Contents

Executive Summary

1. Introduction

2. Climate Change and Coastal Hazards

3. The Legislative and Planning Context

4. Risk assessment

5. Integrating Coastal Hazard Assessment and Climate Change into Council Planning and Decisions

6. Monitoring and review

7. Relevant case law

Glossary and abbreviations

References and bibliography

Web sites

Appendix 1: Relevant legislation

Appendix 2: Hazard drivers and the effects of climate change

Appendix 3: The response of different coastline types to coastal hazards

2. Climate Change and Coastal Hazards

This chapter identifies:

• current evidence for climate change and where the uncertainties exist;
• causes of coastal hazards;
• potential impacts of climate change on the coastal areas of New Zealand.

2.1 Predicting future New Zealand climate change

Literature and scientific studies suggest that not only is global climate subject to significant fluctuations, it is also rapidly changing, indicating an overall warming of the earth's surface and of its oceans and atmosphere. The third assessment report from the Intergovernmental Panel on Climate Change (IPCC) recently concluded that:

"There is new and stronger evidence that most of the global warming observed over the last 50 years is attributable to human activities." (Intergovernmental Panel on Climate Change, 2001)

The broad conclusion from a number of different approaches and scientific disciplines is that the world is warming at a rate faster than at any other time in the last 1000 years, coincident with a rapid rise in greenhouse gases. Sea level has also been steadily rising since the early to mid 1800s. In the future, sea-level rise is projected to accelerate.

Although significant uncertainties exist in projecting these changes into the future, overall it is likely that even at the lower end of projections, the changes over the next 100 years will be more rapid than natural variations over the last 10,000 years.

Changes in climate parameters can only be estimated. For many parameters (such as temperature and sea level) the direction of change is virtually certain (i.e., increasing), but the magnitude of that change is less certain. With other parameters, the direction of change may vary with region (e.g., rainfall is likely to reduce in many eastern regions but increase in the west). For others (such as wave structure and ocean currents) there is only limited understanding of how climate change may affect them, both in terms of magnitude and direction.

The Overview Manual provides the most up-to-date information on the expected changes in the range of climate parameters.

2.2 Causes and "drivers" of coastal hazards

2.2.1 Introduction

Hazards are created by a conflict between human use of the land, and physical processes at the coastline. The nature and extent of human use of the coastline must therefore be taken into account in a hazard assessment. For example, erosion of a cliffed coastline may not constitute a significant hazard in a pastoral farming area, but could be more significant in an urbanised area.

The three main types of natural coastal hazards are considered in this Guidance Manual. These are:

• 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.

These hazards are influenced by a range of natural causes and hazard "drivers". The main drivers that govern coastal erosion and inundation are shown in Figure 2.1. Both coastal inundation and coastal erosion arise from intricate interactions between several drivers, as also shown in Figure 2.1.

Human-induced factors can worsen the risk posed by coastal hazards. Examples of these are the effects of:

a) dams on rivers and irrigation abstraction that reduce sediment supply to the coast;

b) extraction of sand or shingle from the coastal zone, which can reduce the buffering ability of beaches to absorb storms;

c) ill-conceived shoreline protection works that worsen or shift the erosion problem 'downstream' or increase the wave run-up height;

d) dredging of harbour entrances and channels;

e) removal of coastal vegetation; and

f) the artificial lowering of dunes for sea-views or access.

A brief summary of the hazard causes and drivers shown in Figure 2.1 is provided below. More background information is provided in Appendix 2.

2.2.2 Sea-level fluctuations

'Sea-level fluctuations' refers to the fluctuations in the mean level of the sea, after taking out the influence of tides and without the influence of long-term sea-level rise. In terms of heightened inundation and coastal erosion risk, for any one month the mean level of the sea could reach up to 0.25 m above the average sea level. This is most likely to occur during La Niña episodes in decades when the 20 to 30-year Interdecadal Pacific Oscillation (IPO) cycle is in its negative phase. In this respect, we are currently in a negative IPO phase, which appears to have started in 1998 and may last until 2020 to 2030. Effects of IPO can mask or increase long-term sea-level rise for 20 to 30 year periods.

