Estuarine turbidity maximum
Convergence zone of suspended sediment transport, where turbidity levels are high due to high suspended sediment concentrations.
Suspended sediment concentration in the estuarine turbidity maximum
The presence of a turbidity maximum is a common feature of many estuaries. The term turbidity maximum suggests that it corresponds to a particular location in an estuary. In fact, it is a broad zone where concentrations of fine suspended sediment are much higher than in the upstream river or in the adjacent sea, where concentrations of fine suspended sediment are generally below 100 mg/l and often even below 10 mg/l. The suspended sediment concentration (SSC) in the turbidity maximum can be very high, up to several g/l averaged over the water column. A typical example of a turbidity maximum is shown in Fig. 1. The suspended sediment concentration in the turbidity maximum depends on the supply of fine sediments from fluvial and marine sources; erosion of ancient mud deposits in the estuary can also play a role. The most important factor, however, is the trapping efficiency of the estuary, i.e. the efficiency of hydrodynamic processes to retain fine sediments that have entered the estuary and to prevent their escape to the sea. These processes will be discussed in the next section.
Formation of the turbidity maximum
The formation of a turbidity maximum is due to hydrodynamic processes that promote upstream (landward) transport of fine suspended sediment. These typically estuarine processes are:
(1) Tidal asymmetry, generated by the non-linearity of tidal propagation in estuarine channels. This nonlinear tidal propagation results in shorter flood and longer ebb periods, with enhanced maximum flood currents and reduced maximum ebb currents, see Tidal asymmetry and tidal basin morphodynamics. Due to the strong dependence of sediment resuspension on flow velocity, the suspended sediment load carried by the flood currents is much higher than the suspended load carried by the ebb currents. This sediment import mechanism is sometimes designated by the term 'tidal pumping'. (It should be noted that in case of substantial intertidal area, tidal asymmetry can be reversed, with stronger ebb than flood currents; however, such estuaries are more the exception than the rule.)
(2) Estuarine circulation, the residual flow pattern in an estuary induced by the horizontal density difference between seawater and river water, see Fig. 2. The residual upstream flow along the bed of the main channel carries a higher load of suspended sediment than the compensating seaward flow near the surface, see Estuarine circulation.
(3) Stratification. Ebb flow in estuaries is typically more stratified than flood flow, mainly as a consequence of convective overturning during flood when saline seawater is advected over less dense estuarine water. This ebb-flood asymmetry in stratification thus enhances the magnitude of estuarine circulation and up-estuary sediment transport. Self-stratification by suspended sediment also plays a role, with (much) larger sediment concentrations near the bed than higher in the water column. This implies that estuarine outflow is mainly concentrated in the upper layer of the water column, with lower suspended sediment concentrations than the vertical average. Flood flow therefore carries more sediment than ebb flow.
Further upstream in the estuary the residual flow is dominated by river discharge. A convergence zone of suspended sediment transport therefore exists where downstream suspended transport by river flow is of the same order as the upstream suspended sediment transport by estuarine circulation and tidal asymmetry. In this convergence zone, fine suspended sediment accumulates and forms a turbidity maximum. The convergence zone is generally situated around the location of maximum seawater intrusion. In estuaries with strong tides, tidal asymmetry is the major mechanism for upstream sediment transport. In these cases, a second turbidity maximum is formed up-estuary to the location of maximum seawater intrusion, which is much more pronounced than the first one near the head of salinity intrusion. In microtidal estuaries, the turbidity maximum is mainly due to estuarine circulation and stratification processes; the location of the turbidity maximum is determined by the seawater intrusion length.
Suspended sediment settles to the channel bed during periods of slack water, especially in the neap tidal phase of the fortnightly tidal cycle. Flocculation plays an important role as an accelerator of the sedimentation process. In estuaries with a strong turbidity maximum, a fluid mud layer forms on the channel bed when the fine suspended sediment concentration exceeds a few tens of g/l  (see Fig. 3 and Dynamics of mud transport). Under spring tidal conditions, the fluid mud layer is (partly) resuspended and contributes to high turbidity levels, as illustrated in Fig. 1. Resuspension is primarily due to turbulence induced by bed shear stress in the fluid mud layer. Overlying water that is entrained into the fluid mud layer raises and softens the interface layer, eventually leading to mixing of fine sediments into the near surface flow. In this way fine sediments can escape the turbidity maximum if the river discharge is sufficiently high.
