A note on cluster bedform behaviour in a gravel-bed river

Earth Surface Processes and Landforms, 2005

Structured gravel river beds clearly exert a major influence on bed stability. Indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure. This study takes a morphological approach to interpreting the critical processes, using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations. The experiment was carried out on the River South Tyne, UK; it uses flow hydrographs measured nearby and also benefits from previous studies of historical development, channel morphology and sediment transport at the same site. More than 30 clusters were monitored over a seven-month period during which clusters occupied 7–16 per cent of the bed. Threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m3 s−1 and 183 m3 s−1; durations of flow at these levels are critical for cluster development, rather than flow peak values. Wake particles are transported most easily. Flow straightening in the wandering channel planform reduces the stability of clusters, since mechanical strength is markedly reduced by this change of direction. The overall area covered by clusters between significant transport events varies little, implying a dynamic equilibrium condition. Copyright © 2005 John Wiley & Sons, Ltd.

The effect of geomorphic features and hydraulic conditions on the formation, evolution, and morphology of sediment cluster microforms in an unregulated gravel-bed stream were investigated at field sites on the Entiat River, which drains the eastern slopes of the Cascade Mountains in central Washington state, USA. Sediment clusters were marked, described, and photographed before and after a series of moderate to high discharges over an 18-month period to quantify the evolution of the cluster morphologies. Individual sediment particles were tracked to calculate the range of flows and bed shear stresses that maintain and destroy clusters. Examination of geomorphic settings, channel morphology, and particle size distributions documented the conditions that favor cluster formation. The investigation tested the hypotheses that clusters in this environment delay sediment entrainment and that their morphology and evolution follow a predictable evolution similar to that found in laboratory studies.Clusters formed on gravel bars adjacent to riffles with slopes ≥ 1%, poor to moderate sorting, and mean and maximum particle size values 1.5 times greater than those associated with noncluster bars. Clusters were more stable under the bimodal sediment size distribution contributed by a tributary alluvial fan and were destroyed with and without anchor clast mobilization at sites where sediment size was more uniform. The six cluster morphologies identified in the field were similar to those in flume studies, but did not follow the same evolutionary cycle over multiple flow events. This contrast was attributed to the flow events resetting the cluster cycle, leading to a high percentage of upstream triangles. The dimensionless critical shear stress required to entrain the mean grain size ranged from 0.06 to 0.08. The hydraulic thresholds and geomorphic features that result in stable vs. mobilized cluster microforms in this setting can serve as a model for regulated rivers and restoration projects aimed at sustaining instream flows to maintain natural sediment transport conditions.

Journal of Engineering Mechanics, 2003

The formation of cluster structures in gravel bed streams is a naturally occurring process that characterizes the response of sediment to the erosive power of flowing water. Cluster structures are formed when two or more particles group together. Their size depends on sediment availability, flow intensity, and sediment material properties such as sediment specific gravity. The scope of the present study was to ͑1͒ quantitatively describe the evolution and disintegration processes of cluster microforms under different sediment availabilities and sediment specific gravities; and to ͑2͒ examine the flow-frictional characteristics around clusters and compare them with those around a single particle that is fully exposed to the flow. Overall, 14 experimental runs were carried out in a laboratory flume for three sediment availabilities, namely the 2%, 35%, and 50%. Six of these tests were performed by using glass spheres ͑Specific gravity, SGϭ2.58͒, the other six by employing an equal combination of glass and Teflon spheres ͑SGϭ2.12͒ of the same diameter to evaluate the role of specific gravity on cluster evolution; the remaining two runs were performed for the 50% sediment availability using heavier lead palettes ͑spheres͒ of the same size with the glass and Teflon spheres to facilitate near-bed flow measurements around a cluster and a single particle without the occurrence sediment movement. The results of this study showed that: ͑1͒ sediment availability and specific gravity affects the architecture and size of cluster microforms; and ͑2͒ if we use the critical stress for the lowest sediment availability ͑2% availability͒, cr2% * , as the reference stress, then particle members of clusters disintegrate at twice the cr2% * for the 2% availability case, at nine times more than the cr2% * for the 35% case and at eleven times more than the cr2% * for the 50% case. Finally, it was determined for the 50% case that computed form resistance comprises 98% of the total when clusters form, which agrees with findings that have been reported in literature.

