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personnel, the bridge has been in existence for decades and there is no knowledge of previous flow <br />impediment. Based on collected bridge parameter data, and a water surface elevation at 75% of the <br />bridge height, maximum flow beneath the bridge is approximately 3,500 cfs. <br />The Chandler Creek flow at the Fremont County Road 11A bridge was used to size a crossing for the <br />site access road (see Site Drainage Map). It is assumed that the design flow will provide protection <br />for the 20-year, 24-hour storm event. The crossing design shall provide passage of smaller storm <br />event flows and incorporate design or maintenance procedures to minimize blockage from debris. <br />Flooding and over-topping of the crossing from larger storm event flows shall be considered during <br />final design. Final design of the structure may also consider Chandler Creek drainage basin <br />hydrologic information to better increase confidence in the structure design and ability to pass the <br />desired storm event. <br />7.0 TOPSOIL STOCKPILE <br />The planned topsoil stockpile at the Northfield Project site has been included just north of <br />Subwatershed B (see Site Drainage Map). The stockpile area will be graded to allow access by earth <br />moving equipment and mine personnel while minimizing the disturbed area. Stormwater flows <br />running onto the stockpile area will be minimized by construction of ditches and berms or the use of <br />other appropriate flow diversion measures. Sediment movement from the stockpile will be <br />controlled by constructing a small containment berm immediately down-gradient of the pile or other <br />appropriate sediment control best management practices. The establishment of required vegetative <br />cover on the stockpile and surrounding disturbed areas will also minimize sediment movement from <br />the stockpile and likely eventually eliminate the need for sediment collection or control structures. <br />8.0 MINE VENT FAN ACCESS AREA <br />The underground mine vent and fan (see Site Drainage Map) will be located south of Chandler Creek <br />and Fremont County Road 79. The access road and vent area will be graded to allow access by mine <br />personnel and construction and repair equipment. The grading shall be performed to minirriize steep <br />and erosive slopes. Where erosive slopes are created or stormwater flows are concentrated, <br />sedimentation control measures such as gravel and straw bales shall be installed, as needed. High <br />use areas will also be stabilized with gravel or pavement to minimize disturbance and erosion <br />potential. <br />9 <br />