A GIS approach to landslide hazard management for the West Coast region, New Zealand. A thesis submitted in partial fulfilment of the degree of Master of Science in Hazard and Disaster Management at the University of Canterbury Kevin A. England University of Canterbury 2011 Abstract Landslides, in their various forms, are a common hazard in mountainous terrain, especially in seismically active areas and regions of high rainfall. The West Coast region of New Zealand is dissected by many active faults, experiences frequent earthquakes and in many locations annual rainfall exceeds ten meters. Consequently, landslides are widespread in the region and since European settlement began, have been responsible for 27 deaths, along with frequent damages to road and rail infrastructure, settlements and agricultural land. This study identifies areas that are susceptible to rainfall triggered landslides in the West Coast region. To achieve this, a landslide susceptibility map was produced using bivariate statistics and the analytical hierarchy process. It has an accuracy that predicts 80% of all the landslides in the top 40% of the susceptibility scores on the map. As part of this process, 3221 rainfall triggered landslides and 522 earthquake (or other trigger) triggered landslides have been mapped and digitised into a Geographic Information System. In parallel with this, a descriptive historical catalogue of 1987 landslides has been compiled from the available sources. These new tools provide decision‐makers with an enhanced means of managing landslide hazards in the West Coast region. In order to avoid misinterpretation the study has been carried out in compliance with the “Guidelines for landslide susceptibility, hazard and risk zoning for land use planning”, which was published in 2008 by the Joint Technical Committee on Landslides and Engineered Slopes. The tools developed in this thesis represent a fundamental step in land‐use planning and set‐up of landslide hazard management in the West Coast region. i Table of contents Abstract ............................................................................................................................... i Table of contents ................................................................................................................ ii List of figures ..................................................................................................................... vi List of tables ..................................................................................................................... ix Acknowledgements ............................................................................................................ x Chapter 1. Introduction ...................................................................................................... 1 1.1 Context and background .................................................................................................. 1 1.2 Objectives ......................................................................................................................... 3 1.3 Terminology ...................................................................................................................... 5 1.4 Methodology .................................................................................................................... 7 1.4.1 Landslide susceptibility modelling ........................................................................ 7 1.4.2 Weights of evidence modelling and the analytical hierarchy process ................. 8 1.4.3 Testing and validation ......................................................................................... 10 1.5 Development of an historic landslide catalogue ............................................................ 10 1.6 Thesis structure .............................................................................................................. 11 Chapter 2. The study area ................................................................................................. 13 2.1 Geography and population ............................................................................................ 13 2.2 Climate ............................................................................................................................ 16 2.3 Geology ........................................................................................................................... 20 2.3.1 Introduction ........................................................................................................ 20 2.3.2 Rocks west of the Alpine fault ............................................................................ 22 2.3.3 Rocks east of the Alpine fault ............................................................................. 26 ii 2.4 Earthquakes .................................................................................................................... 27 2.5 Landslides in the region .................................................................................................. 27 2.6 Summary ........................................................................................................................ 33 Chapter 3. Data collection ................................................................................................ 35 3.1 Introduction .................................................................................................................... 35 3.2 Landslide inventory map ................................................................................................ 36 3.3 Thematic factor maps ..................................................................................................... 43 3.3.1 Geology ............................................................................................................... 44 3.3.2 Slope gradient ..................................................................................................... 45 3.3.3 Slope aspect ........................................................................................................ 45 3.3.4 Land cover ........................................................................................................... 45 3.3.5 Soil drainage ........................................................................................................ 47 3.3.6 Soil induration ..................................................................................................... 48 3.3.7 Proximity to faults ............................................................................................... 49 3.3.8 High intensity rainfall .......................................................................................... 49 3.4 Raster data preparation for weights of evidence modelling ......................................... 50 3.5 Limitations of the data layers ......................................................................................... 55 3.6 The historic landslide catalogue ..................................................................................... 56 Chapter 4. Methodology used for landslide susceptibility modelling ................................ 62 4.1 Introduction .................................................................................................................... 62 4.2 Methodology .................................................................................................................. 68 4.2.1 Bivariate statistics ............................................................................................... 68 4.2.2 Analytical hierarchy process ............................................................................... 70 4.2.3 Success rate testing ............................................................................................ 73 iii 4.2.4 Validation ............................................................................................................ 74 4.2.5 Assigning zones to the landslide susceptibility map ........................................... 76 4.3 Summary ........................................................................................................................ 78 Chapter 5. Results of the landslide susceptibility modelling ............................................. 79 5.1 Introduction .................................................................................................................... 79 5.2 Calculation of total weights using bivariate statistics .................................................... 79 5.3 Calculation of layer weights using the AHP .................................................................... 84 5.4 Weighted sum process ................................................................................................... 87 5.5 Success rate testing ........................................................................................................ 88 5.6 Validation ....................................................................................................................... 90 5.7 Assigning susceptibility zones ........................................................................................ 92 5.8 Spatial characteristics of the landslide susceptibility zones .......................................... 93 5.9 Limitations ...................................................................................................................... 94 Chapter 6. From landslide hazard information to landslide