Abstract: To achieve scientific evaluation and understanding the changes in precipitation pattern and the spatiotemporal variation in precipitation extremes for urban areas in a changing environment, we analyzed the spatial and temporal distribution and the change characteristics of the precipitation extremes in Beijing. The statistical results for the storm with different durations from the Heavy Rain Atlases in Beijing and the daily precipitation data (from June to September) from 45 rain-gauged stations over Beijing during 1960-2012 were used in this study. The annual maximum value method and the percentile threshold value method were selected to define the precipitation extremes, and six durations storm (e. g. 10 min, 30 min, 1 h, 6 h, 24 h, and 72 h) and three extreme precipitation indices (frequency, precipitation amount, and contribution ratio) under the two thresholds (95% and 99%). The results showed that: ① The spatial distribution of precipitation extremes in Beijing was affected by the topographic features and urbanization, with some high-value centers of extreme precipitation for these special areas (e. g. urban areas, the interface between the mountainous area and the plain areas), which was consistent with the annual precipitation; ② All the extreme precipitation indices have significant decreasing trends during 1960-2012, with a rate of 0.13/10 a (0.04/10 a) for the frequency of extreme precipitation events, 11.59 mm/10 a (5.28 mm/10 a) for the extreme precipitation amount, and 2%/10 a (1%/10 a) for the contribution ratio of extreme precipitation, respectively, under the 95th (99th) percentile threshold; ③ The changes in the two periods (1960-1985 and 1986-2012) between the urban and suburban areas were different due to the impact of urbanization on extreme precipitation. The frequency of extreme precipitation events in the suburban areas was higher than that of urban areas during the 1960-1985, but the other two indices in the suburban areas were lower than urban areas. However, all the three indices in the urban area were higher than that in suburban areas during the period of 1986-2012.
Abstract: Accurate and continuous precipitation is vital for water resource management, flood forecasting, and hydrologic process simulation, although traditional gauge-based precipitation data is often insufficient due to the sparse and uneven distributions of stations. Moreover, it's always difficult to improve the precision and performance by interpolation technique. Satellite-based precipitation product solves these deficiencies of gauge-based data and provides an alternative data source for ungauged regions due to its high spatial and temporal resolution. The following research. This study mainly quantitatively evaluates the accuracy and suitability of the latest version (V7) of TRMM (Tropical Rainfall Measuring Mission) 3B42 multi-satellite precipitation estimation product in Pearl River basin, China, and to perform a hydrological simulation to further verify the applicability of the data. Dongjiang River basin and Beijiang River basin, located in the downstream area of Pearl River basin, were selected as the study case. Statistical indices of the Dongjiang River basin and the Beijiang River basin, including correlation coefficient (R), root mean square error (ERMS), mean error (EM), mean absolute error (EMA), relative error (ER) and Nash Sutcliffe Coefficient Efficiency (ENSC) were utilized to measure the accuracy of 3B42-V7 data. By contrasting the quantitative assessment results of 3B42-V7 data and gauge precipitation data derived from 90 rainfall stations during 1998-2006, the R values (correlation coefficients) of majority of the grid cells were greater than 0.60 on a daily scale and greater than 0.90 on a monthly scale, suggesting that the 3B42-V7 data presents high accuracy in the studied area. Moreover, EM (at two time scale) and ER are higher than 0 in most grid cells, which imply that a systematic overestimation of the precipitation in the studied area exist in the 3B42-V7 data. In the moderate heavy rainfall class, the results of intensity distribution of 3B42-V7 data were similar to that of gauge data, while 3B42-V7 data underestimated the small rainfall class while overestimating the heavy rainfall class, indicating that the 3B42-V7 data had relative low accuracy in detection of extreme precipitation events. Spatial distribution of the statistical indices of 3B42-V7 data implies that the accuracy of satellite-based precipitation data is typically affected by altitude. VIC (Variable Infiltration Capacity) model, a large-scale distribution hydrological model, was applied to hydrological simulation in this case. Two scenarios are set in this study: scenario I, 3B42-V7 data were applied to the model calibrated with gauge data while they showed a relatively unsatisfactory performance during the hydrological simulation; scenario Ⅱ, 3B42-V7 data performed preferably with the model recalibrated by the 3B42-V7 data, suggesting that calibrating the model with 3B42-V7 data rather than gauge data before the hydrological simulation would be ideal. The results of the hydrological simulation validation implied that, to some extent, the 3B42-V7 products can be used as the data source in the ungauged regions.
