Longshore Movement Research Articles

Longshore movements and site fidelity of the iconic giant trevally Caranx ignobilis from southern Africa, determined using passive acoustic telemetry

Giant trevally Caranx ignobilis are iconic, apex marine predators globally distributed in tropical to subtropical regions, where they are of high importance to ecosystems and fisheries. During summer, adults aggregate for spawning, making them vulnerable to overfishing. The world’s largest C. ignobilis aggregation has been recorded in southern Mozambique, but the year-round movements of these fish required further investigation. To assess movement patterns of C. ignobilis along the southern African coast, 36 adult fish were acoustically tagged and monitored along the east coast between Santa Maria in southern Mozambique and Port St Johns in the Eastern Cape, South Africa, for over 5 yr using an extensive passive acoustic receiver array. All acoustically tagged C. ignobilis were recorded in southern Mozambique between November and January each year (with minor exceptions), moving distances up to a maximum of 632 km in one direction. These movements were fast and direct, with maximum travelling speeds of up to 130 km d-1. When not migrating, South African-based fish showed consistent inter-annual fidelity to individual linear areas of space use equivalent to home ranges. Although the coastal length of these areas varied considerably between individuals, even the mean length (92 km) was greater than any previously recorded C. ignobilis home range, globally. Findings from this study, including the value of no-take marine protected areas, are of global ecological interest and have implications for local fisheries management. Protecting these vulnerable aggregations is of the utmost importance for the future of this species in southern African waters and elsewhere.

Monitoring sediment transport pathways from an artificial nearshore berm, South Padre Island, Texas, USA, August 2018 to November 2019—Implications for coastal management

During August 2018 – November 2019, the transport pathways of dredge material from a specially constructed nearshore feeder berm were investigated as part of a collaborative study by the City of South Padre Island, U.S. Army Corps of Engineers–Galveston District, U.S. Geological Survey, Partrac GeoMarine Inc., and Texas A&M University, into the efficacy of beneficial use dredge material (BUDM) as a method of replenishing the beach profile and shoreface at South Padre Island, Texas with sediment. Dual-signature (fluorescent and ferrimagnetic) tracer particles, designed to be hydraulically equivalent to the dredge material, were placed on the berm, and a sampling program was initiated to monitor the spatiotemporal movement of tracer particles under the influence of the prevailing hydrodynamic regime. Wave and current data were collected and were used together with available metocean data to identify the forcing mechanisms of sediment transport; improved understanding garnered from the consideration of multiple datasets can be used to inform future coastal management decisions.Tracer analysis results indicated low magnitude, shoreward transport of dredge material from the berm and along shore transport in both northerly and southerly directions. Small amounts of tracer detected in beach face samples demonstrated connectivity between the constructed feeder berm and the shoreface. However, the generally low tracer concentration within beach face samples indicated low rates of sediment transport for berm sediments, and a low magnitude sediment transport pathway from the berm directly to the beach face, with possible storage of sediment in the nearshore bar system.Given that the berm was at or beyond the closure depth and considering the relatively weak prevailing near bottom hydrodynamic conditions, the potential for sediment to be mobilized and transported during the study period was generally low. Periods of higher energy waves, driven by north-northwest winds, were identified as a key driver of sediment transport but due to the infrequent nature of these events the sediment transport regime across the area of interest was temporally limited. Longshore movement of sediment was both north and south, mainly dependent on prevailing wind directions and resulting longshore currents.We discuss implications for coastal zone management and the use of feeder berms as an artificial beach nourishment mechanism to replenish sediments lost by coastal erosional processes. The insight into nearshore emplacement of material can be used by local governments and coastal managers to optimize the efficacy of berm emplacement and implement such schemes as a cost-effective nourishment option, or when onshore placement of material is not feasible.

TWEED SAND BYPASSING TRANSITION PROJECT

The Tweed River is located in New South Wales, Australia, just south of the state border with Queensland. The Tweed River Entrance Sand Bypass Project, aka Tweed Sand Bypassing (TSB), was formulated to overcome problems caused by the extension of the Tweed River entrance training walls in the 1960s (Dyson et al., 2001). These issues were primarily: the interception of natural longshore movement of sand, resulting in loss of sand supply to the southern Gold Coast beaches; and sand movement into the river entrance with consequential impacts on vessel navigation. TSB is a joint initiative of the New South Wales and Queensland State Governments. The TSB is similar to the nearby Gold Coast Seaway Sand Bypass (GCSSB) that was commissioned in 1986 to stabiles the Nerang River entrance (Clausner, 1989). Together, these two facilities have come to represent best practice outcomes in sustainable coastal management. A Transition Project was established by TSB to identify the most appropriate operational system / model post-expiry of the CA and to ensure ongoing effective coastal management that meets the legislated objectives. The Transition Project has provided a unique opportunity to improve how TSB functions through the exploration of available options and analysis of historical operational performance.

