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r 1 <br />The Chandler Creek flow at the Fremont County Road 1 IA bridge was used to size a crossing for the site <br />access road (see Site Drainage Map). It is assumed that the design flow will provide protection for the <br />20 -year, 24 -hour storm event and provide passage of smaller storm event flows. Construction of the <br />crossing shall incorporate structures or maintenance procedures to minimize blockage from debris. <br />Stability and protection of the structure and creek embankments during flooding and over - topping of the <br />crossing from larger storm event flows shall be considered during construction. It is recommended that, <br />prior to construction of the crossing structure, Chandler Creek drainage basin hydrologic information be <br />analyzed in more detail to better increase confidence in the structure design and ability to pass the desired <br />storm event. It is the responsibility of the operator to ensure that materials used during construction can <br />safely carry anticipated vehicle loads. <br />7.0 TOPSOIL STOCKPILES <br />There are two planned topsoil stockpiles at the Northfield Project site; one is included just north of the <br />site facility and subwatersheds B and C and the second is located near the mine ventilation shaft sites <br />south of Fremont County Road 79 (see Site Drainage Map). The stockpile areas will be graded to allow <br />access by earth moving equipment and mine personnel while minimizing the disturbed area. Stormwater <br />flows running onto the stockpile areas will be minimized by construction of ditches and berms or the use <br />of other appropriate flow diversion measures. Sediment movement from the stockpiles will be controlled <br />by constructing small containment berms immediately down - gradient of the piles or other appropriate <br />sediment control best management practices. The establishment of required vegetative cover on the <br />stockpiles and surrounding disturbed areas will also minimize sediment movement from the stockpiles <br />and likely eventually eliminate the need for sediment collection or control structures. <br />8.0 MINE VENT FAN ACCESS AREA <br />The underground mine ventilation shafts (see Site Drainage Map) will be located south of Chandler Creek <br />and Fremont County Road 79. The access road and ventilation shaft bench sites will be graded to allow <br />access by mine personnel and construction and repair equipment. The grading shall be performed to <br />minimize steep and erosive slopes. Where erosive slopes are created or stormwater flows are <br />concentrated, sedimentation control measures such as gravel, straw bales, or silt fences shall be installed, <br />as needed. High use areas will also be stabilized with gravel or pavement to minimize disturbance and <br />erosion potential. <br />40 <br />