Abstract: A new concept of river-lake water exchange coefficient is introduced to investigate the river-lake water exchange in the middle and lower reaches of Changjiang River. The concept is constituted of the ratio of the amount of runoff from tributaries to a lake and the amount of runoff from the lake to the main stream of a river during a certain period of time. The equilibrium theory is used in the derivation of the river-lake water exchange coefficient. Based on the latter, the exchange process in river-lake water can be classified into three types: Diverting flood waters to lake, water balancing, and draining lake water out to river. Results show that the water exchange between the Dongting Lake and the main stream of Changjiang River has evolved from diverting flood waters to lake, to water balancing, and to draining lake water out to river during the past 60 years. No significant changes can be observed in water exchange types between the Poyang Lake and the main stream of Changjiang River, as revealed by fluctuations of the river-lake water exchange coefficient indicating mostly the water balancing type. The river-lake water exchange coefficient can be strongly correlated to the amount of runoff from the main stream of Changjiang River. In contrast, a week correlation is found in lake's tributaries. Thus, the river-lake water exchange is dominantly controlled by the water flow in the main stream of Changjiang River.
Abstract: Understanding of the temporal and spatial variation of soil moisture in sand dunes is a fundamental part of exploring the ecological and hydrological pattern of desert ecosystem. The temporal and spatial variation of soil moisture is analyzed using neutron probe soil water data collected from the Gurbantunggut Desert in central Eurasia. Results show that the spatial variation of soil moisture in different locations over sand dunes exhibits high level of consistency over time. Vertically, the variation behaves differently with soil layers. In the top layer of the soil (0—1 m), the volumetric soil moisture content has the highest value at the top, following by the hill, and the driest value is found at the foot. The opposite is true for the sub-layer of the soil (1—2 m), where the highest value occurs at the foot, following by the hill, and the smallest value is found at the top. The variation of soil moisture exhibits significant seasonality and has a strong vertical structure. The soil moisture is the richest in spring, but changes most rapidly during the season. The values of the coefficient of variation (CV) in soil moisture for the layers of 0—40 cm, 40—140 cm and 140—200 cm are 13.56%, 5.35% and 0.8%, respectively, which correspond well to the source of soil water, the evaporation loss and the distribution of shrub roots in different soil layers. The vertical variation of soil moisture is greater than that in the horizontal ones. The vegetation covers and topography have strong influences on the spatial distribution of soil moisture. There exists a water-rich region in the Haloxylon ammodendron root-zone and the foot of sand dunes.
Abstract: As an indispensable part of the Qinghai-Tibet Plateau (QTP) water cycle, precipitation and surface water in QTP plays an important role in exploring the permafrost hydrological process and in studying the history of permafrost development. Although enormous achievements in studying QTP precipitation and surface water have been made using isotopic methods, there has been a lack of relevant research on the subject in the Beilu River region of central QTP. In order to have a comprehensive understanding of the QTP water cycle and permafrost hydrological process, a study is conducted on the spatial-temporal characteristic of stable isotope variation in precipitation and rivers in the QTP Beilu River permafrost region. The isotopic composition and meteorological data from the region for the June-December period of 2011 are used to analyze the relationships among stable isotopes, daily average temperatures, precipitation amounts and flow water. Results show that temperature has a dominant effect on the stable isotopes in precipitation in the QTP Beilu River permafrost region. The isotopic seasonality of precipitation exists due to influences of both westerly and monsoon on local precipitation from June through December, which suggests that the stable isotopes in precipitation are controlled by both temperature and precipitation amounts. The stable isotope variation in rivers exhibits a similar behavior to that in precipitation, suggesting that rivers are recharged by the precipitation fallen in the QTP Beilu River permafrost region. The influence of precipitation on stable isotope variation in rivers is determined by precipitation amounts, i.e., the smaller the precipitation amounts are, the lighter the influences become. When compared to the local meteoric water line of the QTP Beilu River permafrost region, both slope and intercept of the δ18O-δD relationships are larger for rivers, suggesting that rivers are also recharged by other sources within the regional water cycle and influenced by evaporative fractionation.