Cliff-top boulder morphodynamics on the high–energy volcanic rocky coast of the Reykjanes Peninsula (SW Iceland)

Cliff-top boulder deposits (CBDs) are morphological indicators of high-energy conditions. Since 2014, a monitoring of CBDs dynamics has been undertaken on the south-western coast of Iceland (Reykjanes Peninsula) to monitor their long-term activation (quarrying, transport and deposition) as a proxy of the inter-annual winter storminess variations and basaltic cliff erosion processes in a context of rocky coast progradation. Annual topomorphological surveys of four study sites were conducted and Structure-from-Motion photogrammetry was performed to quantify CBDs displacements. Hydrodynamic conditions were analyzed based on offshore waves and water level. Results show that CBDs activation occurs every winter, regardless of the variability of hydrodynamic conditions. Depending on the site and the year, >2% and 17% of the CBDs accumulated above 8 m to 10 m asl at the top of the cliffs are regularly mobilized. While inland movements represent the main mode of transport of blocks (between 50% to 60%), seaward and longshore movements are also well represented (10% to 20%). Longshore displacement is favored by the wide tabular morpho-structural setting of the wave-scour cliff-top platforms, which is explained by the structure of pāhoehoe lava flows. The activation of CBDs –measured from the volumes of displaced boulders–, shows a good correspondence with the frequency and duration of storms. However, as was the case during the winter of 2018–2019, it was rather the intensity of two highly morphogenic episodes combining storm waves and especially very high spring tide water levels, that generated the largest boulders displacements. Substantial interannual activation of the CBDs confirms that they constitute an important and still understudied proxy of the morphogenic system of high-energy rocky coasts, whose the dynamic in terms of carrying, transport, and deposition, could significantly increase with rising sea level.

The resilience to human foraging of intertidal resources on the south Cape coast of South Africa and the implications for pre-historic foragers

The extent to which modern humans relied upon intertidal resources, and the impact that this food source had on their cognitive and social development, remains contentious. An outstanding question is whether such a resource could have provided a sufficiently reliable and essentially continuous supply of resources to support communities during the Middle Stone Age (MSA). The south Cape coast of South Africa contains abundant evidence dating back to 164 000 years ago (ka) to show that the intertidal zone of this area was highly productive and was intensively used by MSA humans. In this paper we present the results of an experimental approach to answering the question of whether resources could have been sufficiently resilient to support communities similar in size to those of the Middle Stone Age. For a period of 10 months, we monitored the depletion of shellfish in the two dominant types of marine habitat (aeolianite and Table Mountain Sandstone) that occur on the south Cape coast, following intensive harvesting by indigenous foragers every two weeks or every four weeks. We found no evidence of lasting depletion in either type of habitat at either foraging frequency. We also found that replenishment of the foraged areas soon occurred (within two weeks) by repopulation from deeper water rather than by a long-shore movement. The most important prey species in this process was the highly mobile gastropod mollusk Turbo sarmaticus, which made up 81.9% of calorific harvest. Overall, mobile species made up 92.2% of the calorific harvest. Another factor that contributed to the resilience of the system is the nature of the coastline of the south Cape. This is characterised by extended shallow bathymetries of hard substrata that provide “pantries” of mobile invertebrates for restocking depleted intertidal habitats. We argue that the presence of such a reliable, easily procured, and nutritionally beneficial source of food may have played an important role in the development of more complex types of social behaviour than previously existed.

