Sea Bed Morphology Research Articles

Tidal asymmetry in sediment resuspension on a macrotidal beach in northwestern Australia

A field experiment was conducted on a macrotidal beach in northwestern Australia to investigate the relation between sediment resuspension and sea bed morphology. Continuous measurements of waves, currents and suspended sediment concentrations were carried out over a period of several hours around high tide, concurrent with half-hourly visual observations of the sea bed morphology. The incident waves had a significant wave height of 0.35 m and period 11 s. Most of the data were collected outside the surf zone where sediment resuspension was mainly by wave groups, and suspended sediment transport was primarily accomplished by mean longshore and cross-shore currents. The sediment resuspension process was further characterised by a pronounced tidal asymmetry with suspended sediment concentrations and transport rates during the falling tide significantly larger than during the rising tide. The asymmetry is attributed to the combined effects of a larger bed roughness and stronger mean nearshore flows during the falling tide. The increase in bed roughness occurred during the rising tide under the influence of decreasing bed shear stresses when post-vortex ripple morphology developed on a previously plane bed. Around high tide, the ripples were fully developed and had attained a ripple height of approximately 0.005 m and a ripple length of 0.06–0.08 m. Application of the Van Rijn [Van Rijn, L.C., 1989. Handbook Sediment Transport by Currents and Waves. Delft Hydraulics, Report H461] sediment transport model further confirmed that enhanced sediment resuspension during the falling tide was primarily the result of the development of ripple morphology, with increased mean current strengths playing a secondary role. The results have major implications for modelling suspended sediment transport on macrotidal beaches.

The effect of sea breeze on beach morphology, surf zone hydrodynamics and sediment resuspension

Beach morphodynamic processes were investigated over part of a sea breeze cycle on a microtidal, low wave energy sandy beach in southwestern Australia. Prior to the onset of the sea breeze, offshore winds with speeds less than 5 m/s prevailed. During the sea breeze, alongshore winds with speeds higher than 10 m/s were experienced. The sea breeze induced pronounced changes to the nearshore morphodynamics which were similar to that of a storm event: (1) root mean square wave height increased from 0.3 to 0.5 m; (2) zero-upcrossing wave period decreased from 8 to 4 s; (3) mean cross-shore flows reached velocities of 0.2 m/s directed offshore; and (4) the longshore current increased in strength from 0.05 to 1.0 m/s. Before the sea breeze, sediment resuspension typically occurred during isolated high-wave events associated with the passage of wave groups. Flux coupling between the wave-oscillatory currents and the suspended sediment (Jaffe, B.E., Sternberg, R.W., Sallenger, A.H., 1984. The role of suspended sediment in shore-normal beach profile changes. Proc. 19th Int. Conf. Coastal Engineering, ASCE, pp. 1983–1996) induced net onshore suspended sediment transport, resulting in beachface accretion and a steepening of the foreshore profile. In contrast, during the sea breeze, sediment resuspension was almost continuous. The suspended sediment load increased six-fold and, as a consequence, the longshore suspended sediment transport rate increased by a factor of 100. During the sea breeze, cross-shore transport was directed offshore and primarily associated with mean offshore flows. This resulted in erosion of the beachface and deposition in the surf zone and, consequently, a flattening of the beach profile. The suspended sediment load was strongly related to the Shields parameter defined for the combined action of waves and currents. Investigation of the suspended sediment profiles indicated that the degree of vertical mixing, parameterised by the sediment diffusion coefficient, increased with distance from the bed. In addition, the sediment diffusion coefficient exhibited a large temporal variation, ranging between 0.005 and 0.018 m 2/s. The variability in the sediment diffusion coefficient is possibly due to changes in the sea bed morphology.

High-resolution seismic profiling in the Sea of Marmara (northwest Turkey): Late Quaternary sedimentation and sea-level changes

High-resolution sparker seismic profiling was carried out in the Sea of Marmara (northwest Turkey) to study the late Quaternary tectonics and sedimentation of this important active basin at the western end of the North Anatolian fault zone. A sedimentary unit interpreted as the clinoforms of a prograding delta was observed in places along the outer part of the southern shelf. The topset-foreset transitions of the latest clinoforms cluster around 100 m below present sea level; they were probably deposited during the last glacial maximum and early phase of deglaciation, between 25 and 13 k.y. B.P., when the level of the Aegean was below that of the Dardanelles sill (60–70 m below sea level at present). Therefore, the Sea of Marmara was isolated from the Aegean during the last glacial maximum, but it is possible that a fluvial overflow existed across the Dardanelles during this time, stabilizing the level of the “Marmara Lake” and allowing the delta progradation. Rivers that drain into the Sea of Marmara from the south have a much greater combined drainage basin area and sediment flux than those draining the region to the north. This may explain the much broader shelf to the south: half graben bounded by north-dipping normal faults observed within the southern shelf would tend to trap sediment sourced from the south and subdue any sea-bed morphology on the shelf.

