Uncertainty and Risk Analyses for FEMA Alluvial-Fan Method

Abstract

Alluvial fans along mountain bases pose quite interesting problems for the design of hydraulic structures and highway crossings and for flood insurance studies. The Federal Emergency Management Agency (FEMA) alluvial-fan method is subject to uncertainties because it is an approximate method. In this paper, Rosenblueth's point-estimate is applied to the FEMA alluvial fan method to compute the mean and standard deviation for the 100-yr discharge at any point on the fan and the mean and standard deviation for the fan arc width. The mean and standard deviation for the 100-yr discharge are used to obtain the risk that the 100-yr discharge will exceed the discharge capacity of hydraulic structures on the fan. The mean and standard deviation for the fan arc width are used to estimate the risk that a given location on the fan is within the hazard flood zone. The HEC-1 rainfall-runoff computer model is used to compute inputs to the FEMA method. The proposed uncertainty and risk analyses are applied to an alluvial fan in north Scottsdale, Ariz.