Sand Ribbons Research Articles

Flow and sedimentary processes in the meandering river South Esk, Glen Clova, Scotland

AbstractChannel geometry, flow and sedimentation in a meander bend of the River South Esk were studied from bankfull stages (January–February) to low water stages (May) in 1974. Bed topography varied little over the study period, showing a typical pool and ripple geometry. Variation of mean depth and velocity with discharge differed from section to section around the bend, due primarily to locally varying flow resistance with stage.The flow pattern for all stages was dominated by a single spiral over the point bar, with a development zone at the bend entrance. Deviation of bed shear stress from the mean flow direction was in general accord with theory, especially for high stages. The use of a uniform longitudinal water surface slope in the calculation of bed shear stress is not justified because of a complicated water surface topography, also such calculated shear may not represent effective bed shear on grains, as it accounts also for energy losses associated with secondary flows.Dunes covered much of the bar at high stages, with increasing proportions of ripples, sand ribbons and lower phase plane beds at low stages. Local flow resistance generally decreases from dunes, diminished and ripple‐backed dunes, ripples, sand ribbons to plane beds, and bed forms are predicted quite well by the stream power‐grain size scheme.Mean size, sorting and skewness of sediment over the bed changes little with stage. In general, size decreases, sorting improves and skewness changes from positive to negative from the talweg to the inner bank, and in the downstream direction. Allen's (1970a, b) force balance equation for moving bed load particles is supported for bankfull stage, with some reservations, and textural characteristics are explained by progressive sorting in the direction of sediment transport.Large‐scale trough cross stratification (with some flat bedding) formed at high stage by dunes (and lower phase plane beds) dominates the point bar sediments. Alternations of fine‐medium sand (often cross‐laminated) and vegetation‐rich layers result from periodic deposition on the grassed upper bar surface.Fining upwards sequences produced by lateral channel migration are modified by a coarsening upward subsequence in the upstream bar region where spiral flow is developing from the bend upstream.

Observations sur les rides sous-marines du plateau continental

I. - Introduction The growing economic interest shown in the shallower regions of the continental shelf explains the present need to know more about the contribution processes of the various sedimentary deposits. II. - Morphological forms of sedimentary accumulation The regions studied (Figure 1) permitted investigation of the various possible forms of accumulation, from the large sand bodies (Straits of Dover ; La Chapelle bank) to the sand ribbons present at the south-western end of the Channel. The large sand bodies of the Bassurelle running parallel to the principal tidal currents are flanked by large sand dunes whose crests are approximately parallel to the current. The ripple-marks seen on the surface of the dunes are due to present-day movement, as proved by underwater photographs. However, a colony of Lanice Conchilega of uniform age and short life cycle (2-2½ years) points to a period of stability following the probable occurence of a meteorological disturbance. In the south-western region of the Channel, measurement of tidal currents shows that currents are predominantly south-westerly, except to the south of the Bank ar Forest where a south-easterly direction has been observed. Two sectors (I 15 and I 20) illustrate the transition between the Paleozoic substratum and the Lutetian sedimentary platform covered by pebble deposits and sandbanks, e.g. Trezen Vraz and Bank ar Forest (Figure 6). Figure 9 gives an example of a Barkhan-type crescent dune found in the southern pan of the western Channel, to the north of the sand ribbon zone. The dunes on La Chapelle bank at a depth of 165 m (Figures 10 and 11) appear to be due to movement towards the south-west. The large sand bodies themselves are probably now quite stable. Conclusion Measurement of tidal currents and theoretical studies on the effect of waves in a number of sectors have provided an estimate (Table 1) of maximum probable sediment transport values (Bonneville, Sternberg). These are quite high, and range from 27.7 to 0.65 kg/m/s. It is clear that small differences in maximum current velocity give rise to large differences in the mass transport value.

The pattern of large-scale bed forms in the Langeland Belt (Baltic Sea)

The Langeland Belt forms the southern portion of the main water-exchange channel (Great Belt) which connects the brackish Baltic Sea with the fully marine North Sea. Strong, pulsating aperiodic currents generated in this channel are governed entirely by sea-level and density differences between the adjacent basins; they are thus strictly meteorologic currents. Tidal currents are of some importance in superimposing their amplitudes on undirectional currents. Side-scan sonar surveying revealed the direct influence of these currents in molding bed forms. The rich inventory of large-scale bed forms detected in these surveys consists mainly of the following: 1. (1) Sand ribbons: thin streaks of sand migrating over a lag-sediment bottom; width between 1 and 60 m, length generally more than 100 m. 2. (2) Comet marks: erosional areas down-current from obstacles; lengths up to or exceeding 100 m. 3. (3) Sand shadows: tails of sand deposited behind larger obstacles such as isolated megaripples on a lag-sediment bottom. 4. (4) Megaripples with wavelengths between 10 and 100 m which occur in several large fields; smaller megaripples occur on sand ribbons. The bed forms (1)–(3) form new or revised contributions to the family of large-scale longitudinal bed forms already established by other authors using side-scan sonar. The regional distribution of these bed forms is related to sand availability, current strength and water depth. Direction and length of comet marks proved to be the most useful indicators of current direction and of relative current strength. Based on bed-form morphology, the maximum current velocity lies in the center of the channel and is responsible for the largest comet marks, sand ribbons and sand shadows, but not for the megaripples. The distribution of current directions as interpreted from bed-form morphology and orientation, allows a detailed reconstruction of the current paths and shows the domination of inflow conditions. The bed forms mapped represent rather extreme current conditions. This is also indicated by the fact that traverses rerun after 14 months showed essentially no change in the large-scale bed-form pattern.

