Donald J. P. Swift

Barrier-island genesis: evidence from the central atlantic shelf, eastern U.S.A.

Abstract Since most barrier systems appear to have retreated into their present positions from further out on the continental shelf, the continental shelf is a logical place in which to investigate barrier genesis. The Middle Atlantic Bight of North America, one of the best known shelf sectors, does not appear to contain any drowned barriers. Instead, a series of terraces bear on their surfaces a discontinuous carpet of lagoonal sediments beneath a discontinuous sand sheet formed by erosional barrier retreat. Scarps separating terraces are the lower shorefaces of stillstand barriers whose superstructures were destroyed when shoreface retreat resumed. Thus the “origin” of most barriers is that they have retreated in from the position of their immediate predecessors. Barrier genesis, in the classic sense of large-scale, coastwise spit progradation or mainland-beach detachment, could only have occurred at Late Wisconsin lowstand, when the sense of sea-level displacement was reversed. The relative roles of coastwise spit progradation and mainland-beach detachment depend on coastal relief and slope, with steep, rugged coasts favoring spit progradation at the expense of mainland-beach detachment. Since most major barrier systems form on flat coastal plains, it would appear that mainland-beach detachment is the more important mode of barrier formation. During stillstands or periods of reduction in the rate of sea-level rise, coasts can more nearly approach their climax configuration, in which the shoreline is relatively straight, and the shoreface is well developed and of maximum possible slope. Coastal adjustments during such periods may require localized mainland-beach detachment and coastwise spit progradation, in order to attain such a configuration.

Shoreface-connected sand ridges on American and European shelves: A comparison

Abstract Large scale sand ridges, 10 m or more in height and 2–4 km apart have long been accepted as characteristic of shelves experiencing strong tidal flow. However, they also occur on inner continental shelves whose strongest flows are wind-induced. The Middle Atlantic Bight of North America is the best known example, but inner shelf ridge topographies occur extensively on other Atlantic shelves. We wish to call attention to similarities and differences among the inner shelf ridges of North America, South America, and Europe, as a necessary step prior to the framing and testing of hypotheses concerning the hydrodynamics of ridge formation. Inner shelf ridge fields occur primarily on low, unconsolidated coasts whose substrates provide an ample supply of sediment for ridge formation. The constituent materials are commonly coarse to fine sand, but on the Guiana coast of South America, similar ridges appear to be forming in a mud substrate. Here ridges have much greater spacing (tens of km versus km), less relief, and much flatter slopes (1,3000 versus 1,500). Inner shelf ridges are characteristically aligned obliquely to the shoreline, with acute angles opening into the prevailing flow direction. Downcurrent slopes tend to be steeper and finer grained. The ridges tend to migrate downcoast and offshore, extending their crestlines so as to maintain contact with the shoreface. Where ridges are nearly shore-parallel (current parallel), the movement is expressed as downcurrent growth of ridges and downcurrent extension of swales between ridges. The textural and morphologic asymmetry of inner shelf sand ridges can be explained in terms of existing models for sand wave formation. The oblique orientation with respect to the shoreline is also possibly compatible with these models. An alternative, or perhaps complementary explanation for sand ridge genesis requires helical flow structure in the shelf flow field during peak events, but both theory and observation are less well developed for this model. p]Inner shelf sand ridges appear to be responses to periods of intense flow induced by the passage of storms. The extent to which a given shelf sector develops ridge topography may depend on the efficiency with which the local water mass responds to storm passage. Ridge behavior, and especially ridge migration rates, are necessary information for marine environmental management, since ridges are most active in the nearshore zone where the shelf floor is used for sewage outfalls, deep water ports, nuclear reactor sites, and other structures requiring a measure of sea floor stability.