2.2.3 Tides

The height of a tide governs the likelihood of coastal inundation from a storm surge or river flooding. In addition, tidal currents at estuary entrances and constricted straits play a key role in supplying sediment to estuary beaches and adjacent open-coast beaches.

The mean high water springs (MHWS) level is a useful upper-limit against which to assess coastal inundation hazards. However, along central-eastern coasts from East Cape to Banks Peninsula, MHWS is exceeded quite frequently (20-50% of the time) by high tides. In these eastern areas, a 'pragmatical' MHWS that is exceeded only 10-12% of the time, or alternatively a mean high water perigean-spring level (MHWPS), should be used to assess the coastal inundation hazard (refer to Appendix 2 for further details).

2.2.4 Storms

Storms lead to two main hazard drivers [For more information, see the Tephra article by Bell & Gorman (2003).]:

• waves and swell, which de-stabilise and move large quantities of sediment, leading to erosion, and cause coastal inundation and even structural damage; and
• storm surges, where adverse winds and low barometric pressure produced by storms temporarily elevate the ocean level well above the predicted tide level (up to an upper limit of just over 1 m in New Zealand).

"Storm tide" level is a useful measure for inundation from the sea, and comprises MHWS + storm surge + wave set-up (refer to the diagram in Factsheet 2, Appendix 2). Wave set-up is the increase in sea level inside the surf zone (landward of the first wave breaks) relative to the offshore storm-induced ocean level. (See Appendix 2 to estimate wave set-up for the purposes of a screening risk-assessment.)

Wave run-up is the extra height reached, over and above the storm-tide level, as the broken waves run up the beach and coastal barrier (if present) until their energy is finally expended. Wave run-up is treated separately from storm-tide level because it varies widely along the coast, even in the same locality, due to differences in shoreline steepness and type of natural or artificial coastal barrier. In contrast, storm-tide levels can be calculated for large stretches of coast within a district. (See Appendix 2 to estimate wave run-up for the purposes of screening risk-assessment.)

2.2.5 Tsunami & subsidence/uplift

Although infrequent events, New Zealand faces a risk of inundation and damage from both local and remote tsunami sources, particularly along the entire east coast, Southland and Greater Cook Strait (including the South Taranaki Bight and Tasman/Golden Bay). Remotely-generated tsunami from across the Pacific will seldom exceed 5-10 m in maximum wave height at the coast, but there would be a wide area affected e.g., most of the eastern coast of both islands. Locally-generated tsunami will affect a more localised area, but wave heights could exceed 10 m. It is likely that there would be a reasonable warning time (of several hours) for remote tsunami, but there would be little warning of local tsunami because of the short travel distance to the coastline. The risk from tsunami may also be affected by the level of the tide and whether a local storm-surge is likely to be present.

Coastal margins may also be affected by fast-acting subsidence or uplift resulting from an earthquake (e.g., the 0.75 m subsidence of the coast at East Clive south of Napier in the 1931 earthquake). Areas near the coast may also experience slow subsidence as a result of groundwater abstraction (e.g., Christchurch).

2.3 Impacts of climate change on coastal hazard drivers

Climate change will not introduce any new types of coastal hazards, but it will affect existing coastal hazards by changing some hazard drivers. In general, localities that are currently subject to occasional coastal hazards are likely to suffer increased risks with a warming climate, while areas that are currently in a delicate balance may begin to experience more damaging coastal hazards in future.

This subsection summarises the effects of climate change on hazard drivers. More detail on these factors is provided in Appendix 2.

Table 2.1: Projections of climate-change effects

View projections of climate-change effects (large table)

2.3.1 Climate change effects on sea level

Since the early to mid 1800s, sea level around New Zealand has been rising at an average linear rate of 0.16 m per century. However, as global warming is now occurring and the oceans are beginning to warm, the rate of sea level rise is expected to accelerate in the near future.

