Measuring velocity profiles above different substrates on the Glinščica stream Maja Koprivšek 1, Mitja Brilly 1, Mihael Jožef Toman 2 1 University of Ljubljana, Faculty of Civil and Geodetic Engineering, Chair of Hydrology and Hydraulic Engineering, Jamova 2, Ljubljana, Slovenia 2 University of Ljubljana, Biotechnical Faculty, Department of Biology, Jamnikarjeva 101, Ljubljana, Slovenia INTRODUCTION The measurements of the velocity profiles above different substrates were done on the Glinščica stream in the summer Our main aim was to measure the velocities so close to the riverbed as possible, because we were interested in the velocities in the places where the most of water organisms, especially macroinvertebrates, actually live. The velocities are not heterogeneous only in their vertical extension, but also in all directions over the bed surface, due to the morphology of the riverbed and especially due to the substrate. Large spatial and temporal heterogeneity of the velocities is the reason for measurement of near-bottom velocities being very difficult. STUDY AREA The Glinščica stream watershed area is situated in the central part of Slovenia and reaches into the western part of the urban area of the capital city of Ljubljana. The topography of the basin is comprised of hilly areas to the east and west and a plain area in the southern part. The outlet to the Mestna Gradaščica River represents the southernmost point of the watershed. Four measuring sites are located on differently modified reaches of the Glinščica stream: one of them on the reach with concrete channel (Fig. 5), two on the reach with straightened channel with natural substrate (Fig. 2 and Fig. 3) and one on the transition zone (Fig. 4). MEASURING METHODS The instrument used for the velocity measurements was a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter). On each cross- section, we selected at least three points, in which we measured velocities on different depths: just under the water surface, on 2/10, 6/10 and 8/10 of water depth and near the bed surface. Near-bottom velocities were measured on the height 1.6 cm above the bed surface, which is closest to the bed surface possible when using a wading rod. If the water was not too deep, we repeated the near-bottom measurement without the wading rod, so that we held the probe in a hand on the bed surface. In this case, the sampling volume was approximately 0.5 cm above the bed surface. RESULTS We compared vertical velocity profiles between cross-sections on each measuring site. Only thalweg vertical velocity profiles are shown here (Fig. 6 – Fig. 14). Then, the comparison of thalweg velocities at 6/10 of water depth and near-bottom were done for all the cross-sections (Fig. 15, 16, 18 and 19). The same was also done for turbulent kinetic energy density Effects of substratum and morphologic position Cross-section “pure concrete” Fig. 6: Without a stoneFig. 7: Downstream of the stoneFig. 8: Upstream of the stone Cross-sections “semi-natural substrate” and “artificial substrate” Fig. 9: “Semi-natural substrate”Fig. 10: “Artificial substrate” CONCLUSIONS Although our measuring instrument was not the most appropriate for near-bottom measurements and the majority of our near-bottom measurements was done 1.6 cm above the riverbed, we have found out some relations between near bottom velocities and mean water column velocities. Differences in ratio v 6/10 /v 1.6cm were not significant between cross-sections on semi-natural substrate and cross-sections on artificial substrate, but they were significant between cross-sections with hydraulically rough riverbed and cross- sections with hydraulically smooth riverbed. At low discharge the ratio v 6/10 /v 1.6cm did not differ much between cross-sections (its value ranged between 1.3 and 1.8) while at high discharge the value remained the same on cross-sections over hydraulically smooth riverbed (concrete channel, pool) and increased up to 2.8 on cross-sections over hydraulically rough riverbed (riffle, semi-natural substrate). Roughness elements therefore ensure that near-bottom velocities stay low even at the high discharge. Heterogeneous substrate and existence of larger underwater obstacles, such as stones, macrophytes etc., is therefore essential to ensure adequate living conditions for water organisms. Of course, not just ratio between mean flow velocity and near-bottom velocity is important, but also absolute values of velocities are. From the Fig. 19 it can be seen that near-bottom velocities over the semi-natural substrate are significantly lower than those over the artificial substrate (concrete plates). Semi-natural substrate therefore provides more suitable living conditions for water organisms, but the v 6/10 /v 1.6cm ratio suggests that even artificial substrate in not so bad, if it contains some larger obstacles, appropriately distributed across the riverbed. Pool overgrown with macrophytes Fig. 11: “Outside the macrophytes”Fig. 12: “Between the macrophytes” Cross-sections “pool” and “riffle” Fig. 13: “Pool” Fig. 14: “Riffle” Fig. 15: Comparison of maximal velocities 1.6 cm above the bottom at different water stages Fig. 18: Comparison of maximum cross-section velocities on 6/10 of water depth at different discharges Fig. 16: Comparison of maximal velocities on 6/10 of water depth at different water stages Fig. 19: Comparison of maximum cross-section velocities 1.6 cm above bottom at different discharges Fig. 20: Comparison of TKE (turbulent kinetic energy density) at different discharges Fig. 17: Comparison of mean profile velocities in dependence of water level above pure and overgrown concrete channel Thalweg vertical velocity profiles Fig. 3: Pool overgrown with macrophytes Fig. 1: Measuring sites on the Glinščica stream Fig. 2: “Pool” and “riffle” at high discharge Fig. 4: “Semi-natural substrate” and “artificial substrate” Fig. 5: Overgrown concrete channel (Fig. 20). The comparison between maximum profile velocities on the “pure concrete” and overgrown concrete” is shown on Fig. 17.