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Surface Water Modeling <br />Development of a flow model (XP-SWMM). To adequately predict the impact that this <br />site will have on water resources, a tool is needed to incorporate development and <br />infrastructure assumptions. The model used by FOR for this exercise is the XP-SWMM <br />model (XP Software, Inc.). This model is used to contrast existing conditions with <br />proposed changes associated with development within a watershed. The model looks at <br />the change in land use and land cover, and relates the change to runoff behavior. Runoff <br />predictions can be made for variable frequency events, and routed through the proposed <br />drainage system. The model output and routing can then be used to determine areas <br />where flooding or high water will occur, and then can be used to design a system of <br />sloiiiiwater management facilities, which could include detention storage, diversion, <br />infiltration or any number of associated BMPs. <br />Existing Conditions. The first phase of the quantity and quality modeling involved <br />defining the water behavior as it exists currently. The physical characteristics of each <br />sub -watershed noted in Figure 12.1 form the basis for determining the amount of water <br />that will run off of it during specified climatic events, specifically rainfall and snowmelt <br />events of certain statistical frequency. <br />The results of the existing conditions quantity analysis using the XP-SWMM model are <br />displayed in Table 17.1 by sub -watershed for the 100-year frequency, 24-hour rainfall <br />event and 10-day snowmelt. This represents the peak flow and volume discharges that <br />would be expected for an event that would occur with a frequency of once every one - <br />hundred years, or 1% in any given year. Volume discharges are based on a 5-day runoff <br />simulation for the 24-hour rainfall event and a 30-day runoff simulation for the 10-day <br />snowmelt event. <br />Of note in Table 17.1 is the small amount of flow leaving this site at the Highway 10 <br />culvert (subwatershed 30). During a 100-year event, a peak flow of only 28 cubic feet <br />per second (cfs) leaves the AUAR area, reflective of the sandy nature of the watershed <br />and the low intensity agricultural and low density land use. Flow to the Mississippi River <br />from the southeast corner of the site does not occur, but rather soaks into the sandy soils <br />as it flows in a small ditch to the southeast. Because of this reason, the base level for <br />water quantity and quality eventually reaching the Mississippi River is zero. <br />17-4 <br />