Application of marine seismic surveying methods to engineering geological studies in the near-shore environment

Abstract This paper discusses the application of various marine geophysical techniques to engineering studies in coastal and estuarine environments. Sea bed morphology can be examined using echo sounding and sidescan sonar whilst geological structure is investigated using continuous seismic profiling surveys based on sparker and boomer sources. In both cases samples of sea floor sediments and rocks are collected and boreholes drilled to calibrate the seismostratigraphy in terms of geological structure. Current geophysical equipment and methods are reviewed and are illustrated by three case histories. The first describes a geophysical survey undertaken in the Wash to provide information on sediment movement as part of a feasibility study for bunded reservoir construction. The second discusses an appraisal of the engineering characteristics of the near-shore environment carried out in connection with coastal engineering studies in Lyme Bay, Dorset. Finally, a description is given of a survey which formed part of a study to investigate the distribution of sand and gravel aggregate resources off the East Anglian coast. Whilst there have been continuous improvements in seismic profiling equipment and survey techniques, most of the underlying principles and interpretation techniques have remained the same. By combining the results from marine seismic surveys and geological sampling programmes it is possible to achieve an accurate assessment of the sub-bottom geology in the near-shore environment.

Evidence for slope instability and current-induced sediment transport, the RMSTitanic wreck search area, Newfoundland rise

The first map of the sea bed morphology and sedimentary features within the RMSTitanic search area is proposed from the interpretation of SAR side-scan sonar images. Downslope sedimentary features such as erosional furrows and crown scarps constitute a 7 km wide instability corridor. A large field (15 km2) of asymmetrical sediment waves indicating a downslope transport is identified. Current-induced features corresponding to associated sand ribbons and barchan dunes resulting from the Western Boundary Undercurrent action are mapped. The morphology of theTitanic Canyon is also precised from the SAR images. Finally, the origin of the sea bed features is discussed in an attempt to link each bed form to a sedimentary process.

Iceberg scouring and sea bed morphology on the eastern Weddell Sea shelf, Antarctica

A detailed shallow seismic and side scan sonar study, combined with vibrocoring on the continental shelf off Riiser Larsenisen ice shelf, East Antarctica, has provided information on the different morphological patterns present in the area. Features related to the motion of grounded icebergs prevail, with relatively narrow iceberg plough marks being predominant. A‘washboard pattern’probably results from a wobbling motion of grounded, tabular icebergs under influence of direct push by the advancing ice shelf, and a hummocky disturbed sea bed morphology results from small scale sediment slumping, often triggered by iceberg action. Narrow, elongate depressions incise the sea floor sediments, which are composed of glacigenic diamictons. Formation of the depressions is not fully understood, but erosion by subglacial meltwater under an expanded East Antarctic ice sheet is a possibility, although this requires a different glacier thermal regime than that of the present-day. Although this study is restricted in area, the processes of the region are typical of the Antarctic continental shelf. The results may hence have a more regional significance.

Iceberg scouring and sea bed morphology on the eastern Weddell Sea shelf, Antarctica

A detailed shallow seismic and side scan sonar study, combined with vibrocoring on the continental shelf off Riiser Larsenisen ice shelf, East Antarctica, has provided information on the different morphological patterns present in the area. Features related to the motion of grounded icebergs prevail, with relatively narrow iceberg plough marks being predominant. A‘washboard pattern’probably results from a wobbling motion of grounded, tabular icebergs under influence of direct push by the advancing ice shelf, and a hummocky disturbed sea bed morphology results from small scale sediment slumping, often triggered by iceberg action. Narrow, elongate depressions incise the sea floor sediments, which are composed of glacigenic diamictons. Formation of the depressions is not fully understood, but erosion by subglacial meltwater under an expanded East Antarctic ice sheet is a possibility, although this requires a different glacier thermal regime than that of the present-day. Although this study is restricted in area, the processes of the region are typical of the Antarctic continental shelf. The results may hence have a more regional significance.

