Threshold Of Sediment Movement Research Articles

Effect of velocity hiatuses in oscillatory flow on migration and geometry of ripples: wave-flume experiments

A series of wave-flume experiments was conducted to closely look at characteristics of geometry and migration of wave-generated ripples, with particular reference to the effect of velocity ‘hiatuses’ during which the near-bed flow velocity becomes much smaller than the threshold of sediment movement. Three types of wave patterns were generated: two types for simulating waves with intervening velocity hiatuses; and regular waves for comparison purposes. In the former two types, two different wavelengths of water waves were generated alternately in the course of a wave test: the wave with a longer wavelength was set large enough to mobilize the bottom sediment, whereas the wave with a shorter wavelength was set too small to mobilize the sediment. The former two types were designed to be different in sequence of convexity and concavity of wave patterns. The sequence with the convex–concave longer wave and successive convex–concave shorter wave was described as a ‘zero-up-crossing’ wave pattern, and the inverse sequence was described as a ‘zero-down-crossing’ wave pattern. The ripples developed under oscillatory flow with intervening hiatuses manifested the following characteristics in geometry and migration. (i) The morphological characteristics of ripples, namely wavelength, height and the ripple steepness, are unaffected by the intervening hiatuses of velocity. (ii) The directions of ripple migration under the zero-up-crossing and zero-down-crossing wave patterns corresponded well with the directions of the flow immediately before onset of the hiatuses. (iii) The observation of sand particle movement on the ripple surface indicated that, under the zero-up-crossing waves, the velocity hiatus prevents the entrained sediment cloud from being thrown onshore, and thus the sediment grains thrown onshore are fewer than those thrown offshore. As a result of the sediment movement over one wave-cycle, the net sediment transport is directed offshore under the zero-up-crossing wave pattern. (iv) The velocity of ripple migration was highly correlated with acceleration skewness. Under most of the zero-up-crossing (zero-down-crossing) wave patterns, flow acceleration skewed negative (positive) and ripples migrated offshore (onshore).

Sand ridges off Sarasota, Florida: A complex facies boundary on a low-energy inner shelf environment

The innermost shelf off Sarasota, Florida was mapped using sidescan-sonar imagery, seismic-reflection profiles, surface sediment samples, and short cores to define the transition between an onshore siliciclastic sand province and an offshore carbonate province and to identify the processes controlling the distribution of these distinctive facies. The transition between these facies is abrupt and closely tied to the morphology of the inner shelf. A series of low-relief nearly shore-normal ridges characterize the inner shelf. Stratigraphically, the ridges are separated from the underlying Pleistocene and Tertiary carbonate strata by the Holocene ravinement surface. While surficial sediment is fine to very-fine siliciclastic sand on the southeastern sides of the ridges and shell hash covers their northwestern sides, the cores of these Holocene deposits are a mixture of both of these facies. Along the southeastern edges of the ridges the facies boundary coincides with the discontinuity that separates the ridge deposits from the underlying strata. The transition from siliciclastic to carbonate sediment on the northwestern sides of the ridges is equally abrupt, but it falls along the crests of the ridges rather than at their edges. Here the facies transition lies within the Holocene deposit, and appears to be the result of sediment reworking by modern processes. This facies distribution primarily appears to result from south-flowing currents generated during winter storms that winnow the fine siliciclastic sediment from the troughs and steeper northwestern sides of the ridges. A coarse shell lag is left armoring the steeper northwestern sides of the ridges, and the fine sediment is deposited on the gentler southeastern sides of the ridges. This pronounced partitioning of the surficial sediment appears to be the result of the siliciclastic sand being winnowed and transported by these currents while the carbonate shell hash falls below the threshold of sediment movement and is left as a lag. The resulting facies boundaries on this low-energy, sediment-starved inner continental shelf are of two origins which both are tied to the remarkably subtle ridge morphology. Along the southeastern sides of the ridges the facies boundary coincides with a stratigraphic discontinuity that separates Holocene from the older deposits while the transition along the northwestern sides of the ridges is within the Holocene deposit and is the result of sediment redistribution by modern processes.

Spatial Pattern of Flow Competence and Bed Load Transport in a Divided Gravel Bed River

A 14‐month continuous record of the spatial variation in the incidence and magnitude of bed load transport was provided by nine pit traps sunk into the bed of a 21‐m‐wide upland gravel bed river. The bed load data are related to a simultaneous record of average bed shear stress derived from the depth‐slope product. The study reach is straight and includes a small, low‐relief mid‐channel bar which represents the divide between two zones of markedly different bed structure and stability. he bed of the left channel and bar tail is loose with an open framework, while that of the bar head and right channel is tightly interlocking and well imbricated. Bed surface grain size is similar throughout the reach. The two zones of contrasting bed structure have a strong influence on the threshold of sediment movement (τc) and consequently the spatial pattern of bed load transport. Entrainment thresholds for the tightly structured bed are up to seven times higher than those for the loosely structured bed. Sediment transport is therefore most frequent in the left channel and over the bar tail, where flows with excess shear stresses (τ > τc) peaking at 8τc constitute 19% of the flow record. This is in marked contrast to the right channel and bar head, where excess shear stresses of maximum 1.4τc total only 1.3% of the flow record. However, despite the marked difference in the magnitude and duration of competent flows between the two contrasting bed areas, little difference in bed load yield is observed between the nine traps when averaged over the 14‐month study period. This study shows that different zones of bed structure and stability in a channel will control the incidence and spatial pattern of bed load transport but not necessarily the medium and long‐term bed load yield. This has important implications for local and short‐term bed load sampling programs and the numerical modeling of sediment budgets and zone‐to‐zone bed load transfer.

