Laserfiche WebLink
Seneca Coal Company <br />Seneca II-W Mine <br />Pond 016/016A Two-Pond System Sedimentology Demonstration <br />Seneca Coal Company (SCC) has revised the Pond 016/016A system SEDCAD4 demonstration to <br />update the volume of allowable sediment storage in each of the two ponds. With the cessation of <br />mining operations and subsequent revegetation of disturbed areas, the annual sediment yield to the <br />ponds will decrease significantly in future years. Consequently, it is not necessary to maintain the <br />excessive sediment storage capacity which was used to capture the large volumes of sediment <br />generated from disturbed and barren areas during coal resource removal operations. <br />The watershed area reflects a revised estimate of disturbed and undisturbed areas that changed with <br />the revised mine plan (PR-OS) and cessation of coal removal activities in January 2006. The <br />structure elevations are based on physical surveys. The stage/storage curve utilized data presented <br />by SCC in the as-built report approved as part of Technical Revision No. 40 (TR-40). <br />SEDCAD4 uses an Annual R value for a set number of storage years (Y) to determine if the pond <br />capacity is adequate to store the calculated sediment yield from the design storm. Previous <br />SEDCAD4 modeling exercises for the Pond 016/016A System utilized an Annual R factor of 30. <br />This value was better suited for barren soils, not reclaimed landscapes that are revegetated. In <br />addition, the storage year value (Y) was artificially set a one (1) year. Use of these inputs resulted <br />in high yearly sediment yields with minimal annual sediment storage capacity and caused SCC to <br />evaluate the need to clean the ponds on a yearly basis. <br />Pond 016A, which is the upstream pond in the two-pond sequence, is apre-settling basin. It is not <br />covered by SCC's Colorado Discharge Permit System (CDPS) industrial wastewater permit and, <br />consequently, there are no effluent limitations (e.g., settleable solids concentration) for the <br />discharge. The updated SEDCAD4 demonstration shows that the effluent from Pond 016, which is <br />covered by the CDPS permit, meets the requirements of Rule 4.05.2(7). In addition, the SEDCAD4 <br />demonstration shows that both ponds satisfy the operational criteria of Rule 4.05.9(2)(b) and Rule <br />4.05.9(2)(c)(iii). <br />The revised SEDCAD4 modeling uses an annual R-factor of 10. This value was obtained from <br />Figure 8A.1, Isolines of R factor for Western U.S. (see attached), which was taken from Design <br />Hydrology and Sedimentology for Small Catchment (Haan, et al., 1993). A sensitivity analysis was <br />conducted varying the number of storage years (Y) to maximize the sediment storage capacity while <br />assuring that the pond could store the sediment yield from the design storm and maintain <br />compliance with the applicable rules and regulations. <br />The number of storage years (Y) was increased in the sedimentology sub-routine until the effluent <br />settleable solids concentration in Pond 016 discharge exceeded the water quality criterion (0.5 <br />milliliters per liter [ml/1]), or the model indicated there was insufficient storage room in the pond for <br />the sediment yield from the design storm. As noted above, Pond 016A is apre-settling basin and is <br />not regulated under a CDPS permit. The storage years (Y) value was then set at the year preceding <br />TR-56 Preface -Page 2 Revised 09/06 <br />