Effects of simulated submerged and rigid vegetation and grain roughness on hydraulic resistance to simulated overland flow

Better understanding of the role of vegetation and soil on hydraulic resistance of overland flow requires quantitative partition of their interaction. In this paper, a total of 144 hydraulic flume experiments were carried out to investigate the hydraulic characteristics of overland flow. Results show that hydraulic resistance is negatively correlated with Reynolds number on non-simulated vegetated slopes, while positively on vegetated slopes. The law of composite resistance agrees with the dominant resistance, depending on simulated vegetation stem, surface roughness, and discharge. Surface roughness has greater influence on overland flow resistance than vegetation stem when unit discharge is lower than the low-limited critical discharge, while vegetation has a more obvious influence when unit discharge is higher than the upper-limited critical discharge. Combined effects of simulated vegetation and surface roughness are unequal to the sum of the individual effects through t-test, implying the limitation of using linear superposition principle in calculating overland flow resistances under combined effect of roughness elements.     

Channel response to gravel mining activities in mountain rivers

The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.     

Analytical models for velocity distributions in compound channels with emerged and submerged vegetated floodplains

The lateral distributions of depth-averaged velocity in open compound channels with emerged and submerged vegetated floodplains were analyzed based on the analytical solution of the depth-integrated Reynolds-Averaged Navier-Stokes equation with a term to account for the effects of vegetation. The three cases considered for open channels were two-stage rectangular channel with emerged vegetated floodplain, rectangular channel with submerged vegetated corner, and two-stage rectangular channel with submerged vegetated floodplain, respectively. To predict the depth-averaged velocity with submerged vegetated floodplains, we proposed a new method based on a two-layer approach where flow above and through the vegetation layer was described separately. Moreover, further experiments in the two-stage rectangular channel with submerged vegetated floodplain were carried out to verify the results. The analytical solutions of the cases indicated that the corresponding analytical depth-averaged velocity distributions agree well with the simulated and experimental prediction. The analytical solutions of the cases with theoretical foundation and without programming calculation were reasonable and applicable, which were more convenient than numerical simulations. The analytical solutions provided a way for future researches to solve the problems of submerged vegetation and discontinuous phenomenon of depth-averaged velocity at the stage point for compound channels. Understanding the hydraulics of flow in compound channels with vegetated floodplains is very important for supporting the management of fluvial processes.     

Geomorphologic analysis and physico-dynamic characteristics of Zhatai-Gully debris flows in SW China

Zhatai gully is a typical debris flow channel in Butuo county of Sichuan province, southwestern China. The geomorphologic features are analyzed and the physical-dynamic characteristics are discussed on the basis of field investigation and laboratory tests. Geomorphologic analysis indicates that Zhatai-gully drainage in relation to debris flow can be divided into source area, transport area, and deposition area. The source area has a steep slope and has very limited vegetation cover, which favors runoff, allowing loose solid materials to be mobilized easily and rapidly. In the transport area, there are many small landslides, lateral lobes, and loose materials distributed on both banks. These landslides are active and constantly providing abundant source of soils for the debris flows. In the deposition area, three old debris-flow deposits of different ages can be observed. The dynamic calculation shows that within the recurrence intervals of 50 and 100 years, debris flow discharges are 155.77m³/s and 1y8.19m³/s and deposition volumes are 16.39 x 10⁴ m³ and 18.14 x 10⁴ m³, respectively. The depositional fan of an old debris flow in the outlet of the gully can be subdivided into six layers. There are three debris flow deposits on left and two on the right side of the gully. Grain-size tests of sediments from the soil, gulley bed deposits, and the fresh and old debris flow deposits showed that high amounts of clay and fine gravel were derived from the soil in the source area whereas much of the gravel fraction were sourced from the gully bed deposits. Comprehensive analysis indicates that Zhatai gully is viscous debris-flow gully with moderate to high frequency and moderate to large magnitude debris flows. The risk of a debris flow disaster in Zhatai-gully is moderate and poses a potential threat to the planned hydroelectric dam. Appropriate engineering measures are suggested in the construction and protection of the planned hydroelectric station.     

