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06 centimeters/second. Soil permeabilities in the range of lE-OS to 1E-06 have <br />poor drainage characteristics which means good characteristics for temporarily <br />holding storm water within a detention pond. Soil permeabilities in the range of <br />1E-07 to 1E-09 are practically impervious. Based on the geotechnical information <br />provided by Terracon, the site appears to contain adequate soils for a regional <br />detention pond. The import and cost thereto of an impervious liner for the regional <br />detention pond was not considered. <br />Within Terracon's report a Subsidence Hazard discussion may be found. In <br />addition, after Terracon's report, two Subsidence Hazard Maps are included. In <br />summary, the only acceptable land use for the proposed regional detention pond <br />site is agriculture or open space. <br />Preliminary Regional Detention Pond Design: <br />Preliminary design computations for the regional detention pond were provided by <br />Rocky Mountain Consultants and are included within Appendix 4. Three autocad <br />drawings are included within Appendix 4, two being the conceptual drainage maps <br />and one being the regional detention pond preliminary design drawing. Please <br />refer to the conceptual drainage map for the drainage basin designations, areas and <br />flow patterns. The drainage map indicates drainage areas contributing to the <br />proposed regional detention pond. The Colorado Urban Hydrograph Procedure <br />(CUHP) was used to determine the runoff hydrographs for each of the drainage <br />basins delineated on the drainage plan. The Storm Water Management Model <br />(SWMM) was then used to route these individual hydrographs through the study <br />area to determine the combined runoff peak flow, detention requirements, channel <br />and culvert sizes. Both of these design methods are presented within the Urban <br />Drainage and Flood Control District's Urban Storm Drainage Criteria Manual. <br />The 100 year existing condition storm event was analyzed by Rocky Mountain <br />Consultants. As previously noted, the existing open channel between 8th Street and <br />lst Street has a capacity of 335 cfs of storm water. Between 8th Street and <br />Firestone Lake (Milavec Lake) the existing open channel is limited by the reach <br />between 8th Street and lst Street. The Town of Frederick is planning to improve <br />the existing open channel between 8th Street and lst Street to a capacity of 567 cfs <br />plus freeboard, before start of the regional detention pond construction. <br />The release rate for the proposed regional detention pond was established based <br />upon the 8th Street to lst Street improved open channel capacity and the desire to <br />avoid a State of Colorado Jurisdictional Dam. Based upon a maximum 100 year <br />release rate of 567 cfs from the regional detention pond, the required 100 year <br />pond volume will be 145.48 acre-feet. The configuration of the regional detention <br />pond is shown on the preliminary design drawing �vithin Appendix 4. <br />� <br />