Schwartz

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.



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).



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

Schwartz

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