No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Etapes et rythmes de formation d'une flèche sédimentaire à crochets multiples en environnement mégatidal
... Once all the shorelines and limits of the high-tide beach were obtained, the next step consisted of extracting their variations over time. This was carried out using a code developed at the University of Caen [Robin, 2007; Robin and Levoy, 2007]. To this end, 120 working profiles spaced at 20 m intervals were constructed orthogonal to the oldest line. ...
... Enhanced erosion in the downdrift sector induces a change in shoreline orientation at the distal part of the spit. This change in orientation is at the origin of the formation of a new hook spit [Robin and Levoy, 2007]. The recent morphodynamic behavior of Agon spit, between T 0+6 and T 0+10 (2008) (bar 2) is in agreement with this pattern. ...
... Because of low migration rates, the time of presence of the bar near the high-tide beach is much larger in megatidal environments ($7 years for bar 1 (1977– 1984) and at least 4 years for bar 2 (2004 –2008, with a welding phase that is still ongoing) than on coasts with smaller tidal ranges (approximately 1 year). This explains the strong disturbance such bars induce in the sediment dynamics of the high-tide beach and in shoreline evolution in large tide range environments, it also explains the longer cycle of migration and welding of swash bars [Robin and Levoy, 2007]. The transverse bar morphology observed in 1977 for bar 1, and since 2005 for bar 2, also testifies to the influence of these forms on sediment transport on the high-tide beach. ...
Swash bar development has been well documented from coasts with low to moderate tidal ranges, while studies of the effects of these forms on the morphology and dynamics of the adjacent shore in large tide range environments are rare. The analysis of sequential vertical aerial photographs was combined with field work in order to highlight the effects of swash bar development on the adjacent shoreline in the vicinity of a megatidal inlet (mean spring tidal range of 11 m). Swash bars are observed to form on the ebb tidal delta and to migrate landward before welding onto the coast. A close relationship was noticed between the position of the swash bar and shoreline dynamics. The bar protects the shore against wave attack in a sedimentary system controlled by longshore transport. This protective role is, however, modulated by the large tidal range. As the bar migrates upward toward the high-tide level and the subaerial beach, it develops morphologically into a transverse form that acts as a cross-shore obstacle to longshore sediment transport, thus resulting in shoreline accretion updrift and in strong erosion downdrift. This disturbance may persist for years because of the relatively slow speed of movement of the bar at this stage, an aspect characteristic of large tide range environments. This pattern of behavior differs fundamentally from that documented in the literature where the perturbation of the longshore sediment transport occurs over shorter periods. An original conceptual model of swash bar morphodynamics and repercussions on the adjacent shoreline for this megatidal environment is proposed.
... The role of wave climate conditions in hypertidal environments, such as in the present study, is limited by the fact that wave action is not always effective on the spit (Kulkarni et al., 2004;Robin et al., 2009a). Tides do not directly affect hook formation but can control displacement of ebb delta swash bars across the estuary by intertidal channels (Robin & Levoy, 2007;Michel et al., 2017) and modify the shoreline wave processes (Carter & Orford, 1984). The aim of this study is to investigate the sedimentary architecture of a coarsegrained barrier spit, in relation to its morphological evolution. ...
... The role of swash bars in the development of sandy barrier spits has been studied in some macrotidal inlets (Robin & Levoy, 2007;Robin et al., 2009aRobin et al., , 2009bMontreuil et al., 2014;Le Bas & Levoy, 2018). These morphologies are integrated into the terminus of the sandy spits (Robin et al., 2009b(Robin et al., , 2020 and have even been found in the architecture of some barriers in microtidal (Nielsen & Johannessen, 2009), mesotidal (Lindhorst et al., 2008(Lindhorst et al., , 2013Costas & FitzGerald, 2011) and macrotidal environments (Fruergaard et al., 2020). ...
This study focuses on the architectural characterization of a mixed sand‐and‐gravel spit in relation to its multi‐decadal evolution. The spit, located in the Somme estuary in northern France, is a 5 km long sedimentary body made of several amalgamated ridges with a hooked terminus. Flint pebbles originating from the erosion of Cretaceous chalk cliffs and sands reworked from the tidal flat constitute the spit. Aerial photographs demonstrate that the spit formation started in the 1940s and document its growth through time. The architecture of the spit has been characterized by ground‐penetrating radar (GPR) investigation using 400 MHz and 900 MHz antennas. The GPR data are used to distinguish progradational, aggradational and transgressive geometries that are related to cross‐shore and longshore construction, respectively. Three main morpho‐stratigraphic domains characterize the spit from the root to the terminus: (i) a beach‐ridge domain mainly made of progradational structures; (ii) an intermediate domain formed by beach‐drift structures topped by deposits with cross‐shore geometries; and (iii) a spit‐terminus domain mainly composed of beach drift structures which incorporate sand deposits. At the scale of an individual ridge, high‐frequency GPR data indicates five types of sedimentary architectures associated with the evolution from a hook stage to the subsequent elongation stage. The overall development of the barrier spit is the result of complex interactions between wave and storm dynamics, potentially modulated by the variability of submersion time related to the 18.6 year tidal cycle, and anthropogenic activities including groyne construction and gravel reloading.
... The site of Blainville-sur-Mer presented a dominance of gravelly habitat. The shoreline here was composed mainly of sandy dunes undergoing intense erosion (Robin and Levoy, 2007;Robin et al., 2009) and sandy intertidal dunes and flats subject to rapid displacements owing to high-energy hydrodynamics in a megatidal environment (tidal range .12 m during equinoctial spring tide) (Levoy et al., 1997(Levoy et al., , 2013Montreuil et al., 2014). ...
... These currents were directed northward around high tide and southward at low tide. Moreover, waves generated sand transport over the ebb delta platform of Régneville-sur-Mer, located south of Blainville-sur-Mer (Levoy, 1994), which induced longshore sand transport, then leading to the onshore migration of wave-formed bars (Robin and Levoy, 2007;Robin et al., 2009). During spring tides, it was possible to observe the displacement of intertidal sand dunes with a volume between 25 000 and 30 000 t and crested higher than 2 m (Robin et al., 2009;Montreuil et al., 2014). ...
Recreational and professional clam fishing was an important activity on the extensive intertidal zone of the western Cotentin
coast (western English Channel). A variety of fishing gear was used to harvest the target species: the European clam Ruditapes decussatus (Linnaeus, 1758) and the introduced Manila clam R. philippinarum (Adams and Reeve, 1850). In this study, we studied the effect of rake harvesting during the spring tides of February–March
2014, following an experimental design with a control station and three stations impacted by rake harvesting in three sediment
types: sandy, gravelly, and mixed gravelly rocky habitats. No significant sediment and macrofauna changes occurred at the
three sites after rake harvesting. Nevertheless, the number of clams decreased significantly after raking on the gravelly
habitat, whereas in the other two habitats, sediment transport in this high-energy hydrodynamic environment was able to transfer
clams and other macrofauna species across the fishing sites, thus minimizing the effects of rake harvesting. Therefore, although
the effect of rake harvesting appeared limited during winter, the regional impact (high fishing pressure along 60 km of coast)
and increase of recreational fishers during summer needed to be studied in the future.
