Based on the two-dimensional hydrodynamic model of the finite volume method and structured multigrid, the flow characteristics around a square cylinder with boundary constraint are analysed. The gap ratio G/D (G is the distance from the cylinder to the channel boundary, and D is the side length of the square cylinder) does not change the four flow patterns. Under the laminar vortex street phase, the boundary constraint only reduces the scale of the vortices. The vortex centres are pressed toward the boundaries of the channel, and a low velocity zone is formed near the boundary, but the law of vorticity attenuation along the flow direction is not changed. The flow pattern classification map shows that the boundary constraint increases the Reynolds number required to generate the turbulence flow pattern, and the range of the Reynolds number in the flow pattern of the laminar vortex street has a maximum increase range. The correlation between the time-averaged drag coefficient or the vortex shedding frequency and Reynolds number under different gap ratios indicates that the resistance of the square cylinder and the vortex shedding frequency increase accordingly with the strengthening of the boundary constraint. When G/D < 3.5, the increase is particularly obvious. Meanwhile, the correlation characteristics between the resistance or the vortex shedding frequency of the square cylinder and the Reynolds number are unrelated to the boundary constraint strength.

The wind drag coefficient, $C_d$, has a great influence on the numerical results obtained from shallow lakes. To analyze the modeling impacts of $C_d$ on wind-driven currents, a series of numerical simulations of Lake Taihu were conducted at three grid resolutions (800 m × 800 m, 400 m × 400 m, and 100 m × 100 m) using the empirical formulae of Flather (F76), Large and Pond (LP81), Large and Yeager (LY04), Andreas (A12), and Gao (G20). The G20 formula produced the optimum results of all the formulae for both the water level and velocity simulations; however, the grid resolution was found to have a significant influence on simulation in G20 cases. Thus, the G20 formula is only recommended when using a high-resolution grid to meet the accuracy requirements of analyzing wind-driven currents in the numerical modeling of Lake Taihu. A combination of the A12 formula and a coarse grid is preferred when taking computational efficiency into consideration.

Wind waves is an important factor affecting navigation safety and water environment in limited waters such as lakes and bays. Wind wave spectrum represents the frequency domain features of wind waves and has always been the focus of research. Based on the field observation and flume experimental method, the system analysis of similarity of two kinds of situations, discussed nonlinear response of the relationship of the spectral shape parameter of balance field α, β and wind waves basic frequency between factors like wind speed, wind blowing fetch and water depth. By means of wind tunnel flume and prototype observation data of nonlinear regression analysis, The relation formulas of wind wave frequency prediction considering the comprehensive influence of wind speed, wind blowing fetch and water depth is established. Relevant research is of great significance for revealing the evolution characteristics of wind waves in limited waters and guiding navigation safety and water environment management.

Water diversion projects have an important role in coping with water shortage and improving water quality, but they also have an impact on the ecological environment of lakes that cannot be ignored. As an important biogenic element for evaluating the primary productivity and eutrophication of lake water bodies, the influence of phosphorus by water diversion activities and its impact on the production and elimination of phytoplankton is lacking attention. In this study, we analyzed the phosphorus composition and bioavailability of the water channel and the Gonghu Bay of Lake Taihu under the influence of seasonal water diversion, revealed the spatio-temporal distribution patterns of phytoplankton communities in the water receiving river and lake, and analyzed the contribution of phosphorus to the variations of phytoplankton communities and their quantitative coupling relationships. The results showed that short-term water diversion in autumn and winter did not significantly increase the concentrations of particulate phosphorus in the receiving waters, but there was a risk of increasing the concentration of dissolved reactive phosphorus and dissolved organic phosphorus. The difference in total phosphorus concentrations between the diversion and non-diversion periods in Gonghu Bay was an important environmental factor influencing the phytoplankton community, and the bioavailable phosphorus could better fit the logarithm of algal cell density in all seasons, which was significantly and positively correlated with the phytoplankton cell densities. This study implies that the control of bioavailable phosphorus in the water channel can reduce the ecological risks of the water diversion project on the cyanobacterial blooms to some extent. Keywords Water diversion Phosphorus bioavailability Phytoplankton Wangyu River Lake Taihu

Hydraulics and hydrology are ancient disciplines that play a crucial role in ensuring water security, safeguarding water environments, and maintaining water ecology

To clarify airflow allocation and its driving mechanisms in a complex ventilation system during multi-tunnel flood discharge, a prototype-scale three-dimensional air–water two-phase flow model was developed for the three spillway tunnels and the complete ventilation system of the Baihetan Hydropower Station. The model included the upstream reservoir, three spillway tunnels, multi-level ventilation passages, and downstream channel, and was evaluated against prototype air-discharge measurements and outlet surface-velocity observations. At the normal pool level with all three spillway tunnels fully open, the Y-shaped ventilation tunnels, upper-section ventilation adit, and steep-slope-section ventilation tunnels accounted for 17.1%, 37.0%, and 45.9% of the total air inflow, respectively. Although individual ventilation components showed different inter-tunnel allocation patterns, their combined effects produced similar total air discharges among the three tunnels. Flow separation and vortices in the Y-shaped ventilation tunnels reduced the effective flow area, producing local peak air velocities approximately twice the cross-sectional mean. In the upper-section ventilation adit, axial momentum, branch-turning resistance, and the terminal boundary jointly produced sequential flow diversion that increased from Spillway Tunnel No. 3 to No. 1. Air discharge through each branch in the steep-slope section was jointly controlled by the effective pressure difference and passage resistance. Compared with external independent air supply, intake from the tunnel-roof airspace reduced aerator air discharge and shortened aeration cavities. Although the results are specific to the Baihetan configuration, they highlight the need to evaluate complex spillway-tunnel ventilation systems using their complete connected geometry rather than isolated ventilation branches.