Abstract: Tropical cyclone (TC) is the most harmful weather system in China, so analyzing and understanding the new characteristics of Chinese coastal landfall TCs is of great significance for disaster prevention and mitigation. In accordance with the meteorological data of the past 70 years, the characteristics of Chinese coastal landfall TCs are studied by utilizing statistical methods. The results illustrate that there is obviously varying in characteristics of Chinese coastal landfall TCs under the circumstances of climate change. The frequency of landfall typhoon is higher than that of previous years, the intensity and maximum value are increasing and the instability of annual mean intensity is growing in recent years. It is also found that the higher the intensity of TC is, the narrower the latitudinal zone of TC generation is, and the closer it is to the equator. The relationship named "φ—m" is established between the time and latitude of high intensity TCs (STY ＆ SUPER TY), and it is verified that the length of typhoon season is negatively correlated with the first cyclone day and is not affected by El Niño phenomenon. The length of typhoon season is conformed to normal distribution and its probability density formula is given in this paper.
Abstract: The spatial and temporal distribution of surface parameters and evapotranspiration (ET), and their variation, were determined in a typical area of Horqin Sandy Land. Landsat 7 remote sensing images and meteorological data were obtained and the Mapping Evapotranspiration at high Resolution with Internalized Calibration (METRIC) model was used to determine spatio-temporal variations of net radiation (Rn) and ET, ET properties under different land cover types, and the factors influencing ET. Three key results were obtained. ① Model estimates were strongly linearly correlated with observed values, which confirmed that METRIC provided reasonable ET estimates in the Horqin Sandy Land. ② The spatio-temporal variations of Rn and ET in the underlying surface of the study area showed that the inter-annual Rn and ET had the same trend in each year, with the maximum Rn and ET occurring from water bodies followed by farmland and meadows, while the minimum was from sandy dunes. ③ The coefficient of variation (CV) of sandy dunes was larger than that of farmland and meadows. The most obvious inter-annual variation occurred in sandy dunes, whereas farmland and meadows were stable. In all three types of land, inter-annual changes in ET were mainly due to meteorological factors.
Abstract: Areal rainfall, which reflects the overall precipitation in a basin, has always been an important input parameter of hydrological models. In consideration of the influence of geospatial factors on the spatial distribution of rainfall, object-oriented remote sensing information clustering method is used to extract two shape factors (perimeter and area) and five topographic factors (mean elevation, mean slope, mean aspect, ratio of elevation difference to perimeter, and ratio of elevation difference to area) in the Yalong River basin on the basis of the Thiessen polygon rainfall method. The rainfall-runoff correlation results show that the areal rainfall estimation accuracy on the monthly scale is higher than that on the annual scale by using the topographic factor rainfall method. The spatial distribution characteristics of rainfall in different regions of the Yalong River basin on the monthly scale are also reflected effectively by using the mehod. The monthly and interannual rainfall trend results show that the average correlation coefficient between annual scale rainfall and runoff in the first-order difference is 0.903, which is higher than that of 0.629 on the monthly scale. This difference is mainly due to the influence of the regulation and storage process of hydropower station on runoff heterogeneity along with the enlargement in influence degree as time scale narrows.
Abstract: Effective discharge is the flow rate corresponding to the maximum suspended sediment transport within a certain period of time in a natural river, which can reflect the medium to short term bed-forming effect. Flow frequency and features of suspended sediment transport of different particle size groups before and after the construction of the TGD are analyzed, based on the prototype data from 1991 to 2016 in the Jianli hydrological station of the mean monthly flow rate and suspended sediment discharge and the suspended sediment gradation. The magnitude, return period and duration of the effective discharge corresponding to the suspended sediment grouped transport in the Lower Jingjiang River are investigated via theoretical analysis and magnitude-frequency approach. The results showed that, as influenced by factors such as coming flow and sediment, sediment transport capacity and bed material replenishment, the effective discharge decreases with the increase of particle size. The deviation of effective discharge between various particle groups is increased after the construction of the TGD. The effective discharge corresponding to the particle group with 0.062 mm < d < 0.25 mm increases, and decreases for the other particle groups and for all suspended load after the construction of the TGD. The return period of the effective discharge corresponding to the particle group with d<0.125 mm increases, while decreases with d>0.125 mm. After the construction of the TGD, the cumulative discharge and cumulative duration of the cumulative transport of 50% sediment decreases with the increase of the particle size. Because of the severe unsaturation of fine sediment and relatively large transport capacity of coarse sediment, roughening of the suspended sediment gradation is resulted in. 50% of the cumulative sediment transport requires 43%—82% cumulative discharge and 62%—90% cumulative duration. Studying the changes in effective discharge before and after the construction of the Three Gorges Dam (TGD) is of significant importance to analyze the riverbed evolution in the Lower Jingjiang River under erosion conditions.
