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Reflection: Double Dikes, twice the protection with twice the responsibility?

Posted at 17/05/2021 by Prof. mr. Marleen van Rijswick

During the All-Risk webinar, the questions were discussed how a double (twin) dike can contribute to flood risk protection, and what the division of water management responsibilities between goverments is regarding this concept. In the reflection below you will find what has emerged from the discussion between science and practice.

Contact details

Prof. mr. Marleen van Rijswick

Utrecht University

This reflection emerged from the discussion between the webinar team and the 51 to 73 participants to the All-Risk webinar organised in May 17, 2021.

Would you like to view the presentations of the webinar team? You can view them in the above video.

Artist impression of the Double Dike design. Image by Waterschap Noorderzijlvest.

The discussion

The Double-Dike system in the province of Groningen in the north of the Netherlands is a concept where two parallel dikes together provide the required safety for the hinterland during storms. In the area in between, seawater can flow in and out, which offers opportunities for new land use. Think, for example, of aquaculture, saline cultivation, recovery of salt marshes and clay extraction. During the webinar, it became clear that there is some controversy regarding the concept of Double Dikes. On the one hand, the concept offers interesting additional functions (saline cultivation, aquaculture, nature restoration, clay mining). On the other hand, the question arises whether the second dike contributes to flood risk management and how this relates to the costs.

During the discussion, it appeared that the final words have not yet been spoken on this matter. Reference is made to various applications of Double Dikes: from a low outer and a high inner dike (or vice versa) to sandy coastal applications such as on the northeast islands of Vlieland and Terschelling (also in the province of Groningen), with the foredune as the outer barrier and as the inner barrier the inland boundary profile including the intermediate barrier dune valleys. There are also several options to make use of the intermediate area (for example, space for floating homes/recreational homes such as in Zeeland/Waterdunen in the southwest of the Netherlands).

First, it is important to consider what the intended goal is and what added value is created with a double dike. Then it can be considered which dike – the inner dike, the outer dike or both dikes together – can best be regarded as a primary defence, technically and legally. Theoretically, only the inner defence must comply with the safety standards. Participants prefer to classify both dikes in the double dike system as primary defences. In practice, however, it may be most convenient to assess the defence realising the largest reduction in flood risk as to the primary defence.

Participant responses to the question about the double dike concept.

What is the biggest challenge for implementing a Double Dike?

Participants indicated that the question of which flood defence should be the primary flood defence should also be placed in time. For example, accretion of the intermediate area could cause a shift: from an inner primary barrier to an outer primary barrier. The question remains how such a dynamic (eco)system can be regulated legally.

If one of the two defences contributes relatively little to flood risk management, it can be included in the assessment until it is removed. From a legal point of view, it can then be decided not to designate it as a primary flood defence and to regulate activities on this flood defence in the regulation of the Water Authority (Keur). If the construction of the double dike is not for flood risk management reasons, funding from the Flood Protection Programme (HWBP) will not be possible in advance. In case reinforcing only one flood defence is more cost-effective the idea of a double dike could be undermined in the long run. If you only look at the short term, a double dike is not always an obvious choice. From a long-term perspective, a double dike can have an added value, also with regard to flood risk management in a broader perspective.

A perspective for the future?

Participants also point out that the added value of a double dike in the short term can differ from the added value in the long term. This gives rise to follow-up questions about the value of this concept in a scenario in which an accelerated sea-level rise becomes a reality. Can the accretion of the hinterland contribute to the probability of flooding in the long term and can sea level rise be kept up with this? It is also suggested that perhaps two safety standards can be used: one for the intermediate area in which a lower level of safety is offered, and one for the area behind the inner dike (to protect large numbers of victims and capital intensive investments) An example is the scenario in which it is decided to keep the Western Scheldt permanently open for the accessibility of the port of Antwerp. In the back of the Western Scheldt, along the neck of Zuid-Beverland, a floodable outer dike could be realized with an interdike area in which agricultural use will be possible for a long time, the inner dike then serves to protect the hinterland. Another example: for the Wide green dike it has already been estimated that accretion can be used to arm the dike against rising sea levels.

Aerial photo of the outer and inner dike of the Double dike system. Photo by Waterschap Noorderzijlvest.

It is noted that dike improvements can be looked at more broadly: they can act as a driver for area developments. Consider, for example, climate-proof landscapes where the intermediate area offers a buffer for the lower-lying hinterland (with a broad defence zone). It would be of added value to follow this broad and long-term oriented approach in the evaluation and monitoring. The big key question remains who is responsible for allowing it to silt up in the long term and at the same time guaranteeing safety in the short term. Participants noted that the prospect of preserving the value of the land and water safety in the long term is not yet widely available among governments. The Delta Program could offer added value in this regard.

Trust between governments will be necessary before this long-term perspective can get off the ground in administrative and organisational terms. When asked whether the division of responsibilities between governments could stand in the way of an innovative concept such as the Double Dike, three-quarters of the respondents affirmed this. According to participants, the answers to this question illustrate how difficult plurality is in our world of thoughts and policy. The outcome touches upon the relationship between the governments of the general democracy with regard to the spatial planning of an area (municipalities, provinces, national government) and water authorities as functional management, with their focus on functional flood risk management. However, there are also positive voices: participants indicate that a lot can be done with the twelve-yearly assessment and evaluation. A clear story about “why do we do something and how do we do it” and clearly stating what management entails can bring a lot of peace and space for future-oriented discussion and solutions, and… where there is a will, there is often a way.

All-Risk recommendations:

  • First, look at the intended goal and for which aspects the double dike creates an added value. Then consider which dike – the inner dike, the outer dike or both dikes together – can legally and technically best be regarded as primary flood defence.
  • From a technical-theoretical point of view, we recommend considering the inner dike as the primary flood defence in order to calculate whether it meets the safety standard.*
  • From a legal point of view, depending on the importance of the outer dike for water safety, this dike could – in addition to the inner dike – also be designated as primary flood defence or be actively regulated as a flood defence object (or foreland) in the regulations of the Water Authority (keur/legger).*

*The participants indicate to designate both barriers as primary. While current practice prefers the dike with the largest contribution to the reduction of the probability of flooding.

Last modified: 17/11/2021