The energy budget under the influence of topography in the Zhujiang River Estuary in China

The Zhujiang River （Pearl River） Estuary （ZRE） is a very complicated and large-scale estuarine system in China. It consists of two parts： the river networks and the estuarine bays. Not only is the network system one of the most complicated in the world, but also each estuarine bay has a very special morphodynamic feature due to the geological settings. Morphological boundary conditions have direct effects on the energy dissipa- tion and balance. On the basis of a three-dimensional （3-D） barotropic model whose domain includes the river networks and the estuarine bays, the energy budget is discussed under the influence of topography in the ZRE. The elevation and discharge of this model are validated by the observations collected in July 1999 and February 2001. The results show that （1） the source of energy in the ZRE is mainly generated by tides and river runoffs, which have an obvious seasonal change, and （2） there are some typical hotspots where the energy dissipation is 1-2 orders higher than those in the immediate upstream and downstream sections in the ZRE. These hotspots are linked with the small-scale dynamic structures （SSDS） and morphological units. On the basis of the characteristics of the morphology and the energy dissipation, the hotspots can be catego- rized into three types： the outlet of the ZRE, the meandering river, the branch and junction.

Numerical Simulation of Seawater Intrusion from Estuary into River Using a Coupled Modeling System

One of the most important causes of the freshwater shortage in estuarine area is the increasing seawater intrusion into the river.To simulate seawater intrusion properly,two important factors should be considered.One is the bidirectional and time-dependent coupling effects between river discharges and tidal forces.The other is the three-dimensional and stratified structure of dynamic processes involved.However,these two factors have rarely been investigated simultaneously,or they were often simplified in previous researches,especially for the estuary connected with an upstream river network through multiple outlets such as the Pearl River Estuary(PRE).In order to consider these two factors,a numerical modeling system,which couples a one-dimensional river network model with a three-dimensional unstructured-grid Finite-Volume Coastal Ocean Model(FVCOM),has been developed and successfully applied to the simulation of seawater intrusion into rivers emptying into the PRE.By treating the river network with a one-dimensional model,computational efficiency has been improved.With coupling 1D and 3D models,the specification of upstream boundary conditions becomes more convenient.Simulated results are compared with field measured data.Good agreement indicates that the modeling system may correctly capture the physical processes of seawater intrusion into rivers.

Evaluation and attribution of trends in compound dry-hot events for major river basins in China

Concurrent compound dry and hot events(CDHEs)amplified more damange on the ecosystems and human society than individual extremes.Under climate change,compound dry and hot events become more frequent on a global scale.This paper proposes a mathematical method to quantitatively attribute changes of CDHEs to changes of precipitation,change in temperature and change in the dependence between precipitation and temperature.The attribution is achieved by formulating the total differential equation of the return period of CDHEs among Meta-gaussian model.A case study of China is devised based on monthly precipitation and temperature data during the period from 1921 to 2020 for 80 major river basins.It is found that temperature is the main driving factor of increases in CDHEs for 49 major river basins in China,except for the upper and middle reaches of the Yangtze River.In West China,precipitation changes drove the increase in CDHEs in 18 river basins(23%),particularly in parts of North Xinjiang,Qinghai and Gansu.On the other hand,dependence between precipitation and temperature dominated changes of CDHEs in 13 river basins(16%)of China with other factors,including parts of South China,East China and Northwestern China.Furthermore,changes in both the mean and spread of precipitation and temperature can also contribute to changes in CDHEs.

Flume simulation of wave-induced release of internal dissolved nitrogen in Taihu Lake,China

A wave flume simulator was used to study internal nitrogen release from the surface sediment collected from Taihu Lake,China.Particulate nitrogen concentrations were positively correlated with the concentrations of suspended solids,primarily from surface erosion related to the shear stress and duration of wave action.In response to 4 cm-and 10 cm-high wave production representing waves generated in Taihu Lake by gentle and gusty winds,respectively,the mean dynamic release rate of ammonium(NH+4) from the sediment to the overlying water was 1×10-3 mg/(m2.s) and the NH4+ concentration in the overlying water increased by 0.016 mg/L,indicating that waves resulting from strong wind can induce the rapid release of dissolved nitrogen from Taihu Lake sediments.The decrease in interstitial NH+4 concentrations at all sediment depths was associated with an increase in NH4+ concentrations in the overlying water by 0.01 mg/L,showing that sediment below the eroded layer was the main source of internal nitrogen release.Changes in the interstitial dissolved oxygen and NH+4 concentrations showed that wave-induced pore water movement can greatly increase the diffusion rate,and that these effects can influence the sediment to a depth of at least 15 cm.Diffusion induced by pore water movement may be very important for the formation of an active sediment layer in Taihu Lake.

