Abstract: The recorded runoff of the major rivers in China has been changing due to environment change. It is of significance to quantitatively analyze attribution of runoff change for supporting water resources assessment and management in many practices. Based on abrupt change detection of annual runoff series from 1956—2018 measured at the 8 representative hydrometric stations on major rivers in China, impacts of climate change and human activities on runoff changes were investigated by using hydrological simulation approach. Results show that (1) The abrupt change of annual runoff series recorded at the Wujiadu station on the Huai River, Datong station on the Yangtze River and the Wuzhou staion on the Pearl River are not detectable. However, the abrupt changes have been found for the recorded annual runoff series of the major rivers in North China with the earliest and the latest abrupt change years occurring in 1965 for the Tieling station on the Liao River and in 2003 for the Haerbin station on the Songhua River, respectively. The relationships between runoff and precipitation for periods before and after abrupt change year were markedly changed probably due to human activities. (2) RCCC-WBM model not only performs well on monthly runoff simulation for humid South China, but also achieves good simulation result for major rivers in arid North China. The model has been proved being qualified to naturalize runoff in the human-disturbed periods for major rivers in China. (3) In general, human-induced runoff changes are higher than the climatic impacts for major rivers in the North China, while climate change is a dominant driver of runoff variation for the Hai River and major rivers of China in its south.
Abstract: Robust calibration of physically based hydrological models is essential for the quantification of runoff components in snow and glacier melt runoff fed basins. This study evaluates a hydrograph partitioning curve (HPC) based calibration technique utilizing seasonality of precipitation and temperature in the Yarlung Tsangpo River basin. The HPC indicate the separating periods when various runoff components, including baseflow, snowmelt and glacier melt runoff, and rainfall direct runoff, dominate the basin hydrograph. Parameters of the THREW distributed hydrological model are grouped into four categories according to their controls on the runoff processes, and subsequently calibrated in a stepwise procedure using extracted HPC. The HPC-based calibration method is evaluated against traditional methods on the basis of robustness, performance of discharge simulations in sub-basins, and estimates of snow water equivalent (SWE) across the whole basin. Results show that:① The HPC-based calibration method provides results comparable to traditional methods in the calibration period while improving the discharge simulation for the evaluation period relative to single objective calibration methods; ② The HPC-based calibration method shows superiority in producing robust sub-basin discharge and tends to estimate smaller bias for the basin SWE; ③ The HPC-based calibration method estimates the contributions of snowmelt, glacier-melt, and rainfall direct runoff to discharge during 2001—2015 as 20%, 14% and 66%, respectively, while the traditional calibration methods yield a higher contribution for glacier-melt runoff and lower contribution for snowmelt runoff. Our findings indicate the potential of the HPC-based calibration method as a tool to quantify the contribution of runoff components, thus improving the modeling of hydrological behaviors under changing climate conditions for similar basins.
Abstract: The flood discharge capacity of an alluvial river is related to its channel geometry, which also depends on the upstream flow and sediment regime. It is important to analyze the relationship between flood discharge capacity and channel geometry and flow and sediment regime, in order to investigate the channel evolution of the braided reach in the Lower Yellow River. The relationship was developed between bankfull discharge and flow-sediment regime, represented by incoming sediment coefficient and fluvial erosion intensity, and the channel geometry, represented by the pre-flood hydraulic geometry from 1986 to 2015 in the braided reach at section- and reach- scales. Results indicate that:① Serious deposition and main channel shrinkage occurred in the study reach from 1986 to 1999, with the flood discharge capacity declining rapidly, and the main channel became narrower and deeper continuously owing to significant channel degradation after the operation of the Xiaolangdi Reservoir, with the flood discharge capacity recovering gradually from 1999 to 2015;② Empirical formulas (power functions) were developed between section- and reach- scale bankfull discharges and the integrated effects of flow-sediment regime and channel geometry respectively, whose correlations are greater than 0.50, but the correlation coefficient for the reach scale improved by 17% at least, as compared with the section scale; and ③ A higher correlation (0.94) was obtained between the reach-scale bankfull discharge and the previous five-year average fluvial erosion intensity during flood seasons and the post-flood hydraulic geometry in the last year, and the corresponding formula can well reflect the variation in bankfull discharge of this study reach, which also provides a method to analyze the variation in bankfull discharge of other reaches.
