Physical modelling of energy losses at surcharged three-way junction manholes in drainage system.

Motivated by the observation that vortex flow structure was evident in the energy loss at the surcharged junction manhole due to changes of hydraulic and geometrical parameters, a physical model was used to calculate energy loss coefficients and investigate the relationship between flow structure and energy loss at the surcharged three-way junction manhole. The effects of the flow discharge ratio, the connected angle between two inflow pipes, the manhole geometry, and the downstream water depth on the energy loss were analyzed based on the quantified energy loss coefficients and the identified flow structure. Moreover, two empirical formulae for head loss coefficients were validated by the experimental data. Results indicate that the effect of flow discharge ratio and connected angle are significant, while the effect of downstream water depth is not obvious. With the increase of the lateral inflow discharge, the flow velocity distribution and vortex structure are both enhanced. It is also found that a circular manhole can reduce local energy loss when compared to a square manhole. In addition, the tested empirical formulae can reproduce the trend of total head loss coefficient.

Integrated modelling of flow-sediment transport and power generation in the Three Gorges Reservoir.

Reservoirs play a crucial role in regulating runoff and generating energy. However, they also lead to significant sedimentation in the reservoir area. In this study, we propose an integrated model that combines a 1-D hydro- and sediment dynamic module with a power generation module. The model considers both suspended and bed load transports. This model is applied to the Three Gorges Reservoir (TGR) and evaluate its performance against corresponding measurements. The results demonstrate that:① the proposed model accurately reproduces the processes of flow and sediment transport, bed deformation, and power generation during the hydrological years of 2019 and 2020. The relative errors for average discharge and bed deformation volume are <6 % and 10 %, respectively. Moreover, the calculated total power (982 × 108-1115 × 108 kW·h) closely agree with the measured values (969 × 108-1118 × 108 kW·h); ② the inflows of small tributaries have a noticeable impact on the calculated water discharge in the TGR. This impact will lead to a 16 % increase in average discharge and alter the magnitudes and occurrence times of flood peaks; ③ the flocculation of fine sediment particles significantly affects sediment transport, particularly in the sub-reach close to the dam. This flocculation will result in a 37 %-57 % reduction in average suspended sediment discharge and a 63 %-93 % reduction in peak sediment discharge. This research provides a comprehensive tool for simulating flow and sediment transport as well as power generation, which can support the optimal regulation of the TGR.

Channel erosion and its impact on environmental flow of riparian habitat in the Middle Yangtze River.

Evaluating environmental flow (EF) is pivotal for conserving and restoring riverine ecosystems. Yet, prevalent EF evaluations presume that a river reach's hydraulic conditions are exclusively governed by inflow discharge, presupposing a state of equilibrium in the river channel. This presumption narrows the scope of EF evaluations in expansive alluvial rivers like the Middle Yangtze River (MYR), characterized by marked channel alterations. Here we show the profound channel erosion process and its impact on EF requirements for riparian habitats within the MYR. Our research unveils that: (i) pronounced erosion has led to a mean reduction of 1.0-2.7 m in the riverbed across four sub-reaches of the MYR; (ii) notwithstanding a 37-107% increase in minimal discharges post the Three Gorges Project, the lowest river stages at some hydrometric stations diminished owing to bed erosion, signifying a notable transformation in MYR's hydraulic dynamics; (iii) a discernible rightward shift in the correlation curve between the weighted useable area and discharge from 2002 to 2020 in a specific sub-reach of the MYR, instigated by alterations in hydraulic conditions, necessitated an increase of 1500-2600 m³ s-1 in the required EF for the sub-reach; (iv) it is deduced that macroinvertebrate biomass rapidly decreases as the flow entrains the riverbed substrate, with the maximum survivable velocity for macroinvertebrates being contingent on their entrainment threshold. These findings highlight the importance of incorporating channel morphological changes in devising conservation strategies for the MYR ecosystem.