Sea level can however vary considerably from year-to-year about the long-term trend, due to seasonal, El Niño-Southern Oscillation and IPO cycles. The extent of this is demonstrated in Appendix 2.

It is important to note that the IPCC expects that sea level will continue to rise for several centuries, even if greenhouse gas emissions are stabilised, due to long lag times for the deep oceans to respond. The expected continued melting of ice sheets or increase in iceberg calving from land-based ice sheets is expected to lead to a sea-level rise in the order of several metres over the next several centuries to millennia, even for the lower range of projected future climate-change scenarios. Apart from sea-level rise, a range of other climate changes can be relevant (summarised below). More detail and quantitative projections can be found in the manual Climate Change Effects and Impacts Assessment.

2.3.2 Climate change effects on storms

It is not clear from current modelling whether the frequency of ex-tropical cyclones reaching central and northern New Zealand will change, but when they do their impact on the coast might be greater due to a higher storm intensity. At the same time, 'storminess' is likely to increase in the Southern Hemisphere this century, so that both the intensity and frequency of mid-latitude storms might also increase in the New Zealand region. However, the levels of certainty for these New Zealand projections are currently low.

2.3.3 Climate change effects on currents, winds, waves, and tides

Ocean currents affect our climate and can influence the way storms develop. For ocean currents, the most likely future outlook for New Zealand is for little change to warm-ocean currents, but perhaps some modification of cold-ocean currents e.g., Antarctic Circumpolar Current.

The average westerly wind component across New Zealand may increase by approximately 10% in the next 50 years. As a result, there would be an increase in the frequency of heavy seas and swell along western and southern coasts, and possibly higher extreme waves during more intense ex-tropical cyclones and mid-latitude storms.

Deep ocean tides will not be directly affected by climate change, but tidal ranges in shallow harbours, river mouths and estuaries could be altered by deeper channels (following sea-level rise) or conversely by shallower channels if increased run-off from more intense storms increases sediment build-up in estuaries.

2.3.4 Climate change effects on sediment supply to the coast

Climate change will affect the intricate array of factors that govern the supply of sediment to the coast - some factors leading to more sediment delivery (e.g., more frequent heavy rainfall), others to less (e.g., likelihood of more droughts in eastern areas). The overall future effects on sediment supply to the coast for different regions of New Zealand are as yet poorly defined and likely to vary significantly between different locations. However, in vulnerable areas the overall impact on sediment supply to the coast and estuaries needs to be assessed by detailed investigations, which involve not just the coastal system, but also contributing rivers and their catchments.

2.3.5 Climate change effects on tsunami

The geological causes of tsunami (such as earthquakes, underwater landslides and volcanic activity) will not be directly affected by climate change. However, the coastal effects of tsunami will be altered somewhat by sea-level rise, increasing the risk of coastal inundation. A more important factor in assessing risks will be the height of the tide at the time a tsunami wave hits the coast.

2.4 Coastal hazard effects on different coastline types

Coastal hazards are not only dependent on the 'hazard drivers', but also on the geomorphology of the coast. Geomorphology relates to the features, sediment/geology composition, shape and topography of the coastal margins and beaches. There are four main types of geomorphology on the New Zealand coast:

• open-coast sand beaches;
• open-coast gravel (shingle) beaches;
• cliffed coasts; and
• estuary shorelines.

Each of these responds to coastal hazards differently. The key features that determine the vulnerability of a coast to erosion and inundation hazards are:

• the elevation of the coast above mean sea level;
• the geology of the coast (e.g., hard and soft rock, gravel and sand beaches);
• sediment supply and its availability for beach building;
• the width of the coastal barrier (if present);
• coastal setting and orientation (e.g., exposure to wave energy);
• the stabilising effect of vegetation; and
• shape, slope and height of any pre-existing coastal protection works.

The details of how different types of coast respond to hazards are given in Appendix 3.

