The Volkerakdam (English: Volkerak Dam) or Volkerakwerken (Volkerak Works) is the name given to a group of hydraulic engineering structures between Goeree-Overflakkee and North Brabant in the Netherlands. The works are not a single dam, but are composed of three distinct structures: a dam between Goeree-Overflakkee and Hellegatsplein, a series of locks from Hellegatsplein to North Brabant, and a bridge from Hellegatsplein to Hoekse Waard. The works cross three separate bodies of water: the Haringvliet, Hollands Diep and Volkerak. The works together comprise the fifth project of the Delta Works.
The works were conceived in response to the North Sea flood of 1953, as a result of which it was decided to close the inlets in Zeeland and South Holland. The committee which oversaw the Delta Works concluded that it was necessary to construct two compartmentalisation dams, the Grevelingendam and the Volkerakdam, to close the inlets. It was also deemed necessary to separate the Haringvliet from the waters to the south, to ensure the transport of drift ice from the Hollands Diep to the sea, and to keep the water quality of the (planned) Zeeland Lake acceptable, preserving it from contaminated Rhine water. In addition, a fixed water level on the Zeeland Lake was desirable for the purpose of shipping between Antwerp and Rotterdam, as the water level on the Haringvliet would vary due to the varying discharge of the Rhine. The construction of the Volkerakdam modified tidal movements across the entire area of the lower rivers, with extensive calculations made using the Deltar analogue computer to analyse these tidal variations. The Volkerakdam has a supporting environmental function with regard to the primary dams of the Oosterscheldekering, the Brouwersdam and the Haringvlietdam, controlling volumes of freshwater from the Maas and the Waal which enter the waters around Zeeland. As a result of the closure of the Volkerak, the volume of fresh Rhine water entering the estuary decreased. Consequently, the salinity of the Oosterschelde remained relatively constant, increasing the ecological value of the estuary. The Volkerakdam works also removed the difficulties experienced by shipping from the limited (but very unpredictable) tidal currents in the Hellegat, which had previously resulted in many ships being lost.
Feasibility, planning and design The shipping channel which formed the connection between the Hollands Diep and Volkerak was noted for the complex and unpredictable nature of tidal currents and the presence of dangerous shoals in the area of the Hellegat (English: Hellhole), which had led to a number of incidents with shipping navigating between Rotterdam and Antwerp. Johan van Veen made extensive studies of the Volkerak with a view to improving the situation, and in 1929 he came up with a solution in the form of guiding dams. Van Veen's work was successfully completed in 1931, and along with increasing the safety for navigation in the area, the works and his extensive studies provided significant detailed information essential for the design of the Volkerakdam over twenty years later. The work started by van Veen continued with an extensive study of the tidal movements and associated phenomena such as sediment transport, tidal channels and detailed bathymetry being made over a number of years, with the objectives of increasing safety from storm surges and providing protection against saltwater intrusion from the sea. The body responsible for design of the overall Delta Works scheme, known as The Delta Works Committee (Deltacommissie), had originally proposed to dam the Volkerak at the level of the Dintel river, and selected the location of the works in a more southerly location than the present position, with a shorter dam. During initial feasibility studies, the advantages of moving the works northward with a bridge crossing between Hellegatsplein and Hoekse Waard were identified, resulting in a decision to build the works in the present location. Due to the potential effects of the works on shipping between Antwerp and Rotterdam, the authorities in Belgium were involved in the development of the design. The increased cost of the resulting longer dam was mitigated by the fact that the more northerly location facilitated construction at a much reduced water depth.
Effects on tidal behaviour As was the case with two other Delta Works projects, the Grevelingendam and the Zandkreekdam, the Volkerakdam was designed as a secondary compartmentalisation dam, with the goal of reducing tidal current velocities in four surrounding estuaries to allow subsequent construction of primary dams. The effects of the construction of the Volkerakdam on tide levels in the area of the lower rivers has been identified at several locations around adjacent water bodies, as shown in the table and graph below. The changes to the tidal range were dependent on the direction and distance from the dam to the station location. In areas south of the dam, the tidal range increased considerably, whereas in areas on the north side the tidal range decreased. Towards Rotterdam, the influence of the Volkerakdam closure on tides became smaller. Although the tide could still penetrate the Biesbosch through the Haringvliet and the Rotterdam Waterway immediately after the construction of the dam, the works nonetheless had a considerable immediate effect. The tide on the Oosterschelde side of the dam increased, the high water at Tiengemeten and Willemstad dropped by 30 centimetres, with the difference becoming smaller along the Dordtsche Kil and the Noord. However, in the Biesbosch (Mouth of the Donge and Werkendam/Bakkerskil), the tidal range has been significantly reduced.
The design of the abutment caissons was undertaken to minimise scour downstream of the dam, arising from vortices generated by the large vertical head above the sill of the structure. The solution adopted was the use of specially adapted caisson units for the abutment ends, with a sloping bottom to fill the triangular void between the first caisson and the slope underneath, ensuring a good fit. The installation of these special units could be undertaken in a matter of days, therefore reducing the amount of time in which large currents were permitted to flow underneath the gradually closing hole, and limiting the potential for scour.
… excerpt ends here. Continue reading the full article.
