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Executive Summary

Climate change will not introduce any new types of coastal hazards, but it will affect existing hazards. Coastal hazards in many areas are expected to increase as a result of the effects of climate change. As development of coastal areas and property values increase, the potential impacts of coastal hazards increase.

There is increasing confidence in the predictions of the effects of climate change. Sea level has risen in New Zealand by about 0.25 m since the mid-1800s (historical sea-level rise has been approximately 0.16 m per century), and this rise is expected to accelerate. Under the most likely mid-range projections, sea level is projected to rise a further 0.14 - 0.18 m by 2050, and 0.31 - 0.49 m by 2100. In developing scenarios, it is recommended that at least the most likely mid-range scenario for sea-level rise is used: it is recommended that council staff use a figure of 0.2 m by 2050 and 0.5 m by 2100 when considering sea-level rise in projects or plans.

Sea-level rise and other climate change effects, such as increased intensity of storms and changes in sediment supply to coastlines, are expected to modify coastal hazards in many areas around New Zealand.

Because climate change effects are very gradual, land-use planning decisions must have long-term horizons to accommodate the lifetimes of structures. It is vital that planning occurs now for climate change effects, particularly where decisions are being made on issues and developments that have planning horizons and life expectancies of 50 years or more.

This Guidance Manual is intended to help local authorities manage coastal hazards by:

  • providing information on the effects of climate change on coastal hazards;
  • presenting a decision-making framework to assess the associated risks;
  • providing guidance on appropriate response options.

Three main types of coastal hazard are addressed:

  • coastal erosion caused by storms and/or long-term processes;
  • coastal inundation caused by storms or gradual inundation from sea-level rise;
  • coastal inundation caused by tsunami.

This document recommends Risk Management standard AS/NZS 4360 as the under-girding standard for all risk assessment procedures and processes relating to coastal hazards in a changing climate.

This document is a companion to "Climate change effects and impacts assessment", a further climate change guidance manual published by the New Zealand Climate Change Office. "Climate change effects and impacts assessment" is an 'umbrella' document that addresses the effects of climate change on the whole range of local authority functions. It can be found at: http://www.climatechange.govt.nz/publications/climate/effects-impacts-may04/index.html.

What are the potential impacts on New Zealand's coastal areas?

In general terms, accelerating sea-level rise around New Zealand may result in:

  • increased coastal erosion in some areas. Parts of the coastline that have historically been eroding may experience increased erosion trends; other areas that may have been relatively stable may begin to erode;
  • permanent high-tide inundation of very low-lying margins that may at present experience only episodic inundation;
  • episodic sea flooding of higher coastal and estuarine margins;
  • salinisation of adjacent rivers and streams and landward intrusion of saline groundwater;
  • drainage problems in adjacent low-lying areas, especially where gravity is relied on;
  • increasing 'coastal squeeze', where shorelines are held and constrained by structures such as seawalls and stopbanks, resulting in a reduction of intertidal area and loss of beach; and
  • increased rates and frequency of episodic wave run-up and overtopping of both natural and man-made coastal defences.
Other aspects of climate change will also affect many of the other physical 'drivers' that shape coastal margins and ecosystems, such as winds, waves, storms, sediment supply, and sea temperature. For example:
  • increase in storm rainfall intensities may lead to increased or more frequent lowland river flooding and impacts on water quality from increased sediment loads to estuaries, although sediment availability will depend on catchment land-use and construction practices;
  • change the way sediment is distributed along a coastline through changing longshore transport patterns and 're-aligning' of beaches;
  • altered hydrological soil processes (e.g., greater extremes of drought versus intense rainfall), that may exacerbate erosion and landslips on unconsolidated coastal cliffs;
  • changes in wind, ocean currents and waves may alter coastal sediment movement and coastal upwelling of cooler nutrient-rich ocean waters that is important for coastal productivity, including fisheries; and
  • aquatic ecosystems will be affected by rising temperatures (air and water), potential loss of habitat from constraining stopbanks in some areas, while in other areas, will be impacted by possible increases in sediment loads entering estuaries during storms.

