Abstract: In this work,the urban flood and waterlogging problems in Beijing were discussed based on the socio-ecologic statistical data and hydrological and meteorological data. Firstly,the results show that the urban waterlogging has become the major type of urban floods in Beijing during the past 50 years. The spatial distribution of waterlogging sites had an outward expansion tendency accompanied by the rapid urban expansion. The artificial depressions in urban areas became the black spots for urban waterlogging because of the land use change and urban micro-topography change. The waterlogging sites had an increasing trend due to the effects of the climate change and urban development. Secondly,the formation mechanism of urban flood and waterlogging was discussed from the perspective of urban water cycle system. The precipitation pattern over the urban areas was changed due to the effects of climate change and rapid urban development. The precipitation in flood season and precipitation extremes both showed a decline trend,whereas the precipitation intensity for short duration precipitation events showed an increasing trend. Additionally,the changes in underlying feature due to the rapid urban development caused the changes in the rainfall-runoff characteristics,which may cause the changes in regional hydrological processes and water resources amounts. Overall,the increasing risk of urban flood and waterlogging was affected by the frequent extreme precipitation events,the changes in drainage pattern,land use/cover change and urban microclimate characters due to the climate change and urban expansion. The increasing vulnerability was also caused by the low standard in waterlogging elimination and drainage control,the deficiency in urban infrastructure and emergency management capacity,which caused the decrease of urban comprehensive response to the urban floods and waterlogging problems.
Abstract: The purpose of the study is to analyze the response of river system to the climate change and rapid urbanization process in Suzhou,Taihu Lake plain area. The study selected the drainage density,water surface ratio,fractal dimension and tributaries development coefficient as representative parameters. Based on GIS technology,the spatial and temporal variation of river networks caused by urbanization and climate change in the study area in the past 50 years was analyzed. On this basis,the study focused on the quantitative response of river network changes to urbanization and precipitation change,and the development trend of river system under highly urbanization. The results showed that:① The river system in Suzhou presented a high spatial heterogeneity and rapid descending tend in the past 50 years. The drainage density,water surface ratio,fractal dimension and evolution coefficient of branch river were decreased by 5.6%,19%,1.2% and 3%,respectively. ② Compared with climate change,highly urbanization was recognized as the stronger driving factors of river system degrading in the past 50 years. Urbanization contributed over 67% to each unit of river network change.③ With the increase of urbanization,the drainage density,fractal dimension and evolution coefficient of branch river first showed growing trend and then started to decay,presenting a curve in "inverted U shape". The influence of high urbanization to river networks grew from weak to strong. Currently,the development of river network and urbanization rate was in the right of the curve. With the mutual adaption between the urbanization and the development of river network systems,they will evolve into a high-level development stage in the future. This research investigated the developing rule and relationship of urbanization and river network systems,providing support for the river network health and green development and protection in similar area.
Abstract: To handle increasingly severe basin-scale drought systematically,this study develops the Yellow River basin drought response system facing the entire process of drought. A drought index is designed based on the evolution of drought. Drought,water demand,and runoff are forecast using the Weather Research and Forecasting model and regression analysis. The drought limit water level is set in the multiyear regulating storage reservoir to realize interannual compensation of water resources. After the characteristics of flood and sediment stages are identified,the staged flood limit water level is applied to increase the utilization of flood resources. A synergetic optimal operation model of cascade reservoirs is developed to allocate water resources in order to resist drought. The case study indicates that the drought response system in the Yellow River basin can prevent severe damage from drought by balancing the damage among different years. Compared with the actual situation,this system could increase 2.24 billion m3 of the water supply in 2014 when severe drought occurred. The drought response system has been applied in the Yellow River basin,enhancing the capacity for regulating basin-scale water resources to handle drought.
Abstract: To investigate how uncertainty in precipitation forecasts impacts flood forecasting,the THORPEX Interactive Grand Global Ensemble (TIGGE) data extracted from the China Meteorological Administration (CMA),the National Center for Environmental Prediction (NCEP) and the European Center for Medium-range Weather Forecast (ECMWF) were used to establish the GR4J hydrological model such that probabilistic ensemble flood forecasting is explored for the Three Gorges Reservoir. The effectiveness of four statistical post-processing methods,including Bayesian Model Averaging (BMA),Copula-BMA,Ensemble Model Output Statistics (EMOS) and the Modified Bayesian Model Averaging (M-BMA) methods,were compared and analyzed. The results showed that each of the four methods could provide a reasonable and reliable confidence interval on prediction. Besides,compared with the raw deterministic forecasts,the forecast accuracy of expected values associated with the four methods was improved,where the forecast error in water volume was significantly reduced. Furthermore,the M-BMA method performed the best because it considered the heteroscedasticity of the predictive distribution,without conducting a normal transformation,which could be much simpler and more flexible in practice.
