Small scale hydraulic modelling of alluvial fans

Abstract

The Waiho River in the vicinity of the Franz Josef Glacier township has been in a state of aggradation for most of this century. This has led to an increase in the flood hazard potential of the river on the facilities and amenities of the township and surrounding areas. River control works were first introduced into the region in the 1930's and around the same time riverbed levels began to rise. Currently there are nine control structures in place on the Waiho River - fan system and they restrict the development of the alluvial fan to the true right of the fans central axis and Canavans Knob. McSaveney and Davies (1999) suggested that a possible link might exist between a series of river control works and the severe aggradation and this has formed the basis for this research.

Experimental investigations into the effect of increasing the angular confinement at the fan apex found that the stopbanks had no effect on the rate of bed elevation change for aggrading and degrading input conditions. For equilibrium input conditions it was found that increasing the angular confinement resulted in an increased fanhead elevation.

This increase in the aggradation rate was due to a reduced level in the degree of the degradation phase of the development cycle, which occurred when the main channel avulsed down the steep flanks of the alluvial fan. By restricting the flow down the steep flanks of the fan the stopbanks have not caused an increase in the deposition rate but have reduced the effectiveness of the fanhead erosion process.

A small-scale physical hydraulic model of the Waiho alluvial fan found that these results were applicable to a specific situation, and that increased fanhead aggradation occurred following the addition of the stopbanks. The resultant aggradation was due to restricting the flow down the step flanks of the fan and the elimination of avulsions across the central axis. This altered the development pattern from an aggradation - degradation cycle to a migratory one.