Testing a theoretical resistance law for overland flow on a stony hillslope

Overland flow, sediments, and nutrients transported in runoff are important processes involved in soil erosion and water pollution. Modelling transport of sediments and chemicals requires accurate estimates of hydraulic resistance, which is one of the key variables characterizing runoff water depth and velocity. In this paper, a new theoretical power–velocity profile, originally deduced neglecting the impact effect of rainfall, was initially modified for taking into account the effect of rainfall intensity. Then a theoretical flow resistance law was obtained by integration of the new flow velocity distribution. This flow resistance law was tested using field measurements by Nearing for the condition of overland flow under simulated rainfall. Measurements of the Darcy–Weisbach friction factor, corresponding to flow Reynolds number ranging from 48 to 194, were obtained for simulated rainfall with two different rainfall intensity values (59 and 178 mm hr⁻¹). The database, including measurements of flow velocity, water depth, cross-sectional area, wetted perimeter, and bed slope, allowed for calibration of the relationship between the velocity profile parameter Γ, the slope steepness s, and the flow Froude number F, taking also into account the influence of rainfall intensity i. Results yielded the following conclusions: (a) The proposed theoretical flow resistance equation accurately estimated the Darcy–Weisbach friction factor for overland flow under simulated rainfall, (b) the flow resistance increased with rainfall intensity for laminar overland flow, and (c) the mean flow velocity was quasi-independent of the slope gradient.     

The impact of single natural vegetation elements on flow characteristics

In this experimental study, measurements were conducted to explore the impacts of different forms of individual natural vegetative elements within the flow domain on velocity and turbulence characteristics. All the experiments were performed in a flume measuring 26 m in length, 0·98 m in width and 0·85 m in depth, and real tree saplings were utilized to represent the vegetation element. In order to analyse this commonly observed nature phenomenon in floodplains, trees with wide trunks were classified into three groups on the basis of their volume versus height relation. Throughout the velocity measurements three acoustic Doppler velocimeters were employed. Time‐averaged velocity, streamwise and vertical turbulence intensities and turbulent kinetic energy parameters were examined. Additionally, a formulation that gives the velocity profile at a certain distance downstream of vegetation was introduced and the validity of the proposed formulation was checked with experimental data. It is seen that despite their porous structures, the presence of vegetation considerably disturbs the flow field and dissipates a remarkable amount of energy by turbulence. Copyright © 2008 John Wiley & Sons, Ltd.     

The role of different forms of natural riparian vegetation on turbulence and kinetic energy characteristics

Flow measurements were conducted by ADV Vectrino to investigate the role of different forms of single natural emergent vegetation elements on kinetic energy characteristics and the flow structure in open channel flow. Experiments were carried out for both forms of “erect” and “compressed” vegetation. The findings revealed that the form of “erect” has greater retaining influence on time-averaged mean velocity characteristics than “compressed” form. Furthermore, the turbulence and kinetic energy patterns behind the single vegetation were also examined. An empirical equation giving the relationship between the kinetic energy components of flow was derived based on the data. The ratio between turbulence kinetic energy and mean kinetic energy at a certain distance downstream of vegetation element was predicted by the empirical equation depending on the vegetation characteristics. These predictions were compared with a different experimental data set and produced satisfactory results. Also, the mean flow and turbulence characteristics behind an isolated natural plant and a volumetrically equivalent rigid cylinder were also compared.     

Rill flow resistance law under equilibrium bed‐load transport conditions

In this paper, a recently deduced flow resistance equation for open channel flow was tested under equilibrium bed‐load transport conditions in a rill. First, the flow resistance equation was deduced applying dimensional analysis and the incomplete self‐similarity condition for the flow velocity distribution. Then, the following steps were carried out for developing the analysis: (a) a relationship (Equation  13 ) between the Γ function of the velocity profile, the rill slope, and the Froude number was calibrated by the available measurements by Jiang et al.; (b) a relationship (Equation  17 ) between the Γ function, the rill slope, the Shields number, and the Froude number was calibrated by the same measurements; and (c) the Darcy–Weisbach friction factor values measured by Jiang et al. were compared with those calculated by the rill flow resistance equation with Γ estimated by Equations  13 and 17 . This last comparison demonstrated that the rill flow resistance equation, in which slope and Shields number, representative of sediment transport effects, are introduced, is characterized by the lowest values of the estimate errors.     