... Therefore, to address these issues, reference to the most appropriate coastline indicators (dune borders and the limits of vegetation and berms) is needed (Boake and Turner 2005); it is also important to take into consideration the coastal types in the study area and the available data sources (Dolan et al. 1980;Robin and Levoy 2007;Faye 2010;MEEDDM 2010). ...
The eastern coast of the Algiers, which stretches over 15 km, is currently experiencing very intense socioeconomic and urban development that is causing severe disturbances to the coastal environment. The main issue of this study concerns itself with understanding the evolutionary trends of this system and assessing its state of vulnerability towards erosion phenomena. This work focuses on the historical study of the variation in the shoreline position by combining photogrammetry data and in situ DGPS measurements (Differential Global Positioning System). Data treatment was carried out using a geographic information system (GIS) and the Digital Shoreline Analysis System (DSAS) geostatistical computing tool. These techniques have enabled identification of the erosion/accretion rates and description of the evolutionary trends over a period of 58 years by calculating the net rates of coastline changes over three time periods (1959–1980, 1980–2003 and 2003–2017). The results show that the net rate fluctuates between sites, with an overall tendency towards erosion (49% of the coastline), associated with a significant variation in the average annual rates. The computed statistics show that the study area was in a state of accretion between 1959 and 1980, with an average end point rate (EPR) equal to 0.72 m/year. This net rate of change turned negative and became alarming during the period between 1980 and 2017 when the EPR decreased to − 0.54 m/year. These trends are due to a combination of the cumulative effects of storms and anthropogenic actions. Hence, sustainable management policies must be developed rapidly by coastal managers to rehabilitate the area.
... It is likely that the nearshore channel ( Figure 7) inhibits swash bar formation west of this point, so that the accretion of some 33,800m 3 recorded from July 2007 to December 2009 would need to be attributed to inputs from littoral drift arriving from the east. Previous research on the west Normandy, France coast has suggested that drift "shadows" develop immediately downdrift of zones of initial swash bar attachment with erosion occurring during a short lag period before the swash bar material becomes distributed downdrift (Monfort 2007;Robin and Levoy, 2007). The trends recorded here could be explained if a similar process were operating To summarise, the data suggests that onshore migrating swash bars deliver inputs of sand along a 1.3km zone between profiles 5a00176 (Cakeham) and 5a00205 (West Wittering). ...
This report aims to provide an analysis and interpretation of changes occurring along the beach and foreshore of the Cakeham, West Wittering and East Head frontage. It updates and extends the geographical coverage of a similar previous study (Bray, 2007). It is concerned primarily with the period August 2004 to October 2009, but includes analysis of some profile data extending up to December 2009. The objectives are as follows:
1. Identify the main changes occurring to the beach and foreshore for 2006-09 covering areas defined by the Bray (2007) study, including potentially sensitive areas such as East Head spit neck and the Hinge;
2. Extend analysis to cover the Cakeham frontage (2004 – 09) including assessment of recent sand accumulation;
3. Attempt to provide explanations for the changes occurring, including estimations, where possible of likely trends for the future;
4. Comment upon potential implications for on-going and future monitoring and management at East Head, West Wittering and Cakeham.
Although the brief was primarily to evaluate and report upon the beach and foreshore changes within the specified areas and time periods, likely future changes and their implications for management will be discussed where applicable.
... ROBIN, N.; LEVOY, F., and MONFORT, O., 2007. Bar morphodynamic behaviour on the ebb delta of a macrotidal inlet (Normandy, France). ...
The purpose of this paper is to describe the morphodynamics of bars located on an ebb delta of a macrotidal inlet system. Although there have been numerous studies on the morphology and physical processes affecting such bars, these concern only microtidal and mesotidal settings. The west Cotentin coast (Normandy, France) is a fine example of a macrotidal coast with one of the largest tidal ranges in the world. The mean spring tidal range is up to 11 m. Fieldwork was undertaken during one month and involved the deployment of an electromagnetic current meter, a pressure sensor, and differential global positioning system surveys to measure topographic changes. Along the western coast of Cotentin, in spite of the very large tidal inlets, bars are generally small (2 m high and 250 m long) compared with other tidal environments. During the campaign, the bar migrated exclusively landward over a distance of 4 m. Its volume did not change significantly. This slow migration rate is attributed to the large tidal range. In a macrotidal setting, the tidal water level fluctuations control wave height during storms, the duration of emergence and flooding periods, and the intensity and direction of longshore tidal currents. Field observations showed that bar migration rate is correlated with wave activity, whereas volume variations are controlled both by mean tidal currents and wave conditions. These results indicate a morphodynamic behaviour strongly related to specific local macrotidal hydrodynamic conditions, the tidal prism influence being less important.
A unique dataset of 27 years of shoreline changes along the western coast of Cotentin Peninsula (Normandy, France) with a high temporal resolution (about 60 fields surveys) provides an opportunity to analyse the space and time variability of a tidal sandy wave-exposed shoreline and to identify the main drivers of this variability. Spatially, as often observed around the world, this coast shows a clear balance between erosional and accretionary sectors. Maximum shoreline retreat and mean annual erosional rates are, however, greater by a factor of two than maximum and mean accretion rates, with hotspots mainly observed in the vicinity of tidal inlets. A cluster analysis highlighted nine groups of shoreline behaviour that show important contrasts, depending on locations, between erosional and accretionary trends. Linear trends over the nearly three decades of monitoring are uncommon and reverse trends are frequent. Clear divergences in behaviour were observed over the same time periods depending on location, e.g., erosion and accretion can be contemporaneous as can accretion and stability, independently of the antecedent multi-annual trends. Ebb delta zones show heterogeneous patterns, and survey locations belonging to the same clusters appear spatially scattered. The temporal shoreline behaviour can be linked to combinations of high wave energy and high spring tides only for at best 37.5% of the survey locations, and this only over the last decade. No obvious link with mean sea-level rise and the 18.6 y lunar tidal cycle was observed. However, as in the case of wave activity, these external drivers can often be masked by local factors such as inlet dynamics or/and anthropogenic influences (essentially shoreline engineering). Further insight on
sand supply and beach elevation changes is necessary to improve the understanding of the main factors driving space and time variability in shoreline mobility, and it is expected that sea-level rise in the future will be of increasing concern, with important, potentially negative, implications for shoreline mobility on the low sandy sectors of the Normandy coast.
