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J.R. Allen

Publications and source records attributed to J.R. Allen.

18 recordsLinked to original sources
USGS2006

Bluff evolution along coastal drumlins: Boston Harbor Islands, Massachusetts

A series of partially drowned drumlins forms the backbone of the inner islands within Boston Harbor. The shoreline of these rounded glacial deposits is composed of actively retreating bluffs formed by continual wave attack. Comparisons of bluffs reveal variability in their height and lateral extent, as well as in the dominant mechanism causing their retreat. Two processes are responsible for bluff erosion and yield distinct bluff morphologies: (1) wave attack undercuts the bluff and causes episodic slumping, yielding planar bluff slopes, and (2) subaerial processes such as rainfall create irregular slopes characterized by rills and gullies. We propose a model of drumlin bluff evolution that is based on processes of erosion and physical characteristics such as bluff height, slope morphology, and the orientation of the bluff with respect to the long axis of the drumlin and its topographic crest. The four phases of drumlin bluff evolution consist of (1) initial formation of bluff, with retreat dominated by wave notching and slumping processes; (2) rill and gully development as bluff heights exceed 10 m and slumped sediment at bluff base inhibits wave attack; (3) return of wave notching and slumping as bluff heights decrease; and (4) final development of boulder retreat lag as last remnants of drumlin are eroded by wave action. These phases capture the important physical processes of drumlin evolution in Boston Harbor and could apply to other eroding coastal drumlin deposits.

Massachusetts↗
USGS2005

Coastal foredune displacement and recovery, Barrett Beach-Talisman, Fire Island, New York, USA

Coastal foredune mobility has been tracked at Fire Island National Seashore since 1976 with annual field surveys and analysis of frequent aerial photography. Sequential mapping of the foredune crestline depicts nearly islandwide displacement during major storm events, such as in 1992, and localized displacement during alongshore passage of inshore circulation cells during other years. An instance of localized landward erosion and curvilinear displacement along approximately 400 m of foredune occurred in 1994, followed by recovery over the next nine years. Data from annual surveys and partially supported by four LIDAR flights establish that volume recovery rates in the foredune ranged from about 1.0 m3/m/yr to nearly 12.0 m 3/m/yr. Analysis of the foredune morphology and location shows nearly complete recovery of foredune shape and dimension during this interval and it also demonstrates that there has been inland displacement of the foredune crestline of up to 40 m. Total volume recovery within the localized foredune erosion site was greatest, between 34 m3/m to 47 m3/m, in areas of greatest displacement and eventually contributed to creation of a foredune of similar dimension along the entire eroded zone. This process of erosion and recovery describes a mechanism for foredune dimension retention during episodic erosion and displacement and may be a model for foredune persistence accompanying barrier island migration. ?? 2005 Gebru??der Borntraeger.

Zeitschrift fur Geomorphologie, Supplementband↗
USGS2003

Longshore sediment transport rates on a microtidal estuarine beach

Longshore sediment transport rates were estimated on a microtidal estuarine beach in Great South Bay, N.Y., during two dyed sand tracer experiments using a temporal sampling method. Mean onshore wind speeds of 5.8 and 9.9 m/s resulted in root-mean-square wave heights of 0.07 and 0.08 m and wave angles of 3.0 and 10.1, causing transport rates of 0.468 and 0.972 m3/h. Rates were 3.1 to 6.5 times greater than predicted by existing equations using standard coefficients. Greater rates are attributed to the concentration of sediment transport in the energetic swash zone under plunging breakers.

Journal of Waterway, Port, Coastal and Ocean Engin↗
USGS2000

Influence of inner-continental shelf geologic framework on the evolution and behavior of the barrier-island system between Fire Island Inlet and Shinnecock Inlet, Long Island, New York

