Historische analyse van de Zeeschelde en haar getijgebonden zijrivieren: 19e eeuw tot heden

The River Scheldt currently has a macrotidal estuary with a tidal reach of 160km. It covers a complete salinity gradient with a polyhaline, mesohaline and oligohaline zone and a fresh water part with long and short retention time of chloride. The estuary and its tidal tributaries Durme and Rupel have been heavily influenced by anthropogenic pressures such as land reclamation, harbour expansion, dredging activities, embankments and urbanisation. A good understanding of the impact of human interventions on the ecological functioning of the estuarine ecosystem is required. Based on this knowledge appropriate compensation or mitigation measures can be taken in response or anticipation to future negative effects of anthropogenic changes and relative sea level rise. In this study, all available data on tidal regime (1850-2000), bathymetric charts (1930, 1950, 1960, 1970, 1980, 1990, 2000) historical maps from the beginning of the 19th century onwards and aerial photographs from 1944 onwards were compiled and analysed for the Zeeschelde (Belgian part of the Scheldt estuary) and the tidal tributary river sections.

The report deals with three main issues:
(1) Ecological changes during the past century are analysed using a hierarchical ecotope approach.The evolution of the acreage of the distinguished physiotopes (physically characterised units) andecotopes such as shallow waters, tidal mudflats and marshes are analysed for each salinity zone or river section;
(2) Changes in hydrodynamic and morphological parameters are analysed over the past century;
(3) An overview of anthropogenic interventions and natural changes with potential effects onhabitat acreage and quality in the Zeeschelde and its tidal tributaries in the past two centuries. Thenature of the interventions is very diverse: reclamations, diking, land use change, infrastructureworks, dredging activities, canalisations, discharge manipulations and changes in river dynamics.

Changes in tidal regime
Because tidal regime is a key factor in an estuarine ecosystem, we analysed the characteristics ofthe ten year mean high water (MHW), mean low water (MLW), tidal asymmetry and the tidalamplitude. Temporal and spatial patterns of these parameters are examined within differentsalinity zones in the Scheldt, the Durme and the Rupel.
MHW shows a gradual increase in the whole Scheldt estuary. The highest rise is located in the freshwater part with short retention time. Maximum MHW shifted 32km upstream from Antwerp to Sint-Amands within one century.
MLW and tidal asymmetry show a more irregular pattern and are more sensitive to anthropogenicactivities such as dredging and deepening in the navigation channel. MLW dropped in the pastcentury in the harbour zone of the Scheldt between the Dutch-Belgian border and the confluence ofthe Rupel and along the Rupel. In the fresh water zone with long retention time, MLW and tidalasymmetry changed little till 1970 (1st deepening of the Scheldt). After this period MLW strongly dropped as in most other river sections. MLW and tidal asymmetry in the fresh water zone with short retention time increased between 1900 and 1970 and were presumably affected by river discharge reductions and canalisations. After the 70’s MLW and tidal asymmetry decreased again. In the Durme MLW dropped at the confluence, more upstream it rised because of sedimentation in the riverbed. Discharge deviation, damming and channel straighthening played an important role here as well. At the same time tidal asymmetry increased in the whole Durme.
MHW and MLW changes resulted in a high increase of tidal amplitude in the Scheldt and Rupel. In the River Scheldt the highest tidal amplitude was recorded at Lillo (62km to North Sea) around 1890, while in the last ten years the maximum occurs in Temse (98km to Sea); a shift of 36km upstream in a bit more than a century. In the Rupel the amplitude increased overall in time but less so in the upstream part. The Durme has an irregular temporal pattern with a decrease in the beginning of 20th century and an increase around the 1930’s. After that period tidal amplitude showed an opposite trend at the confluence (increase) and upstream (decrease).

Although the present tidal asymmetry in the Zeeschelde upstream Antwerp and in the tributaries comes close to the situation in the 19th century, tidal amplitude and volume increased strongly as a result of morphological changes and relative sea level rise.

Changes in morphology
Analysis of the morphological evolutions in the estuary enhances our understanding of changes andthe mechanisms of its hydrodynamic regime. Channel straightening, reduced flooding area andupstream shifts of the main flood zones are the most important changes discussed.
Canalisation has a direct impact on the river ecosystem e.g. by reducing its connectivity with its valley. In the fresh water zone with short retention time the channel was shortened with 22% (10,5 km). Consequently sinuosity declined and the river is now classified as straight instead of meandering. The accompanying dike normalisations induced a loss of 95% or 826 ha of the former flooding area in this zone. In the tributaries canalisations equally resulted in a significant reduction of connectivity with the surrounding valley. The Durme, shortened with 12% (2,5km) of its length compared to 1850. The disruption between the Rupel and its tidal brook the Vliet caused the loss of one third of its tidal area. Fish migration between river and valley was hampered by loss of natural flooding zones by blocking many tidal brooks.

