Abstract: To better understand the mechanisms and response of snowfall to climate change,daily meteorological data from 49 stations in and around the Tianshan Mountains were used to extract the snowfall sequence with a parametrized snowfall identification scheme. The changing characteristics of different snowfall events in the Tianshan Mountains from 1961 to 2016 were analyzed using the percentile threshold method. The results showed that:① The snowfall was larger and more frequent in the mountain region than that in the basin,snowfall on the northern slopes was greater than that on the southern slopes,and snowfall decreased from northwest to southeast. ② In the past 56 years,the snowfall in the Tianshan Mountains has increased significantly and the snowfall frequency has increased slightly. The trends in snowfall amount and snowfall frequency at all types were as follows:light snow decreased significantly, moderate snow had no obvious trend,and heavy snow and extreme snow increased significantly. The regions with significant snowfall changes were concentrated in the central region of the northern Tianshan Mountains and Yili River valley. The increase of snowfall was mainly because of an increase in extreme snowfall and snowfall frequency. ③ Annual snowfall,heavy snowfall and snowfall frequency,and extreme snowfall and snowfall frequency increased abruptly in the mid-1980s,while the snowfall and snowfall frequency of other types showed no obvious change. ④ The increases in annual snowfall and extreme snowfall in the Tianshan Mountains were related to warmer temperatures.
Abstract: To explore the mechanism of changes in the hydrologic cycle under the changing environment in eastern coastal areas of China,based on intensive observations from the hydrological station network and construction of experimental watersheds,we analyzed the distribution and responses of hydrological elements in areas of different land use and urbanization levels. The research results are as follows:① The spatial distribution of extreme rainfall exhibits differences,reaching 50 mm,and is mainly affected by the micro-topography and wind direction of the basin. ② For the runoff response for two basins with different urbanization levels,the flood response time,water level increase,and water level increase per unit rainfall of the highly urbanized basin are higher than those of the low urbanization basin,which is mainly affected by the size of the watershed and degree of urbanization. ③ Forest land plays a critical role in water conservation,and the decreasing process of soil moisture under forest land is slow,whereas the process in urban and cultivated land is faster;the average soil moisture lag times in 10 cm,20 cm,and 40 cm soil layers are 0-0.25 h,0.25-0.75 h,and 0.5-0.75 h,respectively,while the soil moisture response times for deep soil layers (60 cm and 80 cm) are more complex because of the influence of preferential flow. ④ The peak groundwater occurrence time has a hysteresis relationship with rainfall,and the lag time is 6.5-12 h.
Abstract: In order to solve the nonlinear problem of runoff concentration caused by excess rainfall intensity,a method deriving time-variant distributed unit hydrograph (TVDUH) based on the SCS's velocity equation was presented in this paper. A reference rainfall intensity is introduced to represent the hydraulic conditions corresponding to k values. A dimensionless factor formed by the ratio of duration excess rainfall intensity to reference rainfall intensity is added in SCS's equation to make up a new velocity equation that can take into account of the influence of excess rainfall intensity on flow velocity. The modified SCS's equation is used to derive the TVDUH of each rainfall duration during a rainfall event. A rainfall-runoff hydrological model is constructed of TVDUH and the runoff generation module of Xin'anjiang model. The rainfall-runoff hydrological model is used in Peihe River basin to calibrate the reference rainfall intensity. Application is performed in Tanjiahe River basin for validation. Results show that the number of events whose coefficient of efficiency being greater than or equal to 0.9 increases from 42% of time-invariant unit hydrograph to 83%. It is concluded that the presented method can contribute to increase the rainfall-runoff simulation capabilities of watershed hydrologic models observably.
Abstract: In order to better understand the effect of the lake shrinkage on the flood process in a large lake,a quantitative analysis of the relationship between the lake shrinkage and the flooding in Dongting Lake was conducted,for which Jingjiang-Dongting Lake hydrodynamic model was applied and different scenarios of water area in the Dongting Lake were assumed. The results show that the water level and peak discharge will increase,and the peak time will occur earlier with the shrinkage of the lake. The water level in the West and South Dongting Lake would increase by around 2.0 m and the water level in the East Dongting Lake would increase by around 0.4 m if the area of Dongting Lake was shrunk from 2 670 km2to 1 380 km2 when the 1996-type flood occurred in Dongting Lake. Moreover,the arrival time of the peak water level at Chenglingji station would be about 11 hours earlier,and the peak discharge would increase by about 4 300 m3/s. Hence,the flood disaster would be more severe in the Dongting Lake area if the lake continues to shrink from now on. In addition,the water surface slope changes a little in the West and South Dongting Lake. But the water surface slope increases a lot near the Lujiao station due to the water level in the East Dongting Lake is affected by both of the lake shrinkage and the water level in Yangtze River.
