5c. TOPIC 3 – Relation between sediment load and tidal/riverine characteristics
In topic 3 we study the similarities and differences between the estuaries with regard to their suspended sediment loads in relation to their tidal and riverine characteristics. We further look at which factors influence the position of the turbidity maximum. For this topic we used SPM data (§3.1.4) and salinity data (§3.1.3), and we calculated the netto sediment flux for each water level station (§3.2.3) and tidal energy according to the Dalrymple energy concept (§3.2.4).
Suspended particle matter (SPM)
Surface SPM values are the lowest in the Scheldt and Weser estuary (< 150 mg/l), intermediate in the Elbe (up to 250 mg/l), and high in the Humber (up to 750 mg/l) (see Figure 50). In the Humber and Elbe one or more estuarine turbidity maximum(s) (ETM) (i.e. a zone in which the suspended particle matter concentrations are higher than those in the river or further downstream the estuary) are well developed. For the Humber two peaks are observed: one close to Hull and one at the junction with the Trent (see Figure 50 and Figure 55). For the Elbe one distinct ETM is observed just downstream Glückstadt (see Figure 50 and Figure 53). The Schelde and Weser have less pronounced ETM (Figure 50). For the Scheldt higher SPM values are mainly observed between Temse and Dendermonde, and to less extent downstream from Antwerp (see Figure 51). For the Weser higher SPM values occur at the town of Elsfleth and downstream from Bremerhaven (Figure 54). It should be pointed out that the presented surface SPM values are averaged over different seasonal and tidal conditions (more info, see §3.1.4).
Only for the Scheldt, sufficient depth-averaged SPM data were available to present the variation along the estuary. We observed earlier that some important differences exist between surface and depth-averaged SPM data (see Figure 28 upper left panel and §4.1). Firstly the depth-averaged SPM values in the single channel system of the Scheldt are clearly higher compared to the surface SPM values (Figure 28), and secondly the extent of the ETM zones differs between the depth-averaged and the surface SPM. Although the locations of ETM zones are similar for surface and depth-averaged SPM, we observe that the largest zones with increased SPM occurs upstream Temse for the surface SPM, and downstream Antwerpen for the depth-averaged SPM (cf. Figure 51 and Figure 52). Despite the higher observed depth-averaged SPM values, a positive correlation exists between depth-averaged and surface SPM values (r = 0.76, Figure 56). For low SPM values (< 50 mg/l), depth-averaged SPM values are more or less equal to surface SPM values (i.e. in the multiple-channel system of the Western Scheldt, Figure 28 and Figure 56), while for higher SPM values (> 50 mg/l) depth-averaged SPM values are clearly higher (i.e. in the single channel system of the Sea Scheldt, Figure 28 and Figure 56).
5c. Sediment fluxes
Mean riverine discharge
For the Scheldt, Elbe and Weser sediment fluxes for mean tidal range are dominantly in the ebb direction, with sediment flux values between 0 and -1.5 kg/m²/s under conditions of mean riverine discharge (- sign refers to sediment transport in the ebb direction) (Figure 57). We observe for the Scheldt that sediment flux values based on depth-averaged SPM values result in higher sediment fluxes in the ebb direction (cf. blue and dark blue lines in Figure 57) due to the higher depth-averaged values compared to surface SPM values (see Figure 28 and Figure 56). Although sediment fluxes along the Scheldt and Elbe estuaries are dominantly in the ebb direction, values become positive (transport in the flood direction) in the most downstream part of the estuary. Nevertheless, these values are a factor 10-100 smaller (0 till 0.015 kg/m²/s) compared to fluxes in the ebb direction (from 0 till -1.5 kg/m²/s). For the Scheldt, flood transport changes into ebb transport around 26 km from mouthsal, for the Elbe this is at 15 km from mouthsal (see also Figure 51, Figure 52 and Figure 53). For the Weser sediment fluxes are only orientated in the ebb direction (see Figure 54 and Figure 57). Compared to the other estuaries the Humber has a significant transport in the flood direction (up to 1.2 kg/m²/s). However, the sediment transport in the ebb direction is also much larger compared to the other estuaries (up to -5.9 kg/m²/s around km 100 from mouthsal). Sediment fluxes in the flood direction change into sediment fluxes into the ebb direction around 25 km from mouthsal (Figure 55 and Figure 57).
High and low riverine discharge
In general, high riverine discharges result in higher sediment fluxes (for mean tidal range) in the ebb direction and this over a longer distance along the estuary (towards the mouth) compared to low riverine discharges (see Figure 58). For the Elbe, flood transport changes into ebb transport under conditions of high riverine discharge around 12 km from mouthsal, while this is at km 105 from mouthsal under conditions of low riverine discharge (see red arrows in Figure 58). It should be pointed out that the sediment fluxes during low riverine discharge are very low (values range between -5.8*10-4 and 6.75*10-5 kg/m²/s) compared to conditions with high riverine discharge (Figure 58). For the Humber, flood transport changes into ebb transport under conditions of high riverine discharge around 18 km from mouthsal, while this is at km 62 from mouthsal under conditions of low riverine discharge (see yellow arrows in Figure 58). During low riverine discharge, the sediment fluxes in the ebb and flood direction are comparable in the Humber (between -0.8 and 0.8 kg/m²/s). This is not the case under conditions of high riverine discharge where ebb fluxes reach values up to -17 kg/m²/s, while sediment fluxes in the flood direction are limited to 1.9 kg/m²/s. In the Weser, sediment transport is exclusively orientated in the ebb direction, both under conditions of high and low riverine discharge. For low riverine discharge, sediment fluxes are about 1/3 of the sediment fluxes at high riverine discharge (see Figure 58).
