The development of modern sea dikesCoastal populations have always been threatened by flooding. Although fairly helpless against such events at first, over time they learned to build protective structures against storm floods. In some countries buildings were built on stilts, so that water could flow freely underneath, while in other places houses were built on man-made earthen hills. As early as the twelveth century, ring dikes were already being built in northwest Europe for the protection of individual settlements. Through time, the design of dikes changed. In the early sixteenth century the dikes in many places consisted of two-metre-high walls of timbers, backed and stabilized by an earthen wall. But because these kinds of dikes were heavily battered by the surf, the vertical form was soon abandoned in favour of an elongated slope and flatter profile, where the wave energy of the storm floods could be absorbed more gradually. In the mid-eighteenth century, these dikes often had a height of around five metres. Although the gradually sloping profile proved to be effective, water would still spill over the top during high-surging floods. This would wash out material on the back side until the dam collapsed. The trend thus developed toward the construction of increasingly higher dikes with even flatter slopes. Today, the large sea dikes in northwest Europe have heights of around nine metres. They have low slope ratios of at most 1: 6, and are around 100 metres wide at the base. These can withstand even high storm-flood surges. But with climate change and its accompanying rise of sea level, coastal inhabitants are facing new challenges.
- 4.16 > For several centuries the people in the Netherlands relied on “stack dikes” (Ger. – Stackdeiche) to protect the region around Amsterdam, as shown in this illustration of the Zuiderzee in 1702. These were repeatedly damaged during heavy storm surges.
Climate change as a new challenge to coastal protectionIf sea level should rise by one metre by the end of the century, and even by several metres thereafter, today’s tried and tested coastal protection systems will no longer be adequate. They will have to be upgraded in many places. No one knows, however, how strongly or rapidly climate change and sea-level rise will progress. In contrast to past centuries, when it was sufficient for engineers to design structures that were suitable for the existing conditions, precisely this question arises now in the face of climate change: What conditions will exist in the future? Coastal protection will have to account for diverse probabilities and consider the various scenarios of the IPCC (Intergovernmental Panel on Climate Change) in the planning and construction of protection systems.
- 4.17 > Through time, the profile of dikes on the North Sea coast of Schleswig-Holstein changed. There was a trend away from the steep structures to a long and much lower gradient, so that the storm-flood wave energy could gradually run out over a longer distance.
Staying a step ahead of sea-level riseA fundamental question for coastal engineers is how high or strong coastal defence structures should be designed today. Because the future global progression of sea-level rise is uncertain, and a more rapid advance than the global average is expected in some regions, it would be prudent to incorporate flexibility into coastal protection designs for the future. An adaptive pathways approach is now being promoted. This involves the planning of coastal protection measures adapted to the short-term consequences of change, and not rigidly committed to an uncertain assessment scenario to the end of the century. In this way it may be possible to keep pace with the rising water. A large barrier that closes off a river mouth during storm floods has to be completely rebuilt when it no longer provides sufficient protection as a result of rising sea level. The initial investment would thus be completely lost. It is more sensible to plan for smaller measures that build upon one another. Coastal protection is therefore now facing a paradigm shift. While the axiom of preserving a coastline through the use of large immobile structures was considered valid in the past, the adaptive pathways design approach introduces an array of different concepts and measures, including the selective opening of dikes and the creation of emergency flood plains, or “polders”. Specialists today distinguish the following conventional and adaptive protection principles:
Conventional coastal protection
- Resistance: Planning and construction of coastal protection measures at a large economic cost, which are designed for today’s extreme events such as 100-year floods. This approach represents the classical method for designing coastal protection systems. The disadvantage is that extensive damage can result if these systems do fail, as in the breaking of a dike.
- Static robustness: Planning and carrying out coastal defence measures that are already designed for the worst-case climate scenario for today. This principle has clear disadvantages. For one, enormous investments would have to be made today. For another, the construction would be planned according to the present-day knowledge of climate change. This entails the danger that the protection measures will not be adequate if climate change becomes more intense than expected.
