The average sea level along the Dutch coast was the highest ever recorded last year, according to marine research institute Deltares on Friday.

Deltares scientists say sea level reached an average of 11 centimetres above NAP in 2017, two centimetres higher than the previous record in 2007.

NAP stands for Normaal Amsterdams Peil or the normal water level in Amsterdam, which is slightly lower than sea level and is used as a base to measure how high or low water levels are.

Sea level expert Fedor Baart said he is not surprised at the increase. ‘The level has been rising gradually by about 0.2 cm a year since 1890, due to the melting of the ice and the warming up of the ocean, he said.

‘That means you would expect the sea level to be higher every year. The more interesting question is actually why the sea level didn’t rise over the last 10 years.’

There are, Baart says, two main causes. Firstly there have been fewer storm surges recently – in 2007 there were four.  ‘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,’ Baart said.

Tides

‘Last year, again there were again several storm surges. In late October 2017, the water in Eemshaven was 4 metres above NAP, more than 2 metres above the tidal level,’ he said.

Another cause of an 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, Baart said.

Some 26% of the Netherlands is below sea level and a further 29% is susceptible to river flooding. The Dutch coast is protected by a complicated system of dykes, seawalls and sluices built after the devastating floods of 1953 which left over 1,800 people dead.