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Distribution of Surface Pebbles with Changes in Wave Energy on a Sandy Estuarine Beach
Abstract
In Delaware Bay, NJ, 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. 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. There is sufficient 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. -from Authors
... The beaches studied in Paraty exhibit great differences in grain size and sorting, as also observed on several beaches of Ilha Grande (Silva et al., 2020) and Guanabara Bay (Silva et al., 2016). The expressive presence of coarse sediments (coarse sand, very coarse sand, and gravel) on sheltered beaches within bays is commonly reported in this type of environment (Nordstrom, 1977;Nordstrom and Jackson, 1993). In a bay, the sediment distribution pattern is marked by a decrease in grain size towards the interior of the bay in response to a decrease in hydrodynamic energy. ...
... It can be observed that, in general, the beaches present a slight increase in granulometry towards the submerged part, with the exception of the northern portion of Jabaquara Beach (Profile C), where there is a marked presence of mud on the submerged beach due to proximity with the mangrove. For Nordstrom and Jackson (1993), this gradual increase in the sediment size from the emerged to the submerged portion of the beach occurs because the coarser fractions accumulate more easily in the underwater part of the beach. The finer sands are transported more easily by the low-energy waves than gravel, leaving the coarser fractions, which tend to accumulate in this region (Nordstrom and Jackson, 1993). ...
... For Nordstrom and Jackson (1993), this gradual increase in the sediment size from the emerged to the submerged portion of the beach occurs because the coarser fractions accumulate more easily in the underwater part of the beach. The finer sands are transported more easily by the low-energy waves than gravel, leaving the coarser fractions, which tend to accumulate in this region (Nordstrom and Jackson, 1993). ...
This study aimed to characterize the dynamics of sheltered beaches in the Paraty area and their susceptibility to storm events. The adopted methodology consisted of 40 beach topographic profiles, collection of 38 samples for granulometric and morphoscopy analysis, and calculation of the emerged volume of sediments. The sheltered beaches in Paraty have low-energy waves, which reach about 0.5 m under stormy conditions. These beaches have different levels of exposure to storm events, which is directly associated with the direction of wave incidence, bay entrance orientation, and the presence of islands. There are records of wave transposition, damage to urban structures, and erosion, which point to the susceptibility of this coast to storm events. Washover deposits were observed at São Gonçalo, Taquari, and Prainha. On Jabaquara, there is erosion, with a reduction in the sediment volume at the southern end of the beach, with concomitant destruction of buildings and fallen trees. The erosion process in Jabaquara is associated with the low sediment exchange; storm waves, responsible for removing sediment from the beaches, without subsequent replacement (post-storm); and the interventions carried out in the area, which may have altered the local hydrodynamics. The sediments of these beaches vary widely in size, from gravel and sands to mud in areas near mangroves, increasing from the backshore to the nearshore zone. The quartz grains of the sands are in general sub-angular, showing the low maturity of these materials, which is common on coasts subject to the offer of sediments from local fluvial influence.
... Sediment that forms estuarine beaches may be predominantly sand with a smaller gravel fraction ( Nordstrom, 1977;Kennedy, 2002;Freire et al., 2007), mixed sand and gravel ( Curtiss et al., 2009) or predominantly gravel ( Isla & Bujalesky, 2004). The gravel fraction (according to the Wentworth scale) may consist predominantly of granules and pebbles ( Nordstrom & Jackson, 1993) or pebbles and cobbles ( Curtiss et al., 2009). Gravel is a key diagnostic feature on predominantly sandy estuarine beaches, and its presence and distribution are important for the interpretation of palaeodeposits ( Nordstrom & Jackson, 1993) and for assessment of the viability of habitat for biota that utilize the estuarine foreshore ( Wieser, 1959;Rice, 2006;Jackson et al., 2008;Dethier et al., 2010). ...
... The gravel fraction (according to the Wentworth scale) may consist predominantly of granules and pebbles ( Nordstrom & Jackson, 1993) or pebbles and cobbles ( Curtiss et al., 2009). Gravel is a key diagnostic feature on predominantly sandy estuarine beaches, and its presence and distribution are important for the interpretation of palaeodeposits ( Nordstrom & Jackson, 1993) and for assessment of the viability of habitat for biota that utilize the estuarine foreshore ( Wieser, 1959;Rice, 2006;Jackson et al., 2008;Dethier et al., 2010). The purpose of this paper is: (i) to identify changes in the textural properties of sediment in transport in the uprush and backwash of individual swash events on an estuarine beach; (ii) to relate these changes to tidal stage; and (iii) to provide an explanation for the distribution of gravel on the foreshore. ...
... A principal zone of sediment reworking is the step, which represents the zone of interaction between the breaking waves and the backwash of the swash and the location of greatest turbulence, characterized by relatively coarse grain sizes ( Miller & Zeigler, 1958;Strahler, 1966). The plunging waves on estuarine beaches cause entrainment of coarser and finer grain sizes that may be delivered to the swash zone ( Nordstrom & Jackson, 1993). Under relatively constant wind conditions, breaker and swash energies are at a minimum at low tide, when wave energies are dissipated by breaking on the low tide terrace, and at a maximum near time of high tide, when waves pass over the low tide terrace and break on the foreshore ( Nordstrom, 1992). ...
Cross‐shore grading of sediment has been observed on the surface of estuarine beaches but the swash zone processes responsible for this grading have not been measured. This study was conducted to provide an explanation for the cross‐shore grading of sediment on a predominantly sandy estuarine foreshore. Data on wave and swash characteristics and sediment trapped in the uprush and backwash during 25 swash events were gathered from mid‐rising to mid‐falling tide on a small transgressive barrier in Delaware Bay, New Jersey, USA. Sediment is predominantly quartz and feldspar, medium to coarse sands with a gravel fraction of granules and pebbles. Wave energies increased with tidal rise. The percentage of gravel in transport in the uprush and backwash is similar (11% and 13%) during the rising tide when the swash zone is at mid‐foreshore, decreases in the uprush (9%) and increases in the backwash (18%) when the swash zone is on the upper foreshore. When the swash zone is at mid‐foreshore on the falling tide, the quantity of gravel in the backwash (30%) is greater than in the uprush (24%). The low proportion of gravel within the foreshore prior to trapping, and the increase in the percentage of gravel when the waves and swash are on the upper foreshore, suggests that the step is the primary source of gravel high on the foreshore. The size of the step increases as wave heights increase with tidal rise. The rate of delivery of gravel into the swash is enhanced by sediment entrained during wave breaking and interaction of the uprush with the previous backwash. The lag in the rate of step migration relative to breaker migration during the falling tide increases the likelihood of mining gravel from the step and subsequent transport in the uprush and backwash. These findings are important for low energy estuarine beaches sensitive to small changes in tidal range and wave energy that cause sedimentological change across the foreshore.
... Field studies of beach profiles over a range of timescales are especially important in understanding beach responses (CALDWELL and WILLIAMS, 1985;SHERMAN, 1991;BRADBURY and POWELL, 1992;CHILD, 1992). However, as noted by many workers (e.g., SHERMAN, 1991;BRADBURY and POWELL, 1992;CARTER and ORFORD, 1993;SHULMEISTER and KIRK, 1993), there is a lack of such information on coarse clastic beaches in general, and more especially for mixed beaches (e.g., KIRK, 1980;NORDSTROM and JACKSON, 1993). Field studies documenting the spatial and temporal variability of grain size, and its effect on beach response, are potentially important for the calibration of engineering type beach models, where grain size effects are difficult to reproduce (HR WALLINGFORD, 1990). ...
