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Morphological management of the Scheldt estuary
Jean J. Peters
Consulting engineer – river specialist
Port of Antwerp Expert Team leader
44 Rue Philippe de Champagne
Tel: +32 2 512 80 06
Fax: +32 2 502 46 44
email: [email protected]
Abstract: Since centuries, human activities and natural processes affected the morphology
and the ecology of the Scheldt estuary, the maritime access route to the port of Antwerp,
Europe’s second largest harbour. Dredging activities are needed to maintain depths and width
in the shipping route. A first significant dredging programme was executed 35 years ago.
Antwerp has recently requested a further improvement of the shipping route. These works are
located mainly in Dutch territory. Ecologists have blamed dredging works to be responsible
for degradation of nature values. Environmental studies have been initiated by the Flemish
and Dutch governments as part of a ‘Long Term Vision’ project for managing the Scheldt
estuary. The Port of Antwerp requested a team of experts to give an independent advise on the
feasibility of a further improvement of the maritime access. A diagnostic made by these
experts stated that degradation of the estuarine nature values had been caused since long by
land reclamation, while dredging as such had not significantly affected the trends observed
before the first deepening. The Port of Antwerp Expert Team has developed a new approach
for managing the estuary’s morphology based on a beneficial use of the sediment produced by
the dredging, to rehabilitate the estuarine ecosystem.
INTRODUCTION
Several European Directives aim at protecting the estuarine environments, such as the 1979
EC Birds Directive and the 1992 EC Habitats Directive. The latter includes the Natura 2000
network of protected sites in the European Community. This network represents areas of the
highest value for natural habitats and species of plants and animals which are rare, vulnerable
or endangered. Although European directives apply equally to all Member States, different
countries implement them in different ways, often because of local differences in regulations
and cultural factors. Conflicts may arise in their implementation, e.g. when it comes to
dredging activities. A major challenge is to have the various stakeholders working together to
manage the estuaries while balancing economic, environmental and safety aspects. The
overall aim is to harmonize the accessibility of ports (economy) with the preservation of
nature (ecology) and, at the same time, improving safety by reducing risks for flooding and
for marine accidents.
MORPHOLOGY OF THE SCHELDT ESTUARY
The Scheldt estuary has its mouth in the Southern Bight of the North which topography is the
result of the Holocene sea level rise. Some 10,000 years ago, the British Isles were part of
continental Europe and covered with ice. As the ice sheet melted, a large freshwater lake
formed in the southern part of what is now the North Sea. As the northern North Sea
remained frozen longer, the meltwater could escape only to the southwest, entering the
Atlantic Ocean in the region of Dover and Calais. At some point around 6500 BC,
catastrophic erosion created the English Channel. Tidal currents and wave action transport the
eroded sediment along the coasts of France, Belgium, Netherlands, Germany and Denmark.
This ‘sediment river’ formed progressively banks and islands, of which today remain the
islands along the Wadden Seas (Northern Netherlands (Figures 1 & 2, source RIKZ).
Figure 1 and 2: Western Europe at the end of Holocene (left) and during Roman Times (right)
The banks and islands became almost continuous barrier, creating an inner sea, a kind of a
lagoon that received the freshwater inflow of the rivers Scheldt, Meuse, Rhine, Ijsel, Eems,
Weser and Elbe. This lagoon was subsequently transformed by the combined action of the sea
(tides and storms) and rivers (floods and sediment input). Rivers carrying large loads of solids
(e.g. Meuse and Rhine) filled the inner sea with sediment, easing land reclamation (creation
of polders), while that part of the inner sea receiving rivers with little solid load (e.g. Scheldt)
remained inundated. The tidal amplitude in the North Sea increased steadily with the sea level
rise, while this amplitude was higher in the English Channel, decreasing in northwestern
direction. Spring tides in periods of storms breached regularly the sand barrier and strong
floodwaters scoured the bed of the inner sea, creating channels, which occasionally remained
open when tidal flows through the breaches became large enough. At the beginning, tidal
energy was dissipated in the shallow inner sea and did not reach the mouth of the Scheldt
River, located at that time close to Antwerp. However, the progressive scour of channels
eased the tidal propagation that finally joined Antwerp and started entering the river (see
figure 4).
Figure 3: Scheldt-Meuse-Rhine-delta (Source: RIKZ) Figure 4: Tides in Antwerp
Sediment produced by scouring of these channels in previously deposited geological layers
was mixed with sediment coming from the sea and settled in areas between channels, creating
shoals and mudflats. In fact, processes responsible for the geomorphic changes were much
more complex, but the overall result has been one of enlargement of channels and siltation on
shallows. Over past millenary, human activities accelerated these processes and modified
drastically the topography: land reclamation, building dykes for protecting polders, closure of
sea branches, bank protection works, etc. The intricate pattern of sandbars and islands, with
many branching channels developed into four large sea branches, almost parallel and oriented
in East-West direction (see figure 3), of which the most southern one became the Scheldt
estuary. This estuary can thus be divided in roughly two distinct parts: the remains of the sea
branch up to Antwerp (Western-Scheldt) and the upstream sloping river reach (Sea-Scheldt).
