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volume of direct runoff. The design of a typical contour furrow <br />together with the appropriate terminology is depicted on Figure 28-2 <br />Contour Furrow Terminology. By considering the Drainage area of a unit <br />length of contour furrow, the following calculation indicates runoff <br />should be less than the drainage area. <br />6WD~R <br />L <br /> <br />• <br />Where: <br />W = Plax. saturated width of furrow depression (ft.) <br />D = hlax. saturated depth of furrow depression (ft.) <br />L = Average distance between contour furrows (ft.) <br />R = Runoff from design event (in.) <br />When contour furrows are inscribed, the distance between furrows <br />can be kept fairly uniform. For reclaimed slopes less than 10% contour <br />furrow separation of 15-18 feet is recommended; for slopes in excess of <br />10%, an 5-10 ft. distance between furrows is the working guide. <br />Using these distances, furrow dimensions for both slope conditions <br />should be 1.C-1.5 feet deep and 1.5-2.0 feet wide at the point of <br />maximum saturation. These dimensions allow about 0.5 foot of <br />freeboard for furrow dams that are close to horizontal. If the <br />distance between furrows is increased, the depth and width should be <br />increased accordingly. <br />The distance between bridges in individual contour furrows is <br />based on the expected maximum variation of the furrow slope from a true <br />horizontal. It is not possible to cut the furrows er.actly along a <br />horizontal topographic "contour"; however, a maximum slope of 5% is <br />acceptable. bridges prevent significant flow along the furrow and are <br />spaced so that over-topping of the dams and bridges at the down-slope <br />parts of the furrow does not occur. In order to achieve this, bridges <br />should be spaced at least every 25 feet. If the slope of individual <br />furrows is expected to exceed 5%, the furrow depth should be increased <br />or the distance between bridges reduced accordingly. A summary of <br />contour furrow construction guidelines is shown in Table c8-2, Contour <br />Furrow Construction Guidelines. <br />"Lti-12 <br /> <br />