Small-scale Bedforms Research Articles

Steady streaming induced by sea waves over rippled and rough beds

The flow in the boundary layer generated close to the sea bottom by the propagation of a monochromatic surface wave is determined by considering small values of both the wave steepness and the ratio between the thickness of the boundary layer and the local water depth. Nonlinear effects are considered and attention is focussed on the steady streaming. The sea bed is assumed to be horizontal, impermeable and rippled and the small scale bedforms are supposed to act as a large roughness the size of which depends on the ripple height. The analysis of Blondeaux et al. (2012), who considered a flat bed, is thus extended. The geometrical characteristics of the ripples are predicted using empirical formulae and, then, the bedforms are assumed to behave as a bottom roughness, the size of which is related to the size of the ripples. A comparison of the model results with experimental measurements supports the theoretical findings. As in the sheet flow regime (when the sea bed is flat), the steady streaming at the bottom of sea waves is found to be different from that predicted in an oscillating water tunnel (U-tube). Moreover, it is found that ripples interacting with a propagating wave can generate a steady streaming which points either in the onshore direction or in the offshore direction, depending on the value of the physical parameters.

Extraction of morphometric bedform characteristics from profiling sonar datasets recorded in shallow coastal waters of China

The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat, Jiangsu, in 2009, and in the Jiulong Estuary, Xiamen, in 2010, respectively. The “multi-threshold value” method was utilized to reveal the morphological undulations along which bedforms were present. Analyses of the datasets obtained show that: (1) sand ripples can have irregular shapes, and (2) changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles. Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness. As current speed increases, seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures. Furthermore, the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures.

Sediment mixtures, coastal bedforms and grain sorting phenomena: An overview of the theoretical analyses

Recent studies, which investigate the processes leading to the formation of rhythmic morphological features in the coastal region, are reviewed. Emphasis is given to morphodynamic stability analyses which consider the presence of a heterogeneous sediment. The selective transport of the different fractions of a sediment mixture leads to grain sorting phenomena, which significantly affect the dynamics of the bottom forms. Small scale bedforms (ripples), intermediate bedforms (sorted bedforms and tidal dunes) and large scale bedforms (shoreface-connected ridges and sand banks) are considered. References are given to papers describing the new aspects of the stability analyses arising from the simultaneous presence of a time-dependent forcing flow and a sediment mixture.

Bedform characteristics during falling flood stage and morphodynamic interpretation of the middle–lower Changjiang (Yangtze) River channel, China

A survey was conducted to examine river bedforms using side-scan sonar, sub-bottom profiler and an Acoustic Doppler Profiler in the middle–lower (mid-lower) Changjiang (Yangtze) River (downstream of Wuhan), China, during August 14–20, 2003 when the discharge was 27,000–36,000m3s−1. The results show bedforms of a range of sizes occur in the channel and their heights and lengths follow an exponential correlation below Flemming's maximum line. Not all bedforms were formed under the flow conditions experienced during the survey, as shown by comparison to three existing models in the literature. Small-scale bedforms with a length <60m, defined as flat bed, mega-ripple and small dune in the present study, are mostly in equilibrium and are probably formed under the existing flow conditions. The large dunes with a length of >60m are not in equilibrium and are believed to be the relicts of previous large discharges now undergoing adjustment. Different bedform types occur in the five river reaches in this section of the Changjiang (Wuhan–Tianjiazhen, Tianjiazhen–Anqing, Anqing–Maanshan, Maanshan–Jiangyin and Jiangyin–estuary downstream). In the narrow and single channel reaches above Anqing, large/small dunes are limited to the channel with higher water surface slope where stream power is strong enough for riverbed sediments to be moved frequently. In the downstream reaches between Anqing and Jiangyin, characterized by anabranching channels, small-scale bedforms superimposed on large dunes occur widely due to intensive riverbed sediment transport from frequent diverging/converging flow during both floods and low-flow periods. Mega-ripples and flat beds prevail in the lowermost channel below Jiangyin where wider cross-sections result in dispersed stream power.