Human activities can exacerbate these coastal hazards. Some of the more common ways that this can occur are summarised below. In some cases it is quite obvious where human modifications to the shoreline will increase the hazards, while in other cases the effects on the hazard risk are more subtle, and may take a long time to manifest.

• the lowering or total removal of foredunes lowers the height and width of the coastal barrier, reducing the buffering effect.
• 'hard' coastal protection works such as seawalls and stopbanks cut off supplies of sand to the beach and cause erosion in front of the walls, worsening the overall erosion. Erosion at the end of the wall is also increased.
• removal or changing the species of dune vegetation can increase sand loss through wind erosion, reducing sand storage and the system's buffer against erosion.
• mining of the beach or shallow nearshore for aggregate removes material from the sediment system and reduces the protection from dunes, gravel ridges and beach foreshores.
• stormwater discharges cause direct scour of the beach, and the accompanying rise in the water table increases erosion during storm events.
• dams or large water extractions from rivers interrupt sediment supplies to the coast.
• large structures such as breakwaters can affect beach orientation, exposure and sediment supply. Sand may accumulate up-drift against the structure, due to interference with the littoral transport system, while beaches down-drift of the structures may be 'starved' of sediment and erode.

2.5 The response of different coastline types to climate change

Some shoreline types, such as depositional sand and gravel beaches, are more vulnerable to the effects of climate change than others. How different shores may respond is relatively well understood, but it is far more difficult to quantify the effects. The key responses to various drivers of change are summarised below.

2.5.1 Response to long-term sea level rise

Figure 2.2 summarises the potential impacts of sea level rise on shoreline movement for different types of coastal geomorphology. In general, shorelines that have historically exhibited erosion will continue to erode, but faster, under accelerating sea level rise.

Sandy coasts

Sandy open coasts that have been relatively stable over time are likely to show a bias towards erosion under a higher sea level, unless the sand supply to the beaches can keep pace with erosion. In some parts of New Zealand this balance between erosion and sediment supply is quite possible. With sea level rise, accreting open coast beaches may continue to accrete, but more slowly, depending on sediment supply.

It is very difficult to quantify the amount of shoreline retreat for a given rise in sea level, and there are different theories on how to attempt this. Locations with higher dunes may suffer less retreat than locations with low dunes. However it is generally accepted that climate change will increase shoreline erosion for sandy beaches, particularly 'bounded' beaches with low dunes. In some situations, the width of the present foreshore or beach will not be sufficient to accommodate this erosion. Where the beach is 'bounded' this may result in the loss of the beach.

It is important to remember that sea level rise will continue for several centuries beyond 2100, even if greenhouse gas emissions are stabilised. Erosion of sandy beaches is therefore likely to continue well beyond this century.

Gravel beaches

Gravel beaches generally have steeper slopes than sandy beaches, so they are likely to suffer less erosion due solely to sea-level rise. For example, for a 6 m high gravel ridge the estimated increase in erosion rate over existing retreat rates is in the order of 0.1-0.2 m/yr for the next 50 years, and 0.1 to 0.3 m/yr for the next 100 years. However, where shoreline retreat is by sediment 'rollover' due to wave overtopping, rates of retreat may increase more with a higher mean sea level and more intense extreme storms.

Cliffs

Erosion of cliffs that comprise sedimentary rocks and clays/silts will, in most cases, continue at similar or slightly higher rates under climate change for moderate to high cliffs.

For cliffs with a gravel barrier beach at the base, positive changes in the barrier may result in no changes to current cliff retreat rates.

Estuaries

The effects of sea level rise on estuarine erosion will depend on a complex interrelationship between the topography of the estuary, sand storage in the estuary, river inputs of sand, and erosion of adjacent beaches. Shorelines will retreat as a result of both inundation and slow but steady erosion. Erosion will be slow because, compared to open coast beaches, estuaries have a less energetic wave environment and limited exposure time (around high tide) for waves to develop.

