Goal: Management of coastal flooding in Western Australia is faced by a paradigm shift from avoidance of risk to hazard mitigation. Combined pressures of increased development and sea level rise challenge existing practices, which have relied upon avoiding hazard. This research undertakes systems analysis of coastal flooding hazard management, considering historic practices in Western Australia and modern techniques.
Abstract: Management of coastal flooding in Western Australia is faced by a paradigm shift. The combined pressures of increased development and sea level rise challenge existing policies and practices, which have largely relied upon avoiding flood risk. Increasing use of coastal hazard mitigation techniques is anticipated. However, a framework to identify scale-appropriate and locally-relevant hazard management has not yet been established. This paper presents a preliminary systems analysis of coastal flooding hazard management, considering historic practices in Western Australia and modern techniques for flood hazard evaluation.
The need for more detailed understanding of coastal flooding management is enhanced by recent changes to coastal planning in Western Australia. Policy revision has recognised that management of coastal hazards extends beyond identification of a boundary that separates safe and unsafe development. A broader range of possible extreme events and the effects of changing climate are considered within a larger area of potential hazard. Development within this area is acceptable, but must recognise and manage coastal risk. Coastal flooding hazard due to an event in the order of 500 years average recurrence interval is required to be considered, including influence of sea level rise projected over a 100 year time frame. Because expanded coastal setbacks would potentially isolate active present-day use of the coastal zone, including parts of several existing town-sites, it was recognised that other forms of hazard mitigation may be necessary. Further, due to the uncertainties regarding time scales and impacts of sea level rise, use of adaptive management to respond to the impacts of sea level rise has been considered appropriate.
Based upon coastal management patterns faced elsewhere in the world, a challenge to policy application is that intervention may be argued as preferential to avoiding hazard when measured purely by financial criteria. However, further considerations should include the ongoing costs of maintaining flood defences, the reduced opportunity to adapt, the potential for transfer of flood hazard and the risk to human safety posed by events above the defence threshold. These applications defy the use of singular recurrence-based criteria, such as setting a 100-year average recurrence interval minimum floor level. Furthermore, coastal flood hazard assessment requires clear recognition of the uncertainties associated with recurrence estimation.
International Approaches to Coastal Flood Management:
This summary regarding coastal flooding management approaches used outside Australia has been prepared to guide the reader towards a limited set of useful references. It is by no means a comprehensive review of available literature or global practices. For a 'one-stop' reference on flooding management, readers are recommended to Sayers et al. (2013).
It is also acknowledged this summary contains limited reference to Australian flood management practices, with extensive relevant information in NFRAG Guidelines, EMA Manuals and the AEMI guide to flood risk management.
International coastal inundation management provides an important context for establishing future coastal inundation management in Western Australia, as the combined influences of projected sea level rise and population growth place stresses on long-term policies of avoiding coastal hazards through town planning and land-use zoning [i],[ii],[iii]. For other countries or regions that are already dealing with greater coastal inundation pressures, the variety of approaches and practices illustrates potential future requirements for Western Australia.
Coastal inundation is a globally distributed hazard occurring when ocean water levels rise above critical thresholds to threaten human life or cause detrimental impact to human property [iv]. Causes of coastal inundation may include contributions from storm surges, surface waves, extreme tides, mean sea level fluctuations, coastally trapped waves, tsunami or meteotsunami 12. The diversity of causes and the context of human values prompt different approaches to coastal flood assessment and management [v]. Approaches commonly vary within nations between agencies or for different geographic regions, even where national flood management guidelines exist [vi].
Most countries use a mixture of hazard mitigation strategies that fall into broad classes of (i) avoid flooding; (ii) protect against flooding; or (iii) tolerate flooding. Each of these classes potentially includes different actions, such as flood avoidance through land-use planning, or through forecasting and evacuation. These actions cannot be considered ‘alternatives’, as even where there are equivalent financial costs and benefits, there are variable socio-economic impacts. For example, physical flood tolerance developed through building design likely to have substantially different consequences to financial tolerance from insurance or through evacuation and post-flood remediation. Implicitly, the flood strategy balance is influenced by population pressure, economic drivers, and governance, typically with perceived flood threat influencing the resulting level of management.
