Measuring and Modeling Suspended Sediment Yields in Urbanizing Georgia Piedmont Watersheds

This paper presents a comparison of U.S. Geological Survey stream monitoring station measurements of total suspended solids (TSS) yield and stream bank erosion monitoring data with a suspended solids model developed by Brown and Caldwell and used by the Gwinnett County, Georgia Department of Water Resources (DWR) to inform its watershed improvement efforts. A comparison between modeled and average measured TSS yield results at each gauging station indicates that a reasonable match exists over the majority of compared watersheds. When removing two stations with less than five years of data (coinciding with a drought of record) and two outlier stations, the average absolute percent difference between modeled and measured results is 20.1%, which is within the sampling error associated with TSS measurements. In general, the model tends to over predict TSS yield. It assumes a streambank TSS loading rate of 12 lbs/ft/yr per unit area of exposed bank as quantified in the field. Six years of measured streambank erosion rates has documented an average annual production of 7.1 lbs/ft/yr. The average measured TSS yields at two stations were substantially less than the modeled values. It is thought that other factors are influencing TSS export such as the age of build out conditions. A discussion is presented addressing the potential reasons for the divergence between modeled and measured results as well as the potential for model calibration.