Sedimentation rates in most North Island estuaries have been 2-4 mm/year, keeping up with the present rise in sea level of 2 mm/year. Eventually however, the acceleration in sea-level rise is likely to outstrip sedimentation.

In spite of the compensating effect of sedimentation, sea-level rise is likely to cause an increase in the amount of water that flows in and out of estuaries during each tide (the 'tidal prism'), along with larger increases in freshwater run-off during heavier rainfall events. Increased flow volumes will correspond to increases in tidal velocities and scour in the main channels and particularly at tidal entrances. This is because estuary surface areas (and hence volumes) will increase greatly if surrounding flat land is inundated by high tides on the back of higher sea levels. These changes might be quite marked because of the shallowness of New Zealand estuaries. Although there are large uncertainties involved, current thinking is that sea-level rise could lead to increases in shoreline erosion within many estuaries. There is also concern that estuarine ecosystems may be 'squeezed out' if there is no space for them to retreat landwards as sea level rises.

2.5.2 Response to changes in wave climate

Wave direction changes

Changes to wave direction could be caused by a shift in the wind climate, and/or a reduction in wave refraction associated with an increase in water depth. This would be most pronounced where deep water waves are heavily refracted, such as around prominent headlands (e.g., Banks Peninsula and off the East Cape). Longshore sediment transport potential could be increased due to changes in wave direction, increasing the rate of shoreline retreat.

It is difficult to quantify potential changes to wave direction. However, subtle changes in wave direction will have greatest effect on pocket sand beaches by moving sand from one end of the beach to the other, and on cuspate forelands (salients) that form in the wave-lee of an offshore island (e.g., the Paraparaumu-Waikanae coast in the lee of Kapiti Island).

For estuaries, the effect of changes in predominant wind direction on the wave climate will depend on the size and shape of the estuary. The greatest effect will occur in wide shallow estuaries where there is a large wind fetch.

Wave height changes

It is unlikely that a modest increase in storm wave heights will increase erosion markedly on sandy beaches, since bigger waves would break at a similar position or further offshore with a higher mean sea level. However, in combination with rising sea level and possible higher storm tides, waves will generally be able to attack the backshore and foredunes more readily in many localities, leading to a combined adverse effect on beaches from sea-level rise and increased wave height.

For gravel beaches and cliffs where there is not large deposition on the nearshore bed, a slight increase in breaker height is expected as a result of increases in water depth. A small increase in the run-up elevation, and therefore inundation, is likely.

For cliffs, any increases in sea level and wave height will result in erosion at slightly higher levels. However rates of undermining may not increase markedly, except on low cliffs (several metres height).

In estuaries, significant changes in wave height are unlikely to occur in the foreseeable future, until such time as sedimentation rates no longer keep pace with the projected acceleration in the rate of sea-level rise.

2.5.3 Response to changes in storm intensity

Some climate change scenarios suggest that the frequency of low-pressure troughs and depressions passing over New Zealand might increase slightly. This could result in more frequent moderate coastal storms.

These changes cannot be quantified at present (other than the possible direction of change), but there is likely to be greater short-term erosion of sand and gravel beaches at many locations. The recovery of foredune or gravel ridges between storms will also be more limited, particularly during unfavourable El Niño-Southern Oscillation and IPO cycles. Somewhat increased erosion of sedimentary cliffs may occur, especially when combined with more adverse soil-hydrological processes from greater extremes of drought and heavy rainfall. Estuary shores are also likely to suffer from more erosion, with the increased frequency and magnitude of shoreline inundation from storm tides and associated wetting and drying of soils likely to be major factors.

2.5.4 Response to changes in sediment supply and transport

The effect of climate change on fluvial erosion and sediment transport processes will have a large influence on the behaviour of depositional sand and gravel beaches. In some areas (e.g., West Coast), increases in rainfall intensity will increase erosion in upper catchments and sediment transport. In these locations, the additional supply may be sufficient to offset other climate-change effects. However, in areas where there is decreased rainfall (e.g., some east coast areas), sediment supply is likely to be reduced, and shoreline erosion is likely to be exacerbated even further.