Abstract: Variations of inlet water flow and riverbed boundary conditions are the dominant factors that precipitate alternating major-minor branches in anabranched rivers, which have different sensitivities to different subtypes. Distinguishing the main factors affecting the alternating major-minor branches of different anabranched reaches is needed to determine/understand the fluvial evolution. Taking the anabranched reaches in the middle of the Yangtze River as reference, 30 generalized schemes are established for the inlet water flow and riverbed perimeter variations. A depth-integrated two-dimensional mathematical model is adopted to explore the effects of the aforementioned factors on major-minor branch alternation. By numerical simulation, a critical branch length ratio of 1.5 is obtained. Hydrodynamic axis shifting is the dominant factor affecting major-minor branches alternation when the branch length ratio is less than 1.5;otherwise, the dominant factor will become the resistance difference between branches, and effects of inlet hydrodynamic conditions on the diversion patterns will weaken significantly.
Abstract: During flood period, the bed load deposited behind a run-of-river dam on a mountain river could be carried over the dam crest by the flow which would intensify sediment transport. In this study, flume experiments are conducted to reveal the transport process of bed load from upstream to downstream after starting, advancing, and then over the top of a dam. This study analyzes the variation and mathematics of sediment transport parameters; describes the over-dam sediment transport model and its movement characteristics; and clarifies the natural relationship between sediment transport rule and riverbed morphology. In general, the findings of this study are:① The relationship between the volume of sediment transport and time can be expressed using a power function; ② The morphology of riverbed around the dam will stabilize, and form stable curved slope deposits in the upstream and downstream; ③ The over-dam transport modes of bed load are distinct under different flow intensities. For general flow intensities, sediment particles roll or slide on the curved slope surface of the upstream sediment body at the beginning. When they are close to the dam, they are carried over the dam crest by the flow and transported to the downstream. For higher flow intensities, the later over-dam transport mode could change significantly. Specifically, the periodic edge-wall vortex is the key driver of over-dam sediment transport.
Abstract: The amount of sediment discharge into the middle and lower reaches of the Yangtze River continues to decrease under the influence of multiple factors including sediment interception by reservoirs and soil and water conservation. The river channel in the middle and lower reaches of the Yangtze River is subject to long-distance and long-term scouring and siltation adjustments. The storage and discharge capacity of the river channel changes accordingly, which influences flood control in the middle and lower reaches of the Yangtze River. The changes of flood control operation of upstream reservoirs and excessive flood volume in the middle and lower reaches encountering the flood of 1954 were simulated based on the prediction of the fluvial process in the middle and lower reaches of the Yangtze River for the next decade. With the further degradation of the river channel in the middle and lower reaches of the Yangtze River, the river channel storage and discharge capacity increased. The Three Gorges Reservoir can reduce the discharge flow during flood control operations and decrease the total flood interception volume. The total excessive flood volume in the middle and lower reaches of the Yangtze River decreased, but some excessive flood volume shifted from upstream to downstream in the spatial distribution.
