Low Tide Terrace Research Articles

Groundwater Dynamics on Macrotidal Gravel Beaches of Tierra del Fuego, Argentina*

Abstract On bimodal microtidal beaches, there are grain-size segregations and the relationship between mean size and foreshore slope is not clear. Dealing with macrotidal beaches composed of gravel and sand, the dynamics vary in response to the tide: reflective conditions at high tide, and a dissipative behaviour during low tide. These dynamic changes are related to grain-size segregations: the upper beach is dominated by gravel (or gravel with coarse sand) and the low-tide terrace is exclusively composed of fine sand. Along the Atlantic coast of Tierra del Fuego, nineteen beaches analysed vary in mean grain size between −4 and 4 phi units. The slope of the foreshore (reflective portion) is linearly related to grain size. The slope of the low-tide terrace (dissipative portion) has no relationship to grain size (fine to very fine sand). These segregations cause temporal changes in the composition of the groundwater sceping at the low-tide terrace. Test experiments performed at the beach permitted to recogn...

Beach morphodynamics in a strong-wind bay: a low-energy environment?

The morphodynamic behaviour of a multibarred beach in a fetch-limited, strong-wind bay (Seaford Beach, SE Australia) was examined during both high- and low-energy conditions, and considered in the context of a definition of low-energy provided in the literature. Measurements of nearshore waves, currents, and morphology revealed a bimodal behaviour. Under initial low-energy conditions, the beach exhibited a “low-tide terrace” state, and waves and currents were of very low magnitude. During subsequent high-energy conditions, the beach demonstrated dynamic behaviour through the formation of a transitional “transverse bar and rip-rhythmic bar and beach,” and migration of the middle bar, with the morphology remaining in an arrested high-energy state during intervening low-energy periods. Although broadly conforming to the morphodynamic model, the beach did exhibit some distinct characteristics attributable to its fetch-limited location; limited progression through the morphodynamic model; and the importance of wind direction and magnitude in governing morphodynamic behaviour. Furthermore, rip currents were not significant in driving beach change through intermediate states. The presence of infragravity energy in the storm wave spectra; a dissipative, multibarred surf zone; dynamic inner and middle bars; and the attainment of a “transitional transverse bar and rip-rhythmic bar and beach” state during rising wave conditions, underline Seaford Beach as “bimodal”, exhibiting process and morphologic features of both higher- and lower-energy beaches. As an example of a beach in a strong-wind bay, Seaford, illustrates that not all fetch-limited beaches are low-energy. Furthermore, the presence of infragravity energy in a highly fetch-limited environment indicates that infragravity energy may occur commonly in fetch-limited environments that are subject to periodic strong winds; a process that has remained largely unrecognised.

Spatial patterns of the surf zone hyperbenthic fauna of Valdivia Bay (Ecuador)

The present research is a first attempt to study the hyperbenthic fauna of the surf zone of Valdivia Bay (South East Pacific--Ecuador), an exposed low tide terrace--rip beach. The aims of the study were to contribute to the inventory of the surf zone fauna of the tropical southeast Pacific coast, to investigate spatial distribution patterns along the bay and to search for possible tidal effects on the hyperbenthic composition. Samples were taken during daylight with a hyperbenthic sledge on 6 consecutive days in August 1999 at high tide, low tide and mid tide. The hyperbenthic community was characterized by quantifying the higher taxa, their density and the relative composition of the dominant groups. No tidal effect could be observed, but two main geographically distinct hyperbenthic assemblages were distinguished along the bay, related to hydrodynamic factors (local current velocity, intertidal beach slope and suspended particulate matter). In the turbulent northern part of the bay the highest densities were encountered and hyperbenthos was, next to the dominant mysid Metamysidopsissp., mainly composed of planktonic organisms (calanoid copepods, chaetognaths and fish eggs). In the calmer southern part of the bay Metamysidopsissp. dominated as well, but the community was completed with other mysid species, and early life stages of Brachyura and fish. Most striking was the general dominance of the mysid Metamysidopsis sp. in all the samples (97%, max. density of 62000 ind. 100 m-2) and the high amount of unknown species. Penaeid shrimp larvae, important for the local shrimp industry, were only found in very low densities during the sampling period (≈1%). Compared to other studies on the whole surf zone hyperbenthic community of sandy beaches, the average densities encountered in Valdivia Bay can be considered to be relatively high (2500--6000 ind. 100 m-2). Although limited in time and performed with a limited taxonomic resolution, the results indicate a high contribution of early life stages of invertebrate and fish species to the biodiversity of the intertidal hyperbenthic community in comparison with temperate regions.

