Beach Sediment Budget Research Articles

Historical and geological assessment of shoreline changes at an urbanized embayed sandy system in Garopaba, Southern Brazil

Embayed sandy beaches are dynamically complex systems that vary in time and space. The rising demand for the uses and resources associated with these environments may neglect the shoreline mobile nature and lead to imbalances in the beach sediment budget. Climate change imminence has been increasing coastal risks, with a considerable contribution of local and regional causes. This study aims to assess the influence of short to long-term processes on short spatial scales and to identify which processes control the behavior of neighboring beaches. Thereby, a case study was analyzed at Barra Beach (BB) and Ferrugem Beach (FB), a small embayment separated by an intermittent inlet located in Garopaba, Santa Catarina state, southern Brazil. The shoreline dynamic behavior of both beaches was assessed through a multi-temporal approach (historical and geological), investigating the main factors influencing their morphodynamic behavior. The historical variation was investigated through shoreline change rates along 16 years (2003–2019), calculated with Digital Shoreline Analysis System (DSAS) v5.0 – ESRI® ArcGis extension, considering past shoreline positions extracted from satellite imagery. Geological evolution was analyzed in terms of seismostratigraphy through subsurface profiles acquired with a Ground Penetrating Radar (GPR) system. Interdecadal shoreline change rates and GPR records indicated that BB has a transgressive shoreline in a retrogradational barrier, while FB is slightly progradational, with a stable shoreline from the historical scale perspective. The interannual analysis showed that the inlet surroundings are influenced by seasonality, and beach rotation was not detected between the beaches nor within themselves. The distinct morphodynamic behavior between BB and FB is a function of their shoreline geometry, architectural elements, wave energy distribution, and sediment sources/sinks, resulting in variant alongshore sediment budgets. Overall, this paper highlights the importance of local shoreline assessment, and it could enhance efficient present-day coastal management, supporting future sustainable urban planning.

Beach-Foredune Sediment Budget Response to Sea Level Fluctuation. Curonian Spit, Lithuania

Beach-foredune sediment exchange maintains a coastal system’s stability. Sea level fluctuation is one of the most important factors that modifies the beach and foredune sediment budget. This study aims to assess beach and foredune sand budget changes depending on sea level fluctuations. On the basis of annual measurements of cross-shore profiles on the Curonian Spit in Lithuania, the sediment volumes on the beach and foredune and their changes between 2002 and 2019 were calculated. The sea level fluctuations were examined in parallel. The obtained data revealed that in the case of a sand surplus, a relatively low sea level rise does not have a significant impact on the development of a foredune (and a minimal impact on a beach) on a decadal time-scale. Short-term sea level fluctuations are reflected in year-to-year variability in a beach sediment budget. However, no significant relationship between year-to-year variability in sea level fluctuation and the foredune sediment budget has yet been identified, nor is there a reliable year-to-year variability relationship between the foredune and beach sediment budget. The foredune sediment budget remained positive both through an increase and a reduction in the sediment volume on the beach.

Response of the Bight of Benin (Gulf of Guinea, West Africa) coastline to anthropogenic and natural forcing, Part 2: Sources and patterns of sediment supply, sediment cells, and recent shoreline change