Abstract: As the dominant submerged macrophytes in Poyang Lake, the winter buds of Vallisneria natans provide food for wintering waterbirds. In order to quantify the effects of water level fluctuation on the habitat area of Vallisneria natans in Poyang Lake, a numerical model was developed by integrating an EFDC hydrodynamic model and habitat suitability curves. The habitat of Vallisneria natans in Poyang Lake was continuously simulated since the 175 m impoundment of the Three Gorges Reservoir in 2008. The quantitative response functions between potential habitat area and water level at Xingzi gauge station were developed for two habitat criteria:① suitable habitat based on suitability curves and ② regions with water depth≤4 m where Vallisneria natans can grow. It was found that potential habitat area of Vallisneria natans is maximized when the water level at Xingzi gauge station is around 15 m, producing a suitable habitat area is about 1 703 km2 and an area with water depths≤4 m is about 2 336 km2. The impacts of Three Gorges Reservoir and proposed Poyang Lake Lock on the potential habitat area of Vallisneria natans were analyzed. The operation of Three Gorges Reservoir could prevent habitat area getting too small, which could guarantee habitat stability. However, the fluctuation of habitat area decreases significantly, with standard deviations of habitat area declining 27% and 47% for potential suitable habitat and regions with water depth≤4 m respectively compared to its pre-operation. For the proposed Poyang Lake lock, in order to decrease its negetive effect on Vallisneria natans habitat, the recommended operation water level during its refill period should be lower than 16 m, and the water level during the refill period of Three Gorges Reservoir should be higher than 13.5 m. This operation results in a reduction in suitable habitat area by less than 20% from the maximum (1 703 km2), and in a reduction in the area of regions with water depth≤4 m less than 10% from the maximum(2 336 km2). The quantitative influence of water level fluctuation on the potential habitat area of Vallisneria natans in Poyang Lake has been clarified, and the quantitative basis for the conservation of Poyang Lake ecosystem under the modified regime of flow and sediment was provided.
Abstract: The Ω caving in riverbanks is one of the typical bank erosion patterns occurred in alluvial rivers which develops quickly and increases the instability of dikes. The numerical simulation of this type of bank erosion is still a challenging task. The Zhinancun bank caving developed at the lower Yangtze River was investigated in this paper, and a 3-D numerical method for modeling the Ω caving in the riverbank was proposed based on the mechanism of bank erosion. The sediment transport capacity in the circumfluence zone was calculated by considering the influences of turbulence intensity. The simulated results indicated that the velocity close to the riverbed is larger than that in the upper layers in the circumfluence zone during the processes of riverbank collapse, which becomes the driving factors for the riverbed scouring and thus bank erosion. The determination of the sediment transport capacity has a great influence on the simulated shape of the bank which can be more accurately simulated by considering the effect of turbulence intensity. This study could improve our understanding of the mechanism of riverbank collapse.
Abstract: Based on the indoor generalized model, this paper considered the wide-and-narrow reach of Baoxing River as the research object; measured the three-dimensional instantaneous velocity based on the typical section of the indoor model using the acoustic Doppler velocimeter; and analyzed the distribution of longitudinal velocity, turbulent intensity, Reynolds shear stress, and turbulent kinetic energy based on the typical section to explore the flow turbulence characteristics of the river. The experimental results revealed that in the wide-and-narrow flume, the turbulence intensity on the side walls of the diffusion section was higher than that in the central region, and the maximum value was observed at 0.2 times the water depth; the maximum turbulent kinetic energy was generated at the foot of the slope on both sides of the diffusion section; the maximum values of the plane Reynolds shear stress and the vertical Reynolds shear stress on both sides of the wall appeared in the diffusion section, while the maximum Reynolds shear stress value of the central area was observed at the middle segment between the two grooves; and the vortexes generated on both sides of the diffusion section strengthened the lateral wall erosion and widened the channel. This study analyzed the turbulent characteristics of wide-and-narrow channels, and the analysis was beneficial for the mountainous river management and the prevention of natural disasters.
Abstract: The standard exponential and logarithmic flow resistance laws in shallow streams with gravel- and boulder- bed cannot be adapted. Field investigations suggest that the flow velocity near gravel bed is lower than that of the upper flow, similarly to the mixing layer flow. In this study, the newly mixing layer analogy is applied theoretically for deducing the flow resistance law by introducing the Reynolds number, Froude number, and shear velocity, which is respectively affected by flow regime, flow condition, and shear stress. Most field data located in 140 mountainous streams located at the Apennines in Italy are selected to calibrate and compared with other resistance laws of mountainous streams. The results show that the novel multivariable mixing layer law is characterized by the most closed-1 Nash-Sutcliffe efficiency index, lowest root mean square error and relative error. This mixing layer approach is based on an entirely different view of shallow gravel stream turbulence, which interprets the flow structure and improves the calculation accuracy.
