2017-08-11_PERMIT FILE - C1981028 (2)

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13 Waters Earth 119101141411 August 31, 2016 Introduction The Coors Energy Company (CEC) Keenesburg Mine is conducting final reclamation activities. Water & Earth Technologies, Inc. (WET) has been contracted to respond to hydrologic comments stated in Colorado Division of Reclamation, Mining and Safety's (DRMS) June 13, 2016 letter to CEC. These comments are specifically concerned with the adequacy of sediment and diversion ditch sizing in relation to final reclamation stormwater drainage conditions using current climatological information during the 10 -year bonding period. WET has addressed these concerns using current climatological information to evaluate the adequacy of permanent ponds and ditches. The Technical Revision Drainage Control Plan II (TR28) was not available to WET for review, so a new analysis of the site has been completed. This document summarizes hydrologic and hydraulic analyses conducted by WET that demonstrate the permanent ponds and ditches at the Keenesburg Mine have adequate capacity. Background Information Climate information was determined from NOAA Atlas 14 which is the current rainfall atlas for Colorado (Appendix A). Peak discharge and runoff volumes were calculated using SEDCAD 4. The SEDCAD model was set up with two separate structures (watersheds): one for the Dugout Pond, and one for Sediment Pond 2 (Exhibit 1). Time of concentration and watershed delineation were determined from available mapping. Two -foot contour mapping was provided for the site, and USGS 10 -ft contour mapping was used to delineate the remainder of the watershed. Consistent with previous hydrologic studies, a hydrologic soil group A was assigned to the site. The site was modeled as a pasture in fair condition with a hydrologic soil group A which corresponds to a curve number of 49. Stage storage relationships for the two ponds were determined from the site topographic mapping. The spillway geometry for Sediment Pond 2 was available from the Sediment Pond 2 Details "As -Built" Drawing (dated 1/19/96). No as -built drawing was located for the Dugout Pond, thus the spillway geometry was determined directly from the topographic mapping and consultation with on-site personnel. Pond Capacity Analysis The measured capacity for the Dugout Pond is 10.9 ac -ft and the 25 -year, 24-hour storm produces only 2.6 ac -ft. In fact, the Dugout Pond was designed to completely contain a 100 -year event. Sediment Pond 2 has a measured capacity of 12.4 ac -ft and the 25 -year, 24-hour storm produces only 7.6 ac -ft. Therefore, both ponds can completely contain runoff resulting from the 25 -year, 24-hour storm. The ponds were modeled in SEDCAD 4.0 for the 25 -year, 24-hour storm with an assumed water surface elevation at the spillway elevation to ensure the spillway has adequate capacity to pass the 25 -year, 24- hour storm. The Sediment Pond 2 and Dugout Pond spillways have sufficient capacity to safely pass the 25 -year, 24-hour storm. Results of the spillway analysis are displayed in Table 1. The pond analyses are "hydrology only" demonstrations that do not take sediment storage into consideration. This methodology was previously accepted by the DNR and is documented in the August 15, 1994 letter from DNR to Industrial Compliance. The relevant excerpt from this document follows: "The Division recognizes that the demonstrations are for "Hydrology Only" and do not include the sediment loading factor. Since the site's surficial cover is 10-20 feet of topsand for the most part, including the ditch sides and slopes, the infiltration factor and lack of sediment load is justifiable. The Division approves of this assumption." Keenesburg Mine Hydrologic Analysis 1 of 2