Author(s): Wen Zhang; Binrui Gan; Lu Wang; Xingnian Liu
Linked Author(s): Lu Wang, Xingnian Liu
Keywords: Rock weirs; Clear-water scour; Unsteady flow; Flume tests
Abstract: Rock weirs typically placed by loose rocks and span the full width of channel, are eco-friendly grade-control structures (GCSs) commonly built in small and medium-sized rivers. A rock weir in rivers can preventing bed degradation, whereas the over-weir flow can cause local scour that can threaten the structural stability. Current studies focus on the clear-water scour at rock weirs under steady flow (USBR, 2016), remaining the scour characteristics at rock weirs under unsteady flow are unknow. As such, predictors for accurately estimating the scour depth at rock weirs are far from complete. Flume tests using uniform coarse sand (d50 = 1.17 mm) were conducted to study the clear-water scour at rock weirs under unsteady flow. The rock weir was placed by loose rocks (Zhang et al., 2023) spanning the full width of the flume, with a 50 mm protrusion above the initial flat bed (Fig. 1). The rock weir was I-shape in the top view and its cross-section was triangular with the upstream and downstream slope equal to the angle of repose of the pebbles (about 40º). Three types of unsteady flow process with single peak (i.e. symmetrical, advanced and delayed hydrograph) were performed in the flume, and the development of the scour depth at rock weirs under each flow process was manually read using cameras focused through transparent grid sheet attached to both sides of the flume. The results show that (i.e. Fig. 2): (1) during the initial of the unsteady flow process with relatively low discharge, no scour occurs at rock weirs as the flow intensity is below the threshold for scour initiation, which is linearly and positively correlated with submergence ht/z when ht/z < 2.4, and is nearly constant when ht/z > 2.4; (2) As the unsteady flow process develops, the clear-water scour process at rock weirs initiates when the flow intensity exceeds the critical flow intensity; the scour depth increases gradually before the peak discharge, and reaches its maximum quickly after the peak discharge; (3) For a given duration of the unsteady flow process, the maximum clear-water scour depth is insensitive to the location of the peak discharge, but increases with increasing peak discharge and decreases with increasing submergence. The results can improve the understanding of the clear-water scour mechanism of rock weirs, and also provide theoretical support for the scour design of rock weirs.
Year: 2024