The impact of gas extraction and sea level rise on the morphology of the Wadden Sea: Extension and application of the model ASMITA

It is the policy of the Dutch government to aim at the extraction of gas from the smaller gas fields in The Netherlands to spare the large Slochteren field in Groningen. The gas reservoirs below the Wadden Sea are counted among the smaller fields. Their capacity is estimated to 200 thousand million cubic metres, and their economic value is equal to about 20 thousand million guilder. Extraction will disturb the morphologic equilibrium. At the commission of the NAM it is investigated to what extent gas extraction affects the morphology of the Wadden Sea. These effects are studied for combinations of three sea level rise scenarios and three scenarios of bottom subsidence. The sea level rise scenarios consist of the recent scenario (0.18 m/century), the expected scenario (average 0.60 m/century) and the high scenario (average 1.00 m/century). The bottom subsidence scenarios consist of no subsidence, minimum subsidence and maximum subsidence. The effects are studied for the flats, channels, deltas and adjacent coasts of the Friesche Zeegat. To study these effects, the model ASMITA is used. ASMITA is an acronym for: "Aggregated Scale Morphological Interaction between a Tidal inlet system and the Adjacent coast". ASMITA is an aggregated-scale behaviour-model of a tidal inlet system, which can be used for long-term modelling. The model consists of three major morphological elements, i.e. the tidal basin, the ebb-tidal delta and the directly adjacent coast. Each of these elements is primarily influenced by the basin-related tidal prism flow and secondarily by the by wave-related hydrodynamics. From each element the equilibrium state is known. In this equilibrium state each element has the same constant overall equilibrium concentration. Disturbance of this equilibrium (e.g. change in volume) results in an exchange of sediment between the various elements until the former equilibrium state is reached again. The exchange is mainly based on diffusion. The basic model consists of five elements: one coastal element at both sides of the delta, an ebb-tidal delta, a channel and a flat. To study the effects for the coast, the model is extended with three extra coastal elements at both sides ofthe delta. At both sides, the coast now consists of two elements which are placed at the side of the coast, and two elements which are placed seawards. The elements farther away from the inlet have a lower equilibrium concentration because the influence of the waves and/or tides is smaller. Due to the differences in equilibrium concentrations diffusive transports are generated. To create an equilibrium the net transport between the elements should equal zero. In long shore direction the diffusive transport is compensated for with a wave-generated sediment transport, which depends on the curvature of the island head and on the littoral transport. And in cross-shore direction the diffusive transport is compensated for with a slopegenerated transport. The model is also adjusted for sea level rise. As regards the coast one can distinguish two effects of sea level rise: an overall and a local effect. The overall effect comprises the structural erosion of the upstream coast due to the sand demand from the tidal basin, and the local effect comprise the "Bruun-effect" and the "island-head effect". The last effect represents the curvature of the island head due to stronger influence of the tides.