Laserfiche WebLink
® HYMO MODEL DISCUSSION <br />In the Seneca II Permit Revision, Peabody committed to comparing reclaimed area runoff <br />data against HYMO model projections and presenting these results in the 1986 AHR. The <br />following discussion explains why this comparison is not feasible and in what context the <br />HYMO projections and monitoring data will be used. <br />The HYMO model was used to assess the impacts of mining on rates and volumes of runoff <br />from lands to be disturbed by mining. Model runs were made for both premining and <br />postmining conditions. The model runs were intended to provide only a qualitative <br />comparison of anticipated conditions and indicate whether significant, detrimental impacts <br />were likely. Because 1tYM0 was run for specific design rainfall amounts with 70-year <br />return periods, it is difficult to find any natural storm events with the same rainfall <br />amounts for runoff comparison purposes. The greatest natural rainfall amount recorded <br />since 1980 at the Seneca II Mine has been 1.40 inches. This is still considerably lower <br />than the 10-year, 24-hour amount (1.76 inches) used in the HYMO projections. <br />Additional problems for comparing actual versus simulated runoff exist at Ponds 002 and <br />003 where significant amounts of pond water are withdrawn for mining purposes. A <br />significant variable at all the ponds is the development of upgradient spoils springs. <br />HYMO does not have any subroutines that incorporate ground water flow components. The <br />NPDES outfall monitoring is expected to show the following scenario. The postmining <br />ground water flow component will result in increased annual runoff volumes. At the same <br />time, greater infiltration rates in the spoil will result in smaller overland runoff <br />volumes from individual storms with an additional (variable amount), but lagged ground <br />water flow volume in the form of spoil spring discharge. <br />Asa consequence of the above referenced factors, no monitoring is being planned to verify <br />the HYMO model runs, per se. Peabody feels that verification of the model has been <br />adequately described by Williams (1966), Williams (1969), and Williams and Hann (1972). <br />The thrust of the monitoring at the NPDES outfalls and sediment ponds is to substantiate <br />that the conclusions reached from the HYMO simulations (no significant detrimental impacts <br />to the local or regional hydrologic balance) were correct. Peabody will modify the text <br />on Page 7-77 of the Seneca II Permit and Page 7-174 of the Seneca II Permit Revision to <br />clearly reflect this monitoring approach. <br />G-1 <br />