Abstract

The Low Impact Development (LID) practice is an increasingly popu-lar concept in stormwater management for controlling the adverse hydrologic and water quality impacts of urban sprawl (Elliott and Trowsdale 2007). LID uses distributed, small-scale, integrated man-agement practices to infiltrate and treat stormwater runoff at the source (Coffman et al. 1999).

Despite the growing acceptances of the LID concept among government regulators, municipal decision-makers, and designers, approaches for modeling LID hydrologic im-pacts have not keep pace with LID implementations. Past LID simula-tion methods vary from representing LID as a subcatchment and manipulating the curve number, routing the flow from the impervious surfaces to neighboring pervious surfaces, to increasing the depression storage of a pervious subcatchment (Prince George’s County 1999; Huber and Cannon 2002; Paul 2005; PSAT 2005). While these methods provide some general estimates on hydrologic benefits from LID, a consistent approach is needed to more directly account for the real hydrologic processes in LID.

The U.S. Environmental Protection Agency’s Stormwater Man-agement Model (SWMM) is a dynamic rainfall-runoff model capable of continuous simulation of runoff quantity and quality (Rossman 2008). The model has been applied worldwide for analyses related to stormwater runoff, combined sewers, sanitary sewers, and other drainage systems (Guitierrez 2006). The current SWMM (Version 5.0.014) does not have modules to simulate LID components, although work is currently ongoing to incorporate this capability. Numerous suggestions have been made in the past for enhancing SWMM with the capability of LID simulations (Guitierrez 2006; USEPA 2006).

This chapter proposes SWMM representation schemes for two of the most frequently used LIDs-bioretention and porous pavement. The model representations use existing SWMM components to simu-late the hydrologic processes in LIDs, such as infiltration, percolation, ponding, and underdrain discharge. The proposed model representations for the bioretention and porous pavement and have been tested against long-term observed data from the University of New Hampshire Stormwater Center (UNHSC). The test was carried out on two parameters reported in the UNHSC study: average peak flow reduction, and average lag time.

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