Author(s): Wim S. J. Uijttewaal
Linked Author(s): Wim S.J. Uijttewaal
Keywords: Shallow flows; Groyne fields; Particle tracking velocimetry; Turbulencestructures
Abstract: Velocity patterns in groyne fields have been determined experimentally using the particle tracking velocimetry technique. In a shallow-flow flume a river reach has been modelled including the more shallow groyne fields and their sloping bottom. The aspects, that are important for understanding and interpreting exchange processes between the river and a groyne field, are discussed using a number of experimentally obtained examples. First, the groyne field width to length ratio imposes the number and shape of the gyres that are to be found. An aspect ratio close to unity gives rise to a single gyre. By increasing the groyne field length a large primary and a smaller secondary gyre emerges. With even longer groyne fields (width to length ratio of 0. 17) the two gyres remain while the main flow field starts to penetrate into the groyne field. Second, the time averaged flow field shows a rather smooth and quiet pattern with no steep gradients of large lateral motion. This is in contrast with the instantaneous flow pattern where large time dependent motions can be observed, which contribute considerably to the exchange of matter. Third the number of preceding groyne fields in an array appears to be of influence on the flow field that develops. The shear layer present at the interface between the river and the groyne field broadens with downstream position, partly due to large turbulence structures that are generated in the shallow shear flow, and leads to a more smooth exchange flow. Investigations of this kind clarify the need for sophisticated numerical models that take into account the strong anisotropy of the turbulence in shallow flows (e. g. two-lengthscale models) and the time dependent dynamics of the large turbulence structures (e. g. large eddy simulations).