The Bight of Benin in the Gulf of Guinea, West Africa, forms an embayment between the Volta River delta in the west (Ghana) and the Niger River delta (Nigeria) in the east. The bight coast comprises sandy beaches backed by Holocene beach-ridge barriers. Incident swell waves, beachface gradient and the unidirectional longshore sand transport from west to east are intimately linked, generating a classic example of a strongly wave-dominated drift-aligned coast. The stability of this coast, which hosts several major cities in addition to three large international deepwater ports, has been strongly affected by human activities. We analyzed shoreline mobility and coastal area change over the period 1990–2015. Our results show how the stability of this coast has been strongly affected by the three ports therein, and by natural and human-altered shoreline dynamics related to the Volta River delta and to distributaries at the northwestern flank of the Niger delta. The combination of these factors has impacted alongshore sediment redistribution by segmenting the previously unrestrained longshore transport of sand that prevailed along this open coast. The result is a mixture of natural and artificial sediment cells increasingly dominated by shoreline stretches subject to erosion, endangering parts of the rapidly expanding port cities of Lomé (Togo), Cotonou (Benin) and Lagos (Nigeria), coastal roads and infrastructure, and numerous villages. Post-2000, the entire bight shoreline has undergone a significant decrease in accretion, which is here attributed to an overall diminution of sand supply via the longshore transport system. We attribute this diminution to the progressive depletion of sand-sized bedload supplied to the coast through the main Volta river channel downstream of the Akosombo dam, built between 1961 and 1965. Sand mining to cater for urban construction in Lomé, Cotonou and Lagos has also contributed locally to beach sediment budget depletion. Although alongshore sediment supply from the Volta River has been the dominant source of sand for the stability or progradation of the Bight of Benin coast, potential sand supply from the shoreface, and the future impacts of sea-level rise on this increasingly vulnerable coast are also important. The continued operation of the three ports and of existing river dams, and sea-level rise, will lead to sustained shoreline erosion along the Bight of Benin in the coming decades.

A Quantitative Assessment of the Annual Contribution of Platform Downwearing to Beach Sediment Budget: Happisburgh, England, UK

Field and numerical investigations at Happisburgh, East coast of England, UK, sought to characterize beach thickness and determine geologic framework controls on coastal change. After a major failure of coastal protection infrastructure, removal of about 1 km of coastal defence along the otherwise protected cliffed coastline of Happisburgh triggered a period of rapid erosion over 20 years of ca. 140 m. Previous sensitivity studies suggest that beach thickness plays a major role in coastal recession. These studies were limited, however, by a lack of beach volume data. In this study, we have integrated the insights gained from our understanding of the Quaternary geology of the area, a novel non-intrusive passive seismic survey method, and a 3D novel representation of the subsurface source and transportable material into a coastal modelling environment to explore the role of beach thickness on the back wearing and downwearing of the cliffs and consolidated platform, respectively. Results show that beach thickness is non-homogeneous along the study site: we estimate that the contribution to near-shore sediment budget via platform downwearing is of a similar order of magnitude as sediment lost from the beach and therefore non-negligible. We have provided a range of evidence to support the idea that the Happisburgh beach is a relatively thin layer perched on a sediment rich platform of sand and gravel. This conceptualization differs from previous publications, which assume that the platform was mostly till and fine material. This has direct implication on regional sediment management along this coastline. The present study contributes to our understanding of a poorly known aspect of coastal sediment budgeting and outlines a quantitative approach that allows for simple integration of geological understanding for coastline evolution assessments worldwide.

Impact of ocean acidification on the carbonate sediment budget of a temperate mixed beach

The production of sediments by carbonate-producing ecosystems is an important input for beach sediment budgets in coastal areas where no terrigenous input occurs. Calcifying organisms are a major source of bioclastic carbonate sediment for coastal systems. Increased levels of CO2 in the atmosphere are leading to an increase in the partial pressure of CO2 on ocean seawater, causing ocean acidification (OA), with direct consequences for the pH of ocean waters. Most studies of OA focus on its impact on marine ecosystems. The impact of OA on carbonate-producing ecosystems could be to reduce the amount of sediments supplied to temperate coastal systems. The aim of this study was to quantify the effect of the predicted OA on the long-term sediment budget of a temperate Mediterranean mixed carbonate beach and dune system. Based on projections of OA we estimated a fall of about 31% in the present bioclastic carbonate sediment deposition rate, with the biggest decreases seen in the dunes (− 46%). OA is also expected to affect the carbonate sediment reservoirs, increasing the dissolution of CaCO3and causing net sediment loss from the system (~ 50,000 t century−1). In the long-term, OA could also play a primary role in the response of these systems to sea-level rise. Indeed, the reduction in the quantity of carbonate sediments provided to the system may affect the speed with which the system is able to adapt to sea-level rise, by increasing wave run-up, and may promote erosion of dunes and subaerial beaches.

