Characterizing Site Suitability for Stormwater Infiltration

Abstract

Infiltration stormwater control measures (SCMs) have the potential to provide numerous hydrologic benefits to urban environments by promoting infiltration and evapotranspiration, processes that are not well represented in the water budget of urban areas. Unfortunately, infiltration SCMs presently have a relatively high failure rate which likely has slowed their rate of implementation. This research is focused on addressing several common factors contributing to infiltration SCM failure: identifying a suitable site for an infiltration SCM, quantifying the systemic bias of common methods used to estimate field saturated hydraulic conductivity (Kfs), and estimating the spatial correlation of Kfs in green infrastructure (GI). A planning phase tool, referred to as the preliminary infiltration rating (PIR), was developed to assess the likelihood of success of a future surface infiltration SCM at a given location. The PIR was calibrated and validated using rain garden maintenance inspections. The validation set resulted in the PIR predicting an accurate or conservative estimate of the rain garden performance in 85% of rain gardens. Numerical experiments were used to quantify the bias of seven common infiltration measurement methods, removing natural variability and random error from the analysis. In sand through silt loam soils that are typical of infiltration SCMs, the simulated methods have a bias varying from 0.7 to 6.2. Geostatistics were utilized to investigate the spatial correlation of Kfs at 9 GI sites. Horizontal, isotropic, variograms of the surface Kfs were constructed at each site from modified Philip-Dunne infiltrometer measurements to evaluate the spatial correlation range, the separation distance at which measurements become spatially independent. The spatial correlation range of Kfs in GI was estimated to vary from less than 1 meter and up to 22.9 meters, with most sites having a spatial correlation range less than 5 meters. Identifying potential infiltration areas early in the planning phase allows the selected area to be integrated with other design elements. Characterizing the infiltration potential of a given site requires an understanding of the accuracy of the selected measurement method and an understanding of the in-situ spatial heterogeneity. This research is applicable throughout the land development process on project types including transportation, residential, commercial, industrial, and institutional development in both the public and private sectors.