2012-02-21_APPLICATION CORRESPONDENCE - C2010089 (10)

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Unit # Textural Class % Clay % Silt % Very Fine Sand % Fine- Medium Coarse Sand % Very Coarse Sand (or coarser) Unified Classification 10 Clay loam 30 40 10 10 10 SM -SC 71 Silty clay loam 35 55 5 0 5 SM -SC 77 Loam -clay loam 25 30 10 15 20 GM /SM -SC 78 Loam - cobbly clay loam 25 25 10 15 25 GM /GC 81 Loam -clay loam 25 30 10 15 20 SM -SC 82 Loam -clay loam 25 30 10 15 20 SM -SC 88 very cobbly clay or sandy loam 25 20 15 20 20 SW Table 1: Soil Textural Class and Size Distribution The 25 -year 24 -hour precipitation event was used to size the emergency spillways, according to Rule 4.05.9(2)(c)(ii), in order to satisfy the regulation Rule 4.05.9(7)(d) which requires that the minimum elevation of the top of the settled embankment shall be 1.0 foot above the water surface in the pond with the emergency spillway flowing at design depth. The SEDCAD program was used to design the sedimentation ponds. SEDCAD cannot directly model a pond with a gated principal spillway. This is due to the fact that SEDCAD cannot calculate the sedimentology without outflow. But the following 3 step simulation methodology addresses this issue. The methodology was recommended by Professor Warner, the SEDCAD developer. The details of 3 steps are described for Pond NHN- 001 as an example: 1) Model the un -gated pond with the combination of principal and emergency spillways to determine the sediment storage and elevation. The first SEDCAD report in Appendix 2.05.3(4) -2 titled as "10 -Year 24 -hour Storm Event for Pond NHN- 001" is used to determine the following parameters in Table 3. • Contribution area of 114.9 acres and • Runoff Volume of 10.92 ac -ft from Structure Summary on page 4 • The Sediment Storage Volume and Sediment Storage Elevation are 0.77 ac -ft and 5676.02 ft, respectively from the Structure Detail on page 6. 3