Flow Field and Morphology Mapping Using ADCP and Multibeam Techniques: Survey in the Po River

An 8-km-long and 250-m-wide reach of the Po River (Italy) was surveyed with two vessels, aiming to investigate the channel morphology and flow field characterizing this major Italian river and to provide data for the calibration of a numerical model. The first vessel was equipped with a multibeam ecosounder (MBES), whereas an acoustic doppler current profiler (ADCP) was located on the second one. Fixed-vessel ADCP measurements were used to preliminarily investigate the turbulence variability affecting the reliability of moving-vessel measurements. To evaluate the friction velocity by using moving-vessel measurements, the vertical variance of velocity profiles as an alternative to logarithmic fitting of ADCP profiles was estimated. An average of 30-m cells of depth-averaged velocity, friction velocity, and surface-bottom velocity angle are presented in maps as the most relevant flow field features. The flow field and channel morphology at bed-form or channel scales showed noticeable correlations (0.9). In particular, a correlation between the friction velocity and small-scale bed forms (4–5 m mean wavelength) was observed.

Architecture of a modern transient slope fan (Villafranca fan, Gioia basin–Southeastern Tyrrhenian Sea)

The high resolution of geophysical data applied to the study of the modern seafloor provides excellent details on the architecture of deep-water fans. Numerous researches have addressed the study of modern ponded and terminal deep-water fans. In contrast, thorough studies of present-day examples of transient slope fans, i.e. those fans that are connected to a deeper depositional area, are not available. Multibeam bathymetry and CHIRP profiles have been used in the study of the Villafranca transient slope fan located in the northeastern Sicilian margin. Hanging above the 300-m high erosional flank of the Stromboli axial valley, perpendicular to the fan elongation, it represents a particular kind of transient fan. The fan is composed of a leveed channel followed downslope by a depositional lobe. The relief of the eastern levee above the channel axis changes due to longitudinal variations in the rate of sediment overbanking; despite the low sinuosity of the channel, an increased rate of overbank flow is observed in the outside of the channel bends. Downslope from a knickpoint, an increase in levee relief is the result of the entrenchment of the central tract of the channel. The erosional deepening of the channel followed the failure of a mass-transport deposit (MTD) that lowered the distal slope portion, also causing a step along the channel course. The MTD is characterized by an extensional headwall domain and by a slightly deformed main body that has suffered very restricted downslope movement. It was caused by the gliding of the levee wedge as a consequence of its undermining, due to the excavation of the Stromboli valley in the distal slope. Retrogressive sediment failures are widespread in the eastern levee; they form high mobility MTDs such as a debris flow that spread in the eastern portion of the depositional lobe. The Villafranca depositional lobe has an internal architecture that largely differs from that of classic fan models. Straight, approximately 500-m-wide and 10 m deep channels and longitudinal bars are in fact the main elements that compose the Villafranca channel-mouth depositional lobe. The channels narrow and deepen downslope, pointing to a progressive establishment of lateral flow non-uniformity in approaching the step at the edge of the depositional lobe. Thick transparent layers or thin well layered deposits, sometimes arranged in smaller scale bedforms, accumulate on the bars. Conversely, prevailing erosional or bypass processes are responsible for reduced sedimentation in the channels. The spatial arrangement of channels and bars within the depositional lobe results in a downslope increase in stratigraphic complexity. The architecture of the Villafranca fan lobe has the potential to be applied as analog for seismic or outcrop-based studies of fans developed in topographically complex slopes and connected to a deeper base level.

