Unique Features of the Source Loading and Management Model (SLAMM)
A logical approach to stormwater management requires knowledge of the problems that are to be solved, the sources of the problem pollutants, and the effectiveness of stormwater management practices that can control the problem pollutants at their sources and at outfalls. SLAMM is designed to provide information on these last two aspects of this approach. SLAMM can be an important component, along with local receiving water studies, of an effective watershed management program.
SLAMM was initially developed to evaluate stormwater control practices more efficiently. It soon became evident that in order to accurately evaluate the effectiveness of stormwater controls at an outfall, the sources of the pollutants or problem water flows must be known. SLAMM has evolved to include a variety of source area and end-of-pipe controls and the ability to predict the concentra-tions and loadings of many different pollutants from a large number of potential source areas. SLAMM calculates mass balances for both particulate and dis-solved pollutants and runoff flow volumes for different development character-istics and rainfalls. It was designed to give relatively simple answers (pollutant mass discharges and control measure effects for a very large variety of potential conditions). Basic types of control practices evaluated by SLAMM include detention ponds, percolation ponds, infiltration devices, porous pavements, grass swales, catchbasin cleaning, and street cleaning, plus different development alternatives. Most of these controls can be evaluated in many combinations and at many source areas as well as the outfall location. SLAMM also predicts the relative contributions of different source areas (roofs, streets, parking areas, landscaped areas, undeveloped areas, etc.) for each land use investigated. An early version of SLAMM was described by Pitt and Shawley (1982) as part of the Nationwide Urban Runoff Program study conducted in Castro Valley, CA. A detailed description of SLAMM, including examples of its use, was presented by Pitt and Voorhees (1995).
The development of SLAMM began in the mid 1970s, primarily as a data reduction tool for use in early street cleaning and pollutant source identification projects sponsored by the EPA's Storm and Combined Sewer Pollution Control Program (Pitt 1979; Pitt and Bozeman 1982; Pitt 1984). Much of the information contained in SLAMM was obtained during the EPA's Nationwide Urban Runoff Program (NURP) (EPA 1983), especially the early Alameda County, California (Pitt and Shawley 1982), and the Bellevue, Washington (Pitt and Bissonnette 1984) projects. The completion of the model was made possible by the remainder of the NURP projects and additional field studies and programming support sponsored by the Ontario Ministry of the Environment (Pitt and McLean 1986), the Wisconsin Department of Natural Resources (WI DNR) (Pitt 1986), and the U.S. Environmental Protection Agency (EPA) (Pitt and Voorhees 1995).
SLAMM has been commonly used as a planning level model for large watershed projects. As an example, SLAMM has been extensively used by the Wisconsin Nonpoint Source Program in its evaluation of urban areas, as described by Pitt (1986). The WI DNR uses SLAMM to identify sources of pollutants, quantify their discharges, and to evaluate alternative control practices. Its use in Wisconsin in conjunction with geographical information systems (GIS) has also been described by several authors (Thum, et al. 1990; Ventura and Kim 1993; Kim and Ventura 1993; Kim, et al. 1993; and Haubner and Joeres 1996). Another early use of SLAMM on a watershed scale was during the Toronto Area Watershed Management Strategy program (TAWMS). In this project, SLAMM was used to predict drainage area pollutant and flow discharges, SWMM was used to predict combined sewer overflow (CSO) discharges from the older sections of the city, and HSPF was used to evaluate receiving water conditions resulting from these discharges (TAWMS 1986).
SLAMM is used to better understand the relationships between sources of urban runoff pollutants and runoff quality. It has been continually expanded since the late 1970s and now includes a wide variety of source area and outfall control practices. SLAMM is strongly based on field observations, with minimal reliance on theoretical processes that have not been adequately documented or confirmed in the field. Special emphasis has been placed on small storm hydrology and particulate washoff in SLAMM. Many currently available urban runoff models have their roots in drainage design where the emphasis is on very large and rare rains. In contrast, many stormwater quality problems are mostly associated with common and relatively small rains. The assumptions and simplifications that are legitimately used with drainage design models are not appropriate for water quality models. SLAMM therefore incorporates unique process descriptions to more accurately predict the sources of runoff pollutants and flows for the storms of most interest in stormwater quality analysis
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