The Western Scheldt is very wide, multi-channel, its riverbed and banks composed of sand,
with some muddy tidal flats. The Sea Scheldt is rather narrow, single-channel, without middle
bars, its riverbed composed of silty sand or mud.
NATURE VALUES AND HUMAN IMPACTS
The Scheldt estuary is one of the few remaining European estuaries covering the entire
gradient from fresh to salt water tidal areas. A large part of the Scheldt Estuary is designated
either under the Bird and/or Habitats directives as Special Areas of Conservation (SAC 1) or
Special Protection Areas (SPA 2 ). The ecological richness of the estuarine ecosystems is
subjected to protection by these international designations (Bird and Habitat directive) and by
the ‘Ramsar’ Convention on Wetlands. The quite large fresh water tidal flat areas of the Sea
Scheldt (the ‘river’ part) are exceptional, at European and also at World level. Its mudflats
and salt marshes are designated as SAC, part of them also designated as SPA. The channel is
not (yet) considered as Natura 2000 area.
The Western Scheldt (the remains of the sea branch) is almost entirely designated as SAC. Its
sandbars, mudflats, salt marshes and shallow waters are considered of high ecological value
and designated as SPA. Close to the transition between Western Scheldt and Sea Scheldt can
be found the largest uninterrupted salt marsh in Western Europe, the 'Drowned Land of
Saeftinghe’. The channels (navigation route) are not part of the SPA. Due to the quite large
tidal range, extensive intertidal flats are important as feeding area and as a nursery room. The
Western Scheldt and its mouth occupy a very important place in Europe for migratory birds.
All activities related to port development, e.g. navigation, infrastructure development, capital
and maintenance dredging and associated disposal are subject to the procedures of the
European Habitats Directive as well as environmental impact assessment for larger projects.
These procedures imply the risk that some economically important projects could be halted on
the basis of the application of the European Directives.
DREDGING AND APPLICATION OF EU DIRECTIVES
The Port of Antwerp, with its total of
160.054 tons in 2005, requires a further
adaptation of its access, a deepening and
a widening of the channel, mainly in the
Western Scheldt. Dredging is required
on several crossings, also called ‘riffles’
or ‘sills’ (Figure 5).
Environmental impact studies have
indicated that deepening the crossings
has a negligible effect on the
morphological evolution of the estuary
and on its nature values. However, the
disposal of the dredged material may
have a negative impact.
Figure 5: location of the main dredging sites
1
under the EU ‘Habitats Directive’, Council Directive 92/43/EEC on the Conservation of natural
habitats and of wild fauna and flora
2
under the EU ‘Birds Directive’, Council Directive 79/409/EEC on the conservation of wild birds
Mathematical modeling was used for evaluating the possible impact induced by disposing
larger amounts of dredged materials in secondary channels, outside the main navigation
channel. It concluded the secondary channels would fill up and disappear, making the
Western Scheldt evolving from a multi-channel to a single-channel estuary. This was found
unacceptable from environmental point of view. The outcome would have been to export the
dredged materials to the sea, what was not financially feasible or acceptable either. Therefore,
the PAET experts worked out a new way to manage the dredged material. By the way, it is
remarkable that application of the European Directives looks at impact of dredging without
considering the possible negative influence of past works such as the construction of polders,
groynes, bank protections, jetties at harbor entrances, and other.
THE WALSOORDEN PROJECT: A NEW DISPOSAL STRATEGY
The project was based on an analysis of the morphological changes over the past two
centuries, during which the morphological changes were almost exclusively due to the
reduction of the total area by land reclamation. The maps on figure 6 show the evolution over
a period of 60 years since 1938. The ‘Walsoorden’ area shows a change in the shape of
sandbars and channels
mainly due to the
modification
in
channels downstream,
just before the bend,
the orientation of the
flood flow erodes the
sandbar
which
ecological
value
reduced
as
the
sediment
on
the
sandbar
became
coarser, with less silt.
The proposal made by
PAET was to dispose
dredged material on
the tip of the bar, so to
change its shape and
to have the flood flow
divided
into
the
channels instead of attacking frontally the bank. A new system was developed for disposing
sediment quietly on the riverbed, with little dispersion in the environment.
Figure 6: Maps 1938 - 1997
The proposal was studied at Flanders Hydraulics Laboratory with the assistance by PAET,
combining several tools: desk studies, field observations, physical scale modeling and
numerical simulations with a 3-dimensional model (Delft3D). The research demonstrated the
need for good field data, especially flow velocities and their spatial distribution (flow lines)
and sediment transport measurements.
WALSOORDEN DISPOSAL TEST AND MONITORING
The study results were evaluated by a group of independent experts and the authorities
decided to conduct a well-monitored disposal test, according to the proposal made by PAET.
At the end of the year 2004, half a million cubic meters of sand were disposed in one month
time at the tip of the Walsoorden sandbar. Criteria had been established for evaluating the
success of the experiment: a morphological one based on the volume that would remain in
place without being eroded, ecological criteria based on changes in physical and biological
properties on and in the vicinity of the sandbar (changes in sediment characteristics, changes
in elevation, changes in benthos communities and in vegetation (BACI - Before-AfterControl-Impact experiment).
