Coastal flood risk management: A step by step approach
Coastal flood hazards are diverse and highly unpredictable. In order to develop an appropriate risk management response, it is essential to understand these hazards and their potential impacts. The following steps outline the building blocks for formulating a coastal flood risk management plan, which can be tailored according to the context of a country with its own particular hazards, stage of development, socio-cultural characteristics and institutional structure.
Step 1: Define and estimate the hazard
While coasts can experience types of ‘inland’ flooding from rivers and local rainfall, this article focuses on hazards coming from the sea, which can bring the most forceful types of flood, are difficult to predict and prevent.
A storm surge is an abnormal rise in sea level driven by tropical or extra-tropical storms. Storm surges can be hazardous for navigation and port operations, and have the potential to severely damage coastal structures (including flood barriers) and cause significant loss of life.
A tsunami is a wave caused by a sudden rising or lowering of the ocean floor or by large masses of earth falling or sliding into the water. Propagating as consecutive, very long period ocean waves over long distances, a tsunami might have little effect on ships on high seas but is highly destructive on land, running over everything in its path as a wall of water.
A tidal flood occurs when the tide’s range is at its maximum—typically around the new moon and full moon—and particularly when combined with strong winds.
Coastal erosion is the process of wearing away material from a coastal profile. It occurs mainly during strong winds, high waves, high tides and storm surge conditions, and results in coastline retreat and loss of land.
Several different natural hazards can give rise to erosion, high velocity wave action, and flooding. It is therefore critical to collect and use inundation data from all potential disasters and to implement a hazard mitigation program that addresses the protection of inhabitants from all natural hazards1.
Step 2: Predict potential impacts
Coastal floods are capable of causing large numbers of fatalities and significant asset damage, as they often run deep with high flow velocities and powerful waves in densely populated areas. They can also occur unexpectedly, giving little or no time for warning and preventive evacuation, and leaving large populations exposed.
Coastal storms can also have far-reaching negative impacts on the environment. The force of storm surges and tsunamis can damage coral reefs, mangroves, dunes and sea grass beds, and receding waters can drag natural and man-made waste into the ocean.
Saltwater flooding can also bring about salinity problems, causing a shortage of fresh and safe drinking water in the short-term and negatively impacting agricultural crops and soil structure in the long-term.
Finally, the forces from storm surges, waves and tsunamis reshape soft coasts, and can result in long term coastal morphology changes.
Step 3: Assess vulnerabilities
The effects of coastal storms depend on the population’s characteristics, the infrastructure put in place to provide protection, and the community’s location and elevation in relation to the nature of coastal hazards. To assess vulnerability, one must look at its three components: exposure, susceptibility and resilience.
Exposure looks at how predisposed a community is to being disrupted by a flood and relies heavily on the physical conditions where people live in areas liable to flooding. Susceptibility relates to the socioeconomic context of those affected and includes their awareness of and preparedness for the risk they are living with, and access to institutions and measures that can help reduce the effects of the hazard. Resilience looks at coping capacity, which includes the wide array of structural and non-structural countermeasures and mechanisms through which the people reduce their vulnerability.
Step 4: Analyse and select measures
Non-structural measures do not reduce the exposure to coastal floods but rather aim at reducing the vulnerability and susceptibility of people and their goods living in the coastal zone. Coastal zone planning and land management, early warning and evacuation procedures, protection of critical infrastructure, coping and insurance of residual risk, community preparedness, poverty reduction and self-help all fall under this category.
Ecosystem services are the benefits people obtain from ecosystems. Ecosystems can influence the effects of floods and storms positively and negatively. They can be harnessed for wave energy dissipation (through coral reefs, vegetation and geomorphology); as barriers to flooding (through natural terrain elevation and dunes, for example); and for coastal stabilization, erosion control and sediment retention. Building with nature rather than against it falls under this category and entails taking advantage of the forces, interactions and materials present in nature.
A promising use of engineering solutions is through the multifunctional use of infrastructure, as seen in the transition of conventional embankments into super levees in Japan and the combination of dike design with salt-marsh restoration in The Netherlands.
Institutional and legislative measures for coastal flood management include disaster risk reduction instruments and coastal zone planning policies and strategies, applied according each country’s particular framework.
Step 5: Integrate policies and management
Making coasts climate-proof requires new adaptation strategies which are timely, technically sound, economically feasible and socially acceptable. However, both climate change and socio-economic developments come with large uncertainties.
One method that can be used to determine adaptation paths for different sectors is to identify the adaptation tipping point—the critical juncture where a policy in a key sector (for example, water management or spatial planning) needs to be revised or a new strategy needs to be implemented. The degree of climate change to which each key sector can cope is determined. Then, climate change scenarios are used to show in which time period those adaptation tipping points may be reached. Combining the adaptation tipping points with local scenarios will identify the vulnerability of a sector and the possible need for new adaptation strategies.
In the context of coastal risk management, governance concerns creating the proper conditions for sustainable coastal development and can be done through various means. One is to promote cooperation between different government levels and sectors, taking into account decentralization trends and national coordination needs. Another is to facilitate cooperation between the government and the private sector, with a view to balancing privatization trends and safeguarding the public interest. It is also useful to get the buy-in and involvement of stakeholders and citizens to promote societal acceptance of development projects as well the long term sustainability of development projects. Finally, it is important to make arrangements for dealing with uncertainties and sharing of risks.
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1Hwang, D., M. Francis, B. H. Choi, J.P. Sing, S. Stein, J. Borrero, H. K. Thio, C.Ratti & D. Bergado (2005). Mitigating the Risk from Coastal Hazards: Strategies & Concepts for Recovery from the December 26, 2004 Tsunami.