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<br />(e <br /> <br />5. Standing waves over channel bed antidunes. <br />6. Local hydraulic jumps caused by structures or debris. <br /> <br />The HEC-2 program models gradually varied flow conditions and does not account <br /> <br />for possible rapidly varied flow conditions. Rapidly varied flow conditions <br /> <br />and variations in depth are highly indeterminant in high velocity natural <br /> <br />channels. Therefore, a conservative freeboard (distance between calculated <br /> <br />water surface and top of levee) was used. <br /> <br />To provide a conservative degree of protection for the water treatment <br /> <br />plant facilities, a total freeboard of 6.7-feet was used. This freeboard <br /> <br />would protect the levee from overtopping in the unlikely event that some <br />" <br />natural occurrence would cause all the energy of the flowing water to be .". <br />\ <br />converted to water depth at the levee. ," q <br /> <br />v) "r~ <br /> <br />r- The recommended depth of scour protection is five-feet based on the <br />, following considerations. Based on the duration of the storm event and the <br /> <br />... <br />I <br /> <br />Shape, of the hydrograph reported by the U. S. Army Corps of Engi neers (Ref. <br /> <br />8), the IOO-year storm event will be of a very short duration. High velocity <br /> <br />erosive flows are estimated to last less than one hour. Based on the test, <br /> <br />borings, the fill is composed of gravel and cobble size fragments and it is <br />estimated that the short duration of the high flow rate will not erode this <br /> <br />material below five feet below the stream bed. <br /> <br />An additional safety factor is provided in the 20-foot minimum width of <br /> <br />levee between the channel and the Water Treatment Plant facilities. <br /> <br />5-5 <br />