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Coastal Hazards and Climate Change: A Guidance Manual for Local Governmentin New Zealand

Executive Summary

1 Introduction to the Guidance Manual

2 The Changing Climate

3 Implications for New Zealand’s Coastal Margins

4 Responding to Climate Change: Future-proofing Decision-making

5 Understanding Changing Coastal Hazard Risk

6 Managing Coastal Hazard and Related Climate Change Risks

7 Further Resources

8 References

9 Glossary

10 Appendix 1: Relevant legislation

11 Appendix 2: Relevant case law

12 Factsheet 1: Coastal erosion

13 Factsheet 2: Coastal inundation (storms)

14 Factsheet 3: Coastal inundation (tsunami)

15 Factsheet 4: Components of sea level

16 Factsheet 5: Tides

17 Factsheet 6: Mean High Water Spring (MHWS)

18 Factsheet 7: Storm surge

19 Factsheet 8: Long-term sea-level fluctuations

20 Factsheet 9: Datums – Mean Sea Level and Chart Datum

21 Factsheet 10: Waves

22 Factsheet 11: Wave set-up, run-up and overtopping

23 Factsheet 12: ENSO and IPO

3 Implications for New Zealand’s Coastal Margins

3.1 Introduction

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The magnitude of the impacts on coastal margins will differ between regions and even between localities within regions.  Such impacts will depend on the complex interaction between the localised impacts of climate change on the physical drivers that shape the coast, the natural characteristics of the coast, and the influence that humans have had or are having on the coast.

3.2 Coastal inundation

The frequency, extent and magnitude of coastal (saltwater) inundation will be substantially altered by climate change effects and by interactions between the following drivers:

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• mean sea-level rise

• long-term sea-level fluctuations

• tide range

• changes to the frequency and magnitude of storm surges

• changes in storminess and wave conditions.

An increase in mean sea level will allow a gradual encroachment of seawater at high tides on low-lying coastal and estuarine land.  If not constrained by coastal protection works, the inundations of such low-lying areas will transform them into coastal marsh and they will eventually become a permanent part of the coastal or estuarine system.

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• for existing areas prone to coastal inundation, climate change means that coastal inundation during storms could become more likely relative to the present day, given the same specific ground level or barrier height.  Coasts with smaller tide ranges will be more vulnerable (eg, east coast on both the North and South Islands and Cook Strait / Wellington) than coasts with higher tide ranges

• the extent of the area at risk of inundation may well increase relative to the present day (although this will depend on the specific 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 further exacerbate the problem.  Low-lying urban areas will be particularly susceptible.  Figure 3.1 indicates potential areas around New Zealand’s coastal margins where inundation may be influenced by changes in the coastal hazard drivers.

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Where overtopping of a coastal barrier is a primary pathway for inundation, in addition to changing sea levels, small changes in swell wave conditions may have a significant impact on wave set-up and run-up during storms (Box 3.1).  The water tables along coastal margins may also be higher in response to sea-level rise, which may increase inundation directly or potentially increase wave run-up and overtopping.

Source: Bell et al 2006; Acknowledgement: Environment Bay of Plenty

The potential for inundation may also be exacerbated by coastal erosion (see next section) where erosion leads to a loss of either human-made or natural coastal defences (such as dune systems or gravel barriers: Figure 3.2), or where loss of beach increases the exposure during storm conditions (a particular issue in front of hard coastal defences).

Figure 3.2: Wash-out of the gravel barrier on the west coast of the South
Island during a storm in 2006 has significantly increased the risk of inundation
due to wave run-up and overtopping to the properties that back the beach

3.2.1 Assessing the effects of climate change on inundation risk

There have been no peer-reviewed studies on how climate change will affect coastal inundation risk in New Zealand.  In part, this is owing to the lack of high-resolution topography for coastal margins.  This is now changing, with an increasing area of coastal regions being mapped with LiDAR⁴⁴ providing high-quality topography datasets on which to base such assessments (see Box 3.2).

