Search

Coastal zones are dynamic environments where sediment transport significantly influences erosion, deposition, and ecosystem stability. Accurate modelling of the complex interactions between hydrodynamics, sediment transport, and morphological changes in high-energy shoreface areas is crucial for sustainable coastal management. This study aims to assess sediment dynamics and understand the dominant physical drivers of sediment transport in the shoreface by coupling high-resolution numerical modelling with a shoreface sand tracer experiment. Results from the field experiment showed high diffusion and a net onshore and downdrift movement of the tracer in agreement with the observed hydrodynamic data. The numerical modelling strategy, validated with in situ hydrodynamic data and tracer results, demonstrated good performance and highlighted the dominant modes of shoreface sediment transport. While during low energetic wave conditions, the dominant transport mechanism involves the slow-rate bed load sediment movement toward the shore, under moderate energetic wave conditions, longshore wave-induced suspended transport becomes the prevailing mode of transport. The study provides valuable insights to increase the understanding of sediment transport in the shoreface and highlights the importance of coupling numerical modelling with sediment tracer experiments. • Field observations and numerical modelling are used to understand sediment transport in the shoreface • Conceptual sediment dynamic model combines Lagrangian and Eulerian approaches • Energetic wave conditions increase wave-induced currents and sediment transport rates, with waves breaking over the bar crest. • Streaming and wave nonlinearities are the main drivers of sediment transport in the shoreface • Alongshore suspended transport dominates in the surf zone, distributing sediments downdrift [ABSTRACT FROM AUTHOR]

Coastal dynamics are governed by the interface of complex, multidimensional processes that result from the interaction between offshore wave climate and local geomorphology. Nearshore waves are usually the primary driver of sediment transport in the coastal zone. However, nearshore wave processes in sub-tropical to tropical regions are often poorly understood, since deep-water waves are often substantially transformed, via interaction with coral reefs, as they propagate towards the nearshore zone. Here, we produce a suite of nearshore wave conditions using a discrete set of representative deep-water wave parameters to determine the potential for large-scale, wave-driven longshore sediment transport along the sub-tropical to coral reef mediated tropical northeastern coast of Australia (15-28oS). Representative conditions at deep-water source points, determined using a Maximum Dissimilarity Algorithm (MDA), were used as boundary conditions for a coastal wave model. Potential longshore sediment transport is largely dependent on offshore wave direction and the presence (or lack thereof) of the Great Barrier Reef (GBR). There is a decrease in mean, wave-driven transport potential from >500,000 m3/year in southern Queensland to <50,000 m3/year north of Fraser Island and along the section of the coast regulated by wave transformation through the GBR. Seasonal changes in the deep-water wave regime control patterns of sediment transport along the entire coast, even in the low wave energy regions. We also show that there are complex sediment transport pathways on this tropical coast with large variability in both direction (up-coast vs. down-coast transport) and magnitude in the nearshore. Our results provide insights into the potential volumes, pathways, and complexity of sediment transport on tropical and sub-tropical coasts and a methodological approach that can be applied globally to efficiently construct wave-driven sediment flux estimates on tropical coastlines where coral reefs influence nearshore wave processes. • Seasonal changes in deep-water wave regimes control patterns of sediment transport. • Sediment transport directions and rates in tropical Queensland are highly variable. • The Great Barrier Reef greatly reduces potential sediment transport due to waves. • Wave generation and sediment transport mechanisms in the tropics are complex. [ABSTRACT FROM AUTHOR]

In this study, we aim to investigate the overall morphological evolution of the Jiangsu coast after 1855, when the Yellow River shifted northward. We focus on fine sediment transport between two large-scale geomorphological units, i.e., the Abandoned Yellow River Delta (AYD) and the Radial Sand Ridges (RSRs). An existing morphodynamic model, which was established for reproducing the development of the AYD before 1855, is modified and extended. In addition to the tidal forcing, waves and human interventions (i.e., revetments) are considered in the model. The model results are compared with the existing data. Both the evolution trend of the Jiangsu coast and the spatial distribution of the offshore shoals show good agreement. The simulated fine sediment depositions in different periods are consistent with the geological measurements. The results reveal that the old Yellow River-derived sediment not only contributes to the sedimentation in the RSRs but can also be transported to the adjacent zones, especially farther south/southeast. Moreover, the spatial distribution of fine sediment deposits varies in the RSRs. The different sedimentary environments in the Dongsha and Tiaozini ridges result in significant grain size differences in these two neighboring ridges. A sensitivity analysis indicates that tides play a key role in dominating the long-term morphological evolution of the Jiangsu coast and the total erosion from the AYD. On smaller scales, the effect of revetments (built since the 1930s) on the evolution of the nearshore zone and the effect of wind waves on the erosion of offshore shoals are relatively important. The effect of a gradual coarsening process of bottom sediment along the Jiangsu coast, which may be due to continuous fine sediment removal, is identified. Fine sediment depositions in the Tiaozini ridge and in the northern offshore zone of the RSRs are relatively more sensitive to the coarsening trend of bottom sediment than other areas. [ABSTRACT FROM AUTHOR]