Abstract: Dryland rivers have long been subject to the influences of various human interventions in many parts of the world, however, little attention has been paid on how the fluvial processes of these rivers have responded to. Here we report a case study of the Tarim River, the largest inland river in China. By analyzing historical literature, gauged hydrological data and satellite images, this study performed a detailed examination on how the fluvial process of the river has changed under ever-increasing human influences in recent decades. The results demonstrate that human activity in the river basin have exerted significant impacts on the Tarim River, causing major changes in runoff, sediment transport and river morphology. Hydrological data observed over the past five decades shows an obvious reduction in both runoff and sediment load in the Tarim River, even though the runoff from source tributaries exhibits a gently increasing trend. The annual occurrence of low-flow events shows a significantly increasing trend, while the occurrences of both moderate-and high-flow events take a decreasing trend. The upper reach of the Tarim River is dominated with a wandering channel pattern, which is in aggradation and yet in a strong fluctuation form. The mean width of the wandering channel has been in a gently decreasing trend, perhaps due to the reclamation of river floodplains into farmlands and river embankments. The middle reach of the Tarim River takes a typical meandering channel pattern, the sinuosity index of which is distinctively lower than those of the old (abandoned) channels, even though it shows a gentle increasing trend in recent decades probably due to the increased occurrence of low-flow events.
Abstract: The sediment discharge has been reduced sharply since the operation of Three Gorges Project in 2003, leading to the severe erosion of Lower Jingjiang Reach (LJR), a representative meandering channel located in the middle of Yangtze River. Some sharp bends in LJR are unusually undergoing convex bank erosion and concave bank deposition resulting in submerged shoal, which are contrary to the basic fundamentals of curved riverbed evolution. These abnormal riverbed deformations induce changes in flood control, waterway condition and the relationship between Yangtze River and Dongting Lake, and indeed the corresponding mechanism needs to be investigated. In the paper, based on a number of field data, the characteristics of riverbed evolution and channel morphology adjustments are fully demonstrated by quantitative statistics of erosion-deposition distribution and channel morphology for selected curved reaches. The results indicate that the annual redistribution of flow, the reduction of sediment discharge and mutation of sediment gradation are the dominant causes.
Abstract: Saltwater intrusion was the salinity diffusion process by the tide and runoff into the sea. In this paper, the theory of energy accumulation and impedance was used as the basis by which to analyze and infer the diffusion response function of runoff into the sea, tidal range and region of intrusion, and the data of runoff into the sea, tidal range and region of saltwater intrusion over the last 50 years was analyzed and calculated to confirm the theory. The results indicated that the function between the tidal range and region of intrusion exhibited obvious S-shaped regularity, and there was the same S-shaped conclusion between the runoff into the sea and maximum area over the standard salinity, which formed a three-dimensional diffusion response function of saltwater intrusion. Using the data over the last 50 years in the Yangtze River Estuary, the three-dimensional diffusion characteristic was confirmed, the S-shaped function based on the hyperbolic tangent of saltwater intrusion was constructed, and the parameter setting and quantization method was given. Through the theory of this function, the runoff into the sea which controls the saltwater intrusion was ascertained to be 10 000 m3/s, and the loss caused by the saltwater intrusion was further calculated over 2001-2012.
