scholarly journals LOW-MOBILITY TRANSPORT OF COARSE-GRAINED BED MATERIAL

2011 ◽  
Vol 1 (32) ◽  
pp. 13
Author(s):  
Jeroen Van den Bos ◽  
Henk Jan Verhagen ◽  
Jelle Olthof
Keyword(s):  
Breaking Waves ◽  
Coarse Grained ◽  
Steady Current ◽  
Bed Material ◽  
Bed Protection

The Paintal (1967) formula can be used to estimate transport rates of coarse-grained bed material (such as stones from a bed protection) under low hydraulic loads just above the threshold of motion; the application however is limited to steady currents. This paper presents research aimed at extending the range of application to a combination of regular non-breaking waves and a steady current.

scholarly journals Sediment dynamics on a steep, megatidal, mixed sand-gravel-cobble beach

2014 ◽  
Vol 2 (1) ◽  
pp. 117-152 ◽  
Author(s):  
A. E. Hay ◽  
L. Zedel ◽  
N. Stark
Keyword(s):  
Wide Band ◽  
Sediment Dynamics ◽  
Coarse Grained ◽  
Bay Of Fundy ◽  
Wave Forcing ◽  
Structure Of Flow ◽  
Median Diameter ◽  
Field Deployment ◽  
Bed Material ◽  
Cobble Beach

Abstract. Results are presented from a pilot study of shoreface sediment dynamics on a steep, poorly sorted, coarse-grained, mega-tidal beach at the head of the Bay of Fundy, Nova Scotia, Canada. The experiment involved the first field deployment of a prototype wide- band, pulse-coherent, bistatic acoustic Doppler profiling system. Measurements of the vertical structure of flow and turbulence above a sloping bed, as well as bed material velocity, demonstrate the capabilities of this instrument vis-a-vis studies of nearshore sediment dynamics at the field scale. The second focus of the paper is the surprising observation that the surficial sediment median diameter, across the lower two-thirds of the intertidal zone, underwent a pronounced decrease when wave forcing was more energetic, compared to values observed during calmer conditions. The explanation for this result appears to involve the formation – in wave-dominated conditions – of O(1 m)-wavelength, 20 cm high ripples on the rising tide, which are then planed flat by the swash and/or the shorebreak on the subsequent ebb.

scholarly journals STABILITY OF A SAND BED UNDER BREAKING WAVES

1974 ◽  
Vol 1 (14) ◽  
pp. 45 ◽  
Author(s):  
Ole Secher Madsen
Keyword(s):  
Pressure Gradient ◽  
Surf Zone ◽  
Breaking Waves ◽  
Breaking Wave ◽  
Light Weight ◽  
Loss Of Stability ◽  
Vertical Flow ◽  
Sufficient Magnitude ◽  
The Stability ◽  
Bed Material

The possible effect on the stability of a porous sand bed of the flow induced within the bed during the passage of near-breaking or breaking waves is considered. It is found that the horizontal flow rather than the vertical flow within the bed may affect its stability. An approximate analysis, used in geotechnical computations of slope stability, indicates that a momentary bed failure is likely to occur during the passage of the steep front slope of a near-breaking wave. Experimental results for the pressure gradient along the bottom under near-breaking waves are presented. These results indicate that the pressure gradient is indeed of sufficient magnitude to cause the momentary failure suggested by the theoretical analysis. The loss of stability of the bed material due to the flow induced within the bed itself may affect the amount of material set in motion during the passage of a near-breaking or breaking wave, in particular, in model tests employing light weight bed material. The failure mechanism considered here is also used as the basis for a hypothesis for the depth of disturbance of the bed in the surf zone. The flow induced in a porous bed is concluded to be an important mechanism which should be considered when dealing with the wave-sediment interaction in the surf zone.

Bed configuration and microscale processes in alluvial channels

1993 ◽  
Vol 17 (2) ◽  
pp. 123-136 ◽  
Author(s):  
André Robert
Keyword(s):  
Probabilistic Approach ◽  
Field Measurements ◽  
Bedload Transport ◽  
Transport Processes ◽  
Coarse Grained ◽  
Small Scale ◽  
Alluvial Channels ◽  
Gravel Beds ◽  
Sand And Gravel ◽  
Bed Material

Numerous recent studies on fluvial processes, both in Canada and internationally, have focused on small-scale phenomena. Investigations on the characterization of surface roughness in coarse-grained channels and its links with flow resistance and sediment transport processes have been a dominant field of research. Closely related is a second major area of investigation on turbulent flow structures in boundary layers over both sand and gravel beds and their relations with the transport of bed material. Phenomena potentially related to 'bursting' have been shown to control bedload transport processes and the concentration of sediment in suspension. Detailed investigations have also been conducted on the links between flow turbulence, bed material movement, and bed morphology at channel junctions. Finally, selective entrainment and transport of individual coarse particles have been studied from field measurements and laboratory experi ments. Emphasis has been put on bed microtopography, surface structure and texture, and on a probabilistic approach to bedload transport.