Estimates of flow resistance and eddy viscosity coefficients for 2D modelling on vegetated floodplains

The problem of quantifying the effects of flexible plants on flow resistance and eddy viscosity by vegetated floodplains is first addressed with a one‐dimensional (1D) approximation based upon the so‐called lateral distribution method. The estimates so obtained are then tested with two‐dimensional (2D) numerical simulations based on the full shallow water equations through the use of the computational code Telemac‐2D. Data obtained on a physical model of the Besòs River (Spain), whose floodplains were covered with plastic ornamental plants to mimic the effect of flexible vegetation, is used for the validation of the numerical results. Additionally, the values of flow resistance estimated numerically with the 1D and 2D simulations are compared with values obtained in a rectangular flume under flow conditions (slope, water depth and artificial lining) similar to those used on the reduced model. It is then established that as more physical mechanisms are included in the mathematical model used to study the problem, the ratio between the floodplain and the main channel flow resistance coefficient increases. The approach demonstrates that whenever enough flow data is available, the lateral distribution method delivers values of flow resistance and eddy viscosity which are highly consistent with 2D numerical modelling. This finding could mean considerable savings in the burdensome task of specifying flow resistance and turbulence dissipation values for 2D modelling of large compound channel systems. Copyright © 2004 John Wiley & Sons, Ltd.     

Downstream effects of stream flow diversion on channel characteristics and riparian vegetation in the Colorado Rocky Mountains,USA

Flow diversions are widespread and numerous throughout the semi‐arid mountains of the western United States. Diversions vary greatly in their structure and ability to divert water, but can alter the magnitude and duration of base and peak flows, depending upon their size and management. Channel geometry and riparian plant communities have adapted to unique hydrologic and geomorphic conditions existing in the areas subject to fluvial processes. We use geomorphic and vegetation data from low‐gradient (≤3%) streams in the Rocky Mountains of north‐central Colorado to assess potential effects of diversion. Data were collected at 37 reaches, including 16 paired upstream and downstream reaches and five unpaired reaches. Channel geometry data were derived from surveys of bankfull channel dimensions and substrate. Vegetation was sampled using a line‐point intercept method along transects oriented perpendicular to the channel, with a total of 100 sampling points per reach. Elevation above and distance from the channel were measured at each vegetation sampling point to analyze differences in lateral and vertical zonation of plant communities between upstream and downstream reaches. Geomorphic data were analyzed using mixed effects models. Bankfull width, depth, and cross‐sectional area decreased downstream from diversions. Vegetation data were analyzed using biological diversity metrics, richness, evenness and diversity, as well as multivariate community analysis. Evenness increased downstream from diversions, through reduced frequency of wetland indicator species and increased frequency of upland indicator species. Probability of occurrence for upland species downstream of a diversion increases at a greater rate beginning around 0·5 m above active channel. The results suggest that channel morphology and riparian plant communities along low‐gradient reaches in montane environments in the Colorado Rocky Mountains are impacted by diversion‐induced flow alteration, with the net effect of simplifying and narrowing the channel and homogenizing and terrestrializing riparian plant communities. Copyright © 2014 John Wiley & Sons, Ltd.     

Neural network modelling for mean velocity and turbulence intensities of steep channel flows

The main purpose of this study is to evaluate the potential of simulating the profiles of the mean velocity and turbulence intensities for the steep open channel flows over a smooth boundary using artificial neural networks. In a laboratory flume, turbulent flow conditions were measured using a fibre‐optic laser doppler velocimeter (FLDV). One thousand and sixty‐four data sets were collected for different slopes and aspect ratios at different locations. These data sets were randomly split into two subsets, i.e. training and validation sets. The multi‐layer functional link network (MFLN) was used to construct the simulation model based on the training data. The constructed MFLN models can almost perfectly simulate the velocity profile and turbulence intensity. The values of correlation coefficient (γ) are close to one and the values of root mean square error (RMSE) are close to zero in all conditions. The results demonstrate that the MFLN can precisely simulate the velocity profiles, while the log law and Reynolds stress model (RSM) are less effective when used to simulate the velocity profiles close to the side wall. The simulated longitudinal turbulence intensities yielded by the MFLN were also fairly consistent with the measured data, while the simulated vertical turbulence intensities by the RSM were not consistent with the measured data. Copyright © 2007 John Wiley & Sons, Ltd.     

