Effects on hydrology due to urban infrastructures are primarily related to the variation of peak discharge and generation of total volume of runoff in the watershed. Storm Water Management Model (SWMM) was chosen as a primary model to evaluate the effectiveness of low impact development (LID) techniques. It is a data intensive model but applies to comprehensive range of modeling options and is capable of simulating LIDs. Rational method and HEC-HMS model were used in the validation of the SWMM simulation results. For modeling application, Dumbarton watershed located in Washington D.C. was selected due to abundance of data availability for modeling parameterization. In addition, the watershed area is reportedly susceptible to continuous excess runoff and soil degradation. After model validation, four LID techniques: bio-retention cells, infiltration trenches, porous pavements, and rain gardens, were simulated by SWMM for evaluating their benefits in reducing peak discharge and runoff volume. Moreover, a cost-benefit analysis was performed to identify tradeoff relationship between reducing peak discharge and runoff volume for a given cost.

The modeling results demonstrate that LID practices play an important role in reduction of total volume and peak runoff. Each LID was tested independently for their performance; for example, bioretention cells were found to reduce the runoff volume by 24% whereas infiltration trenches were found to reduce volume only by 13%. Simulation scenarios, where all LIDs were introduced in integration, predicted the marginal reduction in peak discharge with percentage reduction varies from 3.6% to 10%. However, a significant reduction in runoff volume was observed. At the watershed outlet, total runoff volume was found to decrease from 2,081,000 ft³ to 1,418,000 ft³, which make up about one-third reduction in volume.

Further investigation using cost-benefit analysis demonstrated that the integrated application of LIDs cost much less than the conventional application of LID for a similar percent reduction of runoff. In some subcatchments, more than 33% reduction in peak runoff was achieved with a relatively smaller cost. A trade-off analysis between cost, reduction in peak discharge and reduction in runoff volume revealed that the rate of reduction was higher (cost effective) in runoff volume than that of peak discharge.