Concept of National Drivers on Coastal Inundation Management
Comparison of flood hazard management used by different countries suggests a mix of approaches is used, even for countries with similar socio-economic conditions [vii]. No single pathway to develop an effective national flood strategy is recommended, with modern guidance leaning towards careful use of a mixture of approaches and actions to deal with flood risk in a sustainable fashion, balancing costs and trade-offs. In a riverine setting, this may involve balancing flood risk against water supply distribution. For coastal settings, there is often a balance between protecting against coastal inundation and providing a pathway for runoff flooding, particularly at breakout points through coastal dunes.
Breakouts reduce runoff flooding threat but provides pathways for coastal inundation.
A generalised sequence of flood risk management phases has been identified, based on international practices over the last 150 years, with parallels to be found in human progress over the last 3000 years. The sequence does not necessarily represent more mature or effective flood risk management, with respect to safety or economic consequences. However, the sequence follows increasing investment in flood-affected zones, which is directly linked to available resources and consequently results in more intensive flood risk assessment practice (Figure 3‑6).
Initial approach of tolerance focuses on flood avoidance
This usually shifts towards a protection approach, where a safety standard is defined for the level of protection (e.g. 100-year ARI).
With increasing investment, the approach typically shifts to one of damage reduction, with increased attention paid to mitigation options and tools for damage limitation, simultaneous with increasing safety standards for flood protection works.
Eventually, the challenges of balancing different forms of intervention and their increasing costs, usually results in a transition towards strategic flood risk management, where resources are prioritised, with an objective to maximise effectiveness.
Flood Risk Management (FRM) Phases
West Australian coastal management has historically focused on the approach of coastal flood tolerance, with limited implementation of strategic protection schemes (Section 4).
Progression between flood risk management phases involves changes to focus on flood risk management elements (i.e. tolerance, protect and damage reduction) and includes shifts in criteria used for decision-making [viii] (Figure 3-4). A transition from evaluation against a single safety standard towards more comprehensive evaluation of flooding across a wider range of events and its consequences is widely proposed [ix]. Incorporation of adaptive techniques, multi-criteria assessment, and recognition of flood assessment uncertainty [x],[xi] are widely considered important to this change [xii]. However, this transition is yet to be widely established in national policies, and in many cases requires considerable development. Some factors which may be considered include economic viability, robustness, sustainability, social equity, environmental impact, and adaptive capacity.
Change from a flood protection approach towards a damage reduction approach has occurred for various nations at different scales primarily due to the combined influences of morphology and governance. For example, the width of low-lying reclaimed areas of the Netherlands restricts evacuation but supports defence, which has been developed through a strong central organization. The narrow barrier chain on the east coast United States limits the effectiveness of defensive works, with flood management focusing on building techniques to tolerate coastal floods, which are the responsibility of individual landowners, heavily influenced by insurance policies.
Development of flood criteria has been influenced by variation of oceanographic conditions, coastal landforms, and prevalent types of flood mitigation. For locations with low waves, flood criteria typically used for runoff flooding are often used, with attributes such as water depth, current speed, or rate of water rise. In locations with energetic wave conditions, safety is reduced, requiring lower equivalent thresholds for tolerable inundation. However, as waves can extend the distance landward and elevation of flooding impacts, they are also widely used as coastal inundation criteria. Attributes such as wave height (and period) or runup extent are more commonly in use where there are few coastal defences and increased use of flood proofing [xiii] (e.g. United States), with rates of overtopping or barrier stability criteria applied where coastal defences are in place [xiv],[xv], which is dominant across Europe.
