Evidence of Micro-Rotational Cells in Fluid Flow and Their Possible Implications for Sediment Transport

Abstract

Considerable attention has been given to the determination of the initiation of sediment transport in both sand and gravel-bed rivers. The underlying purpose of these works has been to establish a predictive relationship between these variables and the rate of sediment transport. However, due to the numerous contributing variables involved, it has been difficult to arrive at the definitive relationship between all variables and rates of sediment transport. This paper investigates the relationship between developing micro-rotational cells and rates of sediment transport in a medium-sized stream containing a sand bed. Analysis of the relationship between flow vectors at different time intervals allows for the quantification of this pressure force which operates either on or off the bed, depending on the direction of rotation. Results from 21 near-bed sediment traps and fluid speed data at those locations show a strong relationship between variability in pressure near the bed and sediment transport rate (r 2 = .516) over a range of transport rates between 0.3 to 83 kg m-1 hr-1. This is an important result because it helps explain variation in transport rates when the relationship between fluid speed and transport rate is poor (r 2 = .138).