Evaluation of bed shear stress under turbid flows through measures of flow deceleration

Abstract

In previous work it has been observed that the direct application of the law-of-the-wall in flows of high concentrations of cohesive sediments, results in overestimation of the bed shear stress. In this experimental work the effect of various turbidity levels on the drag coefficient and shear stress over a smooth bed is investigated. For this purpose, turbid flows were simulated in a laboratory annular flume (the Lab Carousel), using kaolinite suspensions of different concentrations, varying from 0 to 60 g/l. Flow velocity measurements were taken in the turbulent boundary layer of hydraulically smooth flows and the values of bed drag coefficient and shear stress were calculated using the method of Flow Deceleration. The results obtained after the processing of the velocity time-series and the application of the Flow Deceleration method indicate that there is no evidence of shear stress modification for suspended sediment concentrations below 3 g/l. However, at higher concentrations the flow in the Lab Carousel exhibited different boundary layer characteristics. Therefore, the bed drag coefficient showed a significant response to the increase of the suspended clay concentration by decreasing up to ~ 50% from the clear water value over the range 3-60 g/l under a flow speed range of 0.8 m/s. The equivalent decrease in bed shear stress was calculated as ~ 40%. These results are in accordance with earlier findings of various researchers, thus confirming the validity of the Flow Deceleration method, used in accurate estimations of the bed drag coefficient and shear stress within the Lab Carousel.

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