Flow modelling in gravel‐bed rivers: rethinking the bottom boundary condition     

Ponnambalam Rameshwaran  Pamela S Naden  Mark Lawless 《地球表面变化过程与地形》2011,36(10):1350-1366

A key problem in computational fluid dynamics (CFD) modelling of gravel‐bed rivers is the representation of multi‐scale roughness, which spans the range from grain size, through bedforms, to channel topography. These different elements of roughness do not clearly map onto a model mesh and use of simple grain‐scale roughness parameters may create numerical problems. This paper presents CFD simulations for three cases: a plane bed of fine gravel, a plane bed of fine gravel including large, widely‐spaced pebble clusters, and a plane gravel bed with smaller, more frequent, protruding elements. The plane bed of fine gravel is modelled using the conventional wall function approach. The plane bed of fine gravel including large, widely‐spaced pebble clusters is modelled using the wall function coupled with an explicit high‐resolution topographic representation of the pebble clusters. In these cases, the three‐dimensional Reynolds‐averaged continuity and Navier–Stokes equations are solved using the standard k ? ε turbulence model, and model performance is assessed by comparing predicted results with experimental data. For gravel‐bed rivers in the field, it is generally impractical to map the bed topography in sufficient detail to enable the use of an explicit high‐resolution topography. Accordingly, an alternative model based on double‐averaging is developed. Here, the flow calculations are performed by solving the three‐dimensional double‐averaged continuity and Navier‐Stokes equations with the spatially‐averaged 〈k ? ε〉 turbulence model. For the plane bed of fine gravel including large, widely‐spaced pebble clusters, the model performance is assessed by comparing the spatially‐averaged velocity with the experimental data. The case of a plane gravel bed with smaller, more frequent, protruding elements is represented by a series of idealized hypothetical cases. Here, the spatially‐averaged velocity and eddy viscosity are used to investigate the applicability of the model, compared with using the explicit high‐resolution topography. The results show the ability of the model to capture the spatially‐averaged flow field and, thus, illustrate its potential for representing flow processes in natural gravel‐bed rivers. Finally, practical data requirements for implementing such a model for a field example are given. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

On determining the geometric and kinematic characteristics of coherent flow structures over a gravel bed: a new approach using combined PLIF‐PIV     

R.J. Hardy  J.L. Best  D.R. Parsons  G.M. Keevil 《地球表面变化过程与地形》2011,36(2):279-284

In gravel‐bed rivers, the microtopography of the bed is known to exert a significant influence on the generation of turbulent flow structures that owe their origin to fluid shear generated near the bed. Although field and laboratory measurements have indicated that flows over gravel beds contain a range of coherent flow structures, the kinematic and dynamic properties of these structures are still poorly understood. This paper describes a new experimental methodology to quantify simultaneously both the kinematic and dynamic characteristics of coherent flow structures based upon combined planar laser‐induced fluorescence and particle imaging velocimetry (PLIF‐PIV). The results confirm that the primary generative mechanism of coherent flow structures is at the bed, where merging hairpin vortices form around bed clasts and generate larger‐scale fluid motions that advect downstream. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Does the permeability of gravel river beds affect near‐bed hydrodynamics?          下载免费PDF全文

James R. Cooper  Annie Ockleford  Stephen P. Rice  D. Mark Powell 《地球表面变化过程与地形》2018,43(5):943-955

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near‐bed flow dynamics, and thus on fluid advection at the sediment–water interface. This study provides the first quantification of this effect for water‐worked gravel beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double‐averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three‐fold. First, local pressure gradients, and therefore rates of material transport, across the sediment–water interface are likely to differ between impermeable and permeable beds. Second, near‐bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse‐grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

Analysis of bedload transport characteristics of Idaho streams and rivers     

Jasna Muskatirovic 《地球表面变化过程与地形》2008,33(11):1757-1768

Field data are essential in evaluating the adequacy of predictive equations for sediment transport. Each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment. Data collected for 33 sites on 31 mountain streams and rivers in Central Idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological, topographic, morphological, hydrological, hydraulic, and sedimentological characteristics. All of these streams and rivers have armored, poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer. The fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges (generally bankfull or less, and in some cases two times higher than bankfull discharge). The bedload transport is size‐selective. The transport rates are generally low, since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges, or, the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material. Detailed analyses of bedload transport rates, bedload and bed material characteristics were performed for each site. The obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel‐bed rivers. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Do alternate bars affect sediment transport and flow resistance in gravel‐bed rivers?     