Abstract: Riverbed structure in mountainous rivers is the product of the interplay between flow-sediment and riverbed, which determines the magnitude of riverbed resistance and significantly affects the bedload transport. In order to characterize the details of riverbed structure, this study proposed four new dimensionless parameters (concavity-convexity number, average concavity, average concavity, and concavity-convexity) of riverbed structure characterization and their calculation methods based on the topographic survey datasets of seven sections of the main stream and tributaries of Xiaojiang River (a tributary of Jinsha River in the upper Yangtze River) in April and September 2017. The calculation method of four new parameters is simple and intuitive. Four parameters represent the development degree of the riverbed structure and morphological characteristics of riverbed structure. Moreover, the characterization of the concavity and convexity is more distinguishable if the degree of the riverbed development is relatively lower. The results based on the application of the new method show that the bulge of the riverbed structure in the lower reaches of the Diaoga River, Taojia Creek, and Qingshui Creek is relatively high and convex, i. e., the deep and steep depression, high resistance of river bed, and low sediment transport rate. However, the upper reaches of the Diaoga River, Xiaojiang River, and Jiangjia Creek, and Lanniping Creek, the bulge of the riverbed structure is relatively low and flat, i. e., the shallow and gentle depression, low resistance of river bed, and high sediment transport rate.
Abstract: This study investigated the formation and evolution of a braided channel with ‘lotus roots’ through a series of experiments. The experimental channel was originally straight with unit cross-sections and was then successfully shaped into a braided channel with alternating wide and narrow reaches. Similarity law was applied to the design of the model discharge relating to its prototypical counterpart observed at the Huayuankou hydrological station in the lower Yellow River. The results indicated the following: ① the formation of the braided channel with ‘lotus roots’ was dominated by the aggregation process, which seems to be related to the growth of alternative bars in the straight channel; ② the geomorphological features of ‘lotus roots’ were highly related to the channel stability; ③ compound channel patterns were found to vary with discharge, i. e., braided with moderate flow and meandering with low flow, while the high flow tended to destroy both alternative bars and the pattern of ‘lotus roots’. This study represents a useful reference for flood control in the lower Yellow River.
Abstract: The synthesis roughness of natural rivers varies with cross-sections, as well as water level and discharge. However, there are a few methods to parameterize the roughness curves (e. g., a roughness-water level curve). In order to improve the model accuracy, a significant effect of vegetation on channel roughness is considered in this research to parameterize the roughness. The channel cross-section is divided into several zones with different partition roughness based on vegetation distribution. After calibrating partition roughness values, the roughness-water level curve is obtained by using a water level-discharge relationship method. By analyzing the impact of channel cross-section characteristics and vegetation cover on the roughness curve, a formula is given for calculating the synthesis roughness of channel cross-section based on partition roughness values. This formula quantitatively describes the relationship between partition roughness and comprehensive roughness. The proposed method was used to calibrate the roughness of the hydrodynamic model of the Lijiang River. The results show that the synthesis roughness of the main channel varies in relation to water level ranging from 0.022 to 0.180. The critical depth was set at 1.5 m to divide the cross-section of the channel into the bottom-bed vegetation zone (n=0.210) and non-vegetation zone (n=0.006). Thus, the roughness-water level curve can be inverted, causing reasonable water level simulation results to be obtained. Dense vegetation in the Lijiang riverbed is the main factor causing the synthesis roughness changing with water level. The slope change of the channel cross-section is the main driving factor inducing the gradient change of the roughness-water level curve. These two factors change the cross-section synthesis roughness non-linearly in relation to water level. The results can provide a reliable reference for the hydrological forecasting of rivers in mountainous areas.
