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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

5 Understanding Changing Coastal Hazard Risk

5.1 Introduction

A sound risk assessment process is fundamental to ensure that coastal hazards, and the effect that climate change has on coastal hazards, are appropriately taken into account in local government policy, planning and resource consent decision-making.  The process has the advantage of being conducive to building in the sensitivity of outcomes to different levels of uncertainties in climate change drivers.

To implement effective approaches to managing coastal hazard risk, such risk must first be appraised through:

• identifying the coastal margins and describing the associated assets located there that are at risk from coastal erosion, storm and tsunami inundation
• considering how such risk may be induced or exacerbated by climate change or by changing development in coastal margins
• evaluating the likelihood and consequences of such risk over the timeframes of interest.

This process also allows the climate change risks and subsequent adaptive responses to be prioritised and compared equitably with other risks, resource availability and cost issues (including works) that the local authority faces.

The risk assessment framework described in this chapter provides an overall framework for carrying out risk assessment at a range of levels, permitting a structured way to think about, or work through, coastal hazard and climate change issues and associated uncertainties.  It is intended that the framework can be used to assist both proactive policy and planning, and assessing and determining resource consent applications.  This process is not the only one that can be used: where a local authority has an existing risk assessment process, climate change should simply be added into it.  For the purpose of this Guidance Manual, the terminology used is outlined in Box 5.1.

5.2 Fundamental concepts in risk assessment

There are several fundamental concepts that should be incorporated in any assessment of coastal hazard risk, and how such risk may change as a result of climate change effects.

5.2.1 Risk varies over time

Risk varies over time and, for coastal margins, the risk is invariably increasing.  This reflects both the changing probability of the underlying hazard occurring, and the changing scale of consequence should the risk occur.  For example:

• climate varies because of its natural variability and longer-term climate change, both of which will influence the occurrence or magnitude of the hazard
• natural defences change (eg, a narrowing of beach or dune width), influencing the occurrence or magnitude of the hazard
• land use, subdivision and development change, usually intensifying, thereby increasing the extent and therefore the magnitude of the consequences
• the value of assets at risk changes. It usually increases, influencing the direct losses and, therefore, the magnitude of the consequences.

Time is a fundamental consideration in any risk assessment of coastal hazards and the effect climate change may have on these risks.  A risk may not exist now but may evolve, owing to climate change, during the lifetime of development, service or infrastructure.  The time factor or horizon that must be considered is the lifetime of the decision, development, service or infrastructure (Figure 5.1).

In this context, risk assessment can recognise the evolution of risks over time by introducing a planning horizon and considering the risk at various points in the lifetime of the decision, development, etc.

For example, for a lifetime of 100 years, the risk may be evaluated as it is now and as it will be in 25, 50, 75 and 100 years’ time.  This approach allows local government to plan for response options to evolve over time – that is, it allows latitude to be incorporated in the response options to address the risk.  If the risk is not addressed now, despite it being likely to occur in the future, the question arises: Is the community locked into a position where it cannot avoid or adapt to the risk?

Figure 5.1: Example timeframes for various decisions and development

Text description of figure 5.1:  A timeline from 2007 to 2100 and beyond shows the typical timeframes, relative to 2007, for different types of development and development decision-making.

2010: Election cycles / profit and loss.
2010-2020: CoastCare activities, Regional and District Plan cycles, sea defences (unmaintained).
2017: Long Term Community Council Plan (LTCCP) cycles.
2020-2030: Single relocatable residential properties.
2040-2050: Maximum resource consent timeframes, sea defences (maintained).
2070: Single or low-density residential properties.
2090: Long-term biodiversity.
2100: Coastal hazard zones.
2100 and beyond: Predominantly permanent development, for example major subdivision and infrastructure (roads, rail, bridges, storm and wastewater, ports, airports).

