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abstractions in the HEG1 model were set to 0.0 inches and the soil infiltration rates used <br />represent the ultimate or saturated soil conditions. <br />An alternative approach to providing conservative antecedent conditions is to model <br />an arbitrary storm at some time interval before the design storm. One method that has been <br />used is to model the 100 -year, 24-hour rainfall event as occurring four days prior to the PMP. <br />It is our opinion, however, that this antecedent flood condition, coupled with the antecedent <br />moisture conditions as stated above, would be overly conservative for this case. In fact, the <br />USBR states in their Flood Hydrology Manual that: <br />Tailing Pond <br />"For PMF's generated by local PMP events in the entire region west of the 103 <br />meridian, no antecedent event is used. Meteorological conditions are such in this <br />region that hydrometeorologists do not consider it reasonable to assume an event of <br />100 -year magnitude to precede the probable maximum event. However, it is <br />reasonable to assume that a storm of sufficient magnitude has occurred to satisfy <br />initial infiltration losses and provide for minimum or ultimate infiltration loss <br />conditions at the onset of the probable maximum local storm event." <br />The elevation and size of the tailing pond continues to increase as additional tailing is <br />deposited in the impoundment. However, information from a survey performed in the fall of <br />2003 is used to present a "snapshot' of the operating conditions. The elevation capacity <br />curve from that survey is shown on Figure 3. The typical pond water level during this <br />deposition season was at about elevation 8815.0. The lowest point of the dam crest at the <br />time of the fall 2003 survey was at about elevation 8823.0. The deposition berm that is <br />pushed up along the dam crest is not considered in the determination of the crest elevation. <br />The surcharge storage volume available in the pond for this scenario was about 5,700 acre - <br />feet to the dam crest and about 5,250 acre -feet to a level with six inches of residual <br />freeboard. Based on typical surveyed beach slopes, this freeboard will result in at least 50 <br />feet of beach width above the high water line. The surcharge storage amount varies <br />somewhat on a seasonal basis, with a maximum in the fall after the summer crest building <br />10 <br />