2018-01-12_REVISION - M2000041 (32)

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Mr. Elliott R. Russell January 4, 2018 Page 2 capacity of the spillway is appropriately sized to convey the 100 year flood event. The 60 foot wide spillway is centered on an approximate 100-foot wide drainage way. No armoring is proposed on the 20 feet on each end of the spillway. If flood waters exceed the size of the concrete portion of the spillway, these edges will likely erode and compromise the spillway. Please demonstrate, in accordance with 3.1.6(3), all portions of the inflow/outflow path will remain stable during the 100 year flood event, including the 20 feet of unarmored slopes on each side of the spillway. The spillway design described in DRMS Comment No. 17, which consisted of an earthen channel with a concrete crest wall,has been revised. The December 2017 revised plan per your November 15, 2017 comments are attached as Appendix A. The revised spillway design consists of a roller compacted concrete(RCC) rundown channel that is trapezoidal in shape with a bottom width of 20 feet. At the top of the rundown, the concrete will extend across the entire trapezoidal section with side slopes of 10:1 (horizontal to vertical) and a depth of 4 feet,resulting in a total concrete structure width of 100 feet. The side slopes gradually transition to 3:1 at the toe of the rundown with a depth of 2 feet for a total concrete structure width of 32 feet. Concrete cutoff walls will be constructed at the top,midpoint, and toe of the rundown slope to protect the structure from scour as well as provide structural stability. The spillway is more than adequately sized to convey the runoff from the direct precipitation of a 100-year event due to the minor amount of contributing drainage area beyond the limits of the reservoir cell. However,the spillway is not sized to convey the full 100-year flood event of the Arkansas River because the reservoir is below- grade, and the base flood elevations exceed the south and east edges of the reservoir by several feet. Rather,the bi-directional spillway is sized to allow the reservoir to fill to equilibrium in a reasonable amount of time as the Arkansas River flood waters rise. At an Arkansas River flow rate of 8,000 cfs(about a 5-year event), the flood waters will begin to enter the reservoir through the spillway channel. The 1,400 cfs bank-full capacity of the inflow spillway structure will occur when the Arkansas River flood waters rise an additional 4 feet to approximately 13,000 cfs(about a 10-year event). Assuming an average flow rate of 700 cfs over the spillway's operating range, it would take approximately 28 hours to fill the reservoir from empty to the normal water line (total of 1,620 acre-feet). This is a reasonable amount of time to fill the reservoir considering the slow-rising flood hydrograph that is typical in this section of the Arkansas River downstream of the attenuation provided by Pueblo Reservoir. The new spillway design is shown on Sheet 11 of the revised plan set. A technical specification for Roller Compacted Concrete(RCC)has been added to the technical specifications attached in Appendix B. The spillway HEC-RAS model is also provided with this submittal.