2001-05-22_REVISION - M2000158 - Laserfiche WebLink

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ENVIRONMENTS INC. MAY 22, 2001 PAGE 2 in the areas where armoring is necessary, the gradation of the soil and subsoil (sand and gravel) will be examined along with vegeta- tion characteristics to determine if the erosive power of the flood will cut into the vegetation and soil. In those areas were it is determined that erosion will occur the operator will place riprap material, that cannot be lifted and moved by floodwater. This will prevent erosion and down cutting on those slopes subject to scour. As noted in the Division's review, we will use a Dso = 24 to fulfill this requirement. For this mine Pickett Engineering opted to use a 100-year event as the design criteria for the plan. This is consistent with the standard national procedures adopted by the Federal Emergency Manage- ment Agency as the base flood for purposes of floodplain management. PS6G recognizes that the erosion controls installed are only intended to be effective up to the flow rate associated with the design event. There may be residual losses from extreme flood events, above and beyond those for which mitigation is cost effective. As noted in the original flood plain information the 100-year floodwater velocity at the south end of this 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. The increased flows and velocity on the upper end are north of the confluence of the South Platte River and St. vrain Creek. These velocity estimates are on a macroscopic scale, and were the average velocity of the large volume of water flowing through the mine reach during a 100-year flood. Since no specific velocities are known, the operator will follow the general guidelines and conservative formulas outlined in the guidance document. The map in the Pickett study has been revised to include the areas that may be affected by a 100-year flood. These areas incorpo- rate the areas outlined in the division letter, in addition to those shown previously. While the original plans were based on a qualita- tive analysis of the river and floodplain, the operator will begin to develop the qualitative analysis as mining progresses. using the information provided in the review letter, the map was revised to extend the armoring around the south side of the slope in Meadow Lake. Pit slopes that intersect with natural swales, abandoned tail water ditches, et cetera,. will also be armored. These areas do not readily show on the map so as the site is developed, they will be identified and armored using the design criteria outlined in this plan. The last areas of concern are the gaswell pads and access road alignments in Sharkey's Lake where they are in close proximity to the River.