5.2.2 Risk varies spatially

Coastal hazard risk can be extremely variable, even over relatively short distances.  This spatial variability in risk can again be due to both spatial variability in the probability of hazard occurrence and variability in the hazard consequence.  Factors that affect the spatial variability of risk include:

• changing coastal morphology (ie, coastal characteristics) or changing exposure to coastal hazard drivers (eg, waves) along a coast, resulting in differing erosion rates, storm response, etc
• differing hinterland elevations and the influence of inundation pathways, eg, variation in inundation risk
• varying land use, subdivision density and value of human assets
• cultural and environmental assets.

5.2.3 Risk assessment needs to be appropriate

Any risk assessment needs to be:

• conducted at a level of detail appropriate to the scale of the risk and nature of the decision (Table 5.1)
• consistent with the level of data or information available.

This Guidance Manual is aimed primarily at local government staff involved in policy, planning or resource consenting; and those who need to be able to assess the risks posed by coastal hazards and to identify when a more detailed assessment may be required.  As such, it is qualitative in nature and should be able to be conducted by local authority personnel, although some input from coastal hazard specialists is generally desirable.  Where available, coastal hazard personnel from the regional council should be consulted.

The approach outlined in this Guidance Manual assumes that local government staff using the process have a reasonable knowledge of the characteristics of the coastal margins, are aware of past coastal hazard issues, have access to aerial photographs, and have reviewed previous relevant studies and reports.

However, the risk assessment framework used here is also amenable to more detailed levels of risk assessment.  As the level of detail increases, input from suitably qualified and experienced specialists in coastal hazards (possibly available in the regional council) may be required.

5.2.4 Risk needs to be communicated

The purpose of the risk assessment process is to aid decision-making.  Therefore, there is a need to communicate the risk assessment process in language that is as clear and concise as possible.  Within all risk assessments, there is a need to:

• define the overall approach
• clearly define all key assumptions made
• identify all uncertainties and their potential impact of the overall decision
• outline the scope and impact of any sensitivity testing
• be accountable and transparent
• report in a way that the non-specialist can understand the significance of the results.

5.2.5 Uncertainty needs to be considered

All local government business contends with uncertainty.  Nevertheless, local government has developed a range of mechanisms and approaches to deal with uncertainty through all its planning and review processes.

In terms of coastal hazards and climate change, uncertainty defines the quality of our knowledge concerning risk.^{Footnote 54}  Uncertainty may affect both the likelihood of hazard conditions occurring and the consequences of those hazard events (Figure 5.2).  The extent of the impact that future climate change will have is also uncertain.  For example, we cannot predict with any degree of certainty the quantity of greenhouse gases that will be emitted over the coming century.  While ongoing research typically aims to reduce uncertainties, adopting a risk-based approach allows uncertainty to be accommodated and treated accordingly within decision-making.

Text description of figure 5.2:  A summary of four different sources of uncertainty. Along the x-axis uncertainty is related to knowledge of the consequence (defined from poor to good). Along the Y-axis uncertainty is related to knowledge of the probability (defined from poor to good). The combinations of these factors include:

1. Ignorance about the risk. This results from poor knowledge of consequence and poor knowledge of probability. For example: rapidly changing climate, new or unknown processes, complex dependencies such as non-linearity, longer-term forecast, insufficient data and climate surprises.
2. Ambiguity about the risk. This results from poor knowledge of consequence and good knowledge of probability. For example: uncertain or unknown impacts, no impact models, uncertain how to value consequences, lack of concern.
3. Impacts well defined but probability uncertain. This results from good knowledge of consequence and poor knowledge of probability. For example: poor knowledge of likelihood of damage, good impact or process models, well defined impacts if event occurs, longer-term assessment.
4. Good knowledge about the risk. This results from good knowledge of consequence and good knowledge of probability. For example: unchanging climate, good historical data, good impact models, short-term prediction.

Source: Adapted from Willows and Connell 2003.