Link to download the final article before june29, 2023 : https://authors.elsevier.com/c/1h33r8MvAty14R
Sand spits with distal hooks have been well documented from coasts with low to moderate tidal ranges unlike high tidal‐range environments. Datasets from 15 LiDAR and 3 UAV surveys between 2009 and 2019 on the Agon spit in Normandy (France), a setting with one of the largest tidal ranges in the world (mean spring tidal range: 11 m), combined with in‐situ hydrodynamic records between 2013 and 2017, highlight a three‐stage pattern of spit hook evolution. Stage 1 (2009‐2013) commenced with the onshore migration and attachment of a swash bar, followed by persistent spit accretion updrift of the bar and erosion downdrift because of the slow speed of bar migration in this large tidal‐range environment. In stage 2 (2013‐2016), three overwash events and a 220 m‐wide breach culminating in the total destruction of the spit during winter 2015‐2016 involved the landward mobilization of thousands of m³ of sand. These events occurred during short durations (few hours) when spring high tides coincided with relatively energetic waves, underscoring the importance of storms in rapid spit morphological change. Strong spring tidal currents maintained the breach. Stage 3 (2016‐2019) has involved new hook construction through welding of a swash bar and spit longshore extension, highlighting the resilience of the spit over the ten‐year period, and involving a positive sediment balance of 244,000 m³. The three stages bring out, by virtue of the temporal density of LiDAR and UAV data used, a high detail of spit evolution relative to earlier studies in this macrotidal setting. The large tidal range strongly modulates the role of waves and wave‐generated longshore currents, the main process‐drivers of spit evolution, by favouring long periods of inertia in the course of the spring‐neap tidal cycle but also brief episodes of significant morphological change when storm waves coincide with spring high tides.
The shoreline in the vicinity of inlets can exhibit considerable variability in morphology in both space and time. Most studies on inlets and their adjacent shores have focused on the morphodynamics of sediment by-passing mechanisms generated by longshore transport. For the first time, the morphology, sedimentary features, sediment budgets and patterns of evolution of the shoreline and ebb delta in a macrotidal inlet system have been investigated using seven LiDAR topographic surveys in Normandy, France, over a period of 3.7 years from February 2009 to October 2012. The ebb delta shows strong development on the northern flank of the inlet, expressed by a large sand spit and two types of superimposed dynamic sandy features: eight long-crested and highly mobile transverse bars and a large swash bar. Sand transport from N–S on the updrift beach feeds the growth of the distal part of the spit. This sand supply is further augmented by the onshore movement of a large swash bar welding to the upper foreshore. However, the main topographic changes were induced by the northward migration of the transverse bars on the ebb platform. This is driven by strong northward-directed tidal currents parallel to the shore. The bars exhibit a more complex morphology and dynamics along the seaward margin of the ebb delta where their mobility is controlled by wave action. Topographic measurements suggest a clear sand recirculation pattern. In this morphodynamic model, sand coming from the updrift upper beach is transported southward and deposited at the distal end of the spit, where it serves to construct transverse bars close to the tidal inlet. Transverse bar migration ends in the wave-exposed northern margin of the ebb delta, where they are integrated into the shallow dissipative shoreface sand sink. This sink nourishes the southward longshore transport to feed growth of the large swash bar and southward spit elongation. This semi-circular recirculation cell model involves an inversion of sand movement close to the inlet and emphasizes the combined role of tidal currents and waves in the large-scale 3D ebb–delta sediment dynamics in this macrotidal setting, in contrast to the much more commonly reported alongshore sediment by-passing mode of microtidal inlets.
The morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in Normandy, France, was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts. Although there have been numerous studies on ebb-tidal bar morphology and on the processes affecting such bars, these concern only microtidal or mesotidal settings, and are mainly based on observations. No detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features, especially along macro and megatidal coasts. In this study, the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process (swash, surf, shoaling waves) over a representative bar cross-shore profile, and sediment transport patterns deduced from fluorescent tracer. The results show that the bar migrated exclusively shoreward during moderate to storm conditions (Hs>0.7 m at the bottom of the bar seaward slope). During low energy wave conditions, no bar movement was observed. Swash action was not the dominant process, mainly due to its duration, which did not exceed 8 min in the course of a semi-diurnal tidal cycle. The movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents. The increase in the significant wave height disturbed the general mean current behaviour, which was parallel to the bar crest during low energy conditions. Under surf conditions, mean flows are directed onshore, with an absence of bed return flow; this allows sediment to be transported towards the bar crest causing a landward bar migration. An offshore-directed cross-shore current (not favourable for bar migration) is recorded with larger water depths and no breaking waves (shoaling conditions). This study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting, which induce a rapid shift in processes (such as bed shear and sediment re-working) across the shore-normal profile. These conditions explain the relative low rate of migration of the bar over the delta platform (about 33 m/year). This rate is much less than those reported from micro-mesotidal environments. Calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement, notwithstanding the megatidal character of the field site.
L'ensemble des résultats obtenus au cours de cette thèse permet d'aboutir à un bilan sédimentaire global sur le secteur de côte étudié. L'embouchure des havres doit être considérée comme un piège à sédiments majeur vers lequel convergent les matériaux circulant longitudinalement sur les plages voisines. Le bilan sédimentaire des plages est de ce fait déficitaire. Les principales entrées sédimentaires dans le système étudié proviennent d'apports issus des plages au Nord de Carteret et de l'érosion du trait de côte de la zone d'étude. La frontière sud du système est fermée par le cap de Granville. Les extractions de sables, bien que réduites actuellement, ont fortement amaigri le volume total de sédiments depuis 50 ans. Les échanges sédimentaires avec les petits fonds sont limités à la partie méridionale de la zone d'étude. Compte tenu des volumes en jeu, ils interviennent peu dans l'évolution des plages, contrairement à la circulation sédimentaire longitudinale sur les hauts et moyens estrans.
In mixed energy settings, tidal inlets undergo an episodic process known as shoal bypassing, whereby discrete bars are released from the ebb-tidal delta and migrate onshore. Such events can transport large volumes of sand, in the form of landward-migrating shoals, to adjacent beaches. Nine tidal inlets in South Carolina were analyzed to determine if there are predictable relationships among the volume of sand in the ebb-tidal deltas and in the individual bypassing shoals, the time interval between bypassing events, and the tidal prism. Historical photographs spanning up to 58 years yielded 221 discrete shoals at various stages of bypassing for the analysis. Building on earlier work by Kana (1995), mean shoal-bypassing event time intervals and the mean bypassing shoal volumes were found to be related to tidal prism. Larger inlets underwent shoal-bypassing events less frequently than smaller inlets, but produced larger bypassing shoal volumes. The relationship between average event interval and tidal prism was based on the equation, I = 0.046Tp + 4.56, where I is the average shoal-bypassing event interval (years) and Tp is the tidal prism (106 m3). The relationship between average shoal volume and tidal prism was based on the equation, S = 6.42Tp + 113.4, where S is the average bypassing shoal volume (103 m3). Both relationships are statistically significant. Bypassing shoals represented, on average, small volumetric percentages (0.6% to 6.6%) of their respective ebb-tidal delta volumes, yet in mesotidal, moderate wave energy settings such as South Carolina, shoal bypassing can be the single most important process contributing to a locally accreting beach.
The use of aerial photographs to estimate short-term shoreline changes, i.e. coastal changes at a monthly scale reflecting seasonal changes in the underlying hydrodynamics, is presented in this paper. To achieve this a data set of seven vertical aerial photographs with a time span of four months, taken at the Ebro delta (NE Spanish Mediterranean coast) has been used. The method was applied to the analysis of very flat areas, highly dynamic coastal features, storm impacts and to the entire deltaic coast. Although the study area is a microtidal environment, obtained results of the very flat areas analysis do not recommend its use at very short time scales due to meteorological tide influences. The formation, erosion and reformation of a spit at the river mouth was easily monitored being controlled the evolution elite length, perimeter and subaerial surface. Aerial photos permitted to identify vulnerable zones to impacts of very energetic storms by characterising breaching events along the coast (location and magnitude). Finally, when the method was applied to the entire deltaic coast, a detailed seasonal and spatial distribution of shoreline changes was obtained. The comparison of shoreline rates of change obtained from photos with that obtained from beach profiles shows that the method is reasonably accurate at least for the Ebro delta coast.
