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• Area 7-1 <br />The drainage area contributing to this channel is relatively small <br />(38 acres). Consequently, a 6 foot wide channel is sufficient at this <br />location. A channel slope of 19.2 percent with riprap protection <br />extending over a length of approximately 600 feet is proposed at this <br />location (see Sheet 7). The confluence of a small drainageway which <br />enters from the west near the downstream limit of proposed riprap <br />revetment should be adequately stabilized and oriented to provide a <br />smooth transition of flow into the main channel. <br />Areas 31-1 and 31-2 <br />Proposed design slopes for riprap lined channels in Areas 31-1 and <br />31-2 are 21.4 and 15.0 percent, respectively. The limits of proposed <br />protection extend over distances of approximately 640 feet in area 31-1 <br />and 310 feet in area 31-2. Typical cross-section details and profiles <br />are shown on Sheet 4. <br />The drainage areas and geometric characteristics of basins 31-1 and <br />31-2 are similar enough that the peak flow rates were assumed to be <br />additive at their confluence. Since both channels will carry high <br />velocity, turbulent flows down steep slopes, it is important to provide <br />a smooth transition at their confluence. <br />Area 41 <br />• Realignment of the permanent channel to avoid the oversteepened <br />valley side slope is proposed for this location. By altering the <br />channel alignment, as shown in Sheet 2, two reaches of riprap lined <br />channel having slopes of 20.4 and 20.9 percent and respective lengths of <br />340 feet and 240 feet are required. Over most of its length, the <br />revised Area 41-1 drainageway will consist of an excavated earthen <br />channel following a slope of about 3 percent. Riprap on the outside <br />bank of the uppermost and lowermost bendways is recommended to ensure <br />that the flows do not cut through to follow the old route over the <br />steepened fill slope. Sheets 5 and 6 show typical details and the <br />proposed profile. It should be noted that additional riprap protection <br />may be required in the natural drainageway downstream of the Area 41-1 <br />channel to provide baselevel control. <br />Riprap Criteria <br />An in-house computerized procedure for sizing riprap on steep-slopes <br />was utilized to evaluate rock size. The methodology employed utilizes a <br />flow resistance equation developed by Bathhurst (1979) for flow in high- <br />gradient channels to evaluate flow conditions (velocity, depth, etc.) <br />Rock stability is then evaluated using the safety factor approach <br />(Stevens et al., 1976). The overall procedure is similar to the <br />methodology utilized in the Office of Surface Mining (OSM) cesign manual <br />entitled, Surface Mining Water Diversion Manual (1982). A minimum <br />design factor of safety equal to 1.5 was utilized to size channel <br />riprap. <br />