PERFORMANCE EVALUATION OF INFILTRATION BASINS IN FLORIDA

Infiltration basins are a commonly utilized stormwater control measure to reduce or eliminate urban runoff. For permitting purposes, basins in Florida are required to recover their design volume, typically runoff from a one inch rainfall event, within 72 hours to satisfy the design criteria. Groundwater mounding analysis is not required if volume recovery can be demonstrated by filling of soil porosity below the basin surface via vertical infiltration. Forty infiltration basins were included in this field study to evaluate whether each basin was performing as designed. Basins ranged in age from less than one year to over twenty years and land uses were equally divided between Florida Department of Transportation (FDOT) and residential developments. Six test sites within each basin were typically selected to measure infiltration rates using a double ring infiltrometer (DRI). Based on DRI rates, 16 (40%) basins had rates less than their designed rates, 10 (25%) had rates equal to their designed rates, and 14 (35%) basins had rates greater than their designed rates. Basins with coarser soils and FDOT basins were more likely to have DRI rates at or above their design rates. Water levels were also monitored in eleven of the basins between March 2008 and January 2012 to determine whether basins performed as designed. Monitoring data from five basins with DRI rates greater than or equal to designed rates confirmed basins were performing as designed or better. Monitoring data for another basin which had DRI rates less than its design rate also indicated the basin did not recover design volumes within 72 hours. However, the remaining five monitored basins had DRI rates less than their design rates while monitoring data showed that these basins were functioning as designed. When groundwater mounding occurred above the surface basins, resulting from individual or successive events exceeding the design storm, storage recovery rates were at least an order of magnitude below the DRI rates and affected storage recovery durations for subsequent events. Results of this study suggest DRI measurements tended to under predict overall infiltration performance. This was most likely due to lateral seepage through side slopes contributing to storage recovery, which is typically not accounted for in designs.