Natuurherstelplan Zeeschelde: drie mogelijke inrichtingsvarianten

The Schelde estuary, with its tidal flats and marshes along an uninterrupted salinity gradient from a marine to a fresh water system, is almost unique in Europe. On top of its important contributions towards national and international biodiversity it performs multiple socio-economic functions of vital importance. To combine all these functions in a sustainable way an integrated management is needed, based on the comprehension of its functioning and aimed at an optimal tuning of all processes and functions involved. With this report we hope to initiate a productive dialogue, one step ahead towards the integrated management of the Schelde estuary. It forms part of an agreement between the Administration of Waterways and Water infrastructure and some Flemish environmental organisations. The agreement was concluded as a result of protest actions against the deepening of the Westerschelde and it should result in a fruitful cooperation towards a more integrated management for the Flemish waterways. This pilot study explores possibilities to couple ecological rehabilitation and the creation of sustainable river related nature with the security measures of the SIGMA plan (a protection plan against floods) and the navigation requisites for the Zeeschelde. It also integrates the proposals from MEIRE et al. (1992), HOFFMANN (1993) and the AMIS study (general environmental impact assessment of the Sigmaplan). The decreased self-cleaning capacity of the river, the highly fragmented ecologica1 infrastructure, the accelerated sedimentation and increased tidal amplitude inhibit a proper functioning of the estuarine ecosystem. They are combined consequences of land reclamation, channelisation, land use and water management in the catchment area, discharge manipulations and the general sea level rise. They bring about the ecosystems limited carrying capacity and resilience, its increased contribution to the eutrofication of the North Sea, impoverished biotic communities, flood hazzard and reduced navigability. Possible restoration measures can be defined at three different levels: expansion of the intertidal area through dike removal and levelling of raised grounds outside the dikes, enhancement of contacts between the river and its catchment through ecological adjustments of dike structures and restoration of the contact between waters on both sides of the dike, structural and functional restoration of inland territories. Application of these measures can yield a more complete ecological infrastructure, improved migration possibilities for biota, more diverse biotic communities, limited input of energy and matter towards the river, enhanced self-cleaning capacity and retention, reduced contribution towards the eutrofication of the North Sea, increased volume and a slow down of the sedimentation rate. According to the adhered vision for the development of the estuary, the proposed restoration measures can be applied in different combinations to enhance its ecological rehabilitation/restoration. In this report three different scenario's are traced out, each of which set a different direction for the area' s development. The first two scenario's aim at a coherent development direction for the estuary, each from a different viewpoint with respect to hydrodynamics, one of the most important guiding determinants for the estuarine gradiënts along which different biotic communities follow upon each other in space and time. The scenario 'Space for the Estuary' leaves as much space as possible for the estuary to allow the tidal wave to die out laterally and longitudinally. Processes, which are steered by tidal dynamics gradually, merge into processes, which are directed by hydrodynamics of precipitation, ground and surface water. In those areas where estuarine expansion is not possible or advisable the attention is rather focused on inland nature development. The full realisation of this scenario would result in a total of 3.544 ha tidal wetland, 846 ha controlled inundation area with a reduced tidal regime and 3.472 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.207 and maximum 3.907 ha. The scenario' Attention for the Alluvium' judges that the greater part of this alluvium in the Schelde River Basin did not come into existence under tidal influence but through seasonal floods from upstream areas. It rather aims at the restoration of the alluvial plains as nutrient-rich wetlands in a continuous transition towards higher and more dry, nutrient -poor soils. Still, in this scenario attention is also paid to estuarine rehabilitation outside the dikes. The full realisation of this scenario would result in a total of 1.663 ha tidal wetland, 774 ha controlled inundation area with a reduced tidal regime and 5.324 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.787 and maximum 5.687 ha.The scenario ‘Funtional and Structural Basic Quality' does not really adhere to a preconceived vision for the development of the estuary. The proposed measures mainly contain the ecological adaptations to the Sigmaprojects as they were suggested in the environmental impact assessment (AMIS-45) and the ecological restoration of all raised fields and dumping grounds outside the dikes. Implementation of this scenario would suffice for the ecological rehabilitation of the estuary, it would at the very most maintain a structural and functional basic quality with the completion of all sigmaprojects, the deepening of the Westerschelde and the structural adaptations of the Zeeschelde to contemporary navigation demands. The full realisation would result in a total of 1.878 ha tidal wetland, 496 ha controlled inundation area with a reduced tidal regime and 1.075 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.508 and maximum 1.694 ha. Not any of these scenario's constitutes a ready-made blueprint for a ‘restored’ Zeeschelde. They picture a few possible designs for the estuary which, by means of models, can be tested for their probable impacts on the ecosystem, its functioning and potential opportunities for ecotopes, species and biotic communities. As such they should be considered as a contribution towards the social discussion that, hopefully, will yield an integrated ecosystem vision in the end