Improved design of dikes and levees

Dikes and levees are hydraulic structures that are built to retain water:

• Dikes usually run parallel to a water body (such as a river or a sea) and have water only on one side. Dikes were first built to reclaim land from the sea and thus protect land that would naturally be underwater most of the time. They also provide protection against flooding from the sea during extreme events.
• Levees are embankments built for flood protection. They are usually earthen embankments and have water only on one side. Levees protect land that is normally dry but may be flooded during extreme events.

Dikes and levees need regular maintenance and strengthening to ensure their protection capacities and to meet safety requirements. New projections for sea level rise, magnitude and frequency of extreme weather events and increased coastal and river flood risk can lead to reconsidering safety requirements. This can lead to building new protections on identified weak points or heightening and strengthening existing ones. Re-enforcing dikes and levees can increase their stability and resistance against breaching and their safety against flooding. The most common methods to reinforce dikes and levees are:

• Earthworks aiming at flattening the dike or levee slope, raising and widening a dike or a levee, or constructing berms.
• Structural measures aiming to strengthen dikes and levees, including seepage or cut-off walls, surface sealing or the additional building of mobile or non-mobile flood protection walls.
• Improvements of the dike and levee material, for example through soil improvement or the use of geosynthetic materials.
• Surface protection of the dike and levee, for example through rock layers to prevent erosion or allow for overtopping.
•  Protection of dike and levees through the plantation of woody plants.

One of the most common failure mechanisms of dikes and levees is breaching in case water overtops them. Dikes and levees can be built in a way that allows for overtopping (e.g. by strengthening the inner wall or broadening and reinforcing the surface). Such dikes and levees prevent the uncontrolled catastrophic breaks associated with devastating flooding of the hinterland. Damages can still occur due to the water that overtops the structures, but they are much smaller compared to a dike or levee break. A different adaptive approach to dikes and levees strengthening consists in their partial or complete deconstruction, especially in potential floodplains to allow more room for the river or sea (see the adaptation option rehabilitation and restoration of rivers and floodplains). In this case dikes or levees can be dismantled completely, opened up by slitting, or relocated further inland if necessary, offering a more sustainable and longer term adaptation potential. For dikes, developing a parallel dike system with enclosed retention polder is also an option to mitigate extreme flood peaks: the construction of double dike systems allows using the space in between to retain the water that washes over.

Therefore, alternative nature-based solutions should always be evaluated to ensure long term sustainability of flood protection, minimise associated tradeoff and provide multiple benefits for the environment and the society.

IPCC categories

Structural and physical: Engineering and built environment options

Stakeholder participation

The choice of the type of dike or levee intervention to be made has important implications not only for the safety of the people and assets behind these infrastructures, but also in terms of visual and landscape impacts. Stakeholder involvement during the design phase is important to inform people on the positive effects in terms of adaptation and safety. Stakeholder participation can also help in identifying mitigating measures which can reduce the visual and landscape impacts of the improved dikes and levees and improve their social acceptance.

Success and Limiting Factors

Dike or levee reinforcement has strong supporters and opponents, with concerns and preferences changing over time and strongly depending on local priorities. 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 some resistance. Heightening and reinforcement of dikes and levees can negatively affect the surrounding landscape. In addition, heightening river dykes can increase the magnitude of peak flows downstream, thus amplifying flood hazard and risk downstream. Moreover, raising flood protection and the consequent reduction in the frequency of flooding events favours the “loss of flood” memory. This can lead to increasing exposure in flood-prone areas, which is usually referred to as “levee effect”. In case of unexpected and sudden failures of the flood defences this could lead to catastrophic consequences (JRC PESETA IV project, Dottori et al., 2020) Together with transparent stakeholder consultation, it is important to assess and consider alternative and integrative solutions, especially the ones that allow more room for the river or sea.

Costs and Benefits

Cost estimates for adapting dikes and levees differ depending on the type of structure and reinforcing method. Some indicative costs are reported in a review study (Aerts, 2018): $20.8–25 million/km per m sea dike raised in the Netherlands; $21.8–31.2 million/km per m sea dike raised for European cities; $5.3 million/km per m levee raised in Canada; $1.9 million/km per m earthen levee raised in Canada; $5.6 million/km for mobile floodwalls in the United states; $130–330/m2 to armor shorelines with rock or other material in the United Kingdom. These costs do not include maintenance costs that can be particularly high for large infrastructure, especially considering new challenges posed by climate change.

The benefits of implementing different flood adaptation measures are usually expressed as reduced flood risk or reduced damage. Investments in dikes and levees may be economically attractive for reducing flood risk in large parts of the world, but not everywhere. Therefore, alternative solutions should always be evaluated and it should be noted that measures that give more room to the sea or river often provide several co-benefits for ecosystems.

Legal Aspects

Any changes to existing dike and levee systems have to be in line with the Water Framework Directive requirements which call for a good status of Europe’s Water Bodies. Furthermore, starting from the 2nd implementation cycle onwards, the consideration of climate change impacts is compulsory under the EU Floods Directive. To prevent environmental damage, the EU Directive on Environmental Impact Assessment (EIA) requires certain infrastructure projects to undergo an EIA. The construction of coastal works to mitigate erosion and hard sea defences ‘capable of altering the coast’, such as dikes, as well as flood-relief works inland fall into Annex II of the EIA Directive. Member States decide whether projects in Annex II should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria. Maintenance and reconstruction of these works are explicitly excluded. Any infrastructure project, including dikes and levees, likely to have a significant impact on habitats and species protected under the EU Natura 2000 network is subject to an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site.

Implementation Time

The time to implement adaptation measures for dikes and levees varies significantly depending on the typology of structure, the chosen strengthening measure and on whether an EIA has to be performed. Implementation times are in the rough range of 5-25 years.

Life Time

The expected lifetime of adapted dikes and levees is usually more than 30 years. It should be noted though that maintenance plays an important role and that maintenance requirements change over time due to the aging of the structures and changes in river discharges and sea levels.

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