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Contents

Preparing for coastal change

Introduction

Part One: The changing climate

Part Two: Implications for New Zealand’s coastal margins

Part Three: Responding to climate change

Conclusion

Factsheet 1: Components of sea level

Factsheet 2: Tides

Factsheet 3: Storm surge

Factsheet 4: Waves

Factsheet 5: Coastal erosion

Factsheet 6: Coastal inundation

Part Two: Implications for New Zealand’s coastal margins

Climate change will not create any new coastal hazards. At many locations it will exacerbate existing coastal erosion or inundation however, and expose other places to coastal hazards where these were no problem before.

Climate change will affect New Zealand’s coastal margins by:

• increased coastal erosion
• more extensive coastal inundation
• higher storm surge flooding
• increased drainage problems in adjacent low-lying areas
• seawater reaching further inland in estuaries and coastal aquifers
• changes in surface water quality, groundwater characteristics and sedimentation
• increases in seawater temperatures.

The magnitude of the impacts on coastal margins will differ between regions and even between localities within regions (see Case Study 1). This is because of the complex interactions between the:

• different ways climate change will affect the physical processes that shape the coast
• natural characteristics of the coast
• influence that humans have had or are having on the coast.

Case Study 1: Ohiwa Spit. What goes around...

The patterns of coastal change on Ohiwa Spit in the Bay of Plenty illustrate their effect on coastal development. This provides a good example of the problems in land planning and coastal hazard management around the New Zealand coast.

Ohiwa Spit has a long history of changes in the position of the coastline. Between 1867 and 1911, the coastline of the Spit tended to build seawards, or accrete.

In the late 1800s, a hotel was built on the Spit and, in the early 1920s, the area was subdivided. Within a few years, erosion was so rapid that the Ferry Hotel was lost and the township abandoned. A tidal channel ended up where the main street had been.

Further erosion took place until the late 1940s. Then, the Spit appeared to stabilise and during the early 1950s a new subdivision further down the Spit was developed. By 1965 erosion was again affecting property: several houses were lost to the sea over the next decade, despite various attempts to protect parts of the coast with makeshift seawall and railway-iron protection.

A series of storms in the mid- to late 1970s resulted in several properties falling into the sea. Buildings that survived the storms were removed from the coastline in 1976. Some landowners received compensation whereas others retained their titles to the land.

Since these storms, the Spit has been growing, with the beach building in width and dunes that had been lost during the storms, re-establishing. In early 2006, some of the remaining section titles were put up for sale and some have sold.

Sources: Richmond, BM, Nelson, CM, Healy, TR, 1984, Sedimentology and evolution of Ohiwa Harbour, a barrier impounded estuarine lagoon in the Bay of Plenty, New Zealand, Journal of Marine and Freshwater Research, 18, pp.461–478; Environmental Defence Society 2006, Society warns against Ohiwa Spit property sale. Press release 11 January 2006; Environment Bay of Plenty unpublished.

Photographs:
1. Photograph courtesy of RK Smith. (left)
2. 1976, photograph courtesy of EBoP. 3. 2005, photograph courtesy of Monique Ford. (right)

Coastal inundation

Climate change will substantially alter the frequency, extent and magnitude of coastal (saltwater) inundation. Interactions between changing sea levels, tidal ranges, changes to the frequency and magnitude of storm surges, and changes in storminess and wave conditions will add to this problem.

An increase in mean sea level will allow the gradual advance of seawater at high tides on low-lying coastal and estuarine land. Unless constrained by coastal protection works, these inundated low-lying areas will eventually become a permanent part of the coastal or estuarine system.

The most damaging inundation tends to occur during storm events that coincide with a reasonably high tide. Sea-level rise will increase the chance of inundation during such storm events, irrespective of any changes in the frequency or magnitude of storm surges, in storminess or wave conditions.

