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<br />Page 11 <br /> <br />III. TECHNICAL DESIGN CRITERIA <br /> <br />A. Hydrology: The preliminary and final drainage reports shall take into <br />consideration two separate storms: the initial storm and the major <br />storm. Historic (existing development) and developed runoffs shall be <br />determined for both storms for the site including the entire basin inter- <br />cepted by the site. <br /> <br />1) Storm Frequency: The storm frequency to be used in drainage system <br />design will be the storm frequency applicable for the design point <br />under consideration. <br /> <br />a) Initial Storm: (Design Storm) The initial storm occurs at <br />fairly frequent intervals, between two and ten years. The <br />drainage system for the initial storm is to be designed to <br />minimize inconvenience, protect against minor damage, and <br />reduce maintenance costs. The design frequency interval for <br />the initial storm depends upon land use. See Table II. <br /> <br />b) Major Storm: (Check Storm) The major storm shall be considered <br />the 100 year storm. The drainage system for the major storm is <br />to be designed to protect against loss of life or serious <br />substantial property damage. <br /> <br />TABLE II. DESIGN STORM FREQUENCY BY LAND USE <br /> <br />LAND USE <br /> <br />DESIGN STORM FREQUENCY <br /> <br />Description <br /> <br />Zoning <br />District <br /> <br />Initial <br />Storm <br /> <br />Major <br />Storm <br /> <br />Residential (Low <br />Density) <br />Residential (High <br />Density) <br />Light Commercial <br />Office & Ind. <br />Parking, Parks, Other <br /> <br />CN, lR-3R <br /> <br />2 Year <br /> <br />100 Year <br /> <br />4R-6R <br />IC <br />OF, 2C-SC, IN <br /> <br />5 Year <br />5 Year <br />5 Year <br />2 Year <br /> <br />100 Year <br />100 Year <br />100 Year <br />100 year <br /> <br />2) Rainfall Intensity: Runoff for both the initial and major storm <br />shall be based on the Rainfall Intensity - Duration Curves for <br />Lakewood, Colorado shown in Figure 2. <br /> <br />3) Runoff Computations: Both the initial and major storm runoff quan- <br />tities shall be computed using the Colorado Urban Hydrograph Pro- <br />cedure for basins larger than 130 acres. The rational formula may <br />be used for basins less than 130 acres. <br /> <br />a) Rational Formula - The rational formula (Q=CIA) is well known <br />and assumed familiar to the user. Figure 3 should be used to <br />compute runoff for each design point. <br />