Hydrodynamic Effects of Sedimentation on Mass Transport Properties in Dead Water Zone of Natural Rivers

Abstract

We can often see dead water zones composed of consecutive groynes in natural rivers. The groynes are generally constructed in the bank of actual rivers in order to navigate stream direction and to prevent bank erosion. Dead water zones such as side cavities are also observed in harbors of rivers, and it is well known that there are significant differences between streamwise velocities of the mainstream and the cavity zone. Of particular significance is that shear instability related to the velocity difference induces coherent horizontal vortex along the boundary of the mainstream and the cavity. Further, large-scale gyres are formed in the dead water zone which conveys suspended sediment from the main-channel, and local sedimentations are promoted in the cavity as shown in photo 1. So, it is necessary to reveal the hydrodynamic properties included with turbulence phenomena in order to control sedimentation reasonably. Akkerman et al.(2004) conducted the sensitivity analysis with a 1-dimensional morphodynamic model, and they discussed the several effects on sedimentation and flood water depth after the occurrence of groyne damage. Recently, permeable groynes are proposed to realize stable bed condition. For example, Kadota & Suzuki (2010) discussed experimentally effects of the permeability and the scales of the groynes and stone gabion in submerged and emerged flow conditions. Tominaga & Sakaki (2010) conducted ADV measurements around the permeable groynes in a natural river, and they evaluated distributions of bed shear stress accurately. In these cavities, not only sedimentation but also congestion of pollutants is often highlighted. It is thus very important to investigate mass & momentum exchanges between the main-channel and the side-cavity in river environment and hydraulic engineering. The above-mentioned horizontal gyres and coherent turbulent structures play significant roles on mass and momentum exchanges. Uijttewaal et al.(2001) measured distribution of dye concentration and pointed out that aspect ratio of side cavity has significant effects on exchange rate of mass between the mainstream and the dead water zone. Weitbrecht et al.(2007) have also conducted laboratory measurements, in which distribution of velocity components and dye were obtained. They examined the relation between the exchange rate and the bed configuration of the cavity.