HomeMy WebLinkAboutPreliminary Drainage Letter 25 RAHLING CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 PHONE: (501) 214-9141 April 30, 2024 Mr. Chris Smith, P.E. City of Little Rock Planning & Development 723 W. Markham St. Little Rock, AR 72201 RE: King’s Crossing Phase I Preliminary Drainage Analysis Mr. Smith, This letter is to accompany the submitted Preliminary Plat for the subject project and provides a summary of the preliminary drainage analysis. The project will consist of 18 residential lots located east of Copper Run Phase 6 and will include paving, drainage, sidewalk, water, sewer, and additional utility extensions. The existing site is currently undeveloped and wooded. The rational method was used to determine the existing peak runoff for the watersheds. Considering the existing cover and steep grade, an existing rational coefficient of 0.50 was used for the site. Time of concentrations were calculated using the TR-55 method. The existing site drains south to north and has two outlet points into Brodie Creek. The preliminary analysis includes four individual drainage basins separated into on-site and off-site areas. These drainage basins are summarized below in Table 1 and are shown on the Preliminary Drainage Analysis drawing. Table 1: Existing Drainage Areas Release Point Drainage Basin Area (Acres) Tc (min.) C Q100 (cfs) A #1 3.09 17 0.5 10.64 #3 2.3 15 0.5 8.8 Total to Release Point A 19.44 B #2 0.83 11 0.5 3.37 #4 1.47 10 0.5 6.14 Total to Release Point B 9.2 Total Site Discharge 28.64 Drainage Basins #1, #3, and #4 will be captured by the proposed storm drainage apart from the rear yards of Lots 501-510. The flow pattern of drainage basin #2 will remain unchanged. The proposed storm drainage infrastructure will be designed to convey the post-developed run-offs to a detention
25 RAHLING CIRCLE, SUITE A-2 LITTLE ROCK, ARKANSAS 72223 PHONE: (501) 214-9141 facility located in the northwest corner of Copper Run, Phase 6. Any future development of the existing drainage areas will be considered in the design. The detention basin will be sized to mitigate the additional post-developed stormwater runoff per the City of Little Rock Drainage Manual. Sincerely, Jacob White, PE