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• <br />This will prevent an accumulation of a torrent of water within the road <br />ditches. Riprap was used at the exit of all ditch relief drains to prevent <br />erosion. <br />All of the hyraulic features of the haul road were sized to safe]y pass at <br />least the peak runoff resulting from a 10-year, 24-hour precipitation event. <br />The drainage ditch along the Pit No. 4 haul road has the capacity to carry <br />100 cfs, several times larger than the largest drainage area for individual <br />cross-culverts. If cross-culverts were not installed, the drainage ditch has <br />sufficient capacity to carry all the 10-year runoff from the tributary areas. <br />Erosion along the bottom of the drainage ditch should not be a problem since <br />it was cut into fresh sandstone. <br />The hydrologic and hydraulic calculations have been summarized below. The <br />hydrologic calculations follow the recommendations of the USGS (1) and the SCS <br />(2). The drainage basin map is presented on Figure 6. Typical culvert sizes <br />and arrangements are shown on Figures 2 and 3 for the haul road to Pit 4. <br />(1) Technical Manual No. 1, Manual for Estimating Flood Characteristics <br />of Natural Flow Streams in Colorado, C~lerado Water Conservation Board <br />- USGS, 1967, by McCain and Jarrett, <br />(2) Procedures for determining peak flows in Colorado, Soil Conservation <br />Service, U. S. Department of Agriculture, March, 1977. <br />• <br />As ditches are excavated during development of the pit, steep sections <br />will be identified. These sections will be inspected periodically by mine <br />personnel. If erosion is substantially altering the dimensions and/or flow <br />conditions in the ditch, the ditch will be enlarged and rock riprap will be <br />added to prevent further erosion, <br />The parameters and equations used to compute peak flows are as follows: <br />Culverts sized using "Procedures for Determining Peak Flows in Colorado" <br />a. Used 10-year, 24-hour storm <br />b. Used steep slope conditions <br />c. CN = 70 for undisturbed areas <br />d. CN = 86 for disturbed areas <br />e. The 12" diameter culverts are sized for a maximum local drainage basin <br />of 31 acres. Some culverts are oversized because of a smaller basin <br />area, however, 12" diameter was smallest size used for maintenance <br />purposes. <br />• 12/87 III-13 <br />