Improved standard sumps as best management practice for stormwater treatment

There are many standard sumps that may also qualify as a best management practice to pre-treat stormwater runoff before it enters an LID practice by removing suspended sediment from the water column. However, no data on the effectiveness of sediment removal and maintenance schedule of the sumps exist. Such data could justify providing pollution prevention credit for the use of standard sumps for transportation departments, municipalities, counties and other local governments. To determine whether they remove suspended sediment from stormwater runoff, two standard sumps of different size were tested in a laboratory setting. Removal efficiency under low flow conditions as well as resuspension rates under high flow conditions were determined. In the low flow removal efficiency tests sediments of known size distributions were fed at known rates into the influent pipe of a sump. At the conclusion of the test the sediments removed by the sump were collected, dried and weighed. In the high flow resuspension tests a commercially available sediment (e.g. F110 sand) was placed inside the sump, and the amount remaining after the sump had been flushed by high flows for a period of time was determined. The sumps did remove suspended sediment at low flows, but at high flows the scour was substantial. A porous baffle was designed and tested as a possible retrofit to the standard sump. Multiple configurations with varying percent open area and different angles of attack were evaluated in a scale model. An optimum configuration was then constructed at the prototype scale and evaluated for both sediment removal efficiency and sediment retention. Results indicate that, with the right baffle configuration, the scour of sediments accumulated in the sump can be nearly eliminated for flows up to the 10-year design storm runoff (as defined by an assumed watershed and slope), and removal efficiencies can be increased at Peclet numbers above 1.5. Removal efficiency functions have been developed for standard sumps and sumps retrofitted with the porous baffle. In addition, uncertainty analyses have been conducted as part of the data interpretation. The data collected show that standard sumps retrofitted with the porous baffle can be successfully used as pre-treatment for LID practices in a stormwater treatment train.