Swash Bars Research Articles

Numerical investigation of swash–swash interaction effects on beachface evolution using Nonlinear Shallow Water Equations

This paper presents a numerical investigation of multiple identical swash events to study the swash–swash interaction processes and their impacts on beachface evolution. The numerical model, based on the Nonlinear Shallow Water Equations, is first calibrated/validated against two different single-event-based data-sets. Multiple swash events are generated by identical solitary waves separated by different time intervals, to achieve weak and strong wave-backwash interactions. After a small number of weak interaction events the main feature is erosion from lower and mid swash region and deposition seaward of the swash in a bed-step, created by a backwash bore, primarily due to bed-load. As the number of waves increases, the strength of this backwash bore reduces because of the reduced beach slope caused by the growing bed-step. This eventually leads to a net quasi-equilibrium between bed- and suspended-load per period in most of the swash and surf zones. For strong interaction, initial bed evolution per event is much slower, due to interactions, and is bed load dominated. A quasi-equilibrium is also established as the influence of suspended load grows. Overall bed change per period within the domain eventually converges in both cases. Final bed profiles (i.e. after the same elapsed time, but different numbers of waves) are fairly similar, both with an offshore swash bar. Both profiles continue to evolve on the offshore side of this bar. However, this evolution is driven by suspended load for the weak interactions and bed load for strong interactions. The implication is that similar swash morphological features can emerge from different swash processes, and also be maintained distinctly.

Primary drivers of multidecadal spatial and temporal patterns of shoreline change derived from optical satellite imagery

Understanding and predicting shoreline change along sandy coasts requires continuous (in both time and space) long-term (decades) shoreline data at good spatial (e.g. 100 s of metres) and temporal (e.g. months) resolution. Publicly available satellite imagery can now provide such time series. However, satellite-derived shorelines (SDS) are associated with uncertainties, particularly at high-energy meso-macrotidal coasts, which challenge the assessment of long-term trends and interannual variability. In this paper we address the 1984–2020 time- and space-evolution of 269 km of high-energy meso-macrotidal sandy coast in southwest France using uncertain (no tide and runup correction) SDS data. The shoreline trends are validated with field data collected over the period 2008–2019. Over 1984–2020, the shoreline eroded by 0.55 m/yr with maximum erosion (accretion) reaching 15.61 m/yr (6.94 m/yr), with the largest changes observed along coasts adjacent to the inlet and estuary mouths. We show that, away from the presence of ebb-tide deltas and swash bars affecting offshore wave transformation and nearshore circulation, the long-term shoreline trend is well explained by the gradients in longshore drift computed from a regional wave hindcast and an empirical longshore transport formula. By averaging the yearly SDS along the entire coastline, we find that interannual shoreline variability is well correlated with the winter West Europe Pressure Anomaly (WEPA), which outscores the other conventional teleconnection pattern indices. WEPA even explains >80 % of the space-averaged shoreline variability over the recent period 2014–2020 when more and higher quality satellite images are available. A more local assessment of the links between climate indices and shoreline response shows that correlation with all climate indices dramatically drops downdrift of the large-scale estuary mouths and inlets. This suggests that along this 10–20 km stretch of downdrift coast, shoreline response is controlled factors internal to the estuary mouth/inlet system. The rest of the coast is mostly controlled by factors external to the system, which are primarily the variability in winter-mean wave height correlated to winter WEPA index. Overall, we demonstrate that an adapted space-averaging of uncorrected (noisy) SDS dataset can allow addressing the time- and space variability of shoreline change and their primary drivers including large-scale climate patterns of atmospheric variability. We also advocate that such SDS analysis can be performed along any coastline in the world in order to guide future model development and application.