Abstract: Understanding the threshold standard of erosive rainfall is the basis of water erosion prediction. Most previous studies have not fully considered the variability of different underlying surface conditions, and have not effectively combined the thresholds for rainfall amount and rainfall intensity. Based on the rainfall and sediment yield data from 1954 to 2014 for the Nanxiaohegou watershed, a typical small watershed in the Loess Plateau Gully Region of East Gansu in China, the rainfall erosivity deviation coefficient method was applied to analyze threshold standard of erosive rainfall under different land use types and spatial scales. The results of this study can be drawn as follows: ① The average rainfall amount threshold of the underlying surface with soil and water conservation treatments (forestland and grassland) increased by 141% and 116%, respectively, compared to that for bare plot (8.7 mm) and farmland plot (9.7 mm). Thus, vegetation measures can significantly improve the threshold standard of erosive rainfall; ② The erosive rainfall amount threshold of Yangjiagou small watershed was 16.5 mm, which was lower than that for the forestland slope area located in the watershed. In addition, the order of rainfall amount threshold for grassland at different spatial scales was grassland slope scale > grassland total slope scale > grassland watershed scale. These results indicated that the threshold standard of erosive rainfall decreased as spatial scale increased; ③ The best erosive rainfall threshold of each sample was the composite-factor standard: P≥a or I30≥b. For the single-factor standard, it is advisable to apply I30 for farmland and bare land; however, the rainfall amount threshold is considered to be more suitable for the underlying surface with vegetation measures. The results of this study can provide a reference for predicting regional soil erosion in the Loess Plateau Gully Region of East Gansu.
Abstract: This study explores the infiltration law for water-repellent soil and identifies the most suitable mathematical model. A laboratory experiment was conducted using a one-dimensional vertical soil column, and the water infiltration into this column was measured for both hydrophilic and hydrophobic soils from the Guishui River basin. The experimental data were then fitted using a Kostiakov piecewise function, Fourier series, first-order Gaussian function, and Gaussian piecewise function. The results indicate that the cumulative infiltration increases to a plateau, whereas, for hydrophobic soil, the infiltration rate is a single-peak curve that increases and then decreases with time. Model graphical analysis shows that the Kostiakov piecewise function model has a maximum and a minimum value appear simultaneously at inflection point of infiltration rate. The produces multiple peaks that do not reflect the monotone increase and decrease of the infiltration rate either side of the peak for Fourier series model. The first-order Gaussian function model cannot replicate the test phenomenon whereby the infiltration rate after the maximum is greater than before. If we ignore the rapid infiltration process at the beginning of the experiment, the Gaussian piecewise function model not only reflects the monotone increase and decrease either side of the peak infiltration rate, but also produces the experimental phenomenon whereby the infiltration rate after the maximum is greater than before.
Abstract: Under the condition of a low Froude number (Fr) flow, the two-stage stilling basin is an effective way of solving the problem of energy dissipation. However, to date, there have been many applications of the two-stage stilling basin, but little research on the shape parameters and energy dissipation mechanisms. To address this issue, the body parameters and their sensitivity in the two-stage stilling basin during low Fr flow were studied by combining theoretical analysis and a physical model. The results indicated that the parameters of the stilling basin such as length, depth, and end sill height all have thresholds. When a parameter exceeds the threshold, the undular or repelled downstream hydraulic jump tends to occur in the first stage stilling basin, while the water surface fluctuation in the second stage stilling basin increases significantly. When the length of the stilling basin is less than the threshold, the flow regime in the stilling basin deteriorates, and the energy dissipation ratio decreases significantly. The lower the height of the end sill, the better the wave attenuation effect. The energy dissipation ratio of the optimized two-stage stilling basin is obviously improved, and the adaptability to the tail water level is also significantly expanded. In this study, the correction coefficient (ωi) of the end sill is introduced, and the formula for calculating the length as well as the end sill height of stilling basins is proposed.
Abstract: Density currents with low sediment concentration are muddy and consist of a large number of fine granular particles. The extremely small settling velocity of the granules leads to adverse water environment, and immense difficulties in water diversion and water treatment in mountainous and plateau areas. This is a cause of concern for society in general, and design departments in particular. In this work, we have used the upper and lower reaches of the Niulan River, a tributary of the Jinsha River, as prototypes, and studied the formation and movement of currents with low sediment concentration and density using a generalized flume test. In addition, we have explored the variations in the characteristics of the head velocity and sediment concentration along the flow path. The results reveal a gradual decrease of the head velocity and sediment concentration along the flow path. For the same amount of discharge, the head velocity of the middle and bottom layers of the current increases with an increase in the sediment concentration. The top layer exhibits a higher head velocity compared to the middle and bottom layers for the same amount of discharge and sediment concentration. The middle layer exhibits the smallest velocity head, while the velocity head of the bottom layer has an intermediate value. These results can serve as a guide for test simulations of physical models of reservoirs.
