Delineating debris-flow hazards on alluvial fans in the Coromandel and Kaimai regions, New Zealand, using GIS

Abstract

This report describes the development and testing of a method for identifying catchments that may be potential sources of debris flows, using the 25-metre digital elevation model (DEM) that is available for the whole of New Zealand. The objective is to make available to local government organisations a rapid, automated method for preliminary assessment of proposals for development, to see if more detailed investigation of debris-flow hazard is required before permitting a development. The importance of correctly identifying potential sites of debris flows was highlighted by the Matata event of May 2005, where a number of houses were destroyed (miraculously without loss of life) by a debris flow in a location easily identifiable as a debris-flow fan by its geomorphic characteristics.

In this report three different types of flood event are considered: water floods, debris floods and debris flows. 

Technical Abstract

Debris-flows pose serious hazards to communities in mountainous regions of the world and are often responsible for loss of life and damages to infrastructure. Characterised by high flow velocity, large impact forces and long runout, debris-flows have potential discharges several times greater than clear water flood discharges and possess much greater erosive and destructive potential. In combination with poor temporal predictability, they present a significant hazard to settlements, transport routes and other infrastructure located at the drainage points (fan-heads) of watersheds. Thus, it is important that areas vulnerable to debris-flows are identified in order to aid decisions on appropriate land-uses for alluvial fans.

This research has developed and tested a new GIS-based procedure for identifying areas prone to debris-flow hazards in the Coromandel/Kaimai region, North Island, New Zealand. The procedure was developed using ESRI Arc View software, utilising the NZ 25 x 25 m DEM as the primary input. When run, it enabled watersheds and their associated morphometric parameters to be derived for selected streams in the study area. Two specific parameters, Melton ratio (R) and watershed length were then correlated against field evidence for debris-flows, debris-floods and fluvial processes at stream watershed locations in the study area. Overall, strong relationships were observed to exist between the evidence observed for these phenomena and the parameters, thus confirming the utility of the GIS procedure for the preliminary identification of hydrogeomorphic hazards such as debris-flow in the Coromandel/Kaimai region study area.

In consideration of the results, the procedure could prove a useful tool for regional councils and CDEM groups in regional debris-flow hazard assessment for the identification of existing developments at risk of debris-flow disaster. Furthermore, the procedure could be used to provide justification for subsequent, more intensive local investigations to fully quantify the risk to people and property at stream fan and watershed locations in such areas.