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Gravel Bar Formation at a Bridge Replacement

Steven Griffin

CIVE 513 Morphodynamic Modeling

December 1, 2020

Study Site - October 2020

Background and Method

Bridge Replaced and Graded 2015

Concurrent Upstream Grading for Woodward Site

Oct 2020

The site in August 2012

Aug 2012

The site in October 2016

Oct 2016

Objective

Objectives of the Study

1. To replicate the bar formation using a 2D hydraulic model with morphodynamic component

2. To understand the reason(s) for the bar formation and to apply lessons learned for future bridge and associated grading work

Methods

Hydraulic Model using SRH-2D v.13.0.14

Post-Project Topo Mesh as Baseline

24,500 Elements

USGS Gage - Lincoln Avenue

May 7 to 16, 2015 - 239.75 hours

Unsteady Boundary Conditions per USGS data

Approximate Boring Log with Sediment Roughness Layer

Sediment Transport Eqn = Meyer Peter Muller

Results

Sediment Deposition after 240 hours (ft)

Sediment Deposition after 240 hours (ft)

Sediment Deposition after 240 hours (ft)

North of bar only inundated for about 20 hours

Pre-Project vs Post-Project Sediment Deposition

Conclusions

1. Southern portion of bar was able to be approximated using SRH-2D and its sediment transport module, with the MPM relationship.

2. Running the sediment transport model for a longer period of record with higher flows may increase the reproductivity of the bar, especially the northern portions.

3. The engineered channel cross-section upstream and under the bridge was prone to 1-3 feet of sedimentation by gravels and sands even during a single 10-day runoff event.

Conclusions

4. We want to be careful in assuming that a larger bridge opening will necessarily lead to lower BFEs, especially when the active channel is more narrow than the opening. Grading can be optimized through a hydraulic structure by a morphodynamic model.

5. Though the new structure handles the 1% recurrence flood much better than the previous structure, more consideration should be paid to more typical flows throughout the year, especially where good quality real-time data is available. In bridge design, we often prioritize the high flows at the expense of understanding the lower flow regime.

Questions?

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