This paper conducted an undistorted scaled model test (geometric scale λ_L = 1:80; the others are derived scales based on Froude similitude) of a 1.3 km-long river reach in Shiting River, China, investigating the impacts of the grade control datum (GCD, defined as the crest elevation of the grade control structure) drop on the upstream bed morphology. Three GCDs and six flood events (occurrence probability 1–50%, discharge = 600–4039 m³/s) were tested on the model. Experimental results indicate that, for a constant GCD, the increase in discharge deepens and widens the upstream river bed. For a lower GCD, the increase in channel depth and width caused by the increasing discharge is greater. For each discharge, the decrease in GCD induces a lower and steeper upstream river bed, widening the upstream main channel. For lower discharge, the GCD drop induces a head cut erosion area upstream of the grade control structure and the head cut erosion area is filled by the upstream sediment when the flow discharge is high. Experimental data also indicate that the maximum general scour depth at the 105th Provincial Highway Bridge is approximately independent of discharge for a constant GCD. For a lower GCD, the general scour depth at the 105th Provincial Highway Bridge increases slightly with discharge.

Riverbank erosion is a common natural river process that threatens the security of instream structures, as well as public and private property. In this study, two sets of tests were performed (direct shear tests and flume tests) to study the effects of riverbed soil composition on the stability of riverbanks. The results of the direct shear tests demonstrated that the riverbank material became increasingly cohesive with increasing clay content and decreasing water content. The internal friction angle decreased monotonically with increasing clay content, but increased towards a peak value, before decreasing thereafter, with increasing water content. The results of the flume tests showed that, for each riverbank material, an increase in bed mobility (non-cohesive bed material > cohesive bed material > fixed bed) increased the equilibrium channel width, riverbank erosion volume and bed degradation. In the first 25 min of the test, the temporal channel width was greater when the bed mobility was lower; after 45 min, the temporal channel was wider when the bed mobility was higher. For each bed material, the riverbank erosion volume and temporal channel width were greater with a non-cohesive bank than with a cohesive bank.

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.

In this study, the impacts of massive sediment input on channel geometry adjustment were analyzed across decades based on the downstream hydraulic geometry. Massive amounts of field data and evolution models showed that the alternation of degradation and aggradation in short-to-medium-term channel adjustment is common in evolving rivers. This phenomenon has always been challenging in research; most existing studies have focused on unidirectional adjustment in short-term channel adjustment. A few studies have considered the alternation of degradation and aggradation in short-to-medium-term channel adjustment, presuming that this phenomenon is caused by water and sediment changes. However, we found that the alternations also occurred under stable water and sediment transport in the North Fork Toutle River, southwestern Washington, USA. This adjustment across decades was analyzed by downstream hydraulic geometry in this study. It was concluded that the river consumes surplus energy to reach the optimal cross section through this short-to-medium-term adjustment under stable water and sediment transport. The objective of channel adjustment is minimal energy loss.

Submerged swirl-generating piers guide inflow toward an annular weir and promote rotating flow in the upper shaft, but also restrict the inlet passages. This study examines the resulting tradeoff using a 1:50 Froude-scaled physical model and three-dimensional Reynolds-averaged Navier–Stokes simulations with an RNG k–ε closure and a volume-of-fluid (VOF) formulation. Pier number N, connection angle θ, and length L are varied over 4~12, 5–25°, and 12~20 m, respectively, in prototype dimensions. Three inlet regimes are observed within the tested range: free-pier flow at H/P < 0.9, transitional flow at 0.9 ≤ H/P ≤ 1.1, and submerged-pier flow at H/P > 1.1. A local discharge-coefficient minimum is observed near H/P ≈ 1.05, coinciding with the onset of overtopping; interaction between the upper and lower flow paths is a proposed explanation. Increasing pier number or length, or decreasing connection angle, generally increases predicted tangential velocity while reducing discharge capacity at a given head. At Q = 400 m³/s, increasing L from 12 to 20 m reduces the maximum inlet velocity magnitude from approximately 4.17 to 2.77 m/s while increasing predicted tangential velocity. These fixed-discharge comparisons include the accompanying change in head. The results describe the tested configurations; shaft velocities and air-core dynamics have not been independently validated.