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If protection is provided in the upper basin from the P;1T5, then the <br />• storm design criteria in the to;ver basin can be relaxed somewhat. The <br />appropriate design storm is often related to the 100-year storm recurrence <br />interval precipitation. By definition, the 100-year storm is that storm <br />which will occur at an average interval of 1G0 years. Plote that the defin- <br />ition refers to an averaoe interval, hence there is no guarantee that a <br />storm of this magnitude cannot occur--say twice in 10 years. The 100-year <br />storm coincides with about one percent probability of being exceeded in <br />any given year, or the "GPP" storm as used by 1;ESA (1975)• <br />The OPP is the smallest design storm reco~~mended by K~SA (1976) for <br />drainage installations where the hazard potential is high. A high hazard <br />potential is defined as (.'.SA, 1975): <br />Hio_h Hazard Potential--Facili±ies located where a failure could <br />be reasonably expected tp cause less of life, serious damaoe to <br />homes, industrial and com~.ercial buildin,s, and important <br />utilities, highvrays and railroads. <br />The purpose of the proposed drainaoe system in the lo~rer basin would <br />be to limit damaoe to primary structures, limit erosion, and prevent slope <br />instability through pondinc, and subsequent saturation of benches. The <br />design would not provide total hvJraulic control durSng major storms. <br />Sicr,ificant repair costs feTlrnrine normal occurrence rainstorms would <br />not be required. Ho~,+ever, significant repair cost: vrould be necessary <br />after extreme storm events. It is believed that the lOC-year storm is a <br />reasonable compromise for desion given the present mine layout, and the <br />cost of complete hvjraulic control cf the very laree stormc in this <br />confined area. <br /> <br />