Laserfiche WebLink
1 <br /> 4.2 Johnson Pit <br /> The subbasin area tributary to the southern, post-1976 mined portion <br /> of the pit contains approximately 35 acres above the rim of the pit. This <br /> relatively small subbasin ranges in elevation from approximately 6350 feet <br /> at the southern rim of the pit to a maximum elevation of about 6,395 feet. <br /> Gradients within the subbasin range from approximately 5 to 20 percent <br /> with an average gradient of 15 percent. The area tributary to the <br /> iunreleased portion of land under the Division' s jurisdiction is <br /> approximately 5. 1 acres. The design storm event and design runoff <br /> characteristics of the subbasin tributary to the Johnson Pit are also <br /> presented in Section 6.0. <br /> ' 5.0 EROSION CHARACTERISTICS <br /> ` For design purposes, erosion characteristics for both the natural <br /> terrain and the disturbed areas were considered. Erosion potential of the <br /> natural terrain were previously assessed by Chen to estimate potential <br /> sediment yield from upstream basins for the designed drainage improvements <br /> at the pits. In addition, the potential headward erosion of the natural <br /> stream channel below the disturbed area at School Section Pit was <br /> considered in order to mitigate against headcutting the reclaimed areas. <br /> Gully erosion potential due to concentrated flow and cheat flow <br /> erosion were considered in the reclamation analysis and design. <br /> t <br />