Abstract: With the development of economy and society, China has stepped into the rapid urbanization process with the population urbanization rate rising from 36.22% in 2000 to 54.77% in 2014. Under the background of global climate change and rapid urbanization, urban flood and waterlogging hazards become more frequent in this country. In this paper, the mechanism of how global climate change and urbanization affect urban precipitation including extreme rainfall was preliminarily analyzed and the effect of urbanization on flood process was also explained from the aspect of surface runoff yield and flow concentration process. Then, the causes of frequent urban floods in China were analyzed systematically. Moreover, main tactics of urban flood and waterlogging prevention and alleviation were stated. Firstly, the urban infrastructure should be strengthened guided by the idea of low impact develop (LID). Thus, sponge cities with good defense ability for rainstorm could be constructed. Secondly, the urban flood management systems with 3-dimension urban flood monitoring, dynamic forecasting and warning and real time operation should be established to raise the scientifically decision-making for flood disasters reduction. Thirdly, to intensify the disaster relief and recovery for urban flood, the emergence plans should be drawn and improved.
Abstract: Environmental change not only quantitatively affects water resources, but also causes non-stationarity in hydrological processes, which challenges the applicability of traditional drought assessment methodologies. Based on data for 1961 to 2013 from two hydrological stations, 62 rain gauge stations, and 24 meteorological stations within the Weihe catchment, this study evaluated the effect of climate change and human activities on runoff reduction using a macro-scale distributed hydrological model along with the Variable Infiltration Capacity (VIC) model. The spatiotemporal characteristics of hydrological drought were analyzed by the Standardized Runoff Index (SRI) which was calculated by multi-parameterization schemes. The robustness of SRI under non-stationarity as well as the impact of environmental change on the hydrological evolution were analyzed. Results showed that the observed significant runoff reduction at the Xianyang and Huaxian hydrological stations in the period of 1991-2013 was mainly due to human activities, which accounted for -66.7% and -71.0% of the total change, respectively. The SRIv series that were based on the time-variant parameterization scheme, could rationally reproduce the historical hydrological droughts. It was also found that after 1991 human activities mainly resulted in the short-term drought fluctuation and climate change mainly led to the trends of droughts and floods in the Weihe catchment.
Abstract: Temporal clustering of magnitudes and times of occurrence of floods in the Tarim River basin recorded at 8 hydrological stations located in 5 states was analyzed using the Cox regression model and monthly frequencies of flood occurrences and dispersion methods under the impact of low-frequency climate change. Results indicated that:under the effect of climate change, there was a significant temporal clustering in the flood process in Tarim River basin. Periods of high flood frequencies trended to be the periods of high flood mitigation seriously influenced by regional flood control which was the main reason for large flood-induced losses in the Tarim River basin. The positive phase of climate indices resulted in the early occurrences of floods for the same exceedance probabilities and decreasing exceedance probabilities for the same time of flood occurrence at the hydrological stations and in the region, which both had positive coefficient values fitted by the Cox regression model with climate indices as covariates, while the negative phase of climate indices had an opposite effect. The exceedance probability of flood occurrence time preferably responded to low-frequency change, which was advantageous for flood control and flood risk management in the Tarim River basin. The majority of hydrological stations did not reveal any inner-annual clustering of flood occurrences when obtained by POT sampling, while regional flood occurrences had significant clustering.
Abstract: To accurately evaluate the hydrologic drought, it is necessary to objectively construct the drought index. In this study, the data collected from two hydrologic stations in the upper reaches of Fenhe River basin were used as case studies. Firstly, the ability of the logistic, normal, two-parameter log-normal, and Weibull probability distributions was examined to choose the most suitable distribution function for establishing the standardized streamflow drought index (SSDI). Secondly, the hydrologic drought magnitudes were divided based on the characteristics of standard normal curve. Finally, the SSDI was compared with the Z index and departure index of streamflow. The typical records of drought and flood events in the Fenhe River basin were taken as evaluation standard, which was used to evaluate the applicability and rationality of the SSDI and drought magnitudes. The results showed that, the two-parameter log-normal distribution was selected as the best fitting function for monthly streamflow in the study area. The SSDI and the magnitudes were both coincident for the actual drought-flood severity. In addition, the occurrence of various hydrologic drought magnitudes was stable at various time scales. The conclusion could provide scientific basis of drought defense, disaster mitigation, and water supply planning measures for the upper reaches of Fenhe River basin.
