Principal Investigator: David Admiraal
Sponsors & Partners
Nebraska Department of Roads
The National Pollutant Discharge Elimination System (NPDES) was established to reduce the presence of pollutants in stormwater. For large municipalities NPDES has resulted in a permitting process that requires the implementation of Best Management Practices (BMPs) to improve water quality and it also places the burden of proof on the municipalities to show that the implemented BMPs are effective. BMPs are broadly defined and may take on a variety of forms. They include public programs (e.g., free distribution of low-phosphorous fertilizer), regulations (e.g., requiring that contractors cover bare soil to prevent soil erosion), structural mechanisms (e.g., detention basins to capture pollutants), and virtually any approach that can be shown to reduce pollutants in stormwater. Nebraska Department of Roads (NDOR) projects and right of ways produce a large quantity of runoff, some of which has water quality issues. Within municipalities NDOR is subject to some of the same permitting rules by which the municipalities must abide. The characteristics of the sites where NDOR must improve runoff quality limit the types of BMPs that NDOR can implement. BMPs that capture and treat pavement runoff are probably the most likely to succeed. Thus, as part of NDOR’s permit for the NPDES regulations, NDOR will be required to design and construct Treatment BMPs for NDOR projects located within large municipalities. Within a short period of time NDOR may be required to install a large number of these structures, and without sufficient knowledge of how well they operate over the long run the effort may be costly and ineffective. Existing stormwater pollution Treatment BMP outlet structures are designed to collect and/or treat the first flush of stormwater runoff from the highway right of way. First flush runoff generally contains the highest concentrations of sediment, debris, and other pollutants. A preliminary review of potential Treatment BMPs shows that most provide treatment by filtering the debris laden runoff or by collecting it in a detention structure and discharging it over an extended time period of 36 to 72 hours, allowing the suspended solids time to settle out. Some Treatment BMP’s both filter and detain polluted runoff. Thus, there is a need to research the design and functionality of stormwater pollution Treatment BMP discharge structures used to either filter stormwater or extend its release over a long period. The outlet structure must either provide adequate filtration or slow the discharge to over 40 hours while remaining as free of clogging by debris and silt as possible. The research proposed herein would provide information that helps NDOR to design outlet structures that operate more reliably and with greater time periods between required maintenance activities. It would also prevent costly missteps before obtaining a reliable design. The research could also benefit sediment control work during construction. Such structures as temporary earthen dikes which at this time need to be drained by breaching or pumping could have sand filtering or extended detention drains built within them instead. Objective The objective of the proposed work is to identify an effective outlet structure design for stormwater treatment BMPs (detention BMPs). This objective will be achieved by (1) an extensive review of literature on detention based BMPs and BMP outlet structures, and (2) laboratory testing of structures that are not sufficiently documented but hold promise for being effective. The effectiveness of each structure that is tested will be assessed based on efficiency for removing suspended solids, long-term reliability and susceptibility to obstruction by debris, and long-term maintenance requirements.