The Cost Effectiveness of Standard and Alternative Sediment and Turbidity Control Systems on Construction Sites in North Carolina

Storm water runoff and sediment transport from construction sites is a major concern as it is a major contributor to non-point source (NPS) pollution. Sediment control and stormwater management is typically implemented by applying Best Management Practices appropriate to the source, location and climate. There is a need for more research in this field since current BMPs installed on construction sites have been demonstrated to be relatively ineffective in retaining a significant portion of sediment during runoff events.To determine the cost effectiveness of erosion and sediment control measures two parameters were estimated. The first parameter concerns the environmental effectiveness of the control measure, for which we used the reduction in the rate of sediment transported off the construction site in tons per studied site per year. The second parameter is the cost of the control practices. To simulate the sediment retention of erosion and sediment control measures for specific test sites we used a previously calibrated process-based model WEPP/GeoWEPP. For the cost assessment we used data from four sources: bids data from the North Carolina Department of Transportation; the National Management Measures Guidance to Control Non Point Source Pollution from Urban areas (EPA); a URS Greiner, Woodward & Clyde report, and SedSpec, an on-line tool for selection of control measures that estimates cost based on the US Army data.The case study (NCSU's Centennial Campus) results confirm findings by other experimental and modeling studies that the current standard design of sedimentation basins does not provide adequate sediment control. The model predicts a 50-60% trapping efficiency and sediment yields that are more than 10-times higher than those predicted for the pre-construction state. In North Carolina sedimentation basins should capture at least 70% of sediment, but that does not prevent high sediment yields and turbidity levels that exceed the state required 50 NTU.The results of modeling and analysis suggest a need for (1) re-evaluation of the assumptions underlying the existing rules for sediment basin design, and (2) consideration of measures to bring the site conditions closer to the assumptions (e.g., mandatory baffles; further reduction in the time allowed for exposed bare soil). Alternative approaches were tested to improve the sediment control effectiveness with the approaches that integrate sediment control with stormwater control measures emerging as the most cost effective and least complex in terms of management and planning.In addition, simulations for phased construction demonstrated that the effectiveness of this approach is highly site specific. The number and type of measures required is greatly dependent on configuration of terrain and grading. Phased development may well require the same number of structures as without phased development along with the additional costs due to scheduling of the construction.The study also found that emerging process-based simulation systems and interactive solid terrain interfaces have a potential to evaluate various sediment control alternatives by taking into account the changing topography on the construction sites and optimizing the entire system integrated with post-construction storm water control.