Unit Process Simulation of a BioInfiltration Stormwater Control Measure

In many areas the design standards for bioinfiltration stormwater control measures (SCM) are based upon generalized parameters or rules of thumb. While these procedures were valuable in initiating the green infrastructure approach to stormwater management, the rules of thumb are conservative, as it is unclear how weather, soil conditions, storm frequencies, and the "Treatment Train Approach," affect the stormwater control measure's performance. This approach leads to a conservative design, and in some cases steers the SCM selection away from green practices. Current stormwater management simulation software do not have the ability to precisely model an infiltration SCM utilizing site and climate factors for continuous events. Models of this type are generally too simple (e.g. specify an infiltration rate) or too complex (e.g. discretization for Richards equation solution); either way they are not effective design tools. Due to this deficiency, current SCM design guidelines do not accurately reflect a basin's capacity to dynamically infiltrate stormwater runoff over varying conditions. Excluding dynamic infiltration can — and will — generally result in BMP over-design or under-reporting of its effectiveness, especially in continuous flow modeling. A new model for ponded infiltration is introduced based on Philip's ponded infiltration equation and modified to work for infiltration devices. The model is easily parameterized for engineering use, requiring only knowledge of the bowl shape and USDA soil classification. The current limitation of the model and area of ongoing research is that for the model to be used in a continuous simulation environment, it requires an effective soil moisture accounting model. When the soil moisture can be estimated, then the model can be evaluated on a per-storm basis. This single-event correlation has been simulated at an infiltration basin at Villanova University in southeast Pennsylvania (sandy clay loam soil type). This correlation shows that the model has the ability to accurately reflect the physics of the ponded infiltration process at an infiltration BMP, and clearly shows how variable the infiltration is in relation to the soil moisture, and how current methods under-estimate dynamic infiltration.