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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

4b. Elbe

Whereas the Scheldt estuary can be considered as a typical converging estuary with a constant decrease in estuary width (Figure 13), the Elbe (Figure 10) is featured by 3 more or less prismatic channels (from TIDE km 113-68, from TIDE km 68-48, and from TIDE km 48-0, see Figure 13) which step-wise decrease in width. Remarkably, the thalweg depth is constant (due to dredging activities) up to the harbor of Hamburg (Figure 14). The Elbe is a multi-channel system from the mouth up to TIDE km 48, and a single channel in the most upstream part (TIDE km 48 – 0). At TIDE km 40 the Elbe splits into 2 single channels (the Northern and Southern Elbe) and merges again to one single channel at TIDE km 25 (Figure 10). The change in wet section is presented in Figure 16.
The Elbe is a mesotidal estuary with at mean tidal conditions a tidal range of 2.9 m near the mouth, a maximum tidal range of 3.6 m at Hamburg (Saint-Pauli), and a minimum tidal range of 2.15 m at the up-estuary boundary (Figure 18). As the tidal wave enters the estuary (from mouthgeo), the increase in tidal range  (up to 2 cm/km, see Figure 19) is only important in the most upstream part of estuary (from TIDE km 75 towards up-estuary boundary, see Figure 18), whereas the tidal range in the downstream part of the estuary can be considered as more or less constant (from TIDE km 75 to mouth area, Figure 18). Once the maximum tidal range is reached, the strong decrease in tidal range in the upstream direction is caused by a decrease in MLWL (Figure 17 and Figure 19). The tidal asymmetry along the Elbe ranges between 1.1 and 1.6 with an increase in the upstream direction (Figure 20).
The mean freshwater discharge at Neu Darchau (see Figure 10) is 722 m³/s, calculated over the time period 2001-2010 (Figure 21). For a typical dry event (low discharge, P5) the discharge is 247 m³/s, for a typical flushing event (P95) this is 1709 m³/s. Low discharges are common during summer, whereas flushing events are more typical during winter (see Figure 21). For the Elbe, the main channel discharge (i.e. discharge at Neu Darchau) is about 100 times larger than the tributary discharges, and hence tributary discharges are negligible.
From the mouth to TIDE km 40, the mean and maximum ebb and flood flow velocities respectively range between 0.2 and 0.9 m/s (Figure 22), and between 0.4 and 1.3 m/s (Figure 23). High discharge conditions 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 40-0): 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) from TIDE km 125 to 30 (Figure 24). This coincides with an increase in tidal range (see Figure 18 and Figure 24). From TIDE km 30, the tidal damping scale becomes negative (damping more important) and the tidal range decreases (see Figure 18 and Figure 24).
Under conditions with a high freshwater discharge, the fluvial energy in the Elbe estuary is higher and the tidal energy is lower compared to conditions with a low freshwater discharge (Figure 25). The point where fluvial energy becomes more important than tidal energy is for the winter (at TIDE km 40) and summer (at TIDE km 35) located close to each other.
The Elbe is a well-mixed estuary where it takes about 76 km for the mean salinity profile to decrease from 30 PSU to 1 PSU (i.e. a mean salinity gradient of 0.38 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 profiles is about 16 PSU, whereas the maximum variation between low water and high water is about 12 PSU for the winter, and 7 PSU for the summer (Figure 27).
The surface suspended particle matter (Elbe dataset representative for low water conditions, see §3.1.4) ranges between 25 mg/l and 250 mg/l and reaches a clear turbidity maxima at TIDE km 95 (Figure 28).

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