Abstract: Soil moisture is one of the key factors for vegetation restoration and ecological reconstruction on the Chinese Loess Plateau (CLP). To understand the regional spatio-temporal variations of profile soil water content and its potential influencing factors, a south-north transect with 86 observational sites were determined on the CLP. Soil water content was measured using a neutron probe to depth of 5 m from June to November 2013. The distribution and variation characteristics of soil water storage (SWS) were evaluated and simulated using classical statistics. The results showed that SWS within different soil layers exhibited moderate variation and generally decreased from the south to the north of the CLP. The mean SWS within the 0-5 m profile for the transect was 735 mm. Spatial variation in SWS increased with increasing soil depth, while the temporal variation had a decreasing trend, suggesting that soil water content tend to be more temporally stable in deeper layers. Soil moisture was primarily influenced by the aridity, the content of clay, normalized difference vegetation index, and slope gradient on the regional scale. These parameters, therefore, could be used to predict spatial distribution of soil water content at a regional scale in a certain confidence level.
Abstract: The mechanism of soil water migration in the reconstruction and restoration of the Loess Plateau is a key scientific problem that must be solved. Isotopic tracers can provide valuable information associated with complex hydrological problems and are difficult to obtain by other methods. This study investigated soil water migration in the unsaturated zone in the hilly and gully region of the Loess Plateau. Samples were collected from the 0-150 cm unsaturated zone, including precipitation, soil water content, plant xylems, and plant roots, and were analysed. The results showed that the isotopic composition of precipitation in the Yangjuangou River basin was affected by evaporation, as indicated by isotopic fractionation phenomena. The δD and δ18O isotopic profiles of the soil water exhibited monthly variations. The soil water was recharged by precipitation. The soil water and precipitation were taken up by shrubland. Water migration exhibited a transformation pathway from precipitation to soil water to plant water. Shrubland root systems were distributed in the shallow layer. Root and water uptake increased at a depth of 20-40 cm. The isotopic composition of xylem water was close to that of soil water at a depth of 20-40 cm. These values reflect soil water signatures associated with shrubland uptake at a depth of 20-40 cm. This study investigate the migration process of soil water, identify the source of plant water use, and provide a scientific basis for the migration process of soil water, including model structure and parameter identification.
Abstract: In order to investigate the mutual feedback effects of rill morphology evolution on runoff and sediment, variation characteristics of runoff and sediment during the evolution of rill morphology were studied. In the current work, an indoor soil bin rainfall simulation experiment was performed using a 3-D terrain scanning method. Loessial soil was analyzed under rainfall intensities of 66 mm/h, 94 mm/h, and 127 mm/h. Results showed that the evolution of rill morphology had a profound effect on sediment yield, but the impact on runoff was not obvious. The variation in runoff depended mainly on infiltration conditions and crust effect of soil and patterns of runoff production. The runoff production on the slope had a tendency to first increase and then decrease before finally reaching stability. There was an obvious relationship between rill morphology evolution and sediment yield; the correlation coefficients of geomorphologic comentropy and soil loss with erosion rate were 0.954 and 0.916, respectively. The emergence of rills was found to aggravate the erosion, resulting in a significant increase in sediment concentration. The variation in sediment yield in rainfall of different intensities was basically the same. The more intense the rainfall, the higher sediment concentration after the rill network became stable.
Abstract: The large number of debris flow events in the Dayi area, Guizhou, China, due to a large rainfall event in June 2011 provided a good opportunity to study the influence of topographical factors and rainfall factors on the triggering of debris flows by shallow landslides. The triggering mechanism for these debris flows is the transport of sediment provided by shallow landslides into the channel. During the heavy rainfall event in 2011, 37 gully type debris flows caused by shallow landslides were triggered. In some catchments no such debris flows were triggered even though these catchments were in the vicinity of gullies with debris flows. We isolated and analyzed the influence of the topography on the formation of debris flows in gullies with almost identical hydrological and geological conditions and propose a new T-factor as a topographical indicator which is a combination of the catchment surface area, the percentage of the catchment area with a slope of 25-45 degree, and the average gradient of the drainage channel in the catchment. Additionally a new R-factor is proposed as a rainfall indicator which is a combination of the rainfall in 1-hour before the debris flow was triggered, the cumulative rainfall before the debris flow was triggered, and the annual rainfall. The relationship of the T factor and the R factor gives a primary threshold P for this kind of debris flow. The primary probability factor P is the combination of T and R, which gives an indication of the probability of debris flow formation.