Two-dimensional numerical model for debris flows in the Jiangjia Gully, Yunnan Province

Debris flows are recurrent natural hazards in many mountainous regions.This paper presents a numerical study on the propagation of debris flows in natural erodible open channels,in which the bed erosion and sedimentation processes are important.Based on the Bingham fluid theory,a mathematical model of the two-dimensional non-constant debris flow is developed.The governing equations include the continuity and momentum conservation equations of debris flow,the sediment convection-diffusion equation,the bed erosion-deposition equation and the bed-sediment size gradation adjustment equation.The yield stress and shear stress components are included to describe the dynamic rheological properties.The upwind control-volume Finite Volume Method (FVM) is applied to discretize the convection terms.The improved SIMPLE algorithm with velocity-free-surface coupled correction is developed to solve the equations on non-orthogonal,quadrilateral grids.The model is applied to simulate a debris flow event in Jiangjia Gully,Yunnan Province and to predict the flow pattern and bed erosion-deposition processes.The results show the effectiveness of the proposed numercial model in debris flow simulation and potential hazard analysis.     

Influence of fine sediment on the fluidity of debris flows

Debris flows include a great diversity of grain sizes with inherent features such as inverse grading, particle size segregation, and liquefaction of fine sediment. The liquefaction of fine sediment affects the fluidity of debris flows, although the behavior and influence of fine sediment in debris flows have not been examined sufficiently. This study used flume tests to detect the effect of fine sediment on the fluidity of laboratory debris flows consisting of particles with various diameters. From the experiments, the greatest sediment concentration and flow depth were observed in the debris flows mixed with fine sediment indicating increased flow resistance. The experimental friction coefficient was then compared with the theoretical friction coefficient derived by substituting the experimental values into the constitutive equations for debris flow. The theoretical friction coefficient was obtained from two models with different fine-sediment treatments: assuming that all of the fine sediments were solid particles or that the particles consisted of a fluid phase involving pore water liquefaction. From the comparison of the friction coefficients, a fully liquefaction state was detected for the fine particle mixture. When the mixing ratio and particle size of the fine sediment were different, some other cases were considered to be in a partially liquefied transition state. These results imply that the liquefaction of fine sediment in debris flows was induced not only by the geometric conditions such as particle sizes, but also by the flow conditions.     

The properties of dilute debris flow and hyper-concentrated flow in different flow regimes in open channels

Particle Image Velocimetry(PIV) technique was used to test the analogues of hyperconcentrated flow and dilute debris flow in an open flume. Flow fields, velocity profiles and turbulent parameters were obtained under different conditions. Results show that the flow regime depends on coarse grain concentration. Slurry with high fine grain concentration but lacking of coarse grains behaves as a laminar flow. Dilute debris flows containing coarse grains are generally turbulent flows. Streamlines are parallel and velocity values are large in laminar flows. However, in turbulent flows the velocity diminishes in line with the intense mixing of liquid and eddies occurring. The velocity profiles of laminar flow accord with the parabolic distribution law. When the flow is in a transitional regime, velocity profiles deviate slightly from the parabolic law. Turbulent flow has an approximately uniform distribution of velocity and turbulent kinetic energy. The ratio of turbulent kinetic energy to the kinetic energy of time-averaged flow is the internal cause determining the flow regime: laminar flow(k/K0.1); transitional flow(0.1 k/K1); and turbulent flow(k/K1). Turbulent kinetic energy firstly increases with increasing coarse grain concentration and then decreases owing to the suppression of turbulence by the high concentration of coarse grains. This variation is also influenced by coarse grain size and channel slope. The results contribute to the modeling of debris flow and hyperconcentrated flow.     

地下水在各向异性土层中的渗流速度

地下水在多孔土层中渗流速度的达西(Darcy)公式是用实验方法得到的。该公式适用于均质各向同性土层中的海流计算,但对各向异性土层中的渗流计算,则不能直接采用。 本文视地下水在多孔土层中的渗流为不可压粘性流体的缓慢层流运动。不计惯性力的作用,用量纲分析法确定运动方程中的摩擦力项表达式,然后导出了地下水在各向异性土层中的渗流速度公式。所得的结果表明,当土壤各向同性时,该公式即简化为达西速度公式。     

The Effects of Protruding Rock Boulders in Regulating Sediment Intrusion within the Hyporheic Zone of Mountain Streams

