Hydrologic impacts of side inlet storage modifications in an artificially drained agricultural landscape

Surface side inlets are an important component of the drainage infrastructure in Minnesota. Side inlets are small watershed outlets that discharge surface water to drainage ditches. Side inlets can be designed or retrofitted such that they act as temporary storage for surface runoff, decreasing downstream peak flow rates and inducing sedimentation in the storage area. This project is part of a larger, multi-year project that attempts to quantify the extent of side inlet locations in representative areas of Minnesota, seeks to compare, through laboratory, testing the effectiveness of different side inlet designs, and finally seeks to determine the potential effect of implementing side inlet controls on a drainage system-wide basis through modeling. The latter objective is the focus of this paper. The EPA SWMM model was used to evaluate the hydrologic effects of widespread adoption (33% of land area) of side inlet controls in a 12.6 km², tile-drained agricultural watershed, in southern Minnesota. The model was calibrated based on averages of typical water balances from other studies and on the typical drainage coefficient for the area. Four design variants were simulated: 1) existing flush pipe design; 2) Hickenbottom riser; 3) rock weir; and 4) rock filter. Stage-area relationships at each side inlet were developed using an algorithm in GIS utilizing LiDAR data. The stage-discharge relationship for the Hickenbottom riser was developed based on manufacturer data. Stage-discharge relationships for the rock filter were based on previous work. Both the rock inlet and the Hickenbottom designs resulted in peak flow reductions of between 20% and 25% for the flows greater than the 2-yr event. Reductions were not as great for more frequent events. The rock weir design resulted in peak flow reductions of 12% for the 1-yr return period event and approximately 15% for less frequent events. Benefits for individual side inlet locations are strongly dependent on ponded surface area to watershed area ratio and that this ratio should be approximately 1% or greater.