We investigated barrier island-tidal inlet sand sharing systems along the United States' mid-Atlantic coast to determine the impact of tidal inlets on adjacent shorelines. We used two reaches in our analyses with different natural (unstructured) settings: the wave-dominated Outer Banks of North Carolina and the mixed-energy, tide-dominated Virginia barrier islands. Three criteria were used to delineate inlet domination and inlet influence on the adjacent barrier shorelines based on the spatial distribution of shoreline rate-of-change values: (1) the cessation of abrupt changes, i.e., the reduction in variability in the rates of change along-the-shore; (2) a change in the sign of the rate value from erosion to accretion or nice versa; and (3) a change in the increasing or decreasing trends in rate values. The maximum distances of inlet influence extend to 6.8 km updrift and 5.4 km downdrift of inlets along the Virginia barrier islands and 6.1 km and 13.0 km for the updrift and downdrift inlet shorelines along the Outer Banks barriers. Additionally, shoreline changes can be dominated by inlet processes to a maximum distance of 4.3 km. These results support previous conclusions that sand bypassing processes exert greater influence on mixed-energy, short barrier island shorelines than on the wave-dominated, long, linear barrier island shorelines.
The inlet throat is increasingly dominated by ebb-tidal currents and seaward sediment transport as tidal range increases from mean towards spring tides. A similar trend exists in the marginal flood channels with increasingly stronger flood than ebb-tidal currents with increasing tidal range. These flow asymmetries explain the seaward- and inlet-oriented bedforms (sandwaves and megaripples) that floor these channels, respectively. -from Authors
Integration of beach profiles and water-level measurements at three sites on a microtidal, wave-dominated coast reveals that tide-gauge records systematically underestimate the actual elevations and horizontal positions that water reaches on the beach as a result of Wave runup. On low-gradient sandy beaches, natural morphological beach features, such as the erosional scarp and vegetation line accurately reflect the positions of frequent maximum high water levels and the berm crest reflects the position of more frequent ordinary high water levels, whereas tide-gauge records consistently predict lower maximum and average levels of beach flooding. The discrepancies between predicted and actual water positions on the beach have important scientific and legal implications. The scientific implications involve the need to map shoreline features that closely track the long-term trends in beach movement, but are insensitive to short-term fluctuations in water level. Neither the instantaneous high water line (wet beach-dry beach boundary) or the berm crest satisfy this requirement, and therefore, they are not recommended for monitoring shoreline position either in the field or interpreted from aerial photographs unless there is no reliable alternative. The legal implications pertain to land ownership and property boundaries in the United States that currently are surveyed from tide-gauge records but were originally defined by common law on the basis of high water levels that leave physical marks on the upland property. Because water levels are actually higher on the beach than predicted by tide gauges, land surveys based on a tidal datum allocate more littoral property to the upland owner than is justified by the physical facts or was intended by law. Consequently, the publicly-owned state submerged lands encompass less of the beach than that area which is regularly flooded by marine water.
Beach erosion rates are often determined by delineating historical shoreline positions from maps and aerial photographs and more recently global positioning systems (GPS). The high water line is usually selected as the shoreline indicator for mapping purposes; it is defined as the wetted bound and by "markings left on the beach by the last high tide." The high water line that is acquired from field determination or photogrammetric means is assumed to represent the mean shoreline position for that year, but field studies have shown that its position is variable because of changes in water level due to waves, wind, tides, and other factors. This study investigated the short-term variability in the high water line location over tidal cycles, days, and months through field observations and interpretation of videotape data. Studies, undertaken at Assateague Island National Seashore in Maryland and at the Field Research Facility at Duck, North Carolina, indicated that the high water line is a useful shoreline indicator within certain limits. GPS-acquired shorelines based on actual identification of the high water line in the field are deemed more accurate than photo-interpreted shorelines for coastal erosion mapping and management.
The United States Federal Emergency Management Agency (FEMA) is assessing technical methodologies and procedures for the collection, analysis, and computation of coastal erosion rates. It is likely that the methodology selected will involve the use of historical shoreline data compared with current shoreline information. This paper largely addresses the sources of errors inherent in the raw data. Source data include historical and recent National Ocean Service (NOS) T-sheets (produced ca. 1840s to present) and air photos (taken ca. late 1930s to present) as well as any other types of accurate map and photographic data. -from Authors
Prediction of shoreline change around inlets at meso-time scales (years to decades) is the next logical step following verification of microscale models. If mesoscale simulations are a goal, a basic question is how microscale models (that simulate processes at hours to weeks) can be scaled up in time or whether macroscale geomorphic models (that qualitatively describe changes at decades to centuries) can become more quantitative. The authors propose an approach that begins with consideration of tidal inlet morphology and sediment circulation around ebb-tidal deltas. Inlets are the focus because in some barrier island settings such as the southeast U.S. coast, it appears a majority of coastal erosion problems at meso-time scales can be traced to changes in adjacent inlets. Inlet morphology and geomorphic models, typical of mixed-energy coastal plain shorelines, are reviewed to illustrate certain common sediment transport patterns. A simplified conceptual model of inlets at meso-time scale is proposed from which the problem of sediment transport may be spatially partitioned. Four primary inlet domains are considered: (A) main ebb channel where tidally generated ebb currents control sediment discharge, (B) ebb-tidal delta with a broad swash platform that is ultimately in balance between ebb, directed flows and wave- and tide-generated shoreward transport, (C) shoal-bypassing zones at the margins of the ebb-tidal delta where sediment shifts unidirectionally from the delta to the shoreline under wave-generated transport, and (D) recurved spits adjacent to the inlet which receive shoal-bypass sediments. Excess sand accumulating in Domain D becomes subject to longshore advection toward and away from the inlet. A portion nourishes the adjacent beach and the remainder recycles back to the inlet channel (Domain A), completing the inlet transport loop.
