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Rationale for Approval Recommendation May 20, 2002 <br />Four Corners Materials, Inc. <br />AM-01. Thomas Pit. Permit No. M-1976-020 <br />cottonwood trees. The erosion control blanket provides temporary protection for the <br />embankment and mulch cover while vegetation is establishing. <br />Manufacturers specifications for the S-2 erosion control blanket indicate that the material <br />is appropriate to withstand channel flow velocity to 7 feet per second on slopes up to <br />1 H:1 V. Flood data, submitted with the application, indicates that channel flow velocity <br />during the 100 year storm may attain 8 feet per second. However, the jetty structure, <br />located immediately upstream of the reinforced embankment, is designed to absorb and <br />deflect energy, thereby reducing the erosive potential of flood flow contacting the <br />reinforced embankment. <br />The reinforced embankment of material remaining between the river and pond will be <br />approximately 20 feet wide at the crest. However, due to the 2H:1 V outslope (river side) <br />and 3H:1 V inslope (pond sidel, the base of the embankment will be 80 feet wide for <br />portions of the embankment reaching 12 foot height. <br />The application clarifies that the jetty and embankment reinforcement will be completed <br />prior to commencement of mining activities at the pond area. Excavation of material from <br />the pond area will maintain the 3H:1 V pond inslope. Therefore, the area affected by the <br />activities proposed in AM-01 should safely withstand flood events up to and including the <br />100 year storm, throughout the mining and reclamation phases of the operation. <br />The application demonstrates compliance with the performance standards of Rule 3.1 and <br />the applicable geotechnical stability requirements of Rule 6.5(2). <br />6. Comments indicating potentia/ for adverse impact to the river channel and <br />off-site areas resu/ting from the activities proposed in AM-01. /Whee/ing, Rau, Dan <br />James, D & L James, Pierce, Ritz, Mayo1 <br />As noted above, the jetty and embankment structure has been designed, certified, and <br />verified by independent engineering analysis, as providing an appropriate safety factor to <br />be able to resist effects of the 100 year event on the river channel. Therefore, channel <br />stability appears to be enhanced by the activities proposed in AM-01 for flood events up <br />to the 100 year storm. The proposed design will reinforce the existing channel bank and <br />assist in confining river flow to the established river channel. <br />Additionally, the application clearly states that the finished pond will have sufficient <br />capacity to retain an additional 15.6 acre-feet of water beyond its anticipated highest <br />water storage level. Therefore, during flood events which generate flood flows of <br />sufficient height to enter the pond, the finished pond will detain approximately 15.6 acre- <br />feet of the flood flow. The certified designs for the pond fully anticipate inflow from the <br />river entering the pond through the open channel spi{{way and/or overtoping the <br />embankment crest during the 100 year flood event. The proposed operation will enhance <br />6 <br />