2007-08-10_PERMIT FILE - C1980007 (3)

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_ 2qL h' k The resulting thickness for the overall drainage blanket was 3.27 feet. The design thickness was set at 4 feet by rounding the computed thickness up to the nearest whole foot. To account for concentrated flows that might flow through the drainage blanket in the low channel between the proposed drop structure and the outlet plunge pool, another calculation was performed to obtain the increased blanket thickness for the channel area only. This scenario was carried out to ensure that the drainage blanket has enough capacity to handle water that might flow in a northwest direction from the existing bench that comprises the left abutment into the aforementioned channel. The computations assumed that the width (parallel to the dam alignment, or profile) possibly contributing flow to the channel is 240 feet. Thus, the flow rate per unit width described above was multiplied by this length to compute a total flow through the drainage blanket in the channel. This flow was then divided by the typical channel width (50 feet) and the resulting flow rate per unit width was used in the above equation. With an L of 210 feet (from drop structure to-RCP outlet), the resulting drainage blanket thickness h, for the channel area was 5.54 feet. The design thickness was set at 6 feet by rounding the computed thickness up to the nearest whole foot, as done previously. 8.5 Filter Material Design The filter criteria for the material that will comprise the trench drains, interceptor trench, and downstream drainage blanket were based on the US Army Corps of Engineers method in "Seepage Analysis and Control for Dams" (1986). The base material, or the material to be protected, consists of the colluvial material that makes up much of the dam and the left abutment. The finest-grained sample of colluvium obtained during the geotechnical investigation (excluding the high-plasticity clay from Trench 9, which appeared to be an outlier) on which a grain size analysis was performed was from SB-06-04 from 42.5 to 44.5 feet. The material showed 85% passing the #200 sieve, which placed this particular sample on the boundary between the Category 1 and Category 2 designations. Because all of the other samples had percentages less than 85% passing the #200 sieve, the typical colluvial material in the vicinity of the dam met a Category 2 designation. This designation results in a maximum D 15 size of 0.7 mm for the filter material. The minimum D15 was set at 0.1 mm, which is the minimum allowed by the method. The maximum particle size was set at 3 inches and the maximum percentage passing the #200 sieve was set at 5%. These are both required by the method independent of the base material. P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 35