Hydrologic and geomorphic effects on the reduction of channel discharge capacity in the Middle Yangtze River.

A systematic increase in the occurrence of flood events has been noted in rivers worldwide. However, our understanding remains unclear of how the flood level is affected by the change in channel discharge capacity. Therefore, the reduction of channel discharge capacity was quantified in the Middle Yangtze River (MYR) based on extensive measured data. It is confirmed that the variation in channel discharge capacity is a superposition result of channel-morphology and channel-resistance changes, as well as the change in local base-level at the outlet. Then a series of numerical simulations were carried out to quantitatively identify the contribution of each factor to the reduction of channel discharge capacity in the MYR for the years of 2004 and 2020. Simulated results reveal that channel degradation increased the flow passage area, helping the channel to convey floods, with the effect being weakened along the reach; however, the decline in flood level under the same discharge was not observed, which was primarily attributed to the increases in channel resistance and local base-level at the outlet. These two factors accounted for 16-91 % and 9-84 % of the total impact at four hydrometric stations in the MYR. It should be noted that the dominant factor to influence the discharge capacity varied greatly at different stations, which depended on the degree of bed-material coarsening, the distance from the confluence of lakes, etc. At the stations immediately downstream of the dam, the contribution of the increased movable bed roughness usually played a more essential role; while at the stations close to the outlet with the confluence of large lakes, the increase in local base-level became the main factor that accounted for the rise of high-flood levels under the specified large discharge.

Modelling of phosphorus and nonuniform sediment transport in the Middle Yangtze River with the effects of channel erosion , tributary confluence and anthropogenic emission.

Phosphorus (P) transport plays a crucial role in the aquatic ecology of natural rivers. However, our understanding still remains unclear that how P transport is affected in a river-lake connected system downstream of a dam. This system usually undergoes both severe channel degradation and complex exchange of flow-sediment-phosphorus between the mainstem and tributaries. In the current study, a method was proposed firstly to determine the individual contribution of different sources to P recover based on the calculation of phosphorus budget; then an integrated model was developed, covering the modules of flow, nonuniform sediment and phosphorus transport. The application of the proposed method in the 955-km-long Middle Yangtze River (MYR) shows that the type of P transportation was predominantly changed from particulate phosphorus to dissolved phosphorus after the operation of the Three Gorges Project (TGP), but a significant longitudinal recovery of total phosphorus (TP) flux was observed. The TP flux exporting from the MYR was mainly from the Upper Yangtze River (44%), and 12%, 18% and 26% of that were originated from channel erosion, tributary confluence and anthropogenic emission. Moreover, the effects were investigated of nonuniform sediment transport and bed-material coarsening on P transport in the MYR, based on the proposed integrated model. Obtained results show that the TP transport process in the MYR was more reasonable simulated using the nonuniform sediment mode, and it is also confirmed that the process of bed-material coarsening after the TGP operation would lead to the decrease of particulate phosphorus flux in the MYR.

Integrated modeling of 2D urban surface and 1D sewer hydrodynamic processes and flood risk assessment of people and vehicles.

In order to accurately simulate the whole urban flooding processes and assess the flood risks to people and vehicles in floodwaters, a 2D-surface and a 1D-sewer integrated hydrodynamic model was proposed in this study, with the module of flood risk assessment of people and vehicles being included. The proposed model was firstly validated by a dual-drainage laboratory experiment on the flood inundation process over a typical urban street, and the relative importance of model parameters and model uncertainties were evaluated using the GSA-GLUE method. Then the model was applied to simulate an actual urban flooding process that occurred in Glasgow, UK, with the influence of the sewer drainage system on flood inundation processes and hazard degree distributions of people and vehicles being comprehensively discussed. The following conclusions are drawn from this study: (i) The proposed model has a high degree of accuracy with the NSE values of key hydraulic variables greater than 0.8 and the GSA indicates that Manning roughness coefficients for surface and sewer flows, inlet weir and orifice discharge coefficients, are the most relevant parameters to influence the simulated results; (ii) vehicles are vulnerable to larger water depths while human stability is significantly influenced by higher flow velocities, with the overall flood risk of people being less than that of vehicles; and (iii) about 88.7% of the total inflow volume was drained to the sewer network, and the sewer drainage system greatly reduced the flood risks to people and vehicles except the local areas with large inundation water depths, where the sewer drainage increased the local flow velocity leading to higher flood risks especially for people.