Unified modeling guidance based on a sedimentation parameter for beach changes

Review of four available proposals for designing coastal movable-bed models demonstrates that each suggested model-scaling law contradicts all the others. Accurate laboratory models providing the separate empirical bases, however, all appear fairly consistent with the new design guidance developed here. This formal development considers sediment excursions under wave action by means of a parameter including bed slope and the threshold of sediment movement. The consequent relationship for sedimentation similitude between model and prototype indicates that appropriate model sediment and horizontal scale depend on the selected vertical scale along with wave period and prototype sand characteristics. Explicit forms derived from the general design guidance recover two expressions resembling those previously reported by E.K. Noda (1972) and by P. Vellinga (1982); the route towards Noda's result indicates that guidance is not appropriate for small models of large situations, but the present development yields an independent basis and a needed refinement for Vellinga's empirical results. Considerations of sedimentation similitude are also shown to be supported by successful new laboratory models using sand profiles developed in a large wave tank as prototypes. Spatial distortion seems to be the advisable strategy for small models but proper distortion is rather modest with prototype materials, according to present results.

Some guidelines in reconstructing ancient sea conditions from wave ripplemarks

The collection of substantial field data on wave ripple mark spacing (wavelength), steepness, grain size and symmetry index from the Miocene Burdigalian sandstones of western Switzerland has allowed apparently close reconstruction of sea conditions. Results indicate, however, that ancient wave reconstructions must be restricted at present to data from relatively steep ripple marks (VFI less than 7.5, i.e. vortex ripples). Data from the flatter ripple marks (post-vortex types) formed at high oscillatory shear stresses, do not provide “reasonable” estimates of wave conditions. Penecontemporaneous overwashing and/or post-depositional compaction, ripple formation near the destruction condition rather than the threshold of sediment movement, orbital diameters far exceeding ripple spacing and superimposed unidirectional currents are all factors which seriously affect wave reconstructions. Evaluation of ancient wave conditions provides insight into geologically important phenomena such as local water depth and basin size.

Threshold of sediment movement by open ocean waves: observations

Between 8 and 25 March, 1973, a sequence of bottom photographs, sediment samples, bottom current measurements, and bottom pressure measurements was recorded continuously at several locations on the Washington continental shelf. Concurrently, wave conditions were recorded at 35-m depth on the summit of Cobb Seamount 465 km west of the shelf experiment. During the sampling period the bottom turbidity at 167 m on the shelf varied significantly in response to the magnitude of wave activity, and these data are used to evaluate the threshold wave conditions necessary to iniatiate the observed sediment movement. The bottom oscillatory velocity, orbital diameter of wave motion, and the mean sediment size occurring at the shelf location are compared. For unconsolidated silt-sized sediment the expression ρU 2 1 10 (ρ 8 − ρ)gD = 0.13 d 0 D 1 2 where ϱ 8 and ϱ are the sediment and fluid densities, d 0 is the horizontal excursion of water particles at the sea floor, D is the grain diameter, and U 1 10 the higher 10% of bottom oscillatory velocities, describes the threshold of grain motion as observed.

The threshold of sediment movement under oscillatory waves; discussion

The threshold of sediment movement under oscillatory waves; discussion

The Threshold of Sediment Movement Under Oscillatory Water Waves

ABSTRACT As the velocity of the to-and-fro water motion near the bottom under oscillatory waves is increased, there comes a stage when the water exerts a stress on the particles sufficient to cause them to move. This study reviews the analyses and available data on this threshold of sediment motion under wave action. For grain diameters less than about 0.05 cm (medium sands and finer) the threshold is reached while the flow in the boundary layer is still laminar and the threshold is best related by the equation um2/(s-) gD = 0.30 (do/D), where um and do are the near-bottom velocity and orbital diameter of the wave motion, is the density of water, and s and D are respectively the density and diameter of the sediment grains. This relationship is modified after an empirical equation deduced by Bagnold but has a theoretical basis. For grain diameters greater than 0.05 cm (coarse sands and coarser) the threshold occurs after the boundary layer has become turbulent and is best predicted with an empirical curve relating do/D to um/(s - ) g T where T is the wave period. This latter dimensionless number represents the ratio of the acceleration forces to the effective gravity force acting on the grains.