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Numerical Modeling Case Studies in Support of ELJ Restoration Projects
The Hoh River & South Fork Nooksack
byBob Elliot
northwest hydraulic consultantsSeattle, WA
nhc
Objectives of Presentation
• Present examples of higher-end numerical hydraulic modeling in ELJ restoration projects.
• Discuss their use in analyzing, designing, and presenting ELJ alternative.
• Illustrate benefits of two-dimensional (2D) and 1D branch-flow (quasi-2D) modeling, where appropriate.
• Two case-studies: Hoh River & South Fork Nooksack River.
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Eroding Bank 500 ft
Hoh River Erosion Project Site
US 1
01
US 101
Objectives of Hoh Project
• An emergency response to bank erosion and revetment failures along US 101, most recently from October 2003 flood.
• Provide longer-term solution for protecting the highway and minimizing frequency of repairs.
• Emulate natural conditions that deliver net benefit to aquatic and riparian habitat (e.g. increased bank roughness, cover, and channel complexity).
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General Approach to Modeling
• Build “far-field” HEC-RAS reach model, analyze preliminary scenarios under wide range of flows, & develop conceptual plan (Herrera).
• Use SMS to develop “near-field” FESWMS (Flo2DH) model as a more accurate tool to help refine and finalize design.
• Use 2D model results to estimate and simulate potential channel response to ELJs and future river planforms.
• Recognize fixed-bed limitations of models.
Approach to 2D Modeling
• Establish accurate baseline conditions of the complex hydraulics at the site (sharp bend, side channels, etc.), including inundation area, flow depths, velocities.
• Incorporate proposed project elements into model. • Analyze only select key flow conditions (100-yr, 2-
yr, and 1-yr). • Provide inputs for refined design: dynamic forces,
scour, flow distribution. Re-evaluate. Re-simulate. • Finalize design to improve hydraulic performance
and minimize impacts.
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500 ft
Computational Mesh with Ground Contours
Assignment of Roughness Coefficients
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2D Results from SMS Program
• Scalars (contours):– Velocity Magnitude– Water Surface Elevations– Water Depth– Bed Shear– Stream Energy– Difference Plots
• Vectors: – Velocity
Existing 100-yr Velocity Magnitude
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ELJ Configurations #1 and #2
Diversion
100-yr Velocity comparison between ELJ alternatives
ELJ #1
ELJ #2
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ELJ #2 100-yr Velocity Magnitude
100-yr Velocity Increase from Existing to ELJ #2
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HEC-RAS cross-sections compared with FESWMS output
(courtesy Herrera)
170.9 – 173.3171.0110168.0 – 170.7168.4100164.6 – 165.6164.990160.1 – 163.3162.080158.2 – 162.5160.770157.6 – 162.0159.460157.3 – 158.3156.850156.8 – 157.3155.740
156.2154.230152.2 – 154.1152.125151.3 – 153.1151.120
150.3150.310
FESWMSwater surface
elevations (ft)
HEC-RASwater surface
elevations (ft)
100-year flow event
HEC-RAS Channel section
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100-yr Water Surface Increase from Existing to ELJ #2
100-yr Flux Line Discharge Computation
7248 cfs Existing6424 cfs ELJ #2
29542 cfs Existing39362 cfs ELJ#2
30036 cfs Existing21237 cfs ELJ#2
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100-yr Increase in Energy from Existing to ELJ #2
100-yr Bed Shear with ELJ #2
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Estimated Channel Location after Anticipated Westward Shift
100-yr Velocity Increase under ELJ #2 with Assumed Channel Shift
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Comparison of 2-yr Existing to ELJ #2 Velocities
2-yr Water Surface Increase from Existing to ELJ #2
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Comparison of 1-yr Existing to ELJ #2 Velocities
Conclusions
• Improved accuracy of model results leads to increased confidence in performance of project.
• Complex hydraulics of site difficult to characterize with 1D model.
• Detailed 2D results enabled fine-tuning of ELJsfor better flow distribution & velocity dissipation.
• Easy visualization & interpretation of model output, can be understood by non-technical project stakeholders and public.
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• Hoh Project Specifics:– Predicted reduction in 100-yr peak velocities
along Highway 101 from 15 fps to 5 fps. – Flow redistributed across point bar and
diversion channel, away from roadway. – Increases in water level small and localized.
No landowner impacts. – Improved channel complexity.
• Limitations to 2D Modeling: – Cost– Good tool, but only a model– Fixed Bed (most cases)
Photo Comparison Looking upstream from Hwy 101
April 8, 2004
January 14, 2005
1234
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Apex (instream) ELJs
Barb (streambank) ELJs
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South Fork Nooksack RiverHutchinson Creek Study
• Restoration Study consisting of proposed ELJs and other enhancements.
• Using the unsteady (UNET) module of HEC-RAS to simulate branch-flow conditions.
• Calibration and baseline modeling complete, evaluation of restoration plan presently underway.
• Potential avulsions & downstream impacts.
Study area
Acme
South Fork HEC-RAS (UNET) Model Layout
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Hutchinson Creek Study Area
Hutchinson Creek
Old H
utchinson Creek
Remnant
Channels
South Fork Nooksack
erosion
Preliminary Restoration Plan
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The End
nhc