A new approach to estimating hazard posed by debris flows in the Westfjords of Iceland

10 The aim of this study is to improve the assessment of hazard posed by debris flows to 11 the people and settlements of northwest Iceland by studying very recent examples from above 12 the town of Ísafjörður and other nearby localities. Debris flows are a recognised hazard in the 13 region: above Ísafjörður, they occur with particularly high frequency and have appreciable 14 volumes (up to 14 000 m). We have used airborne laser altimeter (LiDAR) and differential 15 Global Positioning System (GPS) data to produce isopach maps of flows that occurred in 16 1999, 2007, and 2008. Our data show that these flows begin depositing at higher slope 17 gradients and are also more mobile than hillslope debris flows reported by other authors. 18 Above a 19° slope, erosion is initiated independent of the distance along the flowpath. Using 19 the isopach maps and associated field observations, we have found a relationship between 20 ground slope and patterns in deposition volume. We have used this finding as a basis for an 21 empirical model that enables an estimate of the total travel distance and final thickness of 22 future debris flows to be calculated. This has enabled us to identify areas of the town which 23 are at risk; some of these are not obvious without this analysis. This model is notable for its 24 simplicity, which allows future debris flow characteristics to be predicted without the need to 25 AC C EP TE D M AN U SC R IP T ACCEPTED MANUSCRIPT determine the precise fluid dynamic parameters of the flow such as viscosity and velocity, 26 which are required to implement more complex models. 27 28