Selection of flood mitigation methods is mainly driven by economics, including availability of resources and technology, as well as the distribution of assets potentially impacted by flooding. This results in diverse approaches, with the SOURCE-PATHWAY-RECEPTOR-CONSEQUENCES framework providing some basis for comparison, although wide scope remains to interpret management implications.
Governance frameworks can also influence the relative balance of mitigation and protection activities, due to geographic extent, jurisdictional control, and economics:
Very large flood protection works are typically associated with strong, centralised management agencies, particularly for rich countries such as the United States, Japan, and Netherlands.
Where flood management is done through smaller agencies, such as at state, district, or city level, protective works are usually more targeted, based on economic and political focus, such as demonstrated for North Sea countries, Jakarta or Kolkata. In some situations where there are several layers of governance, smaller agencies can tap centralised funding and technical resources to achieve strategic protection works – this model applies generally to Australia and United Kingdom, as well as a portion of flood protection works in the United States.
The transition from protection, when managing large areas, towards other forms of mitigation is most strongly demonstrated at the landowner scale, where strategic flood protection is generally impractical, placing increased reliance on property-level protection, such as flood-proofing, or flood resilience.
Criteria for decision-making generally shift as the flood management approach moves from protection to damage reduction. This typically involves feedback between the level of investment in the flood hazard zone and the safety standard applied to protective works. However, in recognition that there is a chance that any degree of flood protection works may be overwhelmed, the principle of ‘as low as reasonably practical’ flood risk is also widely used. Decision-making through this phase is often supported by economic-based assessment such as cost-benefit analyses (CBA) [xvi], which are fundamental decision-tools for aid agencies. However, limitations of these methods, through unknowns and increasing uncertainties, can be significant [xvii], and it is notable that Flood and Coastal Erosion Risk Management grants in the United Kingdom typically require CBA ratios exceeding 3-5 for projects to be considered.
Decision-Making Frameworks with Uncertainty
Modified from Wise & Capon (2016)
Criteria related to the ability of a community to tolerate inundation are not widely established, although they typically underpin coastal defence policy in areas affected by coastal inundation. Applied criteria include relating the acceptable recurrence of inundation to land-use or the importance of key infrastructure during emergency management and recovery [xviii]. A combination of economic analysis and community resilience may be necessary for decision-making when deciding a community’s ability to tolerate a flood. Factors typically considered include the ability to evacuate successfully, the cost of flood damage and the loss of services related to the flood event.
Feedback often occurs between the criteria underlying flood defence policy and its application. In areas with focused flood protection, there is typically increased pressure for development, subsequently leading to justification for higher levels of protection. In areas where flood avoidance is used, the marginal value of occasionally flooded land may reduce the justification for its development or protection. Where flood tolerance is applied, the perceived effectiveness of evacuation systems and design to minimize flood damage may lead to development of increasingly hazardous areas, supported by more intensive engineering.
Key outcomes from assessment of international practices are:
There is a wide range of approaches to manage coastal inundation hazard, generally distributed between avoiding, tolerating, or protecting against the hazard. Geographic and social factors influence the strategies adopted, which will vary between locations and agencies.
Robust management typically involves a combination of hazard mitigation activities, with a blend of strategic (large-scale) and property level protection.
There is a trend toward reducing use of singular, discrete thresholds for hazard management, with increased consideration across a wider range of event likelihoods, including impacts from events exceeding design levels.
Uncertainty when estimating flood likelihoods, including effects of projected sea level rise, is increasingly being considered through adaptive, risk-management frameworks, that consider event likelihood, receptor value, susceptibility to damage and residual risk (the latter describes risk associated with events above a nominated design threshold).
There is a practical need to consider financial viability of flood hazard interventions (e.g. using cost-benefit analysis). However, social, environmental and inter-generational effects require wider socio-economic considerations (e.g. multi-criteria assessment & benefit-distribution analysis, or more advanced techniques).
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This is a brief introduction to coastal inundation management - how inundation is characterised in different settings, and approaches towards hazard mitigation. Examples are presented from sites along the Western Australian coast.