Sediment accumulation and budget in the continental margins provides abundant information of source-sink processes from the land to the sea, including weathering, human activities and sedimentary environment. Here we show the distribution of mud areas, modern sedimentation rates and sediment budget in the Bohai Sea, Yellow Sea and East China Sea. Using grain size data of > 18,000 surface sediment samples and 210 Pb data from 413 sites, we identify five areas with sediments mainly composed of fine-grained fractions (mean grain size > 6 φ) and find a relatively high sedimentation rates of > 1.5 mm/yr in the mud areas. Near the Yellow and Yangtze River deltas sedimentation reaches > 95.0 mm/yr. Approximately 1185.4 × 10 6 tons of fine-grained sediment accumulate annually in the mud areas of the east China seas. Atmospheric deposition contributes < 2% (18.37 × 10 6 tons/yr), while the riverine sediment inputs account for > 75% (917 × 10 6 tons/yr). The remainder comes from all other sources including coastal erosion and resuspension of bottom sediments. In addition, ~ 45% of the fluvial sediment supply deposits on the subaerial delta, ~ 40–50% is trapped on the subaqueous delta and shelf, and the remainder < 5% escapes the shelf edge. The results will be a strong foundation for understanding of the transport, deposit and preservation of sediment and other relevant material (e.g. carbon and nutrient etc.) of terrestrial materials in the sea. [ABSTRACT FROM AUTHOR]

The depth at which significant bathymetric change can be expected is an important morphological factor for science and engineering on sandy coastlines. Although most depth of closure studies have been conducted on a limited number of sites, the great quantity of airborne lidar bathymetry data collected over the past decade allows for a much wider study region. Here, we present depth of closure analysis over 600 km of sandy coastline using the Joint Airborne Lidar Bathymetry Technical Center of Expertise (JALBTCX) dataset. Improved closure predictions resulted when both extreme waves (as represented by the 12 hour exceedance significant wave height over a given time interval), and more typical storm waves (as represented by the 99% significant wave height) were jointly considered. In contrast to some other studies, wave steepness terms were found to be negligible. Consideration of error terms in least-squares closure predictions was used to develop relations with different degrees of conservatism: e.g. to provide closure depth exceeded by only 10% of observations. Results are presented for four closure criteria: root-mean-square depth changes Δh close = [20, 30]cm, and relative depth changes Δh close / h = [0.02, 0.04]. Further improvements in accuracy may occur once decadal wave hindcasts include storm surge variations in water levels, and have increased nearshore resolution. [ABSTRACT FROM AUTHOR]

Large-scale coastal erosion in the Mekong Delta has been dramatically increasing in severity in recent decades. There are several effective hard engineering solutions that have been implemented in this delta to efficiently prevent coastal erosion and stimulate sedimentation while supporting the local ecosystem conservation. These measures include Pile-Rock Breakwaters (PRBW), Hollow Triangle Breakwaters (HTB) and Semicircular Breakwaters (SBW). However, research on the sediment transport, morphological changes and toe erosion for these offshore breakwaters is very limited and is currently in the initial stages of understanding the specific conditions of sediment characteristics and foundations. The objective of this study was to reproduce the morphological changes and toe erosion of three breakwaters due to wave-structure interactions. This was investigated using 2D physical models with 3000 irregular waves during 8 experimental hours (equal to 15,000*T p). To extract the bed morphological changes and toe erosion both specialized laser measurements (SW50M laser ruler) and analysis of high-speed video recording by images digitalization were applied. The experimental results show that the shape and structural design of offshore breakwaters can have a significant influence on the bed morphology on both the seaside and the leeside. We found that generally the toe of the construction on the seaside was eroded due to the occurrence of reflected waves, and that the flow is narrowed while passing through the construction, increasing the flow velocity and causing toe erosion. Additionally, the accretion of sediment at the leeside of the breakwaters was found to be mainly driven by the transport of sediment through the construction. Comparing the breakwater designs the experimental results showed that the HTB has the maximum accretion rate behind the structure, as well as the fastest accretion rate behind the breakwater. The SBW has high wave energy dissipation efficiency, although the toe erosion rate is faster than the other classes of breakwaters. The PRBW shows the fastest toe erosion rate in front of the structure and causes accretion at the leeside of the construction but at a lower rate than the HTB. The findings from this study will help practical designers to reinforce the foot of construction during real breakwater designing and inform stability calculations. We recommendation is to apply these three classes of breakwaters, especially the HTB and SBW, for stimulating sedimentation for mangrove restoration in the mangrove mud-coast delta. • Morphology and toe erosion of three breakwaters were tested using laboratory physical models. • Pile-rock (PRBW), hollow triangle (HTB), and semi-circular breakwaters (SBW) are compared. • PRBW had the largest front erosion and the slowest accretion rate behind the structure. • HTB demonstrates the fastest accretion rate and the largest sedimentation area behind the breakwater. • SBW had high wave energy dissipation efficiency, but a faster toe erosion rate. [ABSTRACT FROM AUTHOR]

