Abstract: Sediment transport capacity of overland flow, which is the maximum equilibrium sediment load that a flow can transport, is a key concept for developing process-based soil erosion models because it plays a pivotal role to determine soil detachment rate and sediment transport. Few studies have been conducted to investigate the mechanism of sediment transport capacity by overland flow on steep slopes. In this paper, based on the systematic definition of sediment transport capacity of overland flow, the influences and their mechanisms of hydraulic properties of overland flow and sediment characteristics on sediment transport capacity were deeply analyzed. Sediment transport capacity of overland flow has great spatial and temporal variations due to the comprehensive effects of precipitation, topography, soil, vegetation, land use, sealing formation, rill development, and gravel covering. Therefore, it is very difficult to measure sediment transport capacity of overland flow directly in field conditions. The commonly used methods to determine sediment transport capacity of overland flow can be categorized into the modification from river sediment transport capacity formula, the rational method, the model simulation, and the direct measurement. The advantages and disadvantages of each method were compared and analyzed. Further studies should be focus on the measuring methodology and techniques for sediment transport capacity of overland flow, the hydro-dynamic mechanism of sediment transport process, sediment transport capacities of typical soil types, transport capacity formula development and performance evaluation, and sediment transport capacity of overland flow under changeable conditions. It is helpful to summarize the conclusions of different studies, promote the research on sediment transport capacity of overland flow, and improve the prediction precision of the process-based soil erosion models for better service to water and soil conservation planning and design.
Abstract: A new evapotranspiration data assimilation system was proposed by using the ensemble Kalman filter to assimilate the remote sensing evapotranspiration and simulated evapotranspiration by the Xin'anjiang model. The assimilated evapotranspiration was then used to estimate the soil moisture content in Xin'anjiang model via Particle Swarm Optimization (PSO) to improve the accuracy of runoff simulation. The Hanjiang River basin in China was used as a case study. The remote sensing evapotranspiration (ETSEBS) based on the surface energy balance system model (SEBS) was validated by the water balance evapotranspiration (ETGRACE) calculated through the water balance equation based on the GRACE water storage anomaly data, and compared with the other evapotranspiration productions (i.e., ETGLDAS, ETZhang and ETMODIS). The results showed that ETSEBS outperformed the other evapotranspiration productions when ETGRACE was used as the reference evapotranspiration, with three statistical criterion (R, ERMSand B values of 0.93, 11.93 mm/month and -3.47 mm/month, respectively. Furthermore, the proposed assimilation system was applied to the Xunhe River basin, a tributary of Hanjiang River. The results indicated that during the period 2005-2007, the Nash-Sutcliffe efficiency coefficient (ENS) was 0.85, which was higher than the ENS value of 0.81 without assimilation, and the evapotranspiration assimilation system improved the accuracy for runoff simulation with a slightly improvement during the drought period and a remarkable improvement during wet period, particularly for the peak values.
Abstract: To solve the problem that there exists large bias between simulated precipitation from global climate models (GCMs) and measured precipitation, a bias correction method based on separated three gamma distributions (STG) was proposed. This method divides the precipitation sequence into three parts, i.e., the minimum values, the normal values and the maximum values, and then the cumulative distribution functions (CDFs) of these three parts are respectively bias corrected. In addition, based on the thought of quantile deviation accumulation, a comprehensive evaluation index C is also put forward to evaluate the bias correction effects of STG method. The STG method is applied to correct the bias between the simulated precipitation from 18 GCMs and measured precipitation for 17 meteorological stations in Yarlung Tsangpo River basin, and is further compared with the existing two gamma-related bias correction methods, i.e., Single Gamma (SG) and Double Gamma (DG). The results show that the STG method can effectively remove the bias between GCMs simulated precipitation and observed precipitation, and the evaluation indexes C are mostly above 0.85. Moreover, STG performes better than SG and DG in the validation period which indicates STG is more robust for bias correcting GCMs simulated precipitation.
Abstract: The empirical power law relationship model and non-equilibrium sediment transport theory are two different methods used to describe the suspended sediment transport in alluvial rivers. The mathematical comparison results show that, for steady uniform flow, the derivatives of sediment concentration of the two above mentioned methods are first order approximate when sediment transport state is near equilibrium condition, and the variation of the saturation recovery coefficient is considered in power law relationship when sediment transport state is far away from the equilibrium condition. Based on the equivalence property, the calculation formula of exponent in power law function is established, showing that the value of exponent varies with the saturation recovery coefficient, sediment fall velocity, unit width discharge and longitudinal distance. Based on the derivative of power law function for the structural features of sediment concentration, the calculation expression of saturation recovery coefficient which is far away from the equilibrium condition is extracted and expressed. Finally, a new power law relationship model with changing exponent is proposed and applied to the sand basin. The application result revealed that the computed suspended sediment concentration along the distance of the sand basin is in suitable agreement with the measured values.
