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�11 <br /> l♦ <br /> hl <br /> �P <br /> ess than 2. 5 feet and the design peak discharge will occur during a period of <br /> my 15 minutes, or less, it would be impractical to design for the maximum <br /> ize of riprap required to resist displacement. For example, the design flood <br /> ischarge velocity for ditch section 4-E is 23 fps. At this design velocity, <br /> t would be necessary to place riprap in excess of 4 feet in diameter to prevent <br /> is displacement and the ditch would have to be excavated to at least a depth <br /> f 9 feet. Since the design peak flood depth for ditch 4-E is only 2. 5 feet, <br /> he use of riprap 4 feet in diameter is impractical from both an economic and <br /> construction standpoint. <br /> It is our opinion that riprap up to 18 inches in diameter will be <br /> :efficient to prevent undue erosion and scour due to normal seasonal runoff. <br /> f <br /> )wever, some maintenance of the ditches will be required should the design <br /> t ood occur. <br /> d <br /> Table 1 lists the hydraulic data and design specifications for all <br /> I <br /> iversion ditch sections outlined on sheet S-3. Refer to Figure II-6 for <br /> ,1 example diversion ditch cross section. A design free board distance <br /> 7tween the top of the bank of the diversion ditches and the maximum water <br /> irface and the maximum thickness of the riprap layer are included into <br /> pie overall design dimensions presented in Table 1 . <br /> ilverts <br /> Three 36-inch diameter circular steel culverts, placed at a minimum <br /> --adient of . 1 f t/ft, will be required to pass runoff from ditch sections <br /> 3--A, 4-A, and 4-E (one each) under State Highway 110. These culverts will <br /> sufficient to pass the peak discharge resulting from 1 .25 inches of <br /> i <br /> p ii 1 <br /> 'I <br /> 22 <br /> W <br />