Coastal barriers are defined as shore-parallel, aggraded-prograded supratidal beach ridge sets and individual single ridge-landforms, composed of sand and/or gravel. Island, spit, and mainland barriers represent the three major topographic subdivisions. In terms of bathymetric and hydrographic characteristics, bottom sediment types and associated habitats, barriers separate the marine offshore from inshore facies. This definition is not uniformly accepted and applied in the literature. Barriers impose reduced salinities inshore, diminish storm impact and moderate inland inundation, protect critical habitats and may mark ancient longshore transport directions, shore and sea-level positions. A wide range of dimensions, topography, and other factors characterize mainland strandplains. Shoal aggradation appears to be the prime island-forming mechanism, followed by accretion around semi-consolidated, elevated cores. Smaller islands may also form by barrier spit fragmentation. Detachment of dune ridges from the mainland shore is an often cited but at best unlikely mode of island genesis. Review of the literature and remote-sensed images created new insights, utilized barrier definitions and genetic theories, as applied in the Mississippi Delta Complex. A three-stage concept, involving dune ridge or barrier spit-detachment by marine encroachment and landward shift of fragmented transgressive islands forms the basis of a recent invited Marine Geology research article. That model assumes spit-growth from delta headlands, followed by splintering into landward-migrating islands. Submerged, they become permanent shoals. For several reasons, this "island-cycle" hypothesis is questionable and a reassessment of the geomorphic and stratigraphic context of this delta coast is required. Contradicting the geomorphic setting of assumed Stage 1, key component of the cycle-model, evidence is lacking for subrecent delta lobes and erosional headland in spawning barrier islands. In addition, the delta headland-flanking islands reveal only progradational (regressive) development and in situ disintegration,. Transgressive landward migrations is known only from the Chandeleur chain's prehistory. Sand from decaying Terrebonne-Lafourche 6 Subdelta headland was the dominant source for the Caminada Strandplain and for the shoal /or island predecessor of Grand Isle. Wave refraction around an inferred Lafourche delta lobe (dated provisionally between ∼2900 - ∼ 2300 yrs. BP) and its decayed erosional headland remnant resulted in diametric reversal of littoral sand transport. An earlier proferred ∼750–300 yrs. BP Caminada Strandplain age range is now replaced by a loosely defined ∼2200- to- ∼800 yrs. BP value, based on ages of Grand Isle and reworked delta hinterland sediments incorporated into the Strandplain. • barrier and barrier categories. • littoral sediment transport reversal. • Mississippi Delta Complex. • strandplain and delta headland interaction. • transgressive island-cycle hypothesis. [ABSTRACT FROM AUTHOR]

Shorelines bordering ebb deltas are often subject to deformation by the approach and attachment of ebb-tidal shoals. Although this is a common behaviour, little attention has been paid to the link between shoal migration and the multi-annual development of the adjacent beach-dune system. This study aims to understand beach-dune systems' behaviour during the approach phase of an ebb-tidal shoal attachment event. Topographic and bathymetric data of the barrier island of Texel (NL) was used to calculate shoreline position, beach and dune volume, dunefoot position, and shoal displacement rates. Two classical beach-dune interaction models were used to understand the observed behaviour: the surfzone-beach-dune model and the sediment budget model. The analysis revealed how a larger bank on the land-facing side of the ebb delta split into two smaller shoals with different morphological behaviours. One of the shoals (S1) moved consistently landwards, recently attaching the shore, whereas the second shoal (S2) is still evolving and presents a milder landward movement than S1. In parallel to the landward movement of S1, the adjacent coastline was subject to erosion, partially counterbalanced by nourishments. However, despite the erosion, dunes are growing in volume, suggesting a net positive transfer of sand from the beach to the dunes. In the surfzone-beach-dune model, regular nourishments positively affect beach state by maintaining sufficient beach size for aeolian sediment transport and reducing shoreline erosion. In the budget model, dune volume growth may occur despite a slightly negative beach budget. Framing Texel in the budget model implies that the beach budget before the start of the approach phase is crucial in defining the behaviour of the dune system during this phase. If the shoal approach significantly reduces the beach budget, the system may cross the budget threshold that dune building is possible. Thus, the dune system may evolve to an erosional state, including dune ridge dissection and blowout development. Hence, nourishments have been essential in keeping the current morphological state during the attachment of the shoal, as they restrict the system from crossing the negative morphological threshold. Thus, without the nourishments, beach erosion driven by the shoal might have led to morphological changes in the dune system that could lead to years of a weakened level of protection until foredune recovery after attachment. • Shoal-induced changes in beach budget influence dune development. • Dune morphology may change depending on the beach state prior to attachment. • Nourishments can retain beach budget at a sufficient level to maintain dune morphology. [ABSTRACT FROM AUTHOR]