... On managed mixed beaches it is important to assess the effect of sea walls, groynes, recharge, and rock armouring on beach responses. The responses of coarse clastic beaches, especially mixed beaches, are not well understood because of the relatively small amount of attention such beaches have received compared with sand-dominated systems (KIRK, 1980;CARTER and ORFORD, 1993;NORDSTROM and JACKSON, 1993;MC-KAY and TERICH, 1992). ...
... In this region the occurrence of sand and gravel together facilitate the seaward transport of gravel over the sand on the ebb tide. At this time the raised watertable augments backwash action (e.g., DUNCAN, 1964;SCHWARTZ, 1966;NORDSTROM and JACKSON, 1993) and a variety of bedforms can be formed, including rill marks, swash marks, part- ing lineations, obstacle marks, pebble clusters, and various lower foreshore gravel accumulations (PONTEE, 1995). The latter bedforms consist of localised accumulations of poorly sorted medium-coarse gravel with a variety of configurations. ...
This paper investigates the spatial and temporal variability of three different mixed sand and gravel beaches on the Suffolk coast, U.K. The beaches consist of a highly variable mixture of medium (−3.3 to −3.47 ), moderately sorted gravel (0.85 to 0.78 ), and 15 to 30% coarse (0.47 ) moderately sorted (0.96 ) sand. Above the high water mark beach sediments are predominantly gravel, whilst the sand fraction is concentrated in a planar region of the lower foreshore. The beaches studied exhibit some characteristics of pure gravel beaches, some of pure sand beaches and some which are unique to mixed beaches. Distinctive sedimentary features include a coarser sand fraction and finer gravel fraction, the existence of multi-modality within the gravel fraction, very limited shape sorting, and high spatial and temporal variability. Beach profile responses are similar to those of pure sand and pure gravel beaches, including those changes occurring over semi-diurnal, spring-neap, and seasonal timescales. Some storm responses were also similar, including the formation of beach scarps, large ridges, reduced lower foreshore levels and a wide planar beachface region. However, beach profiles can flatten or steepen under storm conditions depending on the presence of seawalls at the back of the beach. A conceptual model for beach accretion is developed which involves ridge formation on the upper foreshore and subsequent onshore migration to form beach ridges under high energy conditions. Post storm recovery is seen to rely on the accretion of these beach ridges. On the lower foreshore, accretion does not involve ridge accretion and occurs under lower energy conditions. The importance of sediment supply, beach width and nearshore water depth are also discussed.
... As a consequence, the study did not contribute as significantly to the understanding of differential transport as it otherwise might have. Interestingly, the data collected supported In addition to the major differential transport investigations which have been undertaken (eg Bray, 1996;Nicholls, 1985;Workman, 1997 andWright, 1982), various less extensive / exhaustive studies were carried out in the 1980s and 1990s (Matthews, 1980;Caldwell, 1983;Rosen and Brenninkmeyer, 1989;Tomlinson, 1993;Nordstrom and Jackson, 1993;Cooper et al., 1996;and Miller, 1997). Of the authors who identified differential longshore transport, the majority found that particle size was an influence and that large particles moved faster than small. ...
... In the cross-shore orientation, the high density ironstone was transported preferentially onto the lower foreshore in the backswash, while in the vertical orientation, the low density coal tended to settle out of suspension after the other material and hence was found at the beach surface. Differential cross-shore transport, according to particle 16 Differential Sediment Transport size, was identified by both Caldwell (1983) Nordstrom and Jackson (1993) identified neither size nor shape sorting in the cross-shore orientation during their study. ...
p>Three aspects of coarse sediment movement of importance in coastal protection are: differential transport; vertical mixing and the rate of longshore transport. Within the present contribution state of the art tracer methods are used to improve understanding in these key subject areas.
The analyses conducted show that longshore travel distance is intermittently related to particle size and position (cross-shore and vertical) on a beach. Particle size and shape are intermittently related to cross-shore transport, and size, shape and position (cross-shore) are related to vertical movement. The strongest evidence, to date is presented for the existence of secondary sorting. No clear evidence is found that factors are preferentially correlated with cumulative distance travelled relative to distance travelled per tide.
The first measurements of sediment mixing depth on an engineered beach show that the thickness of the moving sediment is related to significant wave height, as is the case for open beaches. The relationship identified actually agrees very closely with that for the adjacent open beach, and with that for an open beach at another site (Charmouth, UK) which has a broadly similar grain size.
Analysis of longshore transport rate data, indicates that drift efficiency (K) does not increase in value with increasing wave energy. The K values presented are, however, the highest found to date. The use of high quality wave and tracer data and/or inadequate tracer mixing are identified as possible causes of the unusual results. The first quantitative investigation of the degree of tracer incorporation confirms that mixing with the indigenous material was not complete and hence is a source of error. Longshore transport rate measurements are compared for open and engineered sections of the same beach. Drift efficiency is found to be higher on the open beach (K=0.853) than the engineered beach (K=0.534).
A review of tracer theory is undertaken. Literature on the subject is found to be limited and heavily reliant on models, nevertheless, it casts doubt on the validity of the methods used in all tracer based determination of longshore (shingle) transport to date - a fact not recognised or discussed by previous authors.</p
... Post-storm changes at Procchio and Lacona eventually resulted in a conspicuously smaller pebble fraction in surface sediments but no clear fining of the sand and granule fraction across all sampling locations. The number and distribution of gravel clasts can indicate changes in process conditions on a beach (Carter, 1988;Nordstrom and Jackson, 1993;Ciavola and Castiglione, 2009;Saini et al., 2012). Our results indicate that pebbles may be a useful diagnostic variable to distinguish flood inputs. ...
... Under natural conditions, the proportion of pebbles on the surface of a beach depend on the amount of mixing in the swash zone. With greater mixing, pebbles are likely to be buried within a predominately sandy matrix rather than remaining as a surface lag (Nordstrom and Jackson, 1993). Through time, the beaches at Elba become better mixed, resulting in fewer surface pebbles. ...
The significance of sediment delivered via storm-associated stream discharge in altering sediment characteristics, beach form, and volume is evaluated on pocket beaches with different basin characteristics and wave exposures. The focus is on changes on three beaches on Elba Island, Italy caused by a flood event in September 2002 that had an estimated recurrence interval of 200 years. Beach profiles and foreshore sediment samples were gathered in 1999 and 2000 to identify pre-storm characteristics, in September 2002 to reveal the immediate effects of the storm, and in 2003 and 2004 to reveal post-storm recovery.
... Pebbles and shells are often conspicuous on predominantly sandy low energy beaches (Ekwurzel, 1990;Nordstrom and Jackson, 1993). Pebbles are often found on the backshores of high energy beaches as lag features following aeolian deflation. ...
... Pebbles on the upper foreshores of low energy beaches are more likely to be stranded particles that are exhumed and preferentially moved by low energy waves than aeolian lag features. Pebbles low on the foreshore are more likely to be wave-related lag features (Nordstrom and Jackson, 1993). Shell particles are common in wrack lines on both high and low energy beaches. ...