Bed forms of the Mediterranean undercurrent observed with side-scan sonar

Details of bed forms beneath the Mediterranean undercurrent in the Gulf of Cadiz are depicted on sonographs (side-scan sonar views to maximum slant ranges of 1 km, 7 km and 14 km). A sequence of bed-form zones, largely comparable with that of strongly tidal continental-shelf seas, is found to depths of about 1400 m. Thus, with decreasing current velocity, measured near the bottom, there are zones of scour hollows, sand ribbons and sand waves, with related deposits. Beyond the sand-wave zone are smooth mud deposits where the undercurrent is in contact with the floor, and large, muddy sediment waves up to 40 m high where it is intermittently in contact with the bottom. These large, muddysediment waves are a bed form not found in the tidal-current environment, and are possibly formed by a combination of fall-out from suspension together with occasional bed movement as the base of the undercurrent fluctuates in depth. Variations in the bedding style in the deposits beneath the mud waves are figured on sub-bottom profiles. A stratigraphic boundary between an acoustically transparent layer, possibly representing pelagic sedimentation, and the overlying bedded sequence may define the age of commencement of the undercurrent. Flow paths, including several downslope trending channels, are influenced by gravity, Coriolis force and topography.

Equilibrium Cross-section of Meandering and Braided Rivers

THERE are two simple models which describe the equilibrium state of a channel cross-section with constant conditions, one static, the other dynamic. In the first case equilibrium is said to hold when all grains on the bed are just at the threshold of movement1. Because movement is produced by a combination of fluid drag and the downslope gravity component and because the fluid drag is greatest in the centre of the channel, the channel slopes can increase up the sides to compensate for the decrease in drag. The resulting profile is roughly a parabola1 and is very similar to many natural channel forms. This static model is particularly relevant to the design of some canals where discharges are fairly constant and the bed material is normally immobile. Natural rivers transport sediment, however, and the mean channel form develops at high stages when nearly all the bed material is in intermittent motion. When sediment is transported as bed load it must be continually tending to creep downslope into the centre of the channel under the influence of gravity. If, however, the channel maintains a constant profile there must be an equal and opposite tendency at all points for the sediment to move sideways upslope. Apart from buoyant upthrust, the only other force acting on the grains is the fluid drag acting in the direction of the water flow, leading to the second model. When river channels are in dynamic equilibrium the flow along the bed has a sideways component up the channel banks which exactly compensates the tendency of the sediment to move downslope under the influence of gravity. As water flowing sideways along the bed must return to the channel centre along the surface this implies that the river has a double spiral secondary flow (Fig. 1). Although double spiral flow in straight channels has been demonstrated2,3, single spiral flow in meander bends is better known1. As, however, the sense of the spiral changes from one bend to the next, it follows from considerations of continuity and symmetry that in the straight reaches between bends there is a transitional double spiral structure (Fig. 1 C). In short the secondary flow in river channels is a double spiral which is asymmetrically developed in bends so that one vortex momentarily excludes the other at the crest of the meander. The two spirals adjoin where the flow attaches itself to the bed and diverges sideways; this line is also the thalweg or deepest part of the channel. It is no coincidence that the same hypothesis of double spiral flow is commonly used to explain the origin of many sand ridge bed configurations formed parallel to the flow, such as primary current lineation, sand ribbons, tidal current ridges and longitudinal aeolian dunes and draas4,5. River channels are analogous to the troughs between longitudinal sand ridges in certain other respects too: they arise spontaneously during sediment transport, reach a dynamic equilibrium by interaction with the flow and are associated with transverse structures, forming meander bars, that migrate downstream. In short, there is a good case for saying that the entire form of a river channel is a bed configuration sensu stricto.

