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be an error. Please review the inputs in Table 4 and correct as necessary. <br />Response: Table 4 has been revised and is resubmitted. Comment 18 n also resulted in <br />changes to this table. All of the SEDCAD modeling for this pond has been resubmitted. In <br />addition to correcting the riser height, a small change was made to the emergency spillway <br />elevation to match the as -built drawing. <br />n. In Exhibit 7, Item 20, Part C there appears to be a discrepancy between the curve number <br />shown on the map and in the SEDCAD model. The map shows a CN of 74 and the SEDCAD model <br />shows a CN of 77. Please review these curve numbers and correct the one that is in error. <br />Response: The SEDCAD models were corrected from CN 77 to CN 74 and re -run. <br />Revised SEDCAD output is attached. As expected, the runoff volumes did decrease <br />slightly in both case, but due to the small areas involved the reduction was less than <br />1%. All text associated with Exhibit 7-20 Part C is enclosed. The four figures are <br />unchanged and not submitted. The two postmining SEDCAD models were not revised <br />but have been repaginated. <br />In addition, we have also made corresponding changes to Table Exh. 71tem 20, Table <br />4. <br />o. Please explain why all subwatershed lengths in Exhibit 7, Item 20, Part C are limited <br />to 200 feet (as shown in sedimentology detail). <br />Response: This applies to the sedimentation runs for the West Taylor Pond in Exh. 7, Item 20, <br />Part C in Volume 13. The "Length" term used in the Subwatershed Sedimentology Details is <br />not the subwatershed's actual hydraulic length. The actual hydraulic length is used as input into <br />the SEDCAD model in the hydrology data, and is needed along with the elevation drop within a <br />subwatershed to arrive at the "Time of Concentration" (shown in column "Time of Conc. " in the <br />Subwatershed Hydrology Detail table). For runoff volume and flow rate calculations, the <br />hydraulic length and elevation drop for each individual subwatershed are measured on the <br />AutoCAD map and the data are input into the data file. However, SEDCAD does not report <br />back this input, only the resulting "Time of Concentration" in the Subwatershed Hydrology Detail <br />table. <br />Unlike the "Hydraulic Length" term used in the above described hydraulic calculations, the <br />"Length" term shown in the Subwatershed Sedimentology Detail is not a physical distance <br />measured on the ground, but is an abstract distance that represents the longest length on a <br />given slope that overland flow could travel without encountering a rill, rivulet, slope change, or <br />other item that would cause collection or concentration of the overland sheet flow. This term is <br />then combined with the slope (in percent) via a complex formula to arrive at Wischmeier's "LS" <br />factor for use in the Modified Universal Soil Loss Equation (MUSLE), which is the method <br />SEDCAD uses to estimate sediment erosion on a slope. In the February 2013 SEDCAD Short <br />Course given at Craig, Colorado, Dr. Warner (co-author and technical contributor to the <br />SEDCAD computer program) stated that this value should never be input as greater than 200 <br />feet because their research had shown that overland flow cannot survive farther than about 200 <br />feet without encountering a collection point. Accordingly, no change has been made to the <br />Page 8 <br />