This review was undertaken to identify locations where low energy beaches may occur and their diagnostic forms and process controls, including waves, tides and water levels. Examples are drawn from the sheltered coastline of Western Australia near Perth and fetch-limited estuarine environments on the northeast coast of the United States. We suggest that the term low energy be used in locations where: (1) non-storm significant wave heights are minimal (e.g. < 0.25 m); (2) significant wave heights during strong onshore winds are low (e.g. < 0.50 m); (3) beachface widths are narrow (e.g. < 20 m in microtidal environments); and (4) morphologic features include those inherited from higher energy events. Micro-topographic features can persist in the swash zone of low energy beaches under non-storm wave conditions. There is little evidence of cyclic cross-shore sediment exchange. Bars, excepting transverse forms, located seaward of low still-water level do not appear to be part of the sediment exchange system with the foreshore. Developing a better definition of the term low energy requires understanding the occurrence and duration of morphological characteristics and the type, magnitude and frequency of hydrodynamic controls that are responsible for these characteristics. Efforts also should be directed toward: (1) discriminating between processes generated within basins (in true fetch-limited environments) and processes generated outside basins (that affect sheltered environments); (2) identifying the relative contributions of tide- and surge-related water level fluctuations on low energy beach shape; and (3) estimating thresholds for beach change.
... The inverse correlation between mean grain size and bottom stress is not surprising, given the inverse relationship between grain size and wave energy revealed in surface sediments and sediments in transport in the swash on sandy low energy beaches (Nordstrom and Jackson 1993). Relatively high-energy waves and currents on these beaches can mobilize all size fractions, from fine sand to pebbles, whereas velocities near the threshold of movement mobilize only the surface layers that tend to retain a great proportion of coarse lag ma- Fig. 3. Relationship between mean longshore shear stress, mean cross shore shear stress, mean total shear stress and quantity of sediment trapped. ...
... Relatively high-energy waves and currents on these beaches can mobilize all size fractions, from fine sand to pebbles, whereas velocities near the threshold of movement mobilize only the surface layers that tend to retain a great proportion of coarse lag ma- Fig. 3. Relationship between mean longshore shear stress, mean cross shore shear stress, mean total shear stress and quantity of sediment trapped. Crosses point to data on April 17, triangles to data on April 18. (Nordstrom and Jackson 1993). Much of the sediment beneath the surface of the bar and in the basal till at the study site is fine grained but little of this sediment is reworked under low energy conditions. ...
This study was conducted to relate the cross-shore distribution of longshore sediment transport and grain size characteristics
to cross-shore and longshore current velocities on a sandy low-energy beach in a non-tidal embayment of the Baltic Sea. Simultaneous
measurements of current velocities and amount of sand caught in streamer traps were made on 31 sampling runs on 6 d in April
1999 at three fixed sites including the swash zone on the upper foreshore, the lower foreshore, and the crest of the most
landward of four bars. Spilling waves broke frequently on the bar but rarely on the lower foreshore, even during onshore wind
speeds up to 11.0 m s−1. Waves always broke as plunging waves at the step at the base of the upper foreshore and were converted directly into swash.
The greatest longshore current velocities in the swash occurred when wind speeds and water levels were greatest, but wind
direction was nearly directly onshore. Longshore velocities were greater in the swash zone than at other sites except when
relatively strong winds blew nearly parallel to the shoreline, causing a pronounced wind-induced current at the other two
sites. Calculated longshore shear stress and rate of sediment trapped were highly correlated on the bar (r=0.90), less highly
correlated in the swash zone, and least highly correlated (r=0.66) on the lower foreshore. Mean trapping rates in the swash
were 14.6 times greater than on the lower foreshore and 7.2 times greater than on the bar. Greater trapping rates in the swash
are attributed to the greater turbulence mobilizing sediments in the uprush and backwash. Little of the finer-grained sediment
on the offshore sites was reworked under low energy conditions. The study reveals the dominance of swash transport on steep,
reflective, low-energy beaches where wave energy dissipation takes place over small distances on the upper foreshore.
... The unconsolidated gravel layer varies from 0.5 m to as thin as a single grain thickness of gravel armor on the lower foreshore. A unique feature of the beach is the median size of the gravel, which increases with decreasing elevation on the beach as also observed by Nordstrom and Jackson (1993) on a low energy estuarine beach. The cross-shore sorting is related to the relationship between swash energy, tide, and gravity/slope effects. ...
... ment location and were quickly transported to lower elevations. Second, the tracers at both sites become wellmixed across the beach from January 2007 to April 2007 during the storm interval. The tracers at PWA re-sort in the following non-storm interval. PWB remains unsorted with the exception of the largest tracers which remain on the lower beach. Nordstrom and Jackson (1993) also observed the trend for gravel sorting to become homogenous (well-mixed) across the beach during high-energy intervals and heterogeneous (well-sorted) across the beach during low-energy intervals. Grain size appeared to be a much better predictor of behavior than grain shape; no clear trend in transport was found related to grain sh ...
Direct measurements of coarse sediment (gravel) transport are presented and analyzed from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA that is exposed to wind waves, vessel wakes, and tidal currents in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology is implemented in two coincident yearlong tracking studies of sediment particles, and complemented with beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the dominant seasonal transport patterns, which include a range of wave climates. During winter storms, the predominant transport is due to wind waves in an alongshore uni-directional process, whereas tides and wakes play a relatively minor role. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents. Although tidal currents are not sufficient to initiate sediment movement alone, the combination of tidal currents and vessel wakes generate significant transport and provide a mechanism for post-storm recovery, re-distributing sediment onshore. Morphologic response occurs as a seasonal fluctuation of the upper beach profile from steep to flat and in sediment composition from gravel to coarse sand between non-storm and storm conditions respectively. These results, which are unique in their duration, suggest that mixed sand and gravel beaches experience different modes of behavior over the range of forcing conditions observed during a typical year. They point to the need for including grain composition in modeling mixed sand and gravel beach response and the need for long term observations of both forcing and response.
... Lagrangian tracers have a long history of use for characterising sediment dynamics on beaches. In the gravel beach literature, particle tagging with fluorescent paint [16][17][18], or radio frequency identification tags (RFID; [19][20][21][22][23][24][25][26][27]) has lead to improved characterisations of transport dynamics on tide-to-tide timescales. No studies, to the authors' knowledge, have made use of cobble-sized tracers to investigate transport in the swash on runup timescales on a coarse-grained or MSG beach. ...
On mixed sand–gravel beaches, impacts from gravel- and cobble-sized grains—mobilized by the energetic shorebreak—limit the utility of in situ instrumentation for measuring the small-scale response of the beach face on wave period time scales. We present field observations of swash zone morpho-sedimentary dynamics at a steep, megatidal mixed sand–gravel beach using aeroacoustic and optical remote sensing. Coincident observations of bed level and mean surficial sediment grain size in the swash zone were obtained using an array of optical cameras paired with acoustic range sensors. Lagrangian tracking of swash-transported cobbles was carried out using an additional downward-oriented camera. The principal objective of the study was to investigate linkages between sediment grain size dynamics and swash zone morphological change. In general, data from the range sensor and camera array show that increases in bed level corresponded to increases in mean grain size. Finer-scale structures in the bed level and mean grain size signals were observable over timescales of minutes, including signatures of bands of coarse-grained material that migrated shoreward with the leading edge of the swash prior to high tide berm formation. The direction and magnitude of cobble transport in the swash varied with cross-shore position, and with the composition of the underlying bed. These results demonstrate that close-range remote sensing techniques can provide valuable insights into the roles of cobble-sized versus sand-sized particle dynamics in the swash zone on mixed sand–gravel beaches.
