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Gas and salt extraction

The effects of subsidence     authors     20 July 2016

 

Summary

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  • Gas is extracted from deep below the Wadden Sea and surrounding area, and permits for rock salt extraction are also granted; this leads to subsidence with possible negative impacts on wildlife and the environment
  • Extraction takes place with ‘the hand on the tap’; with clear signs of environmental damage the competent authority may decide to reduce or even stop production
  • Based on extensive monitoring, annual assessments are made as to whether the level of subsidence exceeds the limits and whether wildlife and the environment have been negatively impacted by the subsidence
  • The effects of subsidence on the earth’s surface depend on the nature of the topsoil; (1) in areas behind dikes and in the dunes, subsidence mostly affects water management, (2) in salt marshes accretion is not always sufficient to compensate for subsidence, which leads to vegetation changes and an increase in flood risk for salt marsh breeding birds, (3) in the intertidal area, it has not yet been possible to unambiguously determine the surface area of the mud flats
  • The effects of an increased flood risk on the populations of salt marsh breeding birds, of which many species in the Wadden Sea show a negative trend, merit further research
  • Existing monitoring of subsidence, surface subsidence, sedimentation and erosion can be strengthened by implementing remote sensing where this is not yet carried out (introducing InSAR radar reflectors, making aerial photos available from the BM-project, using bathymetric Lidar, measuring vegetation height with Lidar)
  • Monitoring with ‘the hand on the tap’ allows the effects of subsidence on conservation objectives to be measured, this requires a stronger integration of the various components of monitoring programmes for specific extractions (subsidence, surface subsidence, sedimentation, soil chemistry, vegetation, birds), as well as a greater integration of monitoring programmes of various extractions (including existing and new extraction where no monitoring yet occurs)
  • The effect of subsidence on natural values cannot be determined by a neat experimental design. The alternative is the integrated monitoring of the chain of effects; hence a methodology must be developed to determine the cumulative effects of subsidence and other human activities at sea (hand cockle fishery, recreation etc.) and on land (grazing, recreation etc.)