Morphodynamics of intermediate beaches: a video imaging and numerical modelling study

Four years of time-exposure images of Palm Beach, Sydney, Australia, are used to: (a) develop a phenomenological scheme based on surf zone properties for the classification of the four intermediate beach states [low tide terrace (LTT), transverse bar rip (TBR), rhythmic bar beach (RBB), and longshore bar trough (LBT)] identified by Wright, L.D., Short, A.D., 1984. Morphodynamic Variability of Surf Zones and Beaches. Marine Geology, 56 93-118; and (b) determine environmental conditions associated with the different beach states. The overall agreement between beach state classifications obtained by visual classifications of images and those predicted by the phenomenological scheme exceeds 90%. Analysis of time series of intermediate beach states and wave data indicates that the nearshore morphology continuously cycles through the four beach states in response to variations in the incident wave energy. The results also indicate that LBT morphology is produced by high-energy wave conditions (erosive), while the progression towards LTT occurs under lower waves (accretive). A depth-averaged morphodynamic model is employed to determine the morphodynamic processes governing the RBB to TBR and TBR to LTT transitions. Model results indicate that the RBB to TBR transition is governed by the erosion of the original longshore bars (in the initial RBB morphology) and the formation and subsequent welding of shoals in the lee of the original longshore bars. Model results also indicate that RBB to TBR transitions can occur under shore normal and oblique wave conditions. The TBR to LTT model simulations indicate that, under obliquely incident waves, progressive erosion of the transverse shoals and deposition of the eroded sediment in rip channels gradually diminishes the longshore variability in morphology, resulting in an LTT morphology.

Coastal and nearshore morphology, bedforms and sediment transport pathways at Teignmouth (UK)

The beach and nearshore area of Teignmouth (UK) has been the object of a multidisciplinary research project, COAST3D, carried out by 11 European institutions. This paper emphasises the sedimentological and morphological investigations of the intertidal and nearshore area using a variety of techniques. From the results presented, it is clear that the Teignmouth site is very dynamic. Sediment mobility is high during fair-weather conditions owing to the strong jet-like current flows associated with the presence of the narrow estuary mouth. The beach at Teignmouth can be categorised as composite with a dissipative low-tide terrace and a reflective upper beach. From surficial sediment sampling, selective transport processes were identified, imposed by the high current velocities at the estuary mouth. This process also leads to the formation of characteristic depositional lobes with poorly sorted sediment. From digital side-scan sonar mosaicing, cross-shore and longshore-oriented bedforms could be observed, with their characteristics being dependent on the pre-survey hydro-meteo conditions. Although the nearshore system is dominated by the jet-outflow, littoral drift processes can also be important. They proved to be important for beach morphological changes leading to a rythmic alongshore beach behaviour which was attributed to a reversal of the littoral drift imposed by a period of larger waves. Moreover, the influence of waves and associated longshore currents cannot be neglected as they form the link between different sedimentary environments. From combined side-scan sonar imagery and sediment trend analysis, residual sediment transport directions were identified. These showed that fair-weather and storm-dominated processes are needed to explain the textural and morphological differentiation at this site. Further analysis will be required to identify the different time-scales involved in producing these changes.