Where some level of quantification of the potential effects of climate change on inundation is required, the approach adopted in any area will very much depend on the characteristics of the area, the level of detail required for the issue under consideration, and the availability and suitability of datasets such as topography and beach profiles.  Any quantifiable assessment will need to give due consideration to the:

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• interactions between the various coastal hazard drivers, the effects of climate change on these drivers, and how these interactions and effects influence inundation.  Coastal inundation is rarely caused by one factor alone (eg, storm surge); it is normally due to some combination of tide level, storm surge and wave conditions (and, in certain cases, exacerbated by river or land drainage contributions).  These factors are typically correlated in some way but very rarely does an extreme high tide level coincide with both high storm surge and high wave conditions.  Understanding how these different drivers are correlated (known as ‘joint probability’) is important in assessing coastal inundation.⁴⁵  Simply assuming that extreme water levels will always occur at the same time as extreme wave conditions will tend to overestimate inundation risk

• dynamic nature of inundation over land, particularly the mechanism of how seawater inundates a certain area (flood pathways) and the storage potential of a flood area relative to the volume of inundating water flowing into the area.  For example, in an overtopping situation, swell will generally contribute a greater volume of seawater to inundation than will shorter-period wind waves.  Assuming a ‘bathtub’ approach – in which a water level is extrapolated landward until it reaches the equivalent contour height on land (based on a combination of extreme wave and water levels) – will tend to overestimate inundation.  However, where inundation is primarily a result of waves overtopping a barrier, this approach may underestimate inundation levels

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

• uncertainties associated with the assessment methods used, future greenhouse gas emission scenarios and the associated 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 hence how sensitive inundation risk is to these uncertainties.

3.3 Coastal erosion

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• relative sea-level rise

• long-term sea-level fluctuations

• the frequency and magnitude of storm surges

• tide range (coasts with relatively small tide ranges could be more vulnerable)

• storminess and wave and/or swell conditions

• rainfall patterns and intensity, and their influence on fluvial and cliff sediment supply.

Coastal erosion is not only dependent on the above hazard drivers, and changes to them, but also on the geomorphology and geological makeup of the coast, including the modifications that humans have made (perhaps indirectly) to the coast.  Although these factors all influence the rate of coastal erosion, in general terms, the rate is predominantly determined by the natural drivers – waves and water levels.  (Other drivers, such as rainfall and drainage patterns, can be significant for certain types of coast, such as soft cliffs.)

Despite the huge diversity of geomorphology found around the New Zealand coast, the generic sensitivity of different physical coastal environments to the likely effects of climate change is relatively straightforward.  This is summarised for a range of landforms in Table 3.1, discussed for the main coastal geomorphological types in the following sections, and summarised in Figure 3.3.  It is important to realise that both regional and local influences, such as variability in and interrelationships between geomorphology, coastal sediments and human influences, will result in significant local deviations from the generic response, producing variations in the rate of coastal change.

Table 3.1: Relative sensitivity of coastal landforms to changes in different climate change drivers

Note: Sea-level rise = sensitivity to accelerations in sea-level rise; storm surge = sensitivity to changes in the frequency and/or intensity of storm surge; precipitation = sensitivity to changes in the pattern and/or intensity of precipitation; wave height = sensitivity to changes in wave height (storms); wave direction = sensitivity to changes in wave direction (eg, changed longshore sediment transport patterns).

Source: Adapted from Jay et al 2003.

3.3.1 Sandy coasts

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Sea-level rise will permit waves to attack the backshore and foredunes more readily in many localities (Figure 3.4) than at present, particularly on coasts with relatively small tide ranges (irrespective of whether there are changes in wave climate or storminess).  If an increase in the frequency or heights of storm waves also occurs, then this combination (sea-level rise and more frequent or higher storm waves) would tend to have greater adverse effects on sand beach systems than at present.  Where the present width of the back or foreshore of the beach is not sufficient to accommodate this erosion, dunes backing the beach will be eroded.  Locations with higher dunes may suffer less retreat than locations with low dunes, although more frequent mass slumping could occur if high dunes are oversteepened.

The elevation of the water table within a beach profile has an influence on erosion.  Higher water tables increase wave run-up and the velocity of backwash, therefore 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.

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On long open sections of sandy coast, longshore sediment transport potential could increase due to changes in wave climate, particularly wave direction.  This may change the patterns and rates of both retreat and advance of the shoreline.  Subtle changes in wave direction may also have a significant effect on pocket sand beaches by moving sand from one end of the beach to the other.  Spit features that are built and maintained by longshore transport are also likely to be sensitive to changes in the wave climate; they will also be subject to increases in tidal flow volume passing through tidal inlets due to higher sea levels.

