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<br />4. Roughness Coefficient (Major Drainageway) <br /> <br />A Manning I s "nil value range shall be E!stablished by the <br />manufacturer I s da1::a, 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 (Major Drainageway) <br /> <br />Same as for grass lined channels (see SE!ction 5.4.2). <br /> <br />5.5 Design Standards f,or Small Drainageways <br /> <br />These standards cover the design of channl~ls that <br />classified as a major drainageway. Additional flexibility <br />stringent standards are allowed for small dra,inage.rays. <br /> <br />are not <br />and less <br /> <br />5.5.1 <br /> <br />Natural Channels (Small Drainaaewaj'l <br /> <br />The design criteria and ,evaluation techniques for natural channels <br />are: <br /> <br />1. The channel and overbank areas shall have adequate capacity <br />for the lOa-year s'::orm runoff. <br /> <br />2. <br /> <br />Natural channel segments <br />greater than 0.8 for <br />protected from erosion. <br /> <br />which have a calculated Froude Number <br />the lOa-year flood peak shall be <br /> <br />3. Roughness factors :.Manning I s n), which are representative of <br />unmaintained channel conditions, shall be used for the <br />analysis of water flurface profiles. <br /> <br />4. Roughness factors ,:Manning I s n), which are representative of <br />maintained channel conditions, shall bl~ used to determine <br />velocity limitations. <br /> <br />5. <br /> <br />Erosion control str.lctures, such as check drops <br />may be required to control flow velocities, <br />initial storm runoff. <br /> <br />or check dams, <br />including the <br /> <br />6. A channel stability analysis will be complete<l to determine <br />the impact of urbanization on the stabilit:y of t:he channel bed <br />and banks. <br /> <br />7. Plan and profile drawings shall be prepared showing the 100- <br />year water surface profile, floodplain, and details of erosion <br />protection, if required. <br /> <br />5.5.2 Grass Lined Channels (Small DrainaglUUuQ <br /> <br />Key parameters in grass lined channel design include velocity, <br />slope, roughness coefficients, depth, freeboard.. curvature, cross <br />section shape, and lining materials. other fac1:ors such as water <br /> <br />5 - 12 <br /> <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 />