It is important to appreciate and clearly define where uncertainty exists, which uncertainties have the most impact on the decision to be made, and the possible steps that could be taken to reduce uncertainty.  It may be that the scale of the decision does not warrant detailed investigation to reduce such uncertainty, or that adopting a precautionary approach is appropriate.  More detailed approaches to risk assessment allow more robust methodologies for incorporating uncertainty into the assessment procedure.

Irrespective of the level of detail of the risk assessment process, the number of uncertainties that are involved with future climate change will require use of a mixture of quantitative and qualitative information.  While the risk assessment process provides a systematic process, judgement (based on a range of information sources) will still need to applied.

5.3 The risk assessment process

The risk assessment process described in the following sections is based on the New Zealand Standard for Risk Management, AS/NZS4360^{Footnote 55} (see Figure 5.3).  The process can be used to:

• identify and characterise the nature of the risk
• identify qualitative or quantitative estimates of the risk
• compare the sources of risk
• assess the impact of uncertainty within the context of the overall decision
• assess and compare the potential effectiveness of solutions to manage the risk.

This section considers steps 1 to 5 of the risk assessment process detailed in Figure 5.3, with step 6, on managing the risks, being covered in chapter 6.

5.3.1 Step 1: Define the problem and establish the context

This first step ‘sets the scene’ within which the risk assessment process takes place and the context within which coastal hazards and climate change effects fit.  Defining the issue will assist with selecting the level of risk assessment required (Box 5.2).  The significance of the risk and the appropriateness of the adaptation measures can then be judged against these considerations.

5.3.2 Step 2: Identify the relevant coastal hazards and climate change drivers

FS 1, 2, 3

Understanding a community’s coastal hazards, vulnerability and exposure to damage, and how these may change over time, is the foundation for developing effective and appropriate risk-management and -reduction measures.  For a coastal hazard risk to occur, there needs to be a ‘driver’ (such as a storm), a ‘receptor’ (such as property within the coastal margin), and an erosion or inundation pathway between the two, created by the driver.  However, a driver or hazard does not necessarily lead to a harmful impact, only the possibility of harm occurring.

Coastal hazards and their consequences often have multiple sources, pathways and receptors (eg, people, infrastructure, property), which may or may not be related and interacting, may or may not occur at the same time, may occur over different timescales (eg, an event such as a tsunami compared to slow coastal erosion), resulting in the overall consequence.  Appreciating these interactions is important.

The Source-Pathway-Receptor-Consequence (SPRC) framework (Figure 5.4) is a convenient way to consider the key drivers of coastal hazards and how they impact on the range of the human and built environment within particular coastal margins.  Example sources, pathways and receptors for different coastal hazards are shown in Table 5.2.

Figure 5.4: Source-Pathway-Receptor-Consequences (SPRC) framework for assessing coastal hazard risk

Text description of figure 5.4: The Source-Pathway-Receptor-Consequences framework for assessing coastal hazard risk comprises four factors. For some form of consequence to occur, there needs to be a hazard source or driver, which translates over the coastal margin in some way (Pathway) affecting different receptors located within the coastal margin. Examples of these four factors include:

1. Sources (drivers). For example: winds, sea level, tides, surges, waves, storms, ENSO, IPO and tsunami.
2. Pathways. For example: overtopping, overwashing, breaching, overland flow and undermining.
3. Receptors. For example: people, property, communities, infrastructure and environment.
4. Consequences. For example: loss of life, stress, material damage, loss of land, environmental degradation, cultural loss and economic impact.

An example is given where there can be multiple sources (in the example two sources) combining over different pathways (in the example two pathways) to affect a range of receptors (in the example four receptors) resulting in a range of different consequences (in the example five consequences).

Note: A conceptual example of how different source-pathway-receptor-consequence combinations can form is shown in section 5.3.2, Step 2.

Identification of the hazards and what is exposed to these hazards focuses on the first three components (Sources, Pathways and Receptors) of the SPRC framework.