Barrier island evolution, middle Atlantic shelf, U.S.A. Part I: Shoreface dynamics

Abstract In part I of this paper, near-bottom fluid velocity and sediment concentration measurements from the shoreface of a middle Atlantic barrier (Long Island coast) are analyzed to provide insight into the dynamics of erosional shoreface retreat. In Part II ∗ , these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle Atlantic barriers. Wave motions on the Long Island shoreface tend to drive sediment onshore. Calculations of the onshore sediment flux driven by asymmetrical, shoaling waves show that for the wave states commonly encountered on the middle Atlantic shelf, values become strongly positive landward of approximately 10 m water depth (upper shoreface) but drop to very low values shortly seaward of that isobath. On the lower shoreface and inner shelf floor, fair-weather wave-current interaction tends to cause a landward creep of sediment, even though the wave-orbital component of the near-bottom velocity field is nearly symmetrical. This movement occurs because the wave approach direction generally lies within 90° of the direction (up-coast or down-coast) of the alongshore wind-driven current. Therefore, fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke. During major storms, the rate of sediment transport increases by at least an order of magnitude and the role of wind-driven currents becomes important. Much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair-weather swells. But the wind-driven flow component is also much stronger and is now available for transporting the resuspended sediment. Storm flows over the shoreface occur in distinct dynamic zones (surf zone, friction-dominated zone, transition zone, geostrophic zone). The zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress. During peak flow events, the characteristic length scales of the friction-dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf. The storm-intensified, wind-driven, alongshore flows of the lower shoreface are frequently jet-like in nature and may be upwelling or downwelling. Wind-driven coastal flows with a strong downwelling component occur during most storms on the Long Island coast and are particularly important to the coastal sand budget. During these events, sand is entrained by the storm-intensified upper shoreface circulation system of wave-driven alongshore currents and rip currents, and is fed into the main wind-driven coastal current. Because of the offshore component of bottom flow, sand is swept down the lower shoreface and onto the adjacent inner shelf. Fair weather processes may be unable to return storm-deposited sand to the beach from such an offshore position. The shoreface transport regime of the Long Island Barrier coast thus consists of long periods of time (months) during which sand moves slowly toward the beach, punctuated by short intense periods (hours or days) during which sand is transferred from the shoreface to the adjacent inner shelf. The long-term sense of movement of the shoreface, whether retreating (the Long Island case) or prograding, must depend on the loss or gain of sand by shoreface with respect to the inner shelf. The data shows that the coastal sand budget is controlled not only by the upper shoreface cycle (withdrawl of sand from the beach prism, storage in the breakpoint bar and its subsequent return to the beach), but by a cycle of larger spatial and temporal scale, in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf.

Ridge and swale topography of the Middle Atlantic Bight, Nnorth America: Secular response to the Holocene hydraulic regime

Abstract The ridge and swale topography of the Middle Atlantic Bight was originally interpreted as a relict strand plain whose ridges reflect stillstands of the returning Holocene sea. However, close examination indicates that the ridges appear to be instead longitudinal bed forms, responses to a regime of intermittent, south-trending storm currents. Ridges may be initiated on the shore face and detached as the coast retreats to form fields of isolated ridges, or they may be molded into the shelf-transverse sand massifs that mark the retreat paths of littoral-drift depositional centers at estuary mouths and off cuspate forelands. The ridge and swale topography is thus a stable end configuration toward which a variety of near-shore constructional topographies have converged during the Holocene transgression. Morphologic evidence for readjustment of ridge topography to the deepening shelf flow field during the Holocene transgression is discernable. However, the extent to which the offshore topography continues to respond to hydraulic regime is unclear. The role of helical flow structure in the storm flow field remains to be documented. Resolution of these problems will require more detailed information of hydraulic process and substrate response on storm-dominated shelves.

Estimates of sand transport on the New York shelf using near-bottom current meter observations

ABSTRACT Calculations of cohesionless bottom-sediment movement within the New York Bight have been made by applying the transport formulae of Bagnold (1941, 1956, 1963), Einstein (1950), and Yalin (1963) to near-bottom current meter and surficial sand size observations. Current data were drawn from the records of eighteen long-term Savonius rotor current meter deployments at various locations within the Bight during Fall of 1973 and Spring of 1974. The assumptions underlying the calculations are that wave activity was minimal at recording sites, that a drag coefficient of 3 10-3 reflecting small scale roughness and large boundary layer Reynolds number is suitable to convert measured currents to friction velocities, and that laboratory threshold velocities apply in the marine environment. The calculations suggest that oceanic bottom sediment movement reaches maximum intensity during the fall and winter due to the added energy input from strong meterological events. Calculated transport quantities on the inner shelf tend to decrease as depth and distance from shore increase. However, during the Fall of 1973, the deep waters near the head of the Hudson Shelf Valley exhibited current flows directed to the north in excess of 40 cm/sec. These up-channel flow events appear to be in response to strong, sustained westerly winds. The maximum sediment transport rate caused by these current velocities is two orders of magnitude greater than that occurring at much shallower depths along the New Jersey coast during the same measurement period. The Spring 1974 current velocity field yields transport rates of lesser magnitudes relative to the previous fall, with a net down-channel sediment flux at the head of the Hudson Shelf Valley. The New York Harbor entrance appears to be the site of near-continual sediment transport generated by swift tidal currents.

