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The Interreg IVB North Sea Region Programme


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Interestuarine comparison: Hydro-geomorphology

5b. TOPIC 2 – Habitats

In this topic we looked at the effect of different habitats (subtidal, intertidal and marsh habitats) on the horizontal tide (flow velocities) and the vertical tide (tidal range). To study this topic we used tidal data (§3.1.2) and topo-bathymetry data (§3.1.1) to delineate habitat maps according to 6 defined habitat classes (§3.3) (see Appendix B, Figure B 1 - Figure B 10). These habitats were also used for the ecosystem services report within TIDE. Flow velocities were calculated based on the cubage technique (§3.2.1). More information on the historical evolution of habitats can be found in the historical report.

Quantification

Areas
Large differences occur in the total estuary surface area. The Scheldt estuary is for example 3.5 times larger than the Weser estuary and about 2 times the size of the Humber (Figure 39). Consequently this will affect the available area of each habitat within the different estuaries.
In the Scheldt the deep subtidal habitat (Sd) has an area of 17000 ha which is about 2 times the Sd area of the Elbe, and 5-10 times the Sd area of the Weser and Humber (Figure 40). For the moderately deep and shallow subtidal habitat (Sm and Ss) surface areas are comparable for the 4 estuaries, only for the Weser surface areas are clearly smaller. The Scheldt estuary contains almost 8000 ha of intertidal flat habitat (If) which is significantly higher than the other estuaries. Concerning the intertidal steep habitat (Is) surface areas are very comparable. The Elbe and Weser have the highest marsh (M) area (for the Elbe about 5300 ha), whereas the Humber has the smallest area (about 700 ha). It should be pointed out that the used methodology for habitat mapping is based on physiotopes, whereby the marsh habitat is defined as the area above MHWL (see §3.3.2). For the Elbe and Weser several of these marsh areas are used as pastures (with still a connection to the estuary), and hence do not have the typical tidal marsh vegetation. Due to these differences in marsh characteristics, the relative percentages in habitat area are also provided excluding the tidal marshes (Figure 42).

The large area of Sd habitat in the Scheldt covers about half of the total estuary’s surface (Figure 41 ). This is clearly higher than the Elbe (37 %), whereas the Weser and Humber only have a quarter of deep subtidal. All estuaries have 50 - 75% subtidal habitat, except for the Weser where only 35% is subtidal. Remarkably is the dominance of the deep subtidal habitat compared to the other subtidal habitat classes for the Scheldt, Elbe and Weser, and the equal distribution of the subtidal habitats (± 25 %) for the Humber. For all estuaries, the total intertidal area (If + Is) varies between 20 - 30%, whereas the relative marsh area is clearly higher for Elbe and Weser than for the Scheldt and Humber, due to the difference in marsh characteristics (Elbe and Weser include pastures). The relative importance of the subtidal areas versus the intertidal areas is presented in Figure 42 .






Widths
In general, the subtidal is the dominant habitat type along the different estuaries with a relative width of 50% or more (Figure 43). Only for the middle part of the Weser, the subtidal is clearly lower with values around 25%. For the Scheldt, Elbe and Weser, the deep subtidal habitat has the largest portion in the total subtidal, with exception of the upper parts, where the moderately deep and shallow subtidal habitats become more important. For the Humber, moderately deep and shallow subtidal are clearly dominant along the entire estuary, only close to the mouth the deep subtidal is important. Intertidal flat areas are important along the multi-channel systems of the estuaries. Moreover in the mouth areas of the Elbe and Weser they are by far the dominant habitat type. In the Scheldt and Humber, intertidal steep habitats are important in the more upstream parts of the estuaries. Finally, the marsh habitat is in particular abundant along the Weser estuary (15 – 50%) and is rather scarce along the Humber. A comparison between the four estuaries for the relative and absolute widths of each individual habitat can be found in Appendix C).

By plotting the tidal range on the habitat width distribution, we observe for each estuary an increase in tidal range where the deep subtidal habitat is important, and a decrease in tidal range where the deep subtidal habitat is rather scarce or absent. Relationships between the relative habitat width and the tidal range gradient are further elaborated in §5.2.3.
It should be pointed out that in the most upstream parts of the estuaries, the accuracy of the habitat width determination might be affected by the coarse grid resolution compared to the estuary width, especially for the Scheldt estuary were habitat mapping is based on a topo-bathymetry grid of 20 x 20 m (see §3.3.2).



5b. Relation between flow velocity and habitats

To test the initially stated hypothesis in topic 2 (see §2), the maximum flood flow velocity is related to the amount of shallow and intertidal habitat. We observe for all four estuaries that flow velocity does not influence the relative width of these habitats (see Figure 44 and Figure 45).




5b. Relation between habitats and tidal damping/amplification

A clear relationship exists between the percentage of deep subtidal habitat (Sd) and the tidal range gradient (𝛁 TR) (Figure 46). For a restricted width of Sd (< 20%) the tidal range gradient is lower than zero, meaning a damping of the tidal range. If Sd becomes more important (> 30%), only  tidal amplification occurs (with exception of a few points) with 𝛁 TR varying between 0 and 4 cm/km, but without an increase in 𝛁 TR for increasing % Sd values.
For the moderately deep (Sm) and shallow subtidal (Ss) habitat, the relationship between % habitat and 𝛁 TR is opposite to the deep subtidal habitat (Figure 47, cf. Figure 46 and Figure 47). Here, low values (< 25%) for Sms (= Sm + Ss)  result in mostly tidal amplification, whereas for higher values of Sms (> 35%) tidal damping prevails.
In contrast to the subtidal habitats, no relationships between % habitat and 𝛁 TR are observed for the intertidal and marsh habitats. Here, for an increase in % habitat, tidal damping and tidal amplification are equally important (Figure 48 and Figure 49).






5b. Conclusions

Tidal amplification (vertical tide) 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 (i.e. for mean tidal conditions). 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%), which corresponds with a rounded shallow channel shape rather than a deep and wide trapezoidal channel shape. The latter is often the shape of deepened (dredged) channels, representative for the main channels of the Scheldt, Elbe and Weser. The cross section of the subtidal Humber is a naturally formed channel (no artificial dredging), with exception of a small section in the mouth area (from TIDE km 117-122). The subtidal habitat distribution of the Humber (Figure 41) suggests a more round profile and generally shallower average depths, giving a much greater emphasis to tidal damping than the other estuaries.
We further recommend an extensive statistical analysis (regressions for the different estuaries, co-variance between the different % habitats, etc.) of this dataset to improve above stated threshold values (% habitat) for tidal damping/amplification. Before doing these analyses, the determination of the % habitat should be improved (mainly for the most upstream parts of the estuary) by: (1) working with a finer topo-bathymetry grid (especially for the Scheldt for which we worked in this study with a 20 x 20 m grid), and (2) improving the interpolation method for the LW and HW surfaces. Initially, the habitat mapping was aimed to calculate habitat areas for which this methodology was considered as sufficiently (methodology see §3.3.2).
Initially, topic 2 would study the relationship between flow velocities (horizontal tide) and the occurrence of intertidal and shallow subtidal habitats. The hypothesis hereby brought forward was: “higher average flow velocities in an estuary result in less intertidal and shallow water area”. However, we did not find any relationship between these parameters and therefore habitats were also related to tidal damping/amplification (vertical tide). It should be pointed out that the “cubage” technique (§3.2.1) calculates cross-section averaged flow velocities. This could explain why no relationship was found between the flow velocity and the habitats since habitat areas are more likely influenced by local variations in flow velocity.


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