Dikes and dams need regular maintenance and strengthening to keep their protection capacities and meet safety requirements. In addition, climate scenarios for sea level rise and extreme weather conditions can lead to reconsidering safety requirements and building new protections on identified weak points or heightening and strengthening existing ones. The design of existing dikes and dams can be modified to fulfill different purposes.
Re-enforcing dikes and dams can increase their stability and resistance against dike breaching, e.g. by strengthening the inner core of the dike, or improving characteristics of the dike's surface that contribute to the overall stability of the dike. Overtopping resistant dikes are wide and less steep than traditional dikes, and can be multifunctional (for example, for agriculture, recreation or transport).
Dikes can also be re-enforced by heightening, broadening or by adding spatial components. Heightening is the usual way to re-enforce dikes, but other innovative approaches have recently been developed. Heightening provides coastal and riverside defense, but without integrated development or a combination of functions that a spatial solution may offer. In Schleswig-Holstein, the safety standard for all dikes includes a margin for sea-level rise of 0.5m and is supplemented by building a reserve in case of stronger sea level rise of a further 0.5m. Broadening may offer additional benefits, yet may not be always practicable because space is limited or for socio-economic reasons. Dikes could be widened up to 300m land inward and the dike could be used as a space for agriculture or recreation. Wider dikes are more resistant to “overtopping” by storm waves. If re-enforcing the flood defense system becomes necessary due to climate change, recent studies advocate a three-step approach, considering spacing, broadening and raising consecutively. Dike design can have the aim of allowing water in certain conditions to overtop them without breaching. This is usually achieved by strengthening the inner wall of the dike, by dike broadening, or by developing a parallel dike system with enclosed retention polder The construction of double dike systems, and using the space in between dikes to retain the water that washes over. In Belgium, this approach has been used in the controlled flooding areas for the Sigma Plan in the Scheldt River estuary: one such area is Kruibeke, where the space between the outer, overflow dike and an inner, higher dike has been restored as wetlands and other habitats (the outer dike has sluices to allow water flow between the wetlands and the Scheldt estuary).
An overtopping dike can provide more safety against flooding than the typical single-line defenses. The measure reduces flood impacts (population exposed, production affected) by decreasing the sensitivity of people and assets in flood prone areas (i.e. the ability to manage water surplus). As the dike will not breach when overtopped, it prevents the uncontrolled catastrophic dike breaks associated with devastating flooding of the hinterland. The number of potential victims and the resulting damage are therefore much lower than incurred when a traditional dike breaks. The risk, calculated as a product of the probability of occurrence and the resulting damage, is thus reduced. Overtopping can still result in anxiety and minor damages. These can be reduced by spatial planning or subject to compensation and insurance.Another option to reduce flood risk, other than strengthening the primary water defense structures, is to compartmentalize the region to be protected in zones, for example by dike ring areas. Compartmentalization either or both protects critical functions in the flood-prone area and reduces the flooded surface area. It diminishes the flood effects by dividing the area into compartments with the use of dikes.
The choice of the type of dike improvement has important implications not only for safety of the people and assets behind the dikes, but also has visual/landscape implications for the people living close to the dikes. Therefore, stakeholder involvement during the design phase is important.
Costs and Benefits
Experience with dike re-enforcement in the Netherlands has yielded the following indicative estimates of total cost: low river dike: 3 mln €/km; high river dike: 5 mln €/km; estuarine dike: 5 mln €/km; coastal defence: 7.5 mln €/km.
Information is available from the Netherlands on constructing dikes within dunes indicates that this is more complex and costs are significantly higher. The costs to construct a dike within the dunes in Katwijk, over a length of about 900 m, amounts about 45 million €. The advantage is that the water safety against flooding is improved while preserving the existing character of the beach resort. These are costs only for water safety; costs for facilities are not included but an underground parking garage was built afterwards, preserving the existing landscape character. Comparable solutions have been chosen for Scheveningen and Noordwijk.
Success and Limiting Factors
Dike or dam reinforcement has strong supporters and opponents, with concerns and preferences changing over time. Support is typically strong after a flood event. Where reinforcement is planned to pro-actively adapt to climate change it is more likely to meet resistance. Heightening and reinforcement of dikes can affect the landscape in a negative way. In countries like the Netherlands, people started to resist reinforcement programmes. The loss of historic houses and views is perceived as problematic. In addition, raising dike height can increase water levels in the river during high flow. In response, various alternatives to dike reinforcement have been developed, including widening the riparian areas and floodplain, creating overflow channels and lateral diversions do increase the capacity of rivers. “Overtopping resistant dikes” may be more costly, with a typical time horizon of 50 years in economic assessment. For a longer time horizon and including maintenance, the comparison becomes more favourable. At the same time, the overtopping resistant dike can be combined with other functions, raising its multi-functional character and broadening opportunities for financing. Opportunities for this are location specific.
In addition, as a consequence of wave overtopping under extreme conditions, the multifunctional coastal zone should be made adaptive to occasional accommodation of water in that area. Such a spatial adaptation in coastal situations may offer opportunities for salt or brackish ecosystems, recreation, living and wet agriculture.