Examination of FST‐hemispheres for evaluating boundary shear stress in streams

In recent years the Fliesswasserstammtisch (FST)-hemispheres have been used increasingly as a device to measure the in-situ bed shear stress for stream restoration. These hemispheres provide a convenient way of estimating the hydraulic characteristics in the near-bed region, which are often needed in river basin management. FST-hemispheres are especially designed for use in the field by environmental engineers and scientists and ecologists. This paper studies the scope and feasibility of using the FST-hemispheres method from the hydraulics perspective. Experiments have been undertaken using FST-hemispheres in a laboratory flume and the results have been compared with published data. Shortcomings have been found in the use of the FST-hemispheres as a means of measuring the boundary shear stress, for which they were originally designed. Through an analysis of the forces acting on the hemispheres in the flow, it has been shown that the FST-hemispheres are more appropriate for use in estimating the flow velocity. The bed shear stress can be related to the near-bed velocity by a bed friction coefficient and thus can be measured indirectly by the hemispheres. Some preliminary numerical model simulations of the flow around a hemisphere have been performed and confirm the aforementioned force-balancing analysis