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-11- <br />C. FLOOD HYDROLOGY <br />__~ Although in a relatively low precipitation zone, the Meeker area is subject <br />to intensive rainfall and infrequent but major flash floods. Flash flooding <br />can occur from high-intensity rainfall which may be confined to small watersheds <br />and last for only a short time. Rainfall and associated flooding can produce <br />significant geologic as well as hydrologic effects. Local geologic processes <br />triggered by such floods can be intensified by improper land use, causing <br />f_. <br />heavy property and resource damage and even loss of life. <br />1. Probable Maximum Thunderstorm <br />The probable maximum thunderstorm (PMTS) is the design storm computed by <br />engineers when the stakes are high in terms of life and property, and the area <br />of interest is small. It is defined as the simultaneous occurrence of the <br />maximum of several elements or conditions which contribute to the "big" <br />flood (USBR, 1973). This concept is necessary, since data from recorded floods <br />in a given area are not adequate to provide a reliable basis for estimating <br />maximum probable flood flows. <br />The "maximum probable flood" is an essential part of the engineering <br />studies for a mining project of wide slope and complexity. In the case of the <br />~ ongoing mining operation, an estimate of the magnitude of this flood is <br />i <br />f <br />a must from the standpoint of compliance with Federal and State laws. <br />Again, as pointed out by the USER (1973), ". it is the unusual <br />event or series of events that produces the great floods." Recent experience <br />during the past 11 years (1965, 1969, 1973, and 1976) should be proof enough <br />that large floods can and do happen in Colorado. Often, a misconception prevails <br />that the absence of flooding in a particular area during the past 10, 25, or <br />;~ perhaps 50 years somehow makes that area immune from high rater. To ignore the <br />