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Modeling Low Impact Development Alternatives with SWIMM
Citation:
ROSSMAN, L. A. Modeling Low Impact Development Alternatives with SWIMM. Presented at Stormwater and Urban Water Systems Modeling, Toronto, BC, CANADA, February 19 - 20, 2009.
Impact/Purpose:
To Share Information
Description:
The U.S. Environmental Protection Agency’s Office of Water (OW) is actively promoting the use of Low Impact Development (LID) practices to help protect and restore water quality in urban and developing areas. Such practices support the concepts of green infrastructure and sustainability by managing stormwater close to its source so as to mimic a site’s predevelopment hydrology. Computational tools are needed to objectively evaluate the hydrologic benefits of proposed LID controls. The Storm Water Management Model (SWMM) is one such tool that provides an attractive framework for modeling the long term performance of LID controls. However the latest version of SWMM (5.0) has only a limited capability to rigorously model LID alternatives. This paper describes how SWMM’s runoff engine was extended to explicitly model a number of specific LID controls, such as permeable pavement, bio-retention areas (e.g., rain gardens and roof gardens), cisterns, infiltration trenches and basins, vegetative swales, and vegetative buffer strips. Each of these controls is represented as a unique LID object with its own set of properties that describe its behavior, and can be connected in a cascading sequence to SWMM subcatchment objects or other LID objects. Each particular type of LID is a sub-class of a Generic Low Impact Process (GLIP) modeling unit. Conceptually, a GLIP is an area of land with several vertical sub-surface zones and outlet drains. These include an upper free surface water zone, an upper surface water outlet drain, an upper soil infiltration zone, a soil vegetative zone, a lower storage zone, a lower outlet drain, and a lower soil infiltration zone. As examples, a cistern GLIP would consist of just the free surface zone, an upper outlet drain and a lower outlet drain, while a rain garden GLIP would conceivably contain all of the various GLIP elements. A GLIP’s performance model is based on solving a set of time-dependent water balance equations for the fluxes between each of the zones and the outlet drains. Within this model, infiltration is computed using a time-dependent, variable head Green-Ampt formulation while both Manning’s equation and the orifice equation are used to model different types of outlet drain flows. The implementation of this approach to modeling LID alternatives within SWMM will be illustrated with a number of case study examples.