On the use of bioluminescence for estimating shear stresses over a rippled seabed

The bioluminescence emitted by micro-planktonic organisms has been adopted to detect shear stresses within oscillating flows over small scale bedforms. To this aim, an experimental campaign has been planned in order to optimize optical detectors and to address peculiar needs of biological organisms. Oscillating flow over ripples has been simulated in an annular cell, an apparatus that permits one to carry out experiments on ripple formation under well controlled conditions. Bioluminescence emission within the annular cell ascertains the existence of a high shear stress regime induced by the presence of the rippled bed in the range of Reynolds numbers between 2000 and 11000. Stress maps obtained from the experimental campaign showed the stronger stresses close to the rippled bed, with maximum values ranging between 15 and 35 Pa. 2D numerical simulations of oscillating flows over a rippled bed have been also run reproducing the experimental conditions, showing a good agreement on the position of the high stress regions, but underpredicting the maximum shear stress. Bioluminescence appears to be a valid instrument to characterize the flow and in particular to visualize high stress areas, leading to the possibility to measure shear stresses directly. Though limits occur, mostly due to the difficulties of dealing with biological organisms, such an approach can be considered as a starting point for the development of a bio-inspired visually based system for flow measurements.

High-Resolution Mapping of Mines and Ripples at the Martha's Vineyard Coastal Observatory

High-resolution multibeam sonar and state-of-the-art data processing and visualization techniques have been used to quantify the evolution of seafloor morphology and the degree of burial of instrumented mines and mine-shapes as part of the U.S. Office of Naval Research (ONR, Arlington, VA) mine burial experiment at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). Four surveys were conducted over two years at the experiment site with a 455-kHz, Reson 8125 dynamically focused multibeam sonar. The region is characterized by shore-perpendicular alternating zones of coarse-grained sand with 5-25-cm-high, wave orbital-scale ripples, and zones of finer grained sands with smaller (2-5-cm-high) anorbital ripples and, on occasion, medium scale 10-20-cm-high, chaotic or hummocky bedforms. The boundaries between the zones appear to respond over periods of days to months to the predominant wave direction and energy. Smoothing and small shifts of the boundaries to the northeast take place during fair-weather wave conditions while erosion (scalloping of the boundary) and shifts to the north-northwest occur during storm conditions. The multibeam sonar was also able to resolve changes in the orientation and height of fields of ripples that were directly related to the differences in the prevailing wave direction and energy. The alignment of the small scale bedforms with the prevailing wave conditions appears to occur rapidly (on the order of hours or days) when the wave conditions exceed the threshold of sediment motion (most of the time for the fine sands) and particularly during moderate storm conditions. During storm events, erosional ldquowindowsrdquo to the coarse layer below appear in the fine-grained sands. These ldquowindowrdquo features are oriented parallel to the prevailing wave direction and reveal orbital-scale ripples that are oriented perpendicular to the prevailing wave direction. The resolution of the multibeam sonar combined with 3-D visualization techniques provided realistic looking images of both instrumented and noninstrumented mines and mine-like objects (including bomb, Manta, and Rockan shapes) that were dimensionally correct and enabled unambiguous identification of the mine type. In two of the surveys (October and December 2004), the mines in the fine-grained sands scoured into local pits but were still perfectly visible and identifiable with the multibeam sonar. In the April 2004 survey, the mines were not visible and apparently were completely buried. In the coarse-grained sand zone, the mines were extremely difficult to detect after initial scour burial as the mines bury until they present the same hydrodynamic roughness as the orbital-scale bedforms and thus blend into the ambient ripple field. Given the relatively large, 3-D, spatial coverage of the multibeam sonar along with its ability to measure the depth of the seafloor and the depth and dimensions of the mine, it is possible to measure directly, the burial by depth and burial by surface area of the mines. The 3-D nature of the multibeam sonar data also allows the direct determination of the volume of material removed from a scour pit.

