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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

6. General conclusions

Geometrical characteristics

Each of the TIDE estuaries is featured by its own morphological signature. Concerning the longitudinal change in width, all four estuaries have a typical funnel shape. The Humber is the most convergent, followed by the Scheldt (intermediate convergence), the Elbe and Weser are the least convergent (Figure 70). In the past the width of the estuaries has been reduced by land reclamation . Although most of the land reclamation took place over a longer period that started in the Middle Ages, morphological changes in the estuaries can still be influenced by these historical activities.
The mean estuary depth is very comparable for the Scheldt, Weser and Elbe and ranges from 7 – 7.5 m (i.e. the cross-section averaged depth at low water, Figure 70). The Humber on the other hand is much more shallow with a mean estuary depth of about 3.3 m. The large estuary depths for the Scheldt, Elbe and Weser are (partly) a consequence of intense dredging activities. As the most important ports are located deep inland, large parts of these 3 estuaries need to be dredged to maintain and occasionally also deepen, the fairway and in this way enable large ships to reach the harbors of Antwerpen, Hamburg and Bremen (Figure 71). In the Humber, the most important port is located near the mouth and limited dredging activities are only necessary in the mouth area.
Deepening of the fairway not only affects the depth, but also the shape of a cross section. Deepened, dredged channels have a typical wide and deep trapezoidal shape, while naturally formed channels (no artificial dredging) have a more rounded profile and generally shallower average depths. These differences in shape are found in the distribution of the subtidal habitats (deep, moderately deep and shallow, see §3.3.1). The Scheldt, Elbe and Weser are dominated by the deep subtidal habitat, whereas the deep, moderately deep and shallow subtidal habitats of the Humber are equally distributed (Figure 41).