Shoreface morphodynamics on wave-dominated coasts

Abstract An open ocean shoreface typical of long, wave-dominated sandy coasts has been examined through a combination of extensive field measurements of wave and current patterns with computations of marine bedload transport and sedimentation. Sand transport on the upper shoreface is dominantly controlled by waves with only secondary transport by currents. Sand on the middle and lower shoreface, as well as the inner continental shelf is entrained by storm waves and transported by a complex pattern of bottom boundary layer currents. Storm events have been studied and modeled for the shoreface off Tiana Beach, Long Island. The dominant effect of coastal frontal storms is to cause significant shore-parallel bedload transport with important shore-normal secondary components. These storms tend to result in net offshore transport of sand removed from the beach and surf zone systems. The bedload transport during a storm is convergent on the shoreface leading to accretion. Most accretion occurs on the upper shoreface with lesser deposits covering the middle and lower shoreface as well as the inner continental shelf. Longer-term equilibrium can be maintained by slow return of sand up the shoreface during non-storm conditions. Annual and geologic time-scale budgets of shoreface sand transport and sedimentation yield equilibrium, net accretion or net deposition. The annual balance results from an integration of the event-scale bedload transport patterns and morphologic responses. These processes and responses have feedback mechanisms which stabilize the system over longer, but not geologic, time scales. Geologic time scale balances are controlled by relative sea level changes and relative availability of sediment supply with the event-scale shoreface and transporting processes providing the mechanism to produce the changes in long-term morphology and sedimentation patterns. In the area of study, the long-term pattern is one of net shoreface erosion, and the permanent loss of sand to the shelf floor.

Seafloor response to flow in a southern hemisphere sand-ridge field: Argentine inner shelf

Abstract The inner continental shelf adjacent to Buenos Aires Province, Argentina, is characterized by a series of sand ridges, averaging 4.7 m in relief and spaced about 2.7 km apart. The ridges trend north-south, forming a 20 to 35° southward-opening angle with the northeast-southwest-trending coast. Although the adjacent coast is of depositional strand-plain origin, it is presently undergoing erosional retreat, and the formation of the ridges appears to be part of the retreat process. Seafloor grain-size distributions and bedform arrays are responses to the same velocity field that has formed the ridges. Along transects normal to the ridges, grain-size variation is 90° out of phase with the topography: the coarsest sediment is not at ridge crests or trough axes, but instead occurs on the landward slope; finest sands are on the seaward slope. Side-scan sonar reveals bedforms aligned obliquely to both ridges and the coast; these are inferred to be sand waves. The grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom, although flows observed during the study period were dominantly north to south. The grain-size gradients and bedform orientations are equivalent to those characteristic of the Atlantic shelf of North America with a north-south inversion. The comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf.

Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor

Where the Atlantic coastal gradient is low and the substrate is unconsolidated, shoreface processes lead to an equilibrium shoreface surface, characterized by a straightened plan view and a concave-up profile. Maintenance of this surface during the post-glacial sea-level rise may lead to barrier formation by mainland beach detachment, where the outer oceanic shoreface is maintained by wave and current processes. n nDownwelling storm currents sweep sand from the shoreface of the middle Atlantic bight, transport it downcoast and seaward and deposit it on the adjacent inner shelf. Some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather. However, there is a net loss in most areas. The loss is not made good by river sand input, since on the Atlantic Coast all river sand is trapped by estuaries. As a result the shoreface in these areas is undergoing erosional retreat. Mean horizontal retreat rates are on the order of 1–3 m yr−1 but most movement occurs in brief episodes with recurrence rates on the order of several events per century. Erosional shoreface retreat, coupled with aeolian overshoot, storm washover and the movement of sand through inlets into the lagoons, has lead to landward migration of the Atlantic barriers through the Holocene period of sea-level rise. n nThe landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat. Repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon. The resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action. The backbarrier deposits eventually re-emerge at the shoreface, where their upper beds are eroded and recycled. The barrier thus migrates over a pavement of its own flood tidal deltas and washover fans; sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets, which reaches down to the underlying Pleistocene. n nThe modern shelf sand sheet is the debris blanket resulting from the retreat process. The leading edge occurs as a thin veneer of rip-current fallout on the shoreface; this is periodically stripped off by winter storms to expose the underlying backbarrier strata. Further seaward, shelf sands eroded from the shoreface rest disconformably on backbarrier deposits; shoreface, beach and dune strata are always missing. Every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past. However, the sheet as a whole has been reconstituted; its primary structures are those of the inner shelf floor. n nBackbarrier sand and mud deposits, characterized by channeling and landward-dipping reflectors, can be traced for over 100 km seaward across the middle Atlantic shelf, beneath the modern sand sheet. The backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the Atlantic shelf surface during Holocene time. The sheet-like nature of this record indicates that, viewed at the appropriate time scale, barrier migration is a continuous process.

