Under the National Pollutant Discharge Elimination System (NPDES)stormwater program, operators of large, medium, and regulated small municipalseparate storm sewer systems (MS4s) require authorization to discharge pollutantsunder an NPDES permit. MS4s can be linear or complex, open, piped, manmade,natural, or a combination of all of the above. Some carry groundwater or pipedstreams, are tidally influenced, or have some other constant source of non-stormwaterdischarge. An MS4 is not always just a system of underground pipes-it can includeroads with drainage systems, gutters, and ditches. Typically, an outfall from a MS4discharges into a creek system and ultimately into a larger receiving water bodywhich might or might not be tidally influenced. In order to evaluate the existing andexpected water quality of the ultimate receiving water body, mathematical models areused to conduct flushing modeling and water quality modeling of the ambient waters.A hydrodynamic mixing zone model will be used to simulate steady or unsteadydischarge from a single open channel outfall or a system of outfalls from municipalseparate storm sewer systems (MS4s). This model is applicable for a wide range ofhydrologic situations wherein flow and transport are governed by time-dependentforcing functions. Permit requirements will consist of a Water Quality MonitoringProgram to be implemented and samples collected according to permitting agencies’procedures for collection, control and preservation.
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