Landslide Classification, Characterization and Susceptibility Modeling in Kwazulu- Natal
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LANDSLIDE CLASSIFICATION, CHARACTERIZATION AND SUSCEPTIBILITY MODELING IN KWAZULU- NATAL. by Rebekah Gereldene Singh Supervisors: Prof T McCarthy and Dr G.A. Botha A dissertation submitted to the Faulty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Master of Science 2009 DECLARATION I declare that this dissertation is my own, unaided work. It is being submitted for the Degree of Master of Science in the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination in another University. The information used in the dissertation has been obtained while employed by the Council for Geoscience. _______________________________ (Signature of candidate) __________ day of ___________________________________ 200_____ ABSTRACT In eastern South Africa landslides are widespread owing to the dramatic topographic-, climatic-, geological- and geomorphological-gradients across the region. In the KwaZulu-Natal (KZN) province numerous landslides and associated deposits are geohazards that represent threats to development and strategic infrastructure. The regional landslide inventory and susceptibility mapping project, following international classification systems and modeling techniques, has revealed the widespread occurrence of landslides. Landslide types mapped include; falls, topples, flows, translational and rotational slides. The bivariate statistical landslide susceptibility modeling method and Analytical Hierarchy Process (AHP) was used to evaluate landslide susceptibility, using a Geographic Information System (GIS). The huge size of some palaeo-landslides mapped is a revelation in the context of KwaZulu-Natal where recent landslide events are mainly small features triggered by intense rainfall events affecting embankments and steep hillslopes. Radiocarbon dating of organic material derived from sag ponds yielded minimum ages for the large middle to late Holocene landslide events. i ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to the following individuals who have made this work possible: • My supervisors, Prof Terence McCarthy and Dr Greg Botha, who provided scientific guidance, and encouragement. • Many thanks to all individuals who completed the preference rating questionnaire. • Thank you to all my colleagues at the KZN unit of the Council for Geoscience for providing moral and academic support during this project. A special thank you to Dr Nick Richards who provided academic guidance during early stages of the project. • The Chief Executive Officer of the Council for Geoscience, Mr. Thibedi Ramontja and Dr Peter Zawada, Executive Manager of the regional mapping division for their support of this project. • My husband, Rakesh and my family for their constant support, encouragement and inspiration. ii TABLE OF CONTENTS ABSTRACT.........................................................................................................................I ACKNOWLEDGEMENTS................................................................................................II LIST OF FIGURES .......................................................................................................... VI LIST OF TABLES............................................................................................................ XI CHAPTER ONE ................................................................................................................. 1 1. INTRODUCTION, OBJECTIVES AND METHODOLOGY ........................................................1 1.1 Introduction..................................................................................................................................1 1.2 Research objectives......................................................................................................................4 1.3 Previous landslide studies in the KZN region..............................................................................7 1.4 Methodology................................................................................................................................8 CHAPTER TWO .............................................................................................................. 15 2. REGIONAL SETTING .................................................................................................................15 2.1 Physiography..............................................................................................................................15 2.2 Climate.......................................................................................................................................16 2.3 Regional geology ...................................................................................................................... 18 2.4 Terrain morphology and geomorphology of KZN.....................................................................22 CHAPTER THREE .......................................................................................................... 27 3. LANDSLIDE TYPES, CLASSIFICATION, MAPPING AND DATING ...................................27 3.1 Overview....................................................................................................................................27 3.2 Overview of landslide classification systems ............................................................................31 3.3 Classification system derived for the KZN context...................................................................32 3.4 Landslide mapping.....................................................................................................................34 3.5 Landslide dating.........................................................................................................................40 3.6 Site Investigations......................................................................................................................44 3.6.1 Undated sites......................................................................................................................44 3.6.2 Dated sites..........................................................................................................................47 iii 3.7 Interpretation of landslide ages..................................................................................................60 3.8 Ground truthing..........................................................................................................................64 3.9 Landslide characteristics...........................................................................................................69 CHAPTER FOUR............................................................................................................. 70 4. LANDSLIDE SUSCEPTIBILITY MAPPING................................................................. 70 4.1 Overview....................................................................................................................................70 4.2 Previous landslide susceptibility maps ......................................................................................70 4.3 Landslide causal factors.............................................................................................................72 4.3.1 Slope angle.........................................................................................................................72 4.3.2 Seismicity...........................................................................................................................74 4.3.3 Lithostratigraphy and rock type.........................................................................................76 4.3.4 Rainfall...............................................................................................................................77 4.3.5 Dolerite intrusion contact zones.........................................................................................80 4.3.6 Lineaments.........................................................................................................................80 4.3.7 Terrain morphology ...........................................................................................................80 4.3.8 Aspect ................................................................................................................................81 4.4 Landslide susceptibility methodologies.....................................................................................82 4.5 Evaluation of methodologies .....................................................................................................83 4.6 Bivariate statistical susceptibility analysis.................................................................................84 4.6.1 Calculation of ranking values of Pertinent Sub–Classes ...................................................85 4.6.2 Analytical Hierarchy Process evaluation of weighting values ..........................................90 4.6.3 Model computation using the ArcGIS Spatial Analyst......................................................93 4.7. Landslide susceptibility evaluation............................................................................................94 4.8 Landslide Susceptibility Map ....................................................................................................97 4.9 Landslide Susceptibility Map Description and Verification......................................................98