Partial dike reinforcements
After sharp protests by fishermen and environmental groups, it was decided in November 1974 not to close off the Oosterschelde with a dam, but to build a storm surge barrier instead. The drawback was that the safety standard of 1/4000 per annum initially guaranteed would not now be met in 1978 but only in 1985. In Zeeland this proved unacceptable, since at that time the dikes could only withstand storm forces estimated to occur 1/100 per annum.
After much political pressure was brought to bear, the government agreed to intermediate dike reinforcements requiring 140 kilometres of dike to be raised to a safety level of 1/500 per annum within 5 years, a highly ambitious plan!
(A safety level of 1/500 per annum means that the dikes have to be strong enough to withstand a storm of such force as will occur only once every five hundred years).
Often greater safety level than 1/500 per annum
It was not possible to employ Delta Commission's theoretical design guidelines to design these dikes, for this approach required wave measurements in order to be able to test the calculated results in practice. No such wave measurements were available, nor was there enough time to carry them out at this stage.
What was available was a proper series of drift line observations, which made it possible to determine the crest height at which the required safety level (1/500) would be guaranteed.
The importance and urgency of the work meant that there was no time for optimisation of the design; this in turn led to opting for "more rather than less". As a result, the safety level of the dikes generally transcends a risk of flooding of 1/500 per annum.
After the dikes had been made higher (and wider) by the Oosterschelde Dike Reinforcement Office, it was found that many stone revetments were also in need of improvement. For when the storm surge barrier is closed, the water level up against the dike remains unchanged, meaning that a higher load is placed on it at that level for a long time. The dike also has to be able to withstand this force. These works were carried out in 1986 - 1987.