Dynamics of a reflective beach with a low tide terrace

Many beaches are characterised by having steep upper slopes, flat low-tide terraces, and mixed or bimodal sediment distributions. This study compares the surf zone hydrodynamic and sediment transport processes of these two dynamically different regimes on the beach at Teignmouth (UK) as part of the COAST3D project. In-situ measurements were made using pressure transducers, electromagnetic current meters and optical backscatter sensors simultaneously on the steep part of the beach (slope 1/10, D 50 1.1 mm) and on the terrace (slope 1/100, D 50 0.25 mm) during a range of incident wave conditions (H b 0.25–0.75 m). The steep upper beach was characterised by: plunging breakers, high wave height/water depth ( γ) values, large gravity band cross-shore velocity variance, a well-developed subharmonic, strong undertow, high suspended sediment concentrations, and sediment transport dominated by a strong mean offshore component. The shallow shelving terrace was characterised by spilling breakers, low γ values, weak undertow, gravity velocity variance decreasing shoreward, infragravity cross-shore velocity variance increasing shoreward, and stronger longshore currents than undertow. Mean suspended sediment concentrations on the terrace were an order of magnitude less than on the steep beach. The oscillatory cross-shore components of transport were onshore in the incident wave band but increasingly dominated by the infragravity band in progressively shallower water. Longshore sediment transport rates on the terrace were large compared to cross-shore transport rates. The measurements suggest that the dynamics of the terrace/steep beach exhibit characteristics of separate dissipative/reflective sites. However, interaction between the different regions does occur. In particular, the low tide terrace acts to reduce the wave height that is capable of reaching the steep beach, thereby protecting it to some degree from erosion.

Morphodynamics of a mesotidal, exposed, low tide terrace beach (Faro, southern Portugal)

A field assessment of microtopographic changes and disturbance depth was carried out during a tidal cycle at Faro beach, southern Portugal. This mesotidal beach presents a well developed low tide terrace, wide and flat (tan ß = 0.02), and an important (tan ß = 0.11) foreshore slope. Erosive processes, related to sea waves, produced a beach profile rotation around mean sea level. Disturbance depth presented great variations according to beach morphology and sedimentology as well as wave characteristics, factors that controlled breaking wave type. Plunging breakers prevailed in the upper and middle foreshore, greatly affecting bottom sediments that recorded disturbance values of 25% of significant breaking wave height. Spilling breakers prevailed in the low tide terrace, with average disturbance values of about 4%. Lastly, values of disturbance depth obtained with two different methods, rods and plugs or rods with loose-fitting washers, were compared. The second method gave higher values of disturbance with coarser sediments.

Textural and compositional characterization of recent sediments along a paraglacial estuarine coastline, Maine, USA

Textural and mineralogical characteristics of bottom sediments collected from the lower Kennebec River estuary, Maine, and adjacent nearshore region were used to differentiate subtidal sedimentary environments. Five depositional environments, defined on the basis of bottom morphology, bathymetry, and dominant sedimentary processes—estuarine channel, channel margin, outer bar, shoreface, and offshore—possess distinct sedimentological characteristics. Trends in textural characteristics of these environments and those along the adjacent barrier beach include: (1) systematic decrease in mean grain size from the estuarine channel to the offshore facies; (2) similarity between the outer bar and estuarine channel sands; (3) increase in sediment sorting from the channel/margin and offshore to shoreface facies; and (4) fining of the low-tide terrace, beachface, and berm samples away from the bar-welding region. The fine-grained nature of the offshore facies is the result of deposition of suspended sediment delivered to the coast during large flood events, whereas the coarse fraction is contributed through the attrition of carbonate tests of the local macrofauna. Results of compositional analyses reveal: (1) a decrease in shell fragments from the estuarine channel (>50%) to the offshore (<10%); (2) a three-fold increase in mineralogical maturity from channel and outer bar to offshore sands; and (3) increase in mineralogical maturity of the beachface and low-tide terrace samples away from the bar-welding region. The similarity between channel-derived bar sands and those sampled from the bar-welding region of the adjacent barrier beach suggests that the sedimentological signature of subtidal environments is temporarily preserved. The wide range of textural and compositional attributes of sediments in paraglacial settings makes these environments ideal places in which to study the variations in sediment transport mechanisms over relatively short distances. We demonstrate the usefulness of primary physical characteristics of bottom sediments in studying the patterns of fluvial–marine interaction along other physiographically similar coastal settings.