Modern technologies have been used for the monitoring, among other: multibeam
bathymetric surveys, LIDAR topographic maps of the dry parts, ‘False color’ composite
aerial HYMAP pictures to follow up the changes in ‘schor’ vegetation and in
microfytobenthos (MFB). After one year, the morphological success of the experiment was
confirmed, the evaluation of the ecological success required a longer observation period.
Nonetheless, nothing negative could be observed, so that it was decided to make a second
disposal of again half a million cubic meters of sand. On Figure 7 is shown the result of the
disposal (left) and the consequent evolution of the sandbar. This one reveals that the sand
disposed in front of the sandbar moves progressively towards it, becoming finally part of it.
This was exactly what was expected and repeating such disposal would finally adapt the
overall shape of the sandbar, which could then produce a better separation of the flood flow in
the two channels.
The monitoring is still ongoing and the overall success of the proposed disposal strategy is
becoming confirmed. The strategy will become at least as an important component of the
management of the morphology and ecology of the Western Scheldt.
Figure 7: Multibeam bathymetric maps
Left: during the disposal test (06/11-22/12/04 Right: Evolution sandbar 22/10/04-14/10/05
(colors indicate the depths, the rectangles the strips in which the dredges sediment were
disposed in).
As stated in the monitoring
report about the ecology, it
is too early for conclusions.
The direct effects are
visible and these are not
negative
(Figure
8).
Positive effects have to be
the result of the modified
flow and sediment patterns,
a
slow
process
of
adaptation that may take
years.
Figure 8: ‘False color’ composite HYMAP pictures in 2004 in 2005
MORPHOLOGICAL MANAGEMENT OF BAYS AND ESTUARIES
The PAET experts have prepared a new report that presents ideas for a managing the
morphology of the Scheldt estuary, aiming on the one hand at reverting the negative trend for
the nature values and rather enhancing these, while preserving on the other hand the economic
interest of some stakeholders, such as Port Authorities. The new strategy for disposing the
dredged sediments can only be one part of the management. Most important is to establish a
good diagnostic of the present trend, detecting which are (all?) the possible causes for the
degradation of the estuarine ecosystems. This diagnostic is not easy to make and requires
expertise, not only modeling. In the case of Walsoorden, the analysis of historical maps,
going back to the 17th century, have indicated the strong influence of the land reclamation on
the tidal propagation, but also on the development of sandbars and channels. Levees, dikes
and bank protections have made the channels stabilize and move towards the banks, while
isolated sandbars tend to group to form larger ones. Because of the stronger tidal penetration
– a natural process, but enhanced by reducing the width of the estuary – the channels became
wider and deeper, making the banks of the sandbars steeper.
The area of mudflats and marshlands along the borders of the estuary are shrinking because
bank protections ‘attract’ the flow by creating turbulence. Measures imagined by
environmentalists to stop this phenomenon, such as stone (rip-rap) bank protections have had
an effect opposite to what was expected. Unfortunately, as often, engineering was blamed, in
this case dredging. Fortunately, a new culture has developed and dialogue between the
engineering and ecologist communities has started, quite successfully. Dredging is not
anymore considered as THE responsible of all evils and is now seen as a possible means to
improve the nature values of the estuary.
CONCLUSIONS
 In many estuaries around the world, nature values are in danger as a result, on the one
hand from natural trends due to the morphological adaptation at the end of the Holocene
sea level rise, on the other hand by increased and often excessive pressure of human
activities.
 Europe has issued very needed Water Framework, Birds and Habitat Directives, which
implementation poses unfortunately a series of difficulties, certainly in the estuarine and
coastal environments, places where these human activities are ever more concentrated.
 There is an urgent need to resolve in many estuaries the (potential) conflicts between the
various actors and stakeholders, e.g. port authorities and shipping industry, NGO’s,
environmentalists, tourism sector.
 In the case of the Scheldt estuary, it has been demonstrated that it is possible to have these
groups working together to find solutions for a sustainable use of the estuaries and its
resources. It showed that before making plans, a good diagnostic is needed, which in turn
requires a deep understanding of the morphological functioning of bays and estuaries
 Management of estuaries and their nature values requires the use of possibly several
tools: desk studies, fieldwork, physical scale models and numerical simulations.
 The present trend is to rely more and more on numerical models. They are very useful,
but we should recognize the fact that our understanding of the phenomena is still not
complete, which makes it dangerous to rely solely on this kind of tool. Therefore, it is
needed to inform (educate?) the decision-makers about the capabilities and limitations of
the tools.
REFERENCES
Peters J.J., R.H. Meade, W.R. Parker and M.A. Stevens (2001). Improving Navigation
Conditions in the Westerschelde and Managing its Estuarine Environment. How to
Harmonize Accessibility, Safetyness and Naturalness? Port of Antwerp.
Foster R.M., F. Rossi, K. Bonnie, C.H.R. Heip, P.M.J. Herman (2006). Ecological monitoring
of the test disposal at Walsoorden. Netherlands Institute for Ecology (internal, in Dutch)

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