Two barrier systems in southeastern Tasmania afford the opportunity to contrast Holocene barrier evolution and explore large-scale coastal morphodynamics under consistent regional-scale boundary conditions on an embayed wave-dominated coastline. New Optically Stimulated Luminescence (OSL) ages for the previously undated seaward ridges of Seven Mile Beach constrain the deposition of the foredune ridges over the past ~1000 years indicating continued shoreline progradation at ~0.4 m/yr. A further 13 OSL ages collected from the foredune ridges of Nine Mile Beach barrier detail the evolution of this barrier system and indicate progradation commenced approximately ~8500 years ago. Following this, a steady rate of shoreline advance at 0.14 m/yr was evident to near present-day with one phase of rapid accretion ~5000 years ago. This occurred as the barrier system extended eastward to span the full embayment width which resulted in the redirection of the estuarine channel and the subsequent erosion of previously deposited ridges due to channel migration likely over several thousand years. The eroded sediment may have contributed to ongoing positive sand budget in the embayment and continued shoreline progradation. Barrier 'recycling' was also suggested as a potential driver of progradation at Seven Mile Beach based on the truncated margin of the landward ridges. However, at both sites, the dynamics of possible sediment exchange between the back-barrier and open shoreline requires further detailed modelling. The relatively small river systems in this region supply minimal sand to the coast at these sites, however, there is some minor alongshore sediment supply along the western margin of these deep embayment's. In light of this, these two barrier systems demonstrate the importance of internal morphodynamic thresholds and cross-shore sediment exchange from the shoreface as key drivers of barrier evolution and large-scale coastal change in embayed coastal settings. • Holocene barrier deposition in Tasmania was driven by onshore sediment transport. • Shoreline advance at Nine Mile Beach averaged 0.14 m/yr from 8500 years to present. • Erosion of inner margin of the barrier may have augmented sediment budget. • Large-scale coastal behaviour is explained by internal morphodynamic thresholds. [ABSTRACT FROM AUTHOR]

Abstract: In order to understand the influences of tide, waves and sediment sizes on the sand exchange between an estuary and the adjacent coastal region, three estuaries around North West England were chosen for detailed study using a numerical morphological model system, TELEMAC (Hervouet and Bates, 2000). The numerical model was calibrated against available field measurements for both hydrodynamics and sediment transport. Simulations on sediment transport under a representative combined waves and tidal condition were carried out. Comparisons of the model results across the three different estuaries concentrate on effects from seabed bathymetry, hydrodynamics and sediment sizes under the complex tide and wave interactions. It is clear that the dominant hydrodynamic processes of an estuary are influenced by the tidal asymmetry, wave-driven currents and wave-induced stirring effects, which are all affected by the local seabed bathymetry given the same input tide and waves. Generally, it is found that the net sediment transport direction at the estuary mouth depends on the relative strength of landwards transport in the shallow water depths due to tidal asymmetry and seawards transport within the estuary's deep channels. In addition, the overall sediment flux direction is largely dictated by local and surrounding sediment sizes. [Copyright &y& Elsevier]

Abstract: Coastal dunes can act as a method of soft coastal protection against inundation and direct impact of waves during storms if they are substantially large enough in volume to withstand erosion without breaching. However, the time evolution of sand dunes under direct wave impact is not well understood and many available models require site specific calibration and have had limited verification at field scales. Here we test three models of varying complexity in their ability to predict both dry beach erosion volumes and dune to a retreat for an East Coast Low storm event that occurred on the Gold Coast, Australia. The process-based model, XBeach, which models the entire profile was able to reproduce both dune toe retreat and dry beach volume, however, was sensitive to calibration parameters. The two parametric models that only modeled erosion above the initial dune toe position were capable of accurately predicting dune toe retreat, however, under-estimated dry beach erosion volumes. With no calibration, the parametric model proposed by Palmsten and Holman (2012) produced the smallest errors of dune toe retreat with mean error in final dune position of 6.6m, or 18% of the total measured dune retreat. With minimal calibration estimated absolute error in average dune toe retreat was less than 13% of observed retreat for all three models. [Copyright &y& Elsevier]

