Cloudiness zone

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Turbidity zones are zones in estuaries in which there is a particularly high concentration of suspended sediments in the water column.

Emergence

Creation of a cloudiness zone by the onset of the flood current in a partially mixed estuary.
Silt deposition from the turbidity zone of the Weser estuary, which is believed to have arisen from solidified fluid mud.

In a turbidity zone, collision processes between suspended sediment particles take place continuously. This leads to the formation of aggregates and flocculation of the particles. In the Weser , for example, flakes of up to 3 mm in diameter were observed. This in turn increases the sinking speed of the particles or now particle aggregates, which leads to sedimentation of the previously suspended sediments at a sufficiently low flow speed of the water, i.e. shortly before to shortly after still water. These deposits, also known as fluid mud , are initially only weakly consolidated due to the way they are created and reach concentrations of a few grams to several hundred grams per liter. They also show strong thixotropy and, depending on the sediment supply and the sinking speed, can reach a thickness of several meters .

Turbidity zones are always associated with the upstream boundary between fresh and brackish water, which at the same time represents their upstream boundary. The downstream limit is at a salinity of about 8 PSU .

The properties of the cloud zone depend on a complex combination of factors. These include the dynamics of the tides, the shape of the estuarine circulation as well as the deposition and erosion of sediments, which in turn depend on the erodibility of the existing sediments. Their spatial and temporal spread depends in particular on the extent of the tidal range , the characteristics of the spring - nipptide cycle, the volume of water runoff and wind-induced changes in water level in the respective estuary. Due to these factors, the cloudiness zone can shift significantly within an estuary.

Investigations in different estuaries have led to the view that the main factor that leads to the formation and maintenance of a cloudiness zone is a phase delay between the flow velocity of the water and the deposition of the sediments suspended in the water column.

The cloudiness zones are not uniform. Depending on the geomorphological and hydrological conditions, there are places with greater turbidity and others with a lower concentration of suspended matter.

Cloudy zones only arise with a correspondingly large tidal range. This is the case at the estuaries in the North Sea. In Germany, the turbidity zones of the Weser , Elbe and Ems are examined in particular . In the estuaries of the Baltic Sea, there is no formation of turbidity zones because of the lower tidal range.

Consequences for people and nature

In the case of still water, fluid mud , a low-viscosity clay , silt , fine sand , and water mixture with a high proportion of organic components can build up. These deposits are usually immediately eroded again when the flood or ebb current starts again, but the lowest layers can be resistant if the compaction is sufficiently strong. If these deposits accumulate over time, they will sooner or later represent an obstacle to navigability that has to be removed, for example, by means of dredging. An important problem here is that low-viscosity sediments or suspensions are often impossible or difficult to detect using hydroacoustic measuring methods.

The growth of phytoplankton is very limited by the strong cloudiness.

Individual evidence

  1. ↑ a b c d e f g K. Schrottke, M. Becker, A. Bartholomä, W. Burghard, BW Flemming and D. Hebbeln: Fluid mud dynamics in the Weser estuary turbidity zone tracked by high-resolution side-scan sonar and parametric sub-bottom profiler. Geo-Marine Letters, 26, 3, pp. 185-198, 2006
  2. ^ Schrottke, K., Bartholomä, A. & Becker, M. (2005): Bed Mobility in the Weser Estuary Turbidity Zone . Hydro international, vol. 9, no. September 7, 2005.
  3. ↑ a b Schuchardt, B., Schirmer, M., Janssen, G., Nehring, S. & Leuchs, H. (1999): Estuaries and brackish waters. In: De Jong, F., Bakker, JF, van Berkel, CJM, Dankers, NMJA, Dahl, K., Gätje, C., Marencic, H. and Potel, P. (1999): Wadden Sea Quality Status Report. Wadden Sea Ecosystem No. 9. Common Wadden Sea Secretariat, Trilateral Monitoring and Assessment Group, Quality Status Report Group. Wilhelmshaven, Germany. Full text ( memento of the original from January 31, 2012 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 342 kB) @1@ 2Template: Webachiv / IABot / www.aquatic-aliens.de
  4. ^ A b Dyer, KR (1988): Fine Sediment Particle Transport in Estuaries. In: Dronkers, J. & van Leussen, W. (eds) (1988): Physical Processes in Estuaries. Springer, p. 295-310, Berlin.