Experimental study of depth-limited open-channel flows over a gravel bed

Laboratory experiments of depth-limited open-channel flows over a gravel bed were conducted in the study.Two gravel patches with identical individual element size and different lengths(3.81 m and 7.5 m)were tested.The depth-limited uniform flow regime with relative submergence S_r(= D/k_s) ranging from2.68 to 5.94 was produced by adjusting the tailgate weir.The velocity profiles were measured by using both an ultra-sound velocity profiler(UVP) and an acoustic Doppler velocimeter(ADV).The conventional methods used to determine the zero-plane displacement and estimate the bed shear velocity were then reviewed and compared.The measured double-averaged(DA) velocity profiles were found to fit well with the log law and defect law with a non-universal Karman constant κ./κ-value remains nearly constant and in the range from 0.2 to 0.3 for the long patch(LP) cases and κ-values are scattered within a wider range from 0.3 to 0.5 for the short patch(SP) cases.While the Br-value in log law remains constant and equal to 8.5 for LP cases,the Br-value was found to decrease with the increase of the dimensionless roughness height k_s~+ for SP cases.The streamwise turbulence intensity distributions were found to be independent on the patch length and agree well with the available experimental data in the intermediate region and wall region.The Manning resistance coefficient and Darcy-Weisbach friction factor were analyzed.The κ-value decreases to 0.22 for the fitting of the logarithmic flow resistance law under small relative submergence.The value of the integration constant Ar in the logarithmic law falls within the normal range between 3.25 and 6.25.     

Effects of wet meadow riparian vegetation on streambank erosion. 2. Measurements of vegetated bank strength and consequences for failure mechanics

We measured the effect of wet meadow vegetation on the bank strength and failure mechanics of a meandering montane meadow stream, the South Fork of the Kern River at Monache Meadow, in California's Sierra Nevada. Streambanks colonized by ‘wet’ graminoid meadow vegetation were on average five times stronger than those colonized by ‘dry’ xeric meadow and scrub vegetation. Our measurements show that strength is correlated with vegetation density indicators, including stem counts, standing biomass per unit area, and the ratio of root mass to soil mass. Rushes appear better than sedges at stabilizing coarse bar surfaces, while sedges are far more effective at stabilizing actively eroding cut banks. Wet meadow floodplain vegetation creates a composite cut bank configuration (a cohesive layer overlying cohesionless materials) that erodes via cantilever failure. Field measurements and a geotechnical model of cantilever stability show that by increasing bank strength, wet meadow vegetation increases the thickness, width, and cohesiveness of a bank cantilever, which, in turn, increases the amount of time required to undermine, detach, and remove bank failure blocks. At Monache Meadow, it takes approximately four years to produce and remove a 1 m wide wet meadow bank block. Wet meadow vegetation limits bank migration rates by increasing bank strength, altering bank failure modes, and reducing bank failure frequency. Copyright © 2002 John Wiley & Sons, Ltd.     

Bed morphology and generation of step–pool channels

Flume experiments have been carried out to study the formation processes and the bed morphology of step–pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step–pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand‐bed rivers. This observation points out that step–pool field‐data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand‐ and gravel‐bed rivers or step–pool units in boulder‐bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self‐stabilization by modifying the step–pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley & Sons, Ltd.     

A hydro‐economic modelling framework for flood damage estimation and the role of riparian vegetation

A modelling framework for the quick estimate of flood inundation and the resultant damages is developed in this paper. The model, called the flood economic impact analysis system (FEIAS), can be applied to a river reach of any hydrogeological river basin. For the development of the integrated modelling framework, three models were employed: (1) a modelling scheme based on the Hydrological Simulation Program FORTRAN model that was developed for any geomorphological river basin, (2) a river flow/floodplain model, and (3) a flood loss estimation model. The first sub‐model of the flood economic impact analysis system simulates the hydrological processes for extended periods of time, and its output is used as input to a second component, the river/floodplain model. The hydraulic model MIKE 11 (quasi‐2D) is the river/floodplain model employed in this study. The simulated flood parameters from the hydraulic model MIKE 11 (quasi‐2D) are passed, at the end of each time step, to a third component, the flood loss model for the estimation of flood damage. In the present work, emphasis was given to the seasonal variation of Manning's coefficient (n), which is an important parameter for the determination of the flood inundation in hydraulic modelling. High values of Manning's coefficient for a channel indicate high flow resistance. The riparian vegetation can have a large impact on channel resistance. The modelling framework developed in this paper was used to investigate the role of riparian vegetation in reducing flood damage. Moreover, it was used to investigate the influence of cutting riparian vegetation scenarios on the flow characteristics. The proposed framework was applied to the downstream part of the Koiliaris River basin in Crete, Greece, and was tested and validated with historical data. Copyright © 2012 John Wiley & Sons, Ltd.     

