Erosion of the Washdyke-Seadown Lowland Coast
Total Page：16
File Type：pdf, Size：1020Kb
EROSION OF THE WASHDYKE-SEADOWN LOWLAND COAST - PAST, PRESENT AND FUTURE A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science in Geography in the University of Canterbury by John L. Benn University of Canterbury 1987 i ABSTRACT This thesis examines the coastal erosion phenomenon of the Washdyke-Seadown lowland coast. The area consists of 12.25 km of mixed sand-gravel beach between Dashing Rocks and the Opihi River Mouth. The coast is backed by a low-lying hinterland of fluvial origin. Erosion and sea flooding pose a hazard and are threatening many valuable assets. These include a substantial wildlife habitat, farmland, the Sea down drain, State Highway One, the main trunk railway line, and the Washdyke Industrial Estate. A combination of historical, field and laboratory data were used in determining the morphological process and sedimentary characteristics of the area. The beach is dominated by pebbly, moderately-poorly to very poorly sorted greywackes. Grain size was found to have decreased by 0.8 � since 1978. Coarsest sediments are associated with construction works on the beach. Most sedi ment has been lost from the mid section of the beach since 3 1977 (-247 142 m ). In this period the southern end of the 3 Washdyke Barrier has gained 40 194 m of sediment. The hinterland is composed of typical lowland swamp deposits. It was fo und to contain gravel of sufficient size to be used on the beach. However, because of the gravels' oxidised character, its long term value to the beach may be limited. Maximum coastal retreat recorded was -440 m at the southern end of the beach. Erosion decreased towards the Opihi River Mouth. This was due to the presence of stopbanks and net northerly drift of sediment feeding that end of the ii beach. The highest long term erosion rate found was -3.6 m.yr This was considerably less than previous studies have indicated. If current rates of erosion persist the Washdyke Lagoon, Seadown Drain and remaining beach sediment will be lost in about 89 years, 36 years and 51 years respectively. These predictions were considered optimistic because they were based on linear extrapolation. It was found that the unconfined Washdyke Barrier, and the stopbank controlled Seadown coast, behaved in different ways. The Washdyke Barrier has rotated and become very broad while the Seadown Coast has retreated parallel, confin ing the backshore against the stopbanks. -1 -1 iii TABLE OF CONTENTS PAGE ABSTACT i LIST OF FIGURES vii LIST OF TABLES X LIST OF PLATES xi ACKNOWLEDGEMENTS xii CHAPTER ONE: INTRODUCTION 1 1. 1 BACKGROUND 1 1.2 PREVIOUS STUDIES 4 1.3 PURPOSE OF THE INVESTIGATION 6 1.4 THESIS APPROACH 8 1.5 THESIS FORMAT 9 CHAPTER TWO: GENERAL SETTING OF THE STUDY AREA AND ITS MORPHOLOGY 12 2.1 INTRODUCTION 12 2.2 GEOLOGICAL DEVELOPMENT OF THE CANTERBURY PLAINS. 12 2.3 MORPHOLOGY OF THE STUDY AREA 13 2.3. 1 The Present Setting of the Smithfield-Opihi Beach. 13 2.3.2 Major Morphological Features� 14 2.4 THE MIXED SAND-GRAVEL BEACH SYSTEM 20 2.4.1 Natural Features 20 2.4.2 Artificial Features 26 2.5 CONCLUSION 31 iv PAGE CHAPTER THREE: SEDIMENTS 34 3. 1 INTRODUCTION 34 3.2 SAMPLE COLLECTION 34 3.3 SEDIMENT CHARACTERISTICS AND METHODS OF DETERMINATION 35 3.3.1 Grain Size (Mz) 35 3.3.2 38 Sorting (oI) 3.3.3 Skewness and Kurtosis 38 (SKI and K) 3.3.4 Shape and Nominal Diameter 39 3.4 DISCUSSION 45 3.4.1 Grain Size Changes Since 1978 45 3.4.2 Grain Size Distribution 46 3.4.3 Sorting Patterns 51 3.4.4 Grain Size Sorting Relationships 57 3.4.5 Skewness 60 3.4.6 Shape and Nominal Diameter 60 3.5 CONCLUSION 63 CHAPTER FOUR: THE SEDIMENT BUDGET 64 4.1 INTRODUCTION 64 4. 2 METHODS 66 4.3 SEDIMENT THICKNESS DISTRIBUTION 71 4.4 CHANGES IN SEDIMENT VOLUME; 1977-198 7 74 4.5 SEDIMENT BUDGET MODEL, 1977-1987 78 4.5.1 Sediment Budget Model Construction 78 V PAGE 4.5.2 Sediment Budget 1977-1987 80 4.6 CONCLUSION 83 CHAPTER FIVE: THE HINTERLAND 86 5.1 INTRODUCTION 86 5.2 SAMPLE COLLECTION 87 5.3 METHODS 88 5.4 RESULTS 88 5.5 SUBSTRATUM COMPOSITION AND SEDIMENT DISTRIBUTION 92 5.6 SUBSTRATUM INFLUENCE ON BEACH MORPHOLOGY 96 5.7 THE HINTERLAND STRUCTURE 98 5.8 THE INFLUENCE OF THE INTERLAND ON BEACH MORPHOLOGY 103 5.9 CONCLUSION 107 CHAPTER SIX: COASTAL EROSION AND FUTURE SHORELINE PREDI C'rI ONS 108 6.1 INTRODUCTION 108 6.2 CALCULATION OF EROSION 108 6.2.1 Methods 108 6.2.2 Results 112 6.2.3 Comparisons with Recent Studies 116 6.2.4 Coastal Changes Between 1865 and 198 7 118 6.2.5 Changes to Beach Geometry 1865-198 7 124 vi PAGE 6.3 PREDICTION OF FUTURE CONDITIONS 126 6. 