Climate adaptation measures for the coastal zone
This article gives an overview of measures that contribute to climate adaptation in the coastal zone. It is recommended to read this article together with the Coastal Wiki articles Climate adaptation policies for the coastal zone and Integrated Coastal Zone Management (ICZM). The important issue of climate adaptation in coastal cities is dealt with in a separate article Coastal cities and sea level rise.
We do not know how much the sea level will rise in the future. Even if the rise in atmospheric greenhouse gases is curbed over the course of this century, sea levels will likely continue to rise for at least another century and probably longer. We do not know how long this rise will continue and what sea level will eventually be reached. By then there will likely be areas that cannot be inhabited anymore due to the risk of flooding, leaving retreat as the only option. In other areas it may be possible to maintain the current coastline through adaptation measures.
Measures aimed at delaying withdrawal from vulnerable coastal areas and measures to prepare for withdrawal are not mutually exclusive, but can be complementary. Policies for adapting to sea level rise should consider the two types of measures in conjunction.
- 1 Postponing retreat
- 1.1 Risk and damage control
- 1.2 Flood prevention – Hard constructions
- 1.3 Flood prevention - Soft constructions
- 2 Preparing for retreat
- 3 Principles of relocation - Planned Relocation
- 4 Related articles
- 5 Further reading
- 6 References
In areas where good opportunities exist to mitigate the risk of flooding, even at higher sea levels, there is no short-term need to retreat. With good opportunities is meant: feasible from an ecological, economic and social point of view. A reasonable and generally preferred strategy for climate change adaptation is to start with so-called 'low-regret measures' or, even better, 'no-regret measures' (Hallegatte, 2009 ). These are adaptation measures that generate immediate benefits without the need for high additional investments. No-regret (or low-regret) measures are implemented step by step, allowing for adjustment when better knowledge of the impacts of climate change impacts becomes available. They are preferably designed according to the insight that natural dynamics generally offer greater flexibility and long-term resilience (self-regulating capacity) against climate change impacts than hard man-made structures.
The following sections will discuss different types of measures, most of which are of a no-regret nature.
Risk and damage control
Mitigation measures aim to limit damage if (natural or manmade) flood protection structures fail or are overtopped by extreme water levels. Flood waters may rush into urban and industrial areas situated on the coast and cause serious damage and possibly loss of life.
- Flood risk mapping. Flood risk maps are an important and necessary tool for spatial planning in risk areas, for granting building permits and for taking measures to limit the consequences of floods. In order to produce such maps, land elevation and land use should be known in sufficient detail, as well as the extent and frequency of extreme water levels along the coast. Reliable information requires observation records of sufficient length and numerical simulation models of local hydrodynamic conditions. This information must be regularly updated with recent projections for climate-induced extreme events. In some areas, tsunamis must also be taken into account. Flood risk maps exist in many European countries, in Japan and in the US.
- Monitoring and appraisal. Establishing flood risk maps also requires estimating the safety level provided by existing flood defense systems. For this, test methods and indicators must be developed, see for example STOWA 2012. To keep the flood risk maps up to date, these indicators should be monitored with an appropriate frequency along the entire coast (including estuaries, tidal inlets, i.e., all areas subject to tides). Indicators relate, among other things, to the coastal profile and the condition of flood defenses (emerged and submerged parts).
- Awareness. Flood risk plans need to be widely communicated in risk areas to raise awareness among the population. This allows people to better prepare and make the right informed choices to anticipate and avoid emergency situations.
- Climate proofing of coastal development plans. Criteria must be drawn up for testing coastal development plans against the adopted climate adaptation strategy. Every new development must be tested for compliance with these criteria.
- Building codes. Building regulations in high-risk areas must ensure that buildings can withstand extreme conditions and provide safety for residents. In France, building in areas with a high risk of flooding is prohibited by law.
- Urban water infrastructure. An appropriate drainage infrastructure is needed to ensure that flood water can be collected and drained quickly in order to safeguard residential areas from flooding as much as possible.
- Safety of critical infrastructure, communication. Essential infrastructure in risk areas must be constructed in such a way that it can withstand extreme conditions without being disrupted. This primarily concerns communication infrastructure (Internet) and essential facilities such as drinking water and electricity.