全球地幔垂直流动速度研究

用高分辨率地震体波速度成像以及相关的地球物理资料,计算地幔垂直流动形式及流动速度,得到全球地幔流垂直运动模式.从全球尺度来看,地幔流基本可划分为以下几个区域：欧亚大陆—澳大利亚、北美洲—南美洲为两个大规模下降流区域,西印度洋—非洲及大西洋、中南太平洋及东太平洋为两个大规模地幔上升流区域.地幔上升流起源于核幔边界,主要表现在地幔中部和上地幔下部.地幔垂直流动速度约每年1～4cm.地幔流动对地表板块运动、海洋中脊和中隆、俯冲带和碰撞带的分布起着控制作用.地幔上升流与地表现代热点有密切关系.从东亚尺度看,地幔流大体分为三个区域：东亚边缘裂谷系和西太平洋边缘海为上升流、西伯利亚地幔深度表现为物质下降流、青藏高原—缅甸—印度尼西亚特提斯俯冲带地幔下降流,这三个区域地幔流动与地表的西太平洋构造域、亚洲构造域和特提斯构造域相吻合.勾勒出南海地区构造特征：从上到下的大体结构是上部呈“工"字型、中间为圆柱型、底部呈盾形的地幔上升流.     

Experimental study of debris flow caused by domino failures of landslide dams

The formation of landslide dams is often induced by earthquakes in mountainous areas.The failure of a landslide dam typically results in catastrophic flash floods or debris flows downstream.Significant attention has been given to the processes and mechanisms involved in the failure of individual landslide dams.However,the processes leading to domino failures of multiple landslide dams remain unclear.In this study,experimental tests were carried out to investigate the domino failure of landslide dams and the consequent enlargement of downstream debris flows.Different blockage conditions were considered,including complete blockage,partial blockage and erodible bed（no blockage）.The mean velocity of the flow front was estimated by videos.Total stress transducers（TSTs）and Laser range finders（LRFs） were employed to measure the total stress and the depth of the flow front,respectively.Under a complete blockage pattern,a portion of the debris flow was trapped in front of each retained landslide dam before the latter collapsed completely.This was accompanied by a dramatic decrease in the mean velocity of the flow front.Conversely,under both partial blockage and erodible bed conditions,the mean velocity of the flow front increased gradually downward along the sloping channel.Domino failures of the landslide dams were triggered when a series of dams（complete blockage and partial blockage） were distributed along the flume.However,not all of these domino failures led to enlarged debris flows.The modes of dam failures have significant impacts on the enlargement of debris flows.Therefore,further research is necessary to understand the mechanisms of domino failures of landslide dams and their effects on the enlargement of debris flows.     

A comparison of flow intensities in alluvial rivers: characteristics and implications for modelling flow processes

This paper compares flow intensity data obtained with different instruments from a variety of fluvial environments. It examines associations between the root-mean-square of longitudinal velocity fluctuations (flow intensity), local mean velocity, relative depth, and boundary resistance. Results indicate systematic differences in the behaviour of flow intensity which scale with respect to position in the boundary layer (deep sand-bedded rivers), boundary grain resistance (shallow river environments with coarse beds), and possibly form resistance (shallower sand-bedded rivers). Preliminary approaches to prediction and modelling of variations in flow intensity are suggested based upon linear regression relationships. Intensity values are also compared with theoretical and empirical limits to the use of Taylor's substitution, which allows time and frequency properties of a single-point velocity time series to be used to yield a flow length scale. In general, limits are exceeded in all environments for near-boundary flow measurements, but are met for y/d > 0·3 in most cases in sand-bed rivers, and for y/d > 0·4 in some gravel-bed environments. © 1998 John Wiley & Sons, Ltd.     

Morphological development of the retention basin “Hartheim”: A case study

Flow in rivers and on floodplains is complex as it is affected by several interconnected factors such as topography, sediment transport and vegetation characteristics. The resulting processes are explained by the measure “Hartheim” planned for retention purposes at the Upper Rhine river. On the basis of existing formulas and instruments it is demonstrated that a good estimation of the development of the measure is possible. The proposed procedure is a useful tool for estimating morphological developments of restored river sections.     