Patterns of erosion and deposition along the shoreline of Price Inlet are strongly influenced by ebb-tidal delta processes. The ebb delta goes through cycles of growth and decay (15-20% change in volume) which last from 4 to 7 yr. In the growth phase, large intertidal bar complexes are formed on both sides of the main ebb channel by the coalescence of landward-migrating swash bars. During this stage, waves refracting around the southern bar complex produce a reversal in the longshore-transport direction on the downdrift shoreline. This results in little sediment escaping the inlet and delta system. The sand-trapping mechanism ceases and the delta reduces in volume when the bar complexes migrate onshore and attach to the beach. The inlet shoreline also goes through cycles of 7- to 42-yr duration in which its offset configuration changes. The shift in shoreline alignment is controlled by the orientation of the main ebb channel and the resulting ebb-tidal delta morphology.-from Author
The lowermost Kennebec River is a relatively narrow, deep, bedrock-cut estuary situated along the highly indented, central coast of Maine. A large tidal prism and strong tidal currents control bedload sediment transport at the estuary mouth. Riverine sediment is introduced to the estuary during spring freshets when freshwater discharge may increase by an order of magnitude. During the same time, some of the existing estuarine sediment is exported to the nearshore. A barrier adjacent to the estuary is the product of an abundant riverine sand supply and numerous bedrock ridges, which anchor this sand deposit. A variety of sedimentologic, geomorphic, and hydraulic data collected over a 14-year period document a clockwise, sand-circulation cell that involves the exchange of bedload among the entrance channel to the estuary, adjacent beaches, nearshore, and offshore region. Tidal currents transport sand from the estuary through a subsidiary channel, bordered on both sides by bedrock islands, to an offshore subtidal bar. Northeast storm waves breaking along this outer bar move sand westward and landward toward the beach, leading to the development of large bar complexes. The welding of a bar complex to the western end of Hunnewell Beach occurs every 6–9years and adds 2–300,000m3 of sand to the shoreline. Sediment transport calculations along the outer bar are consistent with this volume of sand. The sediment gyre is completed as beach sands are moved eastward along the beach and dumped into the estuarine channel. This movement of sand is produced by wave action and tidal currents that enter the estuary peripherally during the flooding tide. During the ebb cycle, strong tidal currents flowing seaward through the subsidiary channel advect water from along the beach, which enhances sand transport into the estuary. Thirteen years of beach profile surveys demonstrate a good correspondence between bar welding events and short-term (1–4years) accretionary shoreline changes. However, the volumetric changes of Hunnewell Beach cannot be accounted for by bar welding alone. The apparent movement of sand between Hunnewell Beach and other sand is further evidenced by the 135year record of historical shoreline changes that reveal up to 200m of shoreline excursions over a 26year period. These data suggest that erosional–depositional trends along Hunnewell Beach are complex and the result of the exchange of sand among the adjacent beaches, nearshore zone, Fox and Wood Island tombolos, Kennebec River channel, and offshore region. The effects of major spring floods and strong bedrock influences produce patterns of sedimentation and a distribution of sand bodies at the mouth of Kennebec River that differ markedly from accepted estuarine models that are based on the relative importance of wave and tidal energy. This study demonstrates the need to consider structural controls when using these models.
For coastal areas with constant rates of shoreline change through time, the results of all the methods are identical. For a coastline with a non-linear response, a linear estimation method can only approximate the average rate-of-change. As the response of the coastline becomes more non-linear, the differences among the rate-of-change estimates given by the various methods increase. Using data from a 65 km long section of the Outer Banks of North Carolina, we demonstrate the differences in computational methods for estimating shoreline changes and we show how the potential sources of error can bias the final statistics. -from Authors
Analysis of shoreline variability and shoreline erosion-accretion trends is fundamental to a broad range of investigations undertaken by coastal scientists, coastal engineers, and coastal managers. Though strictly defined as the intersection of water and land surfaces, for practical purposes, the dynamic nature of this boundary and its dependence on the temporal and spatial scale at which it is being considered results in the use of a range of shoreline indicators. These proxies are generally one of two types: either a feature that is visibly discernible in coastal imagery (e.g., high-water line [HWL]) or the intersection of a tidal datum with the coastal profile (e.g., mean high water [MHW]). Recently, a third category of shoreline indicator has begun to be reported in the literature, based on the application of image-processing techniques to extract proxy shoreline features from digital coastal images that are not necessarily visible to the human eye. Potential data sources for shoreline investigation include historical photographs, coastal maps and charts, aerial photography, beach surveys, in situ geographic positioning system shorelines, and a range of digital elevation or image data derived from remote sensing platforms. The identification of a "shoreline" involves two stages: the first requires the selection and definition of a shoreline indicator feature, and the second is the detection of the chosen shoreline feature within the available data source. To date, the most common shoreline detection technique has been subjective visual interpretation. Recent photogrammetry, topographic data collection, and digital image-processing techniques now make it possible for the coastal investigator to use objective shoreline detection methods. The remaining challenge is to improve the quantitative and process-based understanding of these shoreline indicator features and their spatial relationship relative to the physical land-water boundary.
A study of the East Frisian Islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing. This island chain is located on a high wave energy, high tide range shoreline where the average deep-water significant wave height exceeds 1.0 m and the spring tidal range varies from 2.7 m at Juist to 2.9 m at Wangerooge. An abundant sediment supply and a strong eastward component of wave power (4.4 × 103 W m−1) have caused a persistent eastward growth of the barrier islands. The eastward extension of the barriers has been accommodated more by inlet narrowing, than by inlet migration.It is estimated from morphological evidence that a minimum of 2.7 × 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system. Much of this sand ultimately bypasses the inlets in the form of large, migrating swash bars. The location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb-tidal delta along the downdrift inlet shoreline. The configuration of the ebbtidal delta, in turn, is a function of inlet size and position of the main ebb channel. The swash bar welding process has caused preferential beach nourishment and historical shoreline progradation. Along the East Frisian Islands this process has produced barrier islands with humpbacked, bulbous updrift and bulbous downdrift shapes. The model of barrier island development presented in this paper not only explains well the configuration of the German barriers but also the morphology of barriers along many other mixed energy coasts.
Tidal inlet sediments make up a significant portion of most barrier island complexes. Inlet-affiliated sedimentary units usually include an ebb-tidal delta (seaward shoal), a flood-tidal delta (landward shoal) and inlet-fill sequences created by inlet migration and recurved spit growth.The morphological components of ebb-tidal deltas include a main ebb channel flanked by linear bars on either side and a terminal sand lobe at the seaward end. This channel is bordered by a platform of sand dominated by swash bars which is separated from adjacent barrier beaches by marginal flood channels. The ebb-delta sand body is coarser-grained than other sedimentary units of the inlet and contains polymodal cross-bedding with a slight ebb dominance.Flood-tidal deltas consist of a flood ramp and bifurcating flood channels o the seaward side, which are dominated by flood currents and flood-oriented sand waves, and ebb shields, ebb spits and spillover lobes on the landward side, which contain an abundance of ebb-oriented bedforms. A proposed stratigraphic sequence for a typical flood-tidal delta contains bidirectional, large-scale crossbedded sand at the base, predominantly large-scale (flood-oriented) crossbedded sand in the middle, and finer-grained tidal flat and marsh sediment at the top.Inlets migrate at rates that vary from a few to several tens of meters per year, depending upon such variables as rate of longshore sediment transport and depth of the inlet. Inlet-fill sequences, which fine upward, contain coarse, bidirectional crossbedded sediments at the base, polydirectional crossbedded sands in the middle, and finer-grained aeolian sand at the top.Both tidal-delta morphology and relative size and abundance of ebb- and flood-tidal deltas are considerably different in different oceanographic settings. Microtidal (tidal range T.R. = 0–2 m) areas tend to have smaller ebb-tidal deltas and larger flood-tidal deltas; whereas, mesotidal (T.R. = 2–4 m) areas show just the opposite trend. Large waves tend to inhibit the development of ebb-tidal deltas and accentuate the growth of flood-tidal deltas.