5c. Relation between tidal energy and SPM
For all the SPM datasets executed during similar tidal conditions (half tide for the Scheldt, low water for the Elbe and Weser), an exponential relationship exists between the tidal energy and the SPM (Figure 59 and Figure 60). As the tidal energy becomes high, there is thus a strong increase in SPM, and consequently turbidity maxima occur at locations around (or close to) the maximum in tidal energy (Figure 59, and Figure 53 and Figure 54). For the SPM data averaged over a tidal cycle (available for Scheldt and Humber, see Figure 60), no exponential relationship exists and data are much more scattered. Nevertheless, we do observe that the highest SPM values occur at relatively high tidal energy values.
Figure 59 – Relative presentation of the mean salinity (grey line, compared to 30 PSU), mean tidal energy (light green line, compared to maximum tidal energy) and SPM data (compared to maximum SPM) for the different estuaries. Salinity zones (relatively to 30 PSU) – freshwater: < 0.017, oligohaline: 0.017-0.17, mesohaline: 0.17-0.6, polyhaline: 0.6-1. (a) surface SPM 2001-2010 for the Scheldt, (b) depth-averaged SPM 2001-2010 for the Scheldt, (c) surface SPM 2009 for the Scheldt, (d) surface SPM 2004-2009 for the Elbe, (e) surface SPM 2005, 2009-2010 for the Weser, (f) surface SPM 2004-2009 for the Humber
5c. Relation between salinity and SPM
As the salinity decreases, we observe for the Scheldt, Elbe and Humber an increase in surface SPM (Figure 61). For the Weser however, SPM values are over the entire salinity range close to the turbidity maximum of 1 (turbidity maximum for the Weser not plotted here since located in the freshwater zone, see Figure 59e). For the Humber, SPM values remain high for the salinity range 0 till 0.4 and then drop to much lower values once the salinity is larger than 0.4 (> 12 PSU). For the Scheldt and Elbe, SPM values already attain lower values once the salinity is larger than 0.1-0.2 (> 3-6 PSU).
SPM data is rather sparse for the different estuaries. Therefore the available data differ for each estuary (samples taken at different stages of the tidal cycle), and an interestuarine comparison of SPM was not evident. It should be mentioned that all conclusions formulated below are done based on the available SPM-data, and that differences due to different sample strategies have been taken into account as much as possible. Where a lot of necessary additional data (e.g. type of sediment, up-estuarine fluxes of sediment coming into the system, spatial (over depth, cross section,…) and temporal variation (tidal cycle, spring-neap cycle,…) was lacking, these conclusions should be rather seen as an indication, than scientific well argumentated.
The Humber is a high turbid system compared to the Scheldt, Elbe and Weser (750 mg/l versus values < 250 mg/l, see Figure 50). The high turbidity of the Humber is explained by the high import of sediment, both at the sea boundary as at the river boundary (Figure 57). For the other estuaries, sediment fluxes are clearly lower compared to the Humber (Figure 57). The Weser for example has suspended sediment transport only in the ebb direction (see Figure 57 and Figure 58), which implies that sediment is flushed much more easily out of the estuary. The Scheldt and Elbe are still featured by sediment transport from the sea, however these sediment fluxes are very small (factor 10-100) compared to the sediment fluxes in the ebb direction (Figure 57). It should be pointed out that all sediment fluxes are calculated under mean tidal conditions, while important variations may occur with spring-neap tide variation. Moreover, the turbidity in the estuaries is not only affected by the tidal conditions but also by the flushing times (or residence times, see also §5.4) and sediment availability and riverine import. In long, more slowly flushed estuaries (e.g., the Scheldt) the turbidity in the estuary is higher during the fast currents of large spring tides, while in short, more rapidly flushed estuaries (e.g., the Weser), fine sediments are quickly lost to the coastal zone during the ebb currents of the spring tides (Uncles, 2002). The importance of riverine discharge variations (and thus variation in flushing/residence time) is also demonstrated by the confluence of ebb and flood dominated sediment fluxes, and by the absolute values of the sediment fluxes (see Figure 58).
For SPM data collected during similar tidal conditions (low water, half tide), we observe an exponential relationship between the tidal energy and the SPM (Figure 60), which means that an estuarine turbidity maximum (ETM) occur at locations with high tidal energy. This exponential relationship is not present for SPM data averaged over a tidal cycle, but nevertheless are high SPM values observed at locations with relatively high tidal energy. This is not surprising since higher flow velocities and higher turbulence keeps fine sediment more easily in suspension. We may conclude that for all estuaries, elevated SPM values occur at locations where the tidal energy is high, but this does not imply that ETM automatically coincide with the absolute maximum in tidal energy (see Figure 51 until Figure 55). Other mechanisms may also play their role. Firstly, in the Humber estuary, the ebb and flood directed sediment fluxes meet in between the two ETM’s. One would expect that at the location where these fluxes meet, an ETM occurs. The fact that ETM’s occur upstream and downstream from this location, may be a consequence of the shifting of the confluence of both sediment fluxes due to variations in riverine discharge (see yellow arrows, Figure 58). Secondly, for the Scheldt and Elbe, an ETM occurs in the oligohaline zone of the estuary which means that besides tidal energy also deflocculation/flocculation processes may lead to an increase in SPM values. Indeed, it has been demonstrated in the Scheldt that for PSU values < 5 deflocculation processes start to occur, and that deflocculation is complete for PSU values < 1 (Wollast, 1967, 1973). This means that in the oligohaline zone, flocculated sediment particles start to fall apart, stay longer in suspension, and hence lead to higher values in suspended sediments. For the Elbe, the turbidity maximum also occurs in the oligohaline zone, here at a PSU value around 3.
Back to top