... Different authors have described the types of systems and their main characteristics [1][2][3][4]. Other works have focused on the dynamics and the evolution of these zones, particularly from a geomorphological point of view [5][6][7][8][9][10][11][12] but also paying attention to anthropic influence in these systems (e.g., References [13][14][15][16]). In this case the research focuses on an integrated analysis using different techniques in the large shifting dune within the Corrubedo Natural Park. ...
This work analyzes the evolution of the large shifting dune included in the Corrubedo
Natural Park from 1956 until the present day. The analysis was carried out using aerial images, orthophotographs, topographic surveys and LiDAR (Light Detection and Ranging) data. Variations were analyzed in terms of four different aspects: evolution of pathways in each period, changes in the position of the mobile dune front and the degree of vegetation cover, all between 1956 and 2017; and variations in elevation and volume of the dune system between 2001 and 2015. To analyze these aspects, geospatial and geostatistical techniques were employed, which revealed that the dune front had undergone two different phases: one until the 1980s, with a mean seaward advance of 32 m
and another one between the 1980 s and 2017, when the front prograded to the continent, with an approximate mean advance of 82 m. Loss of volume, related to the expansion of the mobile dune towards the north, was confirmed in this sector. It is evident that these changes are closely linked to anthropic influence, due to sand extraction until the early 1990s and, more recently, due to an increase in the number of visitors to the Natural Park. The results obtained highlight the need to carry out detailed studies to understand the evolution of this system and to prevent further damage in the future. Moreover, based on these analyses, a strategy to improve coastal and environmental management in the Corrubedo Natural Park could be designed.
... RIVER (1993( TO 2016 ABSTRACT: Fluvial-Marine Plain of the Acaraú River shows several spatio-temporal changes caused by natural processes and anthropic interventions. The aim of this study was to analyze the changes in the period 1993-2016 associated with shrimp farming and natural dynamics of the riverine and marine processes of the Acaraú estuary. ...
... Historically, tracer techniques have included painted sediment (Russell, 1960;Jolliffe, 1964;Nordstrom and Jackson, 1993); radioisotopic tracers (Steers and Smith, 1956;Kidson et al., 1958), magnetic tracers (Osborne, 2005), aluminium tracers (Wright et al., 1978), electronic radio transmitters (Bray et al., 1996) and RFIDs (Dickson et al., 2011;Dolphin et al., 2016) as a means of monitoring littoral sediment transport across a range of particle sizes. The development of tracer techniques has been driven by the need to improve tracer recovery rates over broader timescales. ...
Boulder transport is an area of growing interest to coastal scientists as a means of improving our understanding of the complex interactions between extreme wave activity and the evolution of rocky coasts. However, our knowledge of the response of intertidal boulder deposits to contemporary storm events remains limited due to a lack of quantifiable field‐based evidence.
We address this by presenting a methodology incorporating Radio Frequency Identification (RFID) tagging and Differential Global Positioning Navigation Satellite System (DGNSS) technology to monitor and accurately quantify the displacement of RFID tagged boulders resulting from storm wave activity. Based on preliminary findings we highlight the suitability of the technology and methodology to better understand the spatial and temporal response of intertidal boulders to contemporary storm events.
We inserted RFID tags in 104 limestone boulders (intermediate axes from 0.27 to 2.85m) across a range of morphogenic settings at two sites on the intertidal shore platforms at Bembridge, Isle of Wight (UK). Fifteen topographic surveys were conducted between July 2015 and May 2017 to relocate and record tagged boulder locations (tag recovery rate: 91%). The relocated boulder coordinate data from both sites identified 164 individual transport events in 63% of the tagged boulder array amounting to 184.6m of transport, including the displacement of a boulder exceeding 10 tonnes.
Incidents of boulder quarrying and overturning during transport were also recorded, demonstrating that despite the relatively sheltered location intertidal boulders are created and regularly transported under moderate storm conditions. This suggests that contemporary storm events have a greater propensity to mobilise boulders in the intertidal range than has previously been realised.
Consequently, by documenting our methodology we provide guidance to others and promote further use of RFID technology to enable new hypotheses on boulder transport to be tested in a range of field settings and wave regimes.
... Sediments are often gravelly because they are close to the source in eroding formations. Pebbles are a common surface feature on beaches composed primarily of sand ( Nordstrom and Jackson, 1993), and cobbles and boulders are common lag features where wave energy is insufficient to move them ( Joeckel and Diffendal, 2004) and there is an insufficient supply of sand to bury them. Beaches in low wave energy environments can consist of an erosional surface with a thin veneer of pebbles, cobbles and boulders that can be as little as 1 clast thick, on the surface of, or embedded within, sediments in the underlying wave cut platform. ...
Synthesises data on physical, biological and anthropogenic processes, concentrating on the form of estuarine and similar beaches, and the wave and current processes that shape them. Ecological aspects, recreational use, shore protection and regulatory considerations are also covered, indicating the environmental changes that accompany an anthropogenic disturbance to the natural shoreline. The text is divided into the following chapters: definitions and locations of estuarine beaches; waves, currents and water level changes; beach and shoreline characteristics; changes in beach morphology and sediment volumes; resource values of estuarine beaches; shore protection alternatives; management considerations; research needs. (S.J.Stone)
... Another aspect in need of in-depth investigation is the relationship between the shape of the pebbles and the 405 characteristics of the surface over which they move (Carr et al., 1970;Caldwell, 1981): an irregular coarse 406 bottom determines different types of pebble movements (Isla, 1993), and pebbles are preferentially entrained 407 over sandy surfaces (Nordstrom and Jackson, 1993). Sherman et al. (1993) found that the distribution of 408 shape and size facies is primarily controlled by location within the beach cusp systems. ...
In this paper, two short term experiments with tracers on a mixed beach are presented. The aim was to understand how the size and shape of pebbles can affect their transport under low energy conditions. Sediment transport was studied by means of RFID technology to univocally monitor every single marked pebble. A size subdivision of injected pebbles was conducted based on three classes (“Big” from −5.5 to −6.5 phi; “Medium”,
from −5 to −5.5 phi; and “Small”, from −4.5 to −5 phi). Two recoveries were realised 6 and 24 h after the injection. During a single day, the wave motion was very low in the first experiment and low to moderate in the
second (never exceeding 0.4 m). The results showed that discs are less dynamic than spheres but can cover greater distances. Regarding the sediment size, “Big” pebbles are less dynamic if compared to finer classes, and they move preferentially down the swash zone towards the step or do not move up-slope if already at the step.
