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 s tress and Fliesswasserstammtisch-(FST)-hemisphere movement, hemisphere s were calibrated against shear stress in a laboratory flume with full y developed turbulent flow. In five different runs, substratum roughne ss, water depth and location of hemispheres in relation to the surroun ding particles, were varied. 2. FST-hemisphere results were strongly i nfluenced by bottom topography. In the case of hydraulically rough flo w a linear relationship exists between shear stress (tau(c)) and hemis phere density (rho h), whereas in the case of quasi-smooth flow a powe r law was obtained for the tau(c)/rho 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 magni tude between roughnesses. In water depths, where h/k < 4, the tau(c)/r ho h relationships are dependent on the ratio h/k, due to water surfac e effects on hemisphere movement. In the case of k = d(90) similar or equal to r(h) (d(90) = characteristic diameter of the largest particle s of the bottom substratum; r(h) = radius of the hemispheres), the loc ation of the hemispheres in respect to the roughness elements is of se condary importance. 3. In the case of hydraulically rough flow and wak e interference between the roughness elements, the turbulent flow fiel d close to the substratum is three-layered, each layer being character ized by its own velocity distribution laws (Dittrich and Hammann de Sa lazar, 1993). Depending on the height of the roughness elements, FST-h emispheres will be subjected mainly to flow forces of the near-bed lay er (in rough substrata) or to flow forces distant from the near-bed zo ne (in fine substrata). The dominant flow forces acting on bottom part icles, organisms, or FST-hemispheres are shear force and lift force. 4 . The Local Shear Stress Model (Lamouroux et al., 1992) leads to a cor rect prediction of hemisphere distribution in a stream with a cobble s ize substratum, but to an overestimation of hemisphere numbers in a sa ndy-bottom stream. The substratum-dependent shear stress values theref ore need to be entered into the model and a measure of substratum roug hness included. 5. Macroinvertebrate abundance correlates well with th e movement of FST-hemispheres. Samples from points with very high or v ery low roughness did not contribute to scatter in the data, indicatin g that the sum of the near-bed flow forces is relevant to macroinverte brate distribution, not shear stress alone. We conclude, that FST-hemi spheres are well suited to characterize near-bottom hydraulics and the refore the microhabitat of the benthos.