IDENTIFICATION OF ALLUVIAL FANS PRONE TO DEBRIS-FLOW HAZARD IN NORTHWESTERN BANKS PENINSULA

This study examines the effectiveness of using watershed length (WL), Melton ratio (R), and a combination of WL and R to predict the flow type (i.e. debris-flow, debris-flood, or fluvial) for watersheds on alluvial fans for the purpose of creating a debris-flow hazard map at a regional scale in the volcanic terrain of northwestern Banks Peninsula. ArcGIS was used for delineation, data extraction, and classification of watersheds using the model thresholds. The accuracy of each model’s predictions was tested using field reconnaissance. The model based on WL was 63% accurate, whereas the models based on R and WL and R together were 88% accurate. A hazard map created from assigning debris-flow risk levels to each of the combined categories of WL and R revealed that the highest areas of debris-flow risk in the study area are located along the coast of the Lyttelton Volcanic Complex (LVC) which could be detrimental to established infrastructure. The regional debris-flow hazard map generated by this study can be used to evaluate where precautionary measures should be put in place to protect infrastructure, and where to avoid future development around the LVC.

Keywords: Debris-flows, hazard mapping, Melton ratio, watershed length, Banks Peninsula