Very low and steady energy conditions facilitate cross-shore and offshore movement of pebbles, rather than a
slight raise in wave height producing predominant longshore transport even with non-marginal displacements. Low to moderate energy conditions can also produce some trend displacement based on the pebble shape even though T-tests showed that shape was not statistically significant for pebble displacement. The displacements of
“Medium” and “Small” sized pebbles show a statistical dissimilarity compared to the “Big” ones. To refine the velocity estimation necessary to initiate pebble movement, the threshold velocity formulas known up to now
should involve the shape parameter, especially for the short term. Better knowledge of the relationship between the sediment's characteristics and dynamics is critical to forecast the durability of replenishment material and to establish the suitability of fill material relative to native beach material. Hence, a better understanding of the role of particle characteristics is necessary.
... The swash uprush did not reach the bulkhead during the two tidal cycles when monitoring occurred. Mean grain size (Fig. 2) increases (lower f values) with increasing distance from the bulkhead as is common on estuarine beaches (Nordstrom and Jackson, 1993). ...
... The fact that the coarser fraction becomes more prominent on the beachface under low energy conditions seems a bit controversial, but in reality it was observed by other authors even in completely different geomorphological settings. NORDSTROM and JACKSON (1993) were the first ones to postulate that during high wave activity coarser sediment transport was dominated by cross-shore movement towards the beach step. While we were deciding the instant for sampling the beach before undertaking the experiment, we noticed that after storms the beach was generally richer in sand than before the event. ...
This paper describes a series of experiments undertaken on a mixed sand and gravel beach at Porto Recanati (MC). The aim of the work was to understand the mechanisms of sediment transport on the sub-aerial beach. Sediment transport was studied using tracers under moderate wave conditions. It is interesting to notice that while sand and fine gravel transport was correlated with longshore currents, pebbles moved mainly cross-shore. As wave energy decreased, disk-shaped pebbles moved up on the beach profile and changed the beach sediment characteristics. The work has important implications for beach replenishments and for beach users interested in the characteristics of surface sediment.
... The median grain size for the entire sediment mixture based on sieving is 16.0mm at PWA and 11.0 mm at PWB. A unique feature of the beach is the median size of the gravel, which increases with decreasing elevation on the beach as also observed by Nordstrom and Jackson (1993) on a low energy estuarine beach. ...
Direct measurements of coarse sediment (gravel) transport were obtained over an interval of 14 months from a mixed sand and gravel beach on Bainbridge Island, Puget Sound, WA in order to quantify the relative role of different forcing mechanisms and the corresponding time scales of morphological response. The measurements were applied to validate a system of integrated numerical models that includes: a tidal circulation model, a wind-wave growth and transformation model, a vessel wake model and a one-dimensional, profile-based model. The latter model, which provides a long-term integrated assessment of the beach response to major forcing mechanisms, was the primary tool for investigating the impacts of tides, waves and wakes on the mixed sand and gravel shores of the study area. Radio Frequency Identification (RFID) Passive Integrated Transponder (PIT) technology was implemented in tracking studies of gravel-sized sediment particles, and complemented the beach profile surveys and meteorological and hydrodynamic measurements. The sampling of the gravel tracers provides sufficient resolution to reveal the seasonal transport patterns, which include a range of wave and vessel wake climates. Simulations of cumulative transport rate predicted with the integrated modeling system compare well with the alongshore tracer movements and capture the dominant trends and variations during the time period of the measurements. The measurements and modeling reveal that the transport is dominated by wind waves in an alongshore uni-directional process that occurs mainly in winter. However, beach response is also controlled by site-specific exposure to prevailing winds and car ferry wakes. In non-storm intervals, transport is brought about by the combination of vessel wakes and tidal currents; the sub-critical car ferry wakes provide a mechanism for post-storm recovery, in this low energy restricted fetch environment.
... They came to the conclusion that in that case abrasion was clearly the overriding process producing the change in shape towards predominantly compact forms. Nordstrom and Jackson (1993), who studied a mixed sand and gravel beach in Delaware Bay, confirmed the findings by Nordstrom (1977) and Ekwurzel (1988) of an inverse relationship between the average level of wave energy and the seasonal presence or absence of pebbles and cobbles on the beach, an important observation followed up in the present study. Howard (1992) concluded from his comparison of fluvial, submarine fan, and beach conglomerates in California that shape as defined by maximum projection sphericity, amongst other indices, can be used as a paleoenvironmental indicator despite texturally inherited differences in shape. ...
The 8 km Bianco Beach Is a wave-dominated high-energy beach composed of sand and gravel, The fluvial gravel output of the Laverde River feeding the Bianco Beach is well defined in terms of petrographic composition, sphericity, and roundness, During the high-energy (storm) stage the entire beach gravel population is transported 200 m offshore perpendicular to the coast, Strong attrition occurs and the amount of less resistant gneiss decreases, The net longshore transport is northward; roundness Increases but sphericity remains almost unchanged, During the medium energy (post-storm) stage, when fair-weather conditions begin to rebuild the beach berm, attrition and rounding are no longer significant, Instead, redistribution and redeposition according to grain size and shape dominates the cross shore gravel movement with a decrease of sphericity from the offshore to the backshore, Net longshore transport is still northward, sphericity again remaining almost unchanged, During the geologically ineffective low-energy (fair-weather) stage there is some sediment movement along the foreshore, but without ally significant gravel exchange between individual beach zones normal to the beach, Such extended fair-weather conditions conserve the sedimentological changes that occurred during higher-energy events, As a result, the proportion of gneiss decreases from 56% to 31% in the net transport direction, whereas the sphericity decreases only from 0.68 to 0.64, Perpendicular to shore the proportion of gneiss decreases seawards from 50% to 37% and the sphericity decreases landward from 0.70 (offshore) to 0.60 (backshore), In contrast, the sphericity frequency distribution of both river / beach and beach start / beach end are almost identical, By retaining its provenance signal, sphericity fails as a meaningful tool for the recognition of geological environments along the Calabrian coast, This inherited sphericity signature of the fluvial output is conserved along the beach and is differentiated only in the cross-shore direction, The overall roundness R increases alongshore by 0.14 (from 0.55 to 0.69) but does not show any trend normal to shore, Continuous and effective rounding along the foreshore during low energy stages should create much better rounding in this zone than in neighboring zones, which are active only during high-energy stages, The stable cross-shore roundness reflects the "event character" of sediment modification, Only high-energy events move the entire gravel population to yield homogeneous rounding, The results of this study therefore suggest that the Bianco Beach is almost exclusively the product of such high-energy events.
... Sediments are often gravelly because they are close to the source in eroding formations. Pebbles are a common surface feature on beaches composed primarily of sand ( Nordstrom and Jackson, 1993), and cobbles and boulders are common lag features where wave energy is insufficient to move them ( Joeckel and Diffendal, 2004) and there is an insufficient supply of sand to bury them. Beaches in low wave energy environments can consist of an erosional surface with a thin veneer of pebbles, cobbles and boulders that can be as little as 1 clast thick, on the surface of, or embedded within, sediments in the underlying wave cut platform. ...