Abstract: To explore the evolvement characteristics and driving mechanism of a anabranching river channel under the influence of a water control project, this study classifies the branching river channel on the basis of the dynamic split ratio and measured hydrology and topography data for the lower reach of the Three Gorges Reservoir. The regulation rules of the primary branching river channel, secondary branching river channel, and low river island and their effects on the hydrological process are also investigated. The results reveal the following points. First, when the Three Gorges Reservoir is impounding, the low river island of the anabranching river channel shrinks. The branching river channel shows the specific rule of coexistence of "primary branching river channel goes up and secondary branching river channel goes down" and "primary branching river channel goes down and secondary branching river channel goes up." Second, on the basis of the dynamic split ratio of the classification of the branching river channel, these evolutionary differences are summarized as follows:the development rate of the dry-water-prone branching river channel is greater than that of the flood-prone branching river channel, and the flood-prone branching river channel has basically been characterized by siltation since 2008. Third, the decrease in flood and the continuous increase in the medium flow are the driving factors for the adjustment of channel. The erosion and sedimentation of the branching river channel show a stable pattern of the primary and secondary branching river channels, which is compatible with the hydrological process of decreasing the coefficient of runoff variation.
Abstract: Flocculation experiments on the effect of turbulent shear was conducted on the cohesive sediment collected in Huanghuacheng section of Zhongxian County along Three Gorges Reservoir. The experimental instruments included cylindrical settling column, flocculation separation chamber, and image collecting system. An approximately uniform turbulence flow was generated using oscillating grids in the cylindrical settling column, and the relation between the shear rate and the frequency of oscillating grids was calibrated using an ADV (Acoustic Doppler Velocimetry). The results showed that:① The apparent flocculation of cohesive sediment was observed, with the largest floc size Df,95 ranging 73-126 μm and the mean size Df,50ranging 18-33 μm, which were both strongly affected by the turbulent shear rate and the sediment concentration. ② Df,95demonstrated a trend of increase followed by decrease with the turbulent shear rate (from 3.84 s-1 to 30.07 s-1), with its peak occurring at the shear rate 19.94 s-1, when the percentage of large floc (larger than 96 μm in diameter) also reached the vertex. ③ Floc size distribution corresponding to different concentrations (0.3~1.0 g/L) at the same shear rate basically followed a tendency of Df,95 increases with the sediment concentration.
Abstract: Understanding key factors affecting sediment transport and the threshold of sediment transport in a heavy rainfall is crucial to preventing sediment disasters and reducing soil erosions. In this study, the small watershed of Hemingguan, a purple soil region in Sichuan Province, is chosen as a typical area for research. The methods of statistical classification and comparison are used to analyze sediment transport data in recent 10 years. The influence of sub-rainfall and rainfall intensity and duration on the sudden increase of sediment transport is studied. The key factors and the threshold leading to the sudden increase of sediment transport are clarified. The results indicate that, in a typical Sichuan purple soil area, key factors for a sudden increase in sediment transport are sub-rainfall, rainfall intensity and the maximum runoff modulus. The threshold for rainfall intensity is 5.3 mm/h. The threshold for rainfall is 130 mm, and the threshold for the maximum runoff modulus of slope land is 6 000 dm3/(km2·s), based on tillage. The threshold for the maximum runoff modulus of slope land is 3 000 dm3/(km2·s), based on planting grass and trees.
Abstract: Debris flow as non-Newtonian fluid has a large yield stress and moved unsteadily. Many models have been established to study the impact of erosion on the unsteady motion, but most of the models were studied the role of the bottom erosion and dam break on the debris flow motion. Less researches studied the role of gully bank erosion on the unsteady dynamic of debris flow. This paper analyzed the role of bank erosion on the debris flow dynamics through flume experiments by bank erosion-dominant and bottom erosion models. The experiments showed that the normal stress and pore water pressure of debris flow increased unsteadily along with the increase of the height of the debris flow head, but the bank erosion made the fluctuation more obvious. The mechanical equation demonstrated that bank erosion enhanced the increase of the frictional resistance. However, the greater velocity difference between the flow body and flow head made the additional gradient of the flow head greater under the bank erosion-dominant condition, so the bank erosion increased the average velocity of debris flow. The dynamic balance between head resistance and power lead to the unsteady motion of debris flow.
