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As shown in Table 1, all outlet channel velocities are also below 5 ft/sec. This confirms <br />• that the grass lined spillways and outlet channels are acceptable for the expected flows. <br />4.05.9 (2) (c) (ii) and 4.05.9 (7) (d), Minimum 1 foot freeboard when passing the <br />applicable design storm, 25 year 24 hour event <br />The 25 year 24 hour storm is 2.3 inches at Colowyo Mine. Runoff from this storm was <br />modeled for each structure using SEDCAD. Tributary areas to each stock tank are as <br />shown on Figure 1. Time of concentration parameters for each tributary watershed, as <br />required for input in the SEDCAD model, were also determined from the watershed <br />dimensions shown on that figure. A runoff curve number (CN) of 63 was selected as the <br />post- reclamation condition, reflecting a sagebrush rangeland in fair condition, as <br />described in TR -73. <br />Stage- storage curves for each stock tank were developed based on the as -built topography <br />shown on Figure 1, and input into the model. Each stock tank was conservatively <br />assumed to be full to the emergency spillway elevation at the start of the runoff event. <br />Spillway dimensions were determined based on the as -built geometry in Figure 1. The <br />SEDCAD program was then used to determine the maximum water surface elevation <br />while routing the 25 year, 24 hour design storm. The SEDCAD model results also <br />provide the routed outflow rate in cubic feet/second, which was used as described in the <br />previous section to determine flow velocity in the spillway. As shown in Table 1, all of <br />• the stock tanks provide the required 1 foot minimum freeboard between the routed pool <br />elevation and the crest of the embankment. <br />4.05.9 (7) (e), Combined upstream and downstream slopes flatter than 5HJV, and <br />neither slope steeper than 2H:1V <br />Upstream and downstream slopes were determined from the as -built topography shown <br />on Figure 1. As indicated in Table 1, all combined upstream and downstream slopes are <br />considerably flatter than 5H:1 V and no slope is steeper than 2H:1 V. <br />4.05.9 (8) (b) Minimum slope safety factor of 1.3 under steady state seepage <br />Using embankment geometry measured from the as -built topography presented on <br />Figure 1, a classical Bishop Simplified Method of Slices slope stability analysis was <br />performed for each embankment. The critical cross section selected for each <br />impoundment is presented on Figure 2. For each embankment, the phreatic surface was <br />assumed to be a conventional Dupuit parabola for the simple case of steady state seepage <br />in a homogeneous (non- zoned) dam, assuming a permanent pool at the invert of the <br />emergency spillway. These assumptions are conservative to the extent that such a long <br />term seepage condition will probably never develop given the ephemeral nature of the <br />is impoundments. The unit weight was conservatively assumed to be 125 lb /cu ft. Strength <br />Exh. 7 -13 -5 Revision No.: TR -89 <br />Revision Date: 02/11/11 <br />