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Figure 3.4: Sea-level rise will provide increased opportunity for waves to
attack the backshore and foredunes in many localities

On both sand and gravel beach systems, where catchment-derived sources of sediment provide an important supply to the coast, increases in rainfall intensity will increase upper catchment erosion and sediment transport.  In some locations, the additional supply (excluding silts, muds, clays) may be sufficient to offset other climate change effects.  However, in areas where there is decreased rainfall (eg, some east coast areas), ongoing sediment supply may be reduced (even with episodic storms), with shoreline erosion likely to be exacerbated even further.

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

3.3.2 Gravel beaches

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• where there is a wide and well-nourished gravel barrier (ie, sufficient sediment supply), the barrier will retreat slightly and increase in height (Figure 3.3) in response to the rising sea level, increase in wave height or increase in the frequency or magnitude of extreme storms.

• where the gravel barrier system has a net deficit in sediment supply, as is the case of many New Zealand gravel beaches (particularly on the west coast of the South Island), the barrier will experience an increased rate of retreat, or there may even be a breakdown of the gravel ridge (Figure 3.2).  As most of these systems are recessional, future sea-level rise or increases in wave conditions will accelerate this present-day trend.

Figure 3.5: Gravel barriers will tend to retreat, but where there is sufficient
gravel, the barrier will increase in height

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3.3.3 Cliffs

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For alluvial (unconsolidated) cliffs fronted by a gravel barrier beach at their base, such as found along the South Canterbury and North Otago coastline, changes in the rate of retreat of the cliff will be linked to changes of the gravel barrier.  For such cliffs, it is unlikely that there will be significant changes in the rate of retreat.

Figure 3.6: Cliffs will tend to retreat, but the rate of retreat will be highly
dependent on their geological characteristics

3.3.4 Estuarine coasts

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Sedimentation rates in most North Island estuaries have been 2–4 mm per year thus far, keeping up with the present rise in sea level.  Eventually, however, the acceleration in sea-level rise is likely to exceed sedimentation.  This may occur more quickly in urban areas where catchments are developed and restrict sediment supply.

Estuary and harbour shorelines will retreat as a result of both inundation and erosion, but the rate and extent of retreat will be highly variable within any estuary.  In general, estuary systems have a low-energy wave climate and limited exposure time (around high tide) for waves to develop and to erode the shoreline.  However, raised water levels will permit larger waves on high tides to reach the estuary shoreline, potentially increasing the rate of erosion.  Once erosion or loss of land occurs, recovery – if it occurs – will be a much slower process than on open coasts.  Again, estuaries with a comparably smaller tide range will be more vulnerable for a given sea-level rise (eg, most of the east coast and Wellington/Porirua area).  Along low-lying areas bordering estuaries, erosion may be relatively rapid owing to regular, and leading to permanent, high-tide inundation of areas that presently may experience only episodic inundation.

Figure 3.7: Retreat of estuarine shorelines will be highly variable

Where the landward retreat of the high-water mark is constrained due to morphology, geology (eg, rock outcrop) or coastal defences, intertidal areas and their associated ecosystems may be reduced and potentially ‘squeezed out’.

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.  Changes in increased flow volumes may be quite significant given the shallowness of many of New Zealand’s estuaries; they will correspond to increases in tidal velocities and scour in the main channels and, particularly, at tidal entrances.  It is at river, harbour and estuary mouths and inlets that coastal changes tend to be the most dynamic, particularly those associated with a spit morphology.  The influences of such inlets can extend for up to approximately 4 km along the open coast adjacent to the mouth.  The dynamics of coastal and estuarine / river processes and multi-year cycles of sand exchange between the estuary, ebb and/or flood deltas and the adjacent coastline are very complex.  Thus any reliable statement about how individual inlet systems may respond to climate change effects is extremely difficult to make.

3.3.5 Assessing the effects of climate change on coastal erosion risk

Quantifying how the retreat and advance of coastlines will be influenced by climate change is extremely difficult.  Coastal change is a complex process in which coastal hydrodynamics, morphology, geology, sediment supply and deposition and, in some cases, human modifications all interact over multiple timescales.  Further complicating matters are both positive and negative feedbacks within the coastal system, again all of which operate on a number of different spatial and temporal scales.

