Board of Adjustment Agenda and Packet 2015 08 19

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*ENGINEERING , ?CONSULTANTS "Your Project, Our Pride" TM August 7, 2015 Ms. Mary Ann Heaney CIH, CSP Louisville Sustainability Advisory Board 1117 La Farge Ave Louisville, CO 80027 303 - 665 -8528 RE: Louisville Community Gardens — Drainage Compliance Letter Dear Ms. Heaney, The intent of this letter is to document and communicate that the Louisville Community Garden project will not adversely impact the FEMA regulated floodplain. Proposed Improvements: The improvements proposed will install two 8' x 12' storage sheds and up to 32 4' x 8' raised planter beds. The planter beds will be raised 18 inches above grade to provide accessible beds for people who would need them. Drainage Compliance: This is a summary of the analysis to determine the effect of adding a two 8.0' x 12.0' sheds and 32 4.0' x 8.0' raised planters within the AH zone within the FEMA floodplain in the City of Louisville, Colorado. AH zone is classified as areas of shallow flooding with a depth of 1' — 3'. The effective HEC -RAS model for this unnamed tributary was not available, and since the structures are located in a backwater area and a detention area the effect of the structures would be minimal. The The first approach was to calculate the area of displacement of the structures based on their location and elevation and determined the percent loss of storage volume they caused. The structures are located above the emergency spillway elevation, so the detention area is at design capacity and overflowing before the reduction in volume comes into play. The total volume of the detention pond is 521609 cubic feet (cu ft.) of storage. The sheds and planters will remove 1920 cu ft. for a reduction of 0.3 percent storage volume. Because the reduction in volume was above the flood stage, a second approach was investigated. The second approach was to cut a cross section along the BFE line corresponding to 5377 from the FIRM map and calibrated the flow to achieve the WSE of 5377 using Flowmaster open channel calculations. This is the cross section which passes through the area where the proposed raised planter beds will be constructed. The slope used in this calculation was derived from the slope of the street adjacent to the property. Once the baseline was established, running the same cross section with the planter beds and shed added did not cause an increase in the WSE for the cross section. 6505 South Paris Street, Suite B • Centennial, Colorado 80111 Phone: 303.368.5601 • Fax: 303.368.5603 • J3Engineering.net