Dynamics, Evolution, and Facies Preservation of a Microtidal Chenier Plain, Joal-Fadiouth (Senegal, West Africa)

Youm, C.I.; Morales, J.A.; Doumbouya, M.F.; Garcia-Villalba, E.; Sow, I.S., and Sow, E., 2023. Dynamics, evolution, and facies preservation of a microtidal chenier plain, Joal-Fadiouth (Senegal, West Africa). Journal of Coastal Research, 39(2), 257–265. Charlotte (North Carolina), ISSN 0749-0208. Cheniers are coarse sediment ridges formed above finer sediments deposited on a tidal flat, the origin and development of which have traditionally been attributed to storm waves. Joal-Fadiouth is a microtidal system located on the southern coast of Senegal (West Africa). This system presents a succession of sandy and shelly ridges displaying a facies architecture consistent with a chenier plain facies model. This study combines aerial orthophotographs and sediment cores to interpret the sedimentary dynamics and facies of the chenier plain under analysis. The genesis of the last active chenier is related to the arrival of sand from the frontal ebb-tidal delta. Breaching processes occurred in the ebb delta, inducing the abandonment of the frontal lobe by the ebb-tidal currents. Fair-weather waves rework the sand from the frontal lobe, causing a migration of the sand in a manner similar to the formation of a typical nearshore swash bar. In Joal-Fadiouth, these bars soar and migrate over a tidal flat through the action of normal waves during spring high tides. Relict cheniers present shelly gravel sediments coarser than those observed in active cheniers. Thus, the final stage of stabilization of cheniers was attributed to the action of more energetic storm waves. The cheniers that accumulate during the action of ordinary and continuous processes tend not to persist, and only the coarsest cheniers formed during high-energy events finally remain in the sedimentary record. In addition to discussing the preservation of active coastal forms, this work is of regional interest, given that so far there has been only one work on cheniers in West Africa since 1989.

The formation and morphodynamics of complex multi‐hooked spits and the contribution of swash bars

AbstractNumerous studies have been devoted to recurved and hooked spits in micro‐ and meso‐tidal environments, but none have really focused on the peculiar right‐angle plan shape exhibited by some spit hooks. The morphodynamics of a complex multi‐hooked spit in a macrotidal inlet in Normandy, France, have been investigated using aerial photographs and 12 topographic LiDAR surveys from 2000 to 2017, complemented by hydrodynamic data. Merville Spit is a 2.5 km‐long hooked spit, composed of four ‘nested’ ridges each with a right‐angle hook at its tip, that has developed in the vicinity of the sand‐rich outlet of the Orne River. Monitoring of the two last ridges shows that ridge formation starts with the attachment of the tip of a swash bar to the fulcrum point of the previous ridge, and ends following its westward elongation and counter‐clockwise rotation when its hook impinges on the Orne outlet. The main factor controlling ridge plan‐view shape is swash bar dynamics in the vicinity of the spit. The offshore wave direction has no significant impact on ridge and hook behaviour due to the large number of swash bars on the ebb‐tidal delta. Over the study period, between 5 and 10 swash bars massively assured sand supply to the upper foreshore, favouring the growth of existing ridges, but also enabling the building of a new ridge. The bars contribute to imparting hook right angles at the tips of the ridges by creating local wave refraction or by providing a sheltering effect. Strong tidal currents in the Orne channel act as a hydrodynamic barrier that stops hook elongation at the edge of the river's outlet. The similar behaviour of the two studied ridges has been synthesized into a new and original conceptual model of complex multi‐hooked spit formation and evolution under the influence of swash bars.

Study of shelly shore ridge formation using wave flume experiments: Morphological evolution and depositional patterns