Abstract: Values of rainfall parameters like the cumulative rainfall, the mean rainfall intensity and the rainfall duration, depend on the definition of a single continuous rainfall event. In this study, a revised method is presented for defining a single continuous rainfall event. Using rainfall data collected during the occurrence of debris flows in the Wenchuan earthquake area, single continuous rainfall events and derived values of rainfall parameters are determined with the Jan's method and a revised method proposed in this paper. Empirically-based rainfall thresholds for debris flows occurrence (based on the cumulative rainfall, the mean rainfall intensity and the rainfall duration) are assessed following the two methods for defining single rainfall events. The threshold values obtained using the two methods are validated with debris flows events, which occurred in the Qingping town of Mianzhu city in the Sichuan province. Results show that the cumulative rainfall threshold values slightly differ for the two methods. The mean rainfall intensity threshold obtained by the revised method is generally smaller than that of the Jan's method. This is due to the fact that the revised method is able to improve the shortcoming of the Jan's method often resulting in a shorter duration of a rainfall event. The revised method proved to be both rational and feasible. The results show that the rainfall thresholds obtained by the revised method can be used to predict the debris flows in the Wenchuan earthquake area.
Abstract: In order to ensure rational utilization and to improve rainfall retrieval algorithms, it is of great significance to illustrate the characteristics and the sources of errors in the daily Tropical Rainfall Measuring Mission (TRMM) 3B42V6 precipitation product. In this study, we analyze the accuracy of three high resolution precipitation datasets at a spatial resolution of 0.25°×0.25° degrees using daily rain gauge data observed over the Ganjiang River basin. The three datasets are daily TRMM and other satellites precipitation product (3B42V6 derived), TRMM (3B42RTV6), and NOAA Climate Prediction Center Morphing Technique (CMORPH). Results show that the bias (B) of 3B42V6 is substantially less than that of 3B42RTV6 and CMORPH. However, CMORPH is more accurate than 3B42V6 as measured by other accuracy indices including the absolute bias (AB), the Nash-Sutcliffe efficiency coefficient (C) and the probability of detection (PD). The lower B values obtained from the TRMM 3B42V6 precipitation product are due to the fact that the rainfall monthly ground station data are used to correct the original TRMM data. However, the scheme for merging microwave and infrared (IR) precipitation information in TRMM 3B42V6 may not be as effective as the ones in 3B42RTV6 and CMORPH. As a result, the TRMM 3B42V6 precipitation product has higher AB but lower C and PD values compared to CMORPH. A further comprehensive assessment on the quality of daily TRMM 3B42V6 precipitation product is thus warranted to improve the performance of the scheme for merging microwave and IR precipitation information as well as the method of merging the satellite-gauge data.
Abstract: The hydrodynamic characteristics of staggering peak flows at the Y-shaped confluence of Yiluo River are systemically investigated for the planning and design of a flood-prevention project. A two-dimensional hydrodynamic model is applied to the Y-shaped confluence to simulate staggering peak flows. The hydrodynamic characteristics of staggering peak flows can thus be analyzed including water levels and flow patterns, as well as the interaction of two tributary flows. Results show that model simulations agree well with observations. The surface-water flow pattern in the confluence section shows a saddle-shaped surface moving downstream with an increase in discharge.The overbank flow caused by the stronger tributary flow can obstruct the weaker one during the course of staggering peak flows. The hydrodynamic characteristics in the confluence section are significantly associated with the ratio of stronger tributary flow to the weaker one. The structure and direction of rotation of spiral induced flow downstream of the confluent point are determined by the stronger tributary flow. The variation of water level with time exhibits a quadratic curve with three line segments during the course of staggering peak flows, and the duration of high water levels can be 5 to 7 times longer than that occurred during the course of a single peak flow or a single flow with the equivalent peak value. The stage-discharge relation graph can be fitted with a forked curved line. The latter has two inflection points in the vicinity of the two flood peaks.
