A Storm Runoff Simulator to Evaluate Grass Filter Strips and Other Storm Water Management Systems

A storm runoff simulator is needed to evaluate flow hydraulics, sediment trapping and water quality benefits of grass filter strips, small hydraulic structures such as storm drains, other stormwater management practices and similar bioretention systems. Based on an engineering analysis using MUSLE and HEC2, it was determined that a simulator could be developed for a 1 x 9 m grass filter plot, with resource constraints of 7,580 L stored water, 7.3 L s^–1 peak water flow rate and 0.71 metric tons of soil, for field to buffer area ratios (BAR) greater than 30:1, the maximum design criteria specified for grass filter strips by the Natural Resources Conservation Service. A runoff simulator water output control device operating under gravity flow conditions and using a feedback control system between and a flow meter and an actuated valve was designed, built and tested. A time-compressed 10yr-2hr SCS design hydrograph with a 7.3 L s^–1 peak flow rate was used to calibrate the simulator for optimal performance with large changes in flow rate and for compatibility with a variety of hydrograph shapes. When reproducing the compressed SCS hydrograph, the simulator maintained comparable efficiencies among repetitions; with a 2% error between root means square error and the total flow range. Hydrographs with step changes in flow rate, flow rate plateaus, multiple peaks and flow rates up to 9.8 L s^–1 could be simulated with comparable efficiencies and 4 to 6% error.