Measuring the effects of wave action around piers

The erosion process caused by fluid action in rivers, in near-shore regions, and around offshore platforms is interesting for both environmentalists and engineers. Many studies have been made of this phenomenon. However, sediment transport processes are still far from being clearly understood. Field and laboratory investigations are extremely important for improving our knowledge of the basic forces that control such phenomena. Many difficulties arise when trying to measure erosion or scouring processes. One of the most important features of the measurement system is that the instrumentation should not disturb either the fluid motion or the sediments. Therefore, the scour due to the wave motion around a vertical pile is investigated because of the equilibrium conditions of the erosion process. Techniques for structured light and image acquisition can measure and recover sand bed evolutions. These techniques are noninvasive and provide visual reference points or lines on the bottom during morpho-dynamic evolution. This study used computer vision techniques, based on the use of structured light, for three-dimensional (3D) measurements of the flow-sediment interactions with piers. Extensive laboratory applications have been made in wave flumes and wave tanks for investigating the erosion processes around a vertical pile or for investigating small-scale bedforms formation. These methods can detect changes of the sandy bottom, such as small bed-forms or scours, with great accuracy without interfering with the measured system. They also can observe the dynamic behavior of these morphological features.

Non-marine successions in the northwestern part of Kyongsang Basin (Early Cretaceous): Fluvial styles and stratigraphic architecture

Non-marine successions in the northwestern part of Kyongsang Basin (Early Cretaceous) are divided into successive stratigraphic units based on facies assemblages and architecture of sandstone bodies. In the present study, two stratigraphic units (Sinpyong-Anpyong and Jotap units) are documented in detail in terms of fluvial architecture. The Sinpyong-Anpyong unit is divided into thick sandstone, thin sandstone, and mudstone-dominated bodies, representing major channels, sand splays, and floodplains, respectively. The channel sandstone bodies are characterized by superposition of numerous bars and channels deposits formed in braided rivers. The mudstone-dominated bodies largely consist of purple siltstone in the northern (proximal) part and gray mudstone in the southern (distal) part, reflecting well-drained floodplains and poorly drained floodplains with local shallow lakes, respectively. The Jotap unit is divided into thick sandstone, interbedded sandstone/siltstone, and siltstone-dominated bodies, respresenting major channels, proximal floodplains, and distal floodplains, respectively. The thick sandstone bodies consist of trough cross-stratified sets and horizontally stratified sets with common internal scour surfaces and lenticular beds of purple siltstone, indicative of ephemeral channels dominated by small-scale bedforms. The siltstone-dominated bodies consist of purple siltstone and suggest well-drained floodplains. Along with the marked change in fluvial styles, the successions show systematic variations in the stacking patterns of channel bodies and the distribution of floodplain/lake facies. The basal and uppermost parts of the Sinpyong-Anpyong unit are characterized by sourceward expansion of distal, poorly drained floodplain/shallow lake facies and the low proportion and connectedness of channel bodies. Overlying these gray mudstone-dominated sequences, the middle part of Sinpyong-Anpyong unit and Jotap unit show increases in the proportion, connectedness, and grain size of channel bodies and basinward progradation of proximal, well-drained floodplain facies. Such a stratigraphic architecture can be attributed to the fluctuations in the ratio of accommodation space/sediment supply, regulated by repeated basin subsidence.

Sand wave fields beneath the Loop Current, Gulf of Mexico: reworking of fan sands

Extensive fields of large barchan-like sand waves and longitudinal sand ribbons have been mapped by deep-towed SeaMARC IA sidescan sonar on part of the middle and lower Mississippi Fan that lies in about 3200 m of water. The area is beneath the strongly flowing Loop Current. The bedforms have not been adequately sampled but probably consist of winnowed siliciclastic-foraminiferal sands. The size (about 200 m from wingtip to wingtip) and shape of the large barchans is consistent with a previously observed peak current speed of 30 cm/s, measured 25 m above the seabed. The types of small-scale bedforms and the scoured surfaces of chemical crusts, seen on nearby bottom photographs, indicate that near-bed currents in excess of 30 cm/s may sometimes occur. At the time of the survey the sand transport direction was to the northwest, in the opposite direction to the Loop Current but consistent with there being a deep boundary current along the foot of the Florida Escarpment. Some reworking of the underlying sandy turbidites and debris flow deposits is apparent on the sidescan sonar records. Reworking by deep-sea currents, resulting in erosion and in deposits characterised by coarsening upwards structures and cross-bedding, is a process that has been proposed for sand found in cores in shallower parts of the Gulf of Mexico. This process is more widespread than hitherto supposed.