High-resolution, sea-floor mapping techniques, including sidescan-sonar and subbottom profiling, were used to investigate how the geologic framework of the inner-continental shelf influenced the Holocene evolution and modern behavior of the Fire Island barrier-island system, Long Island, New York. The inner-continental shelf off Long Island is divided into two physiographic provinces by a broad outcrop of Cretaceous coastal-plain strata offshore of Watch Hill; this outcrop was part of a subaerial headland during the Holocene marine transgression. Erosion of the headland during transgression furnished sediment to the inner-continental shelf downdrift to the west. The sediment was, in turn, reworked by oceanographic processes into a series of shoreface-attached sand ridges. The oldest (~1200 yr BP) and most stable part of the barrier-island system is immediately landward of the outcropping coastal-plain strata and thickest sand ridges. East of Watch Hill, Pleistocene sediment either is exposed on the inner-continental shelf or is buried by a veneer of modern reworked sediment. Here the barrier-island system has migrated landward at a faster rate than the segment west of Watch Hill and has been breached by numerous historic inlets. Because the Pleistocene sedimentary deposit is generally of uniform thickness throughout the study area and unconformably overlies the Cretaceous coastal-plain strata, both the Holocene and historical evolution of the Fire Island barrier-island system are controlled by the physiography of this regional unconformity. In particular, the shoreface-connected sand ridges appear to be a significant source of sediment to the western portion of Fire Island. Previous attempts to develop a sediment budget for this coastal system have failed to explain volumetric discrepancies, primarily because poor assumptions were made about the nature of sediment transport in the system. A more realistic sediment budget must include a significantly larger spatial scale, including sediment input from the inner-continental shelf.

New York↗
USGS1999

Space and time scales of shoreline change at Cape Cod National Seashore, MA, USA

Different processes cause patterns of shoreline change which are exhibited at different magnitudes and nested into different spatial and time scale hierarchies. The 77-km outer beach at Cape Cod National Seashore offers one of the few U.S. federally owned portions of beach to study shoreline change within the full range of sediment source and sink relationships, and barely affected by human intervention. 'Mean trends' of shoreline changes are best observed at long time scales but contain much spatial variation thus many sites are not equal in response. Long-term, earlier-noted trends are confirmed but the added quantification and resolution improves greatly the understanding of appropriate spatial and time scales of those processes driving bluff retreat and barrier island changes in both north and south depocenters. Shorter timescales allow for comparison of trends and uncertainty in shoreline change at local scales but are dependent upon some measure of storm intensity and seasonal frequency. Single-event shoreline survey results for one storm at daily intervals after the erosional phase suggest a recovery time for the system of six days, identifies three sites with abnormally large change, and that responses at these sites are spatially coherent for now unknown reasons. Areas near inlets are the most variable at all time scales. Hierarchies in both process and form are suggested.

Massachusetts↗
USGS1996

A field data assessment of contemporary models of beach cusp formation

Cusp formation was observed during an instrumented, daily profiled, time series of a reflective beach in Canaveral National Seashore, Florida on January 5, 1988. The monitored cusp embayment formed by erosion of the foreshore and the cusp series had a mean spacing of approximately 28 m. During this time, inshore fluid flows were dominated by two standing edge waves at frequencies of 0.06 Hz (primary) and 0.035 Hz (secondary) whereas incident waves were broadbanded at 0.12-0.16 Hz. Directly measured flows (and indirectly estimated swash excursion) data support both the standing wave subharmonic model and the self-organization model of cusp formation in this study.

Journal of Coastal Research↗
USGS1995

Beach-steps: An evolutionary perspective

Field observation of contrasting beach-step behavior at Canaveral National Seashore on two subsequent days when incident-wave conditions in the inner surf zone were similar prompted this re-examination of our conceptual and quantitative understanding of beach steps. These lower-foreshore features are more complex than previously assumed, evolving through erosional as well as accretional phases, and displaying equifinality in geometric form but not necessarily internal sedimentary structure. Past and recent evidence is reviewed that links beach steps to incident waves at the surging-plunging transition and to the action of a backwash vortex. Tides and low-frequency waves likely play no direct role in beach-step initiation, although their presence can have pronounced influences on modulating nearshore hydrodynamics, and thus, on beach-step maintenance and evolution. A generalized, conceptual model capturing these aspects of beach-step dynamics is presented. Beach-step initiation proceeds via step “carving”, “excavation”, or “building” depending on the erosional-accretional character of the beach-foreshore system. Subsequent evolution of the step form may take one of several alternative morphodynamic pathways including stepface “retreat”, step “drag down”, or step “infilling/elimination” depending on tidal stage/range or wave set-up/setdown. Additional data on equilibrium beach-step forms and associated morphodynamic and hydrodynamic conditions in the field are necessary before quantitative models of beach-step existence and evolution can be formulated with realistic results.

Marine Geology↗