Methods for delineating flood-prone areas in the Great Basin of Nevada and adjacent states
The Great Basin is a region of about 210,000 square miles having no surface drainage to the ocean; it includes most of Nevada and parts of Utah, California, Oregon, Idaho, and Wyoming. The area is characterized by many parallel mountain ranges and valleys trending north-south. Stream channels usually are well defined and steep within the mountains, but on reaching the alluvial fan at the canyon mouth, they may diverge into numerous distributary channels, be discontinuous near the apex of the fan, or be deeply entrenched in the alluvial deposits. Larger rivers normally have well-defined channels to or across the valley floors, but all terminate at lakes or playas. Major floods occur in most parts of the Great Basin and result from snowmelt, frontal-storm rainfall, and localized convective rainfall. Snowmelt floods typically occur during April-June. Floods resulting from frontal rain and frontal rain on snow generally occur during November-March. Floods resulting from convective-type rainfall during localized thunderstorms occur most commonly during the summer months. Methods for delineating flood-prone areas are grouped into five general categories: Detailed, historical, analytical, physiographic, and reconnaissance. The detailed and historical methods are comprehensive methods; the analytical and physiographic are intermediate; and the reconnaissance method is only approximate. Other than the reconnaissance method, each method requires determination of a T-year discharge (the peak rate of flow during a flood with long-term average recurrence interval of T years) and T-year profile and the development of a flood-boundary map. The procedure is different, however, for each method. Appraisal of the applicability of each method included consideration of its technical soundness, limitations and uncertainties, ease of use, and costs in time and money. Of the five methods, the detailed method is probably the most accurate, though most expensive. It is applicable to hydraulic and topographic conditions found in many parts of the Great Basin. The historical method is also applicable over a wide range of conditions and is less expensive than the detailed method. However, it requires more historical flood data than are usually available, and experience and judgement are needed to obtain meaningful results. The analytical method is also less expensive than the detailed method and can be used over a wide range of conditions in which the T-year discharge can be determined directly. Experience, good judgement, and thorough knowledge of hydraulic principles are required to obtain adequate results, and the method has limited application in other than rigid-channel situations. The physiographic method is applicable to rigid-boundary channels and is less accurate than the detailed method. The reconnaissance method is relatively imprecise, but it may be the most rational method to use on alluvial fans or valley floors with discontinuous channels. In general, a comprehensive method is most suitable for use with rigid-bank streams in urban areas; only an approximate method seems justified in undeveloped areas.