A new disposal strategy for the Western Scheldt, conciliating port accessibility and nature preservation

In 1999, Flanders and the Netherlands agreed to set up a common strategy for managing the Scheldt estuary. In 2002, both parties signed a memorandum of understanding in which was defined a “Long Term Vision” strategy and its objectives. One of these is the preservation in the Western Scheldt of a dynamic flood and ebb channel network, the so-called “multi-channel system”. The present trend, a continuation of past natural morphological evolutions combined with human interference (land reclamation and polder building, dredging and other river works) may jeopardise this objective.An expert team appointed by the Antwerp Port Authority stated the need for morphological management, aiming at steering the estuarine morphology. In a first phase, sediment from dredging works could be used to reshape eroded sandbars where needed, in order for the flood and ebb flows to continue maintaining the multiple channels.Since 2002, this new disposal strategy is being investigated as a pilot project on the Walsoorden sandbar in the Western Scheldt. An extended research was conducted in 2002 and 2003, combining several tools: desk studies with maps on the historical morphological changes, field measurements, physical scale model tests and numerical models. As a result of the research work executed at Flanders Hydraulics Research, it was concluded that none of the results contradicted the feasibility of the new disposal strategy at the Walsoorden sandbar, although final judgement would only be possible after the execution of an in situ disposal test.At the end of 2004, 500.000 m3 of sand was disposed during one month with a diffuser in relatively shallow water at the seaward end of the Walsoorden sandbar. The experiment was thoroughly monitored, morphological as well as ecological. One year after the execution of the in situ disposal test, it was concluded that from morphological viewpoint the test was a success. The ecological monitoring revealed no significant negative changes in trends due to the disposal test.In 2006 a new disposal test was executed, using the traditional dumping (“clapping”) technique with hopper dredgers. Due to practical limitations, the disposal (again 500.000 m3) was spread over 3 months. The new experiment was again thoroughly monitored. Due to larger currents in the disposal area, a higher percentage of the material was transported towards the Walsoorden sandbar. This morphological evolution was seen as positive within the objectives of the disposal strategy. From ecological viewpoint again no significant negative changes in trends have been identified.Due to the success of both in situ tests, the strategy of morphological disposal will be included in the dredging and disposal operations for the future deepening of the navigation channel. This new disposal strategy will introduce benefits for both the economy (deepening and maintenance of the fairway) and the ecology (keeping the sediment in the estuary, creating new valuable areas without endangering the multiple channel system). Therefore further research work has been carried out in 2007-2008 on how to embed this strategy in the future dredging and disposal policy and the possible use of the strategy on other locations. During execution (planned to start in 2009) an intensive monitoring programme will monitor the morphological and ecological effects of the disposed sediments, allowing adjusting the strategy if necessary. However, this is only part of a morphological management of the estuary, which would also have to include morphological dredging and modifying the hard bordering at some locations

A new disposal strategy for the Western Scheldt, conciliating port accessibility and nature preservation

In 1999, Flanders and the Netherlands agreed to set up a common strategy for managing the Scheldt estuary. In 2002, both parties signed a memorandum of understanding in which was defined a “Long Term Vision” strategy and its objectives. One of these is the preservation in the Western Scheldt of a dynamic flood and ebb channel network, the so-called “multi-channel system”. The present trend, a continuation of past natural morphological evolutions combined with human interference (land reclamation and polder building, dredging and other river works) may jeopardise this objective.<p>An expert team appointed by the Antwerp Port Authority stated the need for morphological management, aiming at steering the estuarine morphology. In a first phase, sediment from dredging works could be used to reshape eroded sandbars where needed, in order for the flood and ebb flows to continue maintaining the multiple channels.<p>Since 2002, this new disposal strategy is being investigated as a pilot project on the Walsoorden sandbar in the Western Scheldt. An extended research was conducted in 2002 and 2003, combining several tools: desk studies with maps on the historical morphological changes, field measurements, physical scale model tests and numerical models. As a result of the research work executed at Flanders Hydraulics Research, it was concluded that none of the results contradicted the feasibility of the new disposal strategy at the Walsoorden sandbar, although final judgement would only be possible after the execution of an in situ disposal test.<p>At the end of 2004, 500.000 m³ of sand was disposed during one month with a diffuser in relatively shallow water at the seaward end of the Walsoorden sandbar. The experiment was thoroughly monitored, morphological as well as ecological. One year after the execution of the in situ disposal test, it was concluded that from morphological viewpoint the test was a success. The ecological monitoring revealed no significant negative changes in trends due to the disposal test.<p>In 2006 a new disposal test was executed, using the traditional dumping (“clapping”) technique with hopper dredgers. Due to practical limitations, the disposal (again 500.000 m³) was spread over 3 months. The new experiment was again thoroughly monitored. Due to larger currents in the disposal area, a higher percentage of the material was transported towards the Walsoorden sandbar. This morphological evolution was seen as positive within the objectives of the disposal strategy. From ecological viewpoint again no significant negative changes in trends have been identified.<p>Due to the success of both in situ tests, the strategy of morphological disposal will be included in the dredging and disposal operations for the future deepening of the navigation channel. This new disposal strategy will introduce benefits for both the economy (deepening and maintenance of the fairway) and the ecology (keeping the sediment in the estuary, creating new valuable areas without endangering the multiple channel system). Therefore further research work has been carried out in 2007-2008 on how to embed this strategy in the future dredging and disposal policy and the possible use of the strategy on other locations. During execution (planned to start in 2009) an intensive monitoring programme will monitor the morphological and ecological effects of the disposed sediments, allowing adjusting the strategy if necessary. However, this is only part of a morphological management of the estuary, which would also have to include morphological dredging and modifying the hard bordering at some locations

