Improved design of dikes and levees

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.

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.

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.

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.

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.

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.