The magnitude of the impacts on coastal margins will differ between regions and even between localities within regions, depending on the localised impacts of climate change on the physical 'drivers' that shape the coast, the natural coastal characteristics, and the influence of man-made coastal developments.

This Guidance Manual provides a risk-based decision-making framework, following the cyclical steps in the diagram below.

Thumbnail of image. See figure at its full size (including text description).

A risk 'screening' process is advocated, where the level of detail in the analysis is consistent with the perceived degree of risk. The process can be used to prioritise hazards in a district or region, although it is not intended for detailed analysis of risks.

This document recommends Risk Management standard AS/NZS 4360 as the under-girding standard for all risk assessment procedures and processes relating to coastal hazards in a changing climate.

Risk assessment requires analysis of the probability of a particular hazard event (e.g., erosion or inundation) occurring, and its consequences on people, the natural and built environment and local economy. An important feature of coastal hazards and climate-change effects is that both the probability and consequences of coastal hazards - and hence the associated risks - are likely to increase in the future. The probability of hazards is likely to increase due to climate-change effects, while at the same time the consequences are likely to increase because of increasing coastal development and property values.

This manual provides guidance on assessing the probability of the hazard event, which is a function of the hazard 'drivers' (e.g., storms), the climate-change effects on those hazard drivers, and the morphology of the coastline (e.g., sandy beaches, gravel beaches, cliffs and estuaries). While the Guidance Manual is primarily intended for use by local authority planners, the assessment of coastal hazards should also involve an experienced coastal hazards practitioner. Back-of-the-envelope calculations are not usually possible for coastal hazard assessments because of the complexity of the problems.

Once the coastal hazards are prioritised, the next step is to assess and decide on the most appropriate response options to the hazards through the preparation of a "Response Strategy". The underlying premise in the "Response Strategy" is to manage the consequences of potentially hazardous coastal processes. While it is very difficult to reduce the likelihood of a particular coastal hazard event occurring, it needs to be remembered that the frequency of a particular hazard may increase as a result of climate change. This Manual identifies principles and management options that can be used to address climate change-induced coastal hazards and mechanisms currently available to implement these options.

A general hierarchy of response options is advocated in this Guidance Manual:

  1. activities and land-use practices to protect natural defences such as sand dunes, gravel ridges, cliffs, salt marshes, and vegetation;
  2. management of land use to avoid areas of coastal hazard (e.g., location of development away from coastal hazards, retreat or relocate infrastructure);
  3. undertake 'soft defence works' such as re-vegetation, beach access-ways, beach nourishment, or drainage; and
  4. undertake 'hard structural works' such as seawalls, rock armoring or groynes.

The first two options are more easily applied where there is little existing development but land use intensification is proposed (e.g., greenfield developments). In such areas it is prudent to provide an appropriate buffer between the shoreline and the development to maintain natural defences against coastal erosion and inundation, preserve the natural character of the coastline, and maintain public access to the shore.

The situation is more difficult where the coastline is already well developed. Focus is often put on protecting assets using hard structures such as seawalls or rock armouring. If protection options are to be used, 'soft' options should generally be given priority over 'hard' options.

Structural options may be appropriate when used as part of a management 'package', where the structural works are a short-term solution in conjunction with a longer-term planning option. For example, temporary works (e.g., sand sausage) might be implemented as an interim measure, designed to last until a planning technique, such as managed retreat, has progressively relocated at-risk development.

Long-term monitoring of the effects of coastal hazards should be undertaken to improve our understanding, and ensure that response options are effective and sustainable. Monitoring techniques need not be expensive (e.g., a regular photographic record), but in high-risk situations, robust monitoring programmes that will provide useful information for future assessments of coastal hazards and response options should be considered.