Morphometric controls and geomorphic responses on fans in the southern Alps, New Zealand

Morphometric variables associated with 41 debris-flow and 18 fluvial fans and their basins in the Southern Alps of New Zealand are examined. The results show statistically significant differences in the area, maximum elevation, relief and ruggedness (Melton's R) of the basin and the area, gradient, and apex and toe elevations of the fan between debris-flow and fluvial sites. Concavity of the fan longitudinal profile also differs between the two fan types, although this could not be tested statistically. Most of these morphometric differences reflect differences in processes and environmental controls on them. Discriminant analysis indicates that basin area and fan gradient best differentiate the two fan types by process. Moderately strong correlations exist, on both debris-flow and fluvial fans, between basin area or Melton's R and fan area. Correlations between basin area or Melton's R and fan gradient are generally weaker. The results of this study also indicate that on debris-flow-prone fans the fan gradient and basin Melton's R have lower thresholds which overlap little with upper thresholds associated with basins where only streamflow reaches the fan. These thresholds may therefore have value in preliminary morphometric assessments of debris-flow hazard on fans in the Southern Alps. Copyright (C) 2004 John Wiley Sons, Ltd.