IMPACT PROGNOSIS FOR SALT MARSHES FROM SUBSIDENCE BY GAS EXTRACTION IN THE WADDEN SEA

Subsidence will occur as a result of planned gas extraction activities in the Netherlands Wadden Sea (North-west Europe). The effects of sub sidence on salt marshes are comparable to the effects of a rise in sea level, which mag become a world-wide threat to coastal marshes by aff ecting the marsh vegetation through an increased number oi tidal flood ings and an increase in wave energy. Comparison of the prognosis for s ubsidence with a large dataset on the accretion of various salt marshe s over the past 25 years shows that accretion generally is expected to remain positive, even if at the same time sea level should rise by as much as 6 mm per year. In general, the salt marsh zone in the Wadden Sea can cope with subsidence and/or a sea level rise of 5 mm per year for barrier islands and 10 mm for the mainland, which is in the range of future sea level rise. This is an important marsh characteristic wh en considering its role for nature conservation and coastal protection . An accretional deficit is however expected in the pioneer zone in fr ont of the salt marsh, which is situated on an elevation which is most affected by wave action and currents and lacks the protection of a cl osed vegetation cover. Vertical erosion in the pioneer zone will lead to horizontal cliff erosion of the salt marsh zone. This might become a general effect of future sea level rise on the world salt marsh reso urce. Existing management techniques in the pioneer zone of the Wadden Sea salt marshes (brushwood groynes which decrease wave energy and cu rrents) can be optimized to increase sedimentation and vegetation sett lement in this pioneer zone. Expectations for the impact of subsidence due to gas extraction on Nether;lands salt marshes are in general not negative, if the local accretional balance is made a precondition for the rate of gas extraction. The effects have to be monitored.