- Early warning organization, responsibilities, and support systems. Early warning is essential so that timely precautionary measures can be taken to limit loss of life and damage. This early warning task requires structural organizational embedding, with adequate resources and clear protocols.
- Evacuation plan, rescue places. For high-risk areas, an evacuation plan must be available and places of refuge must be established. Regular practice is necessary to ensure that early warning and precautionary measures are carried out flawlessly in emergency situations.
- Rescue organization, responsibilities and support systems. Precautionary measures cannot rule out the risk of fatalities and the failure of essential services. In this case, a fast rescue operation is necessary to provide assistance in emergency situations. For this, a trained organization with clear leadership and appropriate equipment must be ready for deployment.
- Evaluation. Each emergency holds lessons for the next emergency. These lessons are precious because they can save lives and prevent damage in future situations.
Flood prevention – Hard constructions
Rising sea levels inevitably increase the risk of flooding in low-lying coastal areas. Do nothing is not an option. In order to avoid the option of retreat, it is necessary to strengthen flood protection structures. There are many different solutions for this, which are briefly discussed below. For a more detailed discussion of these solutions the reader is referred to other Coastal Wiki articles in the category coastal protection.
In many areas with an existing flood risk, the coast is already protected with hard structures. Sometimes these are light structures, mainly intended to combat coastal erosion, and sometimes there are heavy structures such as sea walls. This protection is usually not sufficient at a higher sea level - and sometimes even not at the current sea level. Since sea level rise is a gradual phenomenon with uncertain final stage, an adaptive approach is the most obvious strategy. Some measures that can be implemented in an adaptive way are discussed below.
The expected sea level rise is incorporated in the periodic maintenance/renovation scheme of infrastructural works. In the Netherlands, for example, the design conditions for sea defenses and river dikes are evaluated every 12 years and adapted to the latest insights regarding climate projections for the next 50 years. Defenses that do not meet the revised design conditions are reinforced.
Spatial reservations are made for future reinforcement of coastal defenses, which can serve in the meantime for other uses, such as nature development.
Soft with a hard core
In some cases, the existing sea defenses cannot easily be strengthened and reconstruction is required. The opportunity may exist to opt for a soft defense, where the old hard structure serves as the core for a soft defense. Such a solution has been applied at several locations along the Dutch coast, for example near Petten in North Holland, see Fig. 1. Soft defenses are more flexible and adapt more easily to rising sea levels than hard defenses. If a soft defense is not possible, e.g. because the costs are too high, the materials not available or the hydrodynamic conditions inappropriate, hard constructions must be designed in such a way that subsequent reinforcement can easily be realized.
Flood prevention - Soft constructions
Nature-based coastal protection
Soft solutions must be designed such that they are compatible with natural local conditions. Well-designed soft defenses follow the principle of "building with nature". They are in natural equilibrium with the environment as much as possible and consist of the same materials that are naturally present. This means that natural processes help maintain the defenses and help repair damage from extreme events, see Nature-based shore protection. Soft measures such as coastal wetland restoration generally yield higher benefits at lower cost compared to hard structures. A few examples are discussed below.
Many densely populated coastal areas threatened by sea level rise lie in low-lying coastal plains formed by river deltas (see e.g. Coastal cities and sea level rise). Here, possibilities are often present to restore existing foreland or to create new foreland that offers protection against flooding, as illustrated by an inventory by Van Coppenolle and Temmerman (2019). In temperate climate zones, forelands are usually formed by salt marshes, in tropical and subtropical zones by mangrove. In some cases existing foreland has disappeared due to disturbance of natural sedimentation processes. In other cases, existing foreland has been embanked and converted into fish ponds, agricultural land or built-up area. For example, tidal flat reclamation in Hangzhou Bay (China) caused a substantial increase of wave heights at the seawall and concomitant shortening of the flood return period. Model simulations indicated that for every km of tidal flat ranging from high marsh to bare tidal flat being regained, the dike can be lowered by 0.84–0.67 m, when designing for a 1 in 200 years storm event.
