Laserfiche WebLink
Appendix Exh7 -14I - East Taylor Ditch <br />The East Taylor Ditch is located in Sections 4 and 9 of T3N, R93W. Although the <br />upstream end of the ditch is in Section 9, the watershed that it drains extends about a half <br />mile southward to the watershed divide near the center of Section 16. The alignment of <br />the East Taylor Ditch is shown on Map 41 and on Figures 14C -1 and 14C -2 in the <br />discussion of the Taylor Tributary Ditch in Appendix 14C of this Item 14. The design <br />profile of the East Taylor Ditch is provided on Map 33A. <br />The total drainage area reporting to the ditch is 399.2 acres. The ditch is designed to <br />meet the requirements of Rule 4.05.3 for permanent drainages less than one square mile. <br />This requires that the channel be capable of handling flows from a 10 year, 24 hour event. <br />As shown on the profile, the steepest portion of the channel, which is the lower portion, <br />has a maximum slope of about 22 percent. Much of the ditch in this armored reach is less <br />than 22 %. Utilizing the approximate average slope (12.25 %) in the model does not <br />require increasing the width or depth of the currently constructed ditch. The existing <br />ditch dimensions are significantly larger than the minimum required, regardless of the <br />slope utilized in the model. The middle reach of the ditch is a relatively flat <br />(approximately 2% slope) vegetated channel. The upper armored reach of the ditch has a <br />moderate slope of approximately 8.3 %. The overall length of the channel is about 8,185 <br />feet. <br />The assumed hydrologic condition is the post- mining condition wherein the entire <br />watershed is reporting to the lowest portion of the ditch, and for which the density of <br />vegetation will vary depending on the time since the particular area has been mined. As <br />shown on the attached SEDCAD printout, the design storm produces a peak discharge of <br />about 47.2 cfs and a total runoff volume of about 7.3 acre -feet. This design event is <br />based on a Curve Numbers of 62 for the lower (northern) portions of the watershed that <br />will have 3 or more years of revegetation growth, and 80 for the upper portion of the <br />watershed which are assumed to have only 1 to 2 years of revegetation growth, as <br />adopted pursuant to TR -95. That upper portion of the watershed comprises about 56% of <br />the total watershed. <br />For ease of construction, a trapezoidal channel cross- section 12 feet in width will be <br />employed for the rip -rap lined portions of the ditch. Channel side slopes will be 3H:1 V <br />slopes and a minimum depth of 4.0 feet will be used for all sections. Riprap size has <br />been selected based on the SEDCAD modeling as described in the Introductory Text in <br />this Exhibit 7 Item 14, pages Exh. 7 -14 -1 through 3. For this ditch a minimum riprap size <br />of 18- inches has been selected which exceeds the minimum size of 9- inches calculated in <br />SEDCAD. For the flat portion of the channel, the ditch bottom is widened to 20 feet, and <br />a vegetative liner is adequate for the design velocities, as presented in the attached <br />SEDCAD printout. <br />Colowyo has also made a determination that the long -term stability of the lower section <br />of riprap channel in the East Taylor Ditch would be enhanced if peak flow was reduced <br />Exh.7 -14I -2 Revision Date: 12/17/14 <br />Revision No.: TR -104 <br />