Abstract: The Lower Jingjiang Reach (LJR) is experiencing problems with continuous channel degradations, and local riverbank erosion occurs frequently. In order to investigate quantitatively the soil properties and erosion mechanisms of composite riverbanks, a field survey is conducted at 6 sections of riverbank collapses along the LJR. An indoor soil experiment is also performed, and both physical and mechanical properties of soil samples are analyzed. The analysis reveals that the vertical profile of soil compositions in the LJR can be characterized as being a typical composite structure. The lower layer is thicker with non-cohesive soils, and the upper is thinner and loose with cohesive soils. A new method for calculating the safety parameter of overhanging block in a composite riverbank is then proposed. The method uses the principle of cantilever stability with the bank stability degree being closely related to the unit weight of the overhanging block and the corresponding tensile strength. Based on the calculated near-bank hydrodynamic conditions and the experimental results from the indoor soil tests, bank stability degrees and influencing factors of a typical composite riverbank in the LJR are investigated quantitatively. The following conclusions can be drawn from the study. The incipient velocity for the non-cohesive lower layer is obviously less than the mean bank-toe velocity, which easily lead to the occurrence of severe fluvial erosions. The cohesive upper layer before failure has sufficient strength to resist from direct fluvial scour. However, the failed soil blocks deposited in the near-bank zone would be broken down and transported downstream by the near-bank fluvial entrainment due to the soil properties of loose structure and low liquid-limit clay. The safety parameter measuring the degree of the bank stability at each stage during a hydrological year varies periodically and is calculated. The lowest value at the flood recession stage is obtained, which is consistent to the recent statistics from the field surveys on bank erosions.
Abstract: The Shields curve is often used to judge the criterion for incipient sediment motion. The curve for all the flow regimes is derived on the basis of the boundary layer theory. The Shields parameter and curve are modified in order to take into account the effect of viscosity. The modified curve is then re-explained from the viewpoint of boundary layer. Results show that the curve's shape exhibits a linear distribution in the smooth turbulent and laminar regime. In the transition regime, the curve is similar to that representing the friction law for steady flow, and finally becomes a horizontal curve in the rough turbulent regime. The modified Shields curve keeps the consistent shape with the original one. The modified curve is applicable to predict the incipient motion of both fine and coarse sediment particles. The prediction agrees well with measured data. In fact, the Shields curve represents the relationship between the threshold bed shear stress and the flow status around the particle. The threshold bed shear stress of one particle is different under different flow regime conditions.
Abstract: Tsunami scour around a coastal structure is one of the primary causes of structure damages. However, the mechanisms of tsunami-induced scour and beach erosion are less understood compared to that of tsunami run-up and tsunami inundation. A series of two-dimensional laboratory experiments of tsunami scour is conducted in accordance with coastal road conditions situated on a sandy beach. The initial 1/10—1/20 composite beach slope is respectively exposed to the N-waves. Measurements are made for the free surface elevations, the wave uprush and backwash processes, run-down jump, and the change of cross-shore beach profiles. The experimental result shows that there exists not only a profile change in the beach due to the impact of back sheet flows, but also the scour around the coastal road resulting from the helical flow. The location of roadway is a key factor affecting the scour depth along the coastal road. The incident wave height is also a contributing factor, but the depth of road embedment has little effect on the scour depth as shown in the experiments. The experiments reveal that coastal roads are vulnerable to tsunami damages due to serious scouring. The information reported in this study is useful for local authorities to better plan for tsunami disaster relief.
Abstract: Some of the natural incised river channels have unique features in fluvial morphology and longitudinal profiles. In this study, the relevant mechanisms of fluvial morphology during incision and their influence on longitudinal profiles are investigated through field surveys, and the use of data analysis and geographic information system (GIS) techniques. Results show that there exists a loss and compensatory mechanism of sediment transport in response to riverbed incision, which is an important factor driving this type of river systems towards a state of equilibrium. After deepening, riverbed sediments consume more erosive energy and sustain high gradient channels, leading to special longitudinal profiles. River discharge and riverbed sediment are the dominant elements controlling the formation of longitudinal profiles. After introducing a dimensionless parameter derived from river discharge and riverbed sediment, the channel gradient and the new dimensionless parameter shows a good linear relationship.