Abstract: The excessive concentration of precipitation in its spatial and temporal distributions could lead to occurrence of flood and drought disasters. Investigating the abrupt changes in precipitation concentration and the driving forces is fundamental for understanding precipitation responses to the changing environment,thus facilitating regional water resources management and early warning of natural disasters. This study investigated the historical changes in precipitation concentration at daily and monthly timescales in the Hanjiang River basin,based on the monthly precipitation concentration index (CIM) and daily precipitation concentration index (CID). The Mann-Kendall trend test and heuristic segmentation algorithm were used to analyze the abrupt changes in precipitation concentration. The potential influences of sunspots and anomalous atmospheric circulation factors on the abrupt changes in precipitation concentration were explored using the cross wavelet analysis technique. Results showed that:① Large values of CIM were found in the northern Hanjiang River basin,while small CIM values were observed in the south part of the study region. As for the CID,large values were found in the east part,with small values observed in the west. CIM has showed an insignificant downward trend,while a significant upward trend of CID was detected at 6 stations;② Mutation points of the CID series were detected at some stations in the basin,while the CIM is shown to be relatively stable,indicating that the CID is more sensitive to the changing environment than the CIM;③ Sunspots and anomalous atmospheric circulation factors have exerted strong influences on the changes in precipitation concentration. Notably,sunspots have exerted the largest influences,as they can indirectly affect precipitation concentration of the Hanjiang River basin through modulating the anomalous atmospheric circulation factors.
Abstract: Aiming at the problems of soil and water losing in the Huaibei area,the difference of runoff and sediment yielding from the Lime Concretion Black soil (LCBS) slope and the Fluvo-aquic Yellow soil (FYS) slope with gradients of 5°,10°,15° under different rainfall intensities (40 mm/h,60 mm/h,80 mm/h) was experimentally investigated. The results show that the initial runoff yielding time from the LCBS slope is longer than that from the FYS slope while the total runoff yield is relatively smaller for the LCBS slope. The initial flow sediment concentration from LCBS slope tends to decrease once slope rills appearing and eventually to a stable level. However,the flow sediment concentration from the FYS slope exhibits a fluctuating change. Sediment yield from the LCBS slope is less than that from the FYS slope under the various experimental conditions with exception of the combining situations of slope gradient 10° and rainfall intensity of 60 mm/h or 80 mm/h. The rills were well developed to an interconnected dendritic structure on the LCBS slope surface with a higher density rills occurring in the middle and lower part of the slope. Relatively,the rill density on the FYS slope surface is smaller. The runoff,sediment yield highly correlated to the slope gradient,rain intensity with multivariate linear and multivariate power relationships respectively for the both soil type slopes. The rainfall intensity gains a higher effect on runoff and sediment yield than slope gradient does.
Abstract: The aim of this work is to confront the problem of the high rate of missed alarms for flash flood warnings and forecasts due to the random nature of rainfall. In order to improve the accuracy of flash flood warnings,considering the transmission and diffusion principle of probability distributions,and by taking the characteristic parameters of rainfall patterns as the control conditions,a random rainfall pattern method based on parameter control is proposed. In addition,a critical rainfall model of flash floods based on random rainfall patterns and an early warning model considering the risk appetite of decision makers are established. Taking the Peihe watershed in Henan Province,as an example,we performed a comparative analysis of the 6 h critical rainfall under different rain pattern sets,and determined the 6 h critical rainfall threshold space for the study area. The results are as follows:the influence ranges of the precipitation peak position coefficient (r),peak multiple ratio (bmax) and antecedent precipitation (Pa) on the critical rainfall are 32% to 34%,33% to 34%,and 15% to 22%,respectively;the rainfall pattern is found to have a greater impact on the critical rainfall than that of the antecedent precipitation;the 6 h critical rainfall threshold space determined by the double factors of r and bmax is 98 to 185 mm. Therefore,the critical rainfall warning model considering the different risk preferences of decision makers is effective,and has the potential for guiding the more reliable implementation of flash flood warning systems.
