Hydraulic Analysis of Suspended Sediment Removal from Storm Water in a Standard Sump

Abstract

Standard sumps (manholes) are common features of urban storm water collection systems, and there are anecdotes suggesting that standard sumps can improve storm water quality. However, no data on the effectiveness of sumps as treatment devices for suspended sediment removal and the associated required maintenance schedule of the sumps could be found. Such data could justify giving pollution prevention credit for the use of standard sumps to transportation departments of cities, counties, and state agencies. To assess the effectiveness of standard sumps as storm water treatment devices, a laboratory study was conducted. Three goals were achieved in this study: (1) sediment capture and sediment washout were measured in four configurations of a straight flow through standard sump; (2) performance functions for the efficiency of suspended sediment removal in a standard sump were developed; and (3) performance functions for sediment washout from a standard sump were developed. To determine whether they remove suspended sediment from storm water runoff, two standard sumps of different sizes were tested in a laboratory setting. Removal efficiency under low flow conditions and washout rates under high flow conditions were measured. The sumps did remove suspended sediment at low flows, but at high flows the washout rate was substantial. The data collected were used to develop two performance functions: one for suspended sediment removal (deposition) in a sump at low flow, and one for sediment washout from the sump at high flow. Four sump configurations were tested under a wide range of flow characteristics. The principal independent variables such as sump dimensions, sediment settling velocities, and hydraulic parameters were grouped into dimensionless numbers that were related to performance of all designs tested. These performance functions can be used to select appropriate designs and analyze the performance of existing standard sumps. Overall, the data collected show that standard sumps can be used as pretreatment devices for storm water if properly selected and maintained.