Coastal Hazards in the Northland Region–Part II: Coastal

Overview of Weather and Coastal Hazards in the Northland Region–Part II: Coastal NIWA Client Report: HAM2003-114 October 2003 NIWA Project: NRC04301 Overview of Weather and Coastal Hazards in the Northland Region–Part II: Coastal Hazards Robert G. Bell Richard M. Gorman Prepared for Northland Regional Council NIWA Client Report: HAM2003-114 October 2003 NIWA Project: NRC04301 National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone +64-7-856 7026, Fax +64-7-856 0151 www.niwa.co.nz All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Contents Executive Summary iv 1. Introduction 1 2. Coastal setting for the Northland region 2 3. What are the coastal issues in the Northland Region? 3 4. Responses by NRC 6 5. Coastal hazard “drivers” 8 5.1 Tides 9 5.2 Sea-level fluctuations 12 5.3 Storms 15 5.3.1 Waves 16 5.3.2 Storm tides 21 6. Climate-change effects on coastal “drivers” 26 6.1 Sea-level rise 26 6.2 Climate change effects on ocean currents, winds, waves, and tides 27 7. Coastal sediment systems and coastal inundation in Northland 29 7.1 Overview of past studies and information 29 7.1.1 West coast Northland 29 7.1.2 Harbours and estuaries of Northland 30 7.1.3 Tidal inlets of Northland 32 7.1.4 Embayments, pocket beaches and continental shelf (east coast) 32 7.2 Climate-change impacts on sediment supply and coastal erosion 35 8. Conclusions—Coastal hazards 36 9. Acknowledgements 36 10. Bibliography and references 37 Reviewed by: Approved for release by: T.M. Hume T.M. Hume Formatting checked ………………… …… Executive Summary As a long peninsular, the Northland region is exposed to significant risks from both weather and coastal hazards. The long coastlines and the exposure to intense weather systems, be they ex-tropical cyclones or north Tasman-Sea lows, results in a region often confronted by natural hazard events. These events can lead to damage to properties and infrastructure, disrupting transport and lifelines, impacting upon the regional economy and in extreme cases they can put peoples lives at risk. This report (Part II) documents the current level of knowledge of coastal hazards for the Northland region, while Part I (Gray, 2003) covers the weather hazards. These two reports form part of an overview of the knowledge base on natural hazards for Northland, carried out under the umbrella of the NIWA/GNS Natural Hazards Centre, and commissioned by Northland Regional Council. The following sections provide a brief overview of the issues associated with coastal management in Northland, known information (publications and data) that exist on physical coastal environments and coastal hazards, and a broad outline of the effects of climate variability and climate-change. Generally, there is a reasonable pool of knowledge on coastal sediment systems, particularly on the east coast, and latterly within the Kaipara Harbour (west coast), especially by university and NIWA researchers. There are some gaps in developing fundamental databases on coastal “drivers”, such as wave climate, tides, winds and storms, to which these reports (Part I and II), partially address. Some projections are given for possible future impacts of climate change on the Northland region, with sea-level rise, effects on sediment supply to the coast and more intense storms likely to be the main impacts on coastal systems. Future work firstly needs to fill some gaps in the databases on coastal “drivers” and secondly usher in a more integrated “all-hazards” approach to coastal hazards, which examines the hazards from several different sources, including flooding. The latter will hopefully will be spurred on by the new Guidance Note for territorial authorities (MfE, 2003), which is based on a risk-management framework. Overview of Weather and Coastal Hazards in the Northland Region–Part II: Coastal Hazards iv 1. Introduction As a long peninsular, the Northland region is exposed to significant risks from both weather and coastal hazards. The long coastlines and the exposure to intense weather systems, be they ex-tropical cyclones or north Tasman-Sea lows, results in a region often confronted by natural hazard events. These events can lead to damage to properties and infrastructure, disrupting transport and lifelines, impacting upon the regional economy and in extreme cases they can put peoples lives at risk. This report (Part II) documents the current level of knowledge of coastal hazards for the Northland region, while Part I (Gray, 2003) covers the weather hazards. These two reports form part of an overview of the knowledge base on natural hazards for Northland, carried out under the umbrella of the NIWA/GNS Natural Hazards Centre 1, and commissioned by Northland Regional Council. The following sections provide a brief overview of the issues associated with coastal management in Northland, known information (publications and data) that exist on physical coastal environments and coastal hazards, and a broad outline of the effects of climate variability and climate-change. 