Patterns and processes of sediment sorting in gravel-bed rivers

1998 ◽  
Vol 22 (1) ◽  
pp. 1-32 ◽  
Author(s):  
D. Mark Powell
Keyword(s):  
Spatial Scales ◽  
Coarse Grained ◽  
Transport Pathways ◽  
Spatial And Temporal Scales ◽  
Rough Bed ◽  
Grain Size Distributions ◽  
Coarse Surface ◽  
Sediment Sorting ◽  
Patterns And Processes ◽  
Bed Material

Sedimentological studies of coarse-grained alluvial rivers reveal patterns of bed material sorting at a variety of spatial scales ranging from downstream fining over the length of the long profile to the vertical segregation of a coarse surface layer at the scale of individual particles. This article reviews the mechanisms that sort bed material by size during sediment entrainment, transport and deposition and discusses some of the inter-relationships that exist between patterns and processes of sediment sorting at different spatial and temporal scales. At initiation of motion, sorting can arise from the preferential entrainment of the finer fractions from the heterogeneous bed sediments. Bedload grain-size distributions are modified during transport as different size fractions are routed along different transport pathways under the influence of nonuniform bed topography and associated flow patterns, and during deposition as the variable pocket geometry of the rough bed surface and turbulence intensity of the flow control the size of the particles that deposit. The review highlights the poor understanding of the many feedback linkages that exist between patterns and processes of sediment sorting at different scales and the need for a greater awareness of the spatial and temporal bounds of these linkages.

scholarly journals Short communication: Field data imply that the sorting (<i>D</i><sub>96</sub> / <i>D</i><sub>50</sub> ratios) of gravel bars in coarse-grained streams influences the probability of sediment transport

2020 ◽  
Author(s):  
Fritz Schlunegger ◽  
Romain Delunel ◽  
Philippos Garefalakis
Keyword(s):  
Water Discharge ◽  
Bedload Transport ◽  
Swiss Alps ◽  
Coarse Grained ◽  
Water Runoff ◽  
Shear Stresses ◽  
Sediment Discharge ◽  
Peruvian Andes ◽  
Gravel Bars ◽  
Bed Material

Abstract. Conceptual models suggest that the mobility of fluvial gravel bars is mainly controlled by sediment discharge. Here we present field observations from streams in the Swiss Alps and the Peruvian Andes to document that for a given water runoff, the probability of bedload transport also depends on the sorting of the bed material. We calculate shear stresses that are expected for a mean annual water discharge, and compare these estimates with grain-specific thresholds. We find a positive correlation between the predicted probability of transport and the sorting of the bed material, expressed by the D96 / D50 ratio. These results suggest that besides sediment discharge, the bedload sorting exerts a measurable control on the gravel bar mobility.

scholarly journals Formation of rhythmic sorted bed forms on the continental shelf: an idealised model

2011 ◽  
Vol 684 ◽  
pp. 475-508 ◽  
Author(s):  
Tomas Van Oyen ◽  
Huib de Swart ◽  
Paolo Blondeaux
Keyword(s):  
Sediment Transport ◽  
Coarse Grained ◽  
Qualitative Comparison ◽  
Fine Sediments ◽  
Bed Elevation ◽  
Steady Current ◽  
Bed Forms ◽  
Current Shift ◽  
Idealised Model ◽  
Sediment Mixture

AbstractAn idealised model is presented to study the formation of sorted bed forms generated by a wind-driven along-shore current. The study employs a linear stability analysis to describe the time development of perturbations of both bottom composition and bed elevation, superimposed on a flat bed composed of a sediment mixture homogeneously distributed in space. The model considers both bed and suspended loads and takes into account the averaged influence of waves on the flow field and the transport of sediment. The results show that the positive coupling between waves, along-shore current and the erodible heterogeneous bed leads to the amplification of two modes, which exhibit distinct characteristics. A first mode is found to be dominant when moderate hydrodynamic conditions are considered and is primarily amplified by the convergence of sediment transport induced by the changes in the bed elevation. This mode has wavelengths of the order of hundred metres and has coarse (fine) sediments in its troughs (crests). By increasing the height of the waves and/or the strength of the steady current, the second mode can become dominant. This mode is characterised by shorter wavelengths and results from the interaction between the convergence of sediment transport related to changes in the bottom composition and that induced by perturbations of the bed elevation. These bed features can have an up-current or a down-current shift between the centre of the coarse-grained bands and the trough of the bottom wave. Typical growth times of the amplified features are of the order of hundreds of days and the migration rates, in the direction of the along-shore current, range between 0.1 and 10 m per day. A qualitative comparison of the model results with field observations indicates that the generation of two distinct modes provides a possible explanation for the broad range of characteristics of the natural bed features.

