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Water temperature in the Yangtze River mainstream has been experiencing significant changes due to the climate change and the operation of a series of world-class large dams, i.e. the Three Gorges Dam (TGD) and upstream cascade dams (CDs). However, quantitative effects of these factors are not fully known, which hinders our understanding on the thermal regime alterations and further prediction in ecosystem response. Here, we will simulate the riverine water temperature (RWT) variations by building a physics-based model, and quantify the respective impacts from TGD, CDs and climate change through a model-based framework. In the framework, both the dam-regulated hydro-thermodynamic processes and the spatial heterogeneity of the meteorological condition in this large river-reservoir system are thoroughly considered. The results show a fluvial warming of 0.31 °C–0.56 °C/10a in recent three decades, mainly attributed to climate change (44%–80% for different reaches). The dam has caused a substantial seasonal thermal lag, e.g. ∼40 d near the TGD in the dry season, and accompanying severe alterations in the monthly RWT. A reduction of 10% in seasonal RWT range is identified, which is attributed to both dam and climate change.

A comprehensive study on a 350-cm long sediment core revealed rich geochemical information in an interesting location of Xiangxi He (river) in TGR. The vertical profiles of trace metals (TMs) and nutrients displayed an irregular pattern, fluctuating around their background values, and no gradual increase was detectable. For TMs, average concentrations were Cd: 0.35 ± 0.21, Co: 21.91 ± 3.30, Cr: 27.06 ± 5.03, Cu: 27.09 ± 4.96, Ni: 34.08 ± 5.23, Pb: 20.58 ± 5.84 and Zn: 80.18 ± 69.16, all in μg/g (n = 61). For most of TMs, their bioavailable forms were only a minor fraction. Therefore, the conventional total metal concentrations will inevitably overestimate their potential risk to ecosystem. For nutrients, average concentrations were total phosphorus (TP): 0.81 ± 0.14, total nitrogen (TN): 1.46 ± 0.28, total sulfur (TS): 0.54 ± 0.19, total carbon (TC): 32.02 ± 8.35, total organic carbon (TOC): 20.90 ± 7.5, all in g·kg−1 (n = 61), and appeared somewhat higher than the “lowest effect level”, which seems to explain the repeated observations of algal blooms in this relatively stagnant section of the river in TGR. The mineralogical composition identified by XRD agreed well with that obtained by chemical analyses. The study revealed that the Xiangxi sediment core (XX06) was remarkably heterogeneous in mineralogical composition, probably caused by natural processes and the dam management. The increased sedimentation rate within TGR should induce a mass dilution effect, thus decreasing the concentrations of pollutants. Large sampling scale and a universally accepted and uniformed analytical sampling and method, together with more vigorous analytical quality control, must be practiced and a solid database be built to facilitate the ecosystem assessment of TGR area.

Permanently submerged sediment samples (SS) were collected in the center stream of eleven tributaries of Changjiang (Yangtze River) and at eight confluence zones in the Three Gorges Reservoir (TGR) in May and December of 2017. The work showed that aqua regia digestion is a simpler, more reliable and robust method compared to total digestion with hydrofluoric acid (HF) for the determination of trace metals (TMs) in sediment for risk assessment purpose. Our study revealed a remarkable accumulation of TMs at the confluence zones and a trend of their gradual increase toward this zone. The presence of iron and manganese (oxy)hydroxides combined with hydrodynamic conditions created by the Three Gorges Dam (TGD) and its operation are believed to play a crucial role. This work also found that concentrations of NH 4 + − N in May sediment were significantly higher than those in December, which could have been caused by both the cyclic hydrodynamic conditions and the warmer water. TOC and TP were both very low in the sediment. Although TN was 2 times higher than the Lowest Effect Level suggested by the Ontario Ministry of Environment, it is uncertain if it reflects a natural background level or due to anthropogenic activities. A critical discussion is made by comparing the conclusions obtained when using different TMs risk assessment models. Necessary precautions are highly recommended when performing this exercise. In this study, no significant risk from either TMs or nutrients was identified.

