Berm Crest Research Articles

Coupled Longshore and Cross-Shore Models for Beach Nourishment Evolution at Laboratory Scale

A series of three-dimensional laboratory experiments on beach nourishment behavior are described and analyzed. The experiments were designed to isolate the influences of berm height, beachfill median grain size, wave height, and wave period. The results have not been scaled up to prototype conditions, but many features of the laboratory evolution have also been observed in previous field studies. Laboratory results indicate that beachfill half-life (time required for half of the added volume to leave the nourished footprint) is inversely correlated with wave height, and positively correlated with berm height. A weak positive correlation with grain size was found. The influence of wave period was inconclusive. A coupled model describing the effects of both longshore and cross-shore sediment transport was developed and applied. The model accounts for the rapid loss of nourishment material offshore via cross-shore sediment transport, followed by a more gradual redistribution up- and downcoast of the project via longshore sediment transport. The influence of cross-shore sediment transport decreases as the beach slope approaches that of the prenourishment beach. The new model has not been calibrated for application at field scales, but it does reproduce the salient features of the laboratory dataset, and previous field data sets, such as the flattening of the beach profile as the project evolves. By describing the position of three elevation contours (berm crest, waterline, and beachfill toe), it thus provides a more realistic alternative to the “one-line” models often applied to beach nourishment problems by accounting for cross-shore sediment transport.

Morphodynamic evolution of a coastal lagoon entrance during swash overwash

New field observations of beach berm growth resulting in the sub-aerial closure of an intermittently closed and open lake or lagoon are presented. These perched estuarine systems frequently exhibit ephemeral entrances that respond to process forcing almost instantaneously on a geomorphic timescale, with closure by a supra-tidal berm of depositional origin. The observations were made following a mechanical opening of the entrance and show very rapid vertical growth of an initially low beach berm as a result of swash overtopping and sediment overwash. Very little simultaneous seaward progradation was observed. Sediment overwash volumes were obtained from survey data over a period of 12 days either side of spring tide and the observations also provide an accurate measure of swash sediment transport at the berm crest. A process-based parametric model is developed through robust parameterisations of wave run-up, wave run-up distributions and sediment transport and is applied to predict the total overwash transport into and infilling the estuary entrance. The model is tested against the field data and compares well with the observations. While the field data are site-specific, the proposed modelling framework represents a first step in modelling the complex growth and recovery of natural beach berms in broad-scale morphodynamic models.

Efficacy of beach dewatering — Alassio, Italy

The goal of this study is to estimate the efficiency of dewatering operations in Alassio Beach, north Italy by following an integrated approach which included beach volume calculations, daily mapping of the shoreline position, examination of specific beach widening events and daily comparisons of morphodynamic characteristics of the drained beach versus a control section which included wave run-up, bar patterns, rip migration, evolution of the berm and cusp morphology. Alassio beach was documented by a daily time-lapse photography, by repeated DGPS profile measurements and by an hourly wave and sea level monitoring. Following one year of draining the shoreline position and the beach volume of the drained beach and the control section showed little differences; i.e., accumulation of 0.28 m 3/m 2 and a shoreline advance of 1.1 m on the drained beach, whereas on the control beach the volume change was − 0.03 m 3/m 2 and the shoreline retreated 1.2 m. Short episodes of beach widening, 1–7 days long, which coincided with onshore bar movement and a relative low sea level, occurred simultaneously on the drained and the control sector. This data indicated the inefficiency of the draining operation. An improved resolution for determining the draining effect was attained by the sequential snap-shot photography which showed many systematic morphological differences between the drained beach and the reference segment. The drained beach developed a berm crest and a linear, often scarped foreshore with a narrow swash zone. No such cumulative morphologies were observed on the non-drained control beach, which commonly displayed cusps that were usually absent on the drained foreshore. The wave data showed recurrent medium wave events of Hs = 1–2 m, which repeatedly interrupted the calms thus “cutting-off” the beach evolution. The draining in Alassio was unable to promote beach accumulation by counteracting the erosive effect of the medium interrupting storms.