A dynamic, convenient and accurate method for assessing the flood risk of people and vehicle.

With the increase of extreme rainstorm caused by climate change, and the development of urbanization and the improvement of people's living standard, there is an urgent need to draw a dynamic, convenient and accurate flood risk map for different disaster bearing bodies, so as to protect people's lives and properties, as well as improve people's risk awareness and facilitate people's lives. This study mainly researched a method of drawing a dynamic, convenient and accurate flood risk map for people and vehicle. In this study, the surface runoff is simulated by GPU (Graphics Processing Unit) Accelerated Surface Water Flow and Transport model (GAST model), meanwhile, the flood risk of people and vehicle is graded base on the incipient velocity formulas, the most unfavorable principle and grading method, finally, the method is applied in two application cases. The following results are obtained: (1) this method could assess the flood risk of people and vehicle dynamically, conveniently and accurately; (2) the flood risk of people is less than that of vehicle, for the same flood hazard, time and place; (3) the adverse effect of water depth on flood risk for vehicle is greater than that for people. This method of assessing the flood risk map of people and vehicle is of great significance, for flood risk management, land use plan and emergency management department to reduce flood disaster risk.

Urban construction and demolition waste and landfill failure in Shenzhen, China.

On December 20, 2015 at 11:40 am a landslide in one of China's most advanced cities, Shenzhen, killed 73 people and damaged 33 buildings. In the absence of heavy rainfall or earthquakes, the landslide was an unexpected and profound shock to many people. According to China's Ministry of Land and Resources, the landslide was triggered by the collapse of an enormous pile of construction and demolition waste (CDW). With China's rapid urbanization, an increasing amount of CDW is being generated, especially in major cities. In total, China produces some 30% of the world's municipal solid waste and of this about 40% is CDW. To prevent landslides associated with CDW, the volume of waste dumped in landfills should be regulated. More specifically 4-Rs (reduce, reuse, recycle and recover) policies should be implemented more widely and efficiently. Although landfill will continue to be an important disposal option, proper management and careful monitoring of CDW are urgently needed to satisfy pressing safety issues. International collaboration, sharing of knowledge, and use of the latest technologies are needed so that the similar landslides can be prevented in China and elsewhere.

Dynamic channel adjustments in the Jingjiang Reach of the Middle Yangtze River.

Significant channel adjustments have occurred in the Jingjiang Reach of the Middle Yangtze River, because of the operation of the Three Gorges Project (TGP). The Jingjiang Reach is selected as the study area, covering the Upper Jingjiang Reach (UJR) and Lower Jingjiang Reach (LJR). The reach-scale bankfull channel dimensions in the study reach were calculated annually from 2002 to 2013 by means of a reach-averaged approach and surveyed post-flood profiles at 171 sections. We find from the calculated results that: the reach-scale bankfull widths changed slightly in the UJR and LJR, with the corresponding depths increasing by 1.6 m and 1.0 m; the channel adjustments occurred mainly with respect to bankfull depth because of the construction of large-scale bank revetment works, although there were significant bank erosion processes in local regions without the bank protection engineering. The reach-scale bankfull dimensions in the UJR and LJR generally responded to the previous five-year average fluvial erosion intensity during flood seasons, with higher correlations being obtained for the depth and cross-sectional area. It is concluded that these dynamic adjustments of the channel geometry are a direct result of recent human activities such as the TGP operation.