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Laboratory Study of 3 Dimensional Characteristics of Recirculating Flow in a River Embayment

Author(s): E. Jamieson; S. J. Gaskin

Linked Author(s): Susan Gaskin

Keywords: River embayments; Dispersion models; Recirculating flow; Groyne fields; Particle tracking

Abstract: The exchange of matter occurs between the main flow of a river and the dead zones at its periphery, which are due either to the shape of the naturally irregular banks or to man made structures such as groyne fields. Two concerns arise from this exchange, the first is the rate of propagation of a pollutant cloud (or nutrients) along the river and the second is the residence time of the pollutants (or nutrients) within the dead zones. This has led to the development of longitudinal dispersion models that take into account the trapping of pollutants in the dead zones and their subsequent release back into the main flow. These models consider the dead zones as homogeneous regions having a bulk exchange coefficient. For the case of groyne fields and harbours, a recirculating flow (or gyre) develops resulting in a more complex exchange process that is a function of the large scale flow structures present. The current work studies the flow structures and exchange processes within a square embayment adjacent to a channel using both dye studies and particle tracking to show that the three dimensional flow structure is important in determining the exchange rate and that the exchange occurs in a two stage process – from core to gyre and from gyre to main flow. Three dimensional particle tracking data clearly show that the exchange occurs due to intermittent vertical structures. Evidence of the two stage process has also been reported in field tracer concentration data from a groyne field study. Further refinement of longitudinal dispersion models will need to model the exchange as a two stage process. The resulting details of the evolution of concentration in the recirculating dead zones are also relevant for studies of environmental micro-habitats in groyne fields.


Year: 2007

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