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that "the total ditch depth equals the required flow depth plus 0.3 feet of <br />freeboard rounded to the nearest half foot," as specifications in the permit <br />application require (see Illustration 5 of the permit application). <br />Precipitation data for calculating peak flows was taken from U.S. Weather <br />Bureau maps included as Exhibit 2-3 in the SCS Engineering Field Manual. This <br />was checked and found to be acceptable. Curve Numbers (CN) varied from 81 to <br />89, which is conservative but reasonable. The drainage areas for ditches were <br />checked and found to be accurately measured by the applicant. <br />The proposed ditches at the Refuse Disposal Area will route runoff around ref- <br />use, topsoil and non-toxic fill (see Map 71 of the permit application). The <br />ditches are V-shaped, with side slopes, of 2h:ly and were designed using Man- <br />ning's Equation. Flow velocities will be less than four feet per second in <br />grass-lied ditches, and riprap will be used to line .those ditches with higher <br />velocities and a slope greater than 1.2 percent. This was adhered to in every <br />case but the upper section of the east refuse ditch in subarea R4 where the <br />ditch, at slope 1.4 percent, and velocity 4.09 fps will be lined with grass. <br />However, a check of the ditch size using the 10 year-24 hour discharge revealed <br />that the design is conservative and erosional problems should not develop. <br />A check of the calculations used ,to size the Refuse Disposal Area ditches indi- <br />cated that, in three cases, the lower ditch segments were sized with slopes <br />that were steeper than those shown on Map 77. In only one of these cases, the <br />subarea R4 refuse ditch, will a more gradual slope (20 percent used in calcula- <br />tions, 3.7 percent shown on map) decrease the actual ditch capacity to below <br />that of a 10 year-24 hour flow. It is required that the applicant re-size the <br />lower segment of this ditch to provide the required capacity. In addition, Map <br />77 should be corrected to illustrate the 24-inch diameter, 110-foot long cul- <br />vert which allows flow from the non-toxic fill stockpile ditch to cross the <br />haul road and discharge into the subarea R2-3 sedimentation pond (see "Stipula- <br />tions" in this section of the TEA). <br />Ditches in the D-Portal area are designed for two purposes (see Maps 24-27 of <br />the permit application). The north cut-off and the northwest cut-off ditches <br />will divert drainage above the D-Portal area into Scullion Gulch. These flows <br />14 <br />