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Morfologisch advies broedeilanden : broedeiland Plaat van Walsoorden
Dit rapport beschrijft de morfologische haalbaarheidsstudie naar een broedeiland op de locatie Plaat van Walsoorden.
Government facilitation of external initiatives : how Dutch water authorities cope with value dilemmas
Water authorities search for new collaborations with non-governmental actors, with the aim of facilitating societal initiatives. A comparative case study was conducted to analyze the value dilemmas faced by water authorities when they choose to facilitate and how they cope with these dilemmas. The study found that the most prevalent dilemma is between traditional democratic values and efficiency-related values. In the chosen solutions, the latter seem to prevail over the former. Casuistry, cycling and hybridization are common coping mechanisms. The study shows the potential of non-governmental initiatives in the water sector while also reflecting critically on dominant administrative values.
Modeling the morphodynamics of the mouth of the Scheldt estuary
Recent research on the Scheldt estuary mainly focused on the Western Scheldt. There is now a renewed interest in the mouth of the estuary as the Flemish government explores the feasibility of large-scale morphological interventions in that area. This paper describes the ongoing development of a process-based numerical model (Delft3D) of the Scheldt estuary. The so-called Delft3D-NeVla model computes morphodynamics forced by waves, tide, wind and river discharge, and affected by sediment dredging and dumping to maintain navigation channels at the desired depth. After further calibration and validation, the Delft3D-NeVla model will become an important tool to understand and predict the morphodynamics of the mouth of the estuary due to natural processes and large-scale morphological interventions such as relocation of navigation channels.
Simulation of long-term morphodynamics of the Western Scheldt
In this paper we use a 2D process-based model to hindcast morphodynamic behavior of the Western Scheldt estuary. The periods of 1860-1970 (110 years) and 1905-1970 (65 years) are simulated. We compare the results to a historically unique dataset of bathymetric maps. The results show that the model results get better over time. The results show that the model is capable of simulating the large scale erosion and sedimentation that has occurred in the considered period. We attribute this to the self-organization of both the model and reality. The interaction between the major tidal forcing and the estuaries' fixed outline overrules other uncertainties over long time scales. Our research shows that process-based models applied in confined environments and under constant forcing conditions may perform well especially on long time scales. This makes them potentially suitable for centennial time scale forecasts related to, for example, climate change.
Sustainability of the multi-channel system in the Westerschelde under influence of dredging and disposal
This paper presents an on-going study meant to improve our knowledge on the morphological development of estuaries for supporting estuarine management considering accessibility for navigation and the ecological value. We focus on the question whether a multi-channel system in the Westerschelde can be sustained under pressure of future deepening and maintenance. After reviewing the previous work on this subject the remaining questions are inventoried and further research for answering each question is proposed. The results of the proposed study will directly be applicable for developing better strategies for disposal dredged sediments, supporting decision making concerning sand mining and further deepening of navigation channels, and for monitoring the effects of human activities on the morphological development in the estuary.
Transport of fine sediments in a narrow converging estuary : the Sea Scheldt
In this paper we study the transport of fine sediments by river-induced flushing, estuarine circulation and tidal asymmetry in the Sea Scheldt, Belgium. This study is carried out with an idealized schematization of the river, modeled as an exponentially converging river with constant depth and rectangular cross section, using Delft3D. Sediment is only imported from the lower sea boundary. Values for tidal amplitude and river flow, prescribed at the models’ open boundaries are comparable to those in the Scheldt. We show that a turbidity maximum is formed at the head of the salinity intrusion, driven by estuarine circulation, and in balance with river-induced flushing. For the given conditions, the model does not predict any fine sediment transport beyond that turbidity maximum.
Human versus natural mud fluxes in the Scheldt Estuary : are they significant and if so, how can they be optimised?
The mud dynamics of the Scheldt estuary is governed by the interplay between tidal flow, freshwater discharge, marine and fluvial mud supply and local sources and sinks. A question is how large human impacts are on these mud dynamics. Using a process-based mud transport model of the Scheldt estuary, these impacts have been quantified by evaluating different scenarios representative for present or alternative maintenance dredging procedures. The results show that although the ‘human’ fluxes caused by maintenance dredging are typically small compared to natural gross fluxes, they are very significant compared to natural residual fluxes, notably in the narrower section of the estuary near Antwerp. Here more than half of the available mud is ‘second-hand’, i.e. it has been dredged from and released back into the estuary at least once. This implies that an optimization of the dredging and release cycles, including the smart selection of release locations, offers the perspective of smaller human impacts, possibly even at lower costs. A down-estuary shift of release locations would be favourable. Also, locations closer tidal flats may contribute to interrupting the vicious circle between dredged mud dispersion and maintenance dredging by enhancing the accretion rate of these flats. However, the surface area of these flats has to be substantial to provide more than just a short-term solution.
A smooth Scheldt
The Western Scheldt is the seaward part of the Scheldt estuary with a pronounced multichannel morphology and extensive intertidal flats. Observations of the bathymetry and aerial photographs of the intertidal and supra-tidal environments over the last 60 years reveal distinct transformations of the morphology on various spatial scales. The overall change is from an irregular distribution of intertidal flats with branching channels and shallow areas towards smooth tidal flats in between the main channels. A decline in the number of small tidal channels – ebb- and flood chutes and channels that connect the main ebb- and flood channels, is observed throughout the Western Scheldt. The large number of small tidal flats have merged into a limited number of bigger entities, and the jagged edges of the flats have given way to almost straight water lines. The surface area of the intertidal flats that is covered with mega-ripple fields has decreased. Mechanisms that account for the changes on all scales throughout the Western Scheldt have not been recognized. So far intrinsic pattern development and/or the closure of branching tidal basins are recognized as candidates to explain the changes.
From stakeholder to shareholder : organising stakeholder commitment for the Schelde Estuary of the future
The Scheldt Estuary provides the stage for a broad range of stakeholders and (conflicting) interests. These diverging interests led to complex decision-making on estuarine policy and management. The EU Interreg project ‘Estuaries on the MOVE’ (EMOVE) aims to bring stakeholders together and organize bottom-up commitment for a sustainable Scheldt Estuary of the future, by turning them into shareholders, implying ‘ownership’ of a particular development or project. Crucial in the approach were: 1) generating a shared understanding on the physical and ecological functioning of estuary and the different perspectives stakeholders have on the estuary and 2) collectively formulating projects and organizing coalitions. The result of the approach was both successful and promising. Five different projects were formulated – ranging from a change in polder regime (‘growing land’) to a cross-border nature reserve, environmental-friendly sediment disposal techniques, silt agriculture and governance opportunities. The applied methodology of bottom-up project formulation, combined with Group Decision Modelling provided a fruitful ground for measures towards a sustainable Scheldt Estuary that have sufficient support to come to implementation and also important generic lessons for organizing stakeholder commitment in sensitive decision-making environments like estuaries.
Scheldt Estuary physics and integrated management : special session of the 36th IAHR World Congress (28 June - 3 July 2015, Delft and The Hague, the Netherlands)
The 36th IAHR world congress was organized in the Netherlands. This was an excellent opportunity to present the state of knowledge on the Scheldt estuary and special sessions were organized on Monday June 29th. No less than 17 papers and extended abstracts were submitted and accepted for a presentation.