In areas already prone to coastal inundation, climate change means that coastal inundation during storms is more likely than today, given the same ground level or barrier height. Coasts with smaller tidal ranges will be more vulnerable than coasts with higher tidal ranges. The extent of the area at risk of inundation may also increase, depending on the site.

Increased sea levels will also affect rivers and streams, surface and storm water drainage, and sewer systems in low-lying coastal areas. The performance of these systems may be compromised by a back-up of flow due to increased downstream sea levels. Increased rainfall intensities may exacerbate the problem. Low-lying urban areas will be particularly susceptible.

Where overtopping of a coastal barrier is a primary cause of inundation, small changes in wave conditions (swell) may have a significant additional impact on wave set-up and run-up during storms (in addition to rising sea levels). The water tables along coastal margins may rise in response to rising sea levels; this may increase inundation directly, or could increase wave run-up and overtopping.

The potential for inundation may also be increased by coastal erosion. Human-made or natural coastal defences (such as dune systems or gravel barriers: Figure 3) may be lost. Loss of beach may increase the exposure during storm conditions, which is a particular issue in front of hard coastal defences.

Figure 3: Wash-out of the gravel barrier on the West Coast of the South Island during a storm in 2006. This has significantly increased the risk of inundation due to wave run-up and overtopping to the properties that back the beach. Photographs courtesy of Doug Ramsay.

Assessing the implications of climate change for inundation risk

To date, there have been few detailed studies on how climate change will affect coastal inundation risk in New Zealand. This is partly because high-resolution topography for coastal margins was lacking. An increasing area of coastal regions is now being mapped with LiDAR (Light Detection and Ranging), providing high-quality topography datasets on which to base such assessments (see Example 1 overleaf).

For assessing or quantifying the potential effects of climate change on inundation, the following factors need to be considered:

• interactions between various coastal hazard drivers (sea level, waves, storms, tides and sediment supply), the impacts of climate change on these individual drivers, and how they together influence inundation. Simply assuming that extreme water levels will always coincide with extreme wave conditions tends to overestimate inundation

• the dynamic nature of inundation over land. Particularly important are flood pathways (ie, how seawater inundates a certain area) and what the storage potential of a flood area is, relative to the volume of inundating water flowing in. Inundation tends to be overestimated by a ‘bathtub’ approach – extrapolating the water level landward until it reaches the equivalent height on land (based on a combination of extreme wave and water levels). But where inundation is mainly the result of waves overtopping a barrier, this approach may underestimate inundation

• availability and length of record of the datasets for sea level, weather and waves for the locality or region

• uncertainties associated with the assessment methods used, future greenhouse gas emission scenarios and the magnitude of their impact on coastal hazard drivers

• the lack of knowledge of how some of these coastal drivers will change with climate change, and the related sensitivity of inundation risk.

Coastal erosion

In many locations, climate change will influence changes in the position of the coast (and the Mean High Water Springs boundary) through changes to, and interactions between, the following drivers:

• relative sea-level rise
• long-term sea-level fluctuations
• the frequency and magnitude of storm surges
• tidal range (coasts with relatively small tidal ranges could be more vulnerable)
• storminess and wave and/or swell conditions
• rainfall patterns and intensity, and their influence on river and cliff sediment supply.

Rates of coastal erosion are not only dependent on these hazard drivers and changes to them. Landforms and geology of the coast, and any modifications that people have made (perhaps indirectly) to the coast, also play a part.

Even more so is the rate of coastal erosion determined by the waves and water levels. Rainfall and drainage patterns can be significant drivers for certain types of coast, such as soft cliffs.

The New Zealand coastline has a wide range of landforms. Yet, the sensitivity of different physical coastal environments to the likely effects of climate change is relatively straightforward and is summarised in Figure 4. Regional and local influences will result in local variations in the rate of coastal change.

Chapter 3 of the source report provides further discussion on the sensitivity of different coastal environments to climate change.