This review is intended to identify differences between beaches in short-fetch environments and beaches on exposed coasts, while also distinguishing between the different subcategories of fetch-limited beaches. Subcategories are discussed largely in terms of estuaries, lakes and reservoirs. The term fetch-limited refers to basins that are small enough that distance rather than wind duration is always a limitation to wave generation. Attention is focused on basins where fetch distances are < 50 km. The dimensions of small basins provide a limit on the energy potential of the waves, causing geologic and biologic controls to be more significant and wind-induced currents, tidal currents and ice to be relatively more effective than on exposed beaches. Shoreline orientations differ greatly over short distances, causing great differences in exposure to dominant winds and isolating beach segments. Limited longshore sediment exchanges result in beach sediments that closely resemble local source materials. The absence of high-energy waves causes beaches and bar forms to be smaller, and the absence of swell waves following storms and the relatively calm conditions reduces the speed of recovery of post-storm profiles and the cyclic nature of beach response. The beaches are often fronted by flat shallow platforms that undergo little morphologic change and help dissipate waves at low water levels. The narrow beaches are poor sources of sediment for wind-blown sand and dunes are small or frequently absent. The narrow beaches and reduced wave energies allow upland vegetation and algae and seagrass to grow close to the active foreshore. This vegetation, the wrack deposited on the beach, and driftwood logs are better able to resist the low-energy waves and are more effective in resisting beach change. Erosion rates of 2–3 m yr− 1 are common in some estuaries and can be > 7 m yr− 1. Rates of up to 1.5 m yr− 1 can occur in small lakes and reservoirs. Shore parallel protection structures are common and have greater survivability in low-energy environments than high-energy environments; they are cheaper to build; and they have been implemented more frequently to control erosion. Their effect has been to reduce the extent of beach in small water bodies. Beach nourishment projects have been fewer than on exposed shores and the quantities smaller. Many nourishment projects have been implemented for amenity value and have been placed in locations where waves have not been able to create an equilibrium landform.The biggest difference in process controls between estuaries and lakes and reservoirs is in the mechanism for water level change. Tides and surges from external basins are important on estuarine beaches, whereas rainfall, runoff, groundwater flow, evapotranspiration and control by dams are more important in reservoirs and lakes. Future sea level rise will threaten beach environments in estuaries where shore parallel walls will prevent onshore migration of landforms and habitats and will change the number and locations of beaches in unarmored areas. Dam removal will pose a threat to the existence of reservoirs and dammed lakes. Water levels are more dependent on human actions in lakes and reservoirs, so changes can be minimal or increased to a greater extent than in estuaries. Lesser stability and predictability of beaches will complicate future management efforts.
... The small percentage of gravel near the base of the pit high on the foreshore of the Middle Segment is somewhat anomalous. The gravel at 0.15 m depth, high on the foreshore of South Segment is believed to be attributed to preferential delivery of pebbles to the upper-most portion of the active beach, where they are stranded until high swash energies remobilize that portion of the beach(Nordstrom and Jackson 1993).Thin, discontinuous clay layers are present on both segments, beginning 0.50 m below the beach surface. These deposits represent the accumulation of fine-grained materials deposited in the earlier fill.Sediments taken from the scarp on the backbeach are representative of sediments in the previous large-scale nourishment project that have not been reworked by wave action. ...
... The rapid 570 reworking of the upper 0.10 to 0.20 m of beach between 571 12 April and 15-16 May 2002 and the resulting 572 elimination of the layer of fill pebbles in the higher 573 portions of the foreshore imply that adding gravel may 574 result in only a temporary change in bulk sediment 575 characteristics below the surface. Gravel near the 576 surface of an estuarine beach is readily entrained and 577 transported over a primarily sandy surface, resulting in a 578 conspicuous cover, especially during times of low wave 579 energy ( Nordstrom and Jackson, 1993). Accordingly, 580 even small amounts of gravel fill can change the 581 appearance of the beach. ...
This field study evaluates the effect of nourishing an estuarine beach with gravel to enhance spawning rates by horseshoe crabs. A total of 564 m3 of coarse sand and gravel were emplaced in two 90 m-long treatment segments at Bowers Beach, Delaware, USA from 9 to 11 April 2002. Field data were gathered between 6 April and 24 May 2002 to characterize the two fill segments and the un-nourished segments between them as well as two control segments at the adjacent Ted Harvey Beach.Sediment samples were taken from the foreshore surface and at depth before and after the nourishment. Bay water levels, wave heights, and beach ground water characteristics were monitored over a 12-hour tidal cycle at one of the nourished (15 May 2002) and the unnourished segment (16 May 2002) at Bowers Beach and at one of the control segments at Ted Harvey Beach (21 May 2002) using piezometers and pressure transducers inserted in wells. The beaches were cored to estimate the density of horseshoe crab eggs deposited during the spawning season. Horseshoe crab eggs were buried in pouches at 0.15 to 0.20 m depth for 30 to 40 days to evaluate their survival in developing into embryo or larval stage. Bulk sediment samples were taken to evaluate moisture characteristics near locations where egg pouches were buried.Density of spawning females at Bowers Beach was 1.04 m− 2 in 2001 and 1.20 m− 2 in 2002. These rates are lower than at Ted Harvey Beach but reveal an increase in spawning while Ted Harvey Beach underwent a considerable decrease (2.63 m− 2 to 1.35 m− 2). Sediments low on the foreshore remained nearly saturated throughout the tidal cycle at both beaches. The average hydraulic conductivity on the upper foreshore at the non-treatment section at Bowers Beach (0.19 cm s− 1) was less than at Ted Harvey Beach (0.27 cm s− 1), and the finer, better sorted sediments at depth at Bowers Beach resulted in a higher porosity, creating greater moisture retention potential. Egg development was greatest at mid foreshore at all sites. Eggs at the lower foreshore elevation remained viable, but did not develop to the embryo stage. Between-beach differences were limited to high elevations where higher mortality occurred at Ted Harvey Beach due to desiccation. Adding small amounts of gravel to a sand beach may change the appearance of the surface but may not appreciably increase mean grain size and sorting at depth or the hydraulic conductivity over the spawning season. The pebble fraction may be important for site selection, but finer sizes may be more important for egg survival because of moisture retention.
