Abstract

Many complex models that utilize continuous simulation (SWMM, HSPF, SLAMM, SIMPTM, etc.) require information pertaining to the accumulation rate of pollutants on the land surfaces. This is one of the most perplexing issues in stormwater modeling. A representation of the accumulation rates is usually obtained through trial and error during calibration, with little, if any, actual direct measurements. Historically, direct measurements have been misapplied in modeling applications, resulting in unreasonable model predictions. Many modelers therefore forego accumulation rate data, preferring to back into values from outfall observations. This approach makes it very difficult to correctly predict the sources of stormwater pollutants in urban areas and to make reasonable stormwater management decisions using source area controls. This dilemma has come about due to a major misinterpretation of previously collected field data: the assumption that street dirt loadings are zero after most rains. With the correct understanding and modeling of the washoff process, the vast amount of historically collected accumulation data becomes an important modeling resource. This Chapter presents a summary of this useful information. This information has been used in Pitt and Voorhees’ Source Loading and Management Model (SLAMM) and variations have been used in Sutherland’s Simple Particulate Transport Model (SIMPTM) to more accurately predict these important source area processes. Relatively simple modifications can be made to other continuous models that utilize accumulation and washoff functions for more accurate and complete stormwater control predictions.

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