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<br />5.5.1 Design Standards for Detention Basins <br /> <br />Detention requirements for developments less than 50 acres shall be according to the following table. For <br />developed areas of 10 acres or less, the required volume equals the total development area times the <br />appropriate storage coefficient. For areas greater than 10 acres and less than 50 acres the volume is <br />determined by applying Harris County Flood Control District criteria for small watersheds. <br /> <br />Table 2 <br /> <br />Developed Area <br />o to 3 acres <br />3 to 10 acres <br />1 0 to 50 acres <br /> <br />Storage Coefficient <br />0.20 acre ft. / acre <br />0.45 acre ft. / acre <br />per HCFCD criteria <br /> <br />For developments larger than 50 acres, Harris county Flood Control District and the City Engineer shall <br />approve the detention facility criteria. <br /> <br />5.2.2 Outlet Sizing <br /> <br />1. The outlet structure shall be designed using the orifice equation as follows: <br /> <br />Q = CA x (2gh)~ <br /> <br />Where, <br />C=0.8 <br />A = cross sectional area <br />g = 32.2 feet / sec^2 <br />h = head differential <br /> <br />For head differential use 2' or the IOO-year water surface in pond minus the 25-year <br />water surface in receiving channel, if available. <br /> <br />2. Minimum restrictor shall be 6" diameter. <br /> <br />5.5.3 Additional Standards for Detention Basins <br /> <br />The detention facility shall be designed for easy maintenance. For smaller developments the designer is . <br />encouraged to use parking lots, underground piping, swales, green spaces, etc. to achieve the volume <br />required. <br /> <br />For larger developments every consideration shall be given to designing of the facility for multipurpose <br />use, such as playgrounds, miniparks, required green spaces, etc. to assure that maintenance will be <br />accomplished. The design shall include the following: <br /> <br />1. an earthen detention basin shall have minimum side slopes of 4: 1 and a minimum bottom width <br />of ten feet; <br />2. the bottom of the detention basin shall have a minimum 0.50% cross slope to facilitate quick <br />drainage. <br />3. a v-shaped trickle channel a minimum of S' wide, six-inch thick. reinforced concrete shall be <br />constructed through the detention basin at a longitudinal slope of 0.20% to accommodate low flow <br />and facilitate rapid drainage. For developments less than 3 acres, the trickle channel may be 2 feet <br />wide and 4" thick. <br />4. a minimum 12-foot wide maintenance berm shall be provided around the perimeter of the <br />detention facility. <br />S. ingress and egress for maintenance including a dedicated right-of-way if required, shall be <br />provided to the detention basin and clearly shown on the construction drawings or site plan <br />subdivision plat. <br /> <br />Storm water Criteria Page 5 of 10 <br />