Abstract: This study investigates the dissipation of curved flow by applying trajectory energy dissipation to avoid increasing harmful vibration. Based on experimental study, a new type of ski-jump energy dissipater is developed with a narrowing curved dentated miter bucket for a curved spillway with super-elevation. The dissipater is able to facilitate flow diversion under mass force that successfully promotes reduction of the flow range inducing backwater while extending the length of the jet flow. The hydraulic characteristics of flow diversion, backwater and jet flow observed when using the new dissipater with a sidewall with different radius of concavity are compared with those of a curved miter bucket. Additionally, a semi-theoretical and semi-empirical approach is presented to distinguish the occurrence of backwater in the new dissipater, according to the geometric characteristics of streamline and the flow characteristics. Experiments with the new dissipater reveal that contraction of the upper bucket and the radius of concavity of the sidewall play leading roles in the phenomena of flow diversion and backwater. Therefore, an expression is deduced to calculate the criterion γc predicting the occurrence of backwater, by the analysis the flow on the upper bucket via the deflecting angle of the folding flow γ conveyed through an empirical expression that combines the quantities of flow and concavity sidewall radius. The outcomes of the method generally matched the results derived from experiment. The findings of this study could be used as reference in the field of hydraulic engineering of curved flow.
Abstract: Understanding the mechanism of fluid flow and solute transport at the pore scale is of great importance for oil recovery, crop nutrient management and groundwater pollution restoration. This study employed lattice Boltzmann model combining with GPU parallel technology to investigate the porous media of computer-generated structures and synchrotron-based X-ray micro-CT scans of soil aggregates (resolution 3.7 μm). The key parameters of fluid flow and solute transport in the porous media were obtained, and the influence of spatial heterogeneity of porous media on hydraulic properties was explored by high-performance simulation (spatial nodes up to 64 000 000). By comparing the three groups of porous media with different structures, it was found that the permeabilities of the soil sample with the highest structural complexity and beads irregularly stacked are on the order of 100 mD (i.e. 10-13m2), which is much lower than that of the regularly stacked beads (>20 000 mD); The soil sample has a tortuosity of 1.40~1.60, which is significantly higher than that of the regularly stacked beads. Our results show that the porous media with high permeabilities have small degree of tortuosity, indicating that the permeabilities of porous media are related to the spatial heterogeneity of the structure. The permeability and tortuosity of soil aggregate are anisotropic. At given pressure gradient, the longitudinal diffusion coefficient is greater for a sample with higher permeability. The heterogeneity of the pore structure also affects the breakthrough curve. The method established in this work can simulate water flow and solute migration in real soil structure, and can be used to study the hydraulic characteristics of porous media at the pore scale.
Abstract: Hyporheic linkages between riparian zones and stream channels are a key component to riparian and stream system, which greatly influence the solutes transportation and transformation that are fundamental to sustaining biogeochemical cycling, biological community, ecological processes and habitat. The extent to which transportation and transformation processes occur is greatly dependent on the hyporheic residence time (HRT). However, our understanding of how this HRT mechanisms is still poorly understood. To explore the mechanism of HRT in riparian zones, we designed an experimental installment, a two-way flow riparian zone model, to evaluate the rate at which an active hyporheic flow developed. Using sodium chloride (NaCl) as conservative solute tracer, tracer experiment was conducted to explore compounding effects of meandering degree and vegetation density on HRT in riparian zone. As vegetation in riparian zones has the potential to fast the hyporheic exchange, it had resulted in the decreasing of the HRT with vegetation density increasing. But vegetation density had a critical value. When the vegetation density was larger than the critical value, the HRT no longer reduced. Over the entire riparian zone, the influence of morphology had apparent mirror-like effect on the distribution of HRT which depended on the position of meandering topography of riparian zone. The mean HRT of the upstream face was about 1/2 of that of downstream face. What's more, hydraulic, vegetation, morphology and soil features had been identified as the key influencing factors of HRT by using dimensional analysis and their sensitivities had been valued with the method of principal components analysis (PCA). Together, the meandering amplitude of topography and the vegetation space were the most sensitive parameters because their total contribution rate reached 91.07%. Under the action of the both factors, it appeared promotional effects on the HRT of the upstream face of meandering wave while there existed not only the promotional effects but also inhibition effects on the HRT of the downstream face of meandering wave.
