A Methodology for Modeling to Best Manage Storm Water Runoff and Maintain Water Quality

With increasing development and associated runoff, the streams of Fairfax County have been severely eroded and the water quality deteriorated. We must minimize not only the peak flow but the total volume of the runoff to achieve a stabilized condition and ensure the water quality in our streams. The concept of Low Impact Development (LID) has been widely promoted for some time. However, there are no established methodologies to evaluate the true impact of development using LID. A common problem is the lack of sufficient tools and techniques to evaluate the runoff of a site prior to and after its development with various LID techniques. Without effective models and techniques, it becomes almost impossible to evaluate the impact of different LID approaches, thereby hindering the decision-making process on how best to develop the site, which could result in an increased runoff and stream erosion.

This paper discusses a step-by-step methodology to innovatively evaluate a site before and after the development using LIDs with a hydraulic/watershed model(s). It demonstrates the benefits of using the groundwater modules in the model(s) and adjusting the parameters such as evapotranspiration rates, soil moisture, ground water flow and groundwater recharge to mimic the runoff from the sites pre and post development.

A typical site was selected as an example to apply the methodology. The parameters for the groundwater module were determined based on past research and knowledge of local site conditions, in particular the soil characteristics and vegetaion conditions. The runoff from the site prior to development was determined with the groundwater module. A number of different development plans for the site were identified, with various LID techniques. The runoff was established for each different area within the site using the groundwater module. Roof, paved, grassed and bio-retention and infiltration areas were assigned different parameters in the groundwater module.

By varying the parameters of the groundwater module and LID techniques, it was possible to identify ways in which the site could be more effectively developed to minimize both the peak runoff and the total volume. By developing the site to more closely match the pre developed runoff peak flows and volume, the impact to the receiving streams can be minimized, thereby maintaining and improving the water quality in the streams.

The audience will gain insight about the recent advances in applications of the model(s) and the technical issues associated with the integration of groundwater modules to support its applications to minimize not only the peak flow but the total volume of the runoff to achieve a stabilized condition and ensure the water quality in our streams.