3. 1 Methods 126 6.3.2 Predicted Life Expectancy of the Washdyke Lagoon 127 6.3.3 Life Expectancy of the Seadown Drain 131 6.3.4 Future Sediment Budget 132 6.4 CONCLUSION 133 CHAPTER SEVEN: CONCLUSION 136 7.1 SUMMARY OF MAJOR FINDINGS 136 7.2 IMPLICATIONS 140 7.3 RECOMMENDATIONS FOR FUTURE RESEARCH 140 REFERENCES 143 APPENDICES 149 vii LIST OF FIGURES Page Figure 1 .1 Location Map of the Study Coast 2 Figure 1.2 Profile Localities Named in the Text 10 Figure 2.1 1881 Survey of Washdyke Lagoon 15 Figure 2.2 Typical Morphology and Zonation of Mixed Sand and Gravel Beach Profiles 21 Figure 2.3 Comparison of Profiles With and Without the Nearshore Step 23 Figure 2.4 Development of Alternating Beds of Sand and Gravel Due to Cusp 27 Migration Figure 3.1 Sample Location Map 36 Figure 3.2 Shape Triangle 40 Figure 3.3 Sorting Populations Showing Two Modes 43 Figure 3.4 Shape Cumulative Percentages Along the Beach 44 Figure 3.5 Nominal Diameter Values Along the Beach 45 Figure 3.6 Frequency Grain Size Graph 47 Figure 3.7 Grain Size Contour Map. Contour Intervals = 1. 0 <I> 49 Figure 3.8 Sorting Contour Map. Contour Intervals = 0. 5 <I> 53 Figure 3.9 Grain Size-Sorting Trends 54 Figure 3.10 Criteria for the Inference of 56 Transport Direction 58 Figure 3.11 Grain Size and Sorting Scattergrams 59 Figure 3.12 Grain Size Sorting Relationship viii Page Figure 4.1 The Sediment Budget Model 65a Figure 4.2 Comparison of Profile Cross Sections. Shaded Area Shows Beach Sediment. 68 Figure 4.3 Seismic Profile Location 70 Figure 4. 4 Seismic Profile Cross Section 73 Figure 4.5 Comparison of Beach Volumes, 1977- 1987 75 Figure 4.6 Net Volume Changes and Percentage Volume Changes, 1977-1987 77 Figure 4.7 Comparison of Backshore and Foreshore Volumes 79 Figure 4.8 Sediment Budget, 1977-198 7 81 Figure 5.1 Excavation Sites for Substratum Sample Collection 93 Figure 5.2 Major Sedimentary Units of the Substratum 94 Figure 5.3 Relationship Between Substratum Height and Beach Thickness 97 Figure 5.4 Borehole Location Map 100 Figure 5.5 Hinterland Subsurface Sediments a) 1.0 m Below Ground Surface lOla b) 2. 0 m Below Ground Surface 101b c) 4.0 m Below Ground Surface 101c d) 6.0 m Below Ground Surface 101d e) 8.0 m Below Ground Surface lOle Figure 5.6 Changing Morphology of a Retrograding Beach Encountering a Stopbank. 104 Figure 5.7 Changing Foreshore-Backshore Ratios of a Retrograding Beach Being Confined by a Stopbank 106 ix Page Figure 6.1 Shorelines Used by Surveyors in New Zealand 111 Figure 6.2 Average Erosion Rates for Each Time Period 114 Figure 6.3 Average Erosion Rates for the Washdyke Barrier, Seadown Coast and Total Beach 115 Figure 6.4 Comparison of Calculated Average Erosion Rate to a More Probable Erosion Trend 121 Figure 6.5 Changing Geometric Dimensions of the Beach 125 Figure 6.6 Washdyke Lagoon Area Over Time 130 X LIST OF TABLES Page Table 4.1 Observed Gravel Thickness (m) . December 1986. 72 Table 4.2 Comparison of Sediment Transfers, 1977-1987. 84 Table 5.1 Substratum Sediment Data 89 a) % Size Class b) Folk Parameters Table 5.2 Rapid Sediment Analysis of Borehole Samples 91 Table 6.1 Average Erosion Amounts and Rates 113 Table 6.2 Comparison of van Mechelen' s 1978 Erosion Rates and Those of the Present Study (1956-1977) 117 xi LIST OF PLATES Page Plate 1.1 Seaflooding at Aorangi Road 3 Plate 1.2 Construction of the New Sewage Outfall, Seaforth Road 5 Plate 2.1 Dashing Rocks looking South. March 1987 16 Plate 2.2 Buried Forest Exposed at Low Tide 19 Plate 2.3 Internal Structures of the Beach 25 a) Thin layer of Pure Gravel Seaforth Road b) Gravel Concentrated on the Surface, and Sand Below Opihi OlSOOO Backshore c) Beds of Sand and Gravel, Washdyke 200 Upper Foreshore d) Beds of Sand and Gravel, Opihi OlSOOO Foreshore Plate 2.4 Stopbank Buried by Beach Sediments, Connolly' s Road 29 Plate 2.5 Old Timaru City Sewage Outfall, Washdyke 1500. 30 Plate 2.6 Erosion of the Renourishment Struc ture, Washdyke 1500 32 Plate 3. 1 Launching of a Fish Trawler, Beach Rd 52 Plate 4. 1 Digging Through the Beach with an Excavator, December 1986. 67 Plate 6.1 Breaching of the Washdyke Barrier, During the South Canterbury Flood, 14.3.1986 123 xii ACKNOWLEDGEMENTS There are many organisations and individuals who are thanked for their assistance in the production of this thesis.