2012-04-10_REVISION - M1981185 (43)

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DRAINAGE DESIGN CRITERIA W Hydrology Criteria La Plata County Road, Bridge and Drainage Specifications and Standards (STANDARDS), and the Urban Drainage and Flood Control District (UDFCD) "Urban Storm Drainage Criteria Manual" were used for the storm drainage system design. The following criteria were utilized in developing the proposed drainage system. • The proposed drainage system is designed to match, as best as possible, the historic drainage patterns occurring at the site. • The proposed drainage system attempts to limit the diversion of storm runoff from one basin to another (basin transfer). • Runoff generated from drainage sub - basins is conveyed either directly or via storm sewer systems or swales to outfall locations into a detention pond where water quality facilities are located. Design Rainfall: NOAA Atlas' were used to determine peak runoff values. The 5 -year and 100 - year frequency storms are used as the initial and major design storms respectively. Runoff Calculation: Peak storm runoff is determined using the rational formula, Q = CIA: Q = storm runoff in CFS; C = runoff coefficient based on surface impermeability; I = rainfall intensity in inches per hour; and A = drainage basin area in acres. UDFCD Imperviousness Values (Table RO -3) and Runoff Coefficients (Table RO -5) are used to develop basin runoff coefficients. The runoff coefficients are weighted for each applicable sub - basin to more accurately reflect the runoff characteristics of the site. Time of Concentration is determined using the criteria in Sections 3.4.1 and 3.4.2 of the UDFCD Criteria Manual. Rainfall intensities are determined using STANDARDS NOAA Rainfall data and creating Intensity- Duration curves. The recurrence intervals used for this study were based on an industrial land use. The minor drainage system is designed for a 5 -year recurrence interval and the major drainage system is designed for a 100 -year recurrence interval. DRAINAGE FACILITY DESIGN General Concept The majority of runoff from the proposed site will be conveyed to the proposed detention pond. The historic drainage basins and patterns are maintained. Rational calculations as well as swale sizing and riprap calculations can be found in Appendix B. Specific Details The proposed development has been divided into four basins: Basin A, Basin B1, B2, P and Basin OS. A drainage map, presenting the drainage basin delineations as well as proposed drainage facilities, can be found in the back of this report. CLC ASSOCIATES Page 2