Restoration of eroded foreland or stimulating growth of new foreland can often be done with fairly simple techniques. It is crucial to restore or promote natural sedimentation processes, which requires good knowledge of sedimentation processes and a sophisticated design (Winterwerp et al., 2020). In some cases, dredged mud can be used to promote mudflat development. Some examples are shown in Fig. 2 and Fig. 3. Further details on foreland creation/restoration can be found in the articles Restoration of estuarine and coastal ecosystems and Mangroves. Many nature-based coastal protection measures contribute to mitigating climate change by creating new carbon sinks (so-called 'blue carbon'), see Blue carbon revenues of nature-based coastal protection.
Foreland restoration - managed realignment
In many river deltas, intertidal marshes have been embanked and converted into agricultural land. As a result, important ecosystem services have been lost, such as nurture zones for the estuarine and marine ecosystem, biodegradation of chemicals, denitrification and carbon storage. Remaining intertidal areas are "squeezed" in front of the embankment. Threatened by sea level rise, they can no longer fulfill their buffer function against storm surges.
Managed realignment as a response to sea-level rise adaptation is most widely practiced in the UK. Part of the embankment is removed, which restores the tidal influence in the area behind. The embankment on the landward side has to be reinforced to prevent low-lying inhabited areas from flooding, as high water levels are not significantly reduced by the restored tidal wetland and in some cases even amplified. Evaluation of several managed realignment projects shows that the original wetland area and associated ecosystem services are recovered, albeit not completely. It turns out that agricultural land use compacts the soil, reducing its drainage capacity. As a result, the ecosystem develops differently than in marshes that have not been embanked. Because only part of the embankment is removed, the strength of currents and waves in the semi-enclosed wetland is less than in the original situation. This can lead to rapid accretion, especially in areas adjacent to turbid estuaries.
In order to maintain its protection function with sea level rise, the foreland level should rise accordingly. If natural accretion is not sufficient, the foreland has to be raised artificially with appropriate hard or soft structures.
Some sandy coasts have large offshore sand deposits that can be mined and brought onshore to prevent coastal degradation and to strengthen the dune belt. Depending on the available amount of sand, the effects of sea level rise can be overcome in these coastal areas by coastal nourishments aimed at maintaining the natural coastal profile (both the submerged and emerged parts). By maintaining a natural average equilibrium profile, coastal erosion caused by severe storms is naturally restored after the return of average wave and tidal conditions (see Dune erosion). Part of the nourished sand is blown into the dunes by aeolian processes, which strengthens the dune belt. As long as the available sand reserves are sufficient to raise the coastal profile in pace with the sea level, coastal nourishment is an advantageous way of adapting to sea level rise.
Coastal nourishments are carried out in many countries, most often with the aim of restoring eroded tourist beaches. In the Netherlands, coastal nourishment is primarily intended to allow the active coastal profile to grow in elevation with the sea level rise. Mega nourishments are carried out at the most threatened coastal locations, according to the principle that natural processes will redistribute the sand along and transversely to the coast, thus creating a naturally resilient profile (Fig. 4).
Dune management and dune building
Dunes can provide excellent protection against storm damage to coastal settlements and against hinterland flooding. Preservation of existing dune belts should therefore have the highest priority; measures to prevent their deterioration are described in Dynamics, threats and management of dunes. It may also be necessary to strengthen the existing dune belt. This can be done by artificial dune nourishment (see Shore nourishment) or by stimulating natural dune growth. Natural dune growth requires a wide beach that receives sand through natural processes or through artificial nourishments; other conditions for dune growth are described in Dune development. Various plant species that are adapted to the seashore environment contribute to dune growth by trapping and stabilizing wind-blown sand, see Shore protection vegetation. Fences are also used to stimulate sand capture and dune growth. Protecting seaside resorts with artificial dunes is common practice in the Netherlands. Beach nourishments and dune vegetation not only provide attractive beaches, but also ensure that the dune grows in pace with sea level rise (Fig. 5).