1 For more details and quarterly Update, see web site: http://www.naturalhazards.net.nz/ Overview of weather and coastal hazards in the Northland region—Part II: Coastal hazards 1 2. Coastal setting for the Northland region Northland consists of a long, narrow peninsula, only 80 km across at its widest point, and 6 km at its narrowest point. It is bounded by the Tasman Sea on the west and Pacific Ocean on the east. Northland is dominated by rolling hill country, with modest areas of flat low-lying land are restricted to areas adjacent to the Awanui and Northern Wairoa rivers. Northland’s population of 140,130 (2001 Census) live mainly on the east coast, concentrated in Whangarei District and the Bay of Islands area. Northland’s coastline is around 1,700 km long, with no part of the region more than 40 km from the ocean. Consequently, the economic, environmental, social and cultural “goods and services” derived from coastal environments is an important component of life in Northland. Northland’s coast exhibits very contrasting types between the west and eastern coasts. The west coast of the region is a continuous sandy dissipative beach backed by active (transgressive) sand dune systems, broken only by the mouths of four harbours (Kaipara, Hokianga, Whangape and Herekino) and the rocky headland forming Tauroa Point (Ahipara). In contrast, Northland's eastern coast forms a very irregular shoreline with rocky headlands, sheltered deep-water harbours, sandy bays and mangrove forests. Also, there are numerous islands along the region's east coast that provide partial sheltering from easterly waves. The major island groups are those in the Bay of Islands, Poor Knights, Hen and Chickens and Cavalli Islands. The east coast is sheltered from the prevailing west to southwest winds, resulting in a mild wave climate, but is exposed to high wave activity from occasional northeasterly gales and extra-tropical cyclones. In contrast, the west coast is continually subject to active seas and swell originating from the Southern Ocean. Overview of weather and coastal hazards in the Northland region—Part II: Coastal hazards 2 3. What are the coastal issues in the Northland Region? Some of the pressures and issues faced by NRC with regard to natural character and coastal hazards are, as outlined in (NRC, 2002): 2 • Escalating demand for coastal subdivision and use of the coastal area has resulted in increased pressures on coastal margins, particularly foredune environments (especially where there is unformed access). • Extraction of sand in the NRC region has concentrated around the entrances to the Kaipara, Paregarenga and Mangawhai harbours. There is potential for erosion on adjacent shorelines. • Historically, development in coastal catchments, including land clearance and associated catchment development, has resulted in impacts on estuaries and harbours from sediment-laden waters (e.g., Fig. C.1) Figure C.1: Land clearance and partial reclamation of an estuary tidal creek in Bay of Islands area in the early 1980s [ Photo: RG Bell ]. • The threat to existing and future communities from natural hazards, the potential and nature of which may be unknown. 2 http://www.nrc.govt.nz/special/soe.2002/coastal/natural.character.of.the.coast/index.shtml http://www.nrc.govt.nz/special/soe.2002/coastal/coastal.hazards/index.shtml Overview of weather and coastal hazards in the Northland region—Part II: Coastal hazards 3 • Recognition and understanding of the range of existing natural hazard threats and the likely frequency and magnitude of particular events. • Identification of areas of high hazard risk, especially those prone to erosion, flooding (river or sea) and land instability, and provision of related information on avoidance measures to people. • Incorporation of comprehensive systems of hazard identification and analysis into the resource consent and building consent processes. • Damage to natural systems through inappropriate hazard protection measures. • The contribution that certain land-use activities have in increasing the hazard threat especially in high-risk areas. Such activities include: o clearance of vegetation by mechanical or other means in areas exposed to the elements and/or with poor soil structures; o changes to sediment load of rivers and streams (+ or –) due to changing land-use patterns; o earthworks, including mineral extraction, in sensitive foreshore and riparian areas; o erection of structures, especially buildings, in flood plains. • Maintenance of existing protection works, including flood control schemes and coastal protection works, and their future effectiveness.

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