Abstract: The Three Gorges Reservoir has changed the water temperature trend on downstream waters of the Yangtze River and has an important impact on downstream ecological environment. Based on the measured data of the Yichang hydrological station on Yangtze River during a period of 14 years(2000-2013), the one-dimensional longitudinal temperature model was used to analyze the temporal and spatial variation of water temperatures of the expanse of 300 km along the river, from Yichang to Jianli, which occurred as a result of the Three Gorges Reservoir Reservoir. The impact on spawning grounds due to water temperature variation was also discussed in different impoundments. The research results show the following:① As a result of the Three Georges Reservoir, there was a phenomena of planarization and delay of the water temperature process in Yichang. It is also evident that the discharged water is of a lower temperature during the summer and higher temperature in winter on downstream. Compared with the natural water temperature, the water temperature in Yichang changed to -4.3℃ in April and 3.7℃ in December during the period of the 175 m stage. ② As a result of the operation of this project, the water-air heat exchange on downstream river waters has also experienced much change, and the relatively strong flow has caused the water temperature of the nature status along the river to recover slowly. The dam also plays a decisive role in regulating the downstream water temperature. The water temperature of Jianli changed -3.2℃ and 3.0℃ in April and December respectively. ③ After the Three Gorges Reservoir was put into operation, the original water temperature in the Chinese sturgeon spawning grounds of Yichang which reached 20.0℃ in winter began to experience a delay of 10-40 days. The water temperature of 18.0℃ in the Four Major Chinese Carps grounds of Jianli in spring time also experienced a delay of 10-30 days. In accordance with the rising level of Three Gorges Reservoir, the trend of the delay will be more significant.
Abstract: To examine the relationship between thermal stratification and spatial distribution of hydrogen and oxygen isotope in Wulixia Reservoir, Guangxi, spatial characteristics and factors influencing hydrogen and oxygen isotopes were studied in during the summer of 2014. Results showed that ① Wulixia Reservoir showed obvious thermal stratification in 2014 summer which can be divided into three layers, epilimnion (from surface water to a depth of 5 m), thermocline (depths of from 5 m to 20 m) and hypolimnion (below 20 m); ② All the δ18O and δD values of Wulixia Reservoir (line equation:δD=4.66δ18O-10.85, R2=0.76) scattered roughly along Global Meteorological Water Line (GMWL), which showed the reservoir water to originate from modern meteorological precipitation. However, water δ18O and δD of Wulixia Reservoir, located in the right lower area of the Guilin Local Meteorological Water Line (CLMWL) and the slope of their line equation were obviously different from the slope of GMWL and CLMWL line, which indicate that evaporation and water-rock interaction could occur before water entered the reservoir; ③ δ18O and δD values of water generally decreased as depth increased, in the following order epilimnion > inflow water > thermocline > hypolimnion outflow water (here ">" means "more positive"). Results indicate that seasonal variation of inflow water δ18O and δD and water evaporation decreased as water depth increased. These are the two primary factors influencing and controlling the vertical distribution of δ18O and δD in Wulixia Reservoir during the thermal stratification that takes place during summer.
Abstract: In order to study the impact of waves on the movement and dilution process of multiple jets, Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) measurement systems are used to measure velocity field and concentration field under the wave environment. The results have shown that multiple jets can be divided into three zones under the wave environment:initial zone, transition zone and mixing zone. In the initial zone, respective characteristics of jet flows from various nozzles are maintained with smaller mutual influence; in the transition zone, bifurcation occurs on the jet flows, and several confluence points are formed through mutual superposition of the jet flows. The positions of peak values of average speed and concentration migrate from the centerlines of jet flows to the confluence points; in the mixing zone, due to integration of multiple jets flows, average speed and concentration are distributed in the form of single "flat peak". According to the comparative analysis, the position of confluence points will migrate downward and the multiple jets will enter into the transition zone and mixing zone more quickly with the increase of wave height and period, and the decrease of jet initial velocity.
Abstract: Submerged spur dike is a kind of typical navigation project regulating structure. During the project implementation, the protection of local scouring near the head is especially important. In order to study the characteristics of local scouring of submerged spur dike head under bottom protection condition, this study adopted undistorted model experiment method and focused on studying the variation response rules of the form of the local scour pit of submerged spur dike head under bottom protection condition, its depth and the bottom protection width. The result indicates that the local scour pit of the head under bottom protection condition is near the bottom protection edge. When bottom protection edge is less than 20 m, the variation of the maximum scour depth is relatively small, but the scout pit's position is away from the dam body. The variation rule of the maximum scour depth with the bottom protection width can be expressed with exponential relation. Based on principle of dimension analysis, this study established a formula to calculate the maximum local scour depth of submerged spur dike under bottom protection condition on riverbed made of sand.
Abstract: A multistep characteristic-based operator-splitting finite element method is proposed to determine the effects of adding a splitter plate at the back of a cylinder on the characteristics of a flow past a circular cylinder. In each time step, the N-S equations are split into diffusive and convective parts. For the convective part, temporal discretization is performed by the characteristic Galerkin method with a multistep technique. Results of the present model are in good agreement with previously published data. Thus, the present model is competitive in solving incompressible viscous flows in terms of accuracy and efficiency. Furthermore, the flow past a circular cylinder with a splitter plate was simulated by the present model. The flow fields and pressure on the cylinder surface with and without a splitter plate were compared. Findings indicated that the plate can suppress vortex shedding, increase the pressure on the leeward of the cylinder, and reduce the pressure difference between the upper and lower surfaces.