The sediment on the floor of the southern Irish Sea

The southern Irish Sea has been surveyed using Asdic, continuous reflexion profiling, echo sounding and bottom samples and the results presented in the form of a sediment facies chart. In a section on the bathymetry some of the major sea-bed features have been interpreted using seismic data. Bed forms, principally sand waves, sand ribbons and sand streams, have been identified and their hierarchical associations established; these have provided insight into the patterns of sediment circulation. The form of these patterns is directly in the control of the tidal regime, which, in the southern Irish Sea, is the reverse of that normally found around British coasts. Along the Irish coast the tidal form consists of high velocities associated with negligible range: on the Welsh side low velocities and a pronounced range are the usual association. The effects of these tidal peculiarities on the sediment have been to create a bedload separation on the Irish shelf, and a coalescence on the Welsh side. Sediment studies indicate a wide compositional range for the potentially mobile material across the area. The sub-littoral sands are essentially quartzose with a strong rock debris component, carbonate contents being generally low (5%). Offshore and in mid-channel, there is a variable but significant and frequently dominant carbonate component which is bimodal as a result of whole shell imposition. Macrofauna communities, broadly defined in association with bottom characteristics, illustrate relationships between source material and detrital organogenic sediment.

Sand ribbons of European tidal seas

Ribbons of sediment, up to 15 km long and up to 200 m wide, extend parallel to the bed load transport paths of the tidal currents. The ribbons have been found by the side-scan Asdic method in most of those areas of the sea floor around the British Isles where tidal currents have maximum near surface velocities of 2 knots or more. The four types of ribbons, distinguished by their form, are related to tidal current velocity and supply of sand. Underwater photographs taken in a sand ribbon zone reveal bodies of shelly sand usually too thin to mask completely the underlying coarse material, but sometimes thick enough for the formation of small-scale bedforms including ripple marks and sand shadows. There is a variety of underlying coarse material and locally there are accumulations between the sand ribbons of the more durable shells that may be more readily preserved in ribbon form than the sand itself.

THE TIDE‐SWEPT CONTINENTAL SHELF SEDIMENTS BETWEEN THE SHETLAND ISLES AND FRANCE

SUMMARYExtensive data obtained by side‐scan Asdic and echo‐sounder equipments, viewed in conjunction with bottom notations on navigational charts and some seabed samples, show the widespread occurrence of sand ribbons, sand waves and sand patches on the continental shelf west of the British Isles and France. The close similarity of their directional trends with those of the strongest tidal currents strongly suggests a causal relationship which applies to much of the continental shelf, parts of the upper continental slope and also to off‐lying shoals such as Rockall Bank. The sand transport paths broadly parallel the coasts in the epicontinental seas and on the open shelf west of the British Isles. In contrast, the paths are substantially normal to much of the coast of western France.

THE NATURE AND ORIGIN OF BED‐FORM HIERARCHIES

SUMMARYThe bed forms generated by liquid flows over loose‐grain boundaries assume only two orientations relative to flow, being either transverse or longitudinal, but range enormously in physical scale. Two types of bed form, current ripples and parting lineations, take a characteristic length (2) which is independent of boundary conditions as expressed by flow depth (d). We have that:λL≤ 0.025 m (parting lineations)λR≤ 0.60 m (ripples).The characteristic length of other kinds of bed form is correlated with flow depth, according to the equations:λD= 1.16D1.1.55 (dunes)λA 6.3d (antidunes, Fr = 1.0)λSR= 1.35d1.31 (sand ribbons)λM ‐ 672d1.11 (meander bars)λT= 116d2 (tidal current ridges)From these relationships can be deduced the compositions of theoretical hierarchies of bed forms, where a hierarchy is defined as a series of bed forms graded as to size which in combination are adapted to a single overall flow condition. The compositions of these hierarchies, which are closely matched by naturally occurring hierarchies, are broadly related to available flow depth, general flow conditions (lower or upper regime), and availability of sediment.Bed forms are considered to be due to unstable interactions between the bed material and the over‐bed flow, of liquid or grains or both.Hierarchies of bed forms arise because the quantities determining the flow are suficiently numerous that several mutually unstable combinations can exist, each combination being expressed in terms of a bed form of a characteristic physical scale and orientation relative to flow.

Tidal current fashioning of a basal bed

The floor of the northeastern part of the Celtic Sea with its bed transport paths converging from the Bristol and St. George's Channels and tidal currents of up to 3 knots, has been examined by means of Asdic and echo-sounder, grab and corer. A rudaceous deposit, marking the post-glacial rise of sea level, is being buried by the basal bed of the present sea. The new bed shows a progressive decrease in grain size down the velocity gradient of the tidal currents. It can be divided into five successive zones, whose character also depends on the nature of the floor, its hydraulic properties and the supply of material. The distinctive features of these zones are: ( 1) predominance of erosion; ( 2) sand ribbons; ( 3) sand waves; ( 4) continuous beds of sand and muddy sand; ( 5) sand patches with intervening shell-gravel.