Controls on fan depositional processes in the schist ranges of the Southern Alps, New Zealand, and implications for debris-flow hazard assessment

Sixteen morphometric and vegetation cover variables associated with 32 debris-flow and 28 fluvial fans and their basins in the schist ranges of the Southern Alps of New Zealand are examined. The results show statistically significant differences in the area, length, planimetric shape, relief ratio, Melton's ruggedness (R), and forest and total vegetation cover between debris-flow and fluvial basins, and in the apex and toe elevations, area, and upper and average axial gradients between debris-flow and fluvial fans. All of these variables except the apex and toe elevations of the fans reflect differences between debris-flow and fluvial processes and environmental controls on them. Discriminant analysis indicates that the relief ratio and Melton's R of the basin and upper and average gradients of the fan axis are able to correctly classify debris-flow and fluvial fans, with almost equally good results obtained by employing only a pair of variables: either basin relief ratio and upper fan gradient or basin Melton's R and basin shape. The lower thresholds at debris-flow sites and upper thresholds at fluvial sites for the relief ratio and Melton's R of the basin and upper and average gradients of the fan are identified, and these as well as the mean values of these variables show significant differences from fans in nearby sedimentary ranges of the Southern Alps with important implications for the assessment of debris-flow hazard on fans. Fan gradient is found to be closely associated with basin size (area and length) at debris-flow sites, but with basin ruggedness (relief ratio) at fluvial sites. Both fan types show a weak association between their size and the size of the contributing basin.