Abstract: Both of cavitation erosion and air entrainment against cavitation are important aspects of studying high-speed jets. Under the condition of slight submerged outlet and constant ratio of chamber area to outlet of jet, decompression tests are carried to study cavitation characteristics of round jet in stilling basin with sudden enlargement and abrupt drop. In which, the pressured rectangular chamber with gate slots followed by round pipe flow is located before the stilling basin. In experiments, two offset height S (5.5 m and 4.0 m) and two round pipe length L2(L2/d=2, 1, 0, d is round pipe diameter) are tested. The experiments reveal that the probability of cavitation on gate slots and sidewalls of stilling basin reduces by 10% when the offset height decreases from 5.5 m to 4.0 m. Moreover, the probability of cavitation on gate slots with L2/d=2 and 0 increases by 40%-60% compared with layout of L2/d=1, while for sidewall in stilling basin the probability of cavitation with L2/d=0 is higher 260% compared with layout of L2/d=1. But its adverse effect on cavitation becomes smaller as L2 continues to increase. Furthermore, the offset height should not be too low in order to keep flows with enough aeration. Therefore, the effect of offset height S on flow cavitation of gate slot and sidewall in stilling basin is small while the effect of length of round pipe is relatively greater.
Abstract: The two-tank setup was employed to produce linearly stratified salt-water in laboratory tank. Laboratory experiments on buoyant plumes were undertaken by jetting dense salt-water downward into the linear stratification, aiming at investigating plume development in stratified salt-water. The time-resolving Particle Image Velocimetry (PIV) approach was adopted to investigate the flow field through the entire plume development. Plume visualization showed three distinct stages of plume development, namely, the inflow stage, the producing stage of the neutral buoyant layer, and the stabilizing stage of the neutral buoyant layer. It was shown that flow velocity decreased from plume stem to edge. Instantaneous flow fields revealed there were complex vortices both near the plume boundary and inside the plume which indicated entrainments occurred throughout plume development. The experimental results agreed with the MTT(Morton, Taylor and Turner) model in general, although with a slightly modified prefactor to the empirical formula. This paper contributes to plume investigations by adding supplemental data to small-scale plume regime of the MTT model.
Abstract: In order to study the motion law of salt water when the levees break with storm surge in the delta estuary, one-dimensional and two-dimensional coupled salinity numerical models are developed to simulate the salt water movement when the levees break with storm surge. The impact of buildings in flood plains on the salt water movement and the gradient development process of the dyke breach are considered. The water levels and salinity calculated by the river networks model are tested with the measured data of multiple survey stations in 2008. The model is applied to simulate the salt water movement of an offshore dyke on the Pearl River Delta when the levees break with storm surge, and the maximum salinity isosurface diagram is mapped. The results show that the salinity of levee-breach flood in most of the breaches is over 4psu. Therefore, the impact of the high salinity ponding of levee breaches flood cannot be ignored. By comparing the drainage salinity calculated in the cases of "levee breaches" and "non-levee breaches", it is found that the levee breaches shunt of upstream enhances the storage capacity of tidal water, increases the flood tidal volume, raises the trace-back risk of salt water intrusion of downstream river network and weakens the suppressing effect of the upstream flow on salt water intrusion.
Abstract: The Qinghai-Tibet Plateau is the source zone of China and Asian's large rivers, where river network well developed within the plateau. Under the superimposed impacts of plateau uplift, climate change, and human activities, the rivers may be significantly changed in the future, resulting in a series of interdisciplinary issues which are becoming a hot topic. Following our field investigations of the plateau rivers during 2007-2016, we comprehensively summarized the research progress of river morphodynamics in recent decades, such as the related geological background, the influences of the uplift, river morphological characteristics, and fluvial processes. Furthermore, we focus on effects of climate change on river morphodynamics in the middle time scale, and the local, abrupt and accelerating effects of human activities on the short time scale. Meanwhile, a new concept of alluvial river group is proposed as well as its definition, characteristics and distribution are preliminarily given, in order to promote the understanding on the formation and evolution of complex braided and meandering rivers in this new framework.
Abstract: Colloids are widely distributed in groundwater systems and facilitate transport of trace elements and organics. They therefore play an important role in mass transfer with subsequent impacts on groundwater quality. In this contribution, we have summarized the current state of colloid research, including studies of colloid stability, transportation mechanisms, and methods for simulating colloid transport. Our findings suggest that colloid stability is influenced by physical and chemical properties, including the diameter, morphology and zeta potential of colloids, and hydrogeochemical conditions, including pH and the ionic strength of groundwater. Colloid transportation mechanisms in porous media include deposition and straining. Colloid deposition is described as either blocking or ripening because of differences in colloid stability. Current methods for simulating colloid transport are increasingly well-developed, but further methods that simulate the heterogeneity of porous media and associated colloid transport are needed.