Rates of deposition and erosion were derived from old maps and aerial photographs of beach ridges and shorelines on Penouille dating from 1765 to 1981. In July 1989, 41 beach ridges were mapped along 7 traverses and sediment samples were collected along 15 beach profiles. A 6-stage model of spit evolution was derived from the beach ridge and shoreline data. Rates of erosion along West Beach (0.50±0.08 m/yr) and beach ridge accretion (45.5±7.3 yrs/ridge) were used to estimate the net sediment flux (1000±160 m³/yr) and age of the Spit (2110±340 yBP). Wave refraction analysis provided evidence that Sandy Beach Spit on the south shore of Gaspe Bay blocked incoming storm waves and longshore currents, resulting in erosion along West Beach at 500±80 yBP. -from Authors
Shoreline salients, cuspate forelands and tombolos on the Coast of Western Australia were examined photegrametrically to determine the suite of landforms that comprised the structures and whether there were regional differences in their geometries. Aerial photographs from three regions were examined, including the South Coast, from Cape Arid to Esperance; the Central West Coast, between Guilderton and Dongara; and the Ningaloo Coast, from Point Cloates to Exmouth. The landforms were broadly classified according to morphological and genetic criteria, such as growth mechanism, degree of mobility, contemporary activity/processes, main processes involved in the supply of material, sources of supply, conditions of wave regime and stage of development. Second, morphometric information was taken directly from the aerial photographs. Following work reported by SILVESTER and HSU (1993), this was analyzed to determine the constant and exponential values in the curvilinear relationship between the length of the offshore structure relative to its distance offshore and the difference between offshore distance and the ratio of salient protrusion to island length. Separate graphs have been compiled for all observations from Western Australia as well as for each region to indicate geographic differences. An examination of the geometric differences between each coastal region was undertaken using analysis of variance techniques. At the 5% significance level, the results indicated that, there are significant differences between the coastal regions for most ratios when tombolos are excluded from the data sets. Most of the South Coast forelands and tombolos are formed by the deposition of sediment resulting from the convergence of swell behind an obstacle, whereas on the Central West and Ningaloo Coasts the submerged reef provide an offshore barrier. Swell is complexly diffracted and refracted by the reefs and sediment is deposited in a less predictable manner. Flushing of lagoonal waters behind the reef either by longshore currents or offshore movement of water through breaks in the reefs appears to impede the formation of tombolos and provides an explanation for travelling versus stationary forms in the respective environments. It has been shown that the development of these forms cannot be attributable to the length of the offshore obstacle or the distance of the obstacle offshore. Explanation of this requires further investigation of the combined oceanographic processes occurring leeward of the reef chains on the Central West Coast and on the Ningaloo Coast.
Well-developed, multi-component flood- and ebb-tidal deltas exist in the lower portion of the Chatham Harbor estuary, Cape Cod, Massachusetts. Each tidal delta contains several smaller sand bodies, a dominant bedform type, and a preferred bedform orientation, which indicates a distinct sediment transport pattern. The hydrography and overall geometry of the estuary are the critical factors in controlling these features on the tidal deltas. The estuary has two inlets, each facing a separate large body of water whose tidal ranges are significantly different. The tidal-range difference develops a steep hydraulic slope that occurs during flood and results in pronounced time asymmetry and tidal-current segregation.
Describes eroding bluffs of unconsolidated Quaternary sediments, complex embayments formed by the breaching of thermokarst lakes, and spits and barrier islands which are experiencing rapid landward migration. Comparison of air photographs shows that between 1950 and 1985 spits were retreating landward at an average rate of 1.7 m a-1 while offshore barrier islands were migrating onshore at a rate of 3.1 m a-1. Sediment availability is a significant parameter controlling spit and barrier island landward migration. Analyses of retreat rates as a function of incoming deep-water wave power showed that barrier islands are retreating more rapidly than spits even if they are exposed to the same level of wave energy. -from Authors
Monthly beach profiling of a typical northeastern/mid-Atlantic microtidal and wave-dominated shoreline demonstrated short-term shoreline position changes of up to 20 m over a one year period. Average long-term shoreline recession rates in this area were 1.2 m/yr ± 1.0 m/yr. Short-term shoreline position changes were the largest source of error in the long-term recession rate measurements. -from Authors
Configurations of tidal deltas are determined by the interactions between inlet tidal drainage and longshore currents in the near-shore zone. The tidal range along the Georgia shorelines is 2-3 meters and produces moderate to strong tidal currents adjacent to estuary inlets. Wave energy dissipates over a wide (11 kilometers), shallow (less than 10 meters) zone and attenuates significantly before it impinges on the shoreline. Therefore, along the Georgia coast, the zones adjacent to estuary inlets experience moderate-velocity to high-velocity tidal currents, whereas, the wind- and wave-induced longshore currents are generally buffered before they reach the shoreline. Tidal deltas produced by the current interactions at tidal inlets, illustrate the predominant influence of inlet tidal drainage. The peripheral-shoal complexes of the tidal deltas have well-developed marginal shoals that are approximately 2 kilometers apart and parallel each other for approximately 5. 5 kilometers seaward of the inlet. Distal shoals are poorly developed or absent.
Accurate detection of shoreline change depends on a consistent measurement technique so that apparent changes in shoreline are not merely manifestations of inconsistencies in that measurement technique. For marine coastal areas the sought-after consistency is usually based on adhering to a definition using a particular tidally-based vertical reference datum. For the U.S. the legal shoreline is the mean high water (MHW) shoreline, which is depicted on U.S. nautical charts produced by NOAA. Each point on a MHW shoreline should represent the horizontal position of the land-water interface at the time when the water level at that point is at a height equal to MHW elevation value at that point. At the time of measurement any deviation of the water level height from the MHW value will shift the horizontal position of the land-water interface seaward or landward. This paper discusses factors that have made it very difficult to measure consistently defined shorelines and are the main causes of the discrepancies in the shorelines measured by different government agencies. The paper also demonstrates a technique for producing consistently defined shorelines using high-resolution elevation data covering the intertidal zone (obtained by flying Lidar at low water) with RTK-GPS vertical referencing, and the shifting of these data to the MHW datum using a vertical datum transformation tool (such as NOS's VDatum, which incorporates an accurate geographic distribution of tidal datums from a calibrated hydrodynamic tidal model) so that the zero elevation points then determine the MHW shoreline.
The ebb delta and adjacent beaches at a mixed energy (tide dominated) inlet were monitored to identify the influence of the inlet/ebb-delta system on erosion and accretion of the adjacent beaches. Sub-aerial beach profile data were collected at 25 locations at monthly intervals over four years. Beach excursion distances and volumes, dispersion diagrams, principal components analysis, and time-series correlations with wave parameters were used to extract the magnitudes, time scales, and causes of the beach changes. The divergence induced in the regional longshore transport regime by wave refraction over the ebb delta was also investigated. Changes on the adjacent beaches, beyond the wave shadow of the ebb delta and more than 3-4 km from the inlet centreline, were dominated by a quasi-annual signal which reflected cross-shore sand transport forced by storm waves. Lesser changes were linked with longshore transport, which reversed its prevailing direction at inter-annual time scales. The longshore transport caused sand oscillation between the small headlands bounding the western beach, sand inputs to the inlet/ebb-delta system from the beaches either side, and a standing pattern of erosion and accretion about the inlet due to refraction-induced transport divergence. Immediately behind the ebb delta the quasi-annual storm-wave signal was either small or non-discernible against much larger, multiyear changes. The multi-year changes correlated with the longshore transport potential and were interpreted as being associated with scouring by the marginal flood-tidal flows, accretion of sand bars migrating shoreward from the ebb delta platform and flanks, and longshore transport divergence. Most of these changes appeared to be the on-shore signature of part of a cycle of sand circulation between the beaches and the ebb delta bars.
