Assessment of In-Stream Processes in Urban Streams for Development of Sediment Total Maximum Daily Loads

Abstract

The Clean Water Act requires the establishment of Total Maximum Daily Loads (TMDLs) for quantifying allowable pollutant loads for stream reaches in which the biological integrity of the stream is threatened. Sediment TMDLs in urban streams are particularly difficult to establish because they require (1) reliable measurement of sediment loads and (2) the ability to locate sediment sources. The research presented in this paper has attempted to address these challenges through a field study of North Peachtree Creek located in metro Atlanta, Georgia, which has been sampled at the Century Boulevard crossing through automatic point sampling and depth-integrated sampling. Storm events from October 2003 through October 2004 provided a field record of sediment concentration and turbidity data over a wide range of storm events. Bed and bank sediment samples were collected for comparison with the point samples and depth-integrated samples. A methodology is presented whereby point sampling is used to calculate suspended sediment discharge and turbidity analysis is used to locate and characterize sediment sources. Point samples provide the boundary condition in the Rouse solution for the vertical distribution of suspended sediment to obtain suspended sediment discharge, which is then calibrated through comparison with depth-integrated sampling. The computer model HEC-RAS (U.S. Army Corps of Engineers, 1998) was applied to the stream reach to calculate the energy grade line slope throughout each storm event for input into the sediment discharge calculations. A favorable relationship between turbidity and suspended fine sediment was found at the sampling cross-section and, through comparison with bed and bank sediment samples, was used to identify the contribution of eroded bank sediment to the total sediment discharge.