U.S. Department of the Interior U.S. Geological Survey Numerical Analysis of the Performance of River Spanning Rock Structures: Evaluating Effects of Structure.

U.S. Department of the InteriorU.S. Geological Survey

Numerical Analysis of the Performance of River Spanning Rock Structures:

Evaluating Effects of Structure Geometry on Local Hydraulics

http://www.usbr.gov/pmts/sediment/kb/SpanStructs River Spanning Rock Structures Research

Chris Holmquist-Johnson

Background

Rock weirs capabilities include: fish passage, bank protection, channel profile stability, and improved aquatic habitat.

Rock weirs are ideal structures for conditions of…steep slopes, diversions, scalability, habitat, and aesthetics.

Failures or loss of function can result in: disruption of service, expensive repairs, maintenance cost, failure to meet biological and geomorphic requirements, and loss of prestige.

Background

Qualitative field evaluations and Initial laboratory results found widespread performance issues.

We think this is a flaw in the designs, not in the concept.

The Rock Weir Design Guidelines Project was developed to investigate failure mechanisms and provide methods for designing sustainable structures.

Research ComponentsThe Research Project takes a multifaceted approach incorporating mutually

supporting field, laboratory, and numerical studies.

Field

Laboratory Computer

• Provides a controlled setting

• Includes scour processes

• Physically replicates hydraulics

• Inexpensively simulates large numbers of cases

• Increases range of applicability

• Provides a design tool

Evaluates current methods Hypothesizes successful techniques

Identifies failure mechanisms Creates a practical link

Need for Physical Modeling

Field investigation identified growth of a scour hole causing geotechnical movement as the primary failure mechanism for rock weirs

Understanding the scour processes and quantities provides vital information on failure modes and potential counter measures required to design sustainable structures

Understanding failure modes in the field is COMPLICATED Analyzing scour hole development is best answered in a

controlled environment such as the laboratory

Need for Physical Modeling

Assumed

Physical Model(Qbankfull, Small cobble)

1015

2025

3035

4050556065

97

97.297.4

97.6

97.8

9898.2

98.4

98.698.8

9999.299.4

99.6

99.8100

100.2

100.4

100.6100.8

101

1015

2025

3035

4050 55 60 6597

97.2

97.4

97.6

97.8

98

98.2

98.4

98.6

98.8

99

99.2

99.4

99.6

99.8

100

100.2

100.4

100.6

100.8

101Difference in anticipated scour hole definition

and physical model results

Rosgen (2001) -

Max scour

U-weir generated two defined scour hole locations

Physical Modeling Objectives

Rating curves for flood elevations Scour prediction method Potential counter measures and retrofits Low flow depths and velocities for fish passage criteria

The field evaluation included 21 sites consisting of 127 structures and included various structure designs by Reclamation, Natural Resources Conservation Service (NRCS), Dave Rosgen, and Oregon Department of Fish and Wildlife (ODFW). The observed range of conditions included:

Evaluations of Field Performance

• Approximate channel widths from 15 feet to 250 feet• Approximate channel slopes from 0.5% to 2%• 75 U-or V-weirs, 28 J-hooks, 12 A-weirs, 4 W-weirs, and 2 rock ramps

Evaluation of Field Performance

Field measurements provide a practical link to guide and validate laboratory and numerical investigations.

Field evaluations identified several failure mechanisms. Scour and Slumping Flanking Loss of Pool Depth Incipient Motion

Identifying failure pathways allows for design of countermeasures.

A-weir with throat buried by sediment

Displacement of Header rocks along arm

Redirection of flow at bankfull Q

Velocity Vectors Parallel to Bed

Velocity Vectors Returning Parallel

to Bed

Plunging Flow

Hydraulic Jump

Rapid Vertical Contraction and

Expansion over the Weir Crest

Importance of Numerical Modeling

Field and laboratory provide data over a limited range of conditions. Narrower ranges of conditions limit the applicability of results. Numerical modeling provides opportunity to test numerous design

parameters over large ranges for less money and in a shorter amount of time to extend the applicability.

Numerical Model Design Parameters

Bed Material and Slope Coarse Gravel to Large Cobble (23mm to 181mm) Slope .001, .005, and .01

Channel Geometry Regime Equations (Parker et al. 2007)

344.0

25050

101.0

ss

bf

DgD

QS

0667.0

25050

4.0

51

63.4

ss

bfbfbf

DgD

QQ

gW

52

51

382.0bfbf Q

gH

S = bed slope;Qbf = bankfull discharge (m3/s);Ds50 = median particle diameter, (m);Wbf = bankfull width (m);Hbf = bankfull depth (m).