Wind Conditions on the Great Barrier Reef Influenced the Recruitment of Snapper (Lutjanus carponotatus)

Most coral reef fishes have a pelagic larval stage before recruiting to reefs. The survival of larvae and their subsequent recruitment can drive the dynamics of reef populations. Here we show that the recruitment of the snapper Lutjanus carponotatus to One Tree Island in the Capricorn Bunker Group, in the southern Great Barrier Reef, was highly variable over 23 years. We predicted that the currents in the Capricorn Bunker Group, including their wind driven components and the Capricorn Eddy (a nearby transient oceanic eddy), would affect patterns of recruitment. A biophysical model was used to investigate this prediction. L. carponotatus were collected from One Tree Island and the dates when they were in the plankton as larvae were determined from their otoliths. The winds present during the pelagic phases of the fish were examined; they were found to have survived either longshore (SSE) winds that induced little cross shelf movement in the larval plume or cross shelf (ENE) winds that induced little longshore movement. The unidirectional transportation of the larval plume in these conditions was favourable for recruitment as it kept the plume concentrated in the Capricorn Bunker Group. These winds were more prevalent in the periods of peak L. carponotatus production that preceded high recruitment. Dispersal under average winds (6.2 m s-1 from the prevailing ESE) and strong winds (velocity 1.5 times average), with and without the Capricorn Eddy, was also modelled. Each of these combinations were less favourable for recruitment than the longshore and cross shelf winds larval L. carponotatus survived before reaching OTI. The larval plume was comparatively less concentrated in the Capricorn Bunker Group under average winds. Strong winds transported the larval plume far longshore, to the NW, away from the Capricorn Bunker Group, while the Capricorn Eddy transported larvae seaward into oceanic waters. Larval swimming could counteract these dispersive forces; however, significant dispersion had occurred before larvae developed strong swimming and orientation abilities. This study provides a physical proxy for the recruitment of snapper. Further, we have demonstrated that great insights into recruitment variability can be gained through determining the specific conditions experienced by survivors.

Theory for Erosion Wave Propagation in Cua Dai Beach, Vietnam

Tanaka, H.; Duy, D.V.; Mitobe, Y., and Viet, N.T., 2018. Theory for Erosion Wave Propagation in Cua Dai Beach, Vietnam. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 621–625. Coconut Creek (Florida), ISSN 0749-0208.This paper deals with the propagation of erosion wave in Cua Dai Beach in Central Vietnam under the influence of longshore sediment movement using analytical results from the simplified equation of the one-line model. In addition, measured shoreline positions obtained from Landsat images are utilized to validate the calculated results. Effects of the waves on the shapes of the erosion/accretion zones along the Cua Dai Beach are observed. Using the analytical solutions, the propagation speed of the erosion wave is estimated as 37 m/y while this value is 54 m/y in reality. Coastal structures such as seawalls located along the Cua Dai Beach is causing more severe erosion along Cua Dai Beach.

Response of the Snowy River Estuary to two environmental flows

The Snowy River is a major river in south-eastern Australia, discharging to the Tasman Sea via a barrier estuary, with its entrance constricted by marine sands. Since the construction of the Snowy Mountains Scheme, river flows have not been sufficient to maintain the river channel. A program of environmental flow releases (EFR) is returning water to the river to restore the fluvial reaches and is now trialling flow regimes that may also benefit the estuarine reaches. This paper documents the response of the estuarine segments of the Snowy River to two EFRs; the release in 2010 was designed to scour the upper reaches of the Snowy River while the larger 2011 release was intended to extend the scouring downstream. For each release, the effects on the entrance morphology, tides and salinity through the flow peak and recovery are described. Each EFR caused minor increases in depth and very minor longshore movement of the entrance channel, although each EFR had been preceded by a larger fresh flow that would have scoured the channels. The small increase in fresh water inflow in the 2010 EFR pushed salinity contours seawards and steepened vertical salinity gradients. The larger inflow in the 2011 EFR purged the upper estuary of saltwater. After the peak flow, salinity recovery was rapid in the principal estuarine channels but took weeks where poorly connected wetlands could store fresh flood waters. Critical flows for scouring the entrance and purging salinity are estimated.