Morphological and Sedimentological Impacts of Hurricane Ivan and Immediate Poststorm Beach Recovery along the Northwestern Florida Barrier-Island Coasts

Ivan, a strong Category 4 hurricane (downgraded to a Category 3 at landfall), caused widespread erosion and overwash along the northwestern Florida barrier-island beaches. This study examines the storm impact and short-term post-storm recovery along a 200-km stretch of coast from Fort Walton Beach eastward to St. George Island. One prestorm and three poststorm beach-profile surveys were conducted to quantify the storm-induced morphological changes and poststorm recovery. Forty-six trenches were excavated to study the characteristics and thickness of subaerial storm deposits.Extensive inundation and overwash occurred within 100 km from the storm center at landfall. Significant beach/dune erosion was measured as far as 300 km east of the storm center. The highest elevation of beach erosion extended considerably above the measured storm-surge level, indicating that storm-wave setup and swash run-up played significant roles in controlling the elevation of beach erosion. A simple empirical formula reproduced the wave setup and swash run-up reasonably well. Beach recovery began immediately after the storm. Within 90 days, the berm crest recovered to its prestorm elevation, although it was now located approximately 15 m landward for nonoverwashed sites and 30 to 40 m for overwashed sites. The steep prestorm foreshore slope was restored from the gentle storm profile within 30 days.An apparent erosional surface was observed along the impacted foredune and backbeach, extending over 300 km eastward from the storm center. This erosional surface represents a net elevation loss ranging from 0.5 to over 2.0 m. A storm layer of up to 50 cm thick was deposited above the erosional surface. Generally, the storm-layer thickness decreases away from the storm center, as well as landward from the high tide line. Local factors such as beach width and orientation, and sediment properties and supply caused variations in the thickness of storm deposit.

Beach face and berm morphodynamics fronting a coastal lagoon

This study documents two different modes of berm development: (1) vertical growth at spring tides or following significant beach cut due to substantial swash overtopping, and (2) horizontal progradation at neap tides through the formation of a proto-berm located lower and further seaward of the principal berm. Concurrent high-frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes. In mode 1 sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent. The net transport of this sediment is landward only, resulting in accretion onto the upper beach face and over the berm crest. The final outcome is a steepening of the beach face gradient, a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm. In mode 2 sediment is eroded from the lower two-thirds of the active swash zone during the rising tide and is transported both landward and seaward. On the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide. The net result is relatively slow steepening of the beach face, a change of the profile shape towards convex, and horizontal progradation through the formation of a neap berm. The primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face. This depends on the current phase of the spring-neap tide cycle, the wave runup height (and indirectly offshore wave conditions) and the height of the pre-existing berm. A conceptual model for berm morphodynamics is presented, based on sediment transport shape functions measured during the two modes of berm growth.

Thermodynamic solutions for equilibrium beach profiles

Solutions are developed for beach profiles using equilibrium principles of thermodynamics applied to simple representations of the nearshore fluid dynamics. Equilibrium beaches are posed as isothermal shorezone systems of constant volume that dissipate external work by incident waves into heat given up to the surroundings. By the maximum entropy production formulation of the second law of thermodynamics (the law of entropy increase), the shorezone system achieves equilibrium with profile shapes that maximize the rate of dissipative work performed by wave‐induced shear stresses. Dissipative work is assigned to two different shear stress mechanisms prevailing in separate regions of the shorezone system, an outer solution referred to as the shorerise and a bar‐berm inner solution. The equilibrium shorerise solution extends from closure depth (zero profile change) to the breakpoint, and maximizes dissipation due to the rate of working by bottom friction. In contrast, the equilibrium bar‐berm solution between the breakpoint and the berm crest maximizes dissipation due to work by internal stresses of a turbulent surf zone. Both shorerise and bar‐berm equilibria were found to have an exact general solution belonging to the class of elliptic cycloids. The elliptic cycloid allows all significant features of the equilibrium profile to be characterized by the eccentricity and the size of the ellipse axes. These basic ellipse parameters are evaluated by process‐based algorithms and empirically validated parameters for which an extensive literature already exists. The elliptic cycloid solutions displayed wave height, period and grain size dependence and demonstrate generally good predictive skill in point‐by‐point comparisons with measured profiles.