Abstract: The response of main channel migration to the altered flow and sediment regime is characterized by complexly temporal and spatial variations in the Lower Yellow River (LYR),and the investigation into the characteristics of main channel migration is crucial to understand the principles of river regime adjustments and fluvial processes in the LYR. In this study,widths and intensities of main channel migration at section-and reach-scales were calculated based on the data of remote sensing images and 28 observed cross-sectional profiles in the braided reach of the LYR during the period from 1999 to 2016,with the characteristics and the key influencing factor to control main channel migration intensity being identified quantitatively. It is found that the main channel processes in the braided reach migrated back and forth towards the left or right direction,and section-scale migration widths in the middle reach were generally larger than the values in the upper and lower reaches. The width and intensity of main channel migration show a gradual decreasing trend,with the average were 188 m/a and 0.16 respectively. It is confirmed that the incoming flow and sediment regime is a dominant factor to influence the main channel migration,although the channel boundary conditions can influence the intensity of main channel migration,covering the influencing factors of the level difference between main channel and floodplain,and composition of bed material. The main channel migration intensity of the braided reach can be expressed as a power function of the previous three-year average fluvial erosion intensity,and the calculated main channel migration intensities in 1999—2016 using the proposed relation generally agree with the observed data.
Abstract: To examine the effects of rock fragment contents of spoil heaps on hydrological and sediment yielding processes,multiple simulated rainfall experiments were conducted in the laboratory. The results showed that,as the rock fragment content in the spoil heap increased,the duration of runoff generation on the slope was prolonged,the rate of runoff decreased linearly,and the soil detachment rate decreased. Under multiple rainfall events,rock fragments in spoil heaps decreased sediment yield primarily through the effects on runoff generation duration,runoff rate and rock fragment cover,and principal component regression could express the relationship between these factors and the average soil detachment rate. Under a certain rock fragment content,as surface soil was detached under multiple rainfall events,the rock fragment cover increased,which caused the soil detachment rate to decrease exponentially and also had a significant functional relationship with the accumulated total soil loss. The erosion reduction effect of the rock fragments contained in the spoil heaps can provide an important reference for water and soil conservation of the underlying surface of production and construction projects.
Abstract: The hyporheic exchange plays a crucial role in maintaining fluvial ecological processes. In order to examine the influence of streambed heterogeneity on the hyporheic exchange,a surface water-groundwater coupling model is constructed for solving flow interactions over a sand dune. By generating different permeability random fields,the influences of hydrodynamic processes and streambed heterogeneity on the hyporheic exchange flux,exchange space and mean residence time are addressed. The results showed that the coupling model has good performance and it can accurately depict the flow fields near the sediment-water interface. Under homogeneous or heterogeneous scenarios,the mean exchange flux and bulk residence time via Reynolds number are representing power law relationships,and the mean exchanged depth tends to be stable after the surface flow transitions into highly turbulent conditions. Results further shown that higher heterogeneity can effectively enhance the hyporheic exchange flux and spatial exchange frequency along the sediment-water interface. However,it will restrict the exchange volume and shorten the mean residence time of flow in the hyporheic zone.
Abstract: The radial sand ridges (RSRs) along the coast of Jiangsu Province are a rare form of tidal sand body. Some inferences have been made on the hydrodynamic mechanisms of radial sand ridges formation;however,opinions on their formation time and sediment origin source differ. In this study,the formation and evolution of radial sand ridges were simulated by a long-time-scale dynamic geomorphological model. The results showed that the convergence and divergence of sand flow field outside the coast of Jiangsu province existed independently without the influence of seabed topography. In different initial topographies,the radial "channel-sand ridge" geomorphological pattern is formed by tidal power in around 150 years as long as there is sufficient sediment supply. Because the abandoned Yellow River flowed northwards 150 years ago,the Yellow River underwater delta continues to be eroded. The radial sand ridges are the result of redistribution of different sediment sources from convergence and divergence flow and are the products of dynamic-sediment-topographic interaction.
Abstract: The response of tidal flats to storm surges is an important scientific problem in the evolution of tidal flats under climate change. To investigate the evolution of silt-muddy tidal flats in response to storm surges along the Jiangsu coast,the measured surface elevation of tidal flats pre-and post-storms and associated hydrological data were analyzed. The evolution of intertidal flats induced by storm surges is characterized by erosion in the upper intertidal flats while accretion in the lower flats. This is distinct from the evolution of sandy coasts which show erosion in the upper zones and accretion in the lower zones led by offshore sediment transport. A two-dimensional numerical model (Delft3D) is established considering tidal currents and sediment transport to simulate the bed evolution under calm and storm conditions,respectively. The hydrodynamic mechanism is explained for the regional characteristics of tidal flats,which shows an episodic effect on morphology after storm waves. The flood-dominated tidal current plays a key role on the long-shore sediment transport.