Transport trajectories of “smart” pebbles on an artificial coarse-grained beach at Marina di Pisa (Italy): Implications for beach morphodynamics

In this paper, transport pathways for coarse sediments were studied on an artificial pebble beach located at Marina di Pisa (Tuscany, Italy). Radio frequency identification (RFID) technology was used to trace the pebbles. The RFID technique is an inexpensive and reliable methodology that is used to address sediment displacement along the coastline. The use of low-frequency radio signals (125kHz) enabled the detection of the marked pebbles within a range of 40cm on the underwater and subaerial portions of the beach. About one hundred pebbles were sampled on the beach and paired with passive transponders. The tracers were released along a series of cross-shore transects (March 2009). The pebbles were injected on the fair-weather berm, on the swash zone, and on the step crest along each transect. The recovery campaign was carried out in May 2009. Fifty-three pebbles were detected; this number is rather high given the intensity of the three storms that hit the coast during the period of the experiment and the significant reworking that this beach is subjected to even under fair-weather conditions. The analysis of the tracer pathways highlighted twofold convergent longshore movement on the beach. The trajectories indicated that the longshore movement dominated the cross-shore movement, but there was no prevailing transport direction. To explore the morphology of the sea bottom fronting the beach, a topographic survey was carried out using an echo sounder. The survey showed the presence of two distinct shoals 20m off the coastline that are responsible for the resulting coarse sediment transport. These results confirm that even mild sea bottom irregularities may significantly affect sediment transport dynamics along a beach, suggesting that accurate planning for defense schemes such as artificial coarse replenishment must begin with an in-depth knowledge of the morphology of the sea floor. Such information may increase the durability and/or the efficiency of the intervention.

Nature and stability of atoll island shorelines: Gilbert Island chain, Kiribati, equatorial Pacific

AbstractIslands rimming Pacific atolls typically form narrow, low‐lying lands that are commonly perceived to be particularly vulnerable to global changes such as sea‐level rise. As these, low islands form the only habitable land for many island nations, understanding the character of shorelines, and the rates and controls that operate to bring about changes, is an issue of central importance. The purpose of this study is to unravel the characteristics of coastal change on atoll islands of the Gilbert Island chain of the equatorial Pacific nation of Kiribati, especially as they relate to autogenic shoreline processes and El Niño/Southern Oscillation variability. Integration of field observations, differential global positioning system data, historical aerial photographs and ultra‐high resolution remote sensing images demonstrates the nature, spatial patterns and rates of change from 17 islands on Maiana and Aranuka atolls. The results illustrate that, between 2005 and 2009, ca 50% of the shorelines on these islands displayed a discernable shift in position; some shorelines were accretionary (at net rates up to ca 8 m year−1) and others were erosional (up to ca 18 m year−1). Long‐term net rates of change on Maiana between 1969 and 2009 were lower than short‐term net rates measured between 2005 and 2009. Both short‐term and long‐term observations illustrate some of the greatest change occur near terminations of the largest, north–south oriented islands, associated with longshore movement of coarse sand and gravel. Direct hits by tropical depressions and marked seasonality, factors interpreted as being essential in island growth and shoreline dynamics elsewhere, do not directly impact these equatorial atolls and can be eliminated as fundamental controls on shoreline dynamics. Similarly, observations over four years suggested that shoreline variability probably is not influenced directly by marked sea‐level change, although a recent increase in the rates of shoreline change could reflect instability related to the cumulative effect of a long‐term increase in the rate of sea‐level rise. Within this framework of global change, local anthropogenic effects, autogenic shoreline processes and El Niño/Southern Oscillation‐influenced wind and wave variability control many aspects of these dynamic shorelines. These results provide quantitative insights into the character and variability of rates of shoreline change, information essential for evaluating and mitigating the vulnerability of island nations such as Kiribati.

AERIAL PHOTOGRAPH OF SENDAI COAST FOR SHORELINE BEHAVIOR ANALYSIS

The shoreline analysis around the Nanakita River mouth has been performed by utilizing aerial photographs to reveal the different behavior between left and right side of the river mouth. The shoreline on left side of river mouth has been moving more dynamically than the right side. The empirical orthogonal function (EOF) method is also applied on the shoreline data to reveal the behavior of shoreline change. The first mode of variability reflects the cross-shore movement which is shown by uniform advance and retreat movement along the beach. The shoreline change around the Nanakita River mouth is dominated by the cross-shore movement which is shown by around 80% contribution of first mode. The second mode of variability depicts the longshore sediment movement which is shown by different shoreline movement between left and right side of river mouth. The longshore movement only contributes around 10% to the shoreline change around the Nanakita River mouth. The second mode also reveals the influence of river mouth to the longshore sediment transport characterized by different shoreline response between left and right side.