Abstract: The delayed response of sedimentation is a phenomenon commonly encountered in the fluvial processes of rivers,while previous studies on the sedimentation of the Three Gorges Reservoir (TGR) give little consideration. A delayed response model of sedimentation is established considering sediment inflow and pool level to investigate the recent sedimentation characteristics at the TGR. Based on the measured data during 2003-2017 and the delayed response model,characteristics of water and sediment inflows,as well as characteristics and delayed laws of sedimentation in the flood seasons are comprehensively analyzed. The results indicate that the TGR exhibits the characteristics of large (low) water inflow and large (low) sediment inflow,and sedimentation in the flood seasons tended to increase in conjunction with the rise in pool levels during 2003-2012. There was a consistent relationship between the water and sediment change,and sedimentation in the flood seasons decreased owing to the operation of cascade reservoirs during 2013-2017. Accumulated sedimentation in the TGR during the flood seasons are closely related to the 5 years' linearly superimposed pool level. This indicates that sedimentation in the flood season is affected not only by the current year's sediment inflow and pool level but also by the sediment inflows and pool levels of the previous 4 years.
Abstract: The storage capacity of Poyang Lake is influenced by the flow from the main stream of the Yangtze River and five rivers in the Poyang Lake basin. After the Three Gorges Dam (TGD) went into operation,hydrological changes in the main stream of the Yangtze River impacted water exchange between the river and Poyang Lake. Based on the analysis of measured data and factors influencing outflow at Hukou station,empirical formulas describing outflow at Hukou station and critical water level for lake storage capacity was established. The change in lake storage capacity in the months before and after the construction of the TGD was then analyzed quantitatively. The results show that the flows from the main stream of the Yangtze River and the five rivers in the Poyang Lake basin affected outflow and water storage capacity by changing the water level difference between Xingzi and Hukou stations and by changing the water level at Hukou station. But the Yangtze River and the five rivers in Poyang Lake basin dirrered in process and degree. Given no change in the water level at Hukou station,when the flow from the lake basin increased by 1 000 m3/s,the outflow increased by 304 m3/s. When the flow from the main stream of the Yangtze River increased by 1 000 m3/s,the outflow decreased by 723 m3/s. In addition,the TGD already had an impact on the water storage capacity of Poyang Lake. The water volume of Poyang Lake decreased 49.4% in September,but increased 47.7% in May.
Abstract: Estuary deltas and coastal cities are facing with the rising risk of the compound flooding due to the coincidence of storm surge,extreme precipitation,high tide and high river discharge. A new integrated numerical modeling system,which coupled atmosphere,ocean and coast processes,was developed to simulate the extreme compound coastal flooding (including four threats of flooding) in Shanghai. The new developed model was calibrated to provide a valid numerical solution for the integrated compound flooding simulation. The results showed that flooding area (inundation depth > 0.2 m) was reduced by approximately 62% after the enhancement of seawall and levee in 1998. Therefore,it is concluded that seawall and levee construction plays a key role in coastal flooding prevention in Shanghai. The accurate simulation of extreme compound coastal flooding provides rational information for the future planning of property insurance and city development.
Abstract: Flow dynamics simulation in the bottom boundary layer (BBL) has great theoretical and practical significance in analyzing wave-current interaction and sediment transport. There are many BBL models,however,most of them are for single bed forms. An intra-wave process based 1DV model was established to simulate the flow dynamics in the BBL under the combined action of waves and currents,which is applicable for both vortex rippled beds and flat beds. This model is based on the governing equations of wave-current BBL. The k-ε model was employed to simulate the turbulence over flat beds. The combined vortex and k-ε model was employed for vortex rippled beds,and expressions of the turbulence kinetic energy and turbulence dissipation at the interface between the vortex layer and turbulence layer were derived. A number of experimental datasets were collected to verify the model,which showed that the model could properly simulate the flow dynamics in the wave-current BBL,such as the instantaneous velocity,kinetic energy and shear stress at different phases as well as the wave-induced mean velocity and wave-current mean velocity. Using this model,discussions were made on the patterns of the flow dynamics over different bed forms in combined wave-current conditions. In conclusion,this paper provides a tool for the study of flow dynamics in the BBL.