Seasonal Dynamics of a Microtidal Pocket Beach with Posidonia oceanica Seabeds (Mallorca, Spain)

Abstract In this work, we analyze the seasonal evolution of a Mediterranean pocket beach and its response to different storm episodes. Magalluf, an intermediate medium sand beach located in the Bay of Palma (Balearic Islands) was monitored by topographic levelling during 14 months. Near the beach, a Posidonia oceanica meadow covers most of the seabed and appears to influence the cross-shore beach adjustment. The low variability observed during the sampling period was perturbed by two storm events that caused significant beach evolution and sediment transport. The first storm gave rise to waves from the SE, significant height = 2.4 m, cross-shore sediment transport and along-shore net sediment exchange that resulted in decreased dry beach extension to a minimum. The second storm was characterized by strong northeasterly winds and generated a set-up of 0.5 m and a nearshore drift reversal that redistributed sediment from the berm crest to the beach face, thereby increasing beach extension. Results from nume...

Municipal initiatives for managing dunes in coastal residential areas: a case study of Avalon, New Jersey, USA

Abstract The characteristics of foredunes created in a municipal management program on a developed barrier island are evaluated to identify how landforms used as protection structures can be natural in appearance and function yet compatible with human values. Shoreline management zones include a naturally evolving, undeveloped segment; a noneroding, developed segment; eroding and noneroding segments of an “improved beach” where dunes have been built by artificial nourishment; and a privately built, artificially nourished dune on the shoreline of an inlet. A disastrous storm in 1962 resulted in an aggressive program for building dunes using sand fences, vegetation plantings, purchase of undeveloped lots, and sediment backpassing to maintain beach widths and dune elevations. The present nourished and shaped foredune in the improved beach is higher, wider, and closer to the berm crest than the natural dune. Restricted inputs of aeolian sand keep the surface flat and poorly vegetated. A stable section of this engineered shore has a wider beach, and sand fences have created a higher foredune with greater topographic diversity. The cross shore zonation of vegetation here is more typical of natural dunes, but the environmental gradient is much narrower. The privately built dune is low, narrow, and located where it could not be created naturally. Foreshore and aeolian sediments in the undeveloped segment and the improved beach are similar in mean grain size (0.16–0.21 mm) and sorting (0.31–0.39 φ ), but sediment on the surface of the nourished dune is coarser (28.1% gravel) with a more poorly sorted sand fraction (1.30 φ ) representing lag elements on the deflation surface. Willingness to enhance beaches and dunes for protection has reduced insurance premiums and allowed the municipality to qualify for funds from the Federal Emergency Management Agency (FEMA) to replace lost sediment, thus placing an economic value on dunes. Success of the management program is attributed to: (i) timing property-purchase and dune-building programs to periods immediately after storms (causing residents to accept high dunes that restrict access or views); (ii) instituting a vigorous education program (reminding residents of hazards during nonstorm periods); (iii) maintaining control over local sediment supplies (to keep pace with erosion and create new shoreline environments); (iv) investing private and municipal economic resources in landforms (qualifying them for external funds for replacement); and (v) maintaining, augmenting, or simply tolerating biodiversity and natural processes (retaining a natural heritage).

TEXTURAL VARIATIONS OVER A FORESHORE-BACKSHORE ENVIRONMENT, HAWAII

Detailed textural analyses in a carbonate foreshore-backshore coastal environment in Hawaii indicated distinct differences in mean, sorting, skewness, and percent of fines between zones and within zones. A well-developed berm crest partitioned the 1,000 m2 grid into foreshore (n = 79) and backshore (n = 46) zones. As a group, foreshore sediments were finer, better sorted, and more negatively skewed than backshore sediments. This pattern was thought to reflect the decoupling of the foreshore source population by swash-backwash and eolian processes. Spatial variations in texture within the foreshore were also observed, and this led to the subdivision of the foreshore into three zones based on observed textural variations–lower foreshore, mid-foreshore, and upper foreshore. The lower foreshore was found to be statistically coarser, more poorly sorted, and more positively skewed than the other foreshore zones. The mid-foreshore zone was the finest, and the upper foreshore was intermediate in grain size. This patterning was thought to reflect the greatest energy dissipation in the lower foreshore, subsequent competency decrease of swash runup, and deposition of coarsest particles remaining in transport in the upper foreshore, accompanied by infiltration losses in the backwash and deposition of the finest materials in the mid-foreshore zone. Traditional bivariate plots of textural parameters for the foreshore and backshore samples indicated that this approach alone would not be useful in distinguishing subenvironments in paleosequence studies. A mean value approach (MVA) was developed and used in combination with published bivariate plots to identify depositional environments using moment statistics. Encouraging results were obtained from Waimanalo Beach sediments and from data published in the literature. The bivariate (textural) suite statistics approach (Tanner, 1991 b) was used to test for correct environmental recognition using the 125 beach samples. Results at first appeared promising, but assessment of literature data illustrated that the original suite diagrams could usefully be expanded. These results support previous statements that textural studies should not be used as the only tool in paleoenvironmental reconstruction. [Key words: beach, nearshore, backshore, sediment, geomorphology, Hawaii.]

