CHANNEL AND BANK STABILITY OF SAND BRANCH TRIUBUTARY AT STATE HIGHWAY 342 NEAR PONTOTOC, PONTOTOC COUNTY, MISSISSIPPI by D. Phil Turnipseed and K. Van Wilson, Jr. Prepared in cooperation with the MISSISSIPPI STATE HIGHWAY DEPARTMENT Jackson, Mississippi 1989 Open-File Report 89-241 TABLE OF CONTENTS Definition of terms Abstract Introduction     Purpose and scope     General description of Sand Branch tributary     Channel modifications to Sand Branch and Chiwapa Creek     Acknowledgments Channel-bed stability     Botanical evidence of degradation     Degradation analysis Channel-bank stability     Botanical evidence of widening     Stability analysis     Widening analysis Conclusions References ILLUSTRATIONS 1. Map showing the location of Sand Branch tributary at State Highway 342 near Pontotoc, MS 2. Channel-bed profiles for Sand Branch tributary in the vicinity of State Highway 342 near Pontotoc, MS 3. Estimated patterns of degradation on Sand Branch tributary and Sand Branch at State Highway 342 near Pontotoc, MS 4. Cross section showing critical failure surfaces for channel banks on Sand Branch tributary 80 feet downstream of State Highway 342 near Pontotoc, MS 5. Existing channel cross section showing near-future widening for Sand Branch tributary 80 feet upstream of State Highway 342 near Pontotoc, MS 6. Existing channel cross section showing near-future widening for Sand Branch tributary 80 feet downstream of State Highway 342 near Pontotoc, MS 7. Existing channel cross section showing near-future widening for Sand Branch tributary 650 feet downstream of State Highway 342 near Pontotoc, MS TABLES 1. Channel-bed degradation on Sand Branch at State Highway 342 near Pontotoc, MS 2. Channel-bed degradation on Sand Branch tributary at State Highway 342 near Pontotoc, MS 3. Dry bulk-unit weight and shear-strength properties of soil as determined form borehole test about 80 feet downstream form State Highway 342 near Pontotoc, MS ABSTRACT The channel of Sand Branch tributary at the site of the proposed reconstruction of the State Highway 342 crossing near Pontotoc, Mississippi, has degraded 3 to 4 feet since the channelization of Chiwapa Creek in 1911. Channel degradation at the mouth of the tributary has totaled 5 to 6 feet. Botanical evidence indicates that several recent bank failures downstream of the crossing occured during extreme floods in the spring of 1983. Rates of channel degradation and widening--as determined from channel descriptions, discussions with local residents, and botanical evidence along the banks--were used in conjunction with soil properties to estimate probable future channel degradation and widening through the year 2010. By assuming that channel-bed elevations can be expressed as a power function with time, additional channel degradation through the year 2010 is expected to be about 1/2 foot at the highway crossing and about 1 foot at the mouth of the tributary. The channel top-bank width could increase about 10 feet just downstream of the highway crossing in the next 10 to 20 years. These projections are based on the assuption that no additional channel modifications and no unusually large and destructive flooding will occur by the year 2010. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.