Abstract: As a low impact development (LID) practice, rain gardens may increase groundwater recharge in urban areas through focused infiltration of the storm runoff. This paper explored the impact of focused recharge through rain gardens on groundwater level and water quality based on a field monitoring study; the observed groundwater level and water quality metrics between the recharge point and a reference well were compared for both long-term (3 years) and short-term (3 days after the rain) periods. The results showed that the groundwater level in the rain garden recharge area was significantly higher the reference point (confidence level α=0.01, same hereafter). The groundwater quality observations showed that, the average concentrations of NH4-N at the recharge point were significantly lower than that at the reference point for both long-term and short-term values; the TN concentrations increased in 3 days after storms, but the average concentration in 3 years did not increase significantly; the NO3-N concentrations at the recharge points slightly increased, but not significantly different from the reference point. The TP concentrations was observed with both increased and decreased values during the long-term and the short-term periods. Findings from this research indicate that rain gardens may produce effective recharge to groundwater, limited influence on P, and significant influence on nitrogen concentration in groundwater for areas that water table depth is greater than 2-3 m.
Abstract: Many research efforts were performed to find the seepage effects on turbulent open-channel flow, from which results were often inconclusive and sometimes contradictory. A series of experimental studies are carried out to examine how seepage effects the open-channel flow structure and the velocity field is measured using the PIV image acquisition system in this study. The relationship between the flow velocity variation and the relative seepage intensity is analyzed. The results show that the velocity of water flow is not a monotonic function of relative seepage intensity. With the increase of injection intensity, the near bed current velocity decreases firstly, following by slight increase, and then decreases. With regards to the suction, the results are opposite. Measured data show an inflection value in the case of suction or injection, with the corresponding relative seepage intensity related to the Reynolds number. The larger the Reynolds number is, the smaller the relative seepage intensity occurs when the inflection value appears.
Abstract: Increasing water head lose to improve the accuracy of flow measurement is a ubiquitous problem in irrigation districts. This research aims to solve the problem. In this context, this research probed into the applicability of the new-type trapezoidal cutthroat flumes in measuring discharge of last trapezoidal channels. Based on Flow-3D, RNG k-ε turbulent model and Tru-VOF technique, numerical simulation was carried out to analyze hydraulic performance of trapezoidal cutthroat flume and rectangular cutthroat flume. Meanwhile, rationality of simulation results was verified by physical prototype test. Results indicated that the distribution of Froude number of trapezoidal cutthroat flume was unimodal; the critical flow appeared near the throat section and the Froude number was less than 0.5 around the upper throat, which could meet the water measurement requirement in irrigation districts; the position of maximum flow rate varied with the change of the flume's boundary. Compared with the rectangular cutthroat flume, both the head loss of trapezoidal cutthroat flume when the flow was large and the measurement accuracy when the flow was small was significantly improved.
Abstract: The identification of different sources of water in a karst groundwater system can not only explain the system's structure, but also help interpret the reason for hydrography and hydrochemistry variations. Quantitative interpretation of the frequency distribution of specific electrical conductivity (SEC) in a spring based on time series data has the capability of identifying water sources within the system. Lingshui Spring, which is in a covered karst basin, was used as a case study to collect conductance frequency distribution (CFDs) over four hydrologic years. The hidden peaks of CFDs were identified and analyzed to determine the water component represented by each peak, then proportion of each kind of water source was calculated. Four kinds of water components were found in Lingshui Spring. The first one is water stored in matrix or fissures in the system, which accounts for 53%-77% of the total discharge. The second one is a water source carrying contaminants from the area surrounding the outlet, which accounts for 3%-7%. The third one is diffusion seepage water through the mantled karst layer or water stored in conduits and fissures with poor mobility pushed out of the aquifer during heavy rains, which accounts for 13%-42%. The fourth one is surface runoff around the lake or water falling directly into lake during raining events, which accounts for 2%-3% of the total discharge. Two typical karst water systems in the peak cluster depression area were compared with Lingshui Spring, and the meanings of the CFDs were analyzed, indicating that the results of CFDs quantifying were in accordance with traditional hydrogeological ways. The study result provides reference for further classifying the 3 620 karst water systems in South China. It is suggested that quantitative interpretation of SEC should be applied and popularized in karst groundwater research in order to know the structure of the system easily and quickly.
