Comparison of Results from Alluvial Fan Design Methodology with Historical Data

Abstract

The Federal Emergency Management Agency (FEMA) method for delineating floodplains on alluvial fans was modified by French in 1992 to estimate design peak flow rates of specified return periods at transportation alignments crossing alluvial fans. The hydraulic engineer can use these estimates to size the drainage facilities required to protect an alignment from flood damage. The focus of this paper is an examination of the validity of the estimates derived from this modified methodology. The strategy adopted in the study was to identify a limited number of existing transportation alignments that cross alluvial fans and use the design methodology to solve for the return periods associated with the maximum capacities of the drainage structures that protect the alignment from damage. The probability that one or more events would exceed the estimated capacity of each structure in the period of time the structure has been in place can then be estimated in accordance with the U.S. Water Resources Council's 1981 guidelines and compared with the recorded history of flood damage to the alignment. This study showed that for a number of reasons the methodology likely yields conservative estimates.