Biogenic sediments from coastal ecosystems to beach–dune systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise

Abstract. Coastal ecosystems produce and store carbonate particles, which play a significant role in the carbonate dynamics of coastal areas and may contribute to the sediment budget of adjacent beaches. In the nearshore seabed of temperate zones (e.g. Mediterranean Sea and South Australia), marine biogenic carbonates are mainly produced inside seagrass meadows. This study quantifies the contribution of biogenic sediments, mainly produced in Posidonia oceanica seagrass meadows and secondarily in photophilic algal communities, to the sediment budget of a Mediterranean beach–dune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data was used to estimate the sediment volume and composition of the beach–dune system as a whole. The San Giovanni beach–dune system contains 3 797 000 ± 404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which is located in the coastal wedge, 16 % (619 000 ± 88 000 t) in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial beach. The sediments are composed of mixed modern bioclastic and relict bioclastic and non-bioclastic grains from various sources. The system receives a large input of modern bioclastic grains, mainly composed of rhodophytes, molluscs and bryozoans, which derive from sediment production of present-day carbonate factories, particularly P. oceanica seagrass meadows. Radiocarbon dating of modern bioclastic grains indicated that they were produced during the last 4.37 kyr. This value was used to estimate the long-term deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000 ± 5000 t century−1, mainly deposited in the coastal wedge (39 000 ± 4 000 t century−1) and dunes (7000 ± 1000 t century−1), and 46 000 t represents ∼ 1.2 % of the total beach–dune sediment mass. Carbonate production from coastal ecosystems was estimated to be 132 000∕307 000 t century−1, 28 % (15 % ∕ 34 %) of which is transported to the beach–dune system, thus significantly contributing to the beach sediment budget. The contribution to the beach sediment budget represents a further ecosystem service, which our data can help quantify, provided by P. oceanica. The value of this sediment-supply service is in addition to the other important ecological services provided by seagrass meadows. The dependence of the beach sediment budget on carbonate production associated with coastal ecosystems has several implications for the adaptation of mixed and carbonate beaches to the loss of seagrass meadows due to local impacts and the changes expected to occur over the next few decades in coastal ecosystems following sea level rise.

Assessment of beach and dune erosion and accretion using LiDAR: Impact of the stormy 2013–14 winter and longer term trends on the Sefton Coast, UK

An important question for coastal management concerns the importance of individual storms and clusters of storms on longer term beach sediment budgets, beach and dune erosion, and coastal flood risk. Between October 2013 and March 2014 a series of deep Atlantic low pressure systems crossed the Northeast Atlantic, and strong winds, high waves and high water levels affected many coastal areas in the UK and other parts of western Europe. Net dune recession of up to 12.1m occurred around Formby Point. On 5 December 2013 the highest water level ever recorded at Liverpool (6.22m ODN) coincided with waves of Hs of 4.55m and Tp of 9.3s in Liverpool Bay. Wave trimming of the dune toe occurred along the entire length of the Sefton coast, but significant dune erosion occurred only where the upper beach (between the mean high water spring tide level and the dune toe) was <25m wide. Sediment budget calculations based on LiDAR surveys in October 2013 and May 2014 indicated a net loss of 127×103m3 of sediment from the beach (above 0m ODN) and a loss of 268×103m3 from the frontal dune system, mostly at Formby Point. However, some parts of the beach to the south of Formby Point gained sediment, indicating net north to south transport over the winter. When considered in a longer term context, the 2013–14 winter represents only a small perturbation on the longer-term coast trend of erosion at Formby Point and progradation to the north and south. Analysis of LiDAR data over a longer time period 1999–2014 indicated upper beach and dune sediment loss of 780×103m3 from the north-central part of Formby Point, with net gains of 806×103m3 and 2116×103m3 in areas to the north and south, respectively. This indicates a net onshore transport of 2142×103m3 from Liverpool Bay towards the coast between Birkdale and Altcar, with a further net total of 210×103m3 transported towards the shore between Altcar and Crosby. In view of the demonstrated value of airborne LiDAR surveys for the quantification of storm impacts and longer term coastal changes, it is recommended that such surveys should be undertaken before and after each winter storm period, covering the area between mean low water spring tide level and a line 200m landward of the dune toe, of as a part of the regional coastal monitoring programme.