Abstract: This paper focuses on the hydrodynamic and suspended sediment transport processes in response to sequential reclamations over the intertidal flat, Jiangsu coast, which used to be one of the widest intertidal flats in China. Investigations into the sediment transport patterns reveal that net suspended sediment fluxes increased from 102 kgm−1 on the upper intertidal flat to 103–4 kgm−1 on the lower intertidal flat, per tidal cycle. This variation was caused mainly by the phases of the current velocity, suspended sediment concentration (SSC), local bed geomorphology, inundation patterns, and extreme weather conditions. The longshore component of tidal currents intensifies seaward over the intertidal flat. Extreme weather conditions (e.g. storms) and strong winds were important factors affecting the current velocity structures, the intensity of residual currents, and the SSCs. The SSCs were characterized by several peaks throughout the tidal cycle. These peaks were associated with strong currents, combined wave-current interactions and intense turbulences due to the initial flood surge. Furthermore, the suspended sediment was finer and contained more clay materials during the flood phase of the tidal cycle, than during the ebb. Fine particles were transported within the upper water column by landward residual currents, but coarse particles were transported within the lower water column by seaward residual currents. Overall, residual currents were apt to transport fine-grained sediment landward and coarse-grained sediment seaward. This pattern indicates that the suspended sediment fluxes are highly variable in the water column, even for very shallow tidal environments. The cross-shore component accounted only for a small percentage of the total sediment flux. However, this part of sediment supply dominated accretion over the intertidal flat. The intertidal flat has become narrower and steeper, following sequential reclamations. A negative feedback process existed to maintain the convex profile shape, although the human activities (i.e. reclamation) could have modified the natural process. The surficial sediment tended to become finer on the mid-upper intertidal flat but coarser on the lower intertidal flat following reclamations, in response to a reduction in the tidal currents over the intertidal zone and enhancement of wave action over the lower intertidal flat. Accretion, with high rates, was maintained by landward transport of sediment. In such an accretional system with influences of reclamation, the width or slope of the intertidal flat is adjusted continuously. Before the cease of reclamation, the intertidal flat is unable to achieve an equilibrium shape. [Copyright &y& Elsevier]

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

Abstract: Spits are among the most dynamic features in the coastal zones. Their stability is, very often, the result of a fragile equilibrium between the availability of sediments and the forcing hydrodynamics. Due to the complex interactions between the processes shaping such geomorphologic features the investigation is difficult and requires separate analysis for each of the processes. A typical example of a spit is Sahalin, which emerged one century ago at the mouth of the Danube Delta''s southernmost distributary, and has continuously evolved through elongation and lateral migration. In order to investigate and quantify separately each of the main processes shaping a spit we divide our research in two stages. First, wave induced sediment transports were simulated and analyzed using a complex processes based on a numerical model for an idealised spit. This schematized spit was based on the shape of a number of spits. Secondly, the findings were used in a similar approach for a real case: the Sahalin spit. Results show convergence of the wave fields towards the spit and large transport rates for the dominant wave directions. The sediment budget, derived from the predicted transport and the historical maps of the spit, show that the evolution of the spit is the result of a continuous interaction between along- and cross-shore sediment transport. Furthermore, a good match was obtained between the volumes of sediment supplied to the spit system and those feeding the expansion of the spit. The final output is a conceptual model that includes four stages (submarine accumulation, emerging, evolution and merging with the mainland of the spit) based on the findings from the present study as well as on the findings of previous authors. Although the model was constructed to explain the evolution of Sahalin spit, it is suggested that it can be applied more generally for spits formed in wave-dominated deltas, in a microtidal environment and with a wave climate dominated by one direction. [Copyright &y& Elsevier]

Abstract: We examined seasonal variations in offshore and longshore sediment transport along the Shimizu coast, Suruga Bay, Central Japan, facing the Pacific Ocean, and considered the relationship between these variations and coastal erosion, based on coastal topographic data and detailed bathymetric data acquired by narrow multi-beam echo soundings in 2004–2007 and analyses of material collected in 2004–2005, combined with wave data in the nearshore zone. Sediment transport in the study area is divided into two seasonal modes. In winter–spring, a period dominated by the winter monsoon, littoral drift is recognized but offshore-directed sediment transport is minimal. Sediment is deposited around coastal structures during this period; consequently, tombolos are formed from gravelly deposits and offshore banks which represent the bulges of the seafloor (in water depths of 3–7m) are formed seaward of the structures due to the deposition of sandy sediment. In summer–autumn, when typhoons commonly strike the coast, we observe pronounced longshore and offshore sediment transport. Consequently, the deposits that form tombolos and offshore banks are intensely eroded. The bulk of the eroded sediments are transported into the submarine canyons in the case that a canyon is located close to the coastline and the shelf is narrow. As a result of this transport, sand ridges and depressions are formed from sandy and gravelly deposits, respectively. The amount of erosion (−34,000m3) from the seafloor where most of the active transport occurred in the study area during summer–autumn approximately corresponds to the amount of nourishment (35,000m3) recorded throughout the entire study area, corresponding to about 50% of the amount of the sediment carried by longshore transport (70,000m3/year). Long-period deep-water waves and high incident waves generated by typhoons during summer–autumn are essential in transporting sediment from the coast to offshore areas; the winter monsoon makes only a minor contribution in this regard. The amount of sediment outflow generated by high-energy waves appears to increase with the accumulation of sediment around offshore breakwaters built in regions with a narrow shelf. Research on offshore-directed sediment transport, as in this study, is important for managing and preserving the nearshore environment (e.g., to examine the budget between supply and outflow), as deposits that flow out into the deep ocean never return to the beach. [Copyright &y& Elsevier]

