Indicating shear stress with FST-hemispheres - effects of stream-bottom topography and water depth

1. In order to quantify the effects of substratum roughness on shear stress and Fliesswasserstammtisch-(FST)-hemisphere movement, hemispheres were calibrated against shear stress in a laboratory flume with fully developed turbulent flow. In five different runs, substratum roughness, water depth and location of hemispheres in relation to the surrounding particles, were varied. 2. FST-hemisphere results were strongly influenced by bottom topography. In the case of hydraulically rough flow a linear relationship exists between shear stress (χ_c) and hemisphere density (ρ_h), whereas in the case of quasi-smooth flow a power law was obtained for the χ_c/ρ_h, relationship. Shear stress for a given hemisphere and relative roughness h/k > 4 (h= water depth; k= height of the roughness elements) deviated up to one order of magnitude between roughnesses. In water depths, where h/k 4, the χ_c/ρ_h relationships are dependent on the ratio h/k, due to water surface effects on hemisphere movement. In the case of k = d₉₀=r_h (d₉₀= characteristic diameter of the largest particles of the bottom substratum; r_h= radius of the hemispheres), the location of the hemispheres in respect to the roughness elements is of secondary importance. 3. In the case of hydraulically rough flow and wake interference between the roughness elements, the turbulent flow field close to the substratum is three-layered, each layer being characterized by its own velocity distribution laws (Dittrich & Hammann de Salazar, 1993). Depending on the height of the roughness elements, FST-hemispheres will be subjected mainly to flow forces of the near-bed layer (in rough substrata) or to flow forces distant from the near-bed zone (in fine substrata). The dominant flow forces acting on bottom particles, organisms, or FST-hemispheres are shear force and lift force. 4. The Local Shear Stress Model (Lamouroux et al., 1992) leads to a correct prediction of hemisphere distribution in a stream with a cobble size substratum, but to an overestimation of hemisphere numbers in a sandy-bottom stream. The substratum-dependent shear stress values therefore need to be entered into the model and a measure of substratum roughness included. 5. Macroinvertebrate abundance correlates well with the movement of FST-hemispheres. Samples from points with very high or very low roughness did not contribute to scatter in the data, indicating that the sum of the near-bed flow forces is relevant to macroinvertebrate distribution, not shear stress alone. We conclude, that FST-hemispheres are well suited to characterize near-bottom hydraulics and therefore the microhabitat of the benthos.