Shorerise and bar‐berm profiles on ocean beaches

A beach equilibrium model is developed that treats the outer (shorerise) portion of the profile independently from that of the inner (bar‐berm) portion. The two portions are matched at the breakpoint‐bar. The partitioning of the profile in this way is consistent with the different forcing modes on either side of the breakpoint. This formulation utilizes beach profile data not previously available. It is shown that both portions of the profile are well fitted by curves of the formh=Axm, wherehis positive downward andxis the positive offshore coordinate. Surprisingly, the value ofm≈ 0.4 is nearly the same for shorerise and bar‐berm and does not change significantly with seasonal beach changes (summer/winter). The principal difference between seasonal profiles is that in winter (higher waves) the breakpoint‐bar is deeper and farther offshore while the berm crest is displaced landward. Thus the changes in seasonal equilibria are manifested by simple, self‐similar displacements of the bar‐berm and shorerise curves as a consequence of changes in surf zone width andO(1) variations in the factorA.

The physical processes of seabed disturbance during iceberg grounding and scouring

Icebergs drifting in ocean currents over the Labrador continental shelf may contact and move through seabed sediments to create characteristically curvilinear features, commonly referred to as scours, which average 1–2 m in depth and about 30–40 m in width. Such scours may continue for several hundreds of metres or several kilometres. The period of seabed interaction may last from a few minutes to several months and results in modification of both the seabed sediments and the iceberg keel. When observed from submersible soon after their formation, scours exhibit morphological characteristics not seen in old, degraded scours. The scour trough, between two berm ridges, is generally flat-bottomed but is characterized by the presence of ridge-and-groove microtopography, with amplitudes up to 0.3 m developed parallel to the scour axis. These features are formed at the trailing edge of the keel by clastic material embedded in the ice and by open fissures in the ice. In places along the inner berm margins, ridges and grooves may be developed at an angle to the scour axis, reflecting lateral displacement of material towards the berm as the iceberg moves forward. Voids up to 1 m deep and 2 m wide occasionally truncate the ridges and grooves. Voids are formed following the dissolution of small (a few m 3) masses of debris-laden ice that are mechanically broken off from the base of the keel and pressed into the seabed by the scouring iceberg. Initially low areas within the scour trough may preserve seafloor that has not been affected by ice-seabed interaction. In these regions deposition of bulldozed sediment from the surcharge at the leading edge of the keel may partially fill the narrow spaces beneath the keel in areas of initially low seabed elevation. Scour berms consist of in situ fractured but intact blocks of material on the inner flanks, and disarticulated blocks 1–2 m high along the berm crest. The outer berm slopes generally consist of pieces of larger blocks spalled from the berm crest resting in relatively finely comminuted, reworked material. The reworked material originates in the leading edge surcharge before being displaced to either side of the keel. Scour berms have irregular topography ranging in height from a few centimetres to as much as 6 m above the seabed. Isolated irregularly-shaped mounds beyond the outer berm may represent pieces of cohesive sediment that originated at the berm crest as overhangs and which subsequently collapsed.