Abstract: The Poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud-dominated coasts: the NSF MARGINS Source-to-Sink Initiative. 210Pb accumulation rates generally decrease from ~1g/cm2 y in the mid-shelf depocenter on the outer shelf to ~0.1g/cm2 y on the mid-slope plateau (range 0.04–2.53g/cm2 y). Higher accumulation rates are observed all along the outer Poverty shelf, extending over the shelf break onto the upper Poverty slope in canyons and gullies. Rates are fast in gullies that incise into the shelf edge (0.75–1g/cm2 y), particularly in the area between Waipaoa Canyon and Poverty Canyon, and in the axis of Poverty Canyon (1.29–1.89g/cm2 y). Below ~1200m water depth, rates in the axis of Poverty Canyon are no more rapid than those found at open slope cores in similar water depths (0.12–0.15g/cm2 y). Excess 210Pb profiles generally exhibit steady state characteristics, except in the axis of Poverty Canyon, where non-steady-state 210Pb profiles are observed. 137Cs was not found above minimum detection limits or above statistical background levels in any cores from the Poverty margin. 7Be was found distributed widely along and across the margin during both summer and winter periods, observed to depths of up to 4cm. Widespread presence of 7Be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year. A sediment budget based on 210Pb accumulation rate data shows that approximately 13–18% of the Waipaoa annual discharge is accumulating on the outer Poverty margin as a whole, and approximately 11–15% of the annual discharge is accumulating on the continental slope alone. Notably, approximately 28% of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons, although they represent only 6% of the area of the outer margin. [Copyright &y& Elsevier]

Abstract: A quantitative provenance approach is developed and in this study applied for the silt and clay fractions of sandy, coastal deposits. Mineralogical composition of 37 surface samples was determined by X-ray diffraction (XRD). Grain-size distributions were obtained by standard dry sieving and pipette techniques. The characterization of mineralogical provenance is based upon known compositional information of possible sources, logical models for mineral combinations related to regional geological provinces and processes, and geographical features of the documented mineral distribution in the area of investigation. These interpretations are further supported by results of correlation and principal component analyses of mineral varieties, grain-size parameters and bathymetric features of the area. Partitioning of source contributions specific for each site of deposition is derived by solving simultaneous equations. Then, the quantified mineral composition at the initial source is reconstructed in the SE Baltic Sea along the Lithuanian coast, exemplifying this new methodology for quantification of sediment sources. The main sources supplying sediment to the area are: 1) Sambian Peninsula to the south (erosion of Pleistocene till and “Blue Earth” Paleogene sediments), supplying 33% of fine-grained sediment on average, 2) Nemunas River, the discharge of which passes through Curonian Lagoon and supplies an average 17% of the coastal fine sediments, and 3) Pleistocene till, eroded on the sea floor in the north and at the Olando Kepurė shore cliff and contributing an average of 50% of the sediment. Although we use the mineralogy within the environmentally important fine-grained fraction, this mathematically simple approach could be used with any other parameters characterizing the identified sources. A quantification of the separate source contributions to each individual site of accumulation studied, using the stable, mineralogical components of the “sediment archive”, offers a time-integrated, net-effect reflection of the combined processes of an entire environmental system. [Copyright &y& Elsevier]

Abstract: This study presents the results of a long-term monitoring program of bed level changes measured during 8 yr at an intertidal mudflat in a microtidal, temperate coastal lagoon. Additionally, bed level measurements obtained at a 10-min temporal resolution at the same tidal flat and at the bed of a nearby tidal channel are presented. Short-term changes in bed level are one or two orders of magnitude larger than the annual net-deposition rate, which shows that the environment is highly dynamic with respect to erosion, transport and deposition of fine-grained sediment. Some seasonality in the bed level changes was observed and there is a tendency for mudflat deposition in spring, summer and early autumn and erosion during the rest of the year, but interannual variations are large and different parts of the mudflat show different seasonal signals. A close coupling between sub- and intertidal deposition and erosion was observed. The time-series showed that some of the material eroded from the mudflat was not exported to the open sea, but instead temporarily deposited in a nearby shallow tidal channel and later returned to the mudflat during calmer weather conditions. These findings support previously published hypothesis and results of modelling studies. Based on the observed abundance of fine-grained sediment at the study sites and the high accretion rates generally found on fine-grained tidal flats in the Danish Wadden Sea area, it is argued that these fine-grained tidal flats are not seriously threatened by the expected sea level rise in the 21st century. [Copyright &y& Elsevier]

