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Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

The authors are solely responsible for the content of this report. Material included herein does not represent the opinion of the European Community, and the European Community is not responsible for any use that might be made of it.
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Interestuarine comparison: Hydro-geomorphology

4a. Scheldt

The Scheldt (Figure 9) is a converging estuary characterized by a typical decrease in estuary width (from mouth to upstream boundary) (Figure 13). Towards the upstream boundary there is a general decrease in thalweg depth (Figure 14), whereas the decrease in cross-sectional averaged depth is more variable (Figure 15). From the mouth to TIDE kilometer 110, the cross-section averaged depth decreases, but at TIDE km 110 starts to increase again. At this point, the Scheldt estuary converts from a multiple channel system (i.e. several subtidal channels divided by intertidal areas) into a single channel system (i.e. one subtidal channel) (Figure 9). The general decrease in estuary width and estuary depth consequently leads to a decrease in the estuary’s wet section (Figure 16).
The tidal range of the Scheldt is macrotidal with at mean tidal conditions a tidal range of 3.8 m near the mouth, a maximum tidal range of nearly 5.5 m at TIDE km 75, and a minimum tidal range of 2.7 m at the up-estuary boundary (Figure 18). As the tidal wave enters the estuary, the increase in tidal range (1-3 cm/km, see Figure 19) is caused by an increase in MHWL and a decrease in MLWL (Figure 17 and Figure 19). The maximum increase in tidal range is observed around TIDE km 120 (Figure 19). Once the maximum tidal range is reached, the strong decrease in tidal range in the upstream direction is mainly caused by an increase in MLWL (Figure 17 and Figure 19). In these upstream parts the tidal asymmetry (i.e. the ratio between mean tidal fall and mean tidal rise) is the highest with values up to 1.7 (Figure 20).
The mean freshwater discharge into the Scheldt estuary at Schelle (see Figure 9) is 107 m³/s. For a typical dry event (low discharge) the discharge is 34 m³/s (i.e. P(5%) value, see §3.1.5), for a typical flushing event this is 253 m³/s (i.e. P(95%) value, see §3.1.5). Low discharges are common during summer, whereas flushing events are more typical during winter (see Figure 21). For the Scheldt all tributaries have a significant contribution to the total river discharge at Schelle.
The mean and maximum cross sectional averaged flood and ebb flow velocities along the estuary respectively range from 0.1 to 1 m/s and from 0.25 to 1.5 m/s (Figure 22 and Figure 23). The maxima in vmean and vmax are observed around TIDE km 70, which coincides with the maximum in tidal range (cf. Figure 22 and Figure 23 with Figure 18). The lowest flow velocities in the estuary are observed near the up-estuary boundary.
The tidal damping scale, which describes the tidal amplification and tidal damping in an estuary (see §3.2.2), is positive (amplification more important) from TIDE km 160 to 65 (Figure 24). This coincides with an increase in tidal range (see Figure 18 and Figure 19). From TIDE km 65, the tidal damping scale becomes negative (damping more important) and the tidal range decreases (see Figure 24 and Figure 19).
Under conditions of mean freshwater discharge, the fluvial energy in the estuary is more important from TIDE km 0-47, whereas the tidal energy is more important in the rest of the estuary (Figure 25). The tidal energy increases from the mouth to its maximum value at TIDE km 70. At this point the maximum tidal range occurs (cf. Figure 18 and Figure 25). The fluvial energy decreases from the up-estuary boundary towards the mouth, and reaches a value zero at TIDE km 110 where it no longer contributes to the total energy.
The Scheldt is a well-mixed estuary where it takes about 73 km for the mean salinity profile to decrease from 30 PSU to 1 PSU (i.e. a mean salinity gradient of 0.4 PSU/km) (Figure 26). During periods with low discharges (typical during summer) the salinity in the estuary increases, where during periods with high discharges (typical during winter) it significantly decreases (see Figure 26, respectively P(95%) and P(5%) profiles). It takes about  83 km for the P(95%) profile to decrease from 30 to 1 PSU (i.e. a mean salinity gradient of 0.35 PSU/km), for the P(5%) profile this is 85 km (mean salinity gradient of 0.34 PSU/km). The maximum difference between the low and high discharge salinity profiles is about 13 PSU, whereas the maximum variation between low water and high water is about 6 PSU (Figure 27).
The suspended particle matter (representative for the mean over a tidal cycle, see §3.1.4) ranges from 30 mg/l near the mouth up to a maximum value of 300 mg/l (Figure 28). In the multi-channel part of the estuary (WesterScheldt) SPM values are low (30-50 mg/l) and no difference between the surface SPM and the depth-averaged SPM is observed. At about 100-110 km from the upstream weir there is clear increase in depth-averaged SPM which reaches a first peak at TIDE km 95 and a second one at TIDE km 57. Both turbidity maxima reach SPM values up to nearly 300 mg/l. For the surface SPM, the increase at TIDE km 110 is small and values further upstream do not exceed SPM values of 120 mg/l (Figure 28). At half tide conditions, the increase in depth-averaged SPM occurs further into the estuary (compared to the SPM values averaged over a tidal cycle), and SPM values reach maxima up to 400 mg/l (Figure 28).

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