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Stephen F. Wathen <br />Page Two <br />December 30, 1991 <br />3). Ditch 5A-D3A has not yet been constructed. This revision is <br />a request to construct this ditch. Vegetation cannot be <br />established prior to construction. As previously discussed with <br />and approved by the Division, when ditches are initially <br />constructed they are closely monitored and repaired as necessary <br />prior to full vegetative establishment. There is an error on Map <br />26 which involves Ditch 5A-D3A and we would appreciate it if you <br />would correct it. The 5A-D3C label on the upper portion of that <br />ditch immediately north of the rock crusher site should read 5A- <br />D3A. <br />4). This curve number is taken from the SEDCAD manual for <br />sagebrush (with grass understory) in fair condition (hydrologic <br />soil group B). CEC has demonstrated in the past that the sagebrush <br />and mountain shrub communities are in fair to good condition (see <br />vegetation information in permit). Furthermore, CEC has been <br />questioned about this in more recent revisions and CMLRD has <br />accepted this curve number. <br />5). The SEDCAD manual indicates that a control practice factor of <br />.05 is representative of a gravel cover, i.e. rock protection, that <br />dissipates the energy of raindrop impact. Given the large portion <br />of the disturbed area covered by gravel, rock and road base the <br />control practice factor of .6 is quite conservative. <br />The SEDCAD manual shows that a control practice factor of .20 is <br />representative of 20$ grass cover. CEC has assumed only l0$ or <br />less cover on the vegetated portions of the watershed tributary to <br />Pond 5A-P1 when much of the vegetated area is in excess of this and <br />even contains a significant number of shrubs. So, again CEC has <br />been conservative in using a control practice of .35. <br />6). The 5A-P1 Pond is an approved existing certified pond which <br />CEC is r~o~ proposing to alter. We are just demonstrating that it <br />can contain the run off from the additional acres associated with <br />the proposed SA-D3 Ditch. The trickle tube is constructed of cast <br />iron. The design Mannings value for this material from the SEDCAD <br />model is .014. I inadvertently used the minimum value for cast <br />iron of .011. However, there is r4 significant impact on the <br />performance of the pond from this change as can be readily <br />demonstrated by changing this one parameter in the SEDCAD model and <br />rerunning the program. <br />Please contact me if there are questions. <br />Sincerely, <br />~~~~ ~~ <br />Terrell K. o son <br />Sr. Environm ntal Engineer <br />vld <br />