Abstract: To predict the intensity of saltwater intrusion under different riverine discharge, a new empirical model was developed to predict the chlorinity in the upper South Branch of the Yangtze River Estuary, where saltwater intrusion is strongly affected by tidal flow from the North Branch. In the model, only runoff at Datong station and lunar calendar date were required as variables. Firstly, the changes of daily mean tidal range was investigated with measuring data at Xuliujing. Using the theory of harmonic analysis with 11 main tidal components, it was proved that the daily mean tidal range changes every half-month. Thus, a new method was proposed to estimate the daily mean tidal range by using the lunar calendar. Secondly, the quantitative relationships between chlorinity and riverine discharge, tidal range were developed using the measured data at Dongfengxisha. Some functions in previous empirical models were examined to test their effectiveness under the condition of saltwater intrusion from the sub-branch channel. Finally, an empirical model was proposed in the form of exponential function to describe the chlorinity under different riverine discharge and tidal range conditions. The predicted chlorinity process at Dongfengxisha was in good agreement with the observations, with the correlation degree of up to 0.8. Using the proposed model in this study, intensity of saltwater intrusion at a certain location can be estimated with simple input data, which provides a convenient method to conduct prediction of saltwater intrusion.
Abstract: Riverbank erosion occurs frequently on the middle Yangtze River, which may result in the instability of the channel navigation. A new riverbank erosion model for the cohesive material was developed by considering the effects of adjacent cohesion. On the basis of 3-D flow and sediment transport model and the mechanisms of riverbank erosion, a 3-D morphological model for the Taipingkou Waterway on the middle Yangtze River was developed by using the adaptive grid system. The 3-D model was calibrated by the observed data, and the simulated results agreed with the measured data. The influences of riverbank erosion processes on the navigation conditions were simulated by the numerical model. The developed model is able to calculate the complex morphological changes due to bank erosion. Bed degradation and bank erosion are enhanced with less sediment load input. Simulated results indicated that the riverbank stability of Lalinzhou is critical for the stability of navigation conditions on the Taipingkou Waterway.
Abstract: To explore the initiation and development of tidal creek network driven by tidal currents, an experimental physical model was established based on the prototype of a silt-muddy tidal flat-creek system on the Jiangsu coast. The evolutional process was simulated with an initially gentle bed slope of about 1%. The movement of flow and sediment, as well as the influence of tidal range on tidal creek development, was analyzed. The results indicated that tidal creek development was influenced by the bed slope variation. For the cases of uniform bed slope, the connection of neighboring tidal creeks, rather than headward erosion as shown by previous studies, resulted in the development of tidal networks. A strong flow and sediment movement occurred at the early flood and the late ebb stages, and sediment movement was most active in the mid-intertidal flat. Tidal range had little influence on the overall pattern formation and the structure of tidal networks. The relationship between the mean drainage path length and the sub-basin area generally followed a power-law function. Tidal networks approached a dynamic equilibrium state earlier under larger tidal ranges which contributed more to the development of the tidal creek widths.
Abstract: Based on the measured terrain in different stages, the distribution characteristics of back silting in the Sheyang Port channel was analyzed. Velocity and sediment characteristics in Sheyang Port channel were calculated and analyzed by mathematical model. The reasons accounting for back silting in the waterway were discussed. The study showed that there is a widespread deposition in the channel after the channel engineering excavation, especially in the area near the mouth. The main reason of siltation in the channel is that the flood current speed is greater than that of ebb, so sediment is difficult to be taken out of the channel, which attributes to deposition of the channel. There are two reasons contributing to the back silting in estuary. Firstly, large amounts of sediment cross over the southern dike and go into the entrance of channel. In addition, there is an evident recirculation zone in the entrance channel during flood tide, which is easy to lead to gate sediment deposition. The reason for silting near the root of dike is the decrease of estuarine water carrying capacity during ebb tide, which is attributed to the river relaxation resulting from confluence of Sheyang River and Huangsha River.
