2001-05-16_REVISION - M2000158 - Laserfiche WebLink

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Memo to Erica Crosbv 2 May 15, 2001 published computational methods available to determine the sizing and gradation of riprap required to armor against erosive flows of different velocities. Depth and velocity of floodwater increases exponentially in flood events of increasingly lower probability of occurrence. The ten-percent annual chance flood (the so-called ten-year flood) results in a substantially lower flow, depth, and velocity than aone-percent annual chance flood (the 100-year return period event). Designs for armoring against erosion during a 100-year flood would most likely fail during a much larger, but longer return period event. For this reason, a design flood must be selected were it is recognized that the erosion controls installed are only intended to be effective up to the flow rate associated with the design event. In order to provide a standard national procedure, the one-percent annual chance flood has been adopted by the Federal Emergency Management Agency as the base flood for purposes of floodplain management. This standard is the basis upon which the Division conducts many of its flood impact evaluations, consistent with nationally recommended and established procedures. Extreme flood events with much greater magnitude than the one-percent annual chance flood can and do occur, but with a lower annualized probability of occurrence. This is a significant complicating factor for flood plain management decisions in that there will always be residual losses from extreme flood events above and beyond those for which mitigation is cost effective. In a submittal dated April 6, 2001, the Applicant estimated that 100-year floodwater velocity at [he south end of the proposed S&H Mine will be from 1.5 [0 2.0 fee[ per second up [0 3.6 feet per second at the north end of the site. Increased flows and velocity at the north end are due to the confluence of the South Platte River and St. Vrain Creek occurring within the mine reach. As stated by the Applicant in the April 6, 2001 letter, velocities of this order are not likely to cause erosion. However, the velocity estimates of the Applicant are on a macroscopic scale, and can be considered an average velocity of the large volume of water that would be flowing through the mine reach during a 100-year flood. On a smaller scale, and in isolated locations, velocities will be much higher than the overall average, and will cause scouring if armor is not installed. It is the potential for isolated high velocity flow that is the impetus for the Applicant's riprap proposal. The TR application indicates that the data and estimates the Applicant provided were derived from the South Platte River Basin Flood Study prepared by the U.S. Army Corps of Engineers, Omaha District, dated April 1977. The Division requests that the Applicant explain how the results of this study were applied to the location of the S&H Mine Property, how the estimated flow rates, flow depths, and velocities for the south end and north end of the site were derived, and provide the maximum flow depths anticipated within the permit boundaries of the proposed mining operation. Copies of pertinent sections of the 1977 study should be provided in support of the Applicant's conclusions. The locations within the proposed S&H Mine reach that are likely to be subject to erosive flows during a flood are the outside edge of sharp bends in the river channel, and the steep gravel pit slopes at locations where the floodwater will spill into the pits. Recognizing this, the Applicant has identified areas of the pit slopes to be riprapped that are along the south and west perimeters of the proposed pits. and has suggested that riprapping may be necessary at as yet undetermined locations along the entire