We have long believed in the Netherlands that we are safe from flooding as long as our dikes are high enough. However, dikes will always be vulnerable, one reason being the continued sea-level rise. Because of this, the concept of ‘multi-layered safety’ ('meerlaagsveiligheid') was introduced in the Delta Programme. Alterra and HKV have applied this concept to the islands and mainland coast in the northern Dutch provinces of Friesland and Groningen.
Three coastal safety layers are defined within the multi-layered safety concept:
- flood protection (the dikes),
- spatial planning changes in the land behind the dikes, and
- an emergency plan.
‘Our research in Friesland and Groningen focused on the second and third layers of this concept,’ explained Alterra researcher Judith Klostermann. ‘We looked at risk-reduction measures in the region, and where possible also considered the economic benefits that could result from the same measures. The agricultural and touristic values of the Wadden area are also important, for example, but so too is the gas and electricity production infrastructure in the north of the Netherlands, which is of vital importance for the whole of the country. The Eems delta is very vulnerable, for several reasons, and a flood in the area would be rapid and deep and severely affect the energy supply.’
A number of variants were developed for the Eems delta as part of the research project and these were assessed by the researchers in terms of cost-effectiveness and their impacts on nature, the landscape, recreation and the regional economy. The open connection with the Wadden Sea, the lack of inner dikes and on-going soil subsidence due to gas extraction and peat oxidation make the area particularly susceptible to sea-level rise. Of the options investigated, the ‘Potato Valley’ (a large dike with potato fields behind the dike) and ‘Fatty Fish’ (a freshwater/salt water transition zone with aquaculture and nature) variants were considered the most realistic.
Because people have so much faith in the primary safety layer (dikes), the tertiary layer (in particular the level of self-sufficiency in the case of a flood) is poorly-developed in the Netherlands. Detailed information then becomes essential: where is the water coming from, and where is it going? The researchers drew up a new evacuation strategy that combines horizontal and vertical evacuation. The strategy shows that the Wadden islands are a particular area of concern because they would have to fend almost entirely for themselves in the case of a flood and because of the number of tourists on the islands who would not know what to do. Vertical evacuation (ensuring that everyone reaches a safe high-lying place within the affected area) is the only option on the islands, but there are as yet no flood risk maps available for the individual islands.
In the Eems delta, horizontal evacuation is also possible (ensuring that everyone leaves the affected area). This applies to vulnerable people (hospital patients for example) and people who live in low-lying areas. Other people, including livestock holders for example, might be able to remain in safe areas in the flooded area. Once a flood has started, vertical evacuation is the only option because it is unsafe to travel during a flood. Judith Klostermann summarised, ‘Primary layer safety turns out to still be the most cost-effective because everything behind the dike is protected. Where this is not possible, for example in areas that lie outside the dikes and in some areas on the Wadden islands, there needs to be more attention paid to secondary layer measures. Tertiary layer measures have so far been largely ignored; they require a lot of detailed knowledge for the development of safety regions together with the water managers in each area. I hope that this report will encourage these organisations to come into contact with one another, because only then is there any point in telling the general public what to do in their particular situation.’