A new portable in situ flume for measuring critical shear stress on river beds

Abstract A new portable in situ flume (PISF) for measuring critical bed shear stress (CBSS) was developed in this study. The PISF consists of an open bottom sediment erosion chamber and an electrically-driven pump. Unlike most existing in situ flumes with similar designs, the new PISF does not rely on monitoring the flow conditions or particle density in the sediment erosion chamber; instead, it is a pre-calibrated flume. The calibration was performed by first determining CBSS of various selected sediment samples of known particle size and density (using the law of the wall), based on flow velocity-depth profiles measured in a 6 m straight open-channel flume using a Particle Image Velocimetry (PIV) system. These same particles of known CBSS were then used in the new in-situ flume under controlled lab conditions to obtain a series of calibration curves of CBSS vs pump electrical power. A wide variety of particle types and sizes (simulated sediments) were used in this two-step calibration procedure to widen CBSS measurement range and simulate cohesive force effects. The size of the PISF is much smaller and more practical than other similar devices as lamellar flow conditions are not required and it can be applied to a wide range of sediment types including cohesive sediments.