CHANNEL AND BANK STABILITY OF TWENTYMILE CREEK AT U.S. HIGHWAY 45 NEAR WHEELER, PRENTISS COUNTY, MISSISSIPPI

By D. Phil Turnipseed and K. Van Wilson, Jr.

Open-File Report 90-111

Prepared in cooperation with the MISSISSIPPI STATE HIGHWAY DEPARTMENT Jackson, Mississippi

1990

TABLE OF CONTENTS

Definition of terms
Abstract
Introduction
    Purpose and scope
    General description of Twentymile Creek
    Channel modifications to Twentymile Creek
    Acknowledgments
Channel-bed stability
    Botanical evidence of gradation
    Gradation analysis
Channel-bank stability
    Botanical evidence of widening
    Stability analyses
    Widening analyses
Summary
References

FIGURES

1. Map showing the location of Twentymile Creek at existing U.S. Highway 45 and proposed U.S. Highway 45 relocation near Wheeler, MS

2. Graph showing estimated patterns of channel-bed gradation processes on Twentymile Creek at existing U.S. Highway 45 near Wheeler, MS

3. Cross section showing critical failure surfaces for channel banks on Twentymile Creek 300 feet downstream from the proposed U.S. Highway 45 relocation near Wheeler, MS

4. Graph showing estimated patterns of channel widening on Twentymile Creek at existing U.S. Highway 45 near Wheeler, MS

TABLES

1. Channel-bed elevation and total degradation on Twentymile Creek at existing U.S. Highway 45 near Wheeler, MS

2. Dry bulk-unit weight and shear-strength properties of soil as determined from borehole tests on the left (north) bank at the proposed U.S. Highway 45 relocation near Wheeler, MS

3. Factors of safety for rotational bank failures on Twentymile Creek in the vicinity of the proposed U.S. Highway 45 relocaton near Wheeler, MS

4. Bankfull channel width and total widening on Twentymile Creek in the vicinity of existing U.S. Highway 45 near Wheeler, MS

ABSTRACT

The channel of Twentymile Creek at the existing U.S. Highway 45 near Wheeler, MS, has degraded about 8 feet and widened about 73 feet since about 1912; however, at the proposed U.S. Highway 45 relocation, no recent (at least since 1968) degradation or widening is evident. The hard, compacted soils of the channel bed in the vicinity of existing U.S. Highway 45 and proposed U.S. Highway 45 relocation, in combination with grade-control structures that have been installed, indicate no further degradation through the year 2010 is likely. In the vicinity of existing U.S. Highway 45, channel width could increase about 15 feet through the year 2010. In the vicinity of the proposed relocation, both banks are reinforced by existing mature hardwood trees. If these trees are cut and removed, the sandy banks could become unstable and channel width could increase about 20 feet in the next 10 to 20 years. These projections are based on the assumption that no additional channel modifications and no unusually large and destructive flooding will occur by the year 2010.


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