THE IMPACTS OF SEA. LEVEL RISE - ON THE SOlJTH AFRICAN COASTAL ENVIRONMENT., by . Peter Hughes Town Cape of University A Thesis submitted to the Department of Oceanography, University of Cape Town in fulfillment of the requirements for the degree of Doctor of Philosophy February 1.992 I ... · ""'' ·• "' The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University CONTENTS Page Abstract ...... ...... .......... ..............•............................. ... .. ....... .. i Acknowledgements ............................. ·..................................... 111 List of Figures ............................................................. ·........... iv List of Tables ................................ : ..........·:............................. vu CHAPI'ER 1 Introduction ................. ,............................... 1 CHAPI'ER 2 The South African Perspective of Global Change .......... ;................................... 8 2.1 Is there any cause for concern? ........................... 8 2.2 Predicted Sea Level Rise .................................... 15 2.3 Why is global change important in the South African coastal environment? . 17 CHAPI'ER 3 Identifying Impacts of Sea Level Rise: Methods ...................................................... 21 3 .1 Modelling . 21 3.1: 1 Beach erosion ................................................ 23 3.1:2 Flooding and .inundation ....... : ........................... 30 3 .1: 3 Salt water intrusion and elevated coastal groundwater tables ... : .. .. .. .. .. .. .. .. .. .. .. .. .. 36 3.1:4 Storm damage ................... ·............................. 37 3.2 The Approach ................................................ 39 CHAPI'ER 4 Detailed Case Studies of the Impacts of Sea Level Rise in Representative . Environments............................................... 42 4.1 The Impact of Sea Level Rise on the Woodbridge Island/Diep River System . 44 4.1: 1 Impact of Increased Coastal Erosion .......... ; .. .. .. .. 48 4.1:2 Impact of Flooding and Inundation ....................... 50 4.1 :3 Increased Salt Water Intrusion and Elevation of Coastal Groundwater Tables .......·.................................................. 53 6 4.1 :4 Reduced Protection from Extreme Events ......................................................... 53 4.1 :5 Discussion.................................................... 55 4.1:6 Summary ............... : ..................................... 58 4.2 The Impacts of Sea Level Rise on the False Bay Coastline, Cape Town . .. 60 4.2: 1 Site Parameters.............................................. 62 4.2:2 Results ....................... ·................................. 64 4.2:3 Discussion..................................................... 73 4.3 The Impacts of Sea Level Rise on Durban ........................................................ 76 4. 3: 1 Site Parameters . 77 4.. 3:2 Results . .. 80 4.3:3 Discussion.................................................... 87 4.4 The Impacts of Sea Level Rise on Walvis Bay ................................................... 89 4.4: 1 Site Parameters .............................................. 94 4.4:2 Results........................................................ 95 4.4:3 Discussion .................................................... 101 4.5 Case study review ........................................... 103 CHAPTER 5 Regional Impacts........................................... 106 5 .1 The Development of a Small Scale Coastal Vulnerability Index with particular reference to South Africa .. .. .. 108 5.2 Application of the Small Scale CVI to the Southern Cape Coast ................................ 112 5.2: 1 Results ........................................................ 113. 5:2:2 Discussion .................................................... 118 5.3 Application ofthe Small Scale CVI to the Natal South Coast ................................... 121 5.3:1 Results ........................................................ 124 5.3:2 Discussion· .................................................... 129 5 .4 Comparison of the Regional · Vulnerability of the South Cape Coast and the Natal South Coast.......................... 131 CHAPTER 6 Discussion .................................................... 137 6.1 Location typicality ...... :·. ·.·:· ............................. 137 6.2 Observations from the case studies ....................... 140 6.2: 1 Coastal Erosion .............................................. 140 6.2:2 Flooding & Inundation ..................................... 142 6.2:3 Saline Intrusion .............................................. 143 6.2:4 Raised Water-Tables ........................................ 143 6.2:5 Storm Vulnerability ......... ; ............................... 144 6.3 Development and observations of the Coastal Vulnerability Index ................................ 145 6.4 National Vulnerability to Sea Level Rise ............................................................ 149 6.5 Identification of Particularly Sensitive Localities .......................................... 157 6.6 Management Options ............................... : ....... 161 6.6: 1 International approach to sea level rise.;.................................................... 163 6. 7 What can be done in South Africa? ....................... 170 REFERENCES ................................................................. 174 APPENDIX 1 Risk Ratings for the south Cape Coast .......................................................... 181 APPENDIX 2 Risk Ratings for the Natal South Coast ..................... ; .................................... 193 APPENDIX 3 Combined Risk Ratings for the south Cape Coast and the Natal South Coast. .................. 210 .ABSTRACT The effects of rising sea levels on the South African coastal environment are investigated with the intention of providing a first step towards identifying and managing the potential impacts of future higher sea levels. Methods suitable for modelling the impacts of sea level rise in South Africa are discussed and a preferred . procedure laid out. This procedure is applied to the areas of Woodbridge Island (Cape Town), False Bay (Cape Town), Walvis Bay and Durban and provides a series of detailed case studies or Potential. Impact Assessments. These areas are taken to be representative of the major "type-location" environments found around the South African coastline. The application of a regional vulnerability ind~x, intended for global usage, is discussed and found to be inadequate for the South African Scenario. Based on the findings of the case studies a new vulnerability index is developed which provides a quasi-economic rating for risk to sea level rise. This new coastal vulnerability index is applied to the southern Cape and Natal south coasts; areas of contrasting geomorphology but comparable population densities. The findings from these exercises are extended to the whole coastline and a national vulnerability to sea level rise is built up. Over 80% of the South African coastline is made up of sandy beaches and mobile dunefields and therefore has an obvious potential for coastal damage. After considering levels of population pressure on the coast, those most vulnerable environments are found to include tidal inlets and locations where development has taken place on the primary dunes. Over 60% of the Cape coast's major inlets and estuaries contain significant development which may be susceptible to the impacts of sea level rise. Over-development of the primary dune can be demonstrated in most coastal towns and cities and dune development is particularly prevalent along the Natal south· coast and Greater Durban area. Overall four areas of particular risk stand out: Greater Cape Town. Melkbosstrand to Gordon's Bay. South Cape coast. Mossel Bay to Nature's Valley. Port Elizabeth. Natal south coast and Greater Durban. Southbroom to Ballitoville. Coastal management policies from a number of countries dealing with sea level rise are discussed and their applicability to South Africa is considered. On the basis of these comparisons it is suggested that South African sea level rise policy should be i instituted on two levels: On a state governmental level, all relevant government departments should acknowledge the overall problem and potential risk from global warming and associated sea level rise. In doing so planners and coastal engineers would be forced to recognize their professional responsibility in this area. On a local government level, any management plan proposed in a recognized sensitive environment or area, should be subject to a sea level rise hazard test. It is suggested that this procedure forms an integral part of the Integrated Environmental Management policy recently being adopted by land use managers in South Africa. Additional studies are necessary to enhance sea level rise management in South Africa. Suggestions for further research include: an improvement in the quantity and quality of the baseline sea level observations; monitoring of coastal processes that may be symptomatic of sea level rise; and investigation into the most effective responses and precautions to take against sea level rise. In this way,