Modeling Low Impact Development in Watersheds using the Storm Water Management Model

Low impact development (LID), which aims to manage stormwater at the source with decentralized micro-scale control measures, has been widely used. Some studies have done the evaluation on typical projects where the LID has been used, however, the evaluation of the benefits of implementing LID need to extend up to the watershed level. Modeling is an important method to evaluate design plans under different rainfall events.

This paper proposes an approach that focuses on modeling LID in watersheds using the Storm Water Management Model (SWMM). There are two ways to simulate LID in a SWMM. The first is using the LID module, which has been added to the SWMM since version 5.1 and the second is simply adjusting the parameters based on a basic SWMM. In the proposed methodology, these two ways of simulating LID in a SWMM are combined. The first step is to choose a typical LID project in the watershed and perform the LID simulation in the SWMM using the LID module, which involves setting a large number of parameters. The parameters of each kind of LID are determined according to its design plan. If monitoring data exist for the project, the model can be better calibrated and verified. Then the performance of a typical project is represented as a whole in the SWMM of the watershed by adjusting the parameters of the corresponding sub-catchment, mainly percentage impervious area (%imperv) and depth of depressed storage in a pervious area (Dstore-Perv). The %imperv parameter is determined by the type of land use that has been changed through LID application. The difficulty is setting the Dstore-Perv parameter, which represents the maximum surface storage provided by ponding, surface wetting, and interception.

The methodology was applied to one of China’s pilot sponge -cities, Tsingtao. Under monitored and designed rainfall events, sponge construction planning for the pilot area can significantly reduce runoff volume, but the effect is different in different sub-watersheds. For a watershed where LID practices will be applied, the uncertainty of the SWMM simulation can be reduced using the proposed methodology. A credible and accurate model is required for the evaluation of LID plans to be valuable and usable as a basis for construction and optimization.