Cold-front driven storm erosion and overwash in the central part of the Isles Dernieres, a Louisiana barrier-island arc

Tropical and extratropical storms produce significant erosion on the barrier islands of Louisiana. Over the past 100 years, such storms have produced at least 2 km of northward beach-face retreat and the loss of 63% of the surface area of the Isles Dernieres, a low-lying barrier-island arc along the central Louisiana coast. Elevations on the islands within the arc are typically less than 2 m above mean sea level. The islands typically have a washover-flat topography with occasional, poorly developed, dune-terrace topography consisting of low-lying and broken dunes. The central part of the arc consists of salt-marsh deposits overlain by washover sands along the Gulf of Mexico shoreline. Sand thicknesses range from zero behind the beach, to less than 2 m under the berm crest, and back to zero in the first nearshore trough. The sand veneer is sufficiently thin that storms can strip all the sand from the beach face, exposing the underlying marsh deposits. The geomorphic changes produced by cold fronts, a type of extratropical storm that commonly affect the Isles Dernieres between late fall and early spring are described. Between August 1986 and September 1987, repeated surveys along eleven shore-normal transects that covered 400 m of shoreline revealed the timing and extent of cold-front-produced beach change along a typical section of the central Isles Dernieres. During the study period, the beach face retreated approximately 20 m during the cold-front season but did not rebuild during the subsequent summer. Because the volume of sand deposited on the backshore (5600 m 3) was less than the volume of material lost from the beach face (19,200 m 3), approximately 13,600 m 3 of material disappeared. Assuming that underlying marsh deposits decrease in volume in direct proportion to the amount of beach-face retreat, an estimate of the mud loss during the study period is 14,000 m 3. Thus, the decrease in volume along the profiles can be accounted for without removing any sand from the area, suggesting that a major effect of cold fronts is first to strip the sand from the beach face and then to erode the underlying marsh deposits. After being eroded, the mud is lost from the islands because currents transport it away from the islands.

Seasonal changes in beach topography and beach volume in Delaware

A beach study was conducted along a shore segment of the Delaware coast from March 1982 through to April 1983. Results indicate that beach topography and beach volume varied with changes in wave-climatic regimes; the regimes in turn were generally in phase with the winter and summer seasons. Because both the magnitude and frequency of storm waves were higher during winter than in summer, the winter beach shape was concave upward. Following the winter storms of 1981–1982, swells rebuilt the beach in two stages from March through to August 1982. In the first stage, the beach vertically accreted (aggraded) and in the second, it laterally accreted (prograded). The aggrading stage involved the welding of a swash bar to the foreshore; thereafter, swash overtopping caused deposition on and landward of the bar. In time, the swash bar attained a berm and swale topography. The prograding stage started when the swash could no longer overtop the berm crest, which confined deposition to the foreshore. This stage occurred following a minor or moderate storm event that shifted the berm crest upward and landward, or during a neap tidal stage that occurred after the construction of a spring tide berm. Between March and the end of August 1982, 60 m3 of sediments per 1 m of beach were deposited on the shore. Of this value, 67% was attributed to aggradational processes, while the remaining 33% was attributed to progradational processes. Swells as well as storm waves caused vertical accretion; storm waves contributed 20% of the total sediment volume deposited on the beach. When the shore was at its maximum sediment volume for 1982, the beach topography was convex upward.An inverse relationship existed between the mean monthly sediment volume and the mean monthly surge variability. The surge level was used as an index of wave conditions; high surge levels were associated with large waves while low surge levels were associated with small waves. When surge variability was high during the winter, sediment volume on the beach was low, and conversely, when surge variability was low in summer, sediment volume was high. Correlation analysis showed that 80% of the variance of the mean monthly sediment volume was explained by the mean monthly surge variability.

Low-frequency changes of sediment volume on the beachface at Warilla Beach, New South Wales, 1975–1985