Validation of a 2D hydrodynamic model within a study to propose the optimal disposal strategy in the Western Scheldt

In order to work out a disposal strategy along sandbars in the Western Scheldt, Flanders Hydraulics Researchconducted research combining field measurements and 2D high resolution numerical modelling. The Delft3Dsoftware was used for this. For the calibration of the numerical model, a lot of information and experience fromprevious modelling studies was available. In a first phase, the water levels in the model were calibrated byapplying a space varying Manning bed roughness coefficient: in this way the propagation of the tidal wavebetween the different measurement stations could be adapted in the model. In a second phase, the focus of thecalibration was on the flow velocities. The horizontal viscosity was used for this as the main parameter. For theintertidal area, the maps of ecotopes –were used to calibrate the flow velocity. A depth depending bed roughness as well as the critical velocity between high and low dynamic intertidal areas were used for the calibration. Combining the in situ measurements, the calibrated numerical models and the experience from 2 in situ disposal tests, the engineers of Flanders Hydraulics Research succeeded in working out an optimal disposal strategy

The Walsoorden pilot project: A first step in a morphological management of the Western Scheldt, conciliating nature preservation and port accessibility

In the framework of the Flemish-Dutch "Long Term Vision" strategy of the Western Scheldt, the Port of Antwerp Expert Team proposed the idea of morphological dredging aiming at steering the estuarine morphology. As a pilot project, the experts proposed a new disposal strategy, where dredged material would be disposed on the eroded tip of the Walsoorden sandbar. The feasibility of the disposal strategy was investigated at Flanders Hydraulics Research in 2002 and 2003. None of the results of this study opposed the feasibility of the proposed strategy, although final judgement would only be possible after the execution of an in situ disposal test. At the end of 2004 500.000 m3 of sand was disposed at the tip of the Walsoorden sandbar. After a one-year extensive monitoring of the experiment, it was concluded that from morphological viewpoint the test was a success. The ecological monitoring revealed no significant negative changes in trends due to the disposal test. In 2006 the in situ disposal experiment was continued, with a new disposal of 500.000 m3 of sand

Distribution and dynamics of intertidal macrobenthos predicted from remote sensing: response to microphytobenthos and environment

We investigated which variables, including environmental variables and food availability, could predict the spatial distribution and dynamics of benthic macrofauna on an intertidal flat. A time series of macrobenthos and sediment grain size samples was complemented by time series of microphytobenthos and saltmarsh vegetation biomass and sediment grain size from airborne hyperspectral remote sensing, and elevation from laser altimetry. Response models were constructed to predict biomass and species richness of macrobenthos as a function of the environmental variables. Total biomass and species richness was best predicted by a combination of microphytobenthos biomass and sediment characteristics as explanatory variables. Deep deposit feeders and surface deposit feeders also responded best to a combination of variables, with deep deposit feeders responding more strongly to sediment grain size and surface deposit feeders responding more strongly to microphytobenthos biomass. The environmental conditions to reach maximum biomass differed for each macrobenthos species. Application of the response models to the remote sensing-derived maps of the environmental variables enabled significant predictions of the spatial distribution of macrobenthos biomass, demonstrating the differences in distribution of the macrobenthos species. The models also revealed the sensitivity of the macrobenthic community to environmental change. In situ and remote sensing data demonstrated a significant fining of the sediment and a (temporal) increase in average microphytobenthos biomass. Field observations also showed a significant increase in species richness and changes in the relative abundance of species, with a decrease in Bathyporeia pilosa, and an increase in Nereis diversicolor, Pygospio elegans and Heteromastus filiformis. Such changes in macrobenthos biomass and species richness were indeed predicted from the response models. The study demonstrates that the synoptic remote sensing techniques combined with field sampling allow efficient ecological mapping and monitoring.