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~ u • INTRODUCTION Water Engineering & Technology, Inc. (WET) has evaluated the hydrologic response of Ponds A, D, and G at the Eckman Park Mine. This reevaluation was prompted by review comments that you received from the Colorado Mined land Reclamation Division (MLRD) in a letter dated 9 February 1989. The following sections describe the work that we performed and the procedures and/or assumptions, used in these analyses. It should be emphasized that certain components of this analyses rely solely on information available from existing as-built plans and spec cations for the ponds. In particular, configurations for the principal outlet works, including riser elevations and sizes, pipe lengths and flow line elevations, as well as stage- capacity curves for each pond were obtained from the certified as-built drawings. Our work has not included field verification or surveying of any of these facilRies. HYDROLOGY General • Drainage areas for the contributing areas to each of Ponds A, D, and G were digitized using 1'=1000' topographic mapping. The drainage areas are shown in Attachment 1. Drainage areas for Ponds A, D and G are 3370, 918 and 440 acres, respectively. Since the drainage area for Pond A is within 2 percent of the value used in previous hydrologic analyses, the inflow hydrographs for this pond were not re-evaluated. The drainage areas for Ponds D and G differ more appreciably from values used in previous analyses. Consequently, hydrologic analyses were conducted for these drainage basins using procedures provided in Peak Flows in Colorado and unit hydrograph procedures provided in National Enoineerino Handbook 4 - Hvdroloov. Hydrologic routing of the flow through each pond was performed using the Corps of Engineers HEC-1 model. The stage-storage curves were dbtained by digRizing values from the as-built drawings. Discharge through the principal outlet works was determined using a Lotus spreadsheet model that we developed using procedures outlined by Jachens (1989) in a paper titled Computer Aided Pioe Droo Design. The Lotus model considers the controlling factors of flow through the riser perforations, weir flow over the riser, orifice Bow in the riser, orifice flow in the outlet pipe and full pipe flow in the outlet pipe. Tailwater effects are considered in the full pipe flow calculations. For all of the computations provided here, it was assumed that the tailwater elevation equalled the top of the downstream end of the outlet pipe. Flow out the emergency spillways was computed using a weir equation with a discharge coefficient • of 2.8. Results were combined with the flowrates through the principal outlet works to develop stage- Water Engineering & Technology, Inc.