Topographic Influences on Flood Frequency Analyses

Abstract

A model that predicts the formation of variable source areas by using the catchment’s topographic index distribution is used to determine the impact of topography, and the variation in contributing area, on the flood frequency distribution of a small catchment in southern Ontario. Log Pearson Type III distributions were fit to data using four different topographic distributions and 1000 randomly-generated storms and antecedent moisture conditions. The four topographic distributions included one for the actual catchment, a second that produced smaller contributing areas, a third that produced larger contributing areas, and a fourth in which the entire catchment contributed to runoff, if runoff was generated during the storm. All data were fitted reasonably well, except for the fourth case which produced the poorest fit. For return periods of less than 10 years, all topographic distributions produced peak flow rates that were roughly similar in magnitude, with the fourth case having the lowest values. However, at return periods of more than 10 years, the third topographic distribution produced higher flow rates than the first and second topographic distributions, while the fourth distribution produced the highest flows of all.