Fish Communities in the Surf Zone of a Protected Sandy Beach at Doigahama, Yamaguchi Prefecture, Japan

Fish communities comprising postlarval to early adult stages were studied in the surf zone of a protected sandy beach, with runnels and a low tide terrace, at Doigahama, Yamaguchi Prefecture, western Japan. One elasmobranch and 100 teleost species, represented by a total of 17608 individuals (1·85–785mm in TL), were collected by fine and coarse meshed beach seines from May 1994 to April 1999. Species richness, abundance and biomass were greater during the evenings as well as in the warmer seasons (May to October). Larval and smaller juvenile ichthyofauna (collected mainly by fine-mesh seine) was relatively poor compared to larger juvenile ichthyofauna (coarse-mesh seine). Tidal effect was observed in the fine-mesh seine samples, whereas no tide-related trends were evident in the coarse-mesh seine samples. Effects of time of the day and tide on the species diversities were not so evident in both seines. Dominant species were classified according to the developmental stages occurring in the surf zone, as follows: Type-I: Postlarval (transformation) stage (Plecoglossus altivelis altivelis,Sardinops melanostictus , Enneapterygius etheostomus, Tripterygion bapturum, Luciogobius sp.). Type-II: Juvenile stage (Lateolabrax latus, Trachurus japonicus, Gerres oyena, Acanthopagrus schlegeli, Sparus sarba, Girella punctata, Mugil cephalus cephalus, Paralichthys olivaceus, Tarphops oligolepis, Heteromycteris japonica). Type-III: Postlarval and juvenile stages (Spratelloides gracilis, Engraulis japonicus, Gobiidae sp.1). Type-IV: Juvenile and early adult stages (Sillago japonica, Paraplagusia japonica). Type V: Postlarval to early adult stages (Takifugu niphobles). It is considered that the surf zone at Doigahama functions as a nursery area, particularly as a feeding place for larger juveniles than as a shelter for larvae and smaller juveniles.

Flow Kinematics of Low-energy Rip Current Systems

Measurements of hydrodynamics and topography were obtained during two separate field experiments at Palm Beach, NSW, Australia in order to assess flow characteristics within rip current systems during accretionary conditions. At an evolutionary time scale (days-weeks), feeder channel flow decreased and rip-neck flow velocity increased during a cycle of intermediate beach state evolution. At shorter time scales (hours), rip current velocity is inversely related to changes in water depth and is clearly modulated by the tide. Spatially, rip current velocity increases progressively both longshore from the feeders to the rip-neck and offshore within the rip-neck itself. Flow velocity is not constant across the rip channel and increases towards the middle and deeper section of the rip channel. Vertical velocity profiles within the rip-neck and feeder channels show an initial increase in flow magnitude above the bed, with maxima towards the middle of the flow and then a subsequent decrease towards the surface. Evidence of pulsatory rip flow behaviour was found at infragravity frequencies in both rip-feeder and rip-neck environments. Infragravity energy was dominant in the feeder channels during the observed beach state evolution and increased in the rip-neck as the system evolved towards transverse bar and low tide terrace and rip beach states. There is evidence to suggest that topography may exert direct controls on rip flow. The results of this study provide both new quantitative information on the flow kinematics of rip currents and confirm findings of previous studies.

Morphometric approaches to describing rip current behaviour

Rip currents are an integral component of intermediate beaches as described by the WRIGHT and SHORT (1984) model, but their morphologic character and behaviour at various states has yet to be suitably quantified. This study utilises field measurements encompassing an almost complete cycle of low-energy beach state evolution to describe morphometric approaches for quantifying rip current behaviour. First, a subjective analyses of the relative planimetric areal extent of rip feeder, rip-neck and non-rip (bar) components showed that active rip areas (AR) were never greater than 50% of the total system area (AT) with AR decreasing from 40-45% during the longshore bar-trough (LBT) state to less than 20% during the low tide terrace (LTT) state. Second, the concept of overfit and underfit rip currents (SHORT, 1985) is examined by describing topographically confined and unconfined flow conditions through the use of the Rip Fit Parameter; RF = (M-O)/Ar, where M = morphologic cross-sectional rip channel area; O = overbank cross-sectional rip area, and Ar = total cross-sectional rip area. In general, low-energy rips are unconfined at high tide for all intermediate states and are confined when RF > 0.5 during transverse bar-rip and low-tide terrace conditions at low-tide. Confined rips are also associated with maximum rip flow velocities and can be related to planimetric rip area. Both morphometric approaches provide new and useful quantification of intermediate beach state rip current morphology during accretionary conditions.