In this study fecal bacteria processes have been investigated using the EFDC 2D/3D model, based on local refinements using an orthogonal curvilinear grid system and with the model being applied to Ribble river basin, through the estuary, and into the Fylde Coast. The input fluxes from numerous minor rivers and streams within the basin were solved using a grid‐based distributed hydrological model (GDHM) and a river network 1D model (RNM1D), developed by the authors. The detailed measured hydrodynamic data, included tide levels and nearshore ADCP 3D velocity field data at four sites and with data being recorded over a 2‐week period. During this period continuous monitoring using four types of tracers was also undertaken across the Ribble basin, which included sampling for E. coli (EC) and Intestinal Enterococci (IE) parameters. Monitoring devices, in the form of buoys, were also used and moved with currents, driven by upstream discharges, tides, waves and wind. The results showed: (i) the grid system gave a detailed topographical representation of the transition zones from the river system to the estuary and coast, with the hydrodynamic and related solute transport processes being well represented; and (ii) the model predicted results fit generally well with the water stage, 3D flow velocity profiles (with some errors in the bottom and surface layers), and E. coli concentrations. The tracer paths from the injection sites were simulated using a Lagrangian particle tracking method, which showed that the tracer from the north bank outfalls and the Ribble river could propagate to the highly popular bathing beaches at Blackpool, particularly under the action of South Westerly winds. More detailed solutions and refinements (e.g., wave driven by the wind and density flows caused by different salinities, temperatures, and suspended sediment concentrations) need to be considered in the next stage of this study.

With the climate change caused by global warming, the occurrence probability of urban flooding due to flash floods has increased gradually. Flush floods propagate rapidly, which can lead to significant hazard to human life and property. Flood hazard to vehicles has become more noticeable and frequent in recent years. Therefore, it is important to investigate the behaviour of vehicles on flooded streets or roads. In the current study, a formula has been derived to predict the incipient velocity of flooded vehicles according to the mechanical condition of sliding equilibrium. A series of flume experiments were conducted using three types of scaled die-cast model vehicles, with two scales being tested for each type of vehicle. The experimental data obtained for the small-scale model vehicles were used to determine the two parameters in the derived formula, and the prediction accuracy of this formula was validated using the experimental data obtained for the large-scale model vehicles. Finally, the corresponding incipient velocities under various incoming depths were computed using this formula for these three prototype vehicles. It is found that for a specified vehicle, the value of incipient velocity reaches its minimum as the incoming flow depth approaches the height of the vehicle, and the smaller and lighter vehicle like Mini Cooper is the easiest to start sliding in floodwaters. The results can be used as a preliminary assessment to define the hazard to vehicles parking on flooded streets or roads.

The Yarlung Tsangpo River is one of the highest major rivers in the world. The river is known for its pristine landscape. However, in recent years, increasing human activities, such as inhabitation, afforestation, and infrastructure projects, have significantly disturbed this fluvial system, while their impacts are not fully known. In this study, the water and sediment transport processes in the Nugesha–Yangcun (N–Y) reach of the Yarlung Tsangpo River, as well as the impact of human activity, are investigated. The N–Y sub-catchment consists of two parts, i.e., the Lhasa River catchment and the mainstream catchment. Riverine discharge, suspended sediment concentration (SSC), and precipitation data are acquired, and a detailed investigation is conducted. The water yield has not changed considerably in recent years, while the sediment yield has exhibited a sharp decline, from ~5 Mt to ~1 Mt. The sediment decrease is mainly caused by the reduced sediment source, which is considered highly related to afforestation. In addition, the dominant sediment contributor has changed from the mainstream catchment to the tributary catchment (while the sediment yield in the mainstream catchment has decreased to almost zero). An anomalously enhanced SSC occurred in the Lhasa River in two consecutive years from 2015, with the SSC value increasing sharply from 0.2 kg/m3 to 0.8 kg/m3, and maintaining a high level for approximately three months. This phenomenon is considered to be related to infrastructure projects in the same period, with the SSC recovered after road construction ended. The increasing human activities have had significant impacts on the sediment regimes in the Yarlung Tsangpo River; hence, more attention should be paid to river basin management.