Coarse shore/berm ridges comprising mollusk shells are peculiar morphological features occurring in some beaches globally, generally wave-built, generated during storms and high tides. Studies regarding the formation of these shelly shore ridges using wave flume experiments (physical modeling) do not provide a detailed analysis of their morphological evolution and depositional features. A bioclastic beach profile with a constant dip, formed by mollusk shells (whole and fragmented ones, DSV-50 = 4.5 mm) of bivalves and gastropods, was subjected to a storm wave condition simulated in a two-dimensional flume with a scale reduction factor of 1/20. A decimeter-scale shelly shore ridge was successfully formed by swash bar emergence. The seaward surface was steeper than the landward slope. The bar was progressively built by swash currents that accumulated shells along the shoreline. According to Dean's function, the equilibrium beach profile displays a good fit for coefficient A, as related to the settling velocity of the bioclasts. The power of m is lesser in the equilibrium beach than that in beaches composed of siliciclastic sand. Three patterns of shell organization and pore framework represent the back, top, and frontal sectors of the ridge. The ridge front includes poorly organized shell particles of dense packing. The crest comprises bioclast beds of uniformly obliquely oriented, densely packed particles. Back ridge sediments exhibit loosely packed moderate shell organization. The morphological evolution of the bioclastic ridge formed in the flume closely resembled bioclastic beach sediments. The depositional pattern in the flume ridge was directly related to the energy of breaking waves and associated swash-current energy.

Sedimentological and ichnological characteristics on macrotidal Baeksu tidal shoreface, southwestern coast of Korea

Integrated sedimentologic and ichnologic studies along the Baeksu coast of southwestern Korea have revealed that despite the macrotidal conditions, the morphologic characteristics and preserved deposits of the intertidal flats show strong similarities to those commonly reported from wave-dominated shorefaces. Morphologically, a beachface is developed near the high-tide level, which is backed by the high-tide mud flat and fronted by the extensive intertidal shoreface (sensu Dashtgard et al., 2021), where wave-formed swash bars are present. Although sedimentation is strongly influenced by seasonal winds and waves, the preserved deposits conspicuously comprise abundant storm beds that consist mainly of wave-rippled and hummocky cross-stratified sands, because summer muds are typically ephemeral. The storm beds, which are predominant and thick near the low-tide line, become thinner in a landward direction, due to the progressive dissipation of wave-energy by bottom friction. Such physical processes, exerted on tidal-shoreface sedimentation, are also reflected in the ichnofacies distribution, showing that distal expressions of the Skolithos Ichnofacies characterize the lower intertidal zone and are replaced landward by proximal expressions of the Cruziana Ichnofacies. Except for the low-BI beachface (BI 0–1), the intensities of bioturbation generally increase in a landward direction (BI 0–1 to BI 3–4), in correspondence with the physical-energy distribution.The study area reveals that the sedimentological and ichnological characteristics commonly employed to recognize wave-dominated shoreface settings (even including high-wave-energy tidal-shoreface counterparts) apparently show a reverse trend within the intertidal shoreface, probably overlying the normal-trend, subtidal shoreface. Although these findings have not been broadly tested, we, however, believe that these features are representative of the low-energy end of the full spectrum of wave-dominated coastal settings.

Sand spit dynamics in a large tidal‐range environment: Insight from multiple LiDAR, UAV and hydrodynamic measurements on multiple spit hook development, breaching, reconstruction, and shoreline changes

AbstractSand 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 cubic metres of sand. These events occurred during short durations (a 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 10‐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. © 2020 John Wiley & Sons, Ltd.

Rapid Change in Sedimentary Facies from Wave- to Tide-dominated Macrotidal Flat in the Sinduri Bay, West Coast of Korea

Yoon, H.H.; Chun, S.S., and Hong, S.H., 2020. Rapid change in sedimentary facies from wave- to tide-dominated macrotidal flat in the Sinduri Bay, west coast of Korea. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 728-732. Coconut Creek (Florida), ISSN 0749-0208.Macrotidal wave-dominated coastal environments are widely distributed on the western open coasts of Korea, although various muddy intertidal flats occur also in the inner part of embayed areas influenced strongly by geomorphological factor. The surface sediment of wave-dominated sandflat and tidal beach, which are developed on the northern Sinduri Bay coast, consists mostly of very well sorted medium to fine sand. Swash bars, which are parallel to the coastline and continuously migrating landward, play the important role in the formation of this macrotidal sandflat. The surface sedimentary facies in this southern embayed area consists mostly of mud. The mud flat is very gentle in slope with slight concave-up profile, and dominantly shows a highly bioturbated mud facies. The distribution of surface sedimentary facies, slope profile, hydrodynamic analysis and the transport pattern of coarse-grained sediment show a rapid change in sedimentary facies with very narrow transition zone from wave-dominated sandflat facies to tide-dominated mudflat facies. Both wave- and tide-dominated coasts have been formed together within a small single geomorphological unit. The co-occurrence of different types of coasts and the rapid shift of sedimentary facies can be indicative of one of characteristics in the macrotidal open-coast depositional environment in which geomorphology, sediment supply and hydrodynamic factors have been closely associated within a relatively small bay area. Sedimentary processes in the two coastal sytems are separated and independent each other. The development of separated depositional systems are also evident even in cored sediments, indicating that the recent depositional setting has been continued much longer time.