p>The Gulf-Normano Breton lies within the Western English Channel but can be considered distinct in terms of both physical and biochemical oceanography. Datasets covering the physical dynamics of the Gulf, particularly those concerning the stability of the coastline of Jersey, the largest of the Channel Islands within the Gulf, are identified and described. Overall, the coastline of Jersey, is stable in the short-to medium-term. Changes in beach profiles and volumes are discussed. Between 1992-1999, losses and gains of beach material have been balanced, either spatially or temporally over a period of 3-6 years in the major embayments (St Ouen's Bay, St Brelade's Bay, the south east coast and Grouville Bay). Underlying these fluctuations, there is a slow loss of material from the upper beach (possibly, to the lower beach) however, these changes are not statistically significant. In contrast, the beach profile volumes within St Aubin's Bay show a highly significant trend of accumulation. St Ouen's Bay forms the focus of this study. Re-analysis of geophysical data (from May 1998) showed that the sea floor area is dominated by exposed bedrock: namely metamorphic Jersey Shale and granite. The most recent survey (April 2001) has confirmed that sand (and, hence, bedforms) in the embayment are confined to; (a) the intertidal (perched) beach area, (b) within the Great Bank and, (c) in shallow sand lenses and channels contained within the rocky sea floor. Great Bank itself is a type 3a Banner (or Headland) Bank resting on the offshore abrasion platform. Whilst the Bank has many bedform types superimposed (ranging from ripples to large dunes 3m in height), there has been no major change in the nature of the sea floor, between May 1998 and April 2001 and the bank is stable in the medium term. Moreover, the Bank is assumed to be controlled by the present hydrodynamic regime; the ADCP confirms that the sea floor interacts with the tidal currents, to produce local eddy structures. There is only limited interaction between the offshore and onshore sand areas. However, onshore / offshore transport may occur within the nearshore sand lenses. Evidence for this are provided by bedforms (side- scan sonar, April 2001) the on/offshore tidal currents (ADCP) and deposition within the beach cores. The beach of St Ouen's Bay can be classified as ultra-dissipative; as such it is expected to be stable. Conversely, beach erosion has been documented since 1759. Although there is insufficient evidence to assess the long term sediment transport paths, the stratigraphy of the intertidal beach area has been examined on the basis of the collection and analysis of sediment peels. These sections show that sediment transport on the beach was dominated by tidal patterns and cross-shore sediment transport. The beach exhibits considerable short term variations in morphology but, unless the underlying peatbeds are eroded, the beach levels are assumed to recover naturally within a few years. is hypothesised that 'St Ouen's Bay contains a stable volume of sand which is transported around the intertidal area by a series of standing edge waves (although evidence for this mechanism is required). The morphology of the bay appears to be in equilibrium with the present climatic conditions as there is a recycling of material over the medium term (3-5 year period/. However, it is uncertain if the bay will be able to maintain its equilibrium, under conditions of rising sea levels and changes in climatic conditions (such as storm intensity and frequency). Whilst the depth-integrated tidal patterns have been studied, there is little knowledge of the current velocities at the sea floor and there is substantial scope for improving the understanding of both the tidal and wave climate within the embayments of Jersey. In addition, the rates of sea levels change around the Island could be better understood, by establishing the historical datum levels. To determine the significance of the trends in beach volume over the longer term, it is necessary to establish the underlying baseline and to extend the profile lines to the 'depth of closure'.</p
Estuaries are among the most biologically productive and geomorphologically complex environments in the coastal zone. A review of research is presented, focusing on broad scale estuarine morphology and evolution and an examination of contemporary processes and forms in the intertidal zone. The chapter includes a discussion of current issues in estuarine research, including geomorphic-biotic interactions, human-modified estuaries, and restoration practices. The chapter concludes with a brief discussion of future areas of concern given current attention to climate variability and sea level rise.
Sandy beaches in estuaries and bays are distinct from open-coast beaches in that they are partially or fully sheltered from ocean waves. The sheltering increases the importance of other sources of wave energy such as infragravity waves that propagate into bays and estuaries with less dissipation than swell, combined with locally generated wind waves. Moreover, tidal translation and currents may become important and contribute to beach erosion and deposition. In this chapter, we review the importance of sandy beaches in estuaries and bays, and introduce the geological setting and oceanographic conditions that determine where they form and what they look like. We consider the variety of terminology that has been used to describe them (e.g. fetch-limited, low-energy or sheltered beaches) and discuss the relatively limited number of studies on their morphodynamics. We also include a case study from Botany Bay, Sydney, Australia. We conclude that sandy beaches in estuaries and bays should be recognised as a distinct class of beach (’bay and estuary beaches’, or BEBs) that has notable contrasts with open-ocean beaches that are dominated by ocean waves.
O presente estudo objetivou compreender o comportamento da praia de Jabaquara no município de Paraty, borda oeste da Baia da Ilha Grande no litoral sul do estado do Rio de Janeiro. Foram realizados perfis topobatimétricos, análise da textura e cálculo do volume de sedimentos nas estações de verão e inverno entre 2016 e 2018. Os resultados mostram características distintas quanto à dinâmica morfológica e sedimentar deste arco praial. O extremo norte da praia possui uma baixa dinâmica, com perfis semelhantes entre as estações; o setor sul apresentou uma maior dinâmica, com variações na morfologia e na largura da praia. No extremo sul dessa praia há indícios de erosão, com a destruição de construções, pós-praia estreito, tombamento de árvores e redução progressiva do volume total de sedimentos. Os sedimentos dessas praias variam bastante em tamanho, desde cascalho, areias até lama nas áreas próximas a desembocadura de rios.
This scholarly bibliography is a compendium of the existing geographical studies of the state of New Jersey. It includes seven sections: general works (atlases, books, monographs, articles, dissertations, and theses), cultural and social geography, economic geography, historical geography, physical and environmental geography, political geography, and urban geography. The search for these entries started with an examination of the contents of the available scholarly journals that publish works on geography. Then, lists of references were examined to search for other journals that these authors published works in. Also found were the main atlases, books, texts, and monographs related to some aspect of New Jersey’s geographical studies. These authors were also located as to their work places, and if available their vitae were searched. Additionally, WorldCat was searched for New Jersey geography topics. Each section is organized in a simple alphabetical sequence, by author’s last name. Where there is more than one entry per author, the earliest is listed first, and subsequent entries listed chronologically after the first. In the case of single versus multiple authors, the entries with single authors are recorded first, and multiple entries after these. The dates of entries reach back into the nineteenth century, and continue to the present. Most entries are written in English, but works written in other languages were recorded as found.
Estuaries are among the most biologically productive and geomorphologically complex environments in the coastal zone. Estuaries vary considerably in terms of geomorphology, tides, drainage basin and fluvial processes, water chemistry, wave characteristics, sediment provenance, upland land use and land cover, biodiversity, and degree of human modification. This chapter presents background information on the definition, classification, and characteristics of estuaries as well as the physical processes that shape morphology. A review of research is presented, focusing on broad-scale estuarine morphology and evolution and an examination of contemporary processes and forms in the intertidal zone. The chapter includes a discussion of current issues in estuarine research, including geomorphic-biotic interactions, human-modified estuaries, and restoration practices. The chapter concludes with a brief discussion of future areas of concern given current attention to climate variability and sea-level rise. Although the focus is on geomorphic research, there are references to key work within biology, chemistry, and hydrology.
Mixed beaches are a comparatively neglected area of coastal research, other than for sedimentological investigations or relatively long-term geomorphological change. The paper reviews the progress of research into processes of sediment transport on mixed sand and gravel beaches since 1980, including fieldwork, laboratory studies and modelling. First-order and secondary factors are identified. The hydraulic conductivity of the bulk sediment is found to be an influence on swash and backwash infiltration and groundwater flow through the sediment. No existing transport model contains the most significant factors for mixed sediment transport, which should include swash and backwash hydrodynamics, infiltration, steep beach gradients, fractionation and differential hydraulic conductivity. Near-prototype scale physical model tests of profile response are needed, since mixed sediments cannot be scaled correctly for both hydrodynamic response and hydraulic conductivity.