Geometry, migration and evolution of small-scale bedforms generated by regular and irregular waves

Ripples generated by both regular and irregular waves have been experimentally investigated. In particular, their geometry at the equilibrium (i.e. height, length and steepness) has been analyzed and compared with semi-empirical predictive models. It has been observed that in the investigated range no distinction between ripples generated by regular or irregular waves is detectable, even though the equilibrium configuration is reached through different mechanisms. Moreover, it has been found that ripples migrate as soon as they appear and their velocity of migration qualitatively agrees with theories based on stability analyses recently proposed. Starting from results in literature, new semi-empirical formulae are provided for calculation of equilibrium characteristics of ripples.

Three‐dimensional flow structure around small‐scale bedforms in a simulated gravel‐bed environment

AbstractPebble clusters are common small‐scale morphological features in gravel‐bed rivers, occupying as much as 10 per cent of the bed surface. Important links exist between the presence of pebble clusters and the development of flow structures. These links are poorly understood at the three‐dimensional level. Particularly neglected has been the effect of clusters on the lateral flow characteristics. A laboratory study was conducted using a hydraulic flume, within which simulated pebble clusters were superimposed onto a plane bed of gravel material. High‐resolution three‐dimensional flow data were collected above the bed at two different flow depths using an acoustic Doppler velocimeter. The results present evidence of the importance of lateral flow in the development of turbulent flow structure. Narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three‐dimensional separation zone in their lee. This may indicate the presence of horseshoe‐type vortical structures analogous to those identified in less hydraulically rough environments. However, it is likely that these structures are more complicated given the mutual interference of the surrounding medium. The lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters. The results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters: flow acceleration up the stoss‐side of the cluster; recirculation behind the cluster in the wake region; vortex shedding from the pebble crest and shear layer; flow reattachment downstream of the cluster; upwelling of flow downstream of the point of reattachment; and recovery of flow. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Qasim Formation: Ordovician Storm- and Tide-Dominated Shallow-Marine Siliciclastic Sequences, Central Saudi Arabia

ABSTRACT The Middle to Upper Ordovician Qasim Formation is well exposed in the Qasim region of central Saudi Arabia and is recognized from many wells to the north and east. It consists of the Hanadir, Kahfah, Ra’an, and Quwarah members that are arranged in two coarsening-upward progradational sequences that have a total thickness of about 200 m in outcrops. Two sections were measured in each sequence. The lower sequence is a storm-dominated, shallow-marine depositional system composed of the late Llanvirn to middle Caradoc Hanadir and Kahfah. At least five progradational beach parasequences were identified. The Hanadir overlies deltaic deposits of the upper Saq Formation and marks a major marine transgression onto Gondwana. It is composed of laminated fissile shale formed in an offshore marine environment together with siltstone laminae that indicate periodic influxes of low-density turbidity currents. The overlying Kahfah is composed of shale and sandstone. The upper progradational sequence consists of the middle Caradoc to middle Ashgill Ra’an and Quwarah members. The Ra’an is a succession of homogeneous fissile shale and minor siltstone laminae formed in an offshore marine environment following a major transgression onto the Kahfah. The overlying Quwarah consists of sandstone and minor amounts of siltstone and shale in its lower parts, and a thickly bedded sandstone facies above. The diagnostic sedimentary structures in the Quwarah are large-scale lens-shaped tidal sand waves composed of sigmoidal bundles deposited in a mesotidal setting of barrier bars, tidal channels, ebb-dominated deltas and lagoons. In the subsurface, the Formation thickens northeastward (basinward) to over 4,000 ft (1,220 m). In the same direction, the sand-dominated Kahfah and Quwarah gradually become shale-dominated, small-scale bed forms are more abundant, and identification of members based on lithofacies is more difficult. After deposition of the Quwarah, present-day Saudi Arabia was affected by a Late Ordovician (late Ashgill) glaciation. The Qasim and older formations are deeply incised by glacial channels at the base of the Zarqa and Sarah formations. Basinward, the glacial unconformities (or their submarine erosion surface equivalents) become less significant and a thick succession of the Formation is preserved.

