Board of Adjustment Agenda and Packet 2011 08 17

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1303 Empire Road Floodplain 7 "The fields and open space between State Highway 42 and Coal Creek are currently in Zone X according to Firm Map No. 08013C0576 G. Since Zone X represents shallow flooding below one foot in the 100-year storm, it is DBCo.'s opinion that this area will still be inundated by shallow flooding in the 100-year storm of depths less than one foot due to the flat and expansive fields east of State Highway 42. Therefore, there is no need to pursue a more detailed analysis of Special Flood Hazard Boundaries in this area. It must be understood that the 100-year flows from the lower downtown Louisville area are also directed towards Drainageway A and to Coal Creek along the north side of the Wastewater Treatment Plant. Additionally due to the installation of two upstream detention ponds at Lake Park and Middle School Park, the total flow from both Drainageway A and the lower downtown Louisville area was reduced to 609 cfs. It was 1300 cfs in our existing conditions analysis, which is a reduction of more than 50%." Hydraulic Analysis In order to provide a verification of the above analysis, CP Engineering utilized the U.S. Army Corps of Engineers Hydraulic Engineering Center River Analysis System (HEC-RAS) software to perform a preliminary hydraulic modeling of the revised floodplain. Input data for the modeling used the topographic mapping obtained from the City of Louisville and 100-year hydrologic flow amounts from the most recent Drainageway A study and recommendations. As shown on Figure 5, a floodplain station and control line, along with six floodplain cross sections were chosen based on the estimated overflow weir and sheet flooding east of highway 42 as discussed above. As expected, the resulting estimated flows are shown to be less than one foot in depth, with generally unconfined (no existing channel or stream banks) sheet flows spreading across the Lastoka Open Space fields east of the highway. The inundation area is bordered on the north by the existing Drainageway A outfall (previously identified by FEMA as an X (shaded) floodplain zone), and the south by the slight topographic ridge created by the former railroad grade. Due to the shallow and unconfined nature of these flows, it would not be expected that a consistent water surface would actually exist across this area, but rather floodwater would flow and pool in low lying areas (very similar to how the property has historically been flood irrigated from the ditch along the east side of Highway 42). Per FEMA guidelines for shallow flooding analysis and mapping, this area would be expected to be mapped as either an AO 1 or an X(shaded) flood zone. Although the analysis does not show that flows would be expected to extend onto the 1303 Empire Road property due to the slight topographic ridge created by the former railroad grade, it is possible that in order to provide a conservative floodplain mapping, FEMA may require the property to be included in the AO 1 or X(shaded) zone. In addition, if the estimated base flood elevation of 5308 feet for the HEC-RAS cross-section 700 (the cross-section that extends through 7115 Dry Creek Court Niwot, Colorado 80503 Phone: 720-231-3251 CURTPARKER55@hotmail.com