Board of Adjustment Agenda and Packet 2011 08 17

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1303 Empire Road Floodplain 6 provide a more accurate evaluation of base flood conditions based on existing topography, current topographic mapping (with a contour interval of 2 feet) was obtained from the City of Louisville. By comparing the adopted base flood elevations that exist for the AE zone west of State Highway 42 to the current (and much more accurate) topographic mapping, a preliminary determination can be made with respect to a revised flood zone for the area east of the highway. This would essentially provide a revised approximate Zone A that would have resulted if the Drainageway A LOMR study limits would have been extended east of Highway 42. As shown on Figure 4, an existing base elevation of 5319 is shown on FIRM Panel 557 immediately west or just upstream from Highway 42. By using the current topographic mapping and survey data, a centerline profile can be determined for Highway 42 that defines an area of the highway that would be overtopped by the base flood elevation of 5319. The resulting overflow area forms a weir overflow approximately 500 feet in length, located about 450 feet north of the Highway 42 and Empire Road intersection. The maximum depth of flow over the highway would be less than 9 inches. The extent of the actual Highway 42 overtopping is considerably less than that shown on the existing FIRM panel (the FIRM shows overtopping occurring from approximately 300 feet south of Empire Road and extending north almost 1,500 feet in length). The large difference between the currently mapped and actual base flood conditions at Highway 42 can be explained by how FEMA requires that a revised LOMR floodplain boundary must be transitioned to connect to a non -revised floodplain boundary at study limits. In this case it appears that the AE zone between the Burlington northern Railway and Highway 42 was used as a transition zone from the revised mapping west of the tracks to the non -revised Zone A mapping east of Highway 42. By using the actual Highway 42 overtopping weir, a revised Zone A area of inundation can be estimated for the area east of the highway. Assuming flow paths perpendicular to contour lines, an approximated area of shallow flooding would result with a flow pattern generally to the northeast downhill from the Highway 42 overtopping (see Figure 4). A small topographic ridge (approximately 2 to 3 feet in height) created by a former railroad grade can conservatively be considered as the southern boundary of the potential flooding zone. Since the former railroad grade lies between the 1303 Empire Road property and this potential inundation area, it appears that the 1303 Empire Road site is outside of the actual 100-year floodplain. It is our understanding that future flood mitigation projects, including the next phase of the Drainage A drainage and floodplain mitigation project are proposed that would result in the elimination of the remnant AO and AE flood zones in downtown Louisville, and subsequently the existing approximate Zone A overflow area east of Highway 42. The nature and location of the 100-year floodplain east of Highway 42 was also addressed in a report prepared in 1999 by Drexel Barrell and Company titled 'As -Built" Report for the Drainageway A Improvements, Louisville, Colorado. On page 12, the report states: 7115 Dry Creek Court Niwot, Colorado 80503 Phone: 720-231-3251 CURTPARKER55@hotmail.com