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home exhibitionsreconstructionthe delta worksdelta reinforcements

Delta reinforcements

The standard finally set by the Delta Commission for the Delta region (see Delta Standards for Dikes below) required virtually all dikes and large stretches of dunes to be improved. In the Delta region in all over 160 km of flood defences had to be tackled, 25 km of them in the form of dunes.

Pre 1953 the height and width of a dike was determined by experience. Storm tides were taken into account and what was washed up on the dike after a storm tide (= drift line) was also taken into consideration. Drift lines show how far the sea encroaches on the land (= up rush height), and this served as an indication of the height the dike should be raised to (= crest height).

Delta standards for the dikes
The Delta Commission made recommendations regarding a dike design based on probability calculus. On the basis of a rough risk assessment taking into account the economic value of the region, the number of inhabitants, buildings and the like, individual safety standards were drawn up for the different regions of Holland, Delta, Wadden and river region. It was established that in the Delta region the flood defences should be able to withstand a storm tide such as occurs once every 4000 years. This safety standard served as a basis for calculating the necessary dike crest height to guarantee this 1/4000 per annum safety.
In order to ensure that the dikes would not be weakened by water lapping over and penetrating them, it was decided to create a shallow inside bank (= incline on the land side) and a clay coating of substantial thickness.

As it was not yet possible to calculate the stone revetment level, this continued to be determined by practical experience.

Testing of dunes
Initially, there were no methods for designing dunes. On the basis of measurements taken after storm tides along the coast and model analysis it became clear that after storm tide a new beach is formed with a predictable equilibrium profile. If it is not to be breached, the sand mass of dunes and beach has to conform to this profile. In 1972 the Technical Advisory Committee for Flood Defences drew up a preliminary guideline. A dozen years later (1984) the "Guideline for assessment of the safety of dunes as flood defences" was published. Most of the definitive delta reinforcements of dunes were only carried out after this publication.

Improvements
The implementation of the Delta works sometimes required improvement works on dikes. To achieve greater safety, the numerous drainage sluices and small pumping stations were replaced by a few large pumping stations serving a wide drainage area. Nearly all pumping stations currently in use in Zeeland were thus adapted and safely integrated in the dike body between 1965 and 1990.

Another example of dikes being improved resulted from the damming up of the Volkerak. This led to a considerable rise in the high-tide level along the Krammer, Zijpe, Mastgat and Keeten. In view of the relatively short period in which the necessary raising of the dike level had to be realised, the decision was made in 1969 to (temporarily) raise the dike level by constructing many kilometres of coffer dam on the dike crest.

Costs
Initially, the government intended to fund 75% of the costs of the Delta reinforcements. To the relief of the district water boards it reversed its decision in 1969 and agreed to subsidise the implementation of the works 100%. However, the preparatory costs still had to be paid by the district water boards.