The relationship between wave height and depth of sediment activation is evaluated on an estuarine beach to determine whether activation depth is less in pebbles than sand. Rods with washers were used to monitor three excavated beach plots filled with (1) pebbles with mean grain size of 11.5 mm; (2) sand and granules; and (3) sand, granules and pebbles. Plots were monitored for 26 events over 27 days. Significant wave heights ranged from 0.18 to 0.40 m and activation depths from 0.02 to 0.12 m. Activation depths in the pebble plot were less than the other two plots when waves reworked sediment not activated during previous tidal cycles. Proportionality coefficients for activation depth to wave height, when net change was
Background, Aim and ScopeThe distribution of sediments in estuarine beaches is controlled by the interactions between sediment supply, hydrodynamic processes and human intervention. The main purpose of this study is to characterize the sediments of Tagus estuarine beaches in order to understand their origin and to contribute to a better knowledge of the Tagus estuary sediment budget. Methods
Surface sediment samples were collected across beach profiles and sand grain size analysis was performed by dry sieving. Grain size statistics for the median (d50) and standard deviation (SDM) were obtained using the Moment method. This study was complemented by a qualitative evaluation of the sediment composition. Cross-shore topographic surveys were conducted for selected sampling sites. ResultsTagus estuarine beach sediments are mainly composed of quartz sand particles which are fine-grained and well sorted near the mouth of the estuary and medium to coarse-grained and moderately sorted in the inner domain. Compositional results show evidence of active anthropogenic sediment sources, especially in the coarser fractions. DiscussionThe analysis of the textural and compositional characteristics of beach sediments in the inner estuarine domain is compatible with local sedimentary sources, while a marine signature is present at the mouth and inlet channel sediments. In the inner domain, differences in the sedimentary processes are represented by the textural characteristics of the sediments, such as the sorting degree and the gravel content. Sediment characteristics also reflect human intervention in the system, with the introduction of anthropogenic and allochthonous particles and the mixture of sediments from different sources. Conclusions
The sediments of the inner Tagus estuarine beaches are derived from local Plio-Pleistocene outcrops while inlet and outer estuary beaches reveal a dominant marine source. Beach textural variability observed in the inner domain is not related to wave forcing gradients, but mainly to variations in the sedimentary processes along the estuarine margins and to human intervention. Results show that the Tagus estuarine beaches depended, almost exclusively, on sediment input from local sources until the last century. With increasing human occupation, sediment transfers became dominated by anthropogenically related activities mainly connected with the occupation of estuarine margins and dredging. Recommendations and PerspectivesFurther studies should extend the present level of knowledge in what concerns sand transport patterns through additional compositional and geochemical analysis, and the development of new techniques in order to allow the quantitative evaluation of the impact of human activities on the sediment budget.
Knowledge of conditions that favor development of eggs is important for management of species whose population growth is sensitive to early life history survival. Viability and develop- ment of the eggs of horseshoe crabs Limulus polyphemus on a sand and gravel beach were evaluated using data gathered on Delaware Bay, USA, from 18 May to 19 June 2004. Eggs were transplanted to pouches and buried in the foreshore for up to 6 wk. Viability and developmental stage were esti- mated as a function of oxygen and temperature gradients across the foreshore. These gradients were related to the characteristics of the intertidal foreshore sediments, beach water table changes, and frequency of inundation due to tide and swash/backwash processes. Results demonstrate the impor- tance of interstitial temperature for development to larvae and the passive role of sediment charac- teristics on moisture retention and temperature. Percentage of eggs remaining in egg stage was sim- ilar across the foreshore, but more eggs developed to embryos at 0.45 of foreshore width, where moisture and gravel content were greater and interstitial temperature was lower. More eggs devel- oped to larvae at 0.60 and 0.75 of foreshore width, where moisture and gravel content were less but interstitial temperature was higher. The beach above 0.75 of foreshore width came under the influ- ence of wave action or full tidal inundation only during high wave heights or spring tides, and pouches at 0.75 of foreshore width were inundated only 19% of the time. Periodic wetting at this elevation did not reduce overall viability of the eggs. High wave energy events resulted in sediment activation depths to pouches at 0.30 of foreshore width, where loss of eggs due to wave activation was the most important control on the development of eggs.
The result of field experiments, designed to investigate the relative proportions of bedload and suspended load, are described. The ratio of bedload to suspended sediment load in the swash zone is examined in both swash and backwash on four beaches by measuring the amounts collected in a sediment trap. Bedload transport is found to dominate the backwash. The relative proportions of bedload and suspended load change over the tidal cycle, with increasing bedload dominance at low tide. The total amount of sediment transported as swash and backwash is noticeably greater at high tide than at low tide. More sediment is transported on the flood tide than on the ebb.
The effects of wave action and horseshoe crab spawning on the topography and grain-size characteristics on the foreshore of an estuarine sand beach in Delaware Bay, New Jersey, USA were evaluated using data collected over six consecutive high tides. Data were gathered inside and outside a 25 m long exclosure constructed to create a control area free of disturbance by crabs. The density of crabs in the swash zone outside the exclosure was 8·1 organisms m−2. The maximum depth of sediment activation on the upper foreshore where spawning occurred was 0·103 m during periods characterized by low significant wave heights: < 0·08 m. This depth is greater than the depth of activation by waves alone during moderate significant wave heights of 0·16–0·18 m but less than the maximum depth (0·127 m) recorded when spawning occurred during periods of moderate wave heights. Spawning, combined with moderate wave heights, creates a concave upper foreshore that is similar to the type of profile change that occurs during storms, thus lowering the wave-energy threshold for morphological response. Spawning during low wave heights increases the mean grain size and sorting of surface sediments caused by the addition of gravel to the swash. Sedimentological differences are most pronounced on the upper foreshore, and data from this location may be most useful when using grain-size characteristics to interpret the effect of spawning in the sedimentary record. Depths of sediment reworking by horseshoe crabs can be greater than those by subsequent storm waves, so evidence of spawning can be preserved on non-eroding beaches. Greater depth of activation by horseshoe crab spawning than by waves alone, even during moderate-energy conditions, reveals the importance of crab burrowing in releasing eggs to the water column and making them available for shore birds.
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.
The abundance of horseshoe crab eggs in the swash zone and remaining on the beach after tide levels fall was evaluated to identify how numbers of eggs available to shorebirds differ with fluctuations in spawning numbers of horseshoe crabs, wave energies and beach elevation changes. Field data were gathered 1–6 June 2004 at Slaughter Beach on the west side of Delaware Bay, USA. Counts of spawning crabs and process data from a pressure transducer and an anemometer and wind vane were related to number of eggs, embryos and larvae taken at depth and on the surface of the foreshore and in the active swash zone using a streamer trap. Beach elevation changes and depths of sediment activation were used to determine the potential for buried eggs to be exhumed by waves and swash.Mean significant wave heights during high water levels ranged from 0.08 to 0.40 m. Spawning counts were low (50–140 females km−1) when wave heights were low; no spawning occurred when wave heights were high. Vegetative litter (wrack) on the beach provides local traps for eggs, making more eggs available for shorebirds. Accumulation of litter on days when wave energy is low increases the probability that eggs will remain on the surface. High wave energies transport more eggs in the swash, but these eggs are dispersed or buried, and fewer eggs remain on the beach. Peaks in the number of eggs in the swash uprush occur during tidal rise and around time of high tide. The number of eggs in transport decreases during falling tide. Many more eggs move in the active swash zone than are found on the beach after water level falls, increasing the efficiency of bird foraging in the swash. Greater numbers of eggs in the swash during rising tide than falling tide and fewer eggs at lower elevations on the beach, imply that foraging becomes less productive as the tide falls and may help account for the tendency of shorebirds to feed on rising tides rather than on falling or low tides on days when no spawning occurs.