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Cofferdam is widely employed in the construction of underwater bridge foundations. Its crucial attribute lies in providing a dedicated platform for construction activities and enhancing the water resistance dimensions in structural design, consequently amplifying local scour. However, previous research on local scour has seldom investigated the effect of construction facilities on the life cycle development of local scour on foundations. This gap has led to a misunderstanding of protective strategies against local scour throughout the construction period. In this paper, a scour experiment platform was implemented with a unidirectional flume. Physical model experiments were conducted to scrutinize the protective impact of anti-scour rib structures against local scour. The experimentally determined scour depth was compared to assess the performance of the anti-scour rib protection system. Oblique photogrammetry was subsequently used to capture the morphology of the equilibrium scour pit in the experiments. The associated topographical data were imported into Fluent commercial fluid software for in-depth flow field analysis. A numerical flume model was established to examine the hydraulic characteristics under two distinct topographical conditions: a smooth riverbed during the initial stage of scour and a scoured riverbed at the equilibrium stage of scour. To further determine the protective mechanism of anti-scour rib protection, the influence of anti-scour rib protection on shear stress was investigated numerically. Analyses revealed that incorporating scour protection ribs during cofferdam construction alters the flow field characteristics, hindering the downward movement of subsurface flow beneath the structure, reducing bed shear stress, and consequently mitigating scour effects. The instantaneous protective effect of scour protection ribs strengthens as the scour topography develops. The protective effectiveness of scour protection ribs was mainly influenced by rib length, spacing, and shape. [ABSTRACT FROM AUTHOR]

Rock weirs are low-head river restoration structures. The present study comprised two sets of flume tests (i.e. fixed bed and moveable sand bed tests) to investigate the rock dislodgement process, which is a primary cause of the failure of rock weirs. The impacts of approach flow intensity, submergence, void ratio and scour of bed sediment on rock weir dislodgement, and the responses of sediment bed scour at rock weirs to the rock weir dislodgement, are analysed and discussed. Three limits are defined to classify the amount of the rock weir dislodgement: (1) limit I indicates the incipient dislodgement of rock from downstream weir toe; (2) limit II indicates that a few rocks are dislodged from downstream weir face and weir crest; (3) limit III indicates that large numbers of rocks are dislodged from downstream weir face and weir crest, which leads to weir failure. A predictor of each limit is proposed for rock weir design. [ABSTRACT FROM AUTHOR]

Riverbank erosion is a common natural river process that threatens the security of in-stream structures as well as public and private properties. The impacts of riparian vegetation on the riverbank erosion processes are still unclear. This study conducted flume experiments with artificial riverbanks and living vegetation to investigate the effects of riverbank vegetation conditions on the riverbank erosion process. Two riverbank types, nonvegetation and vegetation riverbanks, with three growth times of vegetation (T = 15, 30, and 60 d) were compared and discussed. The results show that the erosion form on vegetation riverbanks is the individual particles, whereas it is a large block of soil on nonvegetation riverbanks. The equilibrium channel width, riverbank erosion volume, and bed deposition of the vegetation riverbank are lower than those of the nonvegetation riverbank. As the vegetation growth time increases, the equilibrium channel width, riverbank erosion volume, and bed deposition volume decrease. The experimental data also indicate that the temporal channel width of the nonvegetation riverbank is greater than that of the vegetation riverbank; the lower the vegetation growth time, the greater the temporal channel width. Highlights: This study was conducted by planting real vegetation on a riverbank. Riparian vegetation can increase the stability of riverbanks because of mechanical, hydrological, and hydraulic effects. The erosion form on vegetation riverbanks is the individual particles, whereas it is a large block of soil on nonvegetation riverbanks. [ABSTRACT FROM AUTHOR]

In mountainous areas, a river can widen from a single channel to a compound channel under the influence of geological conditions or human impacts, bringing about challenges in terms of flood control and channel regulation. This paper reports the results of tests conducted in a 26 m long flume with a uniform sediment bed (grain size = 0.5 mm), investigating the flow characteristics and bed morphology in a compound channel between two single channels. The stage‒discharge relationship in the compound channel and the longitudinal and cross-sectional bed profile in the compound channel between two single channels are presented and analyzed. The experimental results indicate that the flow characteristics and bed morphology in a compound channel between two single channels are significantly different from those in a normal compound channel. Based on the experimental data and observations, the mechanisms of flow and sediment transport in the compound channel between two single channels are illuminated. [ABSTRACT FROM AUTHOR]

• Flume data of effects of sediment supply on scour at submerged GCSs are presented. • Quantitative impacts of sediment supply on scour at submerged GCSs are evaluated. • Calculation methods for scour at submerged GCSs in degrading channels are proposed. Grade control structures (GCSs) are common river training structures constructed to prevent riverbed degradation. Existing scour predictors for GCSs are based on flume experiments with either no sediment supply or equilibrium sediment transport conditions, the quantitative impacts of sediment supply on the scour at submerged GCSs are still unclear. The present study reports a series of flume experiments to investigate the effects of sediment supply on scour at submerged GCSs. The experimental results show that, compared with the equilibrium sediment transport condition, a decrease of upstream sediment supply rate can reduce the upstream scour depth (down to zero), increase the downstream scour depth (maximumly by 2.3 times) and change the scour process at a submerged GCS. If the sediment supply rate is close to the sediment transport capacity of the flow, the downstream scour depth first increases rapidly and then oscillates obviously around an equilibrium value (i.e. quasi-live-bed scour). If the sediment supply rate is close to zero, the downstream scour depth oscillates initially, then increases monotonically with time towards an equilibrium value (i.e. quasi-clear-water scour). Based on the experimental data and theoretical analysis, the quantitative impacts of sediment supply rate on the equilibrium scour depth at submerged GCSs are evaluated; and calculation methods considering the sediment supply are proposed for estimating the equilibrium scour depth at submerged GCSs. [ABSTRACT FROM AUTHOR]