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

The Interreg IVB North Sea Region Programme

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Interestuarine comparison: Hydro-geomorphology

4c. Weser

As for the Elbe, the Weser estuary (Figure 11) is featured by 3 more or less prismatic channels, one from TIDE km 72 to 38, one from TIDE km 38 to 28, and one at the most upstream part of the estuary (Figure 13). The thalweg depth gradually decreases from mouth to up-estuary boundary, with at TIDE km 10 a sudden shallowing of the thalweg depth (Figure 14). The Weser estuary is a multi-channel system from the mouth up to TIDE km 30, and a single channel in the most upstream part (TIDE km 30 – 0) (Figure 11). The wet section of the Weser has a typical decrease from mouth to up-estuary boundary (Figure 16).
The Weser is a predominant mesotidal estuary with only at the most upstream part (from TIDE km 20 to 0) a macrotidal regime (Figure 17). At mean tidal conditions, the tidal range at the mouthgeo is 3.8 m and reaches a maximum value in the most upper part of 4.1 m (Figure 17). Within the estuary there is a more or less constant increase of about 1 cm/km in MHWL. The constant tidal range between TIDE km 30 and 60 is caused by an increase in MLWL. For the rest of the estuary the tidal range increases, where from TIDE km 65-120 the increase in tidal range is caused by a small decrease in MLWL and an increase in MHWL. From TIDE km 0-30 the increase in tidal range is caused by only an increase in MHWL (see Figure 17, Figure 18 and Figure 19). The tidal asymmetry at mouthgeo has a value of 1 (duration of mean tidal fall and tidal rise is the same), but then increases in the upstream direction to a maximum value of 1.4 at the upstream boundary (Figure 20).
The mean freshwater discharge at Intschede (Figure 11) is 331 m³/s, calculated over a time period from 2001 to 2010 (Figure 21). For a typical dry event (low discharge, P5) the discharge is 122 m³/s, and for a typical flushing event (P95) this is 798 m³/s. Low discharges are common during summer, whereas flushing events are more typical during winter. For the Weser, the main channel discharge (i.e. discharge at Intschede) is significantly higher than the tributary discharges, and hence tributary discharges are negligible.
From the mouth to TIDE km 10, the mean and maximum ebb and flood flow velocities respectively range between 0.1 and 0.6 m/s (Figure 22), and between 0.2 and 1.3 m/s (Figure 23). High discharge conditions (winter) result in higher ebb flow velocities and lower flood flow velocities, compared to low discharge conditions (summer) (Figure 22 and Figure 23). At the most upstream part of the estuary, this effect is even more pronounced (TIDE km 10-0): close to the up-estuary boundary, the high freshwater discharge results in the absence of a flood flow velocity (value zero, only vertical tide), but clearly reaches higher values for the ebb flow velocity (up to 1.5-2 m/s for vmax, see Figure 23).
The tidal damping scale which describes the tidal amplification and tidal damping in an estuary (see §3.2.2) is positive (amplification more important) for most of the estuary (Figure 24). Only between TIDE km 45 and 38 the tidal damping scale is negative and there no increase in tidal range is observed (see Figure 19 and Figure 24).
Under conditions of high freshwater discharge, the fluvial and tidal energy in the Weser estuary are respectively higher and lower compared to conditions with low freshwater discharge (Figure 25). The point where the fluvial energy becomes more important than the tidal energy is for the winter located near the mouth (at TIDE km 82), and for summer near the up-estuary boundary (at TIDE km 15). This is in contrast with the Elbe, where both points are located close to each other (Figure 25)
The Weser is a well-mixed estuary where it takes about 68 km for the mean salinity profile to decrease from 30 PSU to 1 PSU (i.e. a mean salinity gradient of 0.43 PSU/km) (Figure 26). During periods with low (typical during summer) and high discharges (typical during winter), the salinity in the estuary is respectively higher and lower compared to the mean salinity profile (see Figure 26, respectively P(95%) and P(5%) profiles). The maximum difference between the summer and winter salinity profile is about 16 PSU, whereas the maximum variation between low water and high water is about 11 PSU for winter and summer (Figure 27).

The surface suspended particle matter (Weser dataset representative for low water conditions, see §3.1.4 ) varies between 20 and 100 mg/l, and reaches a maximum at TIDE km 35 (Figure 28 ).

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