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8.4 Downstream Drainage Blanket Analysis and Design <br />A pervious drainage blanket will be placed against the existing downstream slope of the dam and <br />underneath the proposed earth buttress to drain seepage water and minimize the potential buildup of <br />porewater pressures within the buttress material. The thickness of the drainage blanket was computed <br />based on maintaining unconfined flow conditions at all points within .the blanket under full reservoir <br />and steady-state flow conditions. <br />The blanket thickness computations are based on the unconfined flow equation as follows: <br />_ hi - h2 <br />q k 2L <br />where <br />q =groundwater flow rate <br />k =permeability of drainage blanket material <br />h, =head on upstream side =thickness of drainage blanket (head any higher would produce <br />confined flow conditions) <br />h~ =head on downstream side = 0 for this application <br />L =horizontal length of drainage blanket in the direction of flow. <br />The groundwater flow rate q was computed from the seepage model by determining the groundwater <br />flux through the dam per unit width of the dam, which was 5.00x10"~ ft3/s/ft. This value is higher <br />than the q used for the trench drain spacing calculations because it is considered a typical flow rate <br />across the entire dam. <br />Although the actual permeability of this material will likely be 1.0x10"Z cm/s as described in Section <br />' 8.5, the permeability k of the drainage blanket material was conservatively assumed to be 1.0x10"3 <br />cm/s. <br />' The length L of the drainage blanket was computed as the horizontal distance from the upstream end <br />of the drainage blanket to the farthest west toe of the buttress. For the overall downstream slope of <br />the dam, this length was about 350 feet. <br />Solving for the head h,, which defines the boundary between confined and unconfined groundwater <br />flow, the following equation was used for the computations (assuming the tailwater is at the bottom <br />of the drainage blanket): <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 34 <br />