Costs and infiltration benefits of the Watershed Augmentation Study sites

There is significant interest in parcel-scale best management practices (BMPs) to allow stormwater to infiltrate through the soil, recharging the groundwater while being filtered of pollutants. In this study, five parcels were studied to determine whether the BMPs produced enough value to offset the cost and what scale of rainstorm they should be designed to capture. The engineering design of the five sites, which included an elementary school, a recycling facility, a metal yard, a commercial building, and a park, is described in detail. The WinSLAMM model was used to estimate the amount of runoff reduced by the BMP and therefore contributing to infiltration. The cost per unit of stormwater reduced over the project lifetime was calculated. Noting that the pollution removal benefits were not included in the calculations, only one of five sites produced a benefit that was great enough to justify the costs. The authors conclude that the rainfall patterns of Los Angeles make it too costly to justify BMPs that are designed to manage large storms.