Simona Francalanci  Luca Solari  Marco Toffolon  Gary Parker 《地球表面变化过程与地形》2012,37(8):866-875

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Representing the bed roughness of coarse‐grained streams in computational fluid dynamics     

Shaun K. Carney  Brian P. Bledsoe  Daniel Gessler 《地球表面变化过程与地形》2006,31(6):736-749

Computational fluid dynamics (CFD) applications are increasingly utilized for modelling complex flow patterns in natural streams and rivers. Although CFD has been successfully implemented to model many complex flow situations in natural stream settings, adequately characterizing the effects of gravel and cobble beds on flow hydraulics in CFD is a difficult challenge due to the scale of roughness lengths and the inadequacy of traditional roughness representations to characterize flow profiles in situations with large roughness elements. An alternative method of representing gravel and cobble beds is presented. Appropriate drag forces associated with different grain sizes are computed and included in the momentum equations to account for the influence of a hydraulically rough bed. Comparisons with field measurements reveal reasonable agreement between measured and modelled profiles of spatially averaged velocity and turbulent kinetic energy, and model fidelity to the non‐logarithmic behaviour of the velocity profiles. The novel method of representing coarse beds expands the utility of CFD for investigating physical processes in natural channels with large bed roughness. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Persistence of the surface texture of a gravel‐bed river during a large flood     

Jordan A. Clayton  John Pitlick 《地球表面变化过程与地形》2008,33(5):661-673

We present herein clear field evidence for the persistence of a coarse surface layer in a gravel‐bed river during flows capable of transporting all grain sizes present on the channel bed. Detailed field measurements of channel topography and bed surface grain size were made in a gravel‐bed reach of the Colorado River prior to a flood in 2003. Runoff produced during the 2003 snowmelt was far above average, resulting in a sustained period of high flow with a peak discharge of 27 m³/s (170% of normal peak flow); all available grain sizes within the study reach were mobilized in this period of time. During the 2003 peak flow, the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours), leaving the bed texture essentially frozen in place at the peak of the flood. All locations sampled prior to the flood were resampled following the stream abandonment. In response to the high flow, the surface median grain size (D_50s) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the coarse bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. This may be explained because the response of the bed texture to increases in flow strength depends primarily upon the continued availability of the various grain size percentiles in the supply, which in this case was essentially unlimited for all sizes present in the channel. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)          下载免费PDF全文

Andrew G. Pledger  Stephen P. Rice  Jon Millett 《地球表面变化过程与地形》2014,39(11):1500-1513

The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic‐feeding fish, which disturb bed material sediments during their search for food, has received very little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. An ex situ experiment was conducted to investigate the impact of a benthic feeding fish (Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume changes in bed surface topography were measured and grain displacements examined when an imbricated, water‐worked bed of 5.6 to 16 mm gravels was exposed to feeding juvenile Barbel (on average, 0.195 m in length). Grain entrainment rates and bedload fluxes were measured under a moderate transport regime for substrates that had been exposed to feeding fish and control substrates which had not. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four‐hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes by fish corresponded with an average increase in bedload flux of 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. Together, these results indicate that by increasing surface microtopography and undoing the naturally stable structures produced by water working, foraging can increase the mobility of gravel‐bed materials. An interesting implication of this result is that by increasing the quantity of available, transportable sediment and lowering entrainment thresholds, benthic feeding might affect bedload fluxes in gravel‐bed rivers. The evidence presented here is sufficient to suggest that further investigation of this possibility is warranted. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Mean and turbulent flow fields in a simulated ice‐covered channel with a gravel bed: some laboratory observations     

André Robert  Thang Tran 《地球表面变化过程与地形》2012,37(9):951-956

Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Surveyed and modelled one‐year morphodynamics in the braided lower Tana River          下载免费PDF全文

Eliisa Lotsari  David Wainwright  Geoffrey D. Corner  Petteri Alho  Jukka Käyhkö 《水文研究》2014,28(4):2685-2716