Roughness effect on the correction factor of surface velocity for rill flows

Flow velocity is one of the most important hydrodynamic variables for both channelized (rill and gullies) and interrill erosive phenomena. The dye tracer technique to measure surface flow velocity V_s is based on the measurement of the travel time of a tracer needed to cover a known distance. The measured V_s must be corrected to obtain the mean flow velocity V using a factor α_v = V/V_s which is generally empirically deduced. The V_s measurement can be influenced by the method applied to time the travel of the dye-tracer and α_v can vary in different flow conditions. Experiments were performed by a fixed bed small flume simulating a rill channel for two roughness conditions (sieved soil, gravel). The comparison between a chronometer-based (CB) and video-based (VB) technique to measure V_s was carried out. For each slope-discharge combination, 20 measurements of V_s, characterized by a sample mean V_m, were carried out. For both techniques, the frequency distributions of V_s/V_m resulted independent of slope and discharge. For a given technique, all measurements resulted normally distributed, with a mean equal to one, and featured by a low variability. Therefore, V_m was considered representative of surface flow velocity. Regardless of roughness, the V_m values obtained by the two techniques were very close and characterized by a good measurement precision. The developed analysis on α_v highlighted that it is not correlated with Reynolds number for turbulent flow regime. Moreover, α_v is correlated neither with the Froude number nor with channel slope. However, the analysis of the empirical frequency distributions of the correction factor demonstrated a slope effect. For each technique (CB, VB)-roughness (soil, gravel) combination, a constant correction factor was statistically representative even if resulted in less accurate V estimations compared to those yielded by the slope-specific correction factor.     

A laboratory study on the undrained strength of a silty sand from Central Western Taiwan

The sand deposit in Central Western Taiwan typically contained significant amounts of fines. The assessment of liquefaction potential using the simplified procedure often involved adjustment in field test results to account for the effects of fines. The available fines content (FC) adjustment methods are highly empirical and may lead to very different conclusions. The need and/or level of FC adjustment should be justified based on cyclic behavior of sand and its relationship with fines. This study made an attempt to provide that reference information for a silty sand commonly found in Central Western Taiwan. A series of isotropic consolidation, undrained monotonic and cyclic triaxial tests were conducted on reconstituted Mai Liao Sand (MLS) with various FCs and densities. Concurrent shear wave velocities were measured using bender elements in some of the cyclic triaxial tests. This paper describes the geological background, the laboratory tests and their implications in the assessment of liquefaction potential for MLS.     

A heat pulse technique for the determination of small‐scale flow directions and flow velocities in the streambed of sand‐bed streams

The hyporheic zone (HZ) has the capability to eliminate and attenuate nutrients and contaminants in riverine systems. Biogeochemical reactions and the potential elimination of contaminants are strongly controlled by the flow paths and dynamics in the HZ. Nevertheless, an easily applicable method for the field determination of flow patterns in the HZ is still lacking. Therefore, a heat pulse technique, which traces the movement of a short heat pulse in the upper part of the HZ and other sand beds, was developed. Five rods are vertically driven into the sediment of the streambed; one rod with a heater as point source located in about 10‐cm sediment depth and four rods with four temperature sensors in 3 cm distance, arranged concentrically with 7 cm diameter around the heating rod. Subsequently, a heat pulse is applied and the resulting breakthrough curves are indicative of flow velocities and flow directions in the streambed. A rough data analysis procedure is also suggested. In addition, laboratory experiments were performed to test the heat pulse technique. These experiments were validated based on coupled numerical modelling of flow and heat transport. First field tests of the method prove that the method is easily applicable under field conditions. These first field tests showed highly complex flow patterns with flow velocities from 1·8 to 4·9 cm min^?1 and flow directions from parallel to surface flow to opposite to surface flow. This suggests the need for a robust method to quantify hyporheic flow patterns in situ. Copyright © 2011 John Wiley & Sons, Ltd.     

A stochastic model for initial movement of sand grains by wind

A stochastic model for entrainment of sand grains by wind is presented through analysis of the forces exerted on a single spherical grain, coupled with fluctuations of wind velocity and the change in grain position on the surface. The structure of the stochastic model is consistent with experimental data in the literature. The probability of initial motion increases first, and then decreases, with grain size. It reaches a maximum at diameters of about 0·9 mm. Some sand grains are still in motion at less than the conventional threshold velocity, even at very low velocities. The probability of sand grain movement reaches unity at twice the conventional threshold velocity. Considerable discrepancies amongst conventional threshold formulae may result from the different probabilities of initial movement implied in these formulae. Copyright © 2008 John Wiley & Sons, Ltd.