Depositional controls on a hypertidal barrier‐spit system architecture and evolution, Pointe du Banc spit, north‐western France

AbstractPhysical stratigraphy, architecture and evolution of barrier systems in hypertidal environments (tidal range above 6 m) are understudied, and depositional controls are poorly understood compared with wave‐dominated barrier systems and barrier spits in microtidal, mesotidal and macrotidal settings. Based on vibracores, ground‐penetrating radar, radiocarbon and optically stimulated luminescence age data, the formation and evolution of the hypertidal barrier system of Pointe du Banc, north‐western France has been reconstructed. The study shows that the barrier spit has a complex composite sedimentary architecture consisting of wave‐dominated and tide‐dominated sedimentary bodies. A morpho‐sedimentary model is presented that demonstrates how barrier‐spit progradation resulting from littoral drift was the main manner of spit elongation, whereas sediment convergence caused by landward migration of swash bars and seaward migration of tidal dunes caused the spit terminus to grow in height and width. These results suggest that long‐term accretion rates varied considerably in response to changes in sediment supply. Variations in storminess together with the large‐scale topography of the coast controlled the sediment supply and thus the evolution of the barrier system during latest Holocene. Despite architectural complexity, hypertidal barrier systems preserve records of past climate changes.

Shoreline Dynamics Along a Developed River Mouth Barrier Island: Multi-Decadal Cycles of Erosion and Event-Driven Mitigation

Human modifications in response to erosion have altered the natural transport of sediment to and across the coastal zone, thereby potentially exacerbating the impacts of future erosive events. Using a combination of historical shoreline-change mapping, sediment sampling, three-dimensional beach surveys, and hydrodynamic modeling of nearshore and inlet processes, this study explored the feedbacks between periodic coastal erosion patterns and associated mitigation responses, focusing on the open-ocean and inner-inlet beaches of Plum Island and the Merrimack River Inlet, Massachusetts, USA. Installation of river-mouth jetties in the early 20th century stabilized the inlet, allowing residential development in northern Plum Island, but triggering successive, multi-decadal cycles of alternating beach erosion and accretion along the inner-inlet and oceanfront beaches. At a finer spatial scale, the formation and southerly migration of an erosion “hotspot” (a setback of the high-water line by ~100 m) occurs regularly (every 25–40 yr) in response to the refraction of northeast storm waves around the ebb-tidal delta. Growth of the delta progressively shifts the focus of storm wave energy further down-shore, replenishing updrift segments with sand through the detachment, landward migration, and shoreline-welding of swash bars. Monitoring recent hotspot migration (2008–2014) demonstrates erosion (>30,000 m3 of sand) along a 350-m section of beach in six months, followed by recovery, as the hotspot migrated further south. In response to these erosion cycles, local residents and governmental agencies attempted to protect shorefront properties with a variety of soft and hard structures. The latter have provided protection to some homes, but enhanced erosion elsewhere. Although the local community is in broad agreement about the need to plan for long-term coastal changes associated with sea-level rise and increased storminess, real-time responses have involved reactions mainly to short-term (<5 yr) erosion threats. A collective consensus for sustainable management of this area is lacking and the development of a longer-term adaptive perspective needed for proper planning has been elusive. With a deepening understanding of multi-decadal coastal dynamics, including a characterization of the relative contributions of both nature and humans, we can be more optimistic that adaptations beyond mere reactions to shoreline change are achievable.

