Sea level on Dutch coast higher than ever in 2017

Published: 12 January 2018

The average sea level measured on the Dutch coast was higher than ever before in 2017. The Dutch sea level, the averaged measurements from six tide stations, rose to 11 cm above NAP. The last highest measurement was in 2007, when the average sea level was 9 cm above NAP. Why was the water so high in 2017?

Tide station Hook of Holland (source: Rijkswaterstaat)

No surprise

Sea level expert Fedor Baart of research institute Deltares: ‘I’m not surprised that the sea level was higher than ever last year. The level has been rising gradually since 1890 by about 0.2 cm per year due to the melting of the ice and the warming up of the ocean. That means that, as a rule, you expect the sea level to be higher every year. The more interesting question is actually why the sea level didn’t rise during the last 10 years.’ There are two main causes.

Number of storm surges

Firstly, there have not been as many storm surges. In 2007, the storm Kyrill pushed up the water over a period of several days and it was followed by another three storm surges. That was the last year with several storm surges in a single year. Things were relatively calm after that until our country was battered by a storm on 5 December 2013 [1]. Severe storms lasting a few days push up the water on the coast by more than a metre, and that is seen as a rise in the annual average of one centimetre. Last year, there were several storm surges in a single year for the first time since 2007. The year began with a storm that pushed up the water in the Wadden Sea and Eemshaven on 12 January and in the night of 14 January. In late October 2017, the water in Eemshaven was 4 metres above NAP, more than 2 metres above the tidal level.

Long-term tidal cycle

Another important cause of the elevated water level is the long-term tidal cycle. Every 18.6 years, the sea level rises and falls by about 2 centimetres. The last peak was in 2004, and the level is now rising again to the next peak in early 2023. The combination of relatively low numbers of storm surges and a relatively low tide meant that the sea level stopped rising between 2007 and 2016. All this means that the higher sea level last year is entirely in line with expectations.

Measurements

The annual average sea level for the Netherlands is determined on the basis of the measurements from Rijkswaterstaat’s six main stations (Vlissingen, Hook of Holland, IJmuiden, Den Helder, Harlingen and Delfzijl). These tidal stations measure the sea level every minute. The Deltares and HKV sea level monitor provides annual analyses of these data and determines the current sea level rise. The table below shows the extreme sea levels measured by each station in 2017, all of which were observed during the storms referred to here.

Station Time Highest water level (m + NAP)
Delfzijl 2017-01-14 00:23:00 3.94
Den Helder 2017-01-13 19:58:00 2.13
Harlingen 2017-01-13 21:33:00 2.99
Hook of Holland 2017-12-08 05:58:00 2.38
IJmuiden 2017-01-13 16:20:00 2.37
Vlissingen 2017-01-14 02:54:00 3.36

The sea level rise measured by the tidal stations is relative and it also includes land subsidence. This varies in the Netherlands as a result of geological processes, increasing towards the north from about 2 cm/century at Vlissingen to 7 cm/century at Harlingen. Land subsides locally much faster as a result of human activities. Near Delfzijl, the land has subsided by more than 20 cm in recent decades due to gas extraction: subsidence has been more of a factor in measurements of sea level rise than melting ice and expanding water.

Faster sea level rise

The fact that the sea level was higher last year does not mean that the sea level is now rising faster. To determine what we call the ‘current sea level rise’, we take into account the severity and direction of storms. The nodal tide is also taken into account. At present, the sea level on the Dutch coast is rising by 20 cm every century.

Coastal maintenance

This current sea level rise is mainly used for the purposes of coastal maintenance and designs for structures with a short lifespan. If the sea level starts to rise faster, the coast will be nourished with more sand so that it rises with the sea level. Scenarios for sea level rise are used for designs of structures with a long lifespan. We have been assuming that the sea level will rise faster since the 1950s. So far, this has not been the case and so we still have a margin.  The Dutch coast can cope with extreme water levels. The entire coastal system is periodically inspected and the weak spots are tackled through the High Water Protection Programme.