Laserfiche WebLink
• Appendix Exh7-14H -Generalized Ditch Design Procedures, Permanent Channels <br />The Exhibit 7 summary text provides a discussion of the SEDCAD methodology used to design <br />both temporary and permanent channels. Additionally, the introductory text of Volume 2D <br />provides further information regarding suitable Curve Numbers, soil types, and other parameters <br />to use for the design of associated sediment ponds. All SEDCAD outputs predict a peak flow <br />rate for a given design event. <br />This predicted peak flow rate (for a given design storm) is the basis for a given channel design. <br />All channels are designed to meet the requirements of Rule 4.05.3 or Rule 4.05.4 as appropriate. Rule <br />4.05.3 is for drainage areas less than one square mile and requires a minimum design event based on the <br />10 year storm. Rule 4.05.4 is for drainages in excess of one square mile and requires a 100 year, 24 hour <br />event. <br />All future ditch design channels will be trapezoidal in cross-section with 3H:1 V sideslopes. A <br />minimum completed depth of 3 feet will be used, or deeper if required to maintain 1.0 feet of <br />freeboard under the design event. Riprap materials of a specified mean (D50) dimension will be <br />used where indicated. <br />As described in the Exhibit 7 summary text, SEDCAD contains a separate routine to calculate <br />the appropriate riprap diameter. At least two methods are provided, namely the OSM/Simons <br />method and the PARER method. Interestingly, for a given channel cross-section and channel <br />slope, the resultant riprap diameter is solely a function of the flow rate. To simplify future <br />design, the Colowyo Mine has chosen to use the more conservative OSM/Simons method since <br />the PARER method predicts a smaller diameter riprap specification. Further, the mine has use <br />the results of multiple runs of this routine to create a graphical approach for most design <br />situations. <br />Figure 1 provides the results fora 6 foot ditch width, Figure 2 is fora 10 foot width, Figure 3 is <br />fora 12 foot width, and Figure 4 is fora 20 foot ditch width. These range of widths is expected <br />to cover all anticipated design conditions. In general, most ditches will use a 12 foot bottom <br />width (Figure 3) for ease of construction. Note that separate lines are provided fora 10 percent <br />channel slope, a 20 percent channel slope, and a 30 percent slope. For a given peak flow <br />obtained from SEDCAD, there is a minimum D50 riprap diameter size. <br />Many of these runs have been provided following this text to support the results of the four <br />Figures. The attached Table 1 summarizes the data used to produce the four figures. <br />C: <br />Exh. 7-14H-2 Revision Date: 7/25/06 <br />Revision No.: TR-63 <br />