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~~~• conditions. The modeling scenario consisted of the, pond, initially empty, with <br />subsequent inflow of the 10yr and 25yr events, back to back. <br />To accommodate the required 10yr storage the east and west ponds are connected <br />by a 8" (minimum) diameter pipe which will control the flow between the ponds. The end <br />of the pipe in the west pond will be a vertical stand pipe with a trash rack around the <br />opening and at the elevation specified below. Either schedule 40 pvc or corrugated <br />metal pipe will be adequate. The pipe will be buried beneath the west pond <br />embankment and under the mine entrance road and should enter the east pond through <br />its embankment. A minimum slope of 0.5% should be maintained on the horizontal run <br />to assure it completely drains. The outlet should be covered with a trash rack. The <br />elevation of the inlet of the connector pipe in the west pond will be set such that the west <br />pond completely fills before any flow is diverted to the east pond. The spillway crest of <br />the west pond is at elevation 7500.0; the top of the connector pipe is at 7499.7 and the <br />low outlet is of ~494_SJFor the east pond the spillway crest is at 7497.0 and the low <br />outlet is as 7492.0. - y =~ ~ ~~q r~ f - z,~ <br />The required volume of the. 10yr-24hr event for the west pond is 1.96 AF and the <br />annual sediment load is estimated at 0.10 AF, resulting in a total required pond volume <br />of 2.06AF. The volume in the west pond at the time of the as-built surveys was 1.86 AF <br />below the existing spillway crest. The required volume of the 10yr-24hr event for the <br />east pond is 0.19 AF and an annual sediment load of 0.01 AF, resulting in a required <br />• pond volume of 0.20 AF. The volume in the east pond at the time of the as-built surveys <br />was 0.92 AF below the spillway crest. Combining the volumes for the ponds gives a <br />total required storage volume is 2.26 AF and an existing capacity of 2.78 AF. <br />Each pond will have its own outlet works. Sizing of the tow level outlet is controlled <br />by the requirement that the 10yr volume be drained out in 48 hrs. To achieve this the <br />HEC-1 model was used starting with an initially full pond and no inflow. The orifice size <br />was varied until the drain Time approximately equaled 48 hours. The outlets consists of a. <br />3:L1" (west) and 2.0°(east) diameter orifice plate connected to a culvert with a discharge <br />rate that varies with the elevation of the water surface in the pond. The peak stage in' <br />the west pond was 7499.97 ft which is just below the crest elevation of 7500.0 ft. The <br />peak stage for the east pond was 7496.91, below the crest of 7497.0. This combination <br />of :spillway crest elevations and low ou#let elevations assures storage of the 10yr flood <br />volumes. <br />Y .,Spillway crest lengths were determined by modelling the 25yr-24hr event with the <br />.initial water surface elevation set at the elevation of the top of the connector pipe; <br />7499:7ft for the west pond and at the spillway crest elevation 7497.0 for the east :pond <br />(ponds full with 10yr event). The inflow storm hydrograph was routed through the; .ponds <br />and the spillway lengths were adjusted until they passed the 25yr peak inflow with a .rise <br />. above the; crest not more than 0.5 ft: Both spillways are fnre feet long sharp crested <br />.6 _- <br />.~..~ 11 <br />1, January 1998 <br />