2.5.5 Response to changes in beach water tables

Higher water tables increase wave run-up and backwash velocities, increasing both run-up elevations and sediment losses to the nearshore.

Coastal water tables may rise as a consequence of sea level rise, increasing the potential for beach erosion. However, these effects are dependent on how the beach profile adjusts to the higher water table regime, and cannot be easily quantified.

2.6 Other climate-change effects on the coastline

While the direct effects of climate change on coastal erosion and coastal inundation are the main focus of this Guidance Manual, other climate-change impacts should also be considered in any long-term planning process. These include:

• Flooding and drainage of low-lying coastal land-caused by:
  • increased frequency of storm-tides exceeding a specific level or barrier height;
  • greater rainfall intensity of storms (and hence river floods);
  • longer parched or drought periods in some areas (which create run-off problems during an intense storm); and
  • increase in the intensity of severe storms.
• Salinisation of surface freshwaters, groundwater and land- caused by:
  • sea-level rise, which allows saline marine water to encroach further up river and creek watercourses;
  • longer parched or drought periods in eastern areas (leading to reduced river flows, allowing the tidal flows to reach further upstream);
  • sea-level rise, with higher marine water levels in the sea, estuaries and lower river reaches, exerting a higher hydraulic head of saline water on groundwater aquifers; and
  • increase in the frequency and peak levels of storm tides, leading to more frequent inundation of arable land by saline waters - eventually very low-lying lands will naturally convert to salt marshes if not constrained by coastal defences.

Coastal Hazard Factsheet (1): Coastal erosion

Hazard Description:

Coastal erosion hazards arise where human activity or settlement is threatened by a temporary or permanent cut-back of the shoreline. (Coastal accretion is the opposite, where the shoreline builds out over time.)

In many instances, an entire beach-ocean system in a region (e.g., Canterbury Bight) may be balanced in terms of sediment 'credits' and 'debits'. This means that sand or gravel can be moved around within the coastal system in large quantities, but with little net loss or gain of sediment from the system as a whole. However, from the property owner's viewpoint the temporary or permanent retreat of the coastline at their locality is coastal erosion, even if the coastal system as a whole is in balance. Cliffs mostly are erosional features, but the erosion rate can vary greatly.

Coastal erosion poses many problems to established coastal communities in that valuable property can occasionally be lost to a dynamic beach-ocean system. Where there are 'hard' protection structures and a long-term trend of erosion, valuable natural assets such as beaches, dunes and wetlands can be lost as the continuing erosion results in 'coastal squeeze' between the sea and housing or infrastructure. Additionally, human activity may exacerbate the process of coastal erosion through poor land-use methods or the downstream effects of poorly-designed protection works.

Because there are so many factors involved in coastal erosion (see below), shoreline change from sediment "re-distribution" within a beach-ocean system will not be consistent year after year in the same location, but can occur in alternate erosion and accretion cycles over seasonal cycles up to several decades. This means the prediction of future coastal erosion at any locality is very difficult without adequate data, historic information, and good estimates of future climate-change impacts.

Natural Factors:

Natural factors that affect coastal erosion or coastal stability are a complex interaction of:

Weather (wind, waves and storm surge); oceanography (tides, offshore and alongshore currents); climate (seasonal, El Niño-Southern Oscillation, Interdecadal Pacific Oscillation, sea-level rise, catchment run-off); geomorphology (type of beach/barrier system and how it responds, e.g., mixed gravel/sand versus sandy beach, stability of sandspits, intertidal estuary beaches, cliff erosion processes); sediment supply to the shore zone from cliffs, rivers, estuaries, winds, and the offshore seabed; and seismic/tectonic factors (e.g., coastal uplift or subsidence, tsunami).

Removal and deposition of sediment by natural sediment "drivers" continually changes beach shape, volume and structure. Sediment may be transported landwards of the dunes by wind or overwash during storms, temporarily moved to nearshore bars during storms, moved further along the coast, or lost offshore to the continental shelf.