Tidal deposition systems of China's continental shelf, with special reference to the eastern Bohai Sea

The continental shelf of China has undergone a large-scale marine transgression. Tidal currents have become the main active agent and have had a profound influence on the deposition and geomorphology of the shelf. The continental shelf of China has five modern tidal deposition systems: (1) the eastern part of the Yellow Sea, (2) the eastern part of the Bohai Sea, (3) off the mouth of Yangtze River; (4) to the west of Taiwan, and (5) the Strait of Qiongzhou. When the tidal current speed is above 150 cm/s, erosion dominates and reciprocating currents form deep scour furrows. When the tidal current speed is 50 to 150 cm/s, deposition is dominant, and tidal currents form tidal shoals (i.e., tidal sand ridges and tidal sand sheets) whose margins are near an absolute ellipticity value for the M2 tidal component of 0.4. When the absolute M2 value is less than 0.4, a reciprocating current is strong enough to form tidal sand ridges. When the absolute value is above 0.4, a rotating current increases so as to form a tidal sand sheet. Tidal deposition material came mainly from tidal erosion and locally redeposition of Late Pleistocene sediments, and from the neighbouring sea bottom. The tidal deposition system in the eastern Bohai Sea is a typical example. It consists of the Laotieshan Channel scour furrow, the Liaodong Shoal sand ridges and the Bozhong Shoal sand sheet. The maximum tidal current speed in the Laotieshan Channel where strong erosion occurs, is up to 250 cm/s. In the Liaodong Shoal, the maximum tidal current speed is 64–115 cm/s and the absolute ellipticity value of the M2 tidal component is less than 0.4. In the Bozhong Shoal, the maximum tidal speed is 58–79 cm/s and the absolute ellipticity value of the M2 tidal component is above 0.4. The small-scale bedforms surveyed in detail in the area show that the sand ridges and sand sheet are still under the action of tidal current processes at the present time. Based on the interpretation of tidal currents, bedforms and sediment thickness, it is inferred that the general direction of sediment transport within the study area is NNW from the Laotieshan Channel to the Liaodong and Bozhong Shoals. The small-scale bedforms in the detailed survey area indicate a local net sediment transport from south to north.

The development of small scale bedforms in tidal environments: an empirical model for unsteady flow and its applications

ABSTRACTDimensions and plan morphology of current ripples are generally considered to vary with flow velocity and grain size. Recently, however, it has been shown that for sand of D50=0.095 and 0.238 mm the equilibrium dimensions are identical at all velocities within the stability field of ripples and that the plan form of equilibrium ripples is linguoid. On this basis, an empirical unsteady flow model has been developed and tested with flume experiments in order to predict ripple development in natural depositional environments. The model includes the development of washed‐out ripples and upper stage plane bed. The unsteady flow model explains the development and preservation of small scale bedforms in various tidal environments more accurately than previous models. Such bedforms can serve, therefore, as indicators of prevailing hydrodynamic conditions.

Dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves

Bedform dimensions, bed position changes, near-bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of England. The study was conducted in 0.5-2.25 m water depth under both swell and wind-generated storm waves with both weak and strong currents present. Bed positions and suspended sand concentrations were measured using the 3 transducers of a multi-frequency acoustic backscatter sensor with 5 mm vertical resolution. Two distinct bed types, based on wavelength (λ), were observed, each with two or more subtypes possible: (I) small-scale bedforms ( λ < 20 cm) which include two-dimensional pre-vortex (1), vortex (2), post-vortex (3) and three-dimensional vortex (4) forms; (II) large-scale bedforms ( λ > 20 cm) which include two-dimensional (5) forms and three-dimensional vortex (6) forms. Small-scale forms were dominant under non-breaking conditions while the large-scale forms occurred under both non-breaking and breaking waves; the large-scale forms dominate under breaking conditions. Both types, but particularly low steepness forms, were highly mobile with maximum horizontal migration rates of 5 cm min −1. Large (upto 15 cm) and rapid (upto 3.0 cm min −1) changes in vertical bed elevation were also observed in association with the development and migration of large-scale forms. Large-scale bedforms were also highly variable spatially, often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes. Observations suggest these forms are present even under high energy surf zone conditions (wave Shields ⩾ 1). Models for predicting ripple dimensions did not perform well in this environment. This lack of agreement reflects the complex hydrodynamic regimes associated with random (grouped) shoaling and breaking waves together with the presence of currents, often at large angles to the waves, as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment. Suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration. Estimates of transport rates are subject to potentially large errors (upto 30%) without compensation for bed elevation changes relative to sensor position.

Fractal properties of simulated bed profiles in coarse-grained channels

Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process.

Distribution and Control of Bed-Forms on the Continental Shelf, Gulf of the Farallones, California: ABSTRACT

A side-scan mosaic was constructed from an 800 km{sup 2} bed-form field located in the Gulf of the Farallones on the central shelf between Point Reyes and the Golden Gate. Sediment samples were collected at 1-km grid intervals within a small area (130 km{sup 2}) of the field. Large-scale bed-formed, broad, shallow ({<=} 2 m relief) depressions, floored by medium and coarse sand, are arranged regionally in a digitate pattern. Where fully developed the palms of these features are as wide as 2 km with northerly striking fingerlike projections as long as 8 km that narrow to 150 m. Small-scale bed forms, ripples that have a wavelength of about 1 m and that strike north-northeast, cover the digitate bedforms. No ripples were resolved with the 100 and 120 kHz side-scan systems on the intervening areas of fine and very fine sand. The digitate bed forms are most distinct in the southwestern part of the field where there is an abrupt transition between the coarse sediment of the depressions and the nonrippled areas of finer grained material. Digitate bed forms gradually disappear toward the southeast where they are partly covered with a veneer of finer sediment and the boundaries between depressionsmore » and intervening areas become less distinct. Bathymetry exercises major control on the characteristics of the bed-form field by influencing water circulation. The sharp arcuate northwestern boundary of the field mimics the arcuate 80-m isobath, which is perpendicular to the shelf break Water depths gradually shoal to 50 m toward the southeast and the isobaths translate into shelf-parallel bands. The area of translation coincides with the diffusion of the field boundaries and gradual disappearance of the digitate bed forms. The bed forms are erosional features that reflect the intensity of water flow over the area.« less

FORMATION OF ALTERNATE LONGITUDINAL SORTING AS AN INSTABILITY OF FLUVIAL BED-SURFACE COMPOSITION

In a stream composed sand and gravel, wavy patterns of sorting are often observed and they cause fluctuation of fractional bed-load discharge. In this paper, the formation mechanism of longitudinally alternate sorting is formulated as a linear instability analysis of bed-surface composition with a non-equilibrium bed-load transport model for each grain size of sediment mixture, and the condition that the longitudinal sorting falls unstable and predominant wave length of alternate sorting are predicted. As a result, alternate longitudinal sorting develops under the condition slightly above the threshold of motion of bed-material particles, and it always propagates downstream. The predominant wave length is subjected to the diameter of bed material, and thus its scale is the same order as small-scale bed forms. The theoretical results are consistent with the observed data.