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<br />b. Design velocities, as computed should not generally exceed <br />critical velocity as Indicated by ba"kwater curve computatIons. <br />Exceptions permitted In short reaches where variances are <br />reasonable environmentally. <br /> <br />c. The flood plaIn limits must be defIned. <br /> <br />d. Filling of flood pla!n frInges should b" limited In order to <br />maIntaIn channel storage capability, <br /> <br />e. Hydraulic roughness should be representative of reasonably <br />maIntained channel conditions. <br /> <br />f. Drops, or check dams, should be used to control water surface <br />profIle slope to control erosion from the clear water releases <br />from Standley Lake and the Great I,estern Reservoir. <br /> <br />g. Perform backwater curve computations based upon existIng con- <br />dItions with assumption that f'lood plaIn limits will be re- <br />vIsed as bridges are replaced or new roadways are constructed. <br /> <br />2. Grass-Lined ArtifIcial Channels. Design will generally be based <br />on channels having natural waterway hydri9ullc and appearance <br />characteristics. They should be aesthetically designed to the <br />local environment and useful for local n!!creatlon contlnous along <br />the channel. In most Instances, the artificIal channels shall <br />be used only as necessary at bridges where the channel cross-section <br />will meet geometric requirements for bridge openings. The design <br />frequency for channe 1 Improvements at other I ocat Ions will vary, <br />using channel geometries sufficient In size to obvIate flooding <br />hazards and to meet road crossing criteria. Criteria Include: <br /> <br />a. Use trickle channels for low flows with capacities of about <br />one percent of desIgn flow, but not to exceed 250 cfs. Use <br />rlprap, concrete, or other protection to prevent erosion. <br />Maximum flood depth over trickle channel ~ 8 feet,maxlmum <br />velocltyalO fps. <br /> <br />b. A maxImum average desIgn v1!loclty of eight fps for the 100.. <br />year storm, <br /> <br />c. DesIgn depth on over-bank of up to f':lur f,eet, except where top <br />wldth-to-depth ration exceeds thirty, and then not more than <br />five feet. <br /> <br />d. Design top wIdth to range betwE.en 20 and .40 times the design <br />depth. <br /> <br />V"'2 <br />