Structure of Flow Upstream of Vertical Angled Screens in Open Channels

This paper presents the results of a laboratory study of the structure of flow in a diversion structure with a vertical angled wedge-wire fish screen. This screen had a $10\times 25\mathrm{mm}$ mesh and was tested at three angles of 10.4, 17.5, and 26.8°, to the direction of the approaching flow, for two mean velocities of 0.5 and $0.8\mathrm{m}∕\mathrm{s}$ , with a depth of flow of about $0.75\mathrm{m}$ . In this water and fish diversion (channel or) structure, it was found that the depth of flow at any section is approximately constant with a drop at the screen on the side of the canal and decreased towards the bypass located at the downstream end. The distribution of the velocity component $u$ in the direction of the approaching flow as well as the perpendicular component $w$ and the resultant velocity $V$ was uniform in the vertical direction. The depth averaged mean velocity $\overline{V}$ for different verticals at any section in the diversion structure increased with the longitudinal distance $x$ and was correlated with the relative width, $b_s∕b$ (in the diversion structure) for all five experiments. Correlations have been found for the depth averaged transport velocity $\overline{u^{\prime }}$ and the impinging velocity on the screen $\overline{w^{\prime }}$ in terms of the approach velocity $U$ . A general relation has also been developed for the attack angle of the flow on the screen. The downstream part of the screen carried more flow into the canal compared to the upstream part as a result of the uniform mesh size used in this study. The results of this hydraulic study should be useful, particularly for freshwater adult fish, in designing screens in irrigation canals and for micro-hydro sites that use diversion canals.