Abstract: To investigate the distribution characteristics of the equivalent roughness of open-channel flow over an array of glass spheres, eight groups of open-channel turbulence experiments were performed with four different rough beds. The instantaneous velocity data were obtained through particle-image velocimetry (PIV) along the longitudinal cross-section of the test flume. From an analysis of the experimental data, the longitudinal distributions of the theoretical position of the bed surface, equivalent roughness, and coefficient of drag were obtained, and the intrinsic relationship between equivalent roughness and the coefficient of drag was determined. The result shows that the characteristics of the flow in a coarse-grained open channel distinguish two regions, one spherical and the other inter-spherical. There are significant differences in the theoretical position of the bed surface ξ, the relative equivalent roughness ks0, and the coefficient of drag f in each region. In the spherical region (0.65 ≤ ξ ≤ 0.85), a fluctuation in ks0 is evident, and the value of f is large, whereas in the inter-sphere region (ξ=1), the fluctuation in ks0 is relatively slow, and the value of f is small. Another difference is that the average value of ks0 is proportional to the diameter d of the sphere and inversely proportional to the array spacing Lx and the Reynolds number Re of the flow. When the diameter, array spacing, and Reynolds number are fixed, the average value of ks0 for a quincunx-arrayed bed is greater than that of the rectangular-arrayed bed. Moreover, the coefficient of drag increases with ks0, and the variation in the flow characteristics are closely related to the relative smoothness h/d. These results provide a quantitative reference for the determination of the coefficient of drag for open-channel flows over a rough surface.
Abstract: To explore the evolution mechanism of tidal flats under the influence of multiple factors, a cross-shore profile numerical model is developed based on vegetation growth and tidal processes. For a bare flat driven by tides, a general convex cross-shore profile is obtained after medium-to long-term simulation. Sediment supply determines the width of mudflat, and a higher sediment supply leads to a wider and gentler mudflat. For marsh flats, different spatial distribution of biomass affects tidal hydrodynamics differently. At the upper intertidal flat, the flow reduction effect with parabolic distribution of biomass production is stronger than that with linear distribution, and it is converse at the lower intertidal flat. A scarp is simulated at the edge between the bare flat and marsh flat, and it moves seaward as the sediment deposition on intertidal flats.
Abstract: Riverscape ecology has emerged as a new paradigm for river science and its applications, but the discipline faces challenges before it can be considered a full-fledged academic discipline. We review riverscape ecological literature covering the period from 1986 through 2018. The logical necessity, major processes, study scopes, important theoretical advances, and current core issues of riverscape ecology are summarized. We found that a multi-scale, interdisciplinary, holistic view is required to understand rivers, and the need for landscape-scale insights into the problems faced by river-management projects has driven the rapid development of riverscape research. The central topic in riverscape ecology is to understand the structure, patterns, and functional processes of a river's physical landscape and biological system across multiple scales, with a particular emphasis on their interaction at different hierarchical levels. Some theoretical models have been developed to characterize the critical attributes of riverscapes, such as their multiple scales and hierarchies, pronounced dynamism, and marked directionality. We establish five core issues that must be addressed to advance the state of riverscape ecology study: improving and achieving a generally accepted riverscape classification system, advancing multi- and cross-scale studies, re-recognition of classic river ecology theories in the context of riverscape approaches, exploring and simulating riverscape mechanisms, and modeling responses of riverscapes to the climate changes and the regional land use/cover change.
Abstract: Urban rainfall intensity-duration-frequency (IDF) relationships, capable of reflecting urban rainfall characteristics, are an important premise for the planning and design of the infrastructure and sponge city development. The paper briefly described the derivation process of IDF relationships and reviewed recent advances in several aspects including the form of IDF relationships, sampling methods of rainfall data, selection of frequency curves, parameter estimation of distribution functions, and parameter determination of IDF relationships. We analyzed and summarized advantages and disadvantages of the methods reviewed, and pointed out issues associated with two-step optimization and one-step optimization during the IDF relationship establishment, the phenomenon of equifinality in parameter fitting, the rationality analysis of equations and parameters, and the establishment of long-duration IDF relationships. In conclusion, we emphasized that further research is needed in the comprehensive error analysis, rationality analysis, spatial rainfall distribution, and influences of climate change on future rainfall.