Abstract: Solitary waves can cause intense sediment movement in the coastal area,resulting in local scour around the offshore structures that use submerged pile foundations,and seriously affecting the stability of offshore structures. On a flat sand bed with sediment median size of 0.22 mm in a wave flume,a series of experiment were conducted respectively for two cylinders with different diameters and different submergence ratios under the action of solitary waves with a range of incident wave heights. The evolution of scour around the cylinder was recorded and analyzed,and the profiles of the scour hole were compared between those formed under solitary waves and steady current,and between solitary waves and regular wave conditions. It is shown by experimental results that,under the same submergence ratios,the scour type changes from the Twin-Horn-Shaped to the Transient-Shaped as the wave height increases. Under the same incident wave heights and the hc/D within the range 1—7,the equilibrium scour depth in front of the cylinder is independent on the submergence ratios,while the equilibrium scour depth behind the cylinder decreases with an increase in the submergence ratios,and coefficient Ks as a function of S is obtained. A significant difference is found in the profiles of scour holes formed under regular waves and solitary waves,but no obvious distinction is exhibited for scour under steady current. The comprehensive calculation formula of scour depth is obtained from the measured data. It is shown that the deviation between the values from the empirical formula and measured data is below 7%.
Abstract: The long-distance,non-pressure branch tunnel is a hydraulic structure that affects the flow regime and discharge ratio. In the water-filling stage,there are complex flow regimes in the branch tunnel. In this paper,the numerical simulation calculation in the non-pressure branch tunnel at the water filling stage,considering different inlet flows and diversion angles,was carried out by the FLUENT software. At the same time,the water depth at each monitoring station and the discharge ratio for the branch tunnel,after the water flow is stable for different inlet flows,were verified by the model test. The results show that the inlet flow during the water filling stage has little effect on the discharge ratio,which depends mostly on the diversion angles,between 15° and 45°,considered in the simulation. In addition,a recirculation zone appears in the upper flow outside of the lateral branch when the diversion angle is increased to 60°. The change in the diversion angle has little effect on the main tunnel water surface line. The water level in each tunnel section increases when the inlet flow increases.
Abstract: Landslide-induced surge is a kind of secondary disasters triggered by waterside landslides. Generally,the endangering range in a landslide-induced surge far exceeds the motion area of the landslide,and accurately predicting spatial evolution of landslide-induced surge is of significant importance for disaster prevention. However,the existing models usually simplify landslides as rigid bodies,which is obviously against the fact that many landslides propagate in a flow-like way. Therefore,a numerical model was put forward in this paper to provide more reliable prediction results for landslide-induced surges. By treating slip masses as flow-like materials,the governing equations of landslide-water coupling motion were derived. Next,the governing equations were solved by the finite difference method,and the dynamic model that can simulate the evolutionary process of flow-like landslide-induced surge was established. Finally,the evolutionary process of Gongjiafang landslide located at the Three Gorges of the Yangtze River was simulated by the model and the simulated maximum wave heights in the longitudinal section (i.e.,the flow direction) of the river were compared with the measured data. Results show that the maximum wave height in this section appears in the main sliding direction of the landslide,and the maximum wave heights on both two sides decrease quickly. The simulated results agree well with the measured values. The established model can provide more adequate prediction of the influence range of landslide-induced surge.
Abstract: Agent-based modeling (ABM) has enriched the theory and methods of research in water resources management. Better understanding of the state-of-the-art of ABM and its potential in the field of water resources management can promote institutional development and reform in China's water resources management system. In this paper,while ABM of water resources management (WR-ABM) is defined,its key components—agent decision rules and agent interactions—are identified and their modeling approaches are summarized. Various WR-ABMs applied in basin-scale optimal water allocation,urban household water use and agricultural water management published during the period of 2009—2018 are carefully reviewed. Future research on the use of WR-ABMs that should address the challenges and weakness in the water resources management are discussed and several research directions are recommended herein:① further expanding the use of complex adaptive system theory in the field of water resources management;② coupling ABM and water resources system models that include uncertainties;③ exploring the use of machine learning algorithms in the decision-making modeling;④ improving the methods used in the model parameter calibration,result verification and validation;⑤ using standard documentation protocol,such as the ODD protocol,for the description of models;and ⑥ achieving comprehensive and optimal balance between completeness and simplification in model design.
Abstract: Soil erosion is divided into three subprocesses of soil detachment,sediment transport,and sediment deposition. Exploring the critical conditions in terms of hydraulic,topography,soil as well as near-surface characteristics for the occurrence and development of these processes is the basis of establishing process-based soil erosion model and then accurately predicting soil erosion. Hence,the paper summarized the main factors that influencing soil detachment process by overland flow and its mechanism in respect to overland flow hydrodynamics,soil properties,near soil surface characteristics,and spatial-temporal variability of soil detachment. Finally,main problems requiring deep exploration in future are recommended from six aspects,i.e. the spatial-temporal scale of study on soil detachment process,the dynamic mechanism of soil detachment process,the influencing mechanism of rill morphological evolution and network structure on soil detachment process,the coupling mechanism of soil detachment and sediment transport,the measuring method and criterion of soil detachment,and the obtaining and predicting soil erosion resistance parameters. The paper is conducive to deepening the understanding of soil detachment process by overland flow. Additionally,it provides valuable references for the correlational studies of soil erosion process and establishment of process-based soil erosion models.