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