Characteristic patterns of shore-parallel and shore-normal movements of sediment on a sandy beach are described in this study. The sediment movements are apparent as low-frequency variations in the volume of sediment stored above mean low water spring tide (MLWST) level on Warilla Beach, N.S.W. They were determined by an empirical orthogonal-function (EOF) analysis of beach profile records. The profiles were surveyed from eighteen stations along a 2 km long beach at least monthly, generally fortnightly, for over 10 years, from July 1975 until August 1985. Results from the EOF analysis confirm and substantially extend an earlier study of Warilla Beach when the first 5 years of the beach survey record was examined ∗ ∗ Eliot and Clarke (1982a) and Clarke and Eliot (1982, 1983a). . The importance of low-frequency beachface change identified in the earlier, shorter study was confirmed from the extended data set; zones of maximum variability, linked with rip-current activity, are consistent in both studies, and similar patterns of alongshore sediment movement in the swash zone were determined from both analyses. The principal differences between the two sets of results highlight beachface responses to processes operating in different time scales. Amplitude spectra from time series describing the characteristic modes of sediment movement along and across the beachface have periodicities of 2.0, 1.7 and 1.0 years. Beach changes with periods close to 3 years are apparent in the record, but cannot be resolved by standard spectral analysis techniques. Each period is associated with a characteristic pattern of beach change; the triennial and 1.7 year cycles with alongshore sediment transport, and the biennial cycle with onshore-offshore sediment exchange. Eigenvectors from the EOF analysis identify the characteristic patterns of beach change from survey data specified to highlight the shore-parallel sediment movement along 0.5 m deep, horizontal segments of beach, and shore-normal sediment movement along the beach profiles. In the analysis of shore-parallel variation in beach profile configuration, the fundamental eigenvectors explain between 70 and 80% of beachface variance in each instance, and different patterns of sediment movement were identified for each horizontal slice. On the upper part of the beachface, all eigenvectors identified a zone of low variability along the shoreline skirting a rockwall. Below berm crest, in the swash and intertidal zones of the beach, and away from the rockwall, the northern half of the beach was highly variable. Second mode eigenvectors (cellular-flux modes) explain a further 5 to 14% of the variance and describe sediment exchanges between discrete zones in each horizontal segment of the beachface. Zones of greatest variability occurred landward of places where rip currents frequently develop. Conversely, zones of low variability are coincident with areas of beachface in the lee of places where inshore bars frequently form. Analysis of sediment movement along each profile is consistent with the beach changes described. The fundamental eigenvectors account for 91 to 99% of the variance on each profile. They describe an onshore-offshore movement of sediment through the intertidal zone of the beach.

Antidunes on Modern and Ancient Washover Fans

ABSTRACT Antidune deposits on a washover fan on Seabrook Island, South Carolina display a variety of bedding types. The predominant stratification form comprises nested, very thin lenses which are internally cross-laminated and which become thinner and more closely spaced downflow. In addition, cross laminae within the lenses tend toward backset forms in the bermward portions of the flow, and become more foreset-oriented in a downfan direction. These characteristics, together with the antidune field's relationship to other bed features on the fan, suggest that antidune formation is related to the occurrence of an undular hydraulic jump just downfan from the berm crest. Ancient deposits interpreted as washover sequences, including Ordovician and Pleistocene sandstones in South Africa and a Carboniferous sandstone in Kentucky, display sedimentary structures similar to those observed in the Seabrook antidune field. Because washed-out antidunes and their associated cross laminae are very subtle features, antidunes are possibly responsible for many of the gentle irregularities in otherwise flat beds and are therefore possibly far more common in the geologic record than previously recognized. Their sedimentary structures are distinct from degraded forms of ripple cross-lamination, and in many cases are identical to features usually described in the literature as low-angle truncation surfaces.

Variation in subaerial beach sediment volume on a small sandy beach over a monthly lunar tidal cycle

Three major patterns of sediment movement along 0.5 m deep horizontal segments of beachface on a small sandy beach at Coledale, New South Wales, were determined by empirical orthogonal function (EOF) analysis of a 36 day record of beach change. The fundamental beach response is identified by the first eigenfunction mode. The mode describes sediment transfers occurring over the beachface, between the low-tide zone and the berm crest as well as sediment movement along the beach. The alongshore translocations occurred about a nodal point located approximately 0.75 of distance along the beach, from its southern end. Transfers across the beach were established by comparing the time series associated with the eigenfunctions describing alongshore transfers of sediment in different segments. The slice-to-slice comparisons showed an increasing phase down the beachface from the berm crest segment to the mid-tidal zone. The phase difference was approximately 2.5 days per 0.5 m segment. The fundamental beach response and secondary sediment transfers typical of the supratidal zone occur with an apparent 28–30 day lunar tide cycle. They were unrelated to short-term variations in the incident wave amplitude. Lower down the beachface the time series associated with the eigenfunction describing sediment translocations in the mid-tidal zone is aperiodic. The irregularities can be linked to particular process events. However, similar beach changes were not always related to similar process conditions and they were not simply related to wave-regime fluctuations. The patterns of sediment movement along Coledale Beach were consistent with similar patterns reported from elsewhere by Clarke and Eliot (1982, 1983a) in that they were closely related to variation in the nearshore water circulation system. In particular, nodal points identified by the EOF analysis commonly occurred on the beachface landward of inshore regions where bars most frequently developed. The antinodal positions were similarly associated with regions where rip channels are frequently formed.

