GEOMORPHIC RESPONSE TO CHANNEL MODIFICATIONS OF SKUNA RIVER AT THE STATE HIGHWAY 9 CROSSING AT BRUCE, CALHOUN COUNTY, MISSISSIPPI by K. Van Wilson, Jr. and D. Phil Turnipseed Prepared in cooperation with the MISSISSIPPI DEPARTMENT OF TRANSPORTATION Jackson, Mississippi 1994 Water-Resources Investigations Report 94-4000 TABLE OF CONTENTS Abstract Introduction     Background     Purpose and scope     Approach     General description of Skuna River         Channel-bed material properties         Channel-bank material properties     Channel modifications of Skuna River     Acknowledgments Geomorphic response to channel modifications     Cross sectional evidence'     Botanical evidence     Channel-bed elevation     Channel shape     Channel pattern     Bankfull discharge     Channel slope     Bankfull stream power     Channel-bank stability Summary and conclusions References ILLUSTRATIONS 1. Map showing location of Skuna River at State Highway 9 at Bruce, MS 2. Channel cross sections at the State Highway 9 bridges crossing Old Skuna River Canal tributary at Bruce, MS 3. Channel-bed elevation for Skuna River Canal at Federal Aid Secondary Route 794, State Highway 9, and State Highway 341 4. Annual minimum daily stage and minimum bed elevation for Skuna River Canal at State Highway 9 at Bruce, MS 5. Annual minimum daily stage and discharge for Skuna River Canal at State Highway 9 at Bruce, MS 6. Estimated patterns of channel-bed degradation on Skuna River Canal at State Highway 9 at Bruce, MS 7. Estimated patterns of channel deepening on Skuna River Canal at State Highway 9 at Bruce, MS 8. Estimated patterns of channel-widening on Skuna River Canal at State Highway 9 at Bruce, MS 9. Photographs showing old car bodies on the upstream left (south) bank of Skuna River Canal at State Highway 9 at Bruce, MS 10. Graph showing bankfull channel width divided by channel depth for Skuna River Canal at State Highway 9 at Bruce, MS 11. Photographs showing views upstream and downstream of the State Highway 9 crossing of Skuna River Canal at Bruce, MS, February 20, 1992 12. Map showing historical changes in channel pattern of Skuna River Canal in the vicinity of State Highway 9 at Bruce, MS 13. Bankfull discharge from annual stage-discharge relations for Skuna River Canal at State Highway 9 at Bruce, MS 14. Annual peak stage and discharge for Skuna River at State Highway 9 at Bruce, MS 15. Average channel slope of Skuna River Canal between State Highway 341 and Federal Aid Secondary Route 794 16. Bankfull stream power for Skuna River Canal at State Highway 9 at Bruce, MS 17. Cross sections showing critical failure surfaces for saturated channel banks on Skuna River Canal in the vicinity of State Highway 9 at Bruce, MS, 1989 18. Cross section showing critical failure surfaces for saturated channel banks on Old Skuna River about 130 feet upstream from State Highway 9 at Bruce, MS, 1989 TABLES 1. Bulk-unit weight and shear-strength properties of channel-bank material from borehole tests in 1989 on the left (south) banks of Skuna River Canal and Old Skuna River about 70 and 100 feet, respectively, upstream from State Highway 9 at Bruce, MS 2. Botanical and geomorphic evidence of channel-bed degradation collected in 1989 on Old Skuna River and Skuna River Canal tributary in the vicinity of State Highway 9 at Bruce, MS 3. Botanical and geomorphic evidence of channel-bank widening collecte in 1989 on Old Skuna River, Skuna River Canal, and Skuna River Canal tributary in the vicinity of State Highway 9 at Bruce, MS 4. Estimates of near-future (10 to 20 years) channel-bank widening for Old Skuna River, Skuna River Canal, and Skuna River Canal tributary in the vicinity of State Highway 9 at Bruce, MS 5. Factor of safety and failure-block width for critical channel-bank failures on Skuna River Canal and Old Skuna River in the vicinity of State Highway 9 at Bruce, MS ABSTRACT Skuna River at State Highway 9 at Bruce, Calhoun County, Mississippi, has geomorphically responded to channel modifications by lowering of the channel banks and induced widening. Skuna River Canal (Skuna River) has typically degraded about 16.5 feet and widened about 150 feet from 1925 (when constructed) to 1992. Old Skuna River has degraded and widened about 11 feet and 40 feet, respectively, from 1921 to 1991. Most of the geomorphic response on the Old River and the tributary seems to be a consepuence of modifications of the canal. The bankfull discharge of the canal has increased about 1,450 percent, and the channel slope had decreased about 34 percent from 1925 to 1989. The bankfull stream power has been decreasing since 1980. The bankfull channel width-depth ration has been increasing since 1975, which indicates the canal has been widening more than degrading since 1975. As much as 1 foot of additional degradation and 40 feet of additional widening are projected through 2010 on Skuna River Canal in the vicinity of State Highway 9. About 70 feet of additional widening could occur before the canal reachers quasi-equilibrium, which will likely be reached after 2010. If Old Skuna River and Skuna River Canal tributary degrade as much as the canal, which is doubtful, then about 6 and 11 feet of additional degradation could occur by 2010 on the Old Skuna River and the tributary could both widen an additional 30 feet in the next 10 to 20 years. The channel low-stage thalweg of Skuna River Canal is beginning to meander around sandbars inducing lateral erosion of the channel banks. The widening projections in this report do not directly account for lateral erosion and are considered to be a minimum for the typical channel reach. Lateral erosion will likely have a significant effect on future widening processes at this site. 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.