Human Factors:

Human intervention can markedly alter natural coastal sediment processes through:

• Catchment activities e.g., land-use practices, urbanisation, dams, water abstraction;
• Dredging of tidal entrances and harbour channels;
• Sand or gravel extraction from the coastal marine area;
• Coastal protection works e.g., groynes, breakwaters, artificial reefs, seawalls;
• Beach nourishment;
• Permanent modification of coastal margins e.g., dune removal, vegetation removal or change, reclamations, waterways, wharfs and marinas.

Climate-change Influences:

Global warming will impact on most of the above natural factors that affect coastal erosion, apart from the seismic/tectonic group. The most commonly known effect is that of sea-level rise, but climate change will also alter rainfall and run-off patterns (which may cause a change in sediment supply to the coast) and storms may increase in their intensity for a given return-period event, increasing the impacts of waves, wind and storm-surge.

Typical Ranges of Coastal Erosion Rates:

Sandy beaches: highly variable, even within a locality, but mostly < 5 m long-term retreat p.a., but the impact of extreme storm events in some areas can result in erosion of 10+ m and the end of unstable sandspits can experience erosion of 100+ metres.

Gravel: <1 m p.a. in many areas, but up to 2 to 3 m p.a. (average) in vulnerable areas. Retreat usually occurs episodically during extreme storms (up to 5 to 10 m retreat), with stable periods between storms.

Cliffs: high variable, depending on soil/geology composition and hydrologic processes; hard-rock erosion can be negligible, but unconsolidated materials may reach several cms to a metre or so p.a.

Frequency of Occurrence:

Coastal erosion occurs across a wide range of timescales, ranging from individual storms, through annual and El Niño cycles up to long-term retreat at decadal or century scales. Normal practice is to deal with erosion on two timescales: short-term fluctuations (days to a few months, including storm cut-back) and long-term trend (seasonal to decades/centuries).

Hotspot Regions around NZ:

The only national overview to date of long-term erosion and accretion rates around the New Zealand coast are from the Gibb study published in 1984, but work is in progress to update this work with the latest information.

Erosion vulnerability checklist based on geo-indicators

This table indicates the potential for erosion based on geo-indicators for different morphology types - refer to Section 7.5 for further details (based on Bush et al. 1999).

View the potential for erosion based on geo-indicators for different morphology types (large table)

Coastal Hazard Factsheet (2): Storm inundation

Hazard Description:

Storm inundation is an acute natural event arising from extreme weather events (storms), where normally dry land is flooded occasionally. Most people associate flooding with rivers, but sea flooding can occur in low-lying coastal lands, and sometimes both river and sea-flooding combine to increase the hazard.

Sea flooding is caused by a temporary increase of mean sea level (called "storm surge") and energetic wave activity, over and above the predicted high tide height. Storm surge is generated by a combination of adverse winds and low barometric pressure. Waves contribute to coastal inundation by a combination of wave set-up in water level in the surf zone and wave run-up across the beach, which may overtop low coastal barriers. "Storm tide" is the term used to quantify the total height in sea-level reached at the shore, combining tide, storm surge and wave set-up (refer to diagram on following page), to which wave run-up is added. The force of wave run-up and overtopping can also inflict damage on properties and cause injuries. In some instances, sea flooding can occur during local fair weather, when large swells from a distant storm ride in on the back of a very high tide.

Riverine flooding of coastal and estuarine margins is exacerbated by high tides, especially the fortnightly spring tides or monthly perigean tides (when the Moon is closest to the Earth). In relatively flat low-lying coastal margins (e.g., Lower Heathcote at Christchurch, South Canterbury Plains, Hauraki Plains), land may stay flooded with seawater for several days after an extreme event. This type of occurrence has a dramatic effect on vegetation and pasture production, which can sometimes last for a number of years.