Journal of Hydrology, 2007

A short review of the literature on particle clusters on gravel river beds reveals investigations of both process and form dominated by the intensive study approach, using a restricted geographical sample or evidence from flumes. An alternative, presented here, is extensive sampling -from three climatic zones, several channels in each and at multiple transects at each site. It uses insights provided by a more intensively studied 'base station' . Particle clusters in gravel-bed rivers -an experimental morphological approach to bed material transport and stability concepts. Earth Surface Processes and Landforms 30(11), 1351-1368]. Transect surveys were completed in each selected reach to establish flow_depth, bed material size and bed structure. A total of more than 5000 sample points reveals the vital presence of bed material of %100 mm D 50 for all sub-types of clusters to occur; thereafter, cluster frequency relates directly to the D 90 , with an improving correlation at D 90 > 256 mm. A better integration of data from the diverse hydrological/hydraulic regimes can be achieved by correlating cluster frequency with a sorting index for bed material. Further analysis of hydrological and hydraulic data for all sites is required to develop a dynamic explanation. ª

Sedimentology, 1983

ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane-beds. The influence of clusters is a major determinant of sedimentary sorting.

Earth Surface Processes and Landforms, 2003

The objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics (spacing and size). To isolate the effects of sediment availability and specific gravity on cluster evolution, mono-sized spheres were used to simulate the cluster evolutionary cycle. Overall, twelve experimental runs were carried out in the laboratory flume. Six of these tests were performed by using glass spheres (specific gravity, SG D 2Ð58) and the other six by employing an equal combination of glass and Teflon spheres (SG D 2Ð12) of the same diameter to evaluate the role of specific gravity on cluster evolution. The three sediment availability conditions that were investigated here simulated isolated gravel elements, pool-riffle sequences and densely packed gravel-bed. An advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area. The results of this study showed that: (1) sediment availability affects the architecture and size of cluster microforms; and (2) clusters consisting of mono-sized sediments start disintegrating at twice the incipient conditions. By performing complementary tests for the isolated gravel elements case, it was found that the evolutionary cycle of individual clusters could be described as follows, in order of increasing stress: no cluster ! two particle cluster ! comet ! triangle ! rhomboid ! break up.

Experimental Techniques, 2000

onitoring the bed load motion of rivers is of high importance in sedimentation engineering. Bed load motion refers to the transport of bed material and is the mass or volume of sediment that is entrained within a stream section per unit time and width. Understanding the mechanisms involved in sediment transport is important for the design of stable rivers as well as for the protection of fish spawning grounds. * length was sufficient for monitoring the particle motion since ' A. Schvler is a graduate research assistant in the DeDartment of Ciuil and Enui-. ' : beads diameters3. . A videocassette recorder attached to a personal computer . was used to capture the movies. The Asymetrix Video Cap-* ture 4.0 computer program was used to digitize the captured * movies. Individual images were then digitized at sequential ' instants viz., 0, 0.5, 1, 1.5, 2, 5, 10, 15, 20, 25, and 30 * minutes. The close capturing of the images in the first five

Earth Surface Processes and Landforms, 2002

A two-dimensional simulation model of travel distances of individual particles in a gravel-bed river is presented. The model is based on a number of rules, which include particle size, entrainment, trajectory, distance of movement and entrapment. Particle interactions are controlled by resistance fields defined about each obstacle and critical elevation defined in the model. Resistance fields, particle dropping and critical elevation rules control particle interactions. The interaction rules cause the particles to develop pebble clusters, stone cells and transverse structures (transverse ribs). The simulated travel distances of individual particles are consistent with reported field results. Individual particle travel distances were simulated using two different models; one without interactions between the individual particles and the stationary bed and one with interactions. The case without interactions demonstrates the random nature of sediment transport, and narrow ranges of travel distances. Wider ranges of travel distances, similar to those for natural situations, were obtained for the cases with interactions. The more intense the interaction between the mobile stones and the stationary ones, the wider the range of distances of travel for a given particle size. Modelling the mean travel distance yielded a result similar to that published previously, which was based on empirical data. Well developed bed-surface structures were obtained for relatively poorly sorted sediment with intense interactions between particles. Transverse structures developed when relatively large particles were allowed to move.

The purpose of the study was to further the understanding of unsteady transport of coarse bedload. A particular focus was the interaction of the load with the river bed material and the effects of transport on stream hydraulics. It was anticipated that new methodology would be developed and that data sets suitable for further modelling would result. The nature of coarse gravel transport through a short reach of a mountain river was examined using novel methodology; primarily, magnetic particle detection; acoustic bedload detection, electromagnetic current meters and fine-resolution bed elevation surveys.

Geomorphology, 2010

This study investigates sediment mobility in a large gravel-bed river (Tagliamento River, northeastern Italy). Field data were used to identify the morphological effectiveness of a range of flows (floods with recurrence interval b 1 to 3.5 year) and for a detailed analysis of the partial transport condition. The analyses were carried out on three cross sections where a number of areas representative of different morphological units (main and secondary channels, low and high bars, islands) were painted. Grain size of the painted area was measured using a photographic method. Monitoring of bed mobility during the study period supplied new photographs of the painted areas, measurements of size and travel distance of the mobilized particles, and estimate of flow depth after each flood event.