Investigation on the Nourished Sand Movement in terms of Luminescence Measurements

In this study, monitoring the nourished sand longshore movement was performed based on the feldspar luminescence measurements, including both Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). Nourished sand presents larger TL/OSL properties than the natural beach sand. Investigation on the spatiotemporal distribution of post-filling beach sand luminescence features provides the opportunity for assessment on the nourished sand movement characteristics, which was demonstrated from the present field studies along the Miyazaki coast and Shounan coast. Comparing these two techniques, it was found that the OSL technique is suitable for study on short-term sediment movement within a limited area; whereas, TL technique can be applied to the long-term and large-area nearshore process estimation.

Empirical orthogonal function analysis of shoreline changes behind two different designs of detached breakwaters

This paper compares the shoreline responses immediately shoreward of two adjacent schemes of segmented shore parallel rubble mound breakwaters undergoing the same forcing by waves and tides. Scheme one consists of four longer, emergent breakwaters that have produced tidal tombolos in their lee. Scheme two consists of five shorter, lower breakwaters that are submerged at higher tides with salients in the breakwater's lee. Empirical orthogonal function analysis was used to decompose a video derived shoreline dataset into the dominant modes of shoreline change for both schemes. The two schemes showed similar modes of change. The primary mode of change for both schemes was the cross-shore growth and shrinking of the salients/tombolos. The secondary mode of change was the longshore movement of the salients/tombolos. For all modes of change, the dominant length scale was dictated by the breakwater dimensions and locations. A new manifestation of hydrodynamic parameters is introduced: the cumulative integral of the de-meaned parameters. This parameterisation allowed for meaningful correlation of the temporal EOF components with forcing parameters and identification of the important influence of the tide on observed morphodynamic change. Clear differences were observed between the shoreline responses measured in schemes one and two; including differences in bay erosion/accretion; and the longshore translation of salients/tombolos. The beaches in scheme two showed behavioural patterns similar to unprotected beaches which were not observable in scheme one. It is postulated that these differences are caused both by the different breakwater designs and by variation in longshore sediment supply.

Storm-generated morphological change and longshore sand transport in the intertidal zone of a multi-barred macrotidal beach

The morphology, hydrodynamics and sediment transport processes of a multi-barred macrotidal beach were surveyed over 21 semi-diurnal tides, the first eight of which were dominated by storm conditions with offshore wave heights ranging from 1.2 to nearly 3.5 m. The hydrodynamic conditions prevailing over the intertidal beach showed a rapid response to these storm conditions, notably through strong longshore currents that attained peak velocities of 1.6 m s − 1 , i.e. over three times the peak spring tide velocities of tide-generated longshore currents during calm conditions. The bars showed no cross-shore migration during the storm events. Morphological mapping of the beach, tracer dispersal patterns, and calculations of potential sand transport rates using the energetics-based model of Bailard [Bailard, J.A., 1981. An energetics total load sediment transport model for a plane sloping beach. J. Geophys. Res., 86, 10938–10954.] showed significant longshore sand transport, as well as longshore migration of an intertidal drainage channel. These longshore movements were bi-directional, responded to changes in incident wind and wave directions, and involved wave stirring, coupled with longshore advection of sand along the bar crests in particular. Strong trough and cross-shore channel flows also led to the longshore migration of crescentic dunes. The longshore sediment movements were embedded in a cross-shore gradient associated with increasing overall profile mobility from the lower to the upper beach in response to wave shoaling and post-breaking processes, assessed through examination of the relative wave height Hs h − 1 , based on the relationship between significant wave height (Hs) and water depth ( h). Longshore sediment transport is a characteristic feature of the long-term evolution of Wissant Bay, which is dominated by a net northeastward residual sand transport within a bay sediment cell characterised by significant updrift erosion and downdrift accumulation. Longshore transport should be considered as an essential element of the morphodynamics of bar-trough beaches subject to large tidal ranges and significant wind stress, but there is a need for a better understanding of the role of wind stress on longshore currents in the shallow intertidal zone of these beaches.