Close-range airborne Structure-from-Motion Photogrammetry for high-resolution beach morphometric surveys: Examples from an embayed rotating beach

The field of photogrammetry has seen significant new developments essentially related to the emergence of new computer-based applications that have fostered the growth of the workflow technique called Structure-from-Motion (SfM). Low-cost, user-friendly SfM photogrammetry offers interesting new perspectives in coastal and other fields of geomorphology requiring high-resolution topographic data. The technique enables the construction of topographic products such as digital surface models (DSMs) and orthophotographs, and combines the advantages of the reproducibility of GPS surveys and the high density and accuracy of airborne LiDAR, but at very advantageous cost compared to the latter. Three SfM-based photogrammetric experiments were conducted on the embayed beach of Montjoly in Cayenne, French Guiana, between October 2013 and 2014, in order to map morphological changes and quantify sediment budgets. The beach is affected by a process of rotation induced by the alongshore migration of mud banks from the mouths of the Amazon River that generate spatial and temporal changes in wave refraction and incident wave angles, thus generating the reversals in longshore drift that characterise this process. Sub-vertical aerial photographs of the beach were acquired from a microlight aircraft that flew alongshore at low elevation (275m). The flight plan included several parallel flight axes with an overlap of 85% between pictures in the lengthwise direction and 50% between paths. Targets of 40×40cm, georeferenced by RTK-DGPS, were placed on the beach, spaced 100m apart. These targets served in optimizing the model and in producing georeferenced 3D products. RTK-GPS measurements of random points and cross-shore profiles were used to validate the photogrammetry results and assess their accuracy. We produced dense point clouds with 150 to 200points/m², from which we generated DSMs and orthophotos with respective resolutions of 10cm and 5cm. Compared to the GPS control points, we obtained a mean vertical accuracy less than ±10cm, with a maximum of 20cm in marginal sectors with sparse vegetation and in the lower intertidal zone where water-saturated surfaces generated lower-resolution data as a result of a lack of coherence between photographs. The overall results show that SfM photogrammetry is a robust tool for beach morphological and sediment budget surveys. Our SfM workflow enabled the discrimination of beach surface features at a scale of a few tens of centimetres despite the low textural contrasts exhibited by the quartz beach sand and the relatively uniform upper beach topography, as well as the calculation of beach sediment budgets. 66,000m³ of sand were removed from the northern sector of the beach, of which 22,000m³ were transferred to the southern sector in the course of rotation. Finally, we briefly highlight: (1) the advantages of SfM photogrammetry compared to other high-resolution survey methods, (2) the advantages and disadvantages of, respectively, a microlight aircraft and an unmanned aerial vehicle (UAV) in undertaking SfM photogrammetry, and (3) areas of potential future improvement of the SfM workflow technique. These concern more extensive cross-shore deployment of ground control points to reduce possible tilt, and oblique cross-shore photography to improve parallax.

Climate-driven episodes of dune mobilization and barrier growth along the central coast of Portugal

Abstract Here, we explore the evolution of the coastal stretch between Mira Beach and Quiaios Beach in Portugal to understand how it adapted to climatic oscillations. To accomplish this, we integrate subsurface radar images, and sedimentological and chronological data, of the emerged coastal barrier. Our results show the installation and progradation of a stable barrier anchored to transgressive dunes 400 years ago. This is just the last pulse of barrier growth within a complex approximately 5000 year-long history of shoreline stability/instability. Episodes of inland dune mobility have been related to instabilities in the beach sediment budget driven by enhanced storminess and wave rotation around 4.25 and 1.14 ka ago. Conversely, lagoonal deposits documented in the literature suggest periods of relative barrier stability and growth around 4.3 and 2.7 cal ka BP. Wave and wind climate variability are driven by shifts in one of the major modes of atmospheric circulation in the North Atlantic, the North Atlantic Oscillation (NAO). Episodes of persistent positive mode of the NAO related to barrier growth and enhanced longshore sediment transport; those of persistent negative mode contributed to instabilities in the beach sediment budget and aeolian activity by enhancing storminess, but reduced effective longshore sediment transport.

