A comparison of predicted and actual removal rates for total suspended solids at California Department of Transportation roadside vegetated treatment sites

This study was conducted to assess the applicability of various predictive equations presented in the literature to the Caltrans highway environment. The assessment consisted of using the equations to calculate total suspended solids (TSS) removals at eight experimental sites from the Caltrans Roadside Vegetative Treatment Site (RVTS) Study and comparing the predicted treatment performance against actual performance observed in the already-completed RVTS Study. The eight RVTS Study sites included vegetated roadside slopes with varying site characteristics such as soil type, climate, strip width, slope, and vegetative cover. The sites were distributed across California, and storms from several years were included in the data set. Five sediment removal equations were identified through a literature review. Agreement between the equation results and the field data was generally poor. It is difficult to discern whether the discrepancies result from inadequate data or inadequate equations. Some of the equations did not include site- and storm-specific hydrologic factors that likely affected performance. For other equations which did include multiple factors, field parameters such as particle size distribution, spacing between vegetation, and flow friction factors had to be assumed or estimated because they were not measured as part of the RVTS Study, which was designed for a different purpose. These estimates may not be accurate. Finally, all the equations included empirically-derived coefficients which might not be applicable to RVTS data because they were obtained under different conditions. In the end, none of the equations evaluated were able to accurately and reliably predict sediment trap efficiency for the RVTS monitoring stations.