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Guidelines for Contour Furrow Construction <br />• The frequency and depth of contour furrows and the frequency of <br />check bridges in individual furrows is determined primarily by: <br />1. Pfagnitude of runoff from design precipitation event. <br />2. Topographic slope. <br />3. Drainage area above the region to be contour furrowed. <br />The principle of contour furrowing is to increase depression <br />storage to the extent that the runor'f from the drainage area may be <br />completely contained. Eventually, vegetation will become established <br />and the contour furrows will become less prominent. As vegetative <br />growth increases and thus provides greater soil stability, the <br />importance of contour furrows for the control of erosion and <br />sheetflow will decrease. <br />The design event used for this guideline is the 10-year, 24-hour <br />precipitation event. According to the National Weather Service <br />• publication "NOAA" Atlas 2, "Precipitation - Frequency Atlas of the <br />Western United States", the precipitation from a regional 10-year <br />24-hour event is 1.8 inches. As the runoff from this event would <br />probably exceed most snowmelt runoff events in the area, it is an <br />appropriate design criteria. kunoff volumes from the design event were <br />estimated using the techniques outlined in "Procedures for Determining <br />Peak Flows in Colorado" (SCS, i~77). For this analysis, standard <br />antecedent moisture conditions were assumed; thus, not all the <br />precipitation falling on the reclaimed area contributes to runoff. A <br />curve number of 82 was used in the calculations which is applicable for <br />disturbed areas. As the slope of the reclaimed surface does affect the <br />amount of expected runoff and, consequently, contour furrow <br />requirements, variability of slopes must be considered. The <br />construction modifications due to slope variability is discussed below. <br />The calculation of required depression storage is fairly straight <br />forward. For this analysis, it is assumed that runoff from undisturbed <br />• areas or previously reclaimed areas will be diverted away from the area <br />to be contour furrowed. To contain the runoff from the 10-year 24-hour <br />event, the depressicn storage volume must be at least equal to the <br />28-11 <br />