Impact Prognosis for Salt Marshes from Subsidence by Gas Extraction in the Wadden Sea

Abstract

Subsidence will occur as a result of planned gas extraction activities in the Netherlands Wadden Sea (North-west Europe). The effects of subsidence on salt marshes are comparable to the effects of a rise in sea level, which may become a world-wide threat to coastal marshes by affecting the marsh vegetation through an increased number of tidal floodings and an increase in wave energy. Comparison of the prognosis for subsidence with a large dataset on the accretion of various salt marshes 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 when considering its role for nature conservation and coastal protection. An accretional deficit is however expected in the pioneer zone in front 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 closed 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 resource. Existing management techniques in the pioneer zone of the Wadden Sea salt marshes (brushwood groynes which decrease wave energy and currents) can be optimized to increase sedimentation and vegetation settlement in this pioneer zone. Expectations for the impact of subsidence due to gas extraction on Netherlands 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.