Dual sand sources on Farewell Spit intertidal sand flats, New Zealand: Partitioning during redistribution

Extensive intertidal sand flats (c.10000 ha, c. 100 km2) on the sheltered southern side of Farewell Spit, a 30 km long sand spit located at the northern tip of South Island, New Zealand, extend for up to 8 km into Golden Bay. They consist primarily of fine sand with an upper size limit of 0.36 mm, blown from the spit during northerly storms. In parts of the flats fine sand is supplemented by significant but highly variable amounts of coarse sand, with rare stones up to 40 cm long. Tree trunks with tangled root masses, stranded on the flats, suggest that the coarse sediment is being delivered in the root masses of trees which are washed out of rivers discharging into Golden Bay from the mountainous southern hinterland. The greatest concentrations of coarse sediment are located northeast of the Aorere and Takaka Rivers, the two largest rivers discharging into Golden Bay. We propose that some trees are blown by prevailing southwesterly winds from the river mouths to the sand flat, and that a clockwise tidal current gyre carries trees from all rivers onto the flats in the northwestern corner of the bay. The patchy distribution of coarse sand on the intertidal flats indicates that redistribution of sand across the flats is partitioned: coarse sand (>0.5 mm) is not widely mixed with fine sand by surface processes, while fine sand is widely distributed. The sand flat extends subtidally to the 10 m bathymetric contour, giving the system a total area of c. 200 km2. An estimated volume of Holocene age sand in the spit and the sand flats of c. 5.7 km3 represents only c. 10% of the sand delivered to the northern tip of the South Island by longshore movement up the West Coast. The bulk of the sand may be accumulating on the continental shelf.

Characterization of the marine fish assemblage associated with the nearshore hardbottom of Broward County, Florida, USA

Some shallow (<7 m, water depth) nearshore hardbottom areas of southeast Florida have been reported to function as important juvenile fish habitat. Much of this area has been impacted by one or more local beach renourishments (sand fill to offset erosion). We characterized the nearshore fish communities and compared the fish assemblages adjacent to renourished beach to those adjacent to never-renourished beach along a 30-km stretch of coastline, primarily in Broward County, using three visual census methods. Two hundred transect-counts, 100 point-counts and 98 rover-diver counts were completed during June–August 2001. In transect- and point-counts, abundance of all fish species and their sizes were recorded; the rover-diver counts consisted of a simple species list. In total, 164 species and over 72,000 fish were recorded. The highest number of species (145) was recorded with the rover-diver counts. The transects-counts had 118 species and 109 species were recorded on the point-counts. With either all the sites adjacent to renourished beach pooled and compared to the pooled never-renourished sites or individual comparisons amongst renourished and neighboring never-renourished sites, no consistent differences were noted in fish abundance or species richness (ANOVA) or among fish assemblage structure (MDS plot of Bray–Curtis dissimilarity indices). However, although the data show no obvious distinct difference between the renourished and never-renourished sites, due to several important confounding factors (e.g., census methodology, longshore movement of sand fill) and the absence of baseline data prior to any renourishment, it would be premature to translate these results into management strategies. The assemblage structure, in terms of percentage of juvenile fish (<5 cm) as well as percent contributions by family, was similar for the point-counts and transect-counts. However, in mean density per m 2 of substrate, greater abundance and greater species richness values were recorded with the transect-counts than with the point-counts. Newly settled and early juveniles were the dominant component (>84%) of the inshore fish community, consisting primarily (>90%) of grunts (Haemulidae). After the grunts, the wrasses (Labridae) at about 5%, and damselfish (Pomacentridae) at roughly 2% were the predominant families. It is clear from this study and others that the nearshore hardbottom of Broward County is an important juvenile fish habitat, especially for grunts. However, the nearshore hardbottom does not appear to be obligate habitat for these fishes as fishes associated with this area are, apparently, not unique to the nearshore hardbottom either in species or ontogenic stage.

Influence of Jetty Construction on Morphology and Wave Set-Up at a River Mouth

At the Shiribetsu River mouth located in Hokkaido, Japan, a jetty has been constructed on the right hand side of the mouth from July to October 1999. In this study, the effectiveness of the jetty construction is studied by analyzing field data of river mouth morphology and water level variation. Before the jetty construction, water level rise due to wave set-up has frequently been observed every year from October to March. It is found that the height of the wave set-up increased in winter with the development of sand spit at the river mouth. After the construction, it is confirmed that the interruption of longshore sediment movement due to the construction of the jetty led to the stability of the river mouth opening. Furthermore, it reduces the wave set-up in the river mouth.