Excessive sedimentation in mountain stream ecosystems is a critical environmental problem due to the clogging of streambeds by sediment particles within the hyporheic zone,with detrimental effects on fish spawning habitat.In this research,the effects of an array of boulders in regulating the intrusion of incoming sand within a gravel substrate were evaluated by performing detailed experiments in a laboratory flume.A unique experimental setup and two different sampling techniques were utilized for measuring the infiltrated sand within the gravel bed under two bed shear stress conditions（moderate vs.high）.For comparison purposes,experiments were performed without and with the presence of partially submerged to the flow（protruding） boulders,which is typical for the average flow conditions found in mountain streams.Results indicated that sand infiltrated primarily in the upper part of the gravel bed creating a surface seal which hindered the penetration of sand particles deeper into the bed.An exponential decrease of the amount of the infiltrated sand within the hyporheic zone was observed in all experiments regardless of the presence of boulders.However,the presence of boulders promoted sediment intrusion of sand particles especially for the moderate applied bed shear stress condition,since the total amount of the infiltrated sand was found to be on average 44% greater whenboulders were present.The findings from this study can provide additional insight regarding the role of boulders on promoting downwelling of flow and sediment within the gravel substrate with potential effects on fish habitat.     

Effect of clay content to the strength of gravel soil in the source region of debris flow

The production of runoff in the source area of a debris flow is the consequence of a reduction in soil strength. Gravel soil is widely distributed in the source region, and the influence of its clay content on soil strength is one of the important questions regarding the formation mechanism of debris flows. In this paper, the clay content in gravel soil is divided into groups of low clay content(1%, 2, 5%), moderate clay content(3.75%, 5.00%, 6.25%, 7.5%) and high clay content(10.0%, 12.5%, 15%). Tests of the unconsolidated undrained shear strength and consolidated drained shear strength were performed. The unconsolidated undrained shearing(UU) experiment simulates the rapid shear failure of loose gravel soil under the conditions of brief heavy rainfall. The consolidated drained shearing(CD) experiment simulates creep failure of consolidated sediment during extended rainfall. The pore water pressure first increased and then decreased as the clay content increased, and the increase in pore pressure was relatively high in the gravel soil sample when the clay content is in the range of 3.25-7.50%, and stress in the gravel soil is relatively low for a moderate clay content. Gravelly soils with a moderate clay content are moreprone to debris-flow initiation. This paper presents a mathematical formula for the maximum shear stress and clay content of gravel soil under two conditions. The key processes whereby the soil fails and triggers a debris flow—volume contraction of soil, expansion of clay soil, and rise of pore pressure―cause reductions in the soil friction force and enhancement of the water content in the clay particles, and subsurface erosion of soil reduces the soil viscosity, which eventually reduces the soil strength so that the soil loses its stability, liquefies and generates a debris flow.     

Mechanism of downcutting erosion of debris flow over a movable bed

The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow,and formulas for both calculations are presented.Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties(sand volume concentration and flow depth) and gully properties(longitudinal slope,viscous and internal friction angle of gully materials,and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.     

Impacts of future climate change (2030-2059) on debris flow hazard: A case study in the Upper Minjiang River basin,China

An increase in extreme precipitation events due to future climate change will have a decisive influence on the formation of debris flows in earthquake-stricken areas. This paper aimed to describe the possible impacts of future climate change on debris flow hazards in the Upper Minjiang River basin in Northwest Sichuan of China, which was severely affected by the 2008 Wenchuan earthquake. The study area was divided into 1285 catchments, which were used as the basic assessment units for debris flow hazards. Based on the current understanding of the causes of debris flows, a binary logistic regression model was used to screen key factors based on local geologic, geomorphologic, soil, vegetation, and meteorological and climatic conditions. We used the weighted summation method to obtain a composite index for debris flow hazards, based on two weight allocation methods: Relative Degree Analysis and rough set theory. Our results showed that the assessment model using the rough set theory resulted in better accuracy. According to the bias corrected and downscaled daily climate model data, future annual precipitation (2030-2059) in the study area are expected to decrease, with an increasing number of heavy rainfall events. Under future climate change, areas with a high-level of debris flow hazard will be even more dangerous, and 5.9% more of the study area was categorized as having a high-level hazard. Future climate change will cause an increase in debris flow hazard levels for 128 catchments, accounting for 10.5% of the total area. In the coming few decades, attention should be paid not only to traditional areas with high-level of debris flow hazards, but also to those areas with an increased hazard level to improve their resilience to debris flow disasters.     

