Laserfiche WebLink
Positive Factors: <br />1. Predicted overtopping of the drop structure during a 100 -year event is < 1 foot deep and <br />is anticipated to last only a few minutes; this is not sufficient to initiate and advance <br />significant erosion; the flood event required to initiate erosion that then progresses to a <br />breach of the abutment is likely to be significantly larger and more remote in terms of <br />likelihood than a 100 -year event. <br />2. The downstream slope and base of the drop structure has been recently (Feb -Mar, 2013) <br />armored with a substantial blanket of large riprap; this armoring will either prevent or <br />delay the initiation of the erosion process at the toe of the slope in the vicinity of the drop <br />structure, depending on the duration of the overtopping event. <br />3. Visual observations and topographic mapping indicate that overtopping flows may be <br />concentrated into a natural swale on the left (south) side of the drop structure (opposite <br />side from the dam); this may protect the dam embankment from lateral spreading of an <br />erosion gully if the gully forms and is contained within the swale. <br />4. During a 100 -year event, only a small percentage of the total contributing watershed <br />storm water runoff originating from the north side and precipitation falling directly on the <br />impoundment surface will enter and be stored within the tailing impoundment area; due <br />to the shape of the tailing surface (sloping away from the dam) the level of ponded flood <br />water added to the existing free -water pool in the tailing impoundment area likely would <br />be situated several hundred feet upstream from the dam. <br />5. The hydraulic modeling presented in Section 4 predicts that a substantial portion of the <br />PMF volume would be bypassed and would not enter the impoundment area. The <br />predicted high water line from that modelling during a PMF event is at elevation 8613.2. <br />At that elevation, there would remain several hundred feet of distance between the flood <br />pool and the location of the drop structure where the head - cutting erosion is anticipated to <br />initiate. The likelihood of erosion working its way back far enough to connect to the <br />flood pool is low, even for a PMF event. <br />6. The drop structure has a large diameter (48 -inch) conduit that will not be easily clogged <br />with smaller debris (but which could be clogged by larger brush and trees). <br />7. The tailing deposits upstream from the dam and drop structure have drained, are <br />consolidated under their self - weight, and are unsaturated. These materials will not be <br />subject to static liquefaction and not likely to be highly mobile. The volume of tailing <br />materials displaced and transported by flood flows under this PFM would likely be very <br />limited, and comprised predominantly of materials that are in direct contact with flowing <br />water and which are mobilized by scour erosion in the head - cutting channel. <br />Adverse Factors: <br />1. Based on visual observations during the site inspections, the in -place soil materials in the <br />vicinity of the drop structure and South Diversion Ditch are of low - plasticity and appear <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 42 February 2014 <br />