Field measurements and morphodynamic simulations were carried out along a 5‐km reach of the sandy, braided, lower Tana River in order to detect temporal and spatial variations in river bed modifications and to determine the relative importance of different magnitude discharges on river bed and braid channel evolution during a time span of one year, i.e. 2008–2009. Fulfilling these aims required testing the morphodynamic model's capability to simulate changes in the braided reach. We performed the simulations using a 2‐D morphodynamic model and different transport equations. The survey showed that more deposition than erosion occurred during 2008–2009. Continuous bed‐load transport and bed elevation changes of ±1 m, and a 70–188‐m downstream migration of the thalweg occurred. Simulation results indicated that, during low water periods, modifications occurred in both the main channel and in other braid channels. Thus, unlike some gravel‐bed rivers, the sandy lower Tana River does not behave like a single‐thread channel at low discharge. However, at higher discharge, i.e. exceeding 497 m³/s, the river channel resembled a single‐thread channel when channel banks confined the flow. Although the spring discharge peaks caused more rapid modifications than slower flows, the cumulative volumetric changes of the low water period were greater. The importance of low water period flows for channel modifications is emphasized. Although the 2‐D model requires further improvements, the results were nevertheless promising for the future use of this approach in braided rivers. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

A depth‐averaged two‐phase model for debris flows over erodible beds          下载免费PDF全文

Ji Li  Zhixian Cao  Kaiheng Hu  Gareth Pender  Qingquan Liu 《地球表面变化过程与地形》2018,43(4):817-839

Mass exchange between debris flow and the bed plays a vital role in debris flow dynamics. Here a depth‐averaged two‐phase model is proposed for debris flows over erodible beds. Compared to previous depth‐averaged two‐phase models, the present model features a physical step forward by explicitly incorporating the mass exchange between the flow and the bed. A widely used closure model in fluvial hydraulics is employed to estimate the mass exchange between the debris flow and the bed, and an existing relationship for bed entrainment rate is introduced for comparison. Also, two distinct closure models for the bed shear stresses are evaluated. One uses the Coulomb friction law and Manning's equation to determine the solid and fluid resistances respectively, while the other employs an analytically derived formula for the solid phase and the mixing length approach for the fluid phase. A well‐balanced numerical algorithm is applied to solve the governing equations of the model. The present model is first shown to reproduce average sediment concentrations in steady and uniform debris flows over saturated bed as compared to an existing formula underpinned by experimental datasets. Then, it is demonstrated to perform rather well as compared to the full set of USGS large‐scale experimental debris flows over erodible beds, in producing debris flow depth, front location and bed deformation. The effects of initial conditions on debris flow mass and momentum gain are resolved by the present model, which explicitly demonstrates the roles of the wetness, porosity and volume of bed sediments in affecting the flow. By virtue of extended modeling cases, the present model produces debris flow efficiency that, as revealed by existing observations and empirical relations, increases with initial volume, which is enhanced by mass gain from the bed. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Water‐worked gravel beds in laboratory flumes – a natural analogue?     

James R. Cooper  Simon J. Tait 《地球表面变化过程与地形》2009,34(3):384-397

An investigation has been conducted to identify the key parameters that are likely to scale laboratory sediment deposits to the field scale. Two types of bed formation were examined: one where sediment is manually placed and screeded and the second where sediment is fed into a running flume. This later technique created deposits through sequential cycles of sediment transport and deposition. Detailed bed surface topography measurements have been made over a screeded bed and three fed beds. In addition, bulk subsurface porosity and hydraulic conductivity have been measured. By comparing the four beds, results revealed that certain physical properties of the screeded bed were clearly different from those of the fed beds. The screeded bed had a random organization of grains on both the surface and within the subsurface. The fed beds exhibited greater surface and subsurface organization and complexity, and had a number of properties that closely resembled those found for water‐worked gravel beds. The surfaces were water‐worked and armoured and there was preferential particle orientation and direction of imbrication in the subsurface. This suggested that fed beds are able to simulate, in a simplified manner, both the surface and subsurface properties of established gravel‐bed river deposits. The near‐bed flow properties were also compared. It revealed that the use of a screeded bed will typically cause an underestimation in the degree of temporal variability in the flow. Furthermore, time‐averaged streamwise velocities were found to be randomly organized over the screeded bed but were organized into long streamwise flow structures over the fed beds. It clearly showed that caution should be taken when comparing velocity measurements over screeded beds with water‐worked beds, and that the formation of fed beds offers an improved way of investigating intragravel flow and sediment–water interface exchange processes in gravel‐bed rivers at a laboratory scale. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Experimental investigation on local shear stress and turbulence intensities over a rough non-uniform bed with and without sediment using 2D particle image velocimetry     

