Water temperature (WT) is a critical control for various physical and biochemical processes in riverine systems. Although the prediction of river water temperature has been the subject of extensive research, very few studies have examined the relative importance of elements affecting WT and how to accurately estimate WT under the effects of cascaded dams. In this study, a series of potential influencing variables, such as air temperature, dew temperature, river discharge, day of year, wind speed and precipitation, were used to forecast daily river water temperature downstream of cascaded dams. First, the permutation importance of the influencing variables was ranked in six different machine learning models, including decision tree (DT), random forest (RF), gradient boosting (GB), adaptive boosting (AB), support vector regression (SVR) and multilayer perceptron neural network (MLPNN) models. The results showed that day of year (DOY) plays the most important role in each model for the prediction of WT, followed by flow and temperature, which are two commonly important factors in unregulated rivers. Then, combinations of the three most important inputs were used to develop the most parsimonious model based on the six machine learning models, where their performance was compared according to statistical metrics. The results demonstrated that GB3 and RF3 gave the most accurate forecasts for the training dataset and the test dataset, respectively. Overall, the results showed that the machine learning model could be effectively applied to predict river water temperature under the regulation of cascaded dams.

Spur dikes provide significant control for flow regimes in river regulation engineering, which can help in the regeneration of stream habitats. However, the narrowing of the flow by spur dike changes the turbulence characteristics. To clarify the turbulence characteristics around the spur dike, the method of large eddy simulation (LES) was used to investigate the horizontal turbulence structure around spur dikes with different discharges in an open-channel flume. The simulations were an exact reproduction of large-scale laboratory experiments, which showed agreement with the experimental results. The distributions of time-averaged streamwise velocity, bed shear stress, and second-order turbulence statistics obtained from the LES were analyzed. An examination of the time series of velocity fluctuation as the probability density function, quadrant analysis, the power density spectra, flow instability, and the vortex separation created in the detached shear layer were estimated. The results accurately revealed the flow field under flow separation, the turbulence statistics inside the separated shear layer, and the vortex structure and emphasized the variation in the different water depths. The results demonstrated that the form of turbulence was not significantly affected by discharge. Moreover, vortex and energy transmission displayed the same periodicity, despite variances in the structural form of turbulence at different water depths. Overall, the results of the study provide an efficient basis for understanding the turbulence around spur dikes, which is crucial for their safe design.

The vertical slot fishway (VSF) is one of the most common types of fishway facilitating migratory fish movement past obstacles in rivers, such as dams. The uniform vertical distribution of velocity is friendly to fishes with different depth preferences, but unfriendly to fishes with different swimming capacities. For an established VSF, the insertion of an additional structure is a more convenient and effective way to change the flow field rather than altering the original elements. Numerical experiments were carried out using large-eddy simulation (LES) to optimize a typical VSF with supplementary cylinders for fishes with low swimming capacity. The computational domain of the original design is idealized as a box including two pairs of baffles with the periodic boundary condition used in the streamwise direction. The numerical model is well validated by comparison of time-averaged velocity and turbulence kinetic energy with the ADV measurements at gauging points and lines. Two arrangements of cylinders with different numbers and diameters were investigated and compared with the original design based on the first and second-order hydrodynamic statistics at the half height of the VSF. The insertion of cylinders significantly alters the flow field by introducing a branch path of relatively low-speed current. The arrangement of four slim cylinders slightly outperformed that of one thick cylinder against velocity homogeneity along the minor migration path. Although the turbulence intensity is enhanced due to the additional cylinders, the perturbation on fishes is not significant due to the small size of these vortices.

The placement of artificial reefs (ARs) significantly influences local hydrodynamics and nutrient transport, both of which are crucial for enhancing marine ecosystems and improving marine habitats. Large eddy simulations (LESs) are performed to study the flow field around a cuboid artificial reef (CAR) with three inflow angles ($\alpha$ = 0°, 45°, and 90°). The numerical method is successfully validated with experimental data, and a reasonable grid resolution is chosen. The results demonstrate that the case with an inflow angle of 45° exhibits superior flow field performance, including the largest recirculation bubble length and the maximum volumes for both the upwelling and wake regions. Stronger turbulence is also observed around the CAR at this inflow angle, attributed to the intensified shear layer. The instantaneous flow features torn horseshoe vortices and rollers shed from the shear layer, which further develop into hairpin vortices.

To mitigate the negative impacts of traditional rigid revetments on river ecosystems, this study focuses on perforated plate grid revetments, aiming to reveal the hydrodynamic mechanisms and parameter collaborative optimization pathways that simultaneously achieve anti-scour stability and ecological water exchange. A series of flume scour tests were conducted, combined with high-resolution large eddy simulation (LES) validated by experimental data, to systematically analyze the regulatory effects of key design parameters—such as opening ratio and longitudinal offset angle—on near-bottom flow velocity attenuation, vortex structures, and water exchange efficiency. The results indicate that a prototype parameter combination of 0.25 m grid height and 0.50 m plate grid spacing can reduce local scour depth by about 30% and enhance vertical exchange through the synergy of jetting from the openings and internal vortices. The longitudinal offset of adjacent holes may enhance the transverse water exchange but may also significantly reduce the longitudinal exchange intensity; hence, further research is needed. A hole-to-baffle height ratio greater than 0.40 is identified as a critical threshold for improving exchange efficiency. This study proposes a collaborative design framework in which grid spacing controls scour safety and aperture parameters regulate exchange functions, providing an experimental basis for the precise design and performance enhancement of ecological revetments.