Beach changes from sediment delivered by streams to pocket beaches during a major flood

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 200years. 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.Foreshore sediments were finer and better sorted and contained no pebbles prior to the flood. Coarsening of the sand and granule fraction was evident after the flood, along with pebble accumulations, especially near major streams. Mean size, sorting values and percent pebbles one and two years after the flood were generally greater than pre-flood conditions but less than immediately after the flood. Beach profiles reveal conspicuous erosion immediately after the flood, when sediment delivered by streams is transported to subaqueous deltas. Thereafter, sediment moves onshore and alongshore to adjacent beaches to restore subaerial volumes. The location of streams within a compartment, relative to the location of capes and headlands, is important in determining the movement of sediment added to the beach by streams. The sites are all sheltered from the highest-energy waves from the west, facilitating longshore transport to the west. Where the largest stream is located at the east end of a compartment, sediment discharged from it is source material for the downdrift beaches to the west. Where the largest stream is at the west end of the compartment, the ability to supply sediment to the beaches to the east is restricted. Hence, broad-scale geologic controls (headlands and capes) enhance or diminish the ability of streams to influence beach change throughout the pocket.The inability of beaches on two of the sites to migrate landward, due to human development of uplands, will be an issue in the future. Lack of sediment to replenish beaches through erosion of the upland, places increased emphasis on the role of coastal streams in the beach sediment budget. Changing watershed attributes to allow more sediment discharge during high-energy, low-frequency events (e.g. devoting more land to agriculture) would be a way of helping to restore beach sediment.

Dynamics of coastal landform features along the southern Tamil Nadu of India by using remote sensing and Geographic Information System

This article reveals an application of multi-spectral satellite data for analysing the dynamics of different coastal landform features along the southern coastal Tamil Nadu of India. An integrated approach comprising visual image interpretation and maximum-likelihood supervised classification has been employed to classify the coastal landforms by using IRS data (during the period 1999–2006). The quality of image classification has been assessed by performing the accuracy assessments with the existing thematic maps and finally the coastal landforms have been mapped. The study reveals that the dynamics of coastal landforms such as sandy beaches, mud-flats, sand dunes and salt marshes along the study area are mostly influenced by the coastal processes, sediment transport, geomorphology and anthropogenic activities. Major anthropogenic sources for the perturbation of beach sediment budgets and a cause of beach erosion along the study area are excessive sand mining, removal of sand dunes, coastal urbanization, tourism and developmental activities.

Onshore Bar Migration in a Fetch/Limited Strong Wind Bay

WONG, H and STEPHENSON, W., 2011. Onshore Bar Migration in a Fetch/Limited Strong Wing Bay. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 458-469. Grosseto, Tuscany, Italy, ISSN 0749-0208.This paper investigates bar migration in a fetch limited strong wind bay (Seaford Beach SE Australia). Onshore bar migration on open ocean beaches is dependant on long period swell wave and occurs during low energy conditions. However, in a fetch limited environment such waves are absent and most studies suggest that during low energy conditions the beach remains in an arrested state (Jackson et al. 2002). This paper spans a three month study period (15th March – 8th June 2008). Measurement of near shore morphology and wind data were collected. The two bars at Seaford behaved very differently; both onshore and offshore migration was detected during the three recorded storm events. The paper found that, firstly, wave breaking was incomplete in the middle bar during event 1 — the inner bar migrated offshore. Secondly wave breaking in the middle bar was almost complete during event 2 and the inner bar migrated onshore. Thirdly when wave breaking was complete in the middle bar during event 3 — the inner bar remained arrested on some profiles but bar height increased on others. The wave steepness threshold for onshore migration of the inner bar was parameterized s 0.041 – 0.042. Beach sediment modeling in fetch limited strong wind bays remains understudied. Our results should help to improve the predictive capability of beach sediment budgets in fetch limited strong wind bays.

Optimization of beach profile spacing: an applicable tool for coastal monitoring

This paper presents a simplified and straightforward method for determining optimal beach profile spacing from an economic point of view with an admissible error. This error can be computed theoretically by comparing different profile spacings from two consecutive bathymetries. When a beach sediment budget (from previous monitoring surveys) or the volume density of a beach nourishment project is known, a virtual bathymetry can be designed; therefore, a unique real bathymetry would be needed. The method is applied and analysed for beaches with different characteristics regarding tide, energy, morphology and natural features. The results indicate that the estimated errors are proportional to a beach heterogeneity factor, depending on each particular beach case, such that large spacings do not necessarily induce large errors. In our case, profile spacings of 100 m induce average errors of less than 5%. Moreover, differences in tidal range and the existence of rocky reefs do not seem to affect the results.