Influence of Flow Regime on the Vegetation Zonation along Mountain Streams in the Western Cape,South Africa

Zonation patterns of riparian vegetation have been sampled and described in mountain streams in two catchments in the Hottentots-Holland Mountains, Western Cape, South Africa. Six main vegetation types that differ in structure and species composition, are dominant along these river banks: Aquatic vegetation, Wetbanks, Palmiet, Scrub, Forest and Shrubland(Fynbos). The study aims to correlate the vegetation patterns to flooding patterns, in particular the inundation frequency and stream power. A problem arises: because these catchments are ungauged, like most mountain catchments, with the only weirs at the downstream end of the catchment. Discharge data at the weirs are extrapolated to the sites upstream by multiplication with a factor based on the size of the subcatchment that drains through a sample site. In this way, recurrence intervals for floods in mountain streams are derived. Discharges at sites are also calculated using bed roughness(Manning's n) and slope in straight sections with uniform flow conditions. Stream power is derived from the discharges calculated in this manner. The combination of stream power and recurrence intervals explains the occurrence of most vegetation types occurring on the banks, except for one type: Afromontane Forest. This type is probably more dependent on other factors, such as protection from fire and the depth of the groundwater table.     

秦巴山区软岩地基桥桩承载性状试验研究

为了研究软岩地基桥桩的荷载传递性状、破坏机理,并获取在该地质条件下更为可靠的桩基计算参数,对秦巴山区软岩地基3根钻孔灌注试桩进行竖向静载试验.结果表明:秦巴山区软岩地基桥桩试桩荷载沉降曲线呈陡降型,实测竖向极限承载力为20500kN,桩的破坏方式为桩身材料强度破坏;淤泥质亚黏土地层中的碎石起到一定的骨架作用,增强了此地层桩极限侧阻力,发挥极限侧阻力所需的桩土(岩)相对位移为4~8mm;强风化砾岩表现为加工软化型,发挥极限侧阻力所需的桩土(岩)相对位移为3~8mm;中风化砂砾岩表现为明显的加工硬化型,所需的桩岩相对位移大,且桩极限侧阻力的特征点不明显;淤泥质亚黏土地层桩侧阻力占总荷载的60%~70%,随着桩顶荷载的逐步加大,该地层桩侧阻力所占比例不断下降,而嵌岩段桩侧阻力所占比例逐渐上升,达到55%~65%,嵌岩段桩侧阻力沿桩深的分布曲线表现出非线性的特征;试桩为端承摩擦桩,桩端阻力约占桩顶荷载的20%左右,且未充分发挥,在上部结构允许的沉降范围内,适当增加桩端的沉降有利于端阻力的发挥;桩侧阻力先于端阻力发挥,建议单桩承载力设计时分别采用不同的端阻力和侧阻力安全系数.     

Mapping the vegetation distribution of the permafrost zone on the Qinghai-Tibet Plateau

In this paper,an updated vegetation map of the permafrost zone in the Qinghai-Tibet Plateau(QTP) was delineated.The vegetation map model was extracted from vegetation sampling with remote sensing(RS) datasets by decision tree method.The spatial resolution of the map is 1 km×1 km,and in it the alpine swamp meadow is firstly distinguished in the high-altitude areas.The results showed that the total vegetated area in the permafrost zone of the QTP is 1,201,751 km~2.In the vegetated region,50,260 km~2 is the areas of alpine swamp meadow,583,909 km~2 for alpine meadow,332,754 km~2 for alpine steppe,and 234,828 km~2 for alpine desert.This updated vegetation map in permafrost zone of QTP could provide more details about the distribution of alpine vegetation types for studying the vegetation mechanisms in the land surface processes of highaltitude areas.     

Slope debris flows in the Wenchuan Earthquake area

Avalanches and landslides, induced by the Wenchuan Earthquake on May 12, 2008, resulted in a lot of disaggregated, solid material on slopes that could be readily mobilized as source material for debris flows. Rainstorms triggered numerous slope debris flows with great damage to highways and rivers over the subsequent two years. Slope debris flows (as opposed to channelized debris flows) are defined as phenomena in which high-concentration mixtures of debris and water flow down slopes for short distances to highways and river banks. Based on field investigations and measurements of 19 slope debris flows, their main characteristics and potential mitigation strategies were studied. High rainfall intensity is the main triggering factor. Critical rainfall intensities for simultaneous occurrence of single, several and numerous slope debris flow events were 20 mm/day, 30mm/day, and 90 mm/day, respectively. Field investigations also revealed that slope debris flows consist of high concentrations of cobbles, boulders and gravel. They are two-phase debris flows. The liquid phase plays the role of lubrication instead of transporting medium. Solid particles collide with each other and consume a lot of energy. The velocities of slope debris flows are very low, and their transport distances are only several tens of meters. Slope debris flows may be controlled by construction of drainage systems and by reforestation.     