Numerical Model Design Parameters

Structure Geometry Drop height

0.4, 0.8, 1.2 ft

Throat Width .25W, .33W, and .5W

Arm Length (arm slope & departure angle) .5 and 2 times minimum deviation (4.5% and 25 degrees)

Bank Full2-7 Degrees

0.8 ft

1/3 W1/3 W

20-30

2-7 Percent

U-Weir Geometry Design Range

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10%

11%

12%

0 5 10 15 20 25 30 35 40 45 50

Planform Angle (degrees)

Cre

st

Arm

Pro

file

An

gle

(p

erc

en

t)

Recommended Design Range Target Midpoint Minimum Deviation CG SC LC LabData

Numerical Design Parameters

Numerical Model

3D model U2RANS- (Yong Lai, Reclamation)

Unsteady and Unstructured Reynolds Averaged Navier-Stokes solver

Preprocessor, CFD solver

Mesh Generation and Post Processing Auto mesh generator, SMS, Tec Plot

Automatic Mesh Generator

Footer

Downstream

Top B

ank

Toe

Upstream

Header

Throa

t

Bed

Overba

nk

Header

Bed

Boundary Condition

Boundary Condition

Right

Left

Pool

Flow-Wise Bank-Wise Structure-Wise Laterally

Upstream Left Bed Throat

Downstream Right Header Toe

Footer Top bank

Pool Overbank

Automatic Mesh Generator

Numerical Model Validation

Laboratory comparison

U-Weir

Bank Full2-7 Percent

0.8 ft

1/3 W1/3 W

20-30

Design Concept

Construction Before Test

Numerical Model Validation

Bed shear stress distribution at Qbkfull

Laboratory comparison

U-Weir

Bed topography from LIDAR

scan

Surface velocity distribution with

stream lines

Numerical Model Validation

Laboratory comparison

U-Weir water surface profile comparison

Numerical Model Validation

Laboratory comparison

U-Weir Velocity Comparison

Numerical Model Validation

Field U-weir Velocity at Qbkfull

Field comparison

Numerical Model Validation

Field comparison

Numerical Model Validation

Field comparison

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

3 6 9 12 15 18 21 24 27 30

Distance (m)

Vel

oci

ty (

m/s

)

3D Field-ADV weir crest 2D

Velocity Comparison to Measured Field Data

-20%

-15%

-10%

-5%

0%

5%

10%

15%

20%

25%

30%

35%

40%

0 9 12 14 17 25

location

% d

iff

2D 3D

Numerical Model Results-Varying Flow Rate

1/3W 4.5% slope 25 degrees

Numerical Model Results-Structure Geometry (Qbkf)

½*Arm Length

2*Arm Length

Numerical Model Results-Structure Geometry (1/3Qbkf)

½*Arm Length

2*Arm Length

Numerical Model Results

Results from the numerical model show a significant change in velocity and shear stress distributions when the geometry of the structure is altered

Currently working on a statistical method for describing the variation in velocity and bed shear stress to structure configuration

Numerical Model Results Assist In:

Optimizing structure geometry for given restoration needs (Diversion, Fish passage, bank protection, pool development, etc.),

Analysis of flow hydraulics resulting from differing structure configurations,

Predicting DS scour hole development, and

Providing techniques for analyzing, designing, and constructing sustainable rock weir structures.

Research Summary

The research produces tools and guidelines for structure design or retrofits based upon predictable engineering and hydraulic performance criteria.

Understanding the processes governing success and failure allow designers to construct robust and sustainable structures requiring fewer repairs, less disruption of service, and lower maintenance costs.

Guidelines simplify and reduce future design efforts while increasing the likelihood of successful structures.

We hope that solidly developed criteria will simplify the regulatory permitting processes.

In the End

Reliable structure designs reduce failures,

Fewer retrofits saves money,

Saving money while meeting regulatory requirements governing habitat and fish passage makes everyone happy

Reduced failures results in fewer retrofits,

Colorado State University Dr. Chris Thornton and Dr. Chester Watson Tony Meneghetti, Mike Skurlock

Reclamation Kent Collins, Kendra Russell, Elaina Holburn,

David Mooney Science & Technology program Pacific Northwest Area office

Special Thanks To:

Questions?