Abstract: The evolution of wave shapes on fringing reefs is closely related to nearshore nutrient delivery and sediment transport of coral sand. It also plays a crucial role in determining the design and construction of the structure on the reef flat. Therefore, investigating the evolutions of wave shapes on the reef flat is necessary. In this paper, the FUNWAVE 2.0 numerical model was employed to simulate irregular waves, which propagate over different types of fringing reefs, and the influence of forereef bottom slope and reef roughness on the evolution of wave shapes was discussed. Results showed that an increase in the absolute value of wave asymmetry is moderated by the slope of the forereef bottom becoming steeper. Moreover, the absolute value of asymmetry for the smooth bed condition is slightly larger than that for the rough bed condition. However, the wave skewness on different fringing reefs is almost the same. This indicates that the forereef bottom slope significantly influences wave asymmetry variation and the effect of reef roughness is relatively small. However, the abovementioned factors have a negligible effect on the wave skewness. In conclusion, the empirical formulae for wave skewness and wave asymmetry on the reef flat by combining the influence of forereef bottom slope and reef roughness are recommended.
Abstract: The local scour caused by a pile on a slope differs to that at the flat sand bed. A 1:16 slope with sediment median size of 0.32 mm was set up, and an experimental study of local scour around the pile under the action of regular wave was conducted. Under the condition of small KC number, effect of wave height, wave period and pile position were discussed, and similarities and differences between local scour on the slope and at the flat were compared and analyzed from two aspects of maximum scour depth and scour profiles. It is shown from experimental results that the pile position at the slope brings a great influence to the local scour character; Under the same KC number, the maximum scour depth on the slope is greater than that at the flat sand bed; A distinguishing profile is displayed between clear-water scour on the slope and at the flat sand bed, but no obvious distinction is exhibited during muddy-water scour.
Abstract: The cross-shore profile shape and sediment sorting of tidal flats play an important role in the research of estuarine and coastal morphodynamics, and bear practical significance for maintaining biodiversity, exploring sedimentation history, and assessing the impacts of coastal engineering projects. This study reviews the advancements in the research of cross-shore profile shape and sediment sorting dynamics on intertidal flats in terms of field observation, theoretical analysis, as well as numerical modeling. The classic theoretical analytic solutions for tidal flat profiles are reviewed, and the mechanisms of the major governing factors, such as tidal current, wave, sediment characteristics, and supply, in addition to additional factors including vegetation, sea level rise, and reclamation project, are discussed. Several other scientific problems needed to be addressed are proposed, particularly tidal flat morphodynamic modeling and quantitative forecasting under the interaction and coupling of multiple factors, namely hydrodynamics, sediment dynamics, biological activities, and anthropogenic interventions.
Abstract: Current drought-related laws, regulations, plans and standards are targeted to deal with droughts once in 5 to 10 years, without any consideration for extreme droughts characterized by large scope and long duration. Once extreme drought like Chongzhen drought in the late of Ming dynasty or Guangxu drought in Qing dynasty occurs, it will exert severe impacts on the all aspects of our economic society. In the paper, we carry out literature review on the following three aspects:reconstruction of historical flood and drought sequence, restoration and repetition of historical extreme drought. Although the past researches have made great achievements, there are still many needs to be improved. In respect of historical sequence reconstruction, we found that the flood and drought sequences reconstructed by different methods cannot be compared with each other, and the sequences characterized by low temporal and spatial resolution. With regard to historical extreme drought restoration, there is a lack of consideration of the dual attributes of drought. As for historical extreme drought repetition, seldom attention has been paid on drought mitigation capacity and global climate change. Finally, the future research focus and practical demands are forecasted. Breakthroughs need to be made in the following aspects:cross-check between different reconstruction methods, sequence reconstruction with high temporal-spatial resolution, historical extreme drought restoration based on the dual attributes of natural disaster, quantification repetition of historical extreme droughts, and catastrophe risk coping strategy.
Abstract: Wind waves are a ubiquitous characteristic of aquatic environments and also an intrinsic feature of planktonic habitats. In lakes, especially in large and shallow lakes, wind waves greatly influence the physico-chemical properties of the water column, and directly or indirectly play a crucial role in plankton ecology. This paper reviewed studies of the effects of wind waves on the zooplankton, phytoplankton, and bacterioplankton in lakes. Accumulating studies showed that wind waves affect the spatial-temporal distribution of plankton and play a crucial role in shaping the structure and function of planktonic systems. In particular, it strongly affects the behaviour, nutrient uptake rate and flux, predation, growth, competition and succession of planktonic organisms, which in turn significantly influence the structure of aquatic food webs, potentially affecting the overall structure and function of lake ecosystems. This paper highlights the major future research directions to further reveal the ecological effects of hydrodynamics in lakes under global climate change scneario, which could provide new insights and support for decision making in terms of management and control of eutrophic lakes.