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<br />I. <br /> <br />4. Roughness Coefficient (Small Drainageway) <br /> <br />. <br /> <br />A Manning's "n" value range shall be established by the <br />manufacturer's data, with the high value used to determine <br />depth/capacity requirements and the low value used to <br />determine the Froude Number and velocity restrictions. <br /> <br />5. Cross Sections (Small Drainageway) <br />Same as for grass lined channels (see Section 5.5.2). <br /> <br />iig~.~;iQIl~,!.\~r~!L~J~'1 <br /> <br />The criteria for the design of roadside ditches is similar to the <br />criteria for grass lined channels with modification for the special <br />purpose of initial storm drainage. The criteria is as follows <br />(refer to Figure 5-6): <br /> <br />. <br /> <br />1. Capacity - Roadside ditches shall have adequate capacity for <br />the initial storm runoff peaks. During the initial storm <br />runoff event, encroachment shall not extend beyond the street <br />right-of-way. Where the storm runoff exceeds the capacity of <br />the ditch, a storm sewer system shall be required. <br /> <br />2. Flow Velocity - The maximum velocity for the initial storm <br />flood peak shall not exceed 5 feet per second for a type I <br />ditch, and 7 fps for type II or type III ditches. <br /> <br />3. Longitudinal Slope - The slope shall be limited by the average <br />velocity of the initial storm flood peaks. Check drops may be <br />required where street slopes are in excess of 2%. Maximum <br />permissible slope is 3%. <br /> <br />4. Freeboard - Freeboard shall be equal to the velocity head, or <br />a minimum of six inches. <br /> <br />5. curvature - The minimum radius of curvature shall be 25 feet. <br /> <br />6. Roughness Coefficient - Manning'S "n" values presented in <br />Figure 5-1 shall be used in the capacity computation for <br />roadside ditches. <br /> <br />7. Grass Lining - The grass lining shall be in accordance with <br />USDCM, Section 2.3.2, "Major Drainage - Grass Lined Channels <br />-Grass." <br /> <br />8. Driveway Culverts - Driveway culverts shall be sized to pass <br />the initial storm ditch flow capacity without overtopping the <br />driveway. The minimum size culvert shall be a 12-inch <br />diameter pipe (or equivalent) with flared end sections. More <br />than one culvert may be required. <br /> <br />. <br /> <br />5 - 18 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />