Field-Monitoring Sediment Basin Performance during Highway Construction

Abstract

Stormwater regulations require erosion and sediment control practices to be implemented during construction to prevent discharging polluted water offsite and mitigate downstream effects. Sediment basins are a common practice used to detain suspended sediment from stormwater runoff by providing residence time and storage to promote gravitational settling. Sediment basin design, and thus pollutant removal efficiency, vary regionally due to local design standards and preferences. This manuscript presents the results of a case study from Highway U.S. 30 construction in Tama County, Iowa, USA where two sediment basin systems were created within a conveyance channel by constructing an earthen berm across the channel to detain sediment-laden stormwater. A dewatering riser pipe was routed through the earthen berm to provide primary dewatering. The in-channel sediment basin was constructed with a 3% slope and a 10 ft. bottom width. The first system consisted of one basin created by a single earthen berm damming sediment-laden runoff, whereas the second system included two earthen berms, creating two in-channel sediment basins in series. Field monitoring was conducted on in-situ basins by deploying a rain gauge and automated water samplers positioned at the inflow and discharge points of a (a) single basin and (b) two basins in series within a roadside channel. During the monitoring period, no maintenance or dredging was recorded. Water samples were taken from the monitored basins at regular time intervals and analyzed for turbidity. Inflow turbidities often reached magnitudes up to the 103 NTU and discharge samples indicated negligible turbidity reduction after residence. On several occasions, the in-channel sediment basins acted as a sediment source, with discharge turbidities measuring higher than inflow. Despite their initial performance, there was interest in improving the in-channel basin design due to the potential to maximize length-to-width flow ratios, and use of existing infrastructure, which reduced the amount of right of way needed for basin construction, installation time and cost. As a result, several potential design improvements and techniques were recommended to enhance in-channel sediment basin performance.