2001-05-17_REVISION - M2000158 - Laserfiche WebLink

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M-2000-158 TR Memo 2 Mav 17, 2001 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 the south end of the proposed S&H Mine will be from 1.5 to 2.0 feet per second up to 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 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 riverbank west of the S&H Mine location. The areas to be armored under the Applicant's proposal appear to have been selected based on qualitative analysis of the river and floodplain geomorphology, as opposed to quantitative analysis of flood flows and velocities and related potential for scour. In general, it appears that the areas identified by the Applicant as requiring pit slope armor are the most likely locations for pit slope erosion during a flood. However, unless the Applicant can furtherjustify the extent of the proposed armoring through quantitative analysis, the Division will require that the following locations also be armored: • The south terminus of the Applicant's proposed armoring must be extended around [he groin of the pit slope at the southwest corner of Meadow Lake and continued over to the southeast corner of the lake. • The gas well pads and access road alignments for the wells in Sharkey's Lake must be armored around their entire perimeters including the groins in ttte pit slopes where the access roads join the lakeshore. • Any law spots, such as natural swales or abandoned tail water ditches, et cetera, that connect the proposed gravel pits and the river must be armored were the pi[ slope intersects the low spot. • Areas of existing or potentially unstable riverbank requiring the installation of riprap armor as identified by the applicant and verified by the Division. The pit slope armoring proposed in the TR as illustrated on a permit area map truncates abruptly in [he various reclaimed lakes where armor is to be installed. Also, based on the language in the TR pertaining to riverbank armoring, there may be a number of locations within the mine reach where riprapping will end and natural riverbank will resume. The Division's published guidance for riprap end protection recommends doubling the