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CIVII. RFS'I?URCFS <br />The constructed model estimated floodplain limits that were relatively close to what is shown on FEMA Panel # 080266 <br />0750 C. See Figure 1 for the FEMA floodplain as well as the floodplain from the Civil Resources project model. <br />Significant portions of the area shown within the floodplain is backwater or shallow flooding areas outside of the <br />floodway. Two methods were used to estimate the floodway of the South Platte adjacent to the project site. The first <br />method was to allow equal encroachment on both sides of the floodplain to induce a 1-foot rise in the base flood <br />elevation. This method is the standard accepted methodology, however, in the physical setting of this project it was not <br />realistic necessary to strictly adhere to this procedure. Specifically, the project aspects that allow for the full <br />encroachment to be allowed on one side of the floodplain include: 1) project site includes both sides of the river in some <br />areas; 2) steep embankments or outcroppings exist on the west side of the floodplain which limits the feasibility of filling; <br />and 3) Public Service Company owns much of the land west of the river in order to "buffer" there Fort St. Vrain facility <br />from future development and therefore this may be acceptable for there intended land use. The floodway with equal and <br />only east side encroachment are shown on Figure 1 and on the HEC-RAS Cross-Sections included with this memo. <br />Conclusion <br />The purpose of determining the location of the floodway on the South Platte River is to set the limits of fill to allow for <br />mining and above grade water storage options to be analyzed on a conceptual basis. The floodway significantly limits <br />the construction of above grade storage based on preliminary analysis. The location of the floodway should be <br />considered approximate and should be finalized and submitted to FEMA for map revision approval prior to significant <br />investment. Significant differences could occur with refined 100-year flow data. <br />JAUnited Water & San -1411Gilcrest ResourcelMemo-Floodplain Analysis021309.doc