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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

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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

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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

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