Modeling the effects of wave climate and sediment supply variability on large-scale shoreline change

The application of an integrated data analysis and modeling scheme reveals that decadal-scale shoreline evolution along a U.S. Pacific Northwest littoral cell is highly dependent on both sediment supply and wave climate variability. In particular, accurate estimates of (Columbia River) sediment supply and sediment feeding from the lower shoreface are critical components of balancing the barrier beach sediment budget and are therefore essential to making sensible shoreline change hindcasts and forecasts. A simple deterministic one-line shoreline change model, applied in a quasi-probabilistic manner, enables evaluation of the influence of sediment supply and wave climate variability through simulation of historical shoreline change. Through iteration, a range of realistic scenarios are developed to constrain decadal-scale shoreline change predictions. Modeled shoreline changes are significantly sensitive to directional changes in the incident waves, and therefore sensitive to the occurrence of interannual climatic fluctuations such as major El Niño events. A predicted increase in the intensity of the east Pacific wave climate (1.0 m increase in significant wave height in 20 yr) affects forecast shoreline positions only when this increase occurs during the winter storm season. However, the effect of this increase in storm power during any given year is small relative to the impact of major El Niño events. The model has significant skill in decadal-scale hindcasts suggesting that alongshore gradients in sediment transport dominate coastal change at this scale at this site. However, both data and model results suggest that net onshore feeding from the lower shoreface is responsible for approximately 20% of the decadal-scale coastal change. Field measurements and poor model skill at annual scale indicate that cross-shore processes likely dominate coastal change at shorter time scales.

Shoreline Evolution and Management of Hawke's Bay, New Zealand: Tectonics, Coastal Processes, and Human Impacts

The evolution of the coast of Hawke's Bay, New Zealand, and its erosion problems have been governed by multiple factors, including its tectonic setting with an earthquake in 1931 that altered land elevations along its shore, ranging from a 2-m uplift at its north end to 1-m subsidence at its south. Human environmental impacts have also been important, including the deforestation of the watersheds of rivers and the mining of gravel and sand from their channels, having decreased the sediment supplies to the beaches. Significant erosion has occurred at the south end of the Hawke's Bay shore, attributed in part to its subsidence in 1931, together with the net northward longshore transport of the beach sediment that is greater than the volumes being supplied by the rivers, the beach-sediment budget being significantly “in the red.” Midway along the Bay's shore, the construction of the Port of Napier's breakwater in 1887–90, extending seaward from the Buff Hill headland within that city, is interpreted by some investigators to have been the cause of the erosion experienced at Westshore, a development located immediately north of the breakwater. The assumption has been that this erosion was the result of the breakwater having blocked the northward longshore transport of sediment that formerly had bypassed Bluff Hill, this being an example of down-drift beach erosion. However, a reexamination of the history of the Port's development, the resulting shoreline changes, differences in beach gravels on the shores north and south of Bluff Hill, as well as other evidence, support the conclusion that the breakwater was not the cause of the erosion; instead, the impacts to Westshore at the time of the breakwater construction can be attributed primarily to a series of major storms, which produced erosion at a number of sites along the Hawke's Bay shore. There has been minimal erosion at Westshore since the completion of the breakwater a century ago, its northward extending arm acting to shelter the development from the waves of major storms. Investigations by coastal scientists and engineers of the Hawke's Bay coast, together with a comprehensive beach-survey monitoring program, support its sound management based on an understanding of the multiple factors that are important to its responses during storms, and to its long-term changes.

