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Abstract:
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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. |