Icebergs rework shelf sediments to 500 m off Antarctica

Icebergs and sea ice rework the sediments of high-latitude shelves, producing modern diamicts (ice-keel turbates) unrelated to glacial proximity. Off Antarctica, sidescan sonar data indicate the presence of ice-gouge features formed by the physical interaction between ice keels and the sea bed. These are recognized as incisions a few metres deep and tens of metres wide, in water depths up to 500 m. On the submarine bank tops and slopes off Wilkes Land and in the Weddell Sea, subcircular depressions 30 to 150 m in diameter, a washboard pattern, and hummocky bed features also represent iceberg-resting sites. The freshness of sea-bed morphology, nearby Holocene sediment ponding, and active hydraulic sedimentary processes indicate that the sea floor is being reworked by iceberg keels. Tabular iceberg drafts in excess of 330 m have been measured, and modeling studies suggest that nontabular iceberg drafts of 500 m are possible. We conclude that a modern ice-keel turbate deposit in the form of a poorly stratified diamicton is probably widespread on that part (54%) of the Antarctic shelf less than 500 m deep.

SEDIMENT TRANSPORT ON THE SOUTH-EAST AUSTRALIAN CONTINENTAL SHELF

Engineering projects on the continental shelf off Sydney, Australia, have stimulated investigation into the sediment transport system of the shelf. Investigation activities associated with these projects have included: definition of sea bed morphology, sediment distribution and bedform characteristics; monitoring of steady and wave induced currents; wind data collection; suspended sediment sampling; bottom camera sediment movement investigations and analytical studies of sediment reaction to sea bed forcing functions. Sea bed velocity exceedence relationships for both wave oscillations and steady currents have been determined at depths of 24 m, 60 m and 80 m. Thresholds of sediment movement have been defined. Relative sediment transport computations have been undertaken and studies of suspended sediment concentration profiles are in progress so that absolute transport rates can be determined. The prevailing conditions, which include a mainly south bound current, are seldom sufficient to induce entrainment of shelf sediments. Transport events mainly result from major storms in the Tasman Sea which produce both high energy waves and north bound currents. Although these events are rare and short lived, the combined wave and current shear produced at the sea bed during the events gives rise to entrainment conditions which result in their dominance of the shelf sediment transport system.

The sea floor morphology seaward of Terre Adélie (Antarctica)

All available sounding lines were used to construct general (Fig. 2) and local (Figs. 3, 4 and 5) bathymetric charts of the Antarctic sea floor between 136° E and 147° E (Terre Adelie, Southern Indian Ocean). Major coastal and submarine features of the area are delineated and described (Figs. 6 and 7). The sea bed morphology is related clearly to the tectonic and climatic influences. Complex faulting has ruptured the Gondwana crust into a series of deep basins separated by irregular plateaus. The Pleistocene maximal extension of inland ice produced the following morphological effects: (1) the overdeepening of valleys, (2) the construction of outer banks (terminal moraines), and (3) the unusual depth of the shelf edge by incomplete post-glacial isostatic readjustment.

Plough Marks in the Weddell Sea

Abstract The 1976/77 Norwegian Antarctic Research Expedition carried out studies of the sea bed by side-scan sonar. The equipment was operated from the expedition vessel down to about 350 m depth by personnel from the Continental Shelf Institute, Trondheim. Various types of plough marks mostly ranging from 10 to 100 m in width were observed. These included several generations of crossing plough marks as well as plough marks with abrupt changes in trend reflecting changing iceberg motion. The investigations will be expanded during the 1978/79 expedition to include towing at greater depths, and mapping of sea-bed morphology by mosaic towing patterns.

Plough Marks in the Weddell Sea

AbstractThe 1976/77 Norwegian Antarctic Research Expedition carried out studies of the sea bed by side-scan sonar. The equipment was operated from the expedition vessel down to about 350 m depth by personnel from the Continental Shelf Institute, Trondheim. Various types of plough marks mostly ranging from 10 to 100 m in width were observed. These included several generations of crossing plough marks as well as plough marks with abrupt changes in trend reflecting changing iceberg motion. The investigations will be expanded during the 1978/79 expedition to include towing at greater depths, and mapping of sea-bed morphology by mosaic towing patterns.