Double dikes can defend coasts from rising seas

Posted on 26 January 2021

Innovative climate adaptation initiative is a safe, natural and cheaper way to protect coastal communities
Allowing low and high tide to return to a so-called ‘transitional polder' between two dikes will make coastal defence much more sustainable, natural and affordable as climate change causes the seas to rise - according to a report commissioned by WWF.
The authors from the NIOZ Royal Netherlands Institute for Sea Research calculated the possibilities of realising transitional polders along parts of the coast in the Southwestern Delta of The Netherlands. Compared to the current custom of constantly building the dikes higher, which causes subsidence and silting up of the hinterland, coastal defence based on transitional polders would be cheaper due to the new economic functions these have to offer.
Transitional polders between double dikes capture valuable silt
The principle of a transitional polder is based on a double dike. An opening is made in the current sea dike, so that the ebb and flow of the tide once again have free play on the land behind it. The sea is then held back by a second dike: an existing, former sea dike or a new dike to be constructed. This second dike can be lower and less expensive than the current sea defence, because the water has already lost much of its force behind the first dike.

The polder between these double dikes will then slowly rise because of the silt that builds up with each tide - as much as 3 to 5 centimetres per year in some areeas. So, after half a century, the land between the dikes will be up to three metres higher. In the meantime, the expanding land can be used for aquaculture and growing saline crops. Later on, the fertile sea clay can be used for regular agriculture once more. In addition, part of the alternating polder can be set up as a nature reserve for wading birds, and other plant and animal species that depend on tidal areas.
Boosting the regional economy
Particularly in the Dutch province of Zeeland, hit hard by disastrous flooding in 1953, returning land to the sea is a sensitive issue, as evidenced by the lengthy discussion that preceded the managed realignment of the Hedwige-Prosperpolder.

"The situation is very different for a transitional polder," emphasised ecologist Jim van Belzen, one of the authors of the report. "Unlike previous managed realignment areas that were done for nature conservation, the most important goal of double dikes with transitional polders is flood safety. Putting safety first is deeply rooted in the souls of the people of Zeeland. For this reason, these pieces of land are only covered with water temporarily.

"During that time, a great deal of value can be gained from them, for example by cultivating mussels or pickleweed, and through nature, which is also good for tourism. Because of the revenue generated, the cost-benefit analysis for a transitional polder is also much more favourable than for constantly raising the current sea dikes," he added.

"In addition to the favourable cost-benefit analysis, a transitional polder also gives an enormous boost to the regional economy," said economic advisor Gerlof Rienstra. "Whatever you invest in such a polder will result in a permanent increase in value through new uses such as saline cultivation, aquaculture, recreation in the resulting nature and agriculture, among other things."
Do not wait too long
The authors argue that the Western Scheldt, which will deposit a relatively large amount of silt in the intertidal polders, will yield the fastest returns in terms of coastal defence. In the Eastern Scheldt, where there is less silt suspended in the water, it will take longer before the land behind the dike will be sufficiently elevated.

"In view of the ongoing sea level rise, it is therefore of the utmost importance to start implementing this type of measures not in 30 years but now," said Van Belzen. 
Double dikes can protect coasts from rising seas
Double dikes building resilience to rising seas