Flume experimental study on evolution of a mouth bar under interaction of floods and waves

Based on the characteristics of hydrodynamics and sediment transport in the bar area in the Modaomen Estuary,a flume experiment was performed to study the evolution of the longitudinal profile of the mouth bar.The mouth bar evolution was investigated under the impacts of floods with different return periods as well as flood-wave interaction.The results showed that floods with different return periods had significant influences on the evolution of the river mouth bar.Particularly on the inner slope of the mouth bar,the sediment was substantially active and moveable.The inner slope and the bar crest tended to be remarkably scoured.The erosion was intensified with the increase of the magnitude of floods.Moreover,the bar crest moved seawards,while the elevation of the bar crest barely changed.Under the flood-wave interaction,a remarkable amount of erosion on the inner and outer slopes of the mouth bar was also found.The seaward displacement of the bar crest under the interaction of floods and waves was less than it was under only the impact of floods,while more deposition was found on the crest of the mouth bar in this case.

Dynamic analysis of impulse waves generated by the collapse of granular pillars

Impulse waves generated by the collapse of pillar-shaped rock masses in Three Gorges, China,have attracted the attention of both researchers and local authorities owing to their catastrophic consequences. In this work, particle imaging velocimetry(PIV) was used to study impulse waves generated by the collapse of granular pillars during a series of physical experiments. Subsequently, the scenes of particles collapsing into water and the resulting impulse waves were analysed in terms of the solid/fluid fields. The energy obtained by the water during this process is mainly derived from the volume encroachment and continuous thrusting of particles.As indicated by the experimental results, as the aspect ratio(a) of the pillar and water depth increased, the potential energy of the granular pillar became more prone to reduction, whereas the efficiency of energy conversion to the liquid phase reduced. At constant water depth and granular pillar width, the maximum amplitude generated by the collapse of the granular pillar remained essentially the same(i.e., "saturation"was achieved) once the aspect ratio exceeded a certain threshold. The maximum impulse wave(the primary wave) formed before the main body of particles collapsed, resulting in the "saturation" of the maximum amplitude. When the kinetic energy of the particles reaches the maximum, the ratio of energy dissipation of the particles is the lowest;as the energy of water reaches the maximum, the particle collapse process does not end. The dynamic analysis of the impulse waves generated by the collapse of granular pillars provides a new approach to obtain an in-depth understanding of landslides and impulse waves. This can provide technical guidelines for disaster prevention and mitigation of impulse waves generated by bank collapse or coastline collapse.

MULTIPLE TIME SCALE ANALYSIS OF CLIMATE VARIATION IN Macao DURING THE LAST 100 YEARS

The multiple time scale climate changes are studied and calculated with statistical analysis and wavelet transformation on the basis of daily series of observed data over the period 1901-2007 in Macao.The result shows that statistically significant oscillations with 2 to 5 years of period generally exist in the series of climate variables(e.g.annual mean surface air temperature and precipitation as well as evaporation etc.),but with obvious locality in time domain.The variation of annual mean surface air temperature has a quasi 60-year period.The phases of the 60-year variation approximately and consistently match that of Atlantic Multidecadal Oscillation(AMO).The oscillations of seasonal mean surface air temperature in summer and winter have the periods of quasi 30-year and quasi 60-year,respectively.These two periods of oscillations have statistically significant correlation with Pacific decadal oscillation(PDO) and AMO,individually.The multidecadal variations of the precipitation of the annually first flood period and annual evaporation are dominated by periods of quasi 30-year and quasi 50-year,respectively.

THREE-DIMENSIONAL NUMERICAL MODEL FOR WINDING TIDAL RIVER WITH BRANCHES

Natural rivers are usually winding with branches and shoals, which are difficult to be simulated with rectangular grids. A 3-D current numerical model was established based on the orthogonal curvilinear coordinate system and vertical o coordinate system. The equations were discretisized using a semi-implicit scheme. The ＂predictor＂ and ＂corrector＂ steps were applied for the horizontal momentum equations to meet the basic requirement that the depth-integrated currents obtained from the equations for 2-D and 3-D modes have identical values. And a modification of traditional method of dry/wet discriminance was proposed to determine accurately the boundary and ensure the continuity of variable boundary in the simulation. This model was verified with the data measured in a winding tidal river with branches in April, 2004. The simulated data of water levels and velocities agree well with the measured ones, and the computed results reveal well the practical flow characteristics, including the vertical secondary flow in a winding reach.