Detecting ebb-tidal delta migration using Landsat imagery

Ebb-tidal deltas are important coastal sedimentary features that influence broad-scale coastal system behaviour. However, sediment transport within and around tidal deltas is complex, making it very difficult to understand the role they play within coastal sedimentary systems. Here we present details of a new method to examine ebb-tidal delta processes using long records of cloud-masked Landsat 7 and 8 satellite imagery. Our approach is similar to that adopted in nearshore process studies, in which time-averaged collections of digital photographs from terrestrial cameras are used to differentiate between broken and unbroken water based on image tone. Using the Google Earth Engine we are able to rapidly segment 100s of Landsat images into broken and unbroken water classes based on a threshold value. By then calculating the frequency of wave breaking at the Manukau ebb-tidal delta (METD) near Auckland, New Zealand, we can repeatedly map geomorphic features such as channels, linear bar formation, channel margin linear bars, swash bars, and the terminal lobe that marks the outer limit of wave breaking. Using a ~17-year long record of observations we observe >2 km of northward alongshore channel migration and the potential emergence of a new channel to the south. The approach is simple, quick and with the growing constellations of earth observation satellites, has easy application to a range of other coastal systems.

Bar Migrations on a Macrotidal Ebb Delta over a Period of Six Years Using Lidar Survey

Pellerin Le Bas, X. and Levoy, F., 2018. Bar migrations on a macrotidal ebb delta over a period of six years using LiDAR survey. 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. 146–150. Coconut Creek (Florida), ISSN 0749-0208.The migration rates of two swash bars on an ebb delta are studied in a macrotidal environment. A dataset of 12 LiDAR surveys, from 2011 to 2017 is used to compute the movement of the swash bar crests using Digital Shoreline Analysis System (DSAS) version 4.3. Hydrodynamic data are acquired near the ebb-tidal delta during the same period. The relationship between bar migration rate and wave energy flux is analyzed. The correlation is high (r2 = 0.71), but a specific migration rate of the two bars weaken this result. Furthermore, the two swash bars have two different behaviors under the same hydrodynamic conditions. Other analyses are conducted concerning the relationships between the migration of the bar and its volume or the water depth. The correlations are too weak to explain the different behaviors of the bars. Another parameter is highlighted, the number of swash bars in the immediate vicinity of the two bars.

The formation of concealed firmground in open-coast macrotidal flats, southwestern coast of Korea: its sedimentological and ichnological characteristics

Since wave-dominated open-coast tidal flats have sedimentologically strong similarity with typical wave-dominated shorefaces, it is very difficult to distinguish between them in ancient records. Many-year field observations along the southwestern Korean tidal flats have revealed that autogenically generated firmground muds are frequently exposed on the high-tide beach face, as a result of swash-bar migration/welding. Observed ichnology in the firmground muds have clearly shown that ichnocoenoses across the intertidal swash bars are in good agreement with the preserved firmground-mud traces, reflecting different substrate consistencies at the time of colonization. Unlined deep-tier firmground burrows, including Psilonichnus and Thalassinoides, typically crosscut the pre-existing softground suites including Skolithos-, Arenicolites- and Monocraterion-like burrows. Due to the fact that such firmground muds and the autogenic Glossifungites Ichnofacies are very uncommon in a normal wave-dominated shoreface to beach setting, the presence of firmground muds on high-energy beach face may indicate a very specific depositional environment with significant tides and considerable wave energy (i.e., open-coast tidal flats). In this context, the study results would aid in the recognition of wave-dominated tidal flat in ancient deposits.