Abstract: The morphodynamics of inlets and ebb-tidal deltas reflect the interaction between wave and tidal current-driven sediment transport and significantly influence the behaviour of adjacent shorelines. Studies of inlet morphodynamics have tended to focus on sand-dominated coastlines and reference to gravel-dominated or ‘gravel-rich’ inlets is rare. This work characterises and conceptualises the morphodynamics of a meso-tidal sand–gravel inlet at the mouth of the Deben estuary, southeast England. Behaviour of the inlet and ebb-tidal delta over the last 200 yr is analysed with respect to planform configuration and bathymetry. The estuary inlet is historically dynamic, with ebb-tidal shoals exhibiting broadly cyclic behaviour on a 10 to 30 yr timescale. Quantification of inlet parameters for the most recent cycle (1981–2003) indicate an average ebb delta volume of 1×106 m3 and inlet cross-sectional area of 775 m2. Bypassing volumes provide a direct indicator of annual longshore sediment transport rate over this most recent cycle of 30–40×103 m3 yr−1. Short-term increases in total ebb-tidal delta volume are linked to annual variability in the north to northeasterly wind climate. The sediment bypassing mechanism operating in the Deben inlet is comparable to the ‘ebb delta breaching’ model of FitzGerald [FitzGerald, D.M., 1988. Shoreline erosional–depositional processes associated with tidal inlets, in: Aubrey, D.G., Weishar, L. (Ed.), Hydrodynamics and Sediment Dynamics of Tidal Inlets. Springer-Verlag Inc., New York, pp. 186–225.], although the scales and rates of change exhibited are notably different to sand-dominated systems. A systematic review of empirical models of sand-dominated inlet and ebb-tidal delta morphodynamics (e.g. those of [O''Brien, M.P., 1931. Estuary tidal prisms related to entrance areas. Civil Engineering, 1, 738–739.; Walton, T.L., and Adams, W.D., 1976. Capacity of inlet outer bars to store sand. Proceedings of 15th Coastal Engineering Conference, 1919–1937.; Gaudiano, D.J., Kana, T.W., 2001. Shoal bypassing in mixed energy inlets: geomorphic variables and empirical predictions for nine South Carolina inlets. J. Coast. Res., 17, (2), 280–291.]) shows the Deben system to be significantly smaller yet characterised by a longer bypassing cycle than would be expected for its tidal prism. This is attributed to its coarse-grained sedimentology and the lower efficiency of sediment transporting processes. [Copyright &y& Elsevier]

The US Geological Survey conducted a field experiment in the late spring and early summer of 1998 off northern Santa Cruz County, California, to study sediment fluxes along the central California shelf. As part of this study, a bottom-mounted instrument package was deployed in a sediment-filled paleo-stream channel (h=12 m) off a pocket beach to measure waves, currents, suspended-sediment concentration, temperature, salinity, and seabed-level changes. The observations of suspended-sediment concentration revealed intermittent, intense periods of sediment suspension near the bed that were associated with the passage of individual large waves or wave groups. We used an ensemble averaging technique to characterize the temporal structure of near-bed sediment suspension events (SSEs) defined as when the near-bed instantaneous concentration exceeded the mean by three standard deviations. During the 800 h of deployment, over 9500 SSEs occurred. The 3917 SSEs that began under onshore flow were found to have a different temporal structure than the 5650 events that began during offshore flow. The longer mean duration of the onshore flow events caused an average of 83% of the sediment suspended during the course of the events to be carried offshore after the reversal of flow. All of the sediment suspended during the offshore events was carried offshore due to the shorter mean duration of these events. SSEs are shown to contribute 12% on average and up to 95% of the total sediment suspended by mass, demonstrating the importance of these events to sediment transport in this type of environment. Suspended-sediment transport models developed for the shelf, which use the product of the mean current and the mean suspended-sediment concentration, cannot accurately model the magnitude and direction of suspended-sediment transport on this energetic inner shelf due to the lack of information regarding the instantaneous coupling between fluid flow and sediment suspension. We conclude that time-variant models must be used to accurately model suspended-sediment transport in this type of environment. [Copyright &y& Elsevier]

Storminess and sea-level can both have a significant impact on landforms in cyclone-prone coastal regions, although much of our understanding comes from short-timescale modern observations. This study aims to understand the variability of sediment transport and deposition in the Choctawhatchee Bay/Santa Rosa Island in the northern Gulf of Mexico, establishing the dominant sediment transport processes and morphological response of the barrier system to long-term variations in storminess and rising sea-levels. Here, we study the spatial and temporal changes in physicochemical properties of the sedimentary record of Choctawhatchee Bay to examine the character and fidelity of records of storm impacts spanning the Holocene. Proxies for marine and terrestrial conditions in the cores situated closer to the present barrier (proximal) show that sedimentation in coastal areas and marine influence of the bay during the last ~8000 yrs. were mainly determined by barrier response to the Holocene transgression and changes in storminess. In contrast, sedimentation close to the landward shore was governed by terrigenous input. The correlation of grain size and terrigenous proxies with regional hurricane records indicates that hinterland erosion by the rainfall during hurricane events is likely the dominant terrigenous sediment transport mechanism in areas close to the landward shore of the bay. These results suggest that sediment archives in large coastal deposition environments are equally suitable for sea level and cyclone modulated coastal morphological studies and paleo tropical cyclone studies, depending on the location, selected with an understanding of sedimentation processes in the vicinity. • This article provides a high-resolution record of the evolution of Choctawhatchee Bay Florida, USA during the Holocene. • The article discusses the spatial variability of different sedimentary processes within the Choctawhatchee Bay. • It evaluates the suitability of large back-barrier coastal systems in hurricane-prone areas for paleo hurricane studies. • The article gives evidence for the hurricane variability in the area during the last 8 kyrs [ABSTRACT FROM AUTHOR]