Natural Factors:

Natural factors that affect coastal storm inundation are a complex interaction of:

Winds (strong persistent on-shore winds that "pile up" water along the coast); barometric pressure ("inverted barometer" effect, where sea level rises by 0.1 m for every fall of 10 hPa in barometric pressure below the average pressure); sea-level fluctuations (increased elevation of mean sea-level at seasonal, 3 to 5 year El Niño-Southern Oscillation, and 20 to 30 year Interdecadal Pacific Oscillation cycles); tides-timing and height of high tide is critical; waves & swell cause: a) wave set-up - the elevation in water level across the surf zone caused by energy expended by breaking waves, and b) wave run-up - the ultimate height reached by waves after running up the beach and barrier (both are highly dependent on wave height, but also beach slopes and sediment type); river levels near estuaries, lagoons and river mouths following heavy rainfall; geomorphology (type of beach/barrier system, slope of beach and backshore barrier, size of beach sediments, how porous or free draining the sediments are); and seismic/tectonic factors (coastal uplift or subsidence of coastal barrier).

Human Factors:

Human intervention can exacerbate storm inundation hazards through:

• River training works (straightening, stopbanking) that increase river levels at the coast;
• Poor design of existing coastal protection structures (type, slope, smoothness of surfaces, wave focusing by offshore structures or groynes);
• Coastal property development in flood-prone areas (low-lying estuary margins or shore-front properties without an adequate buffer);
• Physical removal, reduction or damage to natural coastal barriers such as sand dunes, gravel barriers (e.g., lower access ways, removal of vegetation, trimming or removing dunes);
• Permanent modification of coastal margins e.g., waterways, canals, marinas, boat ramps.

Climate-change influences:

Global warming will impact on many of the above natural factors that drive coastal storm inundation. The best known effect is the direct contribution of an accelerated sea-level rise, which will lead to two impacts:

• higher likelihood (or probability) that coastal inundation during storms could occur, given the same specific ground level or barrier height
• gradual encroachment of seawater at high tides on low-lying coastal and estuarine land by rising sea levels.

For the latter, low-lying areas will transform into a coastal marsh and eventually become a permanent part of the coastal or estuarine system if the process is not constrained by coastal protection works.

Climate change is likely to affect storms by increasing their intensity for a given return-period event, and therefore increasing the contributors to storm tides (wave, wind and barometric pressure). At present it is uncertain whether climate change will affect storm frequencies. Climate change is likely to alter rainfall and run-off patterns, with the downstream effect of increased river levels. Tide heights may also increase in shallow estuaries and harbours, where siltation by catchment sediments doesn't keep pace with sea-level rise.

Typical Range of Contributors to Coastal Storm Inundation Heights:

Tides: fortnightly spring or monthly perigean high tides range from 2.1 m above the mean level of the sea in Golden Bay to only 0.3 m in Cook Strait at Oteranga Bay, west of Wellington.

Sea-level fluctuations: on seasonal to multi-decade scales, fluctuations of up to ±0.25 m can occur in the mean level of the sea. Historic sea-level rise in New Zealand has been linear at approximately 0.16 m per century.

Storm surge: storm-surge heights above the predicted tide can reach around 0.5 m on a yearly return period, and potentially can reach an upper limit of just over 1 m around New Zealand.

Waves: on a typical sandy coast with offshore wave heights ranging from 4 to 7 m, wave set-up in the surf zone from breaking waves would add an extra 0.6 to 1 m respectively to storm-tide levels at the shore. Finally, wave run-up in storms can add a further 2 to 6 m vertical height onto the storm-tide level (but councils should be aware that various formulae for estimating wave run-up may or may not include wave set-up as well). The large variability arising from widely-differing local shoreline features such as beach slope, type and slope of coastal barrier, presence of a coastal protection structure and its steepness, and the width of the surf zone (e.g., a wave breaking right in close to a seawall will produce high wave run-up heights). Typical sandy coasts with an unmodified backshore will generate wave run-up heights from 2 to 4 m for offshore wave heights in the range 4 to 7 m.

previous next