The largest reduction of storage width in the Zeeschelde occurred in the mesohaline zone by theembankment of some vast polders (Prosperpolder, Hedwigepolder, Groot Buitenschoor andKetenisseschor). As mentioned, huge alluvial and flooding areas such as ‘Kalkense Meersen’ werelost in the fresh water zone with short retention time A reduction of storage width was also foundnear Tielrode and St. Amands. The decline of flooding area along the Durme from 1955 till 1990resulted in an overall loss of 87,4% or 730 ha of the adjacent flood zones compared to 1850. Along the Rupel the major loss came along with the disruption from its tidal brook the Vliet.
The first rise in storage capacity (defined as storage width x tidal amplitude) between 1850 and1920 is mainly attributed to the increase of the tidal amplitude. Since then changes in the storagewidth caused a shift upstream of the maximum storage capacity of the Zeeschelde.

Effects on the ecosystem
All these hydrodynamic and morphological changes had a hughe impact on the Scheldt estuarineecosystem. In the 19th century the Scheldt estuary ranged from mesotidal to microtidal along the complete gradient and tidal influence only reached as far as Kalkense meersen (Wetteren). After the 70s an ever increasing section of the estuary is characterised by a high dynamic macrotidal regime (>5m).

Within the sublitoral zones of the lower Zeeschelde (between the Belgian-Dutch border and the confluence with the Rupel), there is a high net loss of the ecological important shallow waters in favour of deep waters. In 1930 and 1950 the sublitoral zone was still characterised by gradual transitions from shallow to deep waters. At present steep slopes prevail. Shallow waters and weak slopes form a natural protection against destructive erosion for tidal mud flats and marshes. This sheltering border is almost completely lost near Doel, Lillo and the Ballastplaat.

The littoral and tidal mud flat area decreased strongly since 1850. More Recently (1990-2003) we observed retreat of tidal mudflats through erosion at several locations. More detailed information is needed to determine if this is a general trend in the estuary, caused by a ‘coastal squeeze’ effect, or if it is just natural dynamics.

Supralitoral zones (tidal marshes and flood systems) are most susceptible to human pressure. In the 19th and the first half of the 20th century embankments and dike enforcements caused direct loss of tidal marshes. In the last decennia indirect loss through increased tidal amplitude and energy are a bigger threat.
To analyse the evolution of supralitoral zones four different periods were compared:

• 1850 – situation of the Reduced Land Registry. This is an important historical reference because it shows the situation before major canalisations and alignments in the upstream parts of the Zeeschelde;
• 1920 – situation of the topographic map, Dépôt de la Guerre 3;
• 2003 – present situation (last vegetation mapping of the sea Scheldt);
• Future scenario – situation after the implementation of the major nature development projects that are planned within the updated Sigmaplan.

Compared to 1850 the supralitoral zone decreased with 13% by 1920, and with 82% by 2003. The future scenario will restore at the most 71% of the 1850 area, although the dynamics of the Scheldt estuary have increased significantly. In the future scenario the storage area will be bigger compared to 1850 in the oligohaline and fresh water with long retention time zones; in the other sections and in the tributaries it will remain smaller.

Besides the supralitoral extent, embankment degree and land use also changed drastically. Especially in the more upstream area only one third of the supralitoral was embanked in 1850. In 1920 only 28% was left without dikes. In 2003 almost all tidal marshes are without dike, some with breached summer dikes. The system of tidal areas with summer dikes has disappeared completely. In the future scenario similar systems will be partially restored in flood control areas.

Till 1950 almost all supralitoral areas were used as extensive pastures or hay fields. Along the Durme and the fresh water zone, most areas were flooded by breaching summer dikes in wintertime. Willow coppice was a common land use in the beginning of the 19th century until the first World War. After 1920 this trade declined. Since the 1950’s flooding as agriculture practice stopped because of the deteriorating water quality. Ever since all tidal marshes are abandoned and natural succession can take place. Since 1980 some tidal marshes are managed as nature reserves.

To get a more qualitative appraisal of the evolution of the tidal marshes we combined the long term analysis with a more detailed short term investigation. The short term evolution of vegetation was studied by comparing the 1992 and 2003 vegetation maps. This revealed that most of the tidal marsh area evolves to a climax vegetation stage (reed in brackish and willow in fresh water conditions) at the expense of pioneer vegetation, Scirpus maritimus-vegetation and tall herb vegetation. Hence we found not only loss and retreat of tidal marsh area but also loss in habitat diversity. Evolution to a climax vegetation is a natural succession on tidal marshes. On the other hand pioneer and lower marsh vegetations are under extreme hydromorphological pressure and within the current shape of the estuary there is no space for natural erosion/sedimentation dynamics.

Conclusion
This historical analysis lead to the basic hypothesis that under the present geometric and abiotic conditions the shallow waters and intertidal mudflats and marshes along the Zeeschelde can not persist in a sustainable way. The only remedy is to give back space to the estuary where it is possible.
Other hypotheses were advanced to be tested in the future. A model-based analysis of the real cause-effect relationships between anthropogenic and natural evolutions of abiotic parameters and ecotopes will be especially important for their rejection or acceptance.