output analysis of helical patterns.
• Restoration Design: Pool-riffle maintenance may be promoted by considering enhancing hydraulic flow acceleration-deceleration patterns, modifying bank morphology, instead of using bed weir structures.
• Biological Integrity: Restoration designs need to promote pool-riffle development and maintenance by considering 3D hydraulics .
Proposed Restoration Design Concept
• Remove trees on banks except for clusters spaced at approximately 6 channel unit widths -- promote flow acceleration and pool maintenance.
• Increase channel width immediately downstream of tree cluster -- promote flow deceleration and riffle maintenance.
• Use FLOW3D for final proposed design -- examine hydraulic vectors over the reach for helical flow patterns, and local bank/bed shear stress.
Cohesive soil banks -- critical shear stress = 2 to 3 Pa with submerged jet tester; ~ 11 Pa from flume tests.
• Riffles may be augmented with gravel -- current bedload sampling is in progress for development of bedload rating curve.
Flow acceleration at trees: promote bed scour and pool maintenance
Flow deceleration: widen bank and promote riffle maintenance