REV04364 - Laserfiche WebLink

Home Browse Search

February 1998 Hayden Gulch Mane • Phase 1 & 3 Band Release Apy[ication ~ Pape 10 HGCC performed a SEDCAD+3 (SEDCAD) (Civil Sohware Design, 1992) analysis in conjunction with this Phase II/Phase III bond release application for both Pond 003 and Pond HG-8 to demonstrate that the ponds fulfill the intenttons of Rule 4.05.9. Results of the SEDCAD modeling for both the 10-yeaz, 24hour and 25-year, 24-hour storm events which were used to demonstrate chat the ponds meet the requirements of Rule 4.05.9 are presented in Exhibit 6, Permanent Impoundment Information. Ponds 003 and HG-8 were constructed as technology-based sediment control structures. With the successful establishment of the vegetative cover over the reclaimed area, vegetation has replaced the sediment ponds as the primary means of controlling erosion and sedimentation. Per Rule 4.05.2(2), "Sediment ponds ... shall be maintained until ... the disturbed azea has been revegetated and stabilized, the untreated drainage from the disturbed area ceases to contribute additional suspended solids above natural conditions, and the quality of the untreated drainage from the disturbed azea meets the State and Federal water quality standard requirements applicable ... for receiving streams." To demonstrate chat the ponds are no longer required as sediment control strucrures, the SEDCAD sedimentology routine was used to predict sediment loading off the reclaimed azea. SEDCAD utilizes the runoff coefficient (curve number) and revised universal soil loss equation (RUBLE) co calculated runoff and sediment loading. Per the requirements of HGCC's Colorado Discharge Permit System (CDPS) permit, Permit No. COG-850018, the settleable solids limitation for pond discharge is 0.5 milliliter per liter (ml/I). The results of the SEDCAD modeling indicates that the sediment loading in runoff from the reclaimed areas is less than 0.5 ml/1. Table 11, Summary of Design Storm Responses on the Reclaimed Watersheds, presents predicted 10-yeaz and 25-year, 24 hour design storm hydrologic responses including sediment loading, peak dischazge, and runoff volumes. The SEDCAD modeling was also used co assess the emergency spillway design for both Pond 003 and Pond HG-B. As demonstrated in the SEDCAD analyses, neither pond discharges through the emergency spillway as the result of the 25- year, 24-hour storm event. Table I1 also presents spillway specifications and water surface elevations. In addition to the SEDCAD modeling, water quality monitoring performed by HGCC since 1986 indicates that waters in both ponds are suitable for their intended uses and that discharges from the ponds meet applicable water quality scandazds. The approved post-mining land uses at the Hayden Gulch mine aze livestock grazing and wildlife habitat. Observations recorded by HGCC and DMG personnel during site inspections indicate that livestock and wildlife are using the ponds as a water source and that they have been occupied by waterfowl, muskrat and beaver. Hayden Gulch waters are rued for agricultural purposes. HGCC completed an Hydrologic Impact Analysis (HIA) for the Hayden Gulch Mine in conjunction with this Phase II/Phase III bond release application. The HIA, which is presented in Exhibit 7, indicates that constituent concentrations in discharge Montgomtry Watson Americas (dba TenaMatrixJ'PO. Bax 7740/8 Steamboat Springs, Colorado 80477 ' (970) 879~6I60 m ~aniu..1 c ~ Bmd Admnpiiuim\Tm~Av l (.J Bmd Pdeuenpplr~nm /tW! J.