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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />surface profile computation, erosion control, drop structures, and <br />transitions also play an important role. A discussion of these <br />parameters is presented below. <br /> <br />1. Flow Velocity (Small Drainageway) <br /> <br />The maximum normal depth velocity for the 100-year flood peak <br />shall not exceed 7.0 feet per second (fps) for grass lined <br />channels, except in sandy soil, where the maximum velocity <br />shall not exceed 5.0 fps. The Froude Number shall be less <br />than 0.8 for grass lined channels. The minimum velocity, <br />wherever possible, shall be greater than 2.0 fps for the <br />initial storm runoff. <br /> <br />2. Longitudinal Channel Slope (Small Drainageway) <br /> <br />Grass lined channel slopes are dictated by velocity and Froude <br />Number requirements. Where the natural topography is steeper <br />than desirable, drop structures shall be utilized to maintain <br />design velocities and Froude Numbers. <br /> <br />3. Freeboard (Small Drainageway) <br /> <br />A minimum freeboard of 1 foot shall be included in the design <br />of the 100-year flow. For swales (i.e. small drainageways <br />with a 100-year flow less than 20 cfs), the minimum freeboard <br />requirement is 6 inches. <br /> <br />4. Horizontal Curvature (Small Drainageway) <br /> <br />The center line curvature shall have a minimum radius twice <br />the top width of the design flow but not less than 50 feet. <br />The minimum radius for channels with a 100-year runoff of 20 <br />cfs or less shall be 25 feet. <br /> <br />5. Roughness Coefficient (Small Drainageway) <br /> <br />The variation of Manning's "n" with the retardance and the <br />product of mean velocity and hydraulic radius, as presented in <br />Figure 5-1, shall be used in the computation of capacity and <br />velocity. <br /> <br />6. Cross sections (Small Drainageway) <br /> <br />The channel shape may be almost any type suitable to the <br />location and to the environmental conditions. Often the shape <br />can be chosen to suit open space and recreational functions. <br />Representative cross sections are presented in Figures 5-2, 5- <br />3, and 5-4. The limitations on the cross sections are as <br />follows: <br /> <br />a. Trickle Channel - The base flow (except for swales) shall <br />be carried in a trickle channel except for sandy soils. <br />The minimum capacity shall be 1.0 percent to 3.0 percent <br />of the 100-year flow but not less than 1 cfs. Trickle <br />channels shall be constructed of concrete or other <br /> <br />5 - l3 <br />