Full 2D Hydrodynamic Modelling of Rainfall-induced Flash Floods

Mountain catchments are prone to flash flooding due to heavy rainfall. Enhanced understanding of the generation and evolution processes of flash floods is essential for effective flood risk management. However, traditional distributed hydrological models based on kinematic and diffusion wave approximations ignore certain physical mechanisms of flash floods and thus bear excessive uncertainty. Here a hydrodynamic model is presented for flash floods based on the full two-dimensional shallow water equations incorporating rainfall and infiltration. Laboratory experiments of overland flows were modelled to illustrate the capability of the model. Then the model was applied to resolve two observed flash floods of distinct magnitudes in the Lengkou catchment in Shanxi Province, China. The present model is shown to be able to reproduce the flood flows fairly well compared to the observed data. The spatial distribution of rainfall is shown to be crucial for the modelling of flash floods. Sensitivity analyses of the model parameters reveal that the stage and discharge hydrographs are more sensitive to the Manning roughness and initial water content in the catchment than to the Green-Ampt head. Most notably, as the flash flood augments due to heavier rainfall, the modelling results agree with observed data better, which clearly characterizes the paramount role of rainfall in dictating the floods. From practical perspectives, the proposed model is more appropriate for modelling large flash floods.     

High-efficiency calculation method for watershed rainfall-runoff routing using one-dimensional dynamic wave equations

Efficiency in solving the Saint-Venant equations for watershed rainfall-runoff routing is important in flood hydrology. This paper presents a high-efficiency numerical solution of one-dimensional dynamic wave equations (HEDWE) for watershed rainfall-runoff routing, in which the full momentum equation is written as a quadratic equation with only one unknown variable Q, water depth is derived from the continuity equation using the two-step predictor-corrector method, and the discrete scheme is the explicit upwind scheme. The results of numerical tests showed the HEDWE approach has several major advantages. 1) It is a stable numerical method, even for an initially dry area. 2) Its computational efficiency is higher than 4.76E+05 times/s. 3) It can be used for overland flow, river flow, and combinations thereof. The primary disadvantages of the HEDWE approach are its unsuitability for rapidly varying flow, such as dam-break floods.     

Investigation and Assessment of Landslides and Debris Flows in Sichuan Province of China by Remote Sensing Technique

Taking TM images, ETM images, SPOT images, aerial photos and other remote sensing data as fundamental sources, this research makes a thorough investigation on landslides and debris flows in Sichuan Province, China, using the method of manual interpretation and taking topography maps as references after the processes of terrain correction, spectral matching, and image mosaic. And then, the spatial characteristics of landslides and debris flows in the year of 2005 are assessed and made into figures. The environmental factors which induce landslides and debris flows such as slope, vegetation coverage, lithology, rainfall and so on are obtained by GIS spatial analysis method. Finally, the relationships of landslides or debris flows with some environmental factors are analyzed based on the grade of each environmental factor. The results indicate： 1） The landslides and debris flows are mainly in the eastern and southern area of Sichuan Province, however, there are few landslides and debris flows in the western particularly the northwestern Sichuan. 2） The landslides and debris flows of Sichuan Province are mostly located in the regions with small slope degree. The occurring rate of debris flow reduces with the increase of the vegetation coverage degree, but the vegetation coverage degree has little to do with the occurrence of landslide. The more rainfall a place has, the easier the landslides and debris flows take place.     

Simulation of two-phase debris flow scouring bridge pier

Debris flows are typical two-phase flows, which commonly accompany erosion in mountainous areas, and may destroy bridge engineering by scouring. In this study, a physically-based two-phase model is applied for the simulation of debris flow scouring of bridge pier. In this model, the shear stress of debris flow on an erodible bed is considered to be a function of the solid shear stress, fluid shear stress, and volume fraction; accordingly, the erosion is incorporated into the two-phase model. Using a highaccuracy computational scheme based on the finite volume method, the model is employed for simulating a dynamic debris flow over an erodible bed. The numerical results are consistent with the experimental data, and verify the feasibility of the two-phase model. Moreover, a simple numerical test is performed to exhibit the fundamental behaviour of debris flow scouring of bridge pier, which shows that the degree of erosion on each side of the pier is higher compared to other areas. The scouring depth is influenced by the variations of solid volume fraction and velocity of debris flow and pier width.