6. The important role of morphology on tidal amplification/damping

The two most important factors that influence tidal amplification and tidal damping in an estuary are: (1) the funneling of the estuary (i.e. estuary convergence) leading to tidal amplification (the more convergent, the more tidal amplification), and (2) the friction in the estuary (controlled by the estuary depth) which leads to tidal damping. So, if an estuary is strongly convergent and is featured by a large estuary depth (thus a limited friction), it makes the estuary more vulnerable to tidal amplification (Figure 70). Based on the geometric and morphological features of the TIDE estuaries (see Figure 70), we may infer that the Scheldt estuary is most vulnerable to tidal amplification since it has an intermediate convergence and a large estuary depth. Indeed, we observe that the tidal range increases up to a maximum TRx/TR0 value of 1.4 (the highest of the 4 estuaries, see Figure 30), and that increased tidal range (TRx/TR0 > 1) occurs over a distance of 130 km, which is 85% of the estuary length (see Figure 31). The Elbe also reaches large TRx/TR0 values (up to 1.3), but here tidal amplification starts deeper in the estuary (about 10 km from the mouth, see Figure 32). Although we did not look at the mouth area in particular, the shallow character of the Elbe mouth area (which is friction dominated) may possibly play an important role in the damping of the tidal wave as it enters the estuary (1/β < 0, Figure 24). Moreover, from km 0 till 40 from the mouth, the Elbe can be considered as a more or less prismatic channel. It is known that in an ideal prismatic channel no tidal amplification occurs (e.g., Savenije, 2001). The Weser has only a limited maximum TRx/TR0  value of 1.1, but here an increased tidal range (TRx/TR0 > 1) occurs over the entire estuary length. It should be pointed out that the Weser is the shortest estuary (65 km) which is not sufficiently long to reduce the tidal range (TRx/TR0 < 1). The fact that the maximum tidal range only reaches a value of TRx/TR0  = 1.1 is due to absence of tidal amplification between km 15 and 40 from the mouth (see Figure 32). In this area the subtidal width is relatively small compared to the intertidal width and thus the volume of water stored above the intertidal area (which is affected by friction) is probably relatively large compared to the volume of water which is transported through the deep subtidal channel with limited friction (see also Figure 24, 1/β < 0). A second explanation for the absence of tidal amplification is the fact that the Weser estuary in that area is not a converging channel but a prismatic channel (see Figure A 3). The Humber finally is the most convergent estuary, but has a limited maximum TRx/TR0 of 1.15, and the tidal range becomes already damped at 25 km from the mouth (Figure 32). At this point friction becomes strongly dominant in the Humber (no deepened channels), especially in the area between Hull and Trent falls (see also Figure 24, 1/β << 0 ).
Each of the TIDE estuaries has thus an area which can be considered as very important in the protection against flooding, since these areas induce tidal damping or they reduce the tidal amplification. In the Elbe, the mouth area (important friction), and the area in the most downstream part of the estuary between Brunsbüttel and Glückstadt (prismatic channel) prevent an increase in tidal range. For the Weser, no increase in tidal range is observed between Stadland and Elsfleth due to prismatic nature of the Weser channel in combination with an increase in friction. In the Humber, important tidal damping occurs between Hull and the Trent falls due to high friction in that area. The high friction is induced by the shallow character of the subtidal channels (no deepening of the channels by dredging). It should be pointed that this specific area is very important for the safety against flooding along the Humber. No dredging should be carried out in that zone, especially since the Humber is the most convergent (more vulnerable to tidal amplification) of all TIDE estuaries. In the Scheldt estuary we do not observe directly an area which could be considered as important for tidal damping or reduction of the tidal amplification. However, in this study we only looked at mean tidal conditions and consequently the effect of tidal marshes was not evaluated. For the Scheldt, it is known that the Saeftinghe marsh (3000 ha, around TIDE km 110, see Figure 9) stores a large volume of water during spring tides and in this way protects more upstream parts along the estuary.
Despite the important role of the above described areas in reducing or stopping tidal amplification, we cannot consider this as sufficient for a robust protection against flooding, especially along the Scheldt and Elbe where the strongest tidal amplification is observed. Several measures could be introduced to reduce tidal amplification in an estuary. Based on the analysis of all 4 estuaries, we found that tidal damping in estuary becomes important once the estuary depth (i.e. cross-section averaged depth at low water) is smaller than  4.2 - 7.7 m. As analysis were performed  over 5 km blocks, this critical estuary depth should be present for at least 5 km along the estuary. The range in critical estuary depth (4.2 – 7.7 m) is a consequence of the estuary convergence: the more convergent the estuary, the smaller the critical estuary depth. However, we are convinced that it is necessary to include more estuaries in the analysis to improve the accuracy of the found threshold values in estuary depth. We also looked at the effect of habitat occurrence on tidal damping/amplification (i.e. vertical tide) and on flow velocities (i.e. horizontal tide). Tidal damping and tidal amplification in estuaries are to a large extent determined by the subtidal habitats, whereas intertidal and marsh habitats have no significant influence based on the observations. Tidal amplification occurs when the relative width in deep subtidal habitat (> 5 m below LW) is larger than 30% (Sd > 30%) and the sum of the moderately deep and shallow subtidal habitats (5 - 0 m below LW) is smaller than 25%. Tidal damping occurs when Sd < 20% and Sms > 35%. To induce tidal damping in an estuary we thus recommend to have over a distance of 5 km (data were averaged over 5 km blocks), no excessive width in deep subtidal habitat (< 20%) and sufficient width in moderately deep and shallow subtidal habitat (> 35%). It should be pointed out that these analysis were performed only under mean tidal conditions. Possibly, an effect of intertidal and marsh areas on tidal damping/amplification will be observed under spring tide conditions. For the horizontal tide we could not find any relationship between the habitat occurrences and the flow velocity.

Indirect effects of morphology on SPM and tidal marsh evolution

Tidal amplification along estuaries not only affects the flooding risk, but also has a more indirect effect on for example sediment management or ecology. We found that turbidity maxima in estuaries occur at locations where the tidal energy (i.e. common effect of vertical and horizontal tide) is high. For the Scheldt and Elbe also deflocculation/flocculation processes may lead to higher SPM values. Higher amounts of SPM have on their turn an important influence on the ecology, concerning for example primary production or tidal marsh ecology. With regard to tidal marshes, we found that higher SPM values lead to a faster evolution towards a climax vegetation state. At this stage the tidal marsh is from an ecological point of view less valuable due to the limited plant diversity. On the other hand are sufficient high SPM values wanted because it enables tidal marshes to follow up the increase in MHWL which can be considered as favorable for coastal protection.

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