Evaluating the Mixing Mechanism of Outfall Discharges from Municipal Separate Storm Sewer Systems

Under the National Pollutant Discharge Elimination System (NPDES) stormwater program, operators of large, medium, and regulated small municipal separate storm sewer systems (MS4s) require authorization to discharge pollutants under 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 piped streams, are tidally influenced, or have some other constant source of non-stormwater discharge. An MS4 is not always just a system of underground pipes-it can include roads with drainage systems, gutters, and ditches. Typically, an outfall from a MS4 discharges into a creek system and ultimately into a larger receiving water body which might or might not be tidally influenced. In order to evaluate the existing and expected water quality of the ultimate receiving water body, mathematical models are used to conduct flushing modeling and water quality modeling of the ambient waters. A hydrodynamic mixing zone model will be used to simulate steady or unsteady discharge from a single open channel outfall or a system of outfalls from municipal separate storm sewer systems (MS4s). This model is applicable for a wide range of hydrologic situations wherein flow and transport are governed by time-dependent forcing functions. Permit requirements will consist of a Water Quality Monitoring Program to be implemented and samples collected according to permitting agencies' procedures for collection, control and preservation.