Abstract: To study the influence of aquatic vegetation on flow structure under wind-driven waves, field experiments were conducted with two types of typical submerged vegetation (i.e., Vallisneria natans and Potamogeton malaianus) in Lake Taihu in China by measuring flow velocity inside and outside the vegetation patches. By decomposing the wave velocity and turbulent velocity from the spectra of the instantaneous velocity, the influence of vegetation on the distributions of time-averaged velocity, wave velocity and turbulent kinetic energy (ETK) was analyzed in this paper. For each case, a current in a direction consistent with the direction of near-surface wind occurred. The wave velocity was dominated by the vertical component and decreased from water surface to bed bottom. ETK reached its maximum near the water surface and decreased toward the bed bottom. Compared with bare bed, the presence of Vallisneria natans and Potamogeton malaianus decreased the time-averaged velocity, wave velocity, and ETK, and the differences in morphology between these two types of vegetation led to discrepancies in flow alteration. For Vallisneria natans, the canopy frontal area reached a maximum at the middle reach of plant, making the decrease of time-averaged and wave velocities largest near the middle of canopy. With blades concentrated at the top of canopy, the reduction in the time-averaged and wave velocities caused by Potamogeton malaianus was largest near the water surface.
Abstract: As a complex three-phase particle flow and the main carrier of non-point pollution, the coordinated transport processes and environmental effects of the runoff-sediment- pollutant systems on urban surfaces are very complicated. In this paper, the existing research on the transports of runoff-sediment-pollutant systems is summarized and analyzed from the perspectives of, the spatial-temporal distribution of urban surface sediment with absorbed pollutants, the hydrodynamic characteristics and simulation methods of urban surface rainfall-runoff, as well as the coordinated transport and reduction effects of runoff-sediment-pollutant systems. It is considered that the sediment dynamics theory can be used to study the urban surface sediment movement law during the rainfall-runoff process, and that the sediment deposition plays an important role in the reduction of particle pollutants due to the absorption effect. Finally, the following research actions should be carried out in the future: enlarging urban sample size; studying on dynamic characteristics of overland flow on different underlying surface; studying on incipient motion law for non-uniform sediment on urban surfaces, as well as sediment transport capacity for overland flow based on flow power theory.
Abstract: Earth observation satellite remote sensing can provide reliable wide-ranging spatial information and is believed as one of the key technologies for flood risk identification and dynamic modelling. Aiming at illustrating how satellite remote sensing technology promotes flood studies, this paper reviewed the development and technical demands of flood risk identification and dynamic modelling research and analyzed the historical contribution and periodic utility of spatial remote sensing information in flood studies based on the 3-stage evolution trajectory of earth observation satellite remote sensing. Application progresses in 3 typical methods in flood studies, which are risk zoning, hydrological modelling and microwave remote sensing monitoring, respectively, have been discussed and summarized. Focuses of the future research on flood risk identification and dynamic modelling have also been pointed out, including the integration of remote sensing spatial information and model algorithms, the development and application of remote sensing algorithms and systems, the development and application of an integrated system for typical methods and the application of big data methodology and technology, in expectation of providing valuable references for improving the response capacity and risk management level of flood disasters.