Abstract: Understanding histrorical and future morphdynamic evolution processes is of critical importance to the longterm development and regulation of estuarine deltas. Based on a series of bathymetric and hydrologic data from 2001 to 2015, the influence of Yangtze fluvial sediment decline on the bathymetric evolution of the Yangtze River Estuary and adjacent seas is presented. After the completion of the Three Gorges Dam Project, the sediment flux is 135 million ton per year (keep relatively lower level comparing to the past), and an updated stable relationship between riverine discharge and fluvial sediment flux has been taken shape. Impacted by the sediment reduction, the suspended sediment concentration at the South Branch, South Channel and at the upper reach of the North Channel experienced a sharp decline after 2008, accompanied with channel erosion and channel-volume increasing. Meanwhile the channel shape tended to be narrower and deeper than the past. In recent years, the upstream and outlets of the Mouth Bar area are scoured, the length of the shallow area is cut down. Two erosion areas are found at the front of underwater deltas of the Yangtze River Estuary, one is located at the outlet of the North Channel, and another is at the outlet of South and North Passage. From 2007 to 2015, the annual average scour thickness is 0.1 m, and the yearly erosion amount reaches 71 million m3. The influence of fluvial sediment decline on the bathymetric evolutions in the Yangtze River Estuary and adjacent seas is still in progress, and it will likely overturn the traditional seaward sedimentation pattern in this area.
Abstract: In this paper, a layer-averaged fully coupled model is applied for simulating both laboratory lock-release and constant-flux turbidity currents with the aim of analyzing the applicability and uncertainties of empirical relations for the water and sediment entrainment process. For this purpose, the performance of four empirical formulae (i. e., ew59, ew86, ew87 and ew01) for the water entrainment and five (i. e., Es77, Es86, Es87, Es93 and Es04) for the sediment erosion are compared. The following understandings are drawn from numerical case studies. The water entrainment has mild effect on small-scale turbidity currents, though simulation results with the ew01 relation are slightly better than those by others. It might be due to the fact that the ew01 relation has taken into account of the effects of both bottom and upper interface resistances. Lock-exchange turbidity currents are mainly depositional and thus less sensitive to the choice of erosion relations. For constant-flux turbidity currents, the Es87 and Es93 relations appear to perform best. This must be attributed to the fact only these two erosion relations have been calibrated using experimental turbidity current data. This study should help further studies for developing and calibrating empirical relations of mass exchange for turbidity currents.
Abstract: The flow and dilution of a vertical round jet under the combined effect of wave and current are investigated by using the three dimensional large eddy simulation model, and the wave effect on the 3-D concentration characteristics of the jet is discussed. The comparison between the numerical results and the experimental data shows that, the numerical model is capable of reproducing the distribution of the jet velocity and concentration along the water depth in the wave-current environment. The 3-D concentration characteristic indexes of jet, including the minimum dilution and its vertical position as well as the visual area of the section, are adopted in this study. It is found that, the jet in a wave-current environment shows a larger visual area on the sections as the increasing of the wave height or period, which indicates that the wave has a positive role on the acceleration of jet dilution in the wave-current environment.
Abstract: Rapid urbanization have resulted in increasing water supply in urban areas and further led to more water dissipation from urban water uses. When calculating water dissipation, traditional hydrological models only considered evaporation from precipitation, usually ignoring the dissipation due to human water consumption. In order to analyze the urban water consumption process and better understand the urban water dissipation (UWD) mechanism, the UWD calculation model was developed based on the observation of the urban water consumption process and the categorization of UWD units. According to the results from statistical analysis, experiments and the mathematical model, the UWD of one region in Beijing in 2014 was estimated as 820.0 mm, which was significantly higher than that from the traditional hydrological model (494.0 mm averaged over many years). The results showed that the contributions of water dissipation from the green land, building, hardened ground and water surface were respectively 40.58%, 38.70% (including 31.22% from human activities and 7.48% from building roofs), 18.32% and 2.40%. It was concluded that water dissipation inside buildings was one main component of artificial water consumption and was non-negligible for the computation of the total UWD. The quantity of water dissipation of a building was proportional to the number of floors and residents in the building.
