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affect the amount of expected runoff and, consequently, contour furrow • requirements, variability of slopes must be considered. The construction modifications due to slope variability is discussed below. The calculation of required depression storage is fairly straight forward. For this analysis, it is assumed that runoff from undisturbed areas or previously reclaimed areas will be diverted away from the area to be contour furrowed. To contain the runoff from the 10-year 24-hour event, the depression storage volume must be at least equal to the volume of direct runoff. The design of a typical contour furrow together with the appropriate terminology is depicted on Figure 28-2 Contour Furrow Terminology. By considering the drainage area of a unit length of contour furrow, the following calculation indicates runoff should be less than the drainage area. 6 WD > R ~ ~~~~ L Where: • W = Max. saturated width of furrow depression (ft.) D = Max. saturated depth of furrow depression (ft.) L = Average distance between contour furrows (ft.) R = Runoff from design event (in.) When contour furrows are inscribed, the distance between furrows can be kept fairly uniform. For reclaimed slopes less than 10% contour furrow separation of 15-18 feet is recommended; for slopes in excess of 10%, and 8-10 ft. distance between furrows is the working guide. Using these distances, furrow dimensions for both slope conditions should be 1.0-1.5 feet deep and 1.5-2.0 feet wide at the point of maximum saturation. These dimensions allow about 0.5 foot of freeboard for furrow dams that are close to horizontal. If the distance between furrows is increased, the depth and width should be increased accordingly. The distance between bridges in individual contour furrows is based on the expected maximum variation of the furrow slope from a true horizontal. 24-7