Changes in Beach Water Table Elevation during Neap and Spring Tides on a Sandy Estuarine Beach, Delaware Bay, New Jersey

A field investigation of temporal and spatial changes in wind and wave characteristics, runup and beach water table elevation was conducted on the foreshore of an estuarine beach in Delaware Bay during neap (April 9, 1995) and spring (April 16, 1995) tides under low wave-energy conditions. The beach has a relatively steep, sandy foreshore and semi-diurnal tides with a mean range of 1.6 m and a mean spring range of 1.9 m. Data from a pressure transducer placed on the low tide terrace reveal a rate of rise and fall of the water level on April 16 of 0.09 mm s−1 resulting in a steeper tidal curve than the neap tide on April 9. Data from three pressure transducers placed in wells in the intertidal foreshore reveal that the landward slope of the water table during the rising neap tide was lower than the slope during spring tide, and there was a slower rate of fall of the beach water table relative to the fall of the tide. Wave heights were lower on April 9 (significant height from 17.1 min records <0.16 m). The water table elevation was 0.08 m higher than the water in the bay at the time of high water, when maximum runup elevation was 0.29 m above high water and maximum runup width was 2.0 m. The elevation of the water table was 0.13 m higher than the maximum elevation of water level in the bay 74 min after high water, when wave height was 0.12 m and wave period was 2.7 s. The use of mean bay water level at high tide will underpredict the elevation of the water table in the beach, and demarcation of biological sampling stations across the intertidal profile based on mean tide conditions will not accurately reflect the water content of the sandy beach matrix.

Spatial and temporal variations in debris accumulation and composition on an estuarine shoreline, Cliffwood Beach, New Jersey, USA

A twelve month field investigation documents differences in cross shore accumulation and composition of debris on the dune, backbeach, and foreshore of a sandy estuarine shoreline. Debris was gathered monthly within 3 belt transects extending from the dune crest to the low tide terrace (base of the foreshore). Individual pieces of debris were classified by type, function, degree of fragmentation, length, weight, and location on the beach profile (dune, vegetated backbeach, backbeach and foreshore). Plastic comprised 42.5% and glass 29.3% of the 5793 pieces of debris that were collected. Debris was primarily consumer and household related items, and the greatest quantities of debris were found during beach user months. The cross shore spatial distribution of debris reflects the influence of wind or wave processes across the profile with small lightweight debris on the wind-dominated upper profile and heavier debris on the wave-dominated lower profile. A chi-square test revealed that more plastic was observed than expected in the dune and backbeach areas with the greatest quantities found after onshore winds. More glass was observed than expected on the foreshore where low wave energies prevent transport higher on the beach profile.

Field observations of sand-mixing depths on steep beaches

A series of field studies were carried out on three Portuguese beaches (Garra˜o, Faro and Culatra) to assess sand-mixing depths under a range of wave heights (0.34–0.80 m), wave periods (5.1–7.0 s) and mean grain sizes (0.26 and 0.38 mm). All the studied beaches had a reflective profile with a steep upper foreshore (tanβ of 0.10–0.14) and a more gentle low-tide terrace. In all experiments, plunging waves were breaking on the beach face. The study has identified a linear correlation between significant wave height at breaking ( H b), and average sand-mixing depth ( Z m), whereby Z m = 0.27H b. The empirical relationship is ten times larger than a previous one proposed by other authors working on gentle-slope dissipative beaches, and confirms previous findings on similar reflective beaches carried out in the USA. It was also confirmed that Z m is related to wave period, but does not seem to be a function of mean sand size.

COMPARATIVE PHOTOGRAPHS OF THE METULA SPILL SITE, 21 YEARS LATER

ABSTRACT The area subjected to heavy oiling from the 1974 Metula spill of 53,500 tons along the south side of the First Narrows in the eastern Strait of Magellan (Chile) was revisited in December 1995. The marsh/tidal flat at Espora marsh shows some regrowth of vegetation, but oiled areas remain common with 5 to 15 cm of liquid oil present below a thin layer of fine sediment. The asphalt pavement in the sheltered area behind Espora split shows only a minor (≈2 m) reduction in extent. The previous band of asphalt pavement on the exposed, gravel-dominated low-tide terrace along the First Narrows of the strait has mostly been eroded, but disconnected remnants still remain. The Metula spill site continues to provide a field laboratory to study the long-term persistence of spilled oil.

