Sand waves: A model of origin and internal structure

The ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware-Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross-sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression.



A second aspect is the change in bottom sediment characteristics that accompanies these large-scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high-frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea-floor.



A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large-scale bedforms.



Finally, flow was measured and transport calculated on the same ridge during a one-month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43-year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration.



Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.

Evolution of a classic sand ridge field: Maryland sector, North American inner shelf

Hydrology, morphology and sedimentology of the Campos continental margin, offshore Brazil

Bottom-current-controlled sand deposits — a review of modern shallow- to deep-water environments

Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A.

The surficial sand sheet seaward of Diamond Shoals on the North Carolina shelf is molded into a series of coast-parallel ridges as much as 10 m high and 5 km apart. An older cohesive substrate is exposed in the troughs. Fields of sand waves as much as 7 m high occur on the ridges and in the troughs. Their crests are normal to the ridges. Sand size varies across the sea floor in sympathy with the ridge topography. The gently inclined landward flanks are coarser grained; the steeper seaward flanks are finer grained.

There is evidence to indicate that the sand ridges, like the sand waves with which they are associated, are responses to flow. However, it is not possible to demonstrate the nature of coupling between fluid motion and substrate morphology on the basis of the existing data.

The landward part of the study area is subjected to a southward water drift that during winter is punctuated by intense southward pulses associated with storms. The seaward part experiences a strong, predominantly northward flow throughout most of the year, and the zone of shear appears to migrate back and forth across the study area. Regional considerations suggest a southward sand transport. However, during the period of observation, water flow and bedform asymmetry indicated northward transport. It seems probably that the transport direction reverses with time.

Constructional shelf topography, Diamond Shoals, North Carolina

Sands and silty sands are the predominant surficial soils of continental shelves. Cohesive fine‐grained soils are typical off the mouths of large rivers, near bays and estuaries, and in basins located on the shelf. The stratigraphy of shelf soils is very poorly known for most engineering purposes, except in the vicinity of the Mississippi Delta. Vibratory coring is the most common method of sampling shelf sands to depths of about 13 m; greater soil depths are sampled by borings often using drilling and wireline sampling tools. Employment of self‐contained or wireline static cone penetrometers to obtain in situ measurements of sands has not been as common in the United States as in Europe. Dynamic piston corers are the most common samplers in cohesive soils, but rotary and hydraulically activated incremental corers are becoming available for marine use. Self‐contained or wireline vane shear devices and static cone penetrometers are used for the in situ testing of cohesive soils, and the latter dev...

Review of continental shelf marine geotechnics: Distribution of soils, measurement of properties, and environmental hazards

A sand transport detector which detects the discrete impingements of sand particles on an elongated probe member inserted into the sand A piezoelectric transducer element is cemented to the probe and produces an output signal when impingement occurs With an electronic circuit coupled to the transducer, an indication of sand transport may be obtained

Piezoelectric sediment particle transport detector

Observations made from a submersible operating within the sinuous central segment of La Jolla submarine canyon have provided spectacular examples of erosion at two outcrops below a depth of 300 m One outcrop displays extensive destruction through bioerosion of siltstone members Burrowing and grazing activity of galatheid crabs has led to slumping and spalling where cavities cluster, intersect, or parallel the face of this outcrop A second site on the outside wall of a right-angle bend in the canyon axis revealed an undercut extending 23 m beneath the vertical canyon wall A well-established encrusting fauna reaching to the canyon floor indicates that abrasive high-velocity currents have not occurred at this site for at least 6 months The depth of this feature requires that the process responsible for such erosion take place on the sea floor Sediment samples from the fan and fan valley below this part of the canyon reveal coarse deposits lacking features typical of turbidites, yet indicative of high-velocity submarine currents It is concluded that the undercut is a relict erosional feature remaining from a previous period (Pleistocene) when runoff producing the latest rapid rise in sea level must have generated “submarine floods” charged with coarse detritus

Erosion of submarine outcrops, La Jolla submarine canyon, California

Surge velocity, generated by waves near bottom in the inner continental shelf, has been measured using the pendulum principle to indicate current strength. Several designs that can be visually observed, record in situ, or using strain gauges have been field tested and yield comparable results. These are more effective than rotors or other impellers, since the pendulums respond immediately to the changing stresses. An added advantage is that the instruments can be constructed inexpensively.

Harold D. Palmer

Papers

Constructional shelf topography, Diamond Shoals, North Carolina

Review of continental shelf marine geotechnics: Distribution of soils, measurement of properties, and environmental hazards

Piezoelectric sediment particle transport detector

Erosion of submarine outcrops, La Jolla submarine canyon, California

Some simple devices for the study of wave induced surges