Assessment and Prediction of Streambank Erosion Rates in the Town Creek Watershed

Ramirez-Avila, J. J., McAnally, W. H., Langendoen, E. J., Ortega-Achury, S. L., & Martin, James L. (2011). Assessment and Prediction of Streambank Erosion Rates in the Town Creek Watershed. 2011 NGI Annual Conference. Mobile, AL: Northern Gulf Institute.

The Town Creek watershed is a representative area of the Tombigbee River Basin and the Southeastern Plains Ecoregion in Mississippi. The principal channel and at least four main tributaries have been included for several years within the MS Section 303 (d) list of waterbodies biologically impaired due to sediment. The TMDL (Total Maximum Daily Load) developed for Town Creek watershed recommended that streams located near cultivated lands, road crossings and construction activities are priority for streambank and riparian buffer zone restoration and sediment loads reduction. To develop remedial measures and future BMPs (Best Management Practices) within Town Creek watershed for reducing water quality impairment and dredging costs, it is necessary to identify sediment sources and loads currently transported within the watershed. Streambank erosion processes were hypothesized as an important mechanism driving sediment supply from Town Creek watershed. The overall goal of this research was to identify mechanisms and the potential effects of streambank erosion processes and to quantify and model the magnitude and rates of these processes within Town Creek watershed. The overall goal and specific aims of this research were addressed in four substudies by combining field reconnaissance and detailed data collection, laboratory analysis and computational modeling techniques. A first substudy involved a temporal and spatial analysis of observed suspended sediment transport rates and determined the stage of channel evolution for reaches inside Town Creek watershed. Results indicated high spatial and temporal variability of flow and suspended sediment concentrations, discharges and yields within the watershed. Streambank erosion was the major mechanism driving sediment supply within the watershed. Sediment supply from incised streambanks were most frequently caused by gravitational failures occurring during stormflow events, followed by failed material clean out during posterior runoff events. Monitoring and determination of streambank erosion processes and factors on different areas within the watershed was the goal of a second substudy. Results indicated spatial and seasonal changes in rates and locations of streambank detachment and failures. Streambank erosion magnitude can be affected by the hydraulic force produced by a stormflow event, subaerial processes, and type and density of vegetation. Streambank erosion can contribute up to 28.5 Mg per m of streambank. A third substudy evaluated the application of the computational model CONCEPTS (Conservational Channel Evolution and Pollutant Transport System) to an incised reach to assess model performance and capability to simulate spatial and temporal streambanks changes along the studied reach. CONCEPTS was able to accurately predict the time of occurrence and magnitude of top width retreat and streambank failures of streambanks along the modeled reach. The friction angle of the streambank soil was the most sensitive parameter accounting on streambank stability simulation. Empirical and analytical approaches to estimate the rates and the depths of fluvial streambank erosion were determined from a fourth substudy. The empirical model related the depth of fluvial streambank erosion as a function of the hydraulic and the hydrologic properties of flow events, streambank height and type of vegetation on its surface. The analytical approach determined fluvial streambank erosion rates and depths as a function of flow induced forces, streambank geometry and streambank soil properties. Spatial and temporal changes of magnitude of streambank soil properties need to be considered to explain possible differences between simulated and measured depths of erosion. Reduction of suspended sediment loads should focus on attenuation of geomorphic processes, and stabilization of reaches and agricultural lands near streambanks at the northern and western headwaters within Town Creek watershed. Adoption and implementation of management practices such as streambank and riparian buffer zone restoration and establishment of other BMPs are definitively necessary within the watershed to have measurable improvements in water quality.