Owing to this complexity, assessments of future coastal erosion, and the effects that climate change may have on erosion rates, tend to rely on relatively simplistic empirical approaches.  Most commonly, they provide a relationship between past erosion rates, the characteristics of the beach profile, and the relative difference between past and future sea levels (typically based on the ‘Bruun rule’⁴⁷ or a variant of it).

Strictly speaking, such approaches are more suited to providing broad estimates of relative erosion potential along a coastline rather than location-specific assessments of potential change.  Their use in predicting the coastline position at some time in the future should be treated with caution, and tends to imply a level of certainty that is rarely justifiable.  As with assessments of inundation, such approaches require a much more robust incorporation of uncertainties and hence of the sensitivity of future coastal changes to these uncertainties.  For example, consideration needs to be given to:

• uncertainty related to past erosion rates owing to insufficient monitoring data

• the assessment methods

• future emission scenarios and the associated magnitude of their impacts on the various coastal hazard drivers

• the lack of knowledge of how some of these coastal drivers will change with climate change.

Such uncertainty also needs to be communicated more effectively.

However, such approaches, and application of expert judgment, will continue to form the basis of coastal erosion assessments in the foreseeable future.  More rigorous approaches of simulating coastal change at the timescales relevant to the planning of development are still relatively limited due to two main factors.⁴⁸  Firstly, the potential for process-based models to simulate sediment dynamics and the effects climate variability has on these processes over large spatial and temporal scales is limited: this requires that new types of modelling approaches be developed and adopted.  Secondly, there are few high-quality, long-term coastal datasets over the multi-decadal timescale of interest to enable refinement, calibration and validation of such models.

3.4 Salinisation of surface freshwaters and groundwater

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• sea-level rise causing saline water to encroach further up the river and creek watercourses

• longer parched or drought periods in eastern areas leading to reduced river flows, which in turn will enable saline water to encroach further up river

• sea-level rise causing higher water levels at the coast, within estuaries and lower reaches of rivers, which will exert a higher hydraulic head of saline water on unconfined groundwater aquifers.

3.5 Tsunami inundation

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3.6 Coastal defences

Climate change impacts on coastal hazard drivers will also have a significant effect on the integrity and performance of existing human-made coastal defences (Figure 3.8).  In the United Kingdom, it has been estimated that by 2080, the structural improvements required to maintain existing coastal defences to provide protection equivalent of their present standards will cost between 1.5 and 4 times that of today, depending on the emission scenario.⁴⁹

Climate change is likely to reduce the effectiveness of coastal defences for a variety of reasons,⁵⁰ including:

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• higher sea levels will increase the frequency with which defences are overtopped by waves or very high tides (more so for coasts with smaller tide ranges).  This increased overtopping will affect the inundation risk, but is also important to all coastal structures as it can increase erosion of the protected area behind the defence and can lead to failure of the defence itself
  
  
  Figure 3.8: The standard of protection
  provided by coastal defences will
  decrease due to the effects of sea-level
  rise and other climate change impacts
  on coastal hazard drivers

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• an increase in storm-tide levels (due to sea-level rise and/or changes in storm surge) will produce greater water depths at the defence, increasing the magnitude of overtopping during storms and exacerbating the problems detailed in point 1 above

• greater water depths at the structure will increase the exposure of the defence to larger waves.  This increased exposure will increase the risk of damage and failure.  For example, with rock structures, the size of rock required for stability is directly proportional to the cube of the significant wave height.  Therefore, even a small increase in wave conditions at the defence can result in a large increase in the size of rock armour required to achieve the same stability

• with larger waves at the defence, there is likely to be greater reflection from defence structures and increased scour of the beach at the structure’s toe.  This increases the potential for undermining and/or failure of the defence

• steepening of the foreshore in response to sea-level rise where a defence constrains the position of the high water mark but the landward retreat of the low-water position continues (coastal squeeze).  This can further increase the vulnerability of defences to overtopping and structural failure through the processes described above.

Given that many existing coastal defences in New Zealand have not been engineered to provide a high standard of protection, the impacts of climate change could result in substantially increased damage to these defences and lower standard of protection to the land backing it.  Similarly, if defences have been designed with a particular lifetime, defences are unlikely to endure if climate change considerations have not been factored into the design.

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