Board of Adjustment Agenda and Packet 2003 04 16

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SH 42/96th Street Connection Drainage Report for Coal Creek Bridge Proposed Proiect F000dnlain Analysis The peak discharges for Coal Creek from the previous study are still being used in the project area. The 100-year peak discharge at this location is 3900 cubic feet per second. A new hydraulic engineering analysis using the U.S. Army Corps of Engineers HEC-RAS Computer Program (Version 3.1) was done for the proposed project. The new analysis was needed because the mapping for the proposed bridge site is more accurate than the previous analysis (1 foot instead of 2 foot contour interval), the previous analysis did not evaluate the effects of the existing irrigation ditch diversion structures near the proposed bridge site, and the proposed bridge will have an impact on the base flood elevation and floodplain limits. The new hydraulic analysis for the existing condition was based on the channel description discussed previously. The new analysis shows a flow depth of about 5.2 feet at the proposed bridge site with the base flood elevation about 3 feet lower than shown on the FIRM. The proposed condition analysis shows the proposed roadway fills and bridge construction of the project will increase the base flood elevation (from the new analysis existing condition) about 0.6 of a foot with a flow depth of about 5.8 feet. Flood flow velocities in the reach of the project are in the 5 to 9 feet per second range for both existing and proposed conditions. These velocities will not have significant negative erosion or sedimentation effects considering the existing channel conditions and the proposed bridge improvements. Summary The proposed 96th Street Extension / Connection Coal Creek Bridge improvements comply with the City of Louisville floodplain ordinance and FEMA floodplain management regulations. The improvements do not have significant adverse erosion or sedimentation effects on this reach of Coal Creek. Disturbance of wetland and riparian areas caused by the project will be mitigated. A plan and typical section illustrating the bridge improvements and the existing and proposed base flood elevations and floodplain limits, and the detailed output of the computer hydraulic model for floodplain analysis for the project are included in the Appendix. WILSONV 3 &COMPANY IxlenmanlCmneUwn VIIPr