To apply the framework:

1. For the present day, consider the combinations of hazard sources, the range of pathways they use and the extent of the receptors (or potential receptors) each source/pathway combination impacts on, for the location and situation under consideration.
2. Consider how the different source-pathway combinations may interact.  For example, coastal erosion could lead to an increased possibility of dune breaching and hence inundation.
3. Over the timeframe in question, consider the changes that climate change and natural climate variability will have on the identified hazard sources and pathways, how they interact and the resulting impacts on receptors (or potential receptors).
4. Identify whether climate change will result in new source-pathway hazard combinations that will impact on the receptors (or potential receptors).

5.3.3 Step 3: Assess the likelihood and magnitude of the hazards occurring

For each of the potential hazard sources and pathways affecting receptors located within the coastal margin, an assessment is required of the magnitude of these hazard occurrences and how likely they are to occur.

It is important to note that different coastal hazards have different characteristics.  Storm and tsunami inundation tends to be episodic and inundation levels can typically be defined in a probabilistic way; for example, there is a 1% chance of a storm tide of a certain level being exceeded in any one year (see Box 5.4).  Coastal erosion, on the other hand, at present tends not to be expressed probabilistically.  As it is an ongoing process (a creeping hazard), it is usually defined as the expected position of the coast at a certain future point in time.

Information and/or data on hazard probabilities for a particular location, or consideration of coastal hazard zones where data and/or information have been derived, should be used wherever they are available.  For a qualitative assessment, consideration can be given to the following categories, which are based on the terminology for expressing the likelihood of occurrence in the Fourth Assessment Report.^{Footnote 56}  Boundaries between the categories should be considered ‘fuzzy’.  Depending on the situation, for either each source-pathway combination creating a coastal hazard, or cumulatively for each coastal hazard, the following is considered:

For coastal erosion: Over the timeframe of interest (eg, 100 years), consider which terminology best fits the likelihood of the different coastal erosion pathways affecting the issue or receptor under consideration:

• Virtually certain: > 99% probability of occurrence
• Very likely: 90–99% probability of occurrence
• Likely: 66–90% probability of occurrence
• About as likely as not: 33–66% probability of occurrence
• Unlikely: 10–33% probability of occurrence
• Very unlikely: 1–10% probability of occurrence
• Exceptionally unlikely: < 1% probability of occurrence

For storm and tsunami inundation: Again, select the terminology (based on the categorisation above) that best fits the magnitude of the event for each inundation hazard pathway for the planning timeframe in question.

To assist this assessment, Figure 5.5 shows the relationship between Annual Exceedence Probability (horizontal axis) and the likelihood of occurrence within certain planning timeframes (vertical axis).  The coloured lines define the relationship for planning timeframes of 20, 35, 50, 75, 100 and 150 years.

Where the effects of climate change on the likelihood or magnitude of the hazard are significant, the table above could be used to assess how climate change may change the likelihood of the hazard.  However, in many cases, the above categories may be too coarse to define the hazard changes that climate change may bring about.  This is discussed further during the evaluation of the risks in Step 5 below.

5.3.4 Step 4: Assess the scale of the hazard consequences on the receptors

Just as we must consider the magnitude, and likelihood of occurrence, of the different ways coastal hazards may impact on a coastal margin and the receptors (or elements) in it, understanding the potential scale of the consequence is also necessary.  To develop this understanding, we can consider the degree of vulnerability of the existing (and potential) receptors in the coastal margin.

The impact of coastal hazards can have many consequences, only some of which can be expressed in monetary (tangible) terms.  The measure of consequences can include: fatalities, injuries, stress and physical disruption to people; tangible and intangible loss and damage to property, community and lifeline infrastructure, the environment and cultural assets; and direct and indirect impacts on the economy.