In a study of the highly turbid Ems estuary, Winterwerp et al. (2017) have shown that destabilization and resuspension of the fluid mud layer requires flow velocities that are greater than the internal critical flow (the celerity of the internal wave on the mud interface, which is of the order of 1-1.3 m/s). In the Ems estuary, these velocities are exceeded during flood at spring tide, except in case of high river runoff.
Tidal flat accretion
Part of the fine sediments of the turbidity maximum settles on the tidal flats and marshes bordering the main estuarine channel. This reduces the concentration of suspended sediment in the turbidity maximum. The turbidity maximum is advected along the estuary with the tide. It therefore covers a wide estuarine zone. Tidal flats situated in this zone are subject to fast accretion. The landward part of these tidal flats that are not subject to strong wave action, will rapidly grow to a level where flooding only occurs during the highest spring tides , as illustrated in Fig. 4. The growth process is stimulated by the development of marsh vegetation. The concentration of suspended sediment in the turbidity maximum greatly increases when it is no longer possible to store fine sediment on the mud flats, because of their elevation or because they are reclaimed . Fast siltation also takes place in adjacent harbour basins that are situated in the high turbidity zone (see Siltation in harbors and fairways).
Influence of river discharge
The turbidity maximum can be flushed out of the estuary when the river discharge is sufficiently high, see Fig. 5. This happens under spring tidal conditions, if the suspended sediment transport by river flow dominates transport by estuarine circulation and tidal asymmetry over the entire estuary . In estuaries with such high peak discharges, the turbidity maximum does not become as high as in estuaries where the peak discharges are lower.
Infill of estuaries
If the peak discharge is not sufficient to flush the turbidity maximum to the open sea, most of the fine sediments will remain in the estuarine mouth zone. When the river discharge decreases, these sediments are re-imported into the estuary. The estuary then acts as trap for fine sediments. In this case a gradual infill of the estuary takes place. A mud bed forms when the fine sediment concentration exceeds 1000 g/l (see Dynamics of mud transport). The infill stops when the estuarine depth and width are reduced up to a point where peak river discharges can flush the fine sediments far enough into the sea. In this way the estuary evolves towards a dynamic equilibrium.
Influence of dredging
Many estuaries are dredged for navigation purposes. Channel dredging opposes the evolution towards an equilibrium state by strengthening estuarine circulation and reducing the fluvial flushing efficiency. The enhanced fine sediment trapping leads to very high concentrations in the turbidity maximum and requires a corresponding increase of maintenance dredging . A strong increase of the turbidity maxima has been observed in several estuaries (e.g., Ems, Loire) after deepening of the estuarine entrance channel. The observed strong increase in turbidity is also related to reclamation of tidal flats.
'False' turbidity maxima
Data may also reveal elevated suspended sediment concentrations induced by dredging and/or events, which may be erroneously be interpreted as a turbidity maximum. The dumping of dredged sediment can locally increase suspended sediment concentrations to many 100s mg/l for prolonged periods of time. High river flows in e.g. the Scheldt river bring in fine sediment from beyond the weirs near Ghent, which temporarily accumulates in the upper reaches of the river, downstream of the weirs. Data would then suggest the formation of a turbidity maximum at high river flows, but obviously the underlying physical processes are different from the convergence processes explained above.
High turbidity prevents sunlight from penetrating into the water column and therefore suppresses oxygen production by phytoplankton. At the same time, oxygen is consumed by mineralization of the degradable organic components of the fine sediments trapped in the turbidity maximum, as a result of bacterial activity or chemical oxidation . This is illustrated in Fig. 6 for the Loire estuary. High turbidity causes oxygen depletion and mortality of estuarine organisms. This is a serious problem in many estuaries, particularly during warm dry periods, when rapid mineralization occurs while mixing in the water column is suppressed due to density stratification (both salinity and fine sediment concentrations contribute to stratification at small river discharge ). The ecological impact of the turbidity maximum is greatly exacerbated in eutrophic estuaries, see also the article Possible consequences of eutrophication.
- Estuarine circulation
- Dynamics of mud transport
- Tidal asymmetry and tidal basin morphodynamics
- Possible consequences of eutrophication
- Estuarine ecosystems
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