Abstract: At the entrance reach of multi-branched channels, the temporal and spatial distribution of sediment erosion or deposition often changes due to the position of main stream shifting with different income discharge. In order to investigate the relation between the magnitude of bed deformation and flow discharge, numerical simulations of flow fields are carried out and channel morphology comparisons were made across different discharge in several braided reaches of the middle Yangtze. It is found that flow dynamics exhibits a critical character when incoming discharge changes. That indicates the existence of two critical discharges Q1 and Q2. The head of sandbar will be eroded when the incoming discharge ranging between Q1 and Q2. The head of sandbar will be deposited when the incoming discharge is above Q1 or below Q2. Because the impacts of incoming discharge on river bed deformation are different according to different discharge magnitudes, the seasonal or annual adjustment of bed morphology at the head of sandbars will mainly depend on the duration of discharge process.
Abstract: The time-mean velocity profile of turbulent open-channel flows over rough and permeable beds is analyzed both experimentally and theoretically in this study. The velocity profile can be derived from a slip velocity at the bed surface and an origin displacement, a modified logarithmic law is thus obtained. The experiment is conducted in a glass-sided flume where the channel bed is prepared with uniform marbles 1 cm in diameter. The velocity is measured using a laser doppler velocimeter. The profile of the measured velocity is used to validate the theoretically derived profile. The result shows that the modified logarithmic law is able to well describe the measured velocity profile. Under the same flow condition, the shear velocity over the permeable bed would be greater than that over the impermeable bed. The value of the flow at the permeable interface is about 0.35 to 0.45 times that of the average stream flow.
Abstract: The pressure gradient along riverbanks is one of the main driving forces for lateral hyporheic exchange, substance and energy transport in riparian zones. In order to reveal the discipline and main factors of disturbed pressure distribution along sinuous riverbanks, series of experiments are carried out in an adjustable slope test flume. Different geometries of sine-shaped channel models are built and put into the flume to form different boundaries of riverbanks. The pressure head data are measured in a wavelength range of riverbanks to study the characteristics of pressure distributions in different layers, different sinuosity of riverbanks and different hydraulic conditions. A sensitivity analysis of influence factors is also conducted. The experimental results show that the pressure distribution along the sinusoidal riverbank is a curved shape. The pressure distributions in different layers are almost the same. When the riverbank sinuosity is relative low, the pressure variation reaches the crest value and trough value respectively at the most concave and convex location of riverbanks. Under subcritical conditions, the disturbed pressure along the riverbank is mainly affected by the bank amplitude-to-wavelength ratio a/λ and Froude number Fr. The sensitivity analysis reveals that the influence of a/λ is relatively strong.
Abstract: The quality of terrain generalization has an important impact on the result of numerical simulations. Aiming at the practical issue associated with the coexistence of long narrow valleys and vast floodplain, an adaptive grid method with hierarchical structures (HAGM) is introduced. The adaptive criterion is made on the basis of the gradient of free surface elevation and the local Froude number. The local grid density can be adjusted automatically in response to a changing flow regime, which makes it possible to achieve a balance between accuracy and computational efficiency in numerical simulations. The finite volume method is used to solve the two-dimensional shallow water equations based on the HAGM. Gradient limiters and the two-step Runge-Kutta scheme are employed to improve the accuracy in space resolution and time marching, respectively. Results of numerical experiments show that the HAGM has good performance in adapting to a changing flow regime, in capturing sensitive zones for water level calculation, and in achieving local grid refinement. The proposed HAGM is thus suitable for a wide range of applications.
Abstract: The decision maker's experience and knowledge on judgments have a significant influence on the actual flood control operation. In this paper, the decision maker's experience on flood control pre-discharge dispatching and on the required persistent state of releasing flood waters are considered as constraints , and a multi-constrained heuristic progressive optimality algorithm (MHPOA) for flood control operation is thus proposed. In the MHPOA, the general operation rule for flood releasing is seen as heuristic information, which controls the operation of increasing or decreasing discharge rates by judging the variation tendency of inflow and reservoir level. The heuristic information is combined with the progressive optimality algorithm to searching for the optimal operation of flood control with the objective of maximal flood peak reduction. The proposed algorithm is tested for 5 different scenarios designed for optimizing the flood control operation in the Shuikou reservoir. The result shows that the MHPOA is able take a full account of decision maker's experience and knowledge on judgments, reduce the refill risk caused by the issue of too low reservoir levels in the pre-discharge period, avoid the fluctuation of discharge, and obtain the satisfactory solution.