Genesis and Preservation of Antidune Stratification in Modern and Ancient Washover Deposits: ABSTRACT

End_Page 419------------------------------Antidunes are nearly symmetrical, sinusoidal bed forms formed by near-critical flows, and comprise all bed profiles in phase with surface gravity waves in the overlying flow. This definition applies regardless of bed-form propagation direction or mode of formation. They occur in a wide range of depositional environments but are ubiquitous in the swash zones of fine-grained beaches. Antidunes are a common but little-recognized feature of washover fans, where they generate distinct sedimentary structures that are well preserved in the stratigraphic record. On washovers at Seabrook Island, South Carolina, antidune trains form at the fan apex, in phase with an undular hydraulic jump. This jump marks the transition within the overwash surge from supercritical flow of the berm crest to subcritical flow on the fan itself. Bed-form distribution on the fan is controlled not only by this transition but also by downfan energy dissipation within subcritical portions of the flow. From berm crest to fan terminus (i.e., downstream) the lateral sequence of bed geometries is: flat, erosional bed; antidunes; flat bed with primary current lineation and rhomboid rill marks; flat, featureless bed; rhomboid to cuspate ripples; undulatory, linear ripples; washed-out ripples; flat bed. Heavy minerals are concentrated near the fan apex owing to selective down an transport of quartz. Landward fan migration results in a genetic sequence of sedimentary structures in which all of these features are preserved. Streamwise boundaries of the antidune field are abrupt. Within the field itself, both crest amplitude and crest spacing decrease downfan with decreasing velocity. Bedding geometry is controlled by bed-form scale, by the pulsing nature of the flow (which tends to wash out bed forms at the end of each rock event), and by eolian deflation during neap tides and fair weather. Upstream antidunes produce thin lenses of backset laminae which are commonly truncated by flat beds or superposed backsets. These lenses grade downstream into thinner lenses with lower angle backsets, and finally into zones of very low-angle truncation surfaces. Orthoquartzitic washovers in the Carboniferous of Kentucky and the Ordovician of South Africa display well-preserved antidune formset geometries. Although the very well-sorted, mono-mineralic nature of these sandstones renders internal stratification nearly invisible, backset laminae are apparent. Pleistocene washovers in South Africa exhibit antidune bedding types influenced by grain size. Fine to medium-grained sand is organized in en echelon lenses of backset laminae interbedded with flat beds. Coarse sand and granules occur as flow-perpendicular ribbons with lenticular cross sections and spacing similar to antidune spacings on modern fans. Many published photographs of flat beds contain unrecognized examples of antidune stratification. In many places, the antidune cross-laminae would be impossible to detect without heavy minerals or other markers to accentuate bedding. However, in apparent massive beds, x-ray radiographic techniques have revealed cross-laminae with antidunal affinities. Moreover, antidunes may be responsible for much of the low-angle truncation normally associated with flat-bedded facies. End_of_Article - Last_Page 420------------

