Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player
 

  • Nederlands
  • Deutsch
Search in the Encyclopedia

Dieren en planten

Water en land

Mens en Milieu

Natural gas expliotation   Wadden Sea   Western Wadden Sea   Subsidence   
Bottom subsidence due to gas exploitation, 1999., Ecomare
Source: Monitoring Bodemdaling Wad

Subsidence due to gas exploitation

The bottom of the Wadden Sea is dropping, partially due to the exploitation of natural gas. For example, between 1986 and early 2000, the tidal flats to the east of Ameland had dropped 22 centimeters. This subsidence was caused by the exploitation of natural gas on Ameland, which began in 1986. The gas well near Zuidwal, between Harlingen and Vlieland, is also subsiding a measurable amount.

  • Net and gross subsidence

    Subsidence of the bottom due to gas exploitation is largely neutralized because the pit that forms fills up with sand. In 2007, the gross subsidence by Ameland was 19 centimeters, but eventually netted 3 centimeters. This was equivalent to a loss of 53 hectare of exposed tidal flats. Another 23 hectares is expected to disappear by 2025, adding up to around 0.01% of the total surface area of tidal flats in the Dutch Wadden Sea.
    To fill up the pit by Ameland, an extra 270,000 cubic meters of sand nourishments by the shores of Ameland and Schiermonnikoog are necessary per year. When the gas exploitation by the Vierhuizen and Lauwersoog locations begin, another 500,000 cubic meters of extra sand will be needed in the same area to compensate the sand extracted from the coast around Borndiep by the pit near the Lauwersmeer region.
    The exploitation of gas in Groningen and North Friesland causes much more subsidence. This exploitation has substantial consequences for the water balance in the terp and wierd (artificial mounds) countryside, as well as in the Reitdiep valley.

  • Integral Bottom Subsidence Study of the Dutch Wadden Sea

    In March 1999, the Integral Bottom Subsidence Study Wadden Sea (IBW) was published. It is a multi-volume inquiry report on the possible effects of natural gas exploitation in the Wadden region on the geomorphology, the benthic animals, the salt marshes and the waders. The study was commissioned by the NAM.
    Extensive model calculations in the IBW predicted a maximum bottom subsidence due to natural gas exploitation of 28 centimetres in the period till 2050. One expected this total subsidence to be partially compensated by the supply of silt and sand from the North Sea, estimated at around 20 centimeters in this same period of time, so the net bottom subsidence would end up being 6 to 8 centimeters. The number of waders would temporarily decrease somewhat due to the subsidence. Some, also temporary, shifts would take place among the bottom fauna, favourable to the animal species that are adapted to sandbanks which are not exposed as often. No effects would be measured in the salt marshes, according to the IBW, with the exception of those along the coast of Groningen. The loss of area could be counteracted with extra measures to quicken the deposition.
    The IBW was strongly criticised by the environmental movement. They said that the conclusions were influenced by the commissioning organization. Furthermore, they pointed out the uncertainties in the predictions. The changeable prognoses around the bottom subsidence on Ameland clearly indicated that the geological models cannot always faithfully predict what happens in reality.

  • Subsidence in the northern Netherlands

    Exploitation of natural gas in Groningen and North Friesland has caused a much stronger subsidence of the bottom. The water economy in the terp (dwelling mound) region, as well as in the Reitdiep valley have experienced notable effects.

  • Bottom subsidence from salt extraction

    Exploiting natural gas is not the only human activity that caused bottom subsidence in the wadden region. Near the Frisian town Sexbierum, salt is being extracted from the earth at a record depth of 2500 to 3000 meters. This salt extraction had apparently caused a substantial bottom subsidence. Extraction started in 1996. One first thought that extracting 18 million tons of salt over a period of eight years would lead to 35 centimetres of subsidence. This estimate had to be revised after measuring the real rate of bottom subsidence. It appeared that the bottom would drop by 60 to 65 centimetres. Based upon this data, the Ministry of Finances reduced the quota for the amount of salt extraction to 10 million tons.