Abstract: In order to reveal the effects of seepage on the sediment initiation in swash zone,the experimental and numerical studies were performed in this paper. Firstly,the experiments of solitary wave run-up over permeable and impermeable slope was carried out in an indoor flume,the wave height and flow velocity were measured. Secondly,corresponding numerical simulations were performed and validated by the flume test results. Finally,the influence of seepage on the hydrodynamics in the swash zone and sediment initiation were assessed. The test results show that the asymmetry of wave becomes obvious under the effect of seepage. Strong infiltration during uprush increases the shear stress on the seabed surface,and wave downrush leads to the reduction of seabed shear stress due to the flow loss. It is also found that the seepage changes the effective weight as well as the shear stress of seabed soil particles,which leads to the variations of Shields parameter and greatly increase instability of sediment particles. The change of shear stress over permeable slope is the main reason for the Shields parameter variation of seabed particles. This study can provide guidance for the assessment and treatment of sediment transport in the swash zone.
Abstract: Wave-induced set-up brings severe impacts on the safety of the land area behind the reef through increase of the mean water level on the fringing reef flat. Flume experiments were carried out under irregular wave conditions to study the wave-induced set-up on reef flat. The experimental tests were carried out with 3 reef flat depths,4 significant wave heights and 4 spectral peak periods. It was found that:① Stable hydrodynamic conditions on reef flat can be achieved only when the sampling time lasts for more than 200 peak periods;② Irregular wave trains have more complex breaking process than that of regular wave trains,in that the larger waves in the same wave train break on the fore-reef slope in plunging manner,while smaller waves break on the reef flat in spilling manner or do not break;③ The maximum set-up on reef flat was found to increase with the increase of the peak period,and decrease with the increase of the water depth on the reef flat,and be proportional to the significant wave heights. The formula,proposed by Gourlay for estimating set-up on reef flat under regular wave conditions,was extended to irregular wave conditions. It was found that the maximum set-up can be well predicted by replacing the regular wave height and period in the formula with the significant wave heights and peak periods for irregular waves,respectively.
Abstract: Although research on the connotation,goal,content and effect of sponge city is a current topical research area in China,there is lack of investigations on possible adverse effects to sponge city. In recent years,the facilities of rainfall runoff concentrated infiltration have been widely used in the construction of sponge city,however studies have not focused on the influence and risk of concentrated infiltration,a factor which is not conducive to the healthy development and construction of sponge cities in China. Therefore,this study examines the difference between concentrated infiltration and traditional infiltration,and reviews the progress and deficiency of investigations in China and beyond on the influence of concentrated infiltration on soil pollution,groundwater volume and quality. The numerical simulation of the effects of concentrated infiltration on soil and groundwater,and the cumulative effect of pollutants and their risk assessment are also examined. We suggest that future studies should examine the following four aspects:① The risk of soil pollution and its influence factors;② The cumulative effect of rainfall runoff concentrated infiltration on soil pollution;③ The prediction of groundwater pollution risk;and ④ Measures to reduce the risk of groundwater pollution.
Abstract: Consolidated silt is a special depositional landform. The properties,formation and evolution of consolidated silt,which relate to the fields of marine sediment dynamics and marine engineering geology,are of crucial importance for shore protection,navigation safety,and marine construction. This paper introduces the concept,characteristics and distribution of consolidated silt and focuses on mechanisms of formation and evolution. Recent research progress on consolidated silt is reviewed according to the three modes formation and evolution (i.e. primary deposition mode,coarsening and sorting mode,and liquefaction and seepage mode). New research ideas and methods with regards to consolidated silt are considered after analyzing the advantages and disadvantages of emphasizing the three modes. It is proposed that future research directions should:combine basic approaches of marine soil mechanics and sediment dynamics;carry out simultaneous observations of multiple-factors characterizing the water,seabed surface,and internal seabed;build an integrated conceptual model to describe the physical process;establish corresponding mathematical models to evaluate the formation and evolution of consolidated silt.