The Alluvial Fan Method for Analyzing Flood Potential at the Proposed Low-Level Radioactive Waste Isolation Site, Hudspeth County, Texas

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Akhter, M. Saleem
Dutton, Alan R.

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Another method evaluated in this study at the request of the Texas Low-Level Radioactive Waste Disposal Authority is based on procedures developed by the Federal Emergency Management Agency (FEMA) (1985) to estimate flood height in the lower reaches of active alluvial fans. The analysis detailed here shows that use of the FEMA (1985) alluvial fan methodology violates the geologic and geomorphologic evidence available for the study area and also predicts implausible characteristics of surface-water runoff on the study area.

The FEMA (1985) method uses statistical analysis that relates the probability of given discharges at the apex of a fan to the probabilities of a certain depth and velocity of flow occurring at any point on the fan below the apex. The basic assumption is that the area under consideration is an active alluvial fan where surface-water runoff is carried by channels that migrate, randomly avulse, and change course. The flow system is described as a single channel or as multiple channels that exist between the apex of the fan and its extremal boundary. A probability distribution for the peak discharge is required for the analysis. The statistical parameters of this discharge-frequency distribution are used to predict the discharge related to the critical flood height (0.5 ft [0.15 m] for the 100-yr flood category) and to estimate the corresponding arc width of the fan that would be covered by this critical flood height.


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