Preparing for retreat
In some coastal areas, the possibilities for adaptation to higher sea levels while maintaining the existing coastline are very limited. However, even if good options for adaptation are still available at the moment, the time may come when the sea level will be too high. Withdrawal to less vulnerable areas further inland then becomes inevitable. The term "managed retreat" is generally used if this takes place in an organized way before a catastrophic flooding takes place. This is not the same as "managed realignment", which on the contrary is aimed at preventing the relocation of inhabited areas with all associated infrastructure. Even if retreat is not yet necessary in the short term, it may be wise to anticipate with no-regret measures. Several possible measures, many of which are practiced in the US, are discussed below. These measures are of an administrative nature.
Coastal governments (states, municipalities) can apply overlay zones for adaptation to sea level rise. These are additional layers of zoning regulations in specific areas, where rules can be issued as specified below.
Coastal setbacks or erosion allowances are predefined distances from the shoreline that regulate certain or all types of development. Setback areas are introduced to gradually replace settlements in future high-risk areas with nature development that forms a protective buffer (e.g., stimulating dune growth, mangrove development). See Setback area.
Conservation easements (or conservation covenants) are legal agreements whereby a land owner restricts the use of their own property in perpetuity to protect its natural values. It is a real property interest established by agreement between a landowner and a land trust or government. The land owner retains ownership, rights, and privileges on their property, excepting the uses restricted by the easement or covenant. The conservation easement runs with the land, meaning it is applicable to both present and future owners of the land.
Rolling easements are coastal development restrictions that prohibit shore-abutting property owners from “holding back the sea”; the easements “roll” inland with migrating coastal habitats in response to coastal erosion and sea level rise. This allows for some development nearer the coast in the present but with retreat or removal in the future and prevents the construction of more permanent walls and levees that modify slopes or act as barricades, restricting landward migration of habitats.
Transferrable development rights
Transfer of Development Rights is a zoning instrument that redirects development that would otherwise occur on the land (the sending area) to a more suitable receiving area. It is a voluntary program which allows landowners with holdings in a hazard area to trade their development rights to a safe ‘receiving’ area. Owners in the sending area can be compensated for their redirected development rights. Advantages: The landowner receives market-determined financial compensation. Costs for conservation authorities (government, land trust) are lower than acquisition. Developers can make a profit by taking advantage of the regulatory flexibility of a TDR receiving area. The public enjoys the many economic, environmental, and health benefits of conserved land, such as reduced flooding and cleaner drinking water.
A ban on the construction of permanent buildings in vulnerable coastal areas can be part of set-back regulations. In France, all new constructions are prohibited in high-risk areas (red zones subject to a designated high-risk e.g. flooding, fire, earthquake, etc.). Reconstruction is also prohibited if a building has been destroyed as a result of the designated high-risk factor.
Land purchase is an effective, but often expensive, means of controlling development in vulnerable coastal areas. In the UK, large areas along the coast have been purchased with private funds by the National Trust or acquired through donations. In France, many coastal areas are owned by the Conservatoire du Littoral, which has a budget for purchase mainly donated by the French government. These areas are managed by local authorities, who are bound by the ownership rights of the Conservatoire.
Principles of relocation - Planned Relocation
Most people have a strong bond with the area where they have their roots. Relocation is therefore a highly sensitive social, cultural and political issue that easily provokes strong resistance. A few important principles of relocation are:
- Planned Relocation is a measure of last resort when no other adequate risk reduction and/or adaptation options are available.
- Planned Relocation is undertaken for the benefit of Relocated Persons and in a manner that respects and protects their rights and dignity.
- States must have a sound legal basis for undertaking Planned Relocation, in accordance with national legislation and States’ international obligations.
- Relocated Persons and Other Affected Persons should be informed, consulted, and enabled to participate in decisions on whether, when, where, and how a Planned Relocation is to occur, as appropriate.
- Physical relocation to settlement sites should only occur once such sites are capable of sustaining a dignified standard of living for Relocated Persons.
Large-scale relocations as a result of sea level rise have not yet taken place. Plans are being developed for several small low-lying island states. The largest planned relocation concerns Jakarta, of which some districts cannot be properly protected against flooding. The Indonesian government has decided to move the government center to the island Borneo .
- Climate adaptation policies for the coastal zone
- Integrated Coastal Zone Management (ICZM)
- Coastal cities and sea level rise
- Sea level rise
- Setback area
- Shoreline management
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