Mitigating the Drowning Hazard Below Weirs Using Submerged Vanes

In recent decades, public safety at weirs has become more and more of a concern for the owners and authorities responsible for such structures. The hazard at weirs is due to the type of flow called a submerged hydraulic jump, which has been the cause of numerous drownings, thus leading to the nickname “drowning machine” for such structures. A submerged hydraulic jump consists of a large horizontal vortex, or roller, that creates a strong backward current that can pull people and objects back towards the weir and keep them entrapped there indefinitely. Engineers have designed a number of different structural modifications to weirs in order to mitigate the submerged hydraulic jump. In 2019, laboratory flume studies at the University of Saskatchewan showed that by installing an array of angled flat rectangular plates, or submerged vanes, downstream of a weir, it was possible to eliminate the submerged hydraulic jump and create a new flow pattern that consistently carried floating bodies to one wall of the flume. Such a result was promising, since in theory, it meant that someone trapped below a weir could be safely brought to the bank by the flow. However, the range of conditions for which the vanes would be effective was not known. The goal of this research was broadly to investigate the use of submerged vanes below a weir to mitigate the submerged hydraulic jump and therefore the drowning hazard. This was accomplished through the use of a weir and vanes set within a rectangular flume, and the vanes’ effectiveness in conveying an object to the bank was tested by inserting objects into the flow: a spherical ball, and a scaled-down model of a human body, called a test-body. Several parameters were varied throughout different tests: the flow rate, the tailwater depth, the angle at which the vanes were set, the lateral spacing between the vanes, and the distance of the vanes from the weir face. The vanes did bring the test-body toward the channel wall under most conditions. The vanes were effective at tailwater depths just deeper than the vanes up to approximately the height of the weir, but somewhat less consistently so at the highest end of the tailwater range. The higher the vane angle, the more consistent conveyance toward the wall, but this also led to a visibly rougher water surface and more splashing against the channel wall. Vane iii angles, relative to the mean flow direction, of 10° or greater were usable; a vane angle of 30° was thought best due to the consistency in conveyance toward the wall, but with less splashing than higher angles. An array of three vanes, with spacing equal to the vane length, was preferable over two or four vanes, which respectively resulted in less consistent conveyance toward the channel wall, or greater likelihood for the test-body to hit the vanes, without any consistent improvement in conveyance. At a flow rate in the flume of 25 L/s, the test-body quite often hit or became caught on one or more of the vanes, but rarely at 75 L/s. It was believed that the difference in position of the weir’s overflowing nappe in relation to the vanes was a major factor in why the test-body more often hit the vanes at 25 L/s compared to 75 L/s. The first tested position of the vanes was the most effective, where at 75 L/s the nappe intersected somewhere through the middle of the vanes.

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