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Open-File Report 2011–1223

Prepared in cooperation with the Crawford County Conservation District and Fairfield Township, Pennsylvania

Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pennsylvania

By Elizabeth Hittle

Thumbnail of and link to report PDF (113 MB)Abstract

In small watersheds, runoff entering local waterways from large storms can cause rapid and profound changes in the streambed that can contribute to flooding. Wymans Run, a small stream in Cochranton Borough, Crawford County, experienced a large rain event in June 2008 that caused sediment to be deposited at a bridge. A hydrodynamic model, Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH), which is incorporated into the U.S. Geological Survey Multi-Dimensional Surface-Water Modeling System (MD_SWMS) was constructed to predict boundary shear stress and velocity in Wymans Run using data from the June 2008 event. Shear stress and velocity values can be used to indicate areas of a stream where sediment, transported downstream, can be deposited on the streambed. Because of the short duration of the June 2008 rain event, streamflow was not directly measured but was estimated using U.S. Army Corps of Engineers one-dimensional Hydrologic Engineering Centers River Analysis System (HEC-RAS). Scenarios to examine possible engineering solutions to decrease the amount of sediment at the bridge, including bridge expansion, channel expansion, and dredging upstream from the bridge, were simulated using the FaSTMECH model. Each scenario was evaluated for potential effects on water-surface elevation, boundary shear stress, and velocity.

First posted October 14, 2011

For additional information contact:
Director, Pennsylvania Water Science Center
U.S. Geological Survey
215 Limekiln Road
New Cumberland, PA 17070
http://pa.water.usgs.gov/

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Suggested citation:

Hittle, Elizabeth, 2011, Simulations of flow and prediction of sediment movement in Wymans Run, Cochranton Borough, Crawford County, Pennsylvania: U.S. Geological Survey Open-File Report 2011–1223, 25 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Channel Characterization

PA 285 Bridge

Model Selection and Construction

HEC-RAS Model Construction

Channel Bathymetry

Determination of Manning’s n Values

June 2008 Storm Data

Hypothetical Channel Geometries and Bridge Design

MD_SWMS Construction

MD_SWMS Bathymetry

MD_SWMS Computational Grid and Boundary Conditions

Model Calibration-Drag Coefficient

Simulation Results

HEC-RAS Model Results

FaSTMECH Model Results–Water-Surface Elevations

FaSTMECH Model Results–Velocity Distribution

Velocity Distribution for Initial Channel Geometry

Velocity Distribution for Hypothetical Channel Geometries

FaSTMECH Model Results–Shear-Stress Distribution

Shear-Stress Distribution for Existing Channel Geometry

Shear-Stress Distribution for Hypothetical Channel Geometries

Model Limitations

Summary and Conclusions

Acknowledgments

References Cited


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