There have been remarkable advancements in the science and technology of shoreline change mapping; the earlier problems of obtaining accurate erosion rates have been overcome in recent decades. Historical shoreline mapping and trend analysis can provide the requisite data for projection of future shoreline positions for implementation of FEMA's coastal erosion management program. Identification of erosion hazard zones (E-zones) will represent a significant advancement toward fulfilling the intent of the National Flood Insurance Program (NFIP).
A critical need exists among coastal researchers and policy-makers for a precise method to obtain shoreline positions from historical maps and aerial photographs. A number of methods that vary widely in approach and accuracy have been developed to meet this need. None of the existing methods, however, address the entire range of cartographic and photogrammetric techniques required for accurate coastal mapping. This paper provides a conceptual and analytical framework for improved methods of extracting geographic data from maps and aerial photographs. -from Authors
Numerous coastal mapping techniques have been developed over the last twenty-seven years (STAFFORD, 1971; DOLAN et al., 1978; FISHER and SIMPSON, 1979; LEATHERMAN, 1983; McBRIDE et al., 1991; THIELER and DANFORTH, 1994a, OVERTON et al., 1996). These techniques, used to measure shoreline erosion, barrier island migration, and dune erosion, vary in approach, accuracy, expense and training/time requirements. Some of the more recent coastal mapping techniques apply advances in cartography and photogrammetry providing high-resolution measurements with less error than manual methods that use a photographic comparator or stereo zoom transfer scope. However, such techniques are expensive, require extensive training, and may take longer than manual methods. While many coastal mapping studies would benefit from these advanced techniques, not all studies require the high resolution these more recent techniques offer. When beginning a coastal mapping project or choosing to upgrade laboratory facilities, researching established coastal mapping techniques before choosing from among them requires extensive literature review. To assist researchers, engineers and planners who wish to undertake a coastal mapping project, this paper provides an overview of the errors associated with shoreline mapping, and a discussion of factors to be considered when selecting a coastal mapping technique.
On the shoreline of Agon-Coutainville (Manche-France), the natural processes have either been allowed to take their course almost unrestrained or have been thwarted by human interventions (such as sand and gravel extractions or the building of breakwaters and groynes). From archives, maps and personal observations, it appears that, ever since the end of the XVIIIth century, the evolution of strand (backshore) and beach, on a coast characterized by its exceptional tides, provides instances of marine erosion, stability and accretion of the shore.
In Fisher's article (1955) unusual beach projections (cuspate spits) in the lagoons of St. Lawrence Island, Alaska, were described. The question of their formation remained unsettled and led to further discussion by Price and Wilson (1956). The present author reports the data obtained while the problem was studied by the scientists of the U.S.S.R. There exist similar projections in the numerous lagoons of the northeastern part of the U.S.S.R., as well as in a number of other narrow gulfs. Their formation reflects peculiarities of shores of elongated water bodies in which the resultant wave regime is oriented at an acute angle toward the shore. Cuspate spits are formed by shore-drifting processes and do not result from seiches, as suggested by Price and Wilson.
A simulation model of a particular coastal spit is presented to illustrate the value of such models in studying the processes that lead to spit formation. Hurst Castle spit in Hampshire was chosen for the model as it has a distinctive form due to the operation of several easily identified wave types. The processes forming the spit have been discussed by W. V. Lewis. The main ridge, which is composed of shingle, is prolonged by westerly waves, while storm waves build it up. The recurves are formed of material drifted round the end of the main ridge by waves from a southerly point. Waves from the northeast, coming down the Solent, build the lateral recurves. These become longer and more numerous toward the distal end of the spit. The computer program simulates the operation of waves from the west, storm waves, southeasterly waves, and waves from the northeast. In addition, a depth factor simulates the increasing depth of water offshore. This accounts for the increase in number of recurves toward the distal end. A refraction factor allows the curvature of the main ridge to be simulated. The proportion of random numbers allocated to these variables can be adjusted in the data input to elucidate the part they play in the formation of the spit. The close fit of the standard spit to the real spit suggests that the controlling variables are being correctly simulated.
The interrelationship between inlet flow and delta morphology has an important effect upon the bypassing of sediment across inlets. Along the Sea Island Section of the Coastal Plain Physiographic Province the nature of inlet drainage and sediment bypassing profoundly influences patterns of beach erosion and accretion. Small coastal plain inlets generally have arcuate ebb deltas transected by a radially distributed pattern of channels. This arrangement permits an efficient flow of sediment across the inlet with little disturbance to the sediment budget of the adjacent shorelines. Several small inlets also have spits parallel to the shore that divert the flow of river water into the downdrift shore causing erosion. Accretion at the distal ends of these spits takes place at the expense of the proximal end of the spit where the shoreline erodes. Erosion eventually reopens a new channel across the proximal end of the spit and produces a second drainage reentrant. The major inlets along the coastal plain shoreline exhibit shoals with pronounced shore-normal orientations. The proximal ends of these spits may be attached or separated from the shore. When spits are attached to the shore, erosion is observed along the inlet margin and deposition is apparent at the distal end of the spit. When spits are separated from the shore, accretion is observed at shores adjacent to the inlet.
Sand shoals which extend seaward of Georgia estuary entrances are affected by a variety of sedimentary processes. Among these the interaction of waves and tidal currents appears to be most important to the sediment budget. As waves approach the shoreline they are refracted and wrap around the shoal margins. Wave crests interfer over the shoal surfaces and result in surges of water toward the shoreline. Waves also commonly "break" at points of interference and create wave bores which travel shoreward and interact with tidal currents. During the ebbing tide, this interaction is of particular importance to the sediment budget of the shoals. Sediment being transported seaward by tidal flow is diverted landward in gyral-paths by wave bores. The resultant "sediment gyres" are "dynamic sedim nt traps" which cause accumulations of sand in swash platforms. Swash platforms associated with shoals can be recognized by the sand-body geometry and characteristic internal structures.
Three mechanisms are responsible for tidal inlet migration in an updrift direction (counter to net longshore transport): (1) attachment of distal ebb tide delta bars to the downdrift barrier spit; (2) storm-induced breaching and subsequent stabilization to form a new inlet; and (3) ebb tide discharge around a channel bend creating a three-dimensional flow pattern which erodes the outer channel bank and accretes on the inner channel bank. The last two mechanisms can result in either updrift or downdrift inlet migration, depending on channel geometry in the bay and barrier beach configuration. The last mechanism is discussed here for the first time. Analysis of historical charts and aerial photographs, combined with an historical storm synthesis, shows that all three mechanisms are active at a natural tidal inlet along a sandy coast (Nauset Inlet, Cape Cod, MA). On a time scale of 10 years, these mechanisms were effective in producing an updrift migration of more than 2 km. Initiation of updrift migration coincided with a marked increase in storm frequency perturbing the historically stable inlet position. Subsequent updrift migration resulted from ebb-delta bypassing and channel bend flows.