《国际泥沙研究》2020,35(2):193-202

The current work focuses on locally resolving velocities,turbulence,and shear stresses over a rough bed with locally non-uniform character.A nonporous subsurface layer and fixed interfacial sublayer of gravel and sand were water-worked to a nature-like bed form and additionally sealed in a hydraulic flume.Two-dimensional Particle Image Velocimetry(2 D-PIV) was applied in the vertical plane of the experimental flume axis.Runs with clear water and weak sediment transport were done under slightly supercritical flow to ensure sediment transport conditions without formation of considerable sediment deposits or dunes.The study design included analyzing the double-averaged flow parameters of the entire measurement domain and investigating the flow development at 14 consecutive vertical subsections.Local geometrical variabilities as well the presence of sediment were mainly reflected in the vertical velocity component.Whereas the vertical velocity decreased over the entire depth in presence of sediment transport,the streamwise velocity profile was reduced only within the interfacial sublayer.In the region with decelerating flow conditions,however,the streamwise velocity profile systematically increased along the entire depth extent.The increase in the main velocity(reduction of flow resistance)correlated with a decrease of the turbulent shear and main normal stresses.Therefore,effects of rough bed smoothening and drag force reduction were experimentally documented within the interfacial sublayer due to mobile sediment.Moreover,the current study leads to the conclusion that in nonuniform flows the maximum Reynolds stress values are a better predictor for the bed shear stress than the linearly extrapolated Reynolds stress profile.This is an important finding because,in natural flows,uniform conditions are rare.  相似文献   

Examining the physical components of boundary shear stress for water‐worked gravel deposits     

James R. Cooper  Simon J. Tait 《地球表面变化过程与地形》2010,35(10):1240-1246

It is argued in this commentary that, in order to understand better the physical mechanisms that generate boundary shear stress over water‐worked gravel beds, flow velocity data should be re‐evaluated by spatial averaging the Reynolds equations to produce time‐ and space‐averaged (double‐averaged) momentum equations. A series of laboratory experiments were conducted in which the flow velocities were measured using a PIV system over two water‐worked gravel deposits. Combined with detailed data on the bed surface topography and vertical porosity, the physical components of shear stress were obtained. This enabled the various momentum transfer mechanisms present above, within and at the interface of a porous, fluvial deposit, to be quantified. This included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer. It is demonstrated that double‐averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment–water interface exchange mechanisms within the roughness layer in water‐worked gravel deposits. By considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel–sand mixtures and the transfer of solutes into porous, gravel deposits. This analysis reveals the importance of obtaining co‐located, high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Bridging the gap between turbulence and larger scales of flow motions in rivers     

Geneviève A. Marquis  André G. Roy 《地球表面变化过程与地形》2011,36(4):563-568

Although flow turbulence in rivers is of critical importance to earth scientists, ecologists and engineers, its relations with larger flow scales are not well understood, thus leaving a fundamental gap in our knowledge. From an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel‐bed river, we show that the signature of the fundamental turbulent flow structures (e.g. ejections and sweeps) is embedded within increasingly larger flow scales in a self‐similar manner. The imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two‐orders of magnitude from a few seconds to nearly 10 minutes. This property is explained by the clustering of turbulent events creating an emergent pattern at larger scales. The size of the larger flow pulsations scales with the spacing of the pools and riffles in the river. This implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology. These results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport, habitat selection and fish behaviour. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Quantifying River‐Groundwater Interactions of New Zealand's Gravel‐Bed Rivers: The Wairau Plain          下载免费PDF全文

Thomas Wöhling  Moritz J. Gosses  Scott R. Wilson  Peter Davidson 《Ground water》2018,56(4):647-666