Morphodynamics of meso- and macrotidal beaches: examples from central Queensland, Australia

Detailed measurements of the morphology, waves, and longshore and cross-shore currents were conducted on two beaches on the low- to medium-energy, macrotidal coastline of central Queensland (Australia). Nine Mile Beach is characterised by a relatively steep upper profile, a low-gradient intertidal zone with one or two swash bars, transverse bar/rip morphology at low-tide level and a very low-gradient, dissipative subtidal zone. The beach is composed of fine to medium sand and experiences a modal wave height of 0.75 m and a mean spring-tide range of 3.6 m. Lambert's Beach possesses a steep, coarse-grained, reflective upper profile, and a low-gradient dissipative low tide terrace composed of fine to medium sand. Subdued rip channels intersect the low tide terrace and the beach is subject to a modal wave height of 0.6 m and a mean spring-tide range of 4.6 m. The two beaches are classified as Low Tide Bar/Rip and Low Tide Terrace beaches, respectively, following the model of Masselink and Short (1993). During the present investigation, the Low Tide Bar/Rip beach (Nine Mile Beach) exhibited a larger temporal variability than the Low Tide Terrace beach (Lambert's Beach), despite similar hydrodynamic conditions over the survey period. This difference is principally attributed to the differences in the beach morphologies and the types of morphological changes that occurred. On Nine Mile Beach, morphologic changes were primarily associated with secondary features (swash bars and low tide bar/rip system), whereas on Lambert's Beach the changes in morphology were largely the result of adjustments of the overall beach profile. A number of surf zone processes were investigated, including incident-wave attenuation, infragravity-wave energy, bed return flow and longshore currents. The influence of the tide on surf zone dynamics was found to be insignificant, except in the case of longshore currents. On Nine Mile Beach, the longshore current associated with the nearshore cell circulation was stronger during low tide. On Lambert's Beach, the shore-parallel current on the low tide terrace was largely driven by the tide. A beach profile zonation was defined for Nine Mile Beach and Lambert's Beach based on an examination of the varying influence of swash, surf zone and shoaling wave processes. The four-part tidal zonation consists of: (1) an upper intertidal zone across which swash and aeolian processes are the major influence; (2) a mid-intertidal zone dominated by swash and surf zone processes; (3) a lower intertidal zone controlled by surf zone and shoaling wave processes; and (4) a subtidal zone largely influenced by shoaling wave processes.

The mobility of beach fill in front of a seawall on an estuarine shoreline, Cliffwood Beach, New Jersey, USA

A field investigation was conducted to identify beach change associated with emplacement of a stone seawall and beach fill designed to protect an eroding cliff near the west end of a meso-tidal estuary. Visual observations of wave processes reveal a mean breaking wave height of 0·2 m at the most exposed monitoring site, a storm-wave height of 0·43 m and a maximum longshore current velocity of 0·55 ms −1. Maximum net erosion at any one monitoring site over 44 months was 26·3 m 3 m −1 of beach, resulting in a rate of erosion of 2·3 m year −1. Up to 6·8 m 3 m −1 of eolian accretion occurred due to the overly wide nourished beach. Local differences in shoreline orientation, sheltering by headlands, position of the seawall on the beach profile, shore-perpendicular structures and sediment availability resulted in variability in beach processes and changes in beach profile. Flanking occurred at the southeast end of the seawall, where fill material bypassed the upper foreshore and moved along the low tide terrace, favored by a pronounced break in shoreline orientation. Net erosion did not occur at the west end of the seawall because sufficient fill sediment was available from updrift sources. Where no fill remained in front of the seawall, wave energy dissipation on the outer low-tide terrace at low tide and wave reflection off the structure without breaking at high tide diminished the effectiveness of wave-energy concentration and turbulence on the low-tide terrace at the base of the wall, and there was no net scour. Shore-parallel structures constructed on the low-tide terrace on meso-tidal estuarine shorelines may not require a fronting protective beach to prevent net toe scour, but nourishment may delay flanking problems on the upper foreshore at downdrift sites.