Grain size and compositional trends of sediments from Posidonia oceanica meadows to beach shore, Sardinia, western Mediterranean

Inputs of biogenic carbonate sediment from Posidonia oceanica seagrass meadows to four beaches of the Sinis peninsula (Sardinia, western Mediterranean) were evaluated. Beach and continental shelf sediment samples were analysed for grain size distribution and composition, biogenic vs. siliciclastic, in order to identify the provenance of beach sediments and sediment transport pathways. Seabed mapping was carried out in order to identify the distribution of meadows and sediment deposits offshore. Shelf sediments were collected in unvegetated sites and in P. oceanica meadows. Sediments from unvegetated sites were coarse sands and gravel, mainly siliciclastic (biogenic carbonate content is 3–7%). Sediments from P. oceanica meadows were coarse sand, mainly biogenic (carbonate contents varying between 60 and 90%). Beach sediments showed bimodal grain size distribution (59% of samples) resulting from mixing of coarser siliciclastic with finer biogenic materials in variable proportions. Biogenic carbonate contents in beach sediments range from 0 to 90%, reaching the highest values in offshore samples. Analysis of grain size and compositional trends from shelf to beach sediments highlighted that the latter originate from two different sources: erosion of granitic outcrops, providing the siliciclastic component, and export of sediments from P. oceanica meadows, providing biogenic material. P. oceanica meadows also influence shore by contributing towards maintaining the beach sediment budget.

The Variability of Ridge and Runnel Beach Morphology: Examples from Northern France

The morphology of three macrotidal ridge and runnel beaches in northern France was analysed from over 200 profiles in order to identify intertidal spatial and short-term (weeks) profile variability, at both the inter-site and intra-site levels. The beaches, essentially composed of medium to fine sand, are exposed to fetch-limited waves variably dissipated by nearshore sand banks. They have spring tidal ranges of 5.6 to 7.2 m, and sediment budgets ranging from equilibrium to deficient or surplus. The results show that spatial and temporal morphological variability is controlled by: (1) variations in exposure to wave action that depend on the proximity of nearshore sand banks, as well as on protection offered by artificial structures; and (2) by the state of the beach sediment budget. Where equilibrium sediment budget conditions prevail, as in the Dunkerque-Est sector, the beach exhibits a regular alternation of ridges and runnels that represent a cross-shore alternation of fluid-bed interaction domains involving surf/swash activity and channel flow conditions. Energy dissipation at the bed is spent in the construction and destruction of wave and tidal micro- and meso-scale bedforms, leaving little scope for macro-scale ridge migration or change in form, except under exceptionally high wave energy conditions. Chronic sediment losses, as in Wissant Bay, or gains, as in Calais-Hoverport, are recycled respectively alongshore and to embryo dunes and are not necessarily translated in terms of significant meso-scale (years) beach volumetric changes. The short-term beach sediment budget changes however favour active bed readjustments that explain distortion of the regular ridge and runnel form and marked profile mobility, even under low to moderate wave energy conditions.

A Erosão Praial na Costa Atlântica Uruguaia

The Uruguayan Atlantic coast is wave dominated. It appears as a sequence of beaches built between rocky headlands showing all ranges of morphodynamic behaviors from reflective to dissipative. Several beaches are undergoing erosion. All of them coincide with urbanized areas. In most cases the causes have not been established yet but there is no doubt that human activities have contributed to such problems. This fact raises the question about the nature of the erosion process. Does location of beaches under erosion on urbanized areas prove the man-induced origin of it? This paper discusses the possible factors involved on the beach erosion process. Sediment inputs to the beaches are limited. The inner shelf is likely to be the main source as it is covered with sandy sediments and shows a low gradient relief. The most significant continental source are the gullies although this remains a controversial issue as sediments eroded from them are usually finer than beach sediments. Anyway, as minor as it might be, this input contributes positively to the beach sediment budget. Sediment losses occur mainly by the inland migration of dunes and human activities. The latter includes the occupation of the backshore and frontal dunes, the building of seawalls, revetments and other hard structures to protect the houses, and the fixation of mobile dunes by planting. Pinus and Eucalyptus forests. Possible natural causes for beach erosion include storm surges and concentration of wave energy due to wave refraction. The latter hypothesis arouse in this research from wave refraction diagrams that showed a strong influence of the shelf relief on wave climate.

