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• <br />Appendix Exh7-14P -Small Area Exemption <br />The borrow site location is illustrated on Exhibit 7-14P, Figure 1. Development of the borrow is <br />needed to provide a source of embankment materials for the Section 21 Pond and the Section 28 <br />Pond. Sediment control for this site has been designed to meet the requirements of Rule 4.05.5. <br />In particular, Rule 4.05.2.3 describes small area exemptions and Rule 4.05.5 (2) (f) specifically <br />lists the types of measures to be used. In this case, a rock check dam will be used. Downstream, <br />an area of existing vegetation between the check dam and the West Fork of Good Spring Creek <br />provides further sediment control. This area was modeled as a vegetative filter. <br />The borrow site is located in Section 29, T3N, R93W. As shown in the design, the maximum <br />disturbance will be about 1.1 acres. The site is located at the toe of a large colluvial fan deposit. <br />Given the small size of the borrow site and its location relative to the remainder of the fan <br />deposit, this activity will not destabilize or otherwise endanger the overall stability of the slide <br />area located above. The uppermost point in the fan deposit forms a classic passive wedge <br />section which holds the creep surface in place. A shallow circular failure through this surface is <br />much more likely than a large failure through the entire fan. <br />An evaluation of the existing fan deposit shows that it is the result of one or more land slides . <br />• Limits of this slide area are shown on Map 7. <br />A 3.0 foot deep triangular ditch with 2H:1 V sideslopes shall be constructed along the crestline at <br />a design slope of 1.0 percent. This interceptor ditch will route undisturbed area drainage around <br />the borrow site both to the east and west. Total length of these ditches is about 280 feet. <br />Because the upstream drainage area is generally well vegetated, a curve number of 61 was <br />chosen as a conservative assumption. <br />The ditch is designed to meet the requirements of Rule 4.05.3 for drainages less than one square <br />mile. The drainage area of this basin is about 56 acres. A curve number of This undisturbed <br />area does not contribute any runoff under the 10 year-24 hour event. A hydrologic design for <br />this ditch under the 25 year-24 hour event is provided on the attached SEDCAD printout. The <br />ditch is designed for a peak flow of about 6 cfs. <br />The borrow site will be initially developed adjacent to the road and excavated northward as <br />needed. Upon completion of the borrow, all side slopes shall be left at 2H:1 V or less, and all <br />other areas shall be leveled such that grades are less that one percent. <br />As shown on the attached SEDCAD printout, the 10 year-24 hour design storm produces a peak <br />discharge of about 0.85 cfs and a total runoff volume of about 0.06 acre-feet. This design event <br />is based on a Curve Number of 56. As shown of the attached Exhibit 7-14P, Figure 2, a rock <br />filled check dam will be installed at the low point of the finished excavation. Further <br />• downstream, an existing stand of dense vegetation has been modeled as a vegetative filter. As <br />Exh. 7-14P-2 Revision Date: 9/04/07 <br />Revision No.: TR-68 <br />