Adaptive coastal protection
- Resilience: Planning and construction of coastal protection structures that are designed so that their failure does not result in losses and severe damage to infrastructures, buildings or ecosystems, and allows rapid recovery or restoration. This could be achieved, for example, by building floating houses. Another possibility would be to build elevated streets and railways on the tops of dams. This would limit the extent of damage. Ideally, damage would be completely avoided.
- Dynamic robustness: Coastal protection structures are implemented in succession, to a degree that is based on the latest available knowledge about the development of climate change. This principle is based on a “no-regret” strategy. This refers to measures that would have societal benefits even when the extent of climate change turns out to be greater or less than what was expected, and that do not entail irreparable damage if false assumptions were made in the scenarios. One example of a “no-regret” measure is the creation of a “polder” that serves not only for coastal protection, but at the same time can function as a local recreation area or nature conservation area – and thus has an additional societal or ecological value. The disadvantage of this approach is that, in contrast to the concept of static robustness, the coastal protection is not fabricated in a short time by a single measure, but has to be repeatedly evaluated and expanded over a long time period by supplemental and complementary measures. It thus requires long-term and constantly adaptive planning, as well as a management system that can function over time periods of many decades and even has a planning horizon of a full century.
London leads by exampleThe larger bulk of coastal protection measures worldwide today are carried out according to the conventional resistance principle, but in a number of countries initial concepts are being developed that follow the adaptive pathways approach. An ongoing example is the protection of the Thames estuary in England. To protect London from flooding during storm surges, a large flood barrier, the Thames Barrier, was put into operation in 1984. It consists of large movable flood gates that are closed during storm tides and prevent the surge of high water from the sea from reaching the city. At the beginning of this century, because of concerns that the existing barrier will not be able to resist the higher storm tides expected in the future, debate began about whether it should be replaced by a new and even larger barrier further downstream within the estuary of the Thames. The ramifications for the population of London and the expected extent of damage if the barrier were to fail would be immense. The storm floods as a consequence of climate change and sea-level rise could indeed exceed the capacity of the existing Thames Barrier. They would acutely threaten 1.25 million people who live and work in the high-risk flood areas, as well as 500,000 dwellings, 40,000 commercial and industrial properties, important government buildings, 400 schools and 16 hospitals.
The term “polder” originally comes from the Dutch, signifying a piece of land that is protected from high water by dikes. In the context of coastal protection, “polder” designates areas that are purposely allowed to flood in order to diminish the crest of a flood wave.
Coastal protection road map for the futureThe construction of the new barrier in the Thames, which would cost up to five billion pounds, was rejected as an exclusive solution. Instead, in cooperation with scientists, the authorities created a kind of road map for future coastal protection that provides for various measures to be realized with continuing and accelerated sea-level rise, which is commensurate with the adaptive pathways approach. The Thames Estuary 2100 Plan presents a catalogue of measures, and provides clear options in dealing adequately with the risk situation at any given time, in spite of great uncertainties about the progression of climate change. Additionally, the financial burden that would result from construction of a new barrier will be avoided for as long as possible. In this developmental plan, detailed critical points in time by which decisions must be made regarding planned future measures were identified, and by which times the measures must be carried out. Furthermore, and in agreement with the surrounding counties, it was decided which measures should be carried out on various segments of the river between London and the North Sea. In chronological order, the measures include:
- Option 1: Classical defence systems
- Increase height of existing systems (sea walls, dikes, etc.);
- For old systems that need to be replaced, build the new structures higher;
- Design new defence systems so that they are more easily repaired, replaced, or raised.
- Option 2: Create floodplain areas
- Create target areas for flooding – for this purpose four large areas have already been identified in the estuarine area of the Thames.
- Option 3: New barrier
- Construction of a new barrier, for which possible sites have already been identified and the legal framework already established, so that if it becomes necessary construction can be started quickly without the need for elaborate negotiations.
- Option 4: Massive barrier
- Construction of a barrier that, in contrast to the existing one, is always closed, in order to permanently hold back the water under higher sea level conditions. This barrier will include locks for ship traffic.
