REV13604 - Laserfiche WebLink

Home Browse Search

_ ~ File SL-01, Permit No. C81-010 ~ ' Trapper Mining Inc. 11/23/98 Page 3 "allowable" is a concern for A Pit RSA and C Pit RSC because the herbaceous percentage for each of these blocks includes abundant annuals. C Pit RSC has the most abundant annuals, 44.8%. Using the total herbaceous percentage does not consider that erosion control provided by vegetation on these two blocks could be seriously diminished if most of the annuals on these blocks did not return in a growing season. Use of the allowable percentage would be more appropriate for these blocks because the allowable percentage includes no more than 10% annuals. The Division has checked what impact use of the allowable percentage would have in Trapper's sedimentology demonstration. The Division ran SEDCAD+ for C Pit RSC using a post-mining C factor of 0.05 based on the block's 61.8% of allowable cover (compared to Trapper's run using C factor of .015 based on total herbaceous cover of 77.4% and reduced for contouring). The Division's run found that post-mining peak settleable solids from this block would still be less than the pre-mining yield. A copy of the SEDCAD+ printout is attached. 3. Limitations described in TR-55 of using runoff curve number method for predicting runoff. Page 2-11 of SCS TR-55 (1986) describes several limitations of the curve number method for determining runoff. The following two limitations described in TR-55 aze applicable in Trapper's sedimentology demonstration. a) TR-55 recommends that a procedure other than the CN method be used when the CN is less than 40. Trapper's predicted post-mining CN of 42 is very near the CN-40 limit; therefore, an alternative method would be useful at least as a check. b) TR-55 says the CN procedure is less accurate when runoff depth is less than 0.5 inch, and as a check, another procedure should be used to determine runoff. Trapper's predicted 0.01 acre-feet of runoff from 20 acres equates to 0.12 inch of runoff, considerably less than 0.5 inch. Thus, an independent check of the CN method is warranted. 4. Liroitation identified by CDMG of using runoff curve number method for predicting runoff on Trapper Mine. A third limitation of the CN method relates to the soil infiltration capability of Trapper's soils. Trapper initially set out to compaze pre- and post-mining sediment yields from its bond release blocks by running SEDCAD+ for pre- and post-mining conditions. Trapper used pre-mining CNs of 47 and 49 (from its permit), and apost-mining CN of 42 (selected in its bond release submittal). These CNs were too low, though, to generate runoff and sediment in the SEDCAD+ runs. The Division suggested that runoff, and thus sediment, would be generated in the computer nlns if it is assumed that the ground is wet prior to the onset of a 10-yeaz storm (wet antecedent conditions). Table 2.21 of Bazfield, Warner, and Haan (1981) provides CN conversion factors for coverting CNs from dry antecedent conditions to wet antecedent conditions. Trapper re-ran SEDCAD+ using the CNs converted for wet antecedent conditions.