Distribution of Surface Pebbles with Changes in Wave Energy on a Sandy Estuarine Beach

ABSTRACT A 29-day field experiment in Delaware Bay, NJ identified the dispersion of surface pebbles due to cross-shore transport on a mesotidal estuarine beach composed mainly of sand. Wave heights were 0.04-0.56 m. Dissipation of wave energy on the low-tide terrace increases as water levels become lower, creating a spatial gradient in wave energy delivered to the foreshore. Analysis of surface sediment samples taken daily at 4 m intervals across the foreshore and two experiments using dyed pebbles as tracers document increasing quantities of pebbles with distance downslope on the beach, corresponding to the decreasing energy gradient. There is a second locus of high pebble concentration near the upper limit of swash at high water. Numbers of surface pebbles are at a minimum following high-energy events (wind speed > 8.0 m/s, wave height > 0.3 m); the most conspicuous pebble accumulations result from low-energy conditions (wind speed < 3.0 m/s, wave height < 0.2 m). Pebbles move up and down the foreshore within the beach step. They accumulate just above the low-tide terrace on the falling tide when wave energy diminishes at low water under both storm and nonstorm conditions. Finer particles are moved onshore from this location by low-energy post-storm accretional waves and offshore by outflow from the beach water table, leaving pebbles as a surface lag. A fraction of these pebbles is moved up the beach by the swash of low-energy waves or within the beach step in subsequent tidal cycles. There is insu ficient sand in the backwash of low-energy waves to bury pebbles; they project into the flow of the swash and have low pivoting angles, increasing the probability of entrainment and movement over the sand particles to the upper limit of swash.

The morphodynamic effects of incident wave groupiness and tide range on an energetic beach

The roles played by daily tide range and incident wave groupiness in affecting the short-term morphodynamic responses of an energetic beach are examined. Demodulation of original tide and wave time series from the moderate energy Narrabeen Beach, Australia, resulted in new time series of daily tide range and a daily grouping factor which expresses the relative amplitude of the alternation between groups of high and low waves. Using these new time series together with time series of daily beach state and changing breaker conditions, statistical analyses were performed aimed at determining the contributions made by tide and groupiness in explaining time-varying beach state additional to those made by time-varying wave conditions. Eigenvector analyses of monthly surveyed beach and surf zone profiles provided a set of time series on modes of profile configuration change which were also analyzed to determine the relative effects of waves and tide range on profile shape. Both tide range and groupiness are significant determinants of beach state. The six beach states are reflective, low-tide terrace, transverse bar and rip, rhythmic bar and beach, longshore bar trough, and dissipative (Wright and Short, 1984). Provided that wave conditions are within the appropriate range to favor beach states at the reflective end of the sequence, spring tides will tend to favor the low-tide terrace over the transverse bar and rip state. Higher tide ranges are also associated with more subdued bar-trough topography; the more accentuated bar-trough profiles are developed when tide range is minimal. Overall, higher relative incident wave groupiness favors the more dissipative states. Incident wave groupiness is the dominant factor determining whether the low-tide terrace state or transverse bar and rip state prevails, provided incident wave steepnesses permit one or the other of these states. Groupiness is also important in discriminating between the transverse bar and rip and rhythmic bar and beach states.

THE BEHAVIOR AND MIGRATION OF LONGSHORE BARS

The behavior and migration of longshore bars and troughs are investigated over 128 consecutive summer days on the Ventura coast, California. Elevation and sediment data were collected daily at 7.5 m intervals along two 150 m long profiles extending seaward from the berm. Environmental data were also obtained. Two behavioral forms are recognized: stationary bars and troughs that form, persist for several days, and then destruct in situ; and migratory bars and troughs that move landward and diminish to ridges and runnels before welding onto the beach face as low tide terraces. Stationary bars are symmetric or asymmetric in cross-profile and strongly identified with the breaker zone. Migratory bars are mostly asymmetric as net landward sediment flux crosses the bar crest. Controlling mechanisms are complex, different forces producing different bar responses. Plunging breakers can both create and destroy bars. Migratory bars may occur singly or in sets moving landward over several days to a few weeks, migrati...