Abstract

The U.S. Environmental Protection Agency’s (USEPA) Stormwater Management Model, or SWMM, is a large, relatively complex software package capable of simulating the transformation of precipitation to urban runoff and the transport of the runoff from the ground surface through pipe/channel networks and storage/treatment facilities and finally to receiving waters. The model can be used to simulate a single event or a long continuous period. The original model was developed by Metcalf and Eddy, Inc. in association with the University of Florida and Water Resources Engineers, Inc. in 1971. Over the last three decades there have been many significant improvements and enhancements to the model’s capabilities. However, the model’s algorithms used to simulate the accumulation and transport of stormwater pollutants have rarely been addressed or significantly improved. With the recent development of the USEPA’s NPDES stormwater program and the ongoing development of the TMDL program and the continuing interest and concern associated with stormwater pollution throughout the developed world, the need to significantly improve the stormwater quality modeling capabilities of SWMM is greater now than ever before.

This chapter reviews the model’s existing stormwater quality algorithms, discusses the problems associated with these algorithms and offers a specific outline of needed improvements. The modeling insights and improvements are based on knowledge gained through many years of study and research that has led to the development of the Simplified Particulate Transport Model (SIMPTM) (Sutherland and Jelen, 1998) which has been recently applied to projects in Michigan, Oregon and Washington. The suggested improvements are, in general, algorithms that are already incorporated into SIMPTM.

It is the understanding of the authors that an effort is currently underway by USEPA and Camp Dresser and McKee (CDM) to significantly improve SWMM and create SWMM. In an effort to fully cooperate, the authors have provided Mr. Robert Dickinson of CDM with existing SIMPTM computer code and documentation. The hope is that all of the water quality related improvements recommended in this chapter will be included in the new SWMM 5.0. If that occurred, SWMM would more accurately simulate important urban stormwater pollution processes including the pollutant removal effectiveness of both street and catchbasin cleaning practices.

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