As for the Scheldt, the Humber estuary (Figure 12) is a typical converging estuary, mainly from TIDE km 112 to TIDE km 82 (Figure 13). The thalweg depth clearly decreases from the mouth up to TIDE km 90, whereas the decrease from TIDE km 90 up to the up-estuary boundary is more gentle (Figure 14). The Humber-Ouse estuary can be considered as a multi-channel system from TIDE km 95 up to the junction with the Trent. The Ouse, Trent, and the most downstream part of the Humber (downstream TIDE km 95) can be considered as single channel systems (i.e. only one subtidal channel) (Figure 12). The decrease from mouth to up-estuary boundary in estuary width and/or estuary depth results in a decrease of the wet section (Figure 16).
The Humber is a macrotidal estuary with at mean tidal conditions a tidal range of 4.3 m at the mouthgeo
, a maximum tidal range of 5 m at TIDE km 90, and a tidal range of 1.3 m at the Ouse up-estuary boundary (Figure 18). The increase in tidal range from mouthgeo to TIDE km 90 is caused by an increase in MHWL and a decrease in MLWL (Figure 17 and Figure 19). From then on a decrease in tidal range occurs, caused by a stronger decrease in MLWL than the increase in MHWL (Figure 18 and Figure 19).
The mean freshwater discharge at Skelton (Ouse up-estuary boundary) and North Muskam (Trent up-estuary boundary) is respectively 44 and 72 m³/s for the year 2010 (Figure 21). Including the Trent, the Ouse and all tributaries of the Ouse (Wharfe, Derwent, Aire and Don), this results in a mean discharge into the Humber of 209 m³/s. During flushing events (P95, typical during winter) and dry events (P5, typical during summer) the discharge at Skelton is respectively 143 and 9 m³/s, and at North Muskam respectively 177 and 29 m³/s.
The mean and maximum ebb and flood flow velocities respectively range between 0.1 and 1.5 m/s and between 0.1 and 2 m/s (Figure 22 and Figure 23). In Elbe and Weser high discharge conditions (winter) lead to higher ebb flow velocities and lower flood flow velocities in the entire estuary. For the Humber, this is only the case for the most upstream part (from TIDE km 40-0) due to the lower discharges (cf. estuaries in Figure 22). As for the Elbe and Weser, high discharges lead to the disappearance of the flood
The tidal damping scale which describes the tidal amplification and tidal damping in an estuary (see §3.2.2) is positive (amplification more important) in the most downstream part of the estuary (up to TIDE km 95, Figure 24). In the rest of the estuary 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 Humber estuary are respectively higher and lower compared to conditions with low freshwater discharge (Figure 25). Only for the most downstream part of the estuary (TIDE km 71 to mouth) the tidal energy is also higher (i.e. the selection of a slightly higher tide in the estuary for the winter condition (Figure 25). The point where the fluvial energy becomes more important than the tidal energy is for the winter located at TIDE km 80, and for the summer near the up-estuary boundary (at TIDE km 10).
The Humber is a well-mixed estuary where it takes about 60 km for the mean salinity profile to decrease from 30 PSU to 1 PSU (i.e. a mean salinity gradient of 0.48 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 nearly 16 PSU, whereas the maximum variation between low water and high water is about 6 PSU for winter and summer (Figure 27).
The surface suspended particle matter (representative for the mean over a tidal cycle, see §3.1.4 ) varies between 20 and 720 mg/l, and reaches a maximum at TIDE km 88 (Figure 28 ). Surface SPM values are for the Humber estuary clearly higher than for the other estuaries (cf. estuaries in Figure 28 ).
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