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2~~a <br />It is not possible to cut the furrows ex t along a horizontal topographic <br />"contour"; however, a maximum slope of 5 is acceptable. Bridges prevent <br />• significant flow along the furrow and ar_ paced so that over-topping of the <br />dams and bridges at the down-slope parts of the furrow does not occur. In <br />order to achieve this, bridges should be spaced at least every 25 feet. If <br />the slope of individual furrows is expected to exceed 5%, the furrow depth <br />should be increased or the distance between bridges reduced accordingly. A <br />summary of contour furrow construction guidelines is shown in Table 24-1, <br />Contour Furrow Construction Guidelines. <br />Guidelines for Diversion Ditch Construction <br />The construction of the diversion ditches will follow the guidelines as <br />presented below. <br />The design of diversion ditches will follow methodology outlined in <br />"Procedures for Determining Peak Flows in Colorado" (SCS, 1977). The <br />generalized guidelines are given below. <br />• Typically, the channels will be designed to accommodate the peak runoff <br />flow frcm the 10-year, 24-hour precipitation event. Normal antecedent <br />moisture conditions (AMC-II) will usually be assumed for these <br />calculations. Curve numbers (CN) will be assigned to the drainage area <br />based on guidelines outlined in the SCS publication. If several distinct <br />surface conditions exist, then appropriate values will be assigned to each <br />area and the total flows based on area-weighted calculations. <br />Once the design peak flows for the channel are calculated, its <br />dimensions will be designed based on Manning's equation: <br />V = Q = 1.486 R2~3 S: <br />A n <br />Where: <br />V = flow velocity in feet per second. <br />Q = discharge in cubic feet per second. <br />• <br />24-8 <br /> <br />~~ <br />