Progress in arid-lands rainfall-runoff modelling

1994 ◽  
Vol 18 (3) ◽  
pp. 343-365 ◽  
Author(s):  
A.S. EI-Hames ◽  
K.S. Richards
Keyword(s):  
Overland Flow ◽  
Physical Modelling ◽  
Coarse Grained ◽  
Fine Grained ◽  
Physically Based ◽  
Flood Estimation ◽  
Physically Based Models ◽  
Flood Simulations ◽  
Slackwater Deposit ◽  
Bed Material

Hydrological data are lacking in arid regions for a variety of reasons, making emprical methods of flood estimation unsatisfactory. Physically based models which are calibrated in the field using measurements of soil hydrological properties offer a viable alternative modelling strategy, and this article reviews the main elements required in such an approach. These include coupled submodels of 1) infiltration and runoff production; 2) overland flow routing; 3) channel routing to propagate flood waves down wadis beyond runoff source areas; and 4) transmissison losses into wadi beds. Examples are presented of flood simulations using a new physically-based model with such a structure. The role of both coarse-grained (bed material) and fine-grained (slackwater deposit) sediments in wadis in providing additional evidence of flood magnitudes is also discussed, and a methodology is suggested in which physical modelling and sedimentological analyses are combined.

scholarly journals Short communication: Field data reveal that the transport probability of clasts in Peruvian and Swiss streams mainly depends on the sorting of the grains

2020 ◽  
Vol 8 (3) ◽  
pp. 717-728
Author(s):  
Fritz Schlunegger ◽  
Romain Delunel ◽  
Philippos Garefalakis
Keyword(s):  
Water Discharge ◽  
Swiss Alps ◽  
Coarse Grained ◽  
Shear Stresses ◽  
Material Transport ◽  
Peruvian Andes ◽  
Annual Water ◽  
Annual Water Discharge ◽  
Bed Material ◽  
Annual Discharge

Abstract. We present field observations from coarse-grained streams in the Swiss Alps and the Peruvian Andes to explore the controls on the probability of material entrainment. We calculate shear stress that is expected for a mean annual water discharge and compare these estimates with grain-specific critical shear stresses that we use as thresholds. We find that the probability of material transport largely depends on the sorting of the bed material, expressed by the D96∕D50 ratio, and the reach gradient but not on mean annual discharge. The results of regression analyses additionally suggest that among these variables, the sorting exerts the largest control on the transport probability of grains. Furthermore, because the sorting is significantly correlated neither to reach gradient nor to water discharge, we propose that the granulometric composition of the material represents an independent, yet important control on the motion of clasts in coarse-grained streams.

scholarly journals THE INTERACTION OF SMALL AND FINITE AMPLITUDE LONG WAVES AND CURRENTS

1984 ◽  
Vol 1 (19) ◽  
pp. 67 ◽  
Author(s):  
F. Raichlen ◽  
J.J. Lee
Keyword(s):  
Power Plants ◽  
Cooling Water ◽  
Breaking Waves ◽  
Finite Amplitude ◽  
Offshore Structure ◽  
Wave Refraction ◽  
Steady Current ◽  
Waves And Currents ◽  
The Waves ◽  
A Current

The interaction of waves and currents is important for many engineering problems. For example, when considering forces on marine structures, the velocity and acceleration field must be defined, and thus the manner in which a current interacts with small and finite amplitude waves must be understood. When the current is large and oblique to the waves, the direction of the force on an offshore structure may change significantly with depth introducing a torsional moment. Wave refraction and the concomitant attenuation or amplification of waves are also affected by offshore currents. An example is the effect on incident waves of offshore currents induced by the discharge of cooling water from coastal-sited power plants. This current can modify the direction and magnitude of approaching waves, and by these changes the breaking waves at the shore and the nearshore sediment transport associated with these waves may be changed. A number of theoretical studies have been conducted on various aspects of wave-current interactions; see Peregrine (1976). One theoretical study, Thomas (1981), will be used in this investigation. Careful experiments in this area are limited; several are: Iwagaki and Asano (1980), Sarpkaya (1957), and Thomas (1981). Each of these has given attention to certain aspects of small amplitude wave-current interactions. The experiments are difficult to conduct because of the problems inherent in introducing waves into a flume with a steadyuniform current or conversely a current into a wave tank with permanent waves. Certain features of these experimental problems can be seen through the following two examples. If a plunger-wave machine were used and located at one end of a flume in which a steady current is flowing, although the waves would be developing as they interact with the current, the previously steady current would be changed to an unsteady one by the periodic blockage of the flow by the plunger. If the waves are generated at one end of the tank and allowed to develop, and a current is introduced from the bottom of the tank, this current must expand to the full depth of the flow; hence, the waves propagate on a developing current. Therefore, comparisons to theory are, to some extent, difficult to realize, because the theory generally assumes wavecurrent interactions when each is fully developed.

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