ESTIMATING THE LIMITS OF INFILTRATION IN THE URBAN APPALCHIAN PLATEAU

Urban regions with large areas of impervious cover generate high volumes of surface runoff during wet weather, overwhelming antiquated sewer infrastructure and causing overflow into receiving waters. Green infrastructure in urbanized areas commonly uses infiltration systems to convey surface runoff from impervious surfaces into surrounding soils, which helps mitigate the amount of stormwater entering sewer systems. However, the use of infiltration as a sustainable solution can be limited in regions with unsuitable environmental conditions that can lead to rapid degradation of infiltration systems. Two case studies conducted in Allegheny County, PA that look at the limits of infiltration from excessive stormwater loading and unfavorable soil water dynamics are presented. The first study used infiltration rates and precipitation data to model effective precipitation rates of various storm events over increasing levels of imperviousness in order to characterize local impervious cover thresholds. It was found that local soils are unable to effectively infiltration stormwater from a majority of storm events in regions with greater than 60% imperviousness. These regions would be considered less suitable for the use of infiltration-based green infrastructure and would likely require augmentation with other stormwater management strategies. The second study characterized subsurface soil water dynamics along two hillslopes sites where infiltration-based green infrastructure was installed. Prior to installation, soil moisture was monitored continuously at various depths, and the resulting records were then compared to precipitation data to quantify soil water responses to storm events. Prevailing expectations of hillslope soil water dynamics assume well-drained profiles and top-down/horizontal wetting front following storm events, even for clay-rich soils like those found in Allegheny County. However, the study sites often underwent inverted wetting during storm events (bottom to top) and displayed relatively slower drainage rates throughout the soil profile, which led to uncharacteristically high soil moisture conditions. These detailed analyses show that unexpected patterns in soil water dynamics exist which could potentially degrade green infrastructure effectiveness. The findings from these two studies suggest that continuous pre-installation monitoring of hydrological conditions and characterization of region-specific impervious cover thresholds is essential to determine proper placement and design of infiltration-based green infrastructure for optimum performance