ASSET MANAGEMENT GROUP Technical report ISSN 1174 3085 Hawke’s Bay Environmental Change, Shoreline Erosion & Management Issues November 2005 AM06/04 HBRC plan No. 3839 Asset Management Group Technical Report ISSN 1174 3085 Hawke’s Bay Environmental Change, Shoreline Erosion & Management Issues Compiled jointly for: Napier City Council Port of Napier Ltd Hawke’s Bay Regional Council Compiled By: Paul D. Komar, Ph.D. Consulting Oceanographer 3520 NW Elmwood Drive Corvallis, Oregon 97330 USA November 2005 AM 06/04 Reviewed: HBRC Plan Number 3839 Table of Contents Preface and Acknowledgments Executive Summary 1 Introduction: The Coast of Hawke's Bay ..................................................1-1 2 Tectonics and Earthquakes: Their Roles in the Evolution and Erosion of the Hawke's Bay Shoreline .....................................................2-1 2.1 INTRODUCTION ............................................................................................................. 2-1 2.2 THE BIG PICTURE — GLOBAL PLATE TECTONICS................................................... 2-1 2.3 THE TECTONICS OF HAWKE'S BAY ............................................................................ 2-4 2.4 THE 1931 HAWKE'S BAY EARTHQUAKE................................................................... 2-13 2.5 SUMMARY AND DISCUSSION .................................................................................... 2-20 2.6 REFERENCES .............................................................................................................. 2-21 3 Human Settlement and Impacts on the Environments of Hawke's Bay...............................................................................................................3-1 3.1 INTRODUCTION ............................................................................................................. 3-1 3.2 THE ARRIVAL OF THE MAORI AND THE ENVIRONMENTAL CONSEQUENCES .......................................................................................................... 3-2 3.3 EUROPEAN EXPLORATION, SETTLEMENT AND CHANGES IN THE ENVIRONMENT .............................................................................................................. 3-5 3.4 RESEARCH INTO THE CHANGING ENVIRONMENTS OF HAWKE'S BAY .............. 3-15 3.5 SUMMARY AND DISCUSSION .................................................................................... 3-21 3.6 REFERENCES .............................................................................................................. 3-22 4 Ocean Processes in Hawke Bay: Waves, Tides and Changing Sea Levels ..........................................................................................................4-1 4.1 INTRODUCTION ............................................................................................................. 4-1 4.2 PREDICTED AND MEASURED TIDES .......................................................................... 4-1 4.3 GLOBAL AND LOCAL CHANGES IN SEA LEVELS ...................................................... 4-5 4.4 THE WAVE CLIMATE OF HAWKE BAY....................................................................... 4-11 4.5 WAVE REFRACTION AND ENERGY LEVELS ALONG THE HAWKE BAY SHORE.......................................................................................................................... 4-20 4.6 SURF-ZONE PROCESSES ON THE HAWKE'S BAY BEACHES ............................... 4-24 4.7 THE LONGSHORE MOVEMENT OF THE BEACH SEDIMENTS................................ 4-26 4.8 TSUNAMI....................................................................................................................... 4-32 4.9 SUMMARY AND DISCUSSION .................................................................................... 4-35 4.10 REFERENCES .............................................................................................................. 4-37 5 The Dynamics of Mixed Sand-and-Gravel Beaches and the Hawke's Bay Monitoring Program.............................................................5-1 5.1 INTRODUCTION ............................................................................................................. 5-1 5.2 MIXED SAND-AND-GRAVEL BEACHES: A REVIEW ................................................... 5-1 5.2.1 The Compositions and Morphologies of Beaches................................................. 5-2 5.2.2 Ocean Processes on Mixed Beaches.................................................................... 5-8 5.2.3 The Dynamic Responses of Beaches to Changing Wave Conditions................. 5-11 ii 5.2.4 The Longshore Transport of Gravel on Mixed Beaches...................................... 5-16 5.2.5 The Sorting of Sediment Particles on Coarse-Grained Beaches ........................ 5-19 5.2.6 Particle Abrasion.................................................................................................. 5-21 5.2.7 The Beach Responses to Long-Term Changes in Relative Sea Level ............... 5-24 5.3 THE BEACHES OF HAWKE'S BAY.............................................................................. 5-27 5.3.1 Research Investigations of Hawke's Bay Beach Sediments and Morphodynamics ................................................................................................. 5-27 5.3.2 The Hawke's Bay Beach Monitoring Program..................................................... 5-34 5.4 SUMMARY AND DISCUSSION .................................................................................... 5-40 5.5 REFERENCES .............................................................................................................. 5-43 6 The Coastal Response to the Construction of Moles (Jetties) and Breakwaters: The Hawke's Bay Experience.............................................6-1 6.1 INTRODUCTION ............................................................................................................. 6-1 6.2 THE CONSTRUCTION OF JETTIES AND BREAKWATERS: A REVIEW OF THE COASTAL RESPONSES ........................................................................................ 6-1 6.3 HARBOUR DEVELOPMENT AT HAWKE'S BAY AND ITS ENVIRONMENTAL CONSEQUENCES: A RE-EXAMINATION ................................................................... 6-11 6.3.1 The Pre-Construction Environmental Conditions ................................................. 6-14 6.3.2 The Ahuriri Moles.................................................................................................. 6-18 6.3.3 The Port of Napier's Breakwater........................................................................... 6-20 6.4 SUMMARY AND DISCUSSION .................................................................................... 6-24 6.5 REFERENCES .............................................................................................................. 6-27 7 The Hawke's Bay Littoral Cells: Processes, Erosion Problems and Management Strategies .............................................................................7-1 7.1 INTRODUCTION ............................................................................................................. 7-1 7.2 THE HAUMOANA LITTORAL CELL ............................................................................... 7-2 7.2.1 The Beach Sediment Sources ................................................................................ 7-5 7.2.2 The Losses of Beach Sediment.............................................................................. 7-8 7.2.3 The Budget of Beach Sediments ............................................................................ 7-9 7.2.4 The Longshore Transport of the Beach Sediments and Numerical Shoreline-Evolution Models ................................................................................. 7-13 7.2.5 Erosion Problems and Management Responses ................................................. 7-18 7.3 THE BAY VIEW LITTORAL CELL................................................................................. 7-25 7.3.1 Sediment Sources and the Budget of Beach Sediments...................................... 7-27 7.3.2 The Transport and Accumulation of Sand in Hawke's Bay .................................. 7-30 7.3.3 The Longshore Transport of Beach Sediment and the Equilibrium Shoreline ..... 7-35 7.3.4 Beach Erosion in the Bay View Littoral Cell and the Sediment Nourishment Program at Westshore......................................................................................... 7-40 7.4 MANAGEMENT STRATEGIES..................................................................................... 7-51 7.4.1 The Establishment of Hazard Zones .................................................................... 7-51 7.4.2 Shore Protection and Development Strategies .................................................... 7-56 7.5 SUMMARY AND DISCUSSION .................................................................................... 7-60 7.6 REFERENCES .............................................................................................................. 7-63 8 Summary and Discussion: Science and Management of the Hawke's Bay Coast.....................................................................................8-1 8.1 INTRODUCTION ............................................................................................................. 8-1 8.2 TECTONICS AND LAND ELEVATION CHANGES ........................................................ 8-2 8.3 NATURAL PROCESSES AND BEACH DYNAMICS .....................................................