Numerical models provide considerable assistance in the investigation of complicated processes in natural rivers. In the present study, a physics-based two-dimensional model has been developed to simulate the braiding processes and morphodynamic changes in braided rivers. The model applies the basic hydrodynamic and sediment transport principles with bed morphology deformation and a TVD (Total Variation Diminishing) scheme to predict trans-critical flows and bed morphology deformation. The non-equilibrium transport process of graded bed load sediment is simulated, with non-uniform sediments, secondary flows, and sheltering effects being included. A multiple bed layer technique is adopted to represent the vertical sediment sorting process. The model has been applied to simulate the bed evolution process in an experimental river with bed load transport. Comparisons between the experimental river and predicted river are analysed, including their pattern evolution processes, important braiding phenomena, and statistical characteristics. Avulsion activities have been found in the braiding evolution process, representing the primary ways in which channels form and disappear in braided rivers. The increases in the active braiding intensity and total braiding intensity show similar trends to those observed in the experimental river. Statistical methods are applied to assess the scale-invariant topographic properties of the simulated river and real rivers. The model demonstrated its potential to predict the morphodynamics in natural rivers.

Due to the increasing economic and cultural value of bathing waters and the shellfish industry in the UK and worldwide, water quality in estuarine and coastal waters has attracted considerable public attention in recent years. To obtain accurate predictions of the concentration distributions of faecal indicator organisms (FIOs) in coastal waters for better management of bathing water compliance, it is necessary to build an integrated modelling system to predict the complete diffuse and point source inputs from river and catchment basins. In the present paper, details are given of the development of such an integrated modelling system for simulating the transport and decay processes of FIOs, from catchment areas upstream from the coastal region, in which a distributed catchment module, a 1D river network module and a 2D estuarine and coastal module are linked dynamically by boundary inputs and outputs. Extensive measured data from the catchments, river networks and estuaries have been collated to determine the model parameters. Verification results of the distribution of water levels, flows and velocities, and suspended sediment and Escherichia coli concentrations, at controlled monitoring sites are presented, which show that the integrated model predictions generally agree well with the measurements, although locally appreciable errors can occur. The model results also highlight the importance of including the flux of FIOs via sediments being an important factor in terms of assessing the quality of bathing waters. The main factors influencing the relatively high concentration values in the bathing region are analysed, based on the model predictions and measured data, with four categories of FIO concentration levels being reviewed.

Details are given in the paper of the refinement of a three-dimensional layer integrated turbulence model and its application to a scaled physical model of a reservoir, named Hamidieh Reservoir, in Iran. The strong turbulent flows generated in this reservoir are due to the high volumes of flow diversion, with low head differences. In this paper, a refined numerical model is applied to a reservoir, associated with a dam, water intakes and sluice gates, with the aim being to investigate the flow patterns and sediment transport processes in the vicinity of such hydraulic structures. The calibration of the model is undertaken using measurements made from a scaled physical model. The numerical model is able to predict the conditions for a range of scenarios that are difficult to test in the physical model. Different scenarios are introduced to investigate the effects of various intake and sluice gate configurations, as well as their operation schemes on the flow and sediment transport processes into Hamidieh Reservoir. The results indicate that the flow velocity field in the vicinity of one of the intakes varies significantly. Moreover, the sluice gates do not appear to have any considerable effect on the suspended sediment concentrations moving through the intakes.

With a growing number of waterlogging events occurring in large cities, a better understanding and prediction of the urban flooding processes are essential in order to reduce their impacts. A two-dimensional hydrodynamic model was used to simulate a surface water flooding process due to a heavy rainstorm event in the urban area of Beijing. Through incorporating road networks over the large area, an enhanced digital elevation model was presented to describe overland flow through surface pathways and waterlogging forming along the main roads. Rainfall data obtained from a large number of rain gauge stations in Beijing City throughout the whole storm event were acquired. The model was capable of reproducing the hydrodynamic process at the city scale, and it was found that surface runoff generated quickly from the surrounding community areas to roads and accumulated in depression areas. Waterlogging sites reported in the media were extracted, with most of the sites being on roads and predicted reasonably. Waterlogging issues were found generally to be a local feature and closely associated with rainfall intensity.