The Netherlands under pressureBecause large portions of the country lie below sea level, the Netherlands and the Dutch-Belgian border region, which lies in the shallow estuarine area of the Scheldt River, are at risk in the future. Sea-level rise and the processes it triggers present a dual threat for this region. For one, it is feared that the higher storm floods associated with rising sea level will damage or spill over the dikes and protective structures. For another, with climate change, an increase in precipitation is expected for Western Europe, so inland rivers could overflow their banks more frequently. When both processes occur together – higher water levels at the coast and strong rainfall inland – the river water is no longer able to flow into the sea, so it swells and backs up in the inland regions.
Around nine million people live in the low-lying areas of the Netherlands. In addition, there is a high concentration of economic assets, comprising infrastructure as well as business and industry. The city of Rotterdam, for example, incorporating Europe’s largest harbour, presently lies an average of two metres below mean sea level. For many years now, these low-lying areas have been protected by massive structures such as dikes, dams or flood walls. Additionally, since the 1950s flood-defence systems comprising large barriers have been installed that seal off many former bays and rivers from the North Sea, either permanently or during storm surges. In order to upgrade this system to adapt to sea-level rise, expenditures have been calculated for the Netherlands of up to 1.6 billion euros annually to 2050. According to present estimates, if the massive coastal defence installations should fail in spite of these investments, and the region be flooded, the resulting losses could be as high as 3700 billion euros.
Making room for waterIn view of the enormous costs for maintenance of coastal defence structures and the enormous risk that a failure of coastal protection installations entails, an additional strategy has been followed in the Netherlands since 2012 with the “Ruimte voor de Rivier” project (room for the river). While many river channels have already been highly altered by dikes and barriers, more than 30 separate measures will be carried out on the Maas (Meuse), Rhine and Waal Rivers in the Netherlands by the end of the project in 2019. These include:
- Widening river beds so they can hold more water;
- Deepening of rivers;
- New construction of separate canals that will relieve the main river and provide a substantial creative landscape element for new residential areas, which would be built at the same time;
- Relocation of dikes and creation of wide polders, in order to give the high water more room.
- With these measures, the “Ruimte voor de Rivier” project also fits rather well with the concepts of the “Building with Nature” initiative, which was jointly initiated in recent years by Dutch coastal protection experts, engineering agencies, government offices, and researchers, and has already been implemented through a number of pilot projects. “Building with Nature” means that coastal and high-water protection measures are designed to conform to the natural conditions, while at the same time offering new locations for the development of natural areas. An example is the relocation of dikes for the creation of flood polders where species-rich wetlands can develop. The “Building with Nature” concept expands on conventional coastal defence systems, which could more appropriately be described as “building IN nature”. With the conventional measures, rigid artificial structures are imposed on the landscape, looking like foreign objects and often tending to slice through natural spaces.
Major project on the Scheldt estuaryOne of the first major projects to adopt the “Building with Nature” concept is the creation of a number of polders along both the Belgian and Dutch sides of the Scheldt estuary. This involves flattening all of the old dike lines to make lower overflow dikes, and moving the position of the new dike back to form a polder. The polder is bounded on the river side by the overflow dike, which only allows water to spill over at times of high water. In addition, the water level in the polder is regulated by a sluice built into the overflow dike. The purpose of the low overflow dike is to keep water in the polder to sustain wetland habitats. The polders will have a total area of 40 square kilometres. In case of high water they can take on large volumes of additional water and protect against flooding in the hinterland in the future.
- 4.20 > New polders are being built on the Scheldt estuary. The existing levee (A) is lowered to function as an overflow dike. The water level within the polder is regulated through a sluice (B) in order to form a wetland area (C). A new ring dike (D), located behind the original location, protects against high water surge.
- On around 60 per cent of the polder area wetland areas should develop naturally and will serve, among other things, as breeding grounds for birds. The first polder was created in 2006. The project is to be completed by 2030. The total costs will be around 600 million euros. Compared to this, the high-water damages that would result if the polders were not built would be significantly higher. These could be as much as one billion euros annually throughout the period to 2100. >