The goal of this research was to develop a methodology for modeling a bioinfiltration best management practice (BMP) built in a dormitory area on the campus of Villanova University in Pennsylvania. The objectives were to quantify the behavior of the BMP through the different seasons and rainfall events; better understand the physical processes governing the system's behavior; and develop design criteria. The BMP was constructed in 2001 by excavating within an existing traffic island, backfilling with a sand/soil mixture, and planting with salt tolerant grasses and shrubs native to the Atlantic shore. It receives runoff from the asphalt (0.26 hectare) and turf (0.27 hectare) surfaces of the watershed. Monitoring supported by the hydrologic model shows that the facility infiltrates a significant fraction of the annual precipitation, substantially reducing the delivery of nonpoint source pollution and erosive surges downstream. A hydrologic model was developed using HECHMS to represent the site and the BMP using Green-Ampt and kinematic wave methods. Instruments allow comparison of the modeled and measured water budget parameters. The model, incorporating seasonally variable parameters, predicts the volumes infiltrated and bypassed by the BMP, confirming the applicability of the selected methods for the analysis of bioinfiltration BMPs.The urbanization of a watershed, with the associated increase in impervious surface and intensity of use, changes the local hydrology and that of the downstream river system. Paved areas and roofs, as well as compacted turf areas, speed increased runoff volumes to the receiving channels with diminished opportunity for filtration of pollutants. Increased flows downstream cause more frequent flooding as well as accelerated stream channel erosion. Meanwhile, these pulses of excess runoff are not contributing to ground water recharge, leading to lower stream base flows and urban water supply problems during dry periods. Additionally, the pavement, buildings, and turf areas significantly reduce the opportunity for evapotranspiration and the environmental benefits it confers (Schueler, 1995; USEPA, 2002).

To compensate, many BMPs have been developed since the link between increasing impervious areas and watershed scale problems has been recognized. In the past, stormwater management techniques (especially detention basins) emphasized flood control without considering the corollary impacts of development on increasing pollution and decreasing ground water recharge. The long term effects of some of these attempts have now been studied and quantified (Emerson, 2000; Roesner and Nehrke, 2004). The accumulated experience has given rise to new solutions and new governmental regulations. For example, the Pennsylvania Comprehensive Stormwater Policy, announced in 2003, states that

"planners and applicants should evaluate and utilize infiltration BMPs to manage the net change in stormwater generated or otherwise replicate to the maximum extent possible preconstruction stormwater infiltration and runoff conditions so that post construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters" (PaDEP, 2002).

As the above described movement in the state-ofthe-art of stormwater management becomes a part of planning requirements, of engineering practice, and of regulatory review, it becomes necessary to quantify the effects of infiltration BMPs. The designer must be able to demonstrate with confidence that a proposed BMP will behave in a predictable and beneficial way one that suits the needs of the developers, the architects and planners, the reviewing officials, the future residents, and those downstream. Current regulations and design guidelines are still being developed, and some resemble "rules of thumb" rather than specific and objective criteria.

The objectives of this modeling study are to contribute to the acceptance and successful application of bioinfiltration BMPs. By basing a modeling approach on the topography of the site, the geometry of the BMP, and the physical characteristics of the cover and soils, the capability for analyzing the BMP's performance is established. By explicitly considering the physical properties of the site, the modeling approach provides a way to cross-check initial site observations or design assumptions with measurable performance and to make adjustments without resorting to the use of "correction factors" that may have no directly observable reference. And by explicitly considering the different temporal patterns of storms and the variability of the response of the watershed and the BMP over the course of the seasons, the modeling approach provides a more realistic way to develop descriptive statistics. A successful physically based model provides a logical way to move from general performance measures and site observations to the specifics for the design of a BMP.