Observations of morphological change at an ebb-tidal delta

Video observations of depth-limited wave-breaking patterns at an ebb-tidal delta on the energetic west coast of New Zealand at Raglan were used to identify geomorphic features over a 5-year period. The terminal lobe, mouth bar, channel margin linear bars, and swash bars were identified and tracked over the duration. Morphodynamic response was related to environmental conditions by correlating observed movements with concurrent wave and tidal conditions. Movements occurred throughout the record with a slight tendency to occur more during the transition between seasonal forcing trends. Winter deltas were generally broader and extended further seaward than the summer deltas which were more cuspate. The formation of a double-barred ebb-shoal was observed, with the cross-shore position of the outer bar influenced by wave conditions while the inner bar was influenced by ebb-jet strength. Furthermore, swash bars were observed to constrict the seaward extent of the main channel during large wave events, which was subsequently eroded by tidal currents. These observations are consistent with the present consensus that ebb-shoal features are dependent upon competition between ebb-jet strength and opposing waves. Interannual morphological changes were not significantly correlated to any particular environmental forcing, suggesting that either some process or combination of processes not considered were influential, or that the system might be showing signs of emergent behaviour.

Processing and performance of topobathymetric lidar data for geomorphometric and morphological classification in a high-energy tidal environment

Abstract. The transition zone between land and water is difficult to map with conventional geophysical systems due to shallow water depth and often challenging environmental conditions. The emerging technology of airborne topobathymetric light detection and ranging (lidar) is capable of providing both topographic and bathymetric elevation information, using only a single green laser, resulting in a seamless coverage of the land–water transition zone. However, there is no transparent and reproducible method for processing green topobathymetric lidar data into a digital elevation model (DEM). The general processing steps involve data filtering, water surface detection and refraction correction. Specifically, the procedure of water surface detection and modelling, solely using green laser lidar data, has not previously been described in detail for tidal environments. The aim of this study was to fill this gap of knowledge by developing a step-by-step procedure for making a digital water surface model (DWSM) using the green laser lidar data. The detailed description of the processing procedure augments its reliability, makes it user-friendly and repeatable. A DEM was obtained from the processed topobathymetric lidar data collected in spring 2014 from the Knudedyb tidal inlet system in the Danish Wadden Sea. The vertical accuracy of the lidar data is determined to ±8 cm at a 95 % confidence level, and the horizontal accuracy is determined as the mean error to ±10 cm. The lidar technique is found capable of detecting features with a size of less than 1 m2. The derived high-resolution DEM was applied for detection and classification of geomorphometric and morphological features within the natural environment of the study area. Initially, the bathymetric position index (BPI) and the slope of the DEM were used to make a continuous classification of the geomorphometry. Subsequently, stage (or elevation in relation to tidal range) and a combination of statistical neighbourhood analyses (moving average and standard deviation) with varying window sizes, combined with the DEM slope, were used to classify the study area into six specific types of morphological features (i.e. subtidal channel, intertidal flat, intertidal creek, linear bar, swash bar and beach dune). The developed classification method is adapted and applied to a specific case, but it can also be implemented in other cases and environments.

HYDRODYNAMIC DRIVERS OF SEDIMENT TRANSPORT ACROSS A FRINGING REEF

Coral reefs are highly valuable ecosystems, which are under an increasing number of environmental pressures. Sedimentation and sediment transport patterns are among key physical drivers of coral reefs, so it is important to improve our understanding of these poorly studied dynamics on reefs. To this purpose, flume experiments were performed on a scaled fringing reef in the laboratory facilities of Deltares in Delft. The objective was to improve the understanding of hydrodynamic and sediment transport processes across fringing reefs. The water depth and bed roughness were shown to have influence on many processes such as short wave breaking, infragravity (IG) wave generation, IG wave transformation, reef flat seiching, wave-induced setup and wave reflection. The measurements showed that long waves dominate over the short waves at the back of the reef flat. The flow velocities in the rough cases were lower than those in the smooth cases as a result of the bed friction. Analysis of the third order velocity moment and the bed level changes indicates that both the short and the long waves play a role, but that the long waves appear to be the dominant factor in sediment transport and bed profile development - especially close to the beach. Bed roughness affected the shape of a swash bar, which was more pronounced for the smooth than for the rough cases. This demonstrates that the dominance of long waves in a fringing reef lagoon results in different sediment dynamics than, for example, on a regular sandy beach.

