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and, to a lesser extent, to increased volumes of runoff resulting from impervious or paved surfaces. Typical options complement existing water quality treatment options and primarily include the construction of ponds to slow down or detain runoff before it enters natural water bodies. Most regulating agencies require that the rate of runoff from a project not exceed the rate of runoff under existing conditions. This requirement may stand whether the existing condition is a natural or a built environment. Evaluation of water quantity impacts includes an analysis of floodplain areas to ensure that the project does not ultimately reduce the available flood storage. Reduction of floodplain storage volume in one area may require the excavation of new floodplain storage volume in another area. Tools commonly used to evaluate water quantity impacts include engineering hydrologic and hydraulic formulae and computer modeling software, such as the Hydraulic Engineering Center (HEC)- River Analysis System (RAS), the HEC-2, and the Storm Water Management Module (SWMM) in conjunction with topographic maps and storm drainage infrastructure as-built information. Recommended applications of these models are summarized below. Corps of Engineers Hydraulic Engineering Center River Analysis System HEC-RAS is a water surface profile model for steady and unsteady one-dimensional, gradually varied flow in both natural and constructed river channels. HEC-2 HEC-2 is a water surface profile model for steady, gradually varied flow in natural and constructed channels. EPA Storm Water Management Module SWMM is a storm water and wastewater management modeling package for analyzing urban drainage systems and sanitary sewers. The model combines hydrology and hydraulics with water quality. An application known as MIKE-SWMM provides users with a complete, graphical, easy- to-use interface. The data output provided by the models listed above are only as accurate as the quality of the input. For environmental document preparation, the level of necessary input detail is generally less than that needed for final design purposes. Input data generally is obtained from existing topographic maps, groundwater contour maps, and the preliminary plans of the proposed improvements, which illustrate changes in impervious areas, grading, and surface-water routing. SELECTING AN APPROPRIATE METHOD OF ANALYSIS The following is a discussion of methodologies for predicting the extent to which the water quality and drainage components of a transportation system change would differentially or severely affect protected populations. The design of water quality and drainage improvements generally is dictated by natural physical laws and topography, as well as by existing impediments in the built environment. Generally, improved water quality will equally affect all people using the resource, and mitigation improvements related to water quality and drainage will thus benefit society as a whole. Because regulations require either no net change or improvement in water quality and quantity characteristics, there will be little possibility for differential effects in most situations. However, the potential for distributive effects may result from situations such as the following: 125