Stability of a very coarse-grained beach at Carmel, California

Monastery Beach at Carmel, California, is a pocket beach composed of very coarse to granular sediment. In profile, the beach has a well-defined berm crest; a steep foreshore; and a gently sloping, barless offshore covered by large, long-crested oscillation ripples. Carmel Submarine Canyon heads a few hundred meters offshore of the beach, and San Jose Creek, a small ephemeral steam, ponds onshore of the central part of the berm. Wave conditions vary greatly during a year because the beach lies open to the Pacific Ocean for azimuths between 270°–322°N whence come a variety of wave types. Even with a variable wave climate, Monastery Beach has maintained a swell profile for almost three years. Aperiodic beach surveys show that the beach responds little to seasonal changes in wave climate. Four survey lines maintained the same swell profile throughout the study period. The fifth line maintained a stable profile only across the foreshore; the berm was twice artificially breached during storms to prevent upstream flooding along San Jose Creek. In comparison, Carmel Beach, a nearby beach composed of medium sand, commonly alternates between swell and storm profiles. The increased stability of Monastery Beach relative to Carmel Beach is attributed to two factors: grain size differences and location within Carmel Bay. Rebuilding proceeded very slowly along the breached part of the berm at Monastery Beach. The probable cause of such a low recovery rate is that oscillation ripples trapped the sand that was carried offshore when San Jose Creek eroded the beach. The ripples, which are active under high-energy conditions, approach dormancy under low-energy conditions. Each ripple, therefore, acts like a reservoir, retaining sand during most swell conditions.

Distribution of Clay in Recent Sands: ABSTRACT

Shallow cores and surface samples from a beach, point bar, and channel mouth near Pascagoula, Mississippi, were used to study clay distribution in sand-rich sediments. Air photos, permeability measurements, grain-size data, photomicrographs, and X-ray diffractograms provided the data base. Data from clay-silt-sand separations show that, in general, the clay/silt ratio increases as the content increases. The clay/silt ratio increases from 0.12 to 5.92 in beach samples as the percent sand increases from 52.6 to 99.9, from 0.25 to 9.05 in point-bar samples as the percent sand increases from 48.5 to 98.8, and from 0.08 to 1.19 in channel-mouth samples as the percent sand increases from 59.9 to 97.8. The highest clay/silt ratios are in subaerially exposed sediments with generally high vertical permeabilities. Clay/silt ratios in the berm crest of the beach increase to about 5 as permeability increases to about 5 darcys. Clay/silt ratios in the beach portion of the point bar increase to about 9 as the permeability increases to about 1.4 darcys. Scanning electron microscope (SEM) photomicrographs show that the clay can occur as coatings on much larger grains and as composite grains (aggregates of clay, silt, and sand). Only one possible example of authigenic growth in the clay fraction was observed with the SEM. It is concluded that clay may be deposited in sand-rich environments through the settling of large clay-coated grains, as composite clasts, and as floccules. In-situ percolation of clay suspensions and biogenic activity may add to the clay content of sediments. End_of_Article - Last_Page 541------------

Holocene Stratigraphy of Rapidly Transgressive Barrier Island, Cape Romain, South Carolina: ABSTRACT

Cape Romain, South Carolina, a barrier island-cuspate foreland just south of the Santee delta, shows a classic transgressive stratigraphic sequence. The island has eroded landward 150 m in the past 30 years. Today, 75% of the island shoreline is manifest as narrow washover fans and coalesced terraces migrating landward under the influence of normal spring high tides and storm events. The remainder of the island is occupied by eroding ridge and swale topography. Landward of the barrier is a lagoon-marsh system 11 km wide which abuts and overlies the Pleistocene mainland. Using a modified concrete compactor, 40 vibracores 5 to 9 m long were taken from three transects across the island. The coring system permits an undisturbed (preserved primary sedimentary structures and bioturbation) core 7.6 cm in diameter to be removed from any unconsolidated sediment. Cores have been taken in compacted clays, fine to coarse sands, shell lags, and oyster reefs. Recovery ranges from 90 to 100%. Additional data were obtained from box cores and short (1 to 2 m) vibracores taken by divers in specific subtidal locations. Eight meters under the present berm crest is a relict barrier-marsh complex overlain by a shell storm lag. Overlying the shell lag are flaser-bedded sands and muds of a very shallow-water high-energy lagoonal facies which grades upward into highly bioturbated sandy muds of a deeper water lagoonal facies. A tidal-flat oyster reef complex lies above, grading upward into a fringing marsh. The sequence is capped by washover sands and shells, beachface sands, and/or dunes of the present barrier. This sequence indicates a general rise in sea level during the Holocene through a combination of eustatic rise and a relatively high subsidence rate. The sequence shows a barrier island overtopped by the rising sea level. At some later time, another barrier was developed seaward on the shelf. This barrier transgressed landward to occupy the present position of Cape Romain. End_of_Article - Last_Page 518------------