TURBULENT-FLOW STRUCTURE AT CONCORDANT AND DISCORDANT OPEN-CHANNEL CONFLUENCES

Models of flow at river-channel confluences that consist of two concor dant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structur e in the streamwise and vertical directions at both concordant and dis cordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influe nce on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The s patial distribution of normal stress varies with bed geometry as bed d iscordance generates a distortion of the mixing layer between the conf luent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a d ecrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resu lting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.