Magilligan foreland is a Holocene beach-ridge plain comprising three principal sets of sand ridges. Sets I and II are related to the mid-Holocene progradation of the plain, while Set III represents recent deposition at the distal margin. At present, two types of beach ridge are being formed, in both cases as part of the general recovery sequence following storm erosion of the dune cliffs. Mode I beach-ridge development is associated with shore-normal sediment transport under dissipative wave conditions. These ridges form around the High Water Mark of Spring Tides through the migration of coalescence of swash bars within 30 to 55 days of a storm. Mode II formation is more complex; sediment initially stored in nearshore bars is moved alongshore, causing bar elongation. The bar moves into the intermediate and even reflective wave domains. Eventually narrowing of the bar leads to severance of a downdrift portion, usually at or near the major drift pulse to the north west of the foreland. Normally this sediment-starved remnant welds progressively onto the beach face in the direction of the dominant process gradient (east to west) but occasionally locally generated, counteractive seas move sediment onshore employing the bar as a traverse conduit. Mode II beach ridges “emerge” onto the upper beach around 90–150 days after a storm. The beach ridges have distinctive structures, depending on their mode of formation, and both constitute important sources for dune-building sand.
Tidal inlet morphology is strongly dependent on local hydrodynamic conditions that rely on wave/tide dominance. In some places, this hydrodynamic context exhibits a very strong seasonal pattern, showing tide-induced morphologies in summer and wave-induced ones in winter. The Barra Nova tidal inlet in Algarve, South Portugal, presents such characteristics. Short term morphological changes were investigated during an extensive field campaign carried on in February and March 1999, after the winter storm period. This study integrates both topo-bathymetric surveys and remotely sensed video data in order to assess the morphological evolution of the inlet system during the transitional period following storms. Comparison of datasets issued from these two direct and indirect methods has shown a very good agreement (errors usually less than 10 cm in elevation) that allows the quantification of morphological changes of the entire inlet system during the whole period of video observation. During a transitional period after winter storms, ebb shoals adjust rapidly to the new low wave energy conditions. They migrate eastwards (50 m in two weeks for the main sand bar of the downdrift swash platform), resulting in a global downdrift trending of the whole delta. This evolution, closely linked to the previous development of storm-induced features, tends to close the system. Despite a large retreat of Barreta Island during the storm event before the campaign, simultaneous downdrift migration of the inlet channel was not observed. Our results suggest that such a migration can occur only at the end of the transitional phase once the storm induced morphologies have been destroyed.
The English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal.
A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous‐rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply.
A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre‐Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre‐Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre‐Holocene conditions.
Weekly topographic profile measurements across a southward migrating recurved-spit complex throughout a summer period have revealed three different mechanisms of berm development, each reflected by a distinctive sedimentary sequence. Each mechanism dominates berm widening along certain sections of the active spit with transition zones separating each one. Along the straight beach sections where a net longshore transport is well developed, sand accumulates at the distal high-tide swash mark during neap tide. These sandy accumulations are neap berms which are later redistributed over the main berm by swash occurring at spring high water. The main berm grows vertically and horizontally as a result. To the south, along the middle portion of the recurved spit, swash bars or ridge-and-runnel systems actively develop, migrate, and weld onto the established berms. This is the second method of berm widening and results from an excess of sand carried into this portion of the spit due to the steadily decreasing transport of the longshore current system. Berm-ridges develop along the southernmost portion of the active recurved spit and represent the third and most rapid form of beach progradation. Wide, broad swash bars build nearly up to the spring high tide level. At neap high tide, the swash cannot extend over this feature. Wave energy is expended on the seaward margin of the swash bar initially developing a low-angle beach face. Rapidly, this beach face steepens and a new berm (beach face and berm top) is developed on top of the swash bar. This berm structure still retains much of its swash bar or ridge appearance, hence the term‘berm-ridge'. Numerous trenches dug into the beach provide data to model the distribution of primary sedimentary structures in recurved spits. Berm-ridges are the most important features along rapidly accreting spits, and structures associated with these features are volumetrically the most significant. Berm-ridges also develop arcuate, vegetated ridges separated by low lying, marsh-infilled swales. These features are commonly seen within barrier islands and designate former inlets.
Sand transport patterns in the ebb-tidal delta of Texel Inlet were studied using current and wave measurements, calculations of sediment transport rate, bedform orientations, dips of cross beds and the morphological evolution. A qualitative picture of sand transport patterns was formed.Sand supplied from the North Sea and the adjacent coast is transported in and out through several neighboring flood- and ebb-dominated channels in the southwestern ebb delta. Along the seaward margin of the ebb-tidal delta of Texel Inlet, sand is transported in both directions parallel to the coast by tidal currents, but predominantly northwards and landwards. Some sand is desposited where a weak rotational tidal current pattern is produced north of the inlet by the interruption of tidal currents parallel to the coast by the inlet. Waves transport the sand further onshore over the swash platform. Some of it returns to the main circulation of the inlet through the Molengat. Sand transport through the inlet is flood-dominated.It is revealed that not only the interaction of tidal currents and waves but also the interaction between the tidal currents in the North Sea and through the inlet greatly influence the system.
The beaches on the west coast of Cotentin, on the Cherbourg Peninsula in Normandy, France, experience mean spring tidal ranges of 9.3–11.4 m. A study of the morphology, hydrodynamics and grain-size characteristics of these beaches was carried out in order to highlight the influence of large tidally induced water level fluctuations on their morphodynamics. These beaches are herein referred to as “megatidal”, to differentiate them from macrotidal beaches with smaller tidal ranges. They are characterised by a wide and relatively shallow concave profile that lacks bar or ridge and runnel morphology. The typical profile exhibits three segments: a relatively steep high tidal zone exhibiting medium to coarse sand, a mid-tidal zone with moderate slopes and fine to medium sand, and a flat featureless low tidal zone with fine sand. Statistical analyses of currents and wave data from several current meters and pressure sensors show that the incident wave heights and mean current velocities are strongly modulated by variations in water depth during the semi-diurnal tidal cycle. The mean current velocities increase downslope, with strong longshore currents in the low tidal zone driven by tides, storm waves and winds. However, wave orbital velocities are larger in the mid- and high tidal zones than in the low tidal zone. The joint variations in sediment size, slope, wave breaker height and tidally modulated water depth across the beach profile result in marked morphodynamic variability, as indexed by wave-sediment and surf parameters. In addition to the classic combination of a reflective upper beach and a dissipative lower beach exhibited by meso-macrotidal beaches, these megatidal beaches show, at low tide, an extremely dissipative low tidal zone characterised by very rapid breaker migration. Shoaling wave conditions prevail in this zone at high tide. This low tidal zone differs from sandy tidal flats dominated by tidal processes. Unlike the latter, which are generally not affected by waves, except during storms, the low tidal zone of these megatidal beaches is still a wave-dominated one, in addition to being subject to strong tidally driven longshore currents. The study adds a complementary, megatidal, dimension to meso-macrotidal beach classes defined in the literature from various parametric combinations of morphology, wave, sediment and tidal characteristics.