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Abstract for the 18th Physics of Estuaries and Coastal Seas Conference, 2016
Sediment-induced stratification and density current in the Ems estuary
Marius Becker1 and Christian Winter1
Keywords: Ems estuary, measurements, sediment stratification, density current, fluid mud.
Abstract
Feedback of sediment-induced stratification on the flow is widely acknowledged to play a significant role in estuarine
sediment transport (e.g., Winterwerp, 2001). Recently, attention was drawn to the influence of sediment-induced
longitudinal density gradients on the location of the estuarine turbidity zone (ETZ) and, in general, on subtidal dynamics
in the hyper-concentrated Ems estuary (Talke et al., 2009; Donker and de Swart, 2013). However, by contrast to the
increasing number of modelling studies, few experimental results were published regarding the actual vertical structure
of the water column, or the intra-tidal dynamics of high concentration layers, such as fluid mud (e.g., Held et al., 2013).
Figure 1 shows the variability of sediment-induced stratification in the center of the ETZ of the Ems estuary, measured
November 2014 during moderate discharge, at the side of the navigation channel. Layers were separated by strong
gradients in suspended sediment concentration (SSC), i.e., a lutocline on top of a mobile mud layer, and an interface
separating the mobile mud from a higher concentrated fluid mud layer below (SSC > 50 g/l). According to electromagnetic current meter data (ECM, results not shown), the fluid mud layer was stationary.
Figure 1. Vertical distribution of ADCP current velocity (a), SSC interpolated from vertical OBS casts (b), as indicated by vertical
dashed lines, and echo sounder intensity (c, SES-2000, Innomar) during 17 h at Jemgum. The vertical coordinate is height above bed
[m], with an arbitrary datum below the surface of the stationary fluid mud layer. White lines (a, b) separate flood and ebb directed
flow. Salinity (a) is shown by vertical bars. Coloured circles (a) indicate magnitude and location of ECM measured current velocity. A
second ECM deployed in the fluid mud layer is not shown. Coloured circles (b) indicate strength and location of the maximum vertical
SSC gradient associated with the lutocline above the mobile mud layer (top) and the fluid mud surface (bottom). During flood
entrainment (II-II+1.5h), a maximum gradient, corresponding to the lutocline, cannot be determined. SES intensity intensity reveals
details (yellow arrows) of sediment stratification between interpolated vertical OBS casts. Strong reflection in the lower part of the
profile (c) shows the stationary fluid mud layer. During flood (II-III), maximum sediment flux (not shown) was found in the lower part
of the profile, but was confined to the upper part of the mobile mud layer during ebb (V-VI). See text for further description.
1
Marum, University of Bremen, [email protected], [email protected]
During the first half of the flood tide (Figure 1b, II-III), the mobile mud layer was entrained. We observed the reformation of the mobile mud layer during an unexpectedly early stage of the flood phase (III). This is interpreted to result
from a super-saturated situation after the flood acceleration phase. High SSC in the upper part was not sustained during
slightly decelerating flow. The settling flux was increased, inducing the collapse of the SSC profile (Winterwerp, 2001).
Subsequently, the flow was decoupled between the upper and the lower layer, separated by the lutocline,
approximately in the middle of the water column. The flow was flood directed towards the surface, while velocities in
the mobile mud layer were ebb directed (Figure 1a, see also Figure 2c). The mobile mud layer remained unaffected by
entrainment for a period of 4.5 h around high water and moved with an average velocity of 0.08 m/s in ebb direction, with
a peak velocity of 0.12 m/s above the fluid mud surface, directly following flow reversal (III). This ebb directed density
current is seen as the combined effect of the downstream SSC gradient and the downstream bottom slope, supported by
low friction between fluid and mobile mud and the increased water depth.
During re-formation of the mobile mud layer, the region of highest velocity shear shifted vertically from the fluid mud
surface to a location above the lutocline (Figure 2b, III-IV). In addition, significant flow deceleration was observed in the
lower part of the water column, over the vertical extent of the mobile mud layer before the flow reversal (Figure 2a, III).
This indicates a situation of forced convection. While flood entrainment after low water and the related upstream transport
determine tidal pumping of sediments, here, the re-formation of the mobile mud layer potentially reduced near-bed
upstream transport and, thus, reduced the effect of tidal pumping. Vertical velocity profiles were shown to depend on
periodically changing stratification. Consequently, mud-induced straining effects are considered to play an important role
in the estuarine circulation, particularly in the upper reaches of the Ems estuary.
Figure 2. Acceleration and vertical velocity shear. Vertical bars (b) show the gradient Richardson number (Rig). The sketch below
(c) depicts the main periods of flow acceleration (II, V) and deceleration (I, III, VI), vertical shear (red patches), flux (hatched patches),
and the stationary fluid mud layer (> 50 g/l, dark grey). Vertical arrows show settling (green) and entrainment (yellow), i.e., the main
vertical exchange between the mobile mud layer and the water column above. Horizontal arrows indicate tidal currents, flood currents
point from left to right.
References
Donker, J.J.A, de Swart, H.E. (2013) Effects of bottom slope, flocculation and hindered settling on the coupled dynamics of currents
and suspended sediment in highly turbid estuaries, a simple model. Ocean Dynamics, 63, 311-327
Held, P., Schrottke, K., Bartholomä, A. (2013) Generation and evolution of high-frequency internal waves in the Ems estuary,
Germany. Journal of Sea Research, 78, 25-35
Talke, S.A., de Swart, H.E., Schuttelaars, H.M. (2009) Feedback between residual circulations and sediment distribution in highly
turbid estuaries: An analytical model. Continental Shelf Research, 29,119-135
Winterwerp, J.C. (2001) Stratification effects by cohesive and non-cohesive sediment. Journal of Geophysical Research, 106,
22559-22574