Mobility of meso-scale morphology on a microtidal ebb delta measured using remote sensing

Abstract Ebb tidal deltas usually consist of several large lobes of sediment separated by channels in which the bulk of the tidal exchange takes place. The purpose of this paper is to describe and quantify the migration pattern of bedform features associated with an ebb-tidal delta using a new remote video sensing method during a 23 day experiment at New River Inlet (North Carolina). To quantify the migration rates, a Lagged Least Squares Algorithm (LLSA) was developed that found the vector rate for which the suite of lagged images were most similar, computed on a tile-by-tile basis. Our observations revealed a complex set of bedform features that migrated in a circular pattern with movement in offshore regions being away from the inlet mouth and toward the shore while nearshore migration was back toward the inlet. 60% of the wavelength variability of these features is at scales that are smaller than the coherent channel and swash bar structures but much longer than megaripples, i.e., between 10 and 100 m. We have chosen to call these bedform features of meso-scale morphologies. The mean migration rate of these features was found to be 1.53 m/day ± 0.76 m/day. 72% of estimated rates were greater than 1.0 m/day, 31% were larger than 2.0 m/day, and the maximum rate was around 3.5 m/day, averaged over 23 days. Alongshore averages of cross-shore migration rates showed a node at 110 m from the shoreline that separates migration away from the inlet from migration toward the inlet (near the shore). The circular pattern of migration appeared to be consistent with expected residual flow of an ebb-tidal delta system.

Morphological and sedimentological response of a mixed-energy barrier island tidal inlet to storm and fair-weather conditions

Abstract. The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 μm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.

Morphological diversity and complex sediment recirculation on the ebb delta of a macrotidal inlet (Normandy, France): A multiple LiDAR dataset approach

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 3.7years 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.

Nearshore and foreshore influence on overwash of a barrier island

Matias, A., Carrasco, A.R., Loureiro, C., Almeida, S., Ferreira, Ó., 2014. Nearshore and foreshore influence on overwash of a barrier island. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 675–680, ISSN 0749-0208.Accurate prediction of the occurrence and morphological consequences of overwash are important for coastal flood risk assessment and management. A number of morphological and oceanographic factors controlling overwash have been identified by several authors, including nearshore bathymetry. This work intends to identify alongshore variations in storm impact and evaluate the role of sub-aerial and submerged morphological variations in overwash occurrence. For this study, 24 cross-shore topo-bathymetric profiles were set on Barreta Island (Ria Formosa barrier island system, Portugal), extending from the nearshore to the lagoon level on the island backbarrier. Pre- and post-overwash surveys were made between August 2012 and April 2013. Offshore wave characteristics were obtained during overwash events including offshore significant wave height (Hs), peak period and wave direction. Wave propagation and wave parameters at breaking were obtained from SWAN model simulations. During the study period, overwash occurred both under storm conditions (e.g., Hs = 3.8 m) and non-storm conditions (e.g., Hs = 0.8 m), the latter coincident with spring high-tide. Overwash water intrusion distance across the barrier varied alongshore between 0 m (no overwash) and 40 m (reaching lagoon waters). Beach morphology was spatially variable, and changeable from one overwash episode to the next. Important morphological variations of the barrier include modification of barrier crest elevation, beach slope, and presence of swash bars in the vicinity of Ancão Inlet. Predictions of overwash occurrence were made by means of the computation of the Overwash Potential (OP), defined as the difference between the wave runup and the barrier elevation. Several runup equations were tested, and the results of obtained OP were compared to the observations of actual overwash occurrence to determine the most reliable approach. The selected predictor provided an accuracy of 88% for the identification of the locations where overwash processes occurred along the barrier. The predictions were variable alongshore due to variations in breaking waves and beach slope, which in turn are related to the existence of nearshore bars, associated with the presence of the updrift Ancão Inlet. This study proves that nearshore and foreshore morphologies have a major impact on the longshore variation of overwash distribution.