Tourism development, wetland degradation and beach erosion in Antigua, West Indies

Over the past twenty years significant research has been conducted concerning the effects of tourism on local cultures. However, research into the effects of tourism on natural environments is less developed. Though tourism is often represented as a low impact industry, scholars have begun to recognize tourism as an agent of environmental alteration. This study, conducted in 1997 and 1998 indicates that interrelated coastal ecosystems in Antigua, West Indies have been severely degraded as a result of tourism development. Intertidal wetlands, which mediate terrestrial biological nutrient and sediment flows, have been almost completely disrupted. As a result coralgal communities which fix calcium carbonate may lose productivity. Consequently beach sediment budgets are depleted and beach erosion has become significant and endemic on Antigua. This study presents a significant challenge to the representation of tourism, in its present form, as an environmentally sustainable economic activity. Pendant les vingt dernières années des recherches considérables étaient menées sur les répercussions sur les cultures locaux. Pourtant, la recherche concernant les conséquences du tourisme sur le milieu naturel est peux développé. Bien que le tourisme est souvent présente commé une industrie avec un impact très bas, les spécialistes en tourisme ont commencé d'y reconnaite que le tourisme est un agent qui modifie l'environnement. Cette étude, menée en 1997/1998, indique que des écosystèmes cotièrs interdépendants en Antigua, Les Antilles ont étaient sévèrement dégradés par suite du développement du tourisme. Les tèrres marécageuses, qui sèrvent comme médiateurs biologiques tèrrestres, et de substance nutritive et les flux de sédiment avaient été complètement remuées. Par suite, les communités corales qui réparent le carbonate de calcium peuvent perdre leurs productivité. Par conséquent, les budgets de sédiment de plage sont réduits et l'érosion de la plage est devenue considérable et endémique sur l'Antigua. Cette étude presente un challenge pour le tourisme dans sa forme actuèlle, comme une activité économique et écologiquement viable.

Les cordons de galets de la Baie des Anges : histoire environnementale et stratigraphique ; évolution morphodynamique récente en réponse à des aménagements/The gravel barriers of the Baie des Anges (French Riviera): environmental and stratigraphie history; recent morphodynamic responses to coastal management

Abstract The gravel barriers of the Baie des Anges (French Riviera) protect the densely developed area of Nice against marine flooding. The general setting of these barriers is that of a steep coastal margin and shelf, and a fetch-limited, low wave-energy, almost tideless environment. In terms of sequence stratigraphy, these gravel barriers, which have yielded radiocarbon ages ranging from 3000 yr B.P. to Present, are part of a highstand systems tract comprising essentially gravelly river-channel and finer-grained floodplain, delta-plain and nearshore deposits. The local sea-level curve shows a strong slow-down in the Holocene transgression at around 4000 yr B.P. This change coincided with active upland erosion by mountain streams exhibiting catastrophic flow regimes that resulted in downstream bedload channel aggradation and seaward flushing of gravel clasts. The present gravel barriers, which may have been formed and dismantled successively during the eustatic transgression, are an outgrowth of these bioclimatic conditions and of the change from transgression to stillstand. Once gravel supply from river mouths to the littoral drift system became efficient, the gravel barriers became consolidated by bi-directional longshore drift with changing drift termini, in response to short, poorly refracted waves coming from a wide zone spanning east to south-west. The barriers seem to have been morphodynamically stable forms as a result of their highly gravelly composition, relative sea-level stability, and the almost tideless, low energy and fetch- limited hydrodynamic setting of the bay. Stability was enhanced by crestal buildup, a drift-aligned plan-form configuration that enabled longshore transfer of fluvial sediment to make good losses to the steep nearshore shelf, and a textural organisation that favoured segregation between coarse and fine clasts, leading, in the long run, to relative enrichment in coarse clasts and resistance to change. The barrier sediment budgets were balanced, with gains from the coastal rivers compensating for the losses to the steep nearshore zone, which also considerably limited progradation. Over the past century, human activities have resulted in the destabilization of the barrier beach sediment budgets, initiating a phase of slow, uneven erosion and textural disorganisation. To counter this erosion, gravel nourishment has been practised on certain portions of beach in the last twenty years. Optimization of these periodic nourishment campaigns however requires not only a fine understanding of the environmental context and mechanisms of morphodynamic development of these gravel barriers but also of the hydrodynamic processes involved in their breakdown.