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<br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br /> <br />approximated at 1 :1. Although there may be additional lateral erosion of the <br />breach. opening, the rapidly decl ining water surface of the reservoir usually <br />results in a decrease from the peak discharge. For this analysis; a <br />triangular breach opening with 1:1 side slopes was used. The final factor <br />whi ch i nfl uences the peak di scharge is the time for the formati on of the <br />breach openi ng. Based on simul ati ons usi ng the physi cally based NWS BREACH <br />computer model, an erosion rate of 24 feet per minute was selected. The final <br />breach simulation was run using the HEC-l computer model. <br /> <br />The peak discharge resulting from the breach simulation of Twin Lakes <br />Reservoir was 12,200 cfs. The maximum depth over the dam was 0.8 feet <br />(maximum water surface elevation of 9300.8 feet). Little time elapses between <br />the time the dam is first overtopped and the time the reservoir water surface <br />reaches 9300.5 and the breach is initiated. Following initiation of the <br />breach, the reservoir water surface quickly drops. The total duration of <br />overtopping was approximately 10 minutes. At two hours following the breach <br />i ni ti ati on, the water surface computed had dropped to 9286.3 feet and the <br />di scharge through the breach was 820 cfs. At four hours foll owi ng the breach <br />initiation, the water surface was computed to be 8280.9 feet, with a discharge <br />of 120 cfs. <br /> <br />4.0 REHABILITATION ALTERNATIVES <br /> <br />4.1 Technical Alternatives <br /> <br />Consi dering the project defi ci enci es, noted in Secti on 1. 5 of thi s Chapter, <br />three rehabi 1 itati on desi gns were developed for Twi n lakes Dam to establ ish a <br />wide range of possible solutions in terms of construction costs, long term <br />operation and maintenance (0 & M), and technical confidence. The basic <br />components of each alternative design are shown in Figure V.4. At this stage, <br />long term 0 & M costs and the dam's degree of safety are difficult to <br />quantify. Qualitative judgement and past experience with the dam was employed <br />to assess these factors. <br /> <br />-62- <br />