The range in the potential scale of the consequence depends again on the characteristic of the coastal hazard, its interaction with a particular receptor and how vulnerable that receptor is.  For example, a major tsunami event has the potential to result in substantially higher numbers of fatalities compared to ongoing coastal erosion, which would rarely threaten life.  Coastal erosion, on the other hand, may result in irreversible loss of significant numbers of property, or ecosystem or cultural assets.  An episodic event, such as inundation due to a major storm, may cause only disruption for a period of time, or damage that can be repaired.

Again, depending on the situation, the level of consequence for each receptor is assessed: either for each source-pathway combination creating a coastal hazard risk, or cumulatively for each coastal hazard risk.  Some common receptors and suggested levels of consequence are defined in Table 5.3, although alternative types of scaling can be used to suit the particular situation.

Note once again that the categorisation of the consequences in Table 5.3 may be too coarse to detect either:

1. the extent of change that climate change may bring to the level of coastal hazard consequence, or
2. the extent of change that further development may bring to the level of coastal hazard consequence.

This is discussed further during the evaluation of the risks in Step 5.

Note: The criteria for such a table are likely to be specific to each region.

5.3.5 Step 5: Evaluation of coastal hazard risk

The magnitude of risk is commonly expressed as a combination of the magnitude of the hazard occurrence and the magnitude of the vulnerability or consequence.  Before making decisions on how such risks may be managed (see chapter 6), the final step requires assessing the level of risk; what is driving ongoing and longer-term changes in the level of risk; and the significance of such risk in relation to the many other factors that need to be taken into account when considering coastal margin policy, planning and resource consenting decisions.  As such, this step involves assessing:

• the level of risk to the issue or receptor under consideration
• the significance of this risk
• how climate change may affect this level of risk
• what is driving the changing levels of risk.

Figure 5.6 provides a qualitative assessment of the level or risk for each of the source-pathway-receptor-consequence combinations (or cumulatively for each coastal hazard).  It is based on the assessment of the magnitude and occurrence of coastal hazard risk (Step 3), and the potential vulnerability of, or consequence on, the various receptors located (or planned to be located) in the coastal margins (Step 4).

Understanding what is driving changes in the level of coastal hazard risk is the final part in this component of the risk assessment process.  That understanding provides the foundation for informed decisions to be made about the acceptable level of risk, how such risk should be managed (next chapter) or whether a more detailed assessment of risk is required.

Profiles of coastal hazard risk over the next 100 years will change, driven by variations in the:

• magnitude and frequency of hazard events caused by climate change
• human and built environment that are located within coastal margins susceptible to such hazards.

Understanding the relative contribution of each of these factors to coastal hazard risk is important.

In many situations around the coast of New Zealand, changing coastal hazard risk over the next 100 years and longer will be dominated by ongoing development (or increasing value of development) in coastal margins rather than by changes in the occurrence or magnitude of hazard events.

The extent of change of coastal hazard risk (due to either a changing climate or increases in the level or extent of development) can not always be discerned using the methodology outlined above.  If so, then a qualitative assessment of the relative magnitude of influence of these drivers can be made, based on Figure 5.7.

5.4 Detailed risk assessment

Using the above risk framework permits a conceptual approach to assessing coastal hazard risk and the effect climate change may have on it.  For many issues, such a qualitative assessment of risk may be sufficient; the same framework can also be used for a more detailed assessment of coastal hazard risk.

Depending on the particular situation or issue under consideration, a more detailed or quantitative risk assessment may be needed to aid the decision-making process.  Such a more detailed assessment will typically involve some or all of the following:

• analysis of existing datasets or monitoring of hazard sources and receptors to collect further data
• application of quantitative approaches to hazard and risk modelling
• expert opinion.

In general, approaches to quantifying the physical aspects of coastal hazards are relatively well defined. Generally, there is much lower capability for quantitative assessment of biophysical and social, human and cultural impacts. However, whatever methods or approaches are used, there will still be inherent uncertainties or assumptions that need to be made. These, along with the uncertainties relating to projections of future climate, need to be taken into account and clearly communicated.

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