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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 plain 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 hydri9ul Ic and appearance <br />characteristIcs. They should be aesthetically designed to the <br />local environment and useful for local ..!!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 I Improvements at other I ocat Ions wi I I 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,maxImum <br />velocltyalO fps. <br /> <br />b. A maximum average desIgn v1!10clty 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 />'11,,2 <br />