The Ishikari coast of Hokkaido, Japan is a wave-dominated semi-enclosed coast, the sediments of which are primarily sourced from a single river (the Ishikari River), forming a northeast deflected delta. Asymmetric and deflected deltas are generally considered to be formed under net longshore sediment transport downdriftward ; however, through sedimentological and grain-size trend analysis (GSTA) of seafloor sediments, along with some other observational data, we recognised dominant updriftward sediment transport on the updrift-side of the deflected delta system. GSTA showed that the most important trend in the coast is coarser, better-sorted, and more positively skewed (CB+). CB+ trends around the river mouth converge from the lower shoreface zone to the river mouth spit on the updrift side. The trends reflect the combined effects of shore-normal wave action and river sediment discharge. Wave-driven surface currents crossing over the delta lobe converge at the southwestern side of the river mouth. In the deeper zone (lower shoreface), southwest (updriftward) oriented CB+ trends prevail. These trends reflect successive re-deposition of delta front sands, with progressive mud winnowing, driven by the strong south-westerly bottom currents observed in the bay during winter storms. Although GSTA did not show any apparent trend field in the nearshore zone, gradual fining of sand fractions to the southwest, along with the chronological transition of the coastline after the construction of a large port (trap of drifted sands on the downdriftside of the port and regression of coastline on the updriftside of the port), suggest a dominant longshore sediment transport updriftward on the updrift-side of the delta. Because the growth of deflected deltas depends on dominant sediment accretion on the updrift-side, these results can be rationalised in the case of internally sourced deflected deltas. Uncertainties remain regarding the net alongshore sediment transport of the entire Ishikari coast; however, the deflected planform is controlled by deflected mouth bars and river mouth jets due to the waves approaching the river mouth at slightly oblique angle in winter, and is likely independent on the direction of dominant sediment transport. • Dominant updriftward sediment transport was recognised in the seafloor sands on the updrift-side of deflected delta. • Dominant updriftward sediment transport condition can be rationalised in the case of internally-sourced deflected deltas • Planform deflection is controlled by obliquely approaching waves, independent on dominant sediment transport directions. [ABSTRACT FROM AUTHOR]

The different possible factors that influence or drive the observed volumetric and morphological variation in foredunes located along 16 km of a low wave energy, dissipative southern section of the 190 km long Younghusband Peninsula (South Australia) are examined. Grain size, wind energy, coastline orientation, and vegetation species presence/absence vary only slightly along this coast providing a unique site to study other drivers of coastal change. Results show that foredune height and volume progressively increase towards the north coincident with a gradual increase in wave energy. A significant correlation between foredune volume and wave energy is established, while no apparent relationship with other controlling factors was found, confirming the importance of wave energy in the dune building process on wave-dominated beaches. Results indicate that (1) all other factors being equal, wave energy can drive the morphological evolution of foredunes; (2) minor changes in wave energy can result in differences in the long-term beach and dune sediment budget; and, (3) the surfzone-beach-dune interactions model is applicable to coasts with small scale gradients in wave energy. [Display omitted] • Factors that drive volumetric and morphological variation in foredunes on a dissipative beach are examined. • Grain size, wind energy, aeolian transport, orientation, and vegetation species presence/absence vary only slightly alongshore. • Foredune height and volume progressively increase northwards coincident with a gradual increase in wave energy. • The study confirms the importance of wave energy in the dune building process. [ABSTRACT FROM AUTHOR]

Embayed beaches constitute a large proportion of the world's rocky coastlines, but there is a paucity of studies focusing on the longshore sediment exchange between embayed beaches separated by rocky stretches of coast. Here, we investigate the nearshore sediment transport dynamics along a 15-km stretch of the embayed coastline of north Cornwall, SW England, using Delft3D. Numerical simulations (coupled wave and tide) are conducted to compute major circulation modes and sediment fluxes, including order of magnitude for sediment bypassing between bays, for a wide range of modal and extreme wave conditions. Results indicate that extreme events cause (i) multi-embayment circulation and mega-rip formation where an alongshore current is deflected offshore (0.7 m s−1 at >20 m depth) in the down-wave sectors, (ii) large bypassing rates (103–104 m3 day−1 bypassing) and (iii) exchanges extending to depths that exceed the base of the headlands. Accretionary phases over moderate-high swell periods were associated with clockwise intra-embayment circulation with simulated currents inducing redistribution towards the south in the long embayments (>103 m3 day−1 longshore). This circulation mode is combined with significant bypassing rates around the shallower and wider headlands (102–103 m3 day−1). Predictions of sediment fluxes along the lower shoreface are based on the correlation between the modelled sediment fluxes and offshore wave-conditions (r > 0.92), providing insights into the potential sediment budget over multi-annual timescales. Hindcasted yearly bypassing rates around the headlands range between 103 and 105 m3 y−1 under an assumption of unlimited sediment supply, with the bulk of the transport occurring mainly during high energy events. Hence, the magnitude of the computