2021-07-07_REVISION - M1997058 (4)

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CIVIL RES"DURCES, LLC ENGINEERS & PLANNERS June 29, 2021 Colorado Department of Natural Resources Dam Safety Branch Mr. John Batka, P.E. 810 9th Street, Suite 200 Greeley, CO 80631 RE: Public Hazard Evaluation—Walker Reservoir Shallow Fill Area Dear Mr. Batka: Hall-Irwin Corporation has identified a small area of the existing Walker North Reservoir perimeter that appears to have been constructed above natural grade and has requested that Civil Resources, LLC. provide an evaluation to determine the public hazard presented by a potential breach of this facility or mitigative measures that could reduce the same. The Site is located immediately north of Adams-Weld County Line referred to as Baseline Road. The Site was reclaimed as a below grade water storage reservoir consistent with the approved plans submitted to the Department of Mine Reclamation & Safety. The storage is composed of two cells that are physically separated by a demising berm with an approximate crest width of 23 feet and approximate elevation of 4948 feet. Walker North has a 35.1 acre surface area at maximum water surface of 4946 feet and a total stored volume of 170.2 acre-feet above a natural low-point of 4941 feet. Walker South has an area of approximately 63.6 acres and a total stored volume of 308.2 acre-feet above elevation 4941 feet. The slurry wall on the north side of the North Pit was constructed within a berm with a minimum crest elevation of 4947.0 feet and the reported maximum water storage elevation of the reservoir of 4946.0 feet. See Figure 1. The as-built survey(Survey Systems 2008 and 2020, Attachment A) of the reservoir indicates that the subject north bank fill area currently has a minimum elevation of 4946.12 feet and the east side of the reservoir reports an elevation of 4945.83 feet resulting in limited freeboard. Flood Condition Release A detailed floodplain study was completed for this reach of the South Platte and approved by FEMA. Refer to Figure 2. The mapping indicates that the Site is in the floodway as recently redefined with 100-year water surface elevation ranging from 4945 feet on the downstream end to 4947.1 feet on the upstream end of Walker North. - Walker North reservoir is shown to be located within the floodway, however, erosive force in this area is unlikely due to Baseline Road significantly constricting left overbank flows immediately upstream of the reservoir. Further, the flow depths are less than one foot adjacent to the reservoir. - The flood waters will enter Walker North via a shallow side inflow over a wide section of the east bank of the reservoir across natural grade as the flood level rises and the water will recede to the river in a similar manner when the flood level subsides. A proposed two-foot deep by 1-foot wide concrete grade control wall will be set at this location to define the spillway to be maintained. The spillway will be approximately 100 feet wide and can convey approximately 90 cubic feet per second (cfs)of flow at six inches of overtopping depth. The spillway would also provide relief of over-fill flows should the inflow ditch not be shut off when the reservoir is full. 8308 COLORADO BLVD, SUITE 200• FIRESTONE, COLORADO 80504 9 PHONE: 303.833.1416