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