Large-scale current lineations on the central New Jersey shelf: Investigations by side-scan sonar

Abstract Two morphological orders of ridge and trough topography can be recognized on a terraced segment (at 37 m) of the central New Jersey shelf: (1) a first-order system with ridges to 14 m high, 2–6 km apart, in a Z-shaped pattern trending to the NNE, and (2) a second-order system with ridges 2–5 m high, 0.5-1.5 km apart and trends to the NE. Side-scan mapping together with submersible observations and bottom samples indicate a third-order system of large-scale current lineations which has been imprinted across the first- and second-order systems. The lineations are low relief forms (to 1.5 m high) which occur as elongate zones of textural contrast arranged in furrows, bands, patches and ribbons and display a uniform directional trend to the ENE. The morphology of the lineations appear to vary in response to the nature of the bottom. The lineations are narrow (10–25 m apart) and have no detectable relief in troughs and wider (to 75 m apart) and higher (to 1.5 m high) on ridges, especially second-order ridges of fine sand. Also revealed are wave ripple patterns and a pattern related to the outcropping of Pleistocene/Holocene units in trough bottoms and lower flanks. It is suggested that the first- and second-order systems developed during earlier stages of the Holocene transgression in response to a hydraulic regime of the inner shelf. The first-order system may have inherited some of its morphology from an older system, but did respond to a Holocene tidal regime adjacent to a major estuary. The second-order system developed in slightly deeper water, subsequent to the resumption of the transgression after the 37-m stillstand. The third-order lineations appear to be a response to the helical-flow structure within the flow field of a major shelf storm. Ridges of the central shelf may be maintained by alternate periods of oblique sweeping during storms, resulting in a net transport of fine sand out of the troughs and up on the ridges. Subsequent wave reworking returns the fine sand to the troughs.

Quaternary rivers on the New Jersey shelf: Relation of seafloor to buried valleys

The Quaternary evolution of the stream net on the New Jersey shelf has been interpreted on the basis of bathymetric maps and also by means of seismic profiling, with somewhat different results. Maps show the most recent positions of seafloor shelf valleys, but these valleys may have been created by retreating estuary mouths rather than by subaerial stream erosion. Seismic profiles reveal buried valleys of subaerial fluvial origin, which may follow courses that diverge markedly from the trends of associated seafloor valleys. Shelf valleys must be understood in the context of erosional shoreface retreat, a process that largely remade the shelf surface during successive Quaternary transgressions. Most shelf landforms are marine and post-transgressional in origin, having been formed at the foot of the shoreface. Only very large and deeply incised subaerial landforms survive the shoreface-retreat process. The marine landforms that tend to replace or bury subaerial river valleys include shelf valleys created by estuary-mouth scour, shoal-retreat massifs, and shelf deltas. Three distinct shelf valley sets, including both seafloor and buried valleys, are attributable to the ancestral Delaware, Great Egg, and Hudson Rivers, respectively. Individual valleys within valley sets may follow markedly divergent paths. In the case of the Hudson, the estuary retreated up a deeply incised river valley and was confined by it; the shelf valley is a river valley only partially filled by estuarine deposits. In the case of the other two rivers, the estuary mouths became largely decoupled from the underlying river valleys during the transgression, and their retreat paths do not everywhere overlie the buried channels. In each valley set, divergent buried valleys apparently belong to periods of subaerial exposure of the shelf that occurred earlier in Holocene time.