Figure 4: Generalised impacts of sea-level rise on different types of coastal morphology. These illustrations are only indicative: local geomorphology, human impacts and changes to the sediment supply may produce different responses. Photographs courtesy of Doug Ramsay. 1. Ohope Beach, Bay of Plenty 2. Mokau, Waikato 3. Haumoana, Hawkes’ Bay 4. North of Waitaki River, Canterbury 5. Kai Iwi Beach, Wanganui 6. Whatarangi, Cape Paliser, Wellington 7. Manakau Harbour, Auckland 8. Waimea Inlet, Nelson.

Assessing climate change effects on coastal erosion risk

Quantifying how the retreat and advance of coastlines will be influenced by climate change is extremely difficult, because many processes interact over multiple timescales. These processes include coastal hydrodynamics, morphology, geology, sediment supply and deposition and sometimes human modification.

Assessments of future coastal erosion usually look at the relationship between past erosion rates, the characteristics of the beach profile, and the relative difference between past and future sea levels.

Such simple approaches can provide a broad estimate of the potential for erosion along a coastline but cannot give location-specific assessments of potential change. Their use in predicting the coastline position at some time in the future, and in defining setback lines, should be treated with caution, as the implied level of certainty is rarely justifiable. A much more robust consideration of uncertainties is needed, and of how sensitive coastal change is to these. For example, uncertainty:

• about past erosion rates because of insufficient monitoring data
• in modelling assumptions and assessment methods
• in future emission scenarios, and the associated magnitude of their combined impact on the various coastal hazard drivers
• how some of these coastal drivers will change with climate change.

Salinisation of surface freshwaters and groundwater

• sea-level rise will cause salty water to encroach further up rivers and creeks
• longer dry or drought periods in eastern areas will lead to reduced river flows, which in turn will enable salty water to advance further up river
• sea-level rise causing higher water levels at the coast, within estuaries and lower parts of rivers, will exert a higher hydraulic head of saline water on unconfined groundwater aquifers.

Coastal defences

Many existing coastal defences in New Zealand have not been engineered to provide a high standard of protection. Therefore, climate change impacts could substantially increase damage to these defences resulting in reduced protection of the land behind them (Figure 5). If defences have been designed for a particular lifetime, they are unlikely to endure if climate change has not been factored in.

The frequency of defences being overtopped by waves or very high tides will increase as a result of higher sea levels (more so for coasts with smaller tidal ranges). Increased overtopping will not only affect the inundation risk, it will also be important for all coastal structures: it can increase erosion of the protected area behind the defence and can lead to failure of the defence itself.

Increased storm-tide levels will result in deeper water at the defence; this will increase the magnitude of overtopping during storms and exacerbate the above problems. Greater water depths at the structure will also increase the exposure of the defence to larger waves, increasing the risk of damage and failure. For example, the size of rock required for protection is directly related to the significant wave height. Even a small increase in wave conditions at the defence may require a large increase in the size of rock required to achieve the same protection.

Larger waves at the defence are likely to cause greater reflection from defence structures and increased erosion of the beach at the structure’s base. This increases the potential for undermining and/or failure of the defence.

Steepening of the foreshore can further increase the vulnerability of defences to overtopping and structural failure. While a defence may constrain the position of the high water mark, the landward retreat of the low-water position may continue as a result.

Figure 5: The level of protection provided by coastal defences will decrease as a result of climate change impacts on coastal hazard drivers, including sea-level rise. Photographs courtesy of Doug Ramsay.

Tsunami inundation

The geological causes of tsunamis are earthquakes, underwater landslides and volcanic activity; these will not be directly affected by climate change. However, the coastal effects of tsunamis will be altered somewhat by sea-level rise, through increasing the risk of coastal inundation. Estuaries and harbours may also become more vulnerable to tsunamis as entrance channels deepen in response to greater tidal water volumes (tidal prism). As always, the most important factor in determining the magnitude of tsunami impact is the height of the tide when the main tsunami wave reaches the coast.

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