Delineating debris-flow hazards on alluvial fans in the Coromandel and Kaimai regions, New Zealand using GIS Andrew Welsh Non-technical Summary Introduction 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. Types of event In this report three different types of flood event are considered; water floods, debris floods and debris flows. A water flood is what normally happens in a stream or river in flood; the high water discharge carries with it a lot of suspended load (fine sand and silt) which discolours the water, making it dirty; it also moves coarser material along with the flow at the bed, rolling, sliding or bouncing along – this is called “bedload” and in New Zealand rivers is often up to gravel size. i A debris flood is a water flow with a very high concentration of suspended load – perhaps 10% or 20% of the flow by volume may be silts and sands. This is sufficient to change the look of the flow – it becomes somewhat “oily” in appearance and there is much less turbulence than usual. Volcanic lahars often have these characteristics. The flow again moves coarser material as bedload, in the lower part of the flow. A debris flow is a relatively rare type of storm flow in a stream, which comprises a very dense mixture of water, fine sediment and coarse sediment (boulders and trees). In these types of event, the concentration of fine material is so high that it alters the way the flow carries coarse sediment – the flow can now move the very coarsest sediment available, up to boulder size, and can carry these large grains throughout the flow depth. A debris flow can scour the channel bed very deeply. Debris flows are often initiated by small landslides entering a stream in flood. They flow as a slurry, much like wet concrete or mud, and break up into separate waves or surges as they move down a channel. This increases the depth or height of a surge, and causes large boulders to move to the front of the surge, making it potentially very destructive. It also means that a debris-flow surge can leave the stream channel, say at a sharp bend, and move across the adjacent land. Of these three types of flood, debris flows are by far the most destructive, because they can transport large boulders and can leave stream channels. Hence they pose the greatest hazard to development. In fact most debris flow events are accompanied by debris-flood- like phases, in between the surges; it seems that occurrence of debris floods on their own may be fairly unusual. Finally, the distinction between debris flows and debris floods is poorly understood in technical terms. Debris-flow catchment identification The present work does not use deposits to identify likely sites of debris flows, because their relief is too low for the particular characteristics of debris-flow deposits to be identified on the 25-m DEM; instead it uses the characteristics of the catchment areas in which the debris flows are initiated. The catchment areas are much higher and steeper than the fan deposits. A number of investigators have recently carried out studies of the ii catchment geometries that give rise to debris flows, debris floods and water floods, and the results of those studies form the basis for the methodology in this report. The catchment parameters that together best indicate ability to develop debris flows are the Melton Ratio (R) and the catchment or watershed length (WL). R is defined as the ratio of basin relief (the highest elevation in the catchment minus the lowest elevation) to the square root of catchment area. These parameters represent the steepness and size of the catchment; small, steep catchments are most likely to produce debris flows. A Geographical Information System (GIS – ESRI Arcview) was used to enable the 25- metre DEM for a region to be analysed automatically, so that all catchments that had values of R and WL in certain ranges were highlighted. The ranges selected corresponded to catchments able to produce debris flows, debris floods and water floods respectively. Full details of the procedure are given in the report. Results Once this system had been developed, it was used to identify catchments in Coromandel that have the characteristics appropriate to the three types of flood event. Then a field inspection was undertaken, to study the characteristics of the catchments and their fans on the ground and test the predictive ability of the GIS system. This test was very satisfactory – most of the catchments had been correctly identified by the GIS system. The system was then applied to the Kaimai Ranges, a region adjacent and similar to Coromandel in geology and climate; again the test was satisfactory. At this stage it was apparent that the methodology developed was performing adequately in the Waikato area. A final and much more severe test was undertaken; the method was applied to a eighteen known debris-flow-capable catchments throughout the rest of New Zealand, many of them in the very different geologies and climates of the South Island mountains. Rather surprisingly, the method passed this test very well also. In fact, the only catchments known to produce debris flows, which were not thus identified by the GIS, were the iii Awatariki and Waitepuru streams in Bay of Plenty, which had very low R values –these were the sources of the disastrous 2005 Matata debris flows; and four streams in the Alps with slightly too high values of WL. The method thus seems to be about 67% reliable in identifying debris-flow capable catchments; though, interestingly, if R > 0.55 is used as the sole criterion for debris-flow occurrence the reliability increases to 89%. The failure of the method to identify the sources of the Matata disaster is a concern, and suggests that it might also fail to identify other debris-flow prone catchments. Further research is required into this particular aspect. However, the overall reliability of the method indicates its potential. Conclusions This project has demonstrated the feasibility of rapid, automated preliminary identification of debris-flow capable catchments using a GIS-based routine operating on the 25-metre DEM; the reliability appears acceptably high for this purpose. It was found that the geological and climatic setting had little effect on the catchment characteristics associated with debris-flow occurrence; the method developed for Coromandel worked just as well in the South Island mountains. iv v vi vii viii ix x xi xii xiii xiv xv xvi Chapter 1 Introduction Chapter One Introduction 1.1 Background In the last two decades of the twentieth century, over 30 000 people lost their lives to geohazards such as earthquakes, volcanoes, mass-movements such as debris-avalanches and debris-flows, and flash floods (Smith, 2001). Couple this with the billions of dollars worth of property damages and it is apparent that more emphasis needs to be placed on the evaluation and mitigation of such hazards. One of the most destructive of all geohazards is the debris-flow (Takahashi, 1991). Debris-flows can loosely be described as sediment gravity flows, comprising a viscous mix of water, soil, boulders and organic material, capable of reaching speeds of up to 10 meters/sec in mountainous terrain (Stiny, 1910; Sharpe, 1938; Hutchinson, 1969; Varnes, 1978; Jakob and Hungr, 2005). They are amongst the most energetic of geomorphic processes and play a significant role in the denudation of mountainous terrain (Lorenzini and Mazza, 2004; Rowbotham et al, 2005). In combination with normal fluvial processes, debris-flows are a prominent mechanism by which fans are constructed at the mouths of small tributary basins in steep terrain (de Scally and Owens, 2004). When human activity encroaches on such regions, debris-flows transform from being just a natural process of erosion and sediment transport, to become also a natural hazard posing significant danger to settlements, transport routes and other infrastructure located on depositional fans (Figure 1.1) (Jakob and Hungr, 2005; Davies, 1997; Whitehouse and McSaveney, 1992). In recent times, a combination of urban sprawl and an ever-increasing desire of individuals to reside in secluded locations with a view have lead to a greater number of people living in debris-flow prone areas (Staley et al., 2006). This has exacerbated the 1 Chapter 1 Introduction debris-flow hazard in these areas, leading to more instances in which the unexpected occurrence of debris flow has lead to loss of life. A good example of this is seen in Japan, where in the twenty years between 1967 and 1987, 1257 fatalities out of a total 4598 caused by natural disasters were attributed to debris-flows (Takahashi, 1991). Figure 1.1: Debris-flows can present a significant hazard to people and property residing on the depositional fans of steep drainage basins. This photograph shows the devastating amount of sediment and debris transported to fans as a result of major debris-flows in the state of Vargas, Venezuela 1999 (Wieczorek et al., 2001).