FLOW STRUCTURE AND SEDIMENT TRANSPORT WITH IMPACTS OF AQUATIC VEGETATION

Aquatic vegetation plays an important role in the flow structure of open channels and thus changes the fate and the transport of sediment. This article proposes a three-dimensional turbulence model by introducing vegetation density and drag force into the control equations of water flow in the presence of vegetation. The model was used to calculate the impacts of submerged vegetation on the vertical profiles of longitudinal flow velocities, the changes of the depth-averaged flow velocities in a compound channel with emergent vegetation in the floodplain, the removal of suspended sediment from the channels by emergent vegetation, and the bed changes around and in a vegetated island. Numerical investigations show that aquatic vegetation retards flow in the vegetation zone, reduces the sediment transport capacity, and contributes to erosion on both sides of the vegetated island. Calculated results agree well with experimental results.

Simulation of Transport Channel in China's Middle Route South-to-North Water Transfer Project

The unsteady flow in the Middle Route South-to-North Water Transfer Channel was simulated numerically using an implicit solution procedure for the Saint Venant equations. An equivalent roughness was used to simulate the effect of many transfer structures on the water levels in the main channel. Various gate operating and control methods were analyzed to study the response to disturbances produced by varying the flow rates through the Tianjin outlet. The results show that when the inflow at the head changes in the same way as the sum of the flow rates through all the outlets, the transition time and the fluctuation of the water levels using the timed gate operation method are less than when using the simultaneous gate operation method, but the variations of the gate openings and flow rates through each control gate are much larger. The flow disturbances produced by the Tianjin outlet can be rectified within several channel sections and the transition time can be greatly shortened by allowing the water levels immediately upstream of the control gates to vary within proscribed ranges, rather than being held constant.

A three-dimensional solid-liquid two-phase turbulence model with the effect of vegetation in non-orthogonal curvilinear coordinates

A three-dimensional k-ε-Ap solid-liquid two-phase two-fluid model with the effect of vegetation is solved numerically with a finite-volume method on an adaptive grid to study water-sediment movements and bed evolution in vegetated channels. The additional drag force and additional turbulence generation due to vegetation are added to the relevant control equations for simulating the interaction between vegetation and flow. The flow structure and the bed-topography changes in a 60° partly vegetated channel bend are calculated by the model. The numerical results agree well with the measured ones. Calculated and measured results show that the primary flow velocity reduces much in the vegetation zone and increases in the non-vegetation zone, the secondary flow velocity weakens in the vegetation zone and strengthens in the non-vegetation zone, the sediment movement and bed-topography change also weaken in the vegetation zone and strengthen in the non-vegetation zone, a well-planed vegetation arrangement can improve bank stabilization program, and the k-ε-Ap model can deal with bed-load transport with a more reasonable method than the one-fluid model.

Sewage sludge ditch for recovering heavy metals can improve crop yield and soil environmental quality

The treatment and disposal of municipal sewage sludge(MSS)is an urgent problem to be resolved in many countries.Safely using the nutrients within MSS to increase crop yield and enhance the fertility of poor soil could contribute to achieving sustainable development.An indirect use of MSS in ditches alongside Pennisetum hybridum plants was studied in field plots for 30 months and the contents of heavy metals and macronutrients were monitored in soil,sludge and plant samples.We found that the yield of P.hybridum was significantly increased by 2.39 to 2.80 times and the treated plants had higher N content compared with no sludge.In addition,the organic matter(OM)and N contents in the planted soil increased significantly compared with the initial soil.The OM content in the planted soil of the MSS treatment was 2.9 to 5.2 times higher than that with no sludge,and N increased by 2.0 to 3.8 times.However,MSS had no significant effect on the N,P and K contents in the soil at the bottom of the MSS ditch,and the content of heavy metals(Cd,Pb,Cu and Zn)were also within the safe range.Moreover,the moisture content and phytotoxicity of MSS after this indirect use were reduced and the heavy metal contents changed little,which is favorable to the further disposal of recovered MSS.Therefore,this indirect use of MSS is beneficial to agricultural production,soil quality and environmental sustainability.