Abstract: To explore the effect of flow and water salinity on the flocculation-settling of cohesive sediment in estuary and coastal region, we took sediment concentration and floc diameter as parameters, and used a self-made rotary cylinder and electrolyte added method to examine the flocculation-settling process under six flow intensities (0 s-1, 1.38 s-1, 2.54 s-1, 4.67 s-1, 7.19 s-1, 13.2 s-1, 20.3 s-1) and three electrolytes (NaCl, CaCl2, AlCl3). The results showed that: ① The effect of flow on flocculation-settling of cohesive sediment is related to flow intensity. Low intensity flow improves the flocculation-settling at early stage and hinders it at the other stages, while high intensity flow always retards it because of the strong shear stress and turbulent mixing. ② The existence of electrolyte has a net expansionary effect on the action of low intensity flow and a contractile effect for high intensity flow. In the same concentration, when the cation valence of electrolyte becomes from +1 to +3, the influence of flow becomes greater, especially at stabilization period.
Abstract: Water level of the hyporheic zone essentially varies in different hydrological periods. In order to investigate the geochemical characteristics of the lateral hyporheic zone in different hydrological periods, Maanxi Creek in Chongqing was selected to be the study site. The water level, temperature, dissolved oxygen, pH, specific conductance, and selected ions concentrations of the river water, lateral hyporheic zone, and groundwater were monitored during the transitional period of wet season to dry season (October to December). Results show that the water level of the lateral hyporheic zone was changed greatly with the arriving of dry season. The water level gradient decreased between river and hyporheic zone, resulting in a reduction of infiltration from the river water. Because of change of water level and temperature drop of infiltrated water, the microbial activities in the lateral hyporheic zone declined, which increases pH and decreases dissolved oxygen. These changes also reduced the specific conductance and weakened the purification capacity of the lateral hyporheic zone to nitrate and sulfate, and enhanced the fixation capacity to the heavy metals, like manganese and zinc. Based on the analysis of the geochemical dynamics of the hyporheic zone, it can be deduced that the boundary of the hyporheic zone was shifted from a place of 30-50 cm in the wet season to a place of less than 30 cm away from the riverbank in the dry season.
Abstract: With the increasing impacts of both climate change and human activities, river basins were usually facing multiple water issues, which can impact water quantity, water environment, aquatic ecology and sediments. Simultaneously, increased frequencies of extreme hydrological events, such as droughts and floods, were observed. Thus, systematic management and retardation of extreme hydrological events became crucial goals of integrated water basin management. Raising the regulation capacity to multiple water cycling processes in a river basin is an important pathway. In this paper, based on the judgement of the China's major water issues, we analyzed the shortage of traditional water basin management, which relied on changing status of water cycling processes, end-of-pipe control technologies and the process dissociation. We analyzed the overall demand of solving water problems from a systematic level under the changing environment, the general idea of the ecological sponge-smart river basins construction, and proposed the theoretical framework and the critical research questions of the ecological sponge-smart river basins. The construction of ecological sponge-smart river basins should base on ① taking full consideration of the evolutional rules of multiple water cycling processes; ② regulating human activities, including exploring water and land resources; ③ achieving the joint regulation among the surface-soil-underground hydrological processes, including water quantity, water quality, sediments and aquatic ecosystem. We aimed to utmost retard extreme hydrological events, construct and improve comprehensive service functions of a river basin and achieve the conservations of the ecological functions of the ecosystems, including mountains, rivers, farmland, forests and lakes.
Abstract: Local scour depth at weir-like structures is one of the most important engineering design parameters, which significantly affects the stability and safety of the structure. In this paper, the 80 years' research progress on scour at weir-like structures is reviewed. The scour classifications at weirs and the corresponding scour mechanism are elaborated. The research progress on physical modelling, theoretical analysis and numerical modelling of scour at weir-like structures is introduced and discussed. The past research outputs are found mostly confined to clear-water scour at 2-D unsubmerged weirs and relied heavily on physical modelling techniques, while studies on live-bed scour at 3-D weirs, effects of rocky bed materials, coupling relationships of migrating bedforms and developing scour holes, numerical simulation of live-bed scour process, and etc. are limited. The potential research areas are also given in the last section of the paper.