Abstract: Vertical velocities of groundwater flow can be determined by temperature time series measurements at different groundwater depths. The most typical analytic model is the Hatch model, which uses the characteristic of temperature damping or the phase lag of temperature. A local sensitivity analysis is conducted on the parameter of Hatch model in an effort to identify the most important impact factors of the model, and subsequently, the effect of each parameter on the modeling result. The global sensitivity method is then applied to analyze the interaction among parameters and their effects on model simulations. Results show that the distance between the two measuring points (Δz) is the most influential factor on the model simulation, and followed by the heat capacity of solids (ρscs), the effective porosity (ne). The model simulation is least sensitive to the variations of the heat conduction coefficient (λ0) and the degree of heat dispersion (β), and a negative correlation is found between vertical velocities of groundwater flow and the two parameters λ0 and β. While for other parameters, positive correlations exist. As a result, a special attention should be paid on measurement of the distance between the two measuring points, while empirical values can be assigned to the two parameters λ0 and β.
Abstract: In this paper, a new model WESC2D (Two-Dimensional Water Environmental Simulation Code) using an orthogonal curvilinear grid system is developed for the impact study of pollutants discharged into the lower reach of Ganjiang River. The model development is based on the eutrophication principle of the Water Quality Analysis Simulation Program (WASP) model and the Eulerian-Lagrangian alternating direction implicit (ELADI) scheme. The model convection terms are solved by the Eulerian-Lagrangian method based on the bilinear interpolation and sub-grid scale method. The simulation of WESC2D is compared to the analytic solution to the problem of simple concentration peak transport, and to the field measurement of dissolved oxygen from the lower reach of Ganjiang River. The dynamic relationship between the pollution loads and the water quality at sewage outlets along the reach is also examined. Results show that the simulation of WESC2D agrees well with the analytical solution at a substantially reduced numerical cost compared to that of using the general Eulerian-Lagrangian Method (ELM). The new model WESC2D provides an important tool for the study on the relationship between the pollution loads and the water quality in the lower reach of Ganjiang River.
Abstract: The discrete procedure of non-conservative St. Venant equations is carried out by making use of the flux difference splitting scheme. In order to maintain the same dispersed form, characteristic theory is introduced into boundary points discretization. Combining with entropy correction, the flux limiter is introduced into the discrete equations to preserve the property of total variation diminishing (TVD). Based on the implication of TVD, a one-dimensional (1-D) river network model is derived. The 1-D model is used to simulate water levels and discharges in a reach of Chengtong River. Results show that the model is able to handle trans-critical flow fields with large time steps. The model simulates well the water stage and the diversion ratio of bifurcation, indicating the model has practical values in flow simulations.
Abstract: The rainfall threshold is an important indicator of flash flood conditions. In this study, the existing methods for computing rainfall thresholds are divided into two categories and reviewed on the basis of their technical principles. The two categories include the data-driven statistical and inductive methods and the physical process-based hydrologic hydraulic methods. As expansions of rainfall thresholds, the dynamic rainfall threshold and the storm critical curve are also introduced and discussed together with advances in uncertainty analysis of rainfall thresholds. In our review, the statistical and inductive methods have been more widely accepted in China. Moreover, antecedent rainfall (or antecedent soil saturation) and cumulative rainfall at particular time intervals are the two governing factors commonly considered in the calculation of rainfall thresholds. Cumulative rainfall may be the loneliness factor to be considered at times. Further, it is found that the rainfall threshold conveys poorly the magnitude of flash flooding. Understanding of the uncertainty in rainfall threshold calculations would be helpful for the improvement of flash flood warnings. However, how to incorporate the uncertainty into the decision-making process still remains a major challenge.