Longshore Movements of Adult Male Jasus Lalandii: Evidence from Long-term Tag Recaptures

Large-scale longshore movements (>10 km) of adult male Jasus lalandii (>70 mm carapace length) were examined on the west coast of South Africa using tag-recapture information from the period 1968 to 2000. The average rate of recovery of tagged rock lobsters was 15.7% per fishing season. Only 0.48% of 43 885 recaptured rock lobsters moved >10 km, 0.31% southwards and 0.17% northwards. The mean distance moved by those lobsters was 28.6 km and the mean time at liberty was 241.8 days. In recent years, densities of J. lalandii have increased substantially at the south-eastern end of their geographic range. The data indicate that this could not be attributable to a population migration of adult male rock lobsters from the west coast of South Africa.

Determining ridge and runnel longshore migration rate using Spot imagery

High-resolution colour satellite imagery is used to map the morphology and to determine the longshore movements of intertidal ridge and runnel systems that characterise the Atlantic southwest coast of France. A couple of Spot images recorded during the summer of 1989 are processed to extract accurate geographical maps of the coastline defined, in this study, as the level corresponding to the lowest astronomical tide. The coastline extraction method is based on a reflectance model, providing depth measurements that were calibrated in the field. This method enables us to map the coastline regardless of the tide level above the lowest astronomical tide at the moment of the image capture. As this model can be extrapolated to a large area when sea water is clear (considering a restricted range of suspended matter concentration) and bottom colour is homogeneous, the variability of both water turbidity and bottom colour have been analysed on the images. From the remotely sensed coastlines, a statistical analysis of ridge and runnel system length and longshore migration has been performed for a 35 km long coastline located to the north of the Arcachon lagoon inlet. It has been observed that, during the summer of 1989 characterised by fair weather conditions, the typical ridge and runnel morphology remains nearly conservative. Whereas the ridge and runnel mean wavelength remains constant during the summer between 435 and 430 m, about 84% of the systems move southward at the mean rate of 2.4 m d –1 . These results derived from Spot image analysis were validated by in situ topographic surveys undertaken during similar hydrodynamical conditions. L’imagerie satellitaire à haute résolution spatiale est utilisée afin de caractériser la morphologie et les déplacements des systèmes de barre–baïne intertidales situés sur la côte sud-ouest de la France baignée par l’océan Atlantique. Deux images Spot, enregistrées durant l’été 1989, ont été analysées afin de cartographier précisément le trait de côte défini, dans cette étude, comme le lieu du zéro hydrographique. La méthode d’extraction du trait de côte est basée sur un modèle de réflectance calibré sur le terrain, qui met en relation la réflectance avec la profondeur. Cette méthode permet d’obtenir une cartographie du trait de côte, quelle que soit la hauteur d’eau au dessus du zéro hydrographique. Comme ce modèle peut être appliqué à un long linéaire côtier lorsque la turbidité de l’eau varie peu et que la couleur du fond est homogène, la variabilité de ces deux paramètres a été estimée sur les images. A partir des traits de côtes qui ont été extraits, la longueur des systèmes barres–baïnes ainsi que de leurs déplacements parallèlement au rivage ont été statistiquement analysés sur une portion de la côte girondine mesurant 35 km de long, située au nord de l’embouchure d’Arcachon. Durant l’été 1989, caractérisé par des conditions relativement calmes, la morphologie des barres–baïnes change très peu. Les systèmes de barres–baïnes ont une longueur d’onde moyenne comprise entre 430 et 435 m et environ 84 % des systèmes se déplacent vers le sud à la vitesse moyenne de 2,4 m par jour. Ces résultats obtenus par analyse d’images Spot ont été validés par le suivi topographique d’une plage de la zone pendant des conditions hydrodynamiques identiques.

Seasonality of beachwrack at Oakajee in the mid-west region of Western Australia

This study examines seasonal fluctuations in beachwrack accumulations near Oakajee, Western Australia (28°34'5; 114°34'E). Twelve monthly samples were made of beachwrack concentrations at Drummonds Cove and Coronation Beach from 5 May 1998 until 10 April 1999. Four weekly samples were made from 23 July to 12 August 1998 to measure short-term variations in beachwrack volumes. There was no seasonality in beachwrack accumulations at Drummonds Cove. At Coronation Beach beachwrack concentrations were high in late summer and early winter, but declined sharply from late winter to early spring. There was considerable movement of wrack onto and off the beaches. Beachwrack accumulations migrated along the beach within Drummonds Cove, but there was no evidence of such longshore movement on the open coast. Beachwrack accumulates in bays and other protected areas and at the tip of peninsulas, such as Point Moore. Macroalgae and the seagrass genera Amphibolis and Posidol1ia dominated the wrack.