Natural channels often take the form of compound channels during flooding events. The unsteadiness of floods is crucial for flood management and river-floodplain ecosystems. However, the impact of unsteady flow on three-dimensional hydrodynamic processes within compound channels remains poorly understood. This study employed a validated two-dimensional/three-dimensional numerical model to examine how unsteady features influence key hydrodynamic indicators of compound channels, such as velocity distribution, turbulence characteristics, and discharge partitioning between the main channel and floodplain. A definitive positive correlation was found between water level fluctuation amplitudes and integrated discharge during unsteady events, as well as between mean water levels and mean discharges. The proportion of discharge conveyed by the main channel relative to the total was directly linked to the water depth ratio. The exchange at the main channel–floodplain interface exhibits a dual-layer flow pattern: surface waters spill into the floodplain, while deeper waters are siphoned into the main channel. Unsteady conditions principally modulate the intensity of these exchanges, altering hydraulic interactions and water transfer between areas. Pronounced wall shear stresses were detected at the main channel–floodplain interface due to swift channel flows and momentum exchange at the floodplain margin, with maxima adjacent to the floodplain edge correlating with peak flow velocities. Under deep conditions, cross-sectional flow across the compound channel's mixing layer was essentially two-dimensional. These findings elucidate the complex hydrodynamics inherent to unsteady overbank flows while holding implications for enhancing floodplain management and advancing the understanding of unsteady fluvial hydraulics.

Urban runoff serves as a critical pathway for the transport of microplastics (MPs) from terrestrial to aquatic ecosystems. Although fragment-shaped MPs are consistently identified as the dominant morphological type in receiving waters, existing studies have focused on granular or spherical particles, leaving the wash-off dynamics of irregular fragments poorly understood. While the traditional exponential wash-off model has been applied to simulate MP transport, its key parameters remain empirical constants that necessitate case-specific calibration, thereby limiting predictive capability. To address these knowledge gaps, this study conducted systematically controlled laboratory experiments to investigate the wash-off processes of polyvinyl chloride fragments (1-5 mm) under simulated urban road rainfall runoff. The individual and coupled effects of key factors-rainfall intensity, road slope, fragment size, and initial load-were quantified. Results demonstrated that Increasing size from 1 mm to 5 mm not only suppressed the enhancing effect of rainfall intensity but also attenuated the influence of slope changes. Initial load exerts a nonlinear characteristics, first promoting then inhibiting transport The first flush effect, quantified by coefficient b (ranging from 0.055 to 0.778), is most suppressed under low rainfall intensity yet paradoxically strongest for larger fragments on steeper slope. Importantly, new empirical equations were developed to parameterize the exponential wash-off model coefficients (k and C_F), explicitly incorporating the effects of rainfall intensity, slope, initial load, and size, enabling reliable prediction of fragment MP wash-off. These equations enable reliable prediction of fragment MP wash-off without case-specific recalibration. Collectively, these findings advance the mechanistic understanding of urban fragment microplastics transport and provide a scientific basis for optimizing stormwater management strategies to mitigate pollutants export and protect aquatic ecosystem health.

The problem of low water level and uneven distribution of flow in the multi-branch channels at the tail of the Ganjiang River (GJRT) during the dry season has been affecting the local water supply, navigation, and aquatic ecological environment. In recent years, water conservancy projects have been built in each branch of the multi-branch channels at the GJRT. Finding a way to utilize the water conservancy project group to carry out joint regulation and meet the water level and discharge requirements of each branch is an important issue that urgently needs to be solved. This paper analyzes the hydrodynamic process and its impact on water supply, navigation, and ecology in multi-branch channels without water conservation projects through hydrological data analysis and numerical simulation. By conducting numerical experiments on joint regulation of water conservation project group, a multi-objective regulation strategy is proposed to meet the water level and discharge of each branch. The results indicate that the discharge at the GJRT has been continuously decreasing from 1 September. Due to the jacking effect of Poyang Lake, the water level plunges at the GJRT from 1 October, which occurred later than the decrease in water level. The disruption of water levels and discharge makes it difficult to meet the regional water demand. The optimal time to initiate regulation is 1 October, and the target water level of Waizhou Station is 15.5 m, located upstream of the Ganjiang River tail. When the water level before each branch project gate is uniform and exceeds 15.5 m, the water level of Waizhou Station satisfies the requirement. However, the discharge of each branch does not meet the demand. In contrast to a scheduling regulation strategy that maintains the same water level in front of each gate, adopting a strategy with different water levels before each gate can effectively adjust the diversion ratio and fulfill the discharge demand of each branch at the tail of the Ganjiang River.