New Zealand's gravel‐bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river‐groundwater exchange processes in gravel‐bed rivers, we investigate the Wairau Plain Aquifer using a three‐dimensional groundwater flow model which was calibrated using targeted field observations, “soft” information from experts of the local water authority, parameter regularization techniques, and the model‐independent parameter estimation software PEST. The uncertainty of simulated river‐aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null‐space Monte‐Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³/s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³/s, the net exchange flow rarely exceeds 12 m³/s and seems to be limited by the physical constraints of unit‐gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low‐flow periods in the river. We hypothesize that the new insights into the river‐groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.  相似文献   

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde,Sachsen‐Anhalt,Germany)     

Thomas Vetter 《地球表面变化过程与地形》2011,36(12):1647-1657

Riffle‐pool sequences are a common feature of gravel‐bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross‐sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage‐dependant morphometric changes to riffle and pool maintaining processes. In accordance with previous authors, pool cross‐sections of the Mulde River are narrow and riffle cross‐sections are wide suggesting that they should rather be addressed as two general types of channel cross‐sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Q_bf (40% bankfull discharge). An index for the longitudinal amplitude of riffle‐pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes. According to the observed dynamics of bed amplitudes, maintenance of riffle‐pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post‐flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility. The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Grain size and topographical differences between static and mobile armour layers     

Luca Mao  James R. Cooper  Lynne E. Frostick 《地球表面变化过程与地形》2011,36(10):1321-1334

A series of laboratory flume experiments under conditions of sediment starvation (zero sediment feeding) and recirculation were conducted in order to identify the temporal evolution and surface properties of static and mobile armour layers. The experiments were carried out in an 8 m long flume using a bimodal grain‐size mixture (D₅₀ = 6·2 mm) and a range of shear stresses ranging from 4·0 to 8·6 N m^–2. The results confirm that a static armour layer is coarser than a mobile one, and that the grain size of a mobile armour layer is rather insensitive to changes in the imposed flow strength. An analysis of laser scan bed surveys revealed the highly structured and imbricated nature of the static armour layer. Under these conditions the vertical roughness length scale of the bed diminished and it became topographically less complex at higher forming discharges. The topography of mobile armour layers created by rising discharges differed. They exhibited a greater roughness length scale and were less organized, despite the fact that the grain size of the surface material maintained an approximately constant value during recirculation. Also, the mobile armour tended to create larger cluster structures than static armour layers when formed by higher discharges. These differences were mainly due to the transport of the coarser fraction of bed sediments, which diminished to zero over the static armour because of being hidden within the bed, whereas in the mobile armour the coarser particles protruded into the flow and were actively transported, increasing the vertical roughness length scale. Overall, the results show that an examination of the grain size characteristics of armour layers cannot be used to infer sediment mobility and bed roughness. Detailed elevation models of exposed surfaces of gravel‐bed rivers are required to provide critical insight on the sediment availability and sedimentation processes. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Hydrologic and geomorphic flow thresholds in the Lower Brazos River,Texas, USA     

Jonathan D. Phillips 《水文科学杂志》2013,58(9):1631-1648

Abstract

River science and management often require a design or reference discharge. The common (and sometimes unavoidable) use of such discharges may, however, obscure the fact that the magnitude and frequency of critical flows can differ due to various hydrological, geomorphological, and ecological criteria. Threshold stages and discharges were identified for six lower Brazos River, Texas gaging stations corresponding to thalweg connectivity, bed inundation, high sub-banktop flows, channel–floodplain connectivity (CFC), and overbank flooding. Critical flows were also identified for estimated thresholds for sandy bedform and medium gravel mobility, critical specific stream power for potential channel modifications, and cohesive-bank channel erosion. These thresholds have variable relationships to mean, median, and maximum flows. For four of the six stations, daily recurrence probabilities for all but flood flows are at least 1%, and as high as 11%. All stations achieve channel–floodplain connectivity at stages less than banktop. Estimated threshold flows for sediment mobility and channel erosion occur relatively frequently, with daily probabilities of 2–77%. Critical flows for bank erosion occur least often, and for sandy bedform and gravel mobility most often. Thalweg connectivity is always maintained at all sites, while bed inundation flows have a daily probability of about 80% or more. Overall, results suggest that no single flow level is dominant in hydrological or geomorphic dynamics, and that the frequency of a given threshold varies considerably even along a single river. The results support the idea that multiple flow levels and ranges are necessary to create and maintain the hydrological, geomorphological, and ecological characteristics of rivers, and that no single flow level is a reliable determinant of fluvial state.