Effects of the

deepening/widening of the navigation channel on the morphology:

· How can the sand export from the Western Scheldt be related to the deepening/widening of the navigation channel in the 1970’s and 1990’s

When a satisfactory representation of the main trends in the net transport in the Western Scheldt has been provided, systematic runs over a period of decades, with different scenarios of deepening and widening, can isolate the impact of the deepening/widening from the natural evolution.

Applicability ranking: 2-3

Reference: sections on dredge volumes and net sediment transport in Chapter 4 and Chapter 6.

· What will be the consequences of a sand-exporting system on the physical and ecological characteristics?

The present rate of sand export is small for the size of the estuary (bottom changes order 8 mm/yr) and comparable to for instance sea level rise of order 2-8 mm/yr. Longer-term runs may indicate if there is an accelerating trend towards export.

Applicability ranking: 2-3

Reference: sections on area changes per depth class and net sediment transport in Chapter 4 and Chapter 6.

· Is the decrease of the maintenance dredging volume since the 48’/43’ widening caused by either the widening or the deepening or is it the result of the increased sand export from the Western Scheldt?

The effect of widening or deepening can be investigated by systematic scenario runs; the dredging volumes are represented reasonably well in the present model. It seems possible, that an increased sand export leads to less dredging.

Applicability ranking: 3-4

Reference: sections on dredge volumes and net sediment transport in Chapter 4 and Chapter 6.

· What mechanisms govern the development of sills and how can they be modelled morphologically? How are they affected by dredging and dumping?

The main mechanism appears to be the centrifugal force (advective terms) that cause the ebb and flood currents to choose different paths where the channel crosses over. This

essentially 2DH process is modified by 3D divergences of the flow profile from a logarithmic distribution. Also flood flow over shoals seaward of a sill may contribute. The present model is capable in broad terms, though not in all detail, of representing these mechanisms. Possibly, dumping sand right next to the navigation channel may lead to narrower and deeper sill areas.

Applicability ranking: 3

· What can be the explanation that the total intertidal and shallow water area has developed differently from what was predicted in the MOVE-project after the 48’/43’

widening and deepening?

This requires a more thorough analysis of observed and modelled changes at macro-cell level. We have seen that the model, with certain settings, is able to capture some important trends in shallow areas, but much more work is needed here.

Applicability ranking: 2-3 D. Effects of future

large-scale interventions on the morphology:

· How can the relations between the physical processes and an intervention be described in a conceptual model?

The model can help quantifying parameters in a more conceptual model and can serve as a test bed for more simplified approaches.

Applicability ranking: 4

· Can sand, originating from a further deepening, be dumped in the sub-tidal delta (Voordelta), so that it will remain available for the Western Scheldt?

The dynamic link between the sub-tidal delta and the estuary is probably much less active than the connections within the estuary, and there are likely to be losses to the North Sea, so that the effectiveness of this strategy seems questionable.

Model exercises may clarify this.

Applicability ranking: 2-3

Reference: sections on dredge volumes and net sediment transport in Chapter 4 and Chapter 6.

· What will be the effect of a further deepening of the Wielingen/Scheur on the morphology of the Voordelta and the adjacent coastal areas of South-west Walcheren and Zeeuws Vlaanderen?

This depends on what is done with the dredged material; if it

is dumped locally, effects may be local and relatively small.

Further validation of the model is needed with respect to the Voordelta.

Applicability ranking: 3

· What (innovative) large-scale morphological measures can be undertaken to mitigate the steepening of the coast of South-west Walcheren?

Various measures such as long dams (comparable to Eijerland), massive nourishments, permanent bypass schemes, can be examined using a finer version of the model (e.g. at

SCALWEST resolution).

Applicability ranking: 3-4 E. Effects of future

large-scale interventions on the ecology:

· What are the effects of dredging and dumping on the intertidal and shallow water areas (of importance for the ecosystem), such as level, duration of drying, hydrodynamics, morphodynamics and sediment composition?

Some first results for these areas have been generated in terms of areas of certain depth zones, even considering just sand.

Reasonable estimates may be given for parameters following from the morphology; sediment composition has not been considered yet and requires both model improvement and validation.

Applicability ranking: 2

Reference: sections on dredge volumes and area changes per depth class in Chapter 4 and Chapter 6.

· What are the effects of deepening of the navigation channel on the development of the intertidal and shallow water areas?

Direct effects on the tidal propagation can be assessed readily, but shallow area processes are still represented crudely.

Applicability ranking: 2

Reference: sections on dredge volumes and net sediment transport in Chapter 4 and Chapter 6.

· How will the steepness of the tidal flats be affected after morphological interventions?

The processes and parameters governing the steepness of the tidal flats are not known yet.

Applicability ranking: 1-2

· Which interventions influence the development of salt marshes?

First approaches are being developed in the framework of biogeomorphology research and applications in Venice lagoon; simple heuristic methods may provide first-order answers.

Applicability ranking: 2

Summary

In the table below we have summarised the rankings for key aspects regarding the applicability of the model to Western Scheldt management problems. Clearly, much progress has been made in representing the large-scale behaviour and the effects of dredging and dumping, while the overall sediment balance and shallow areas need much more attention.

Type of problems/aspects Applicability

ranking Effects on overall hydrodynamic parameters (e.g. asymmetry). 5 Dredging volumes, effects of management, support of conceptual models.

4

Behaviour of large-scale system and effects of strategies.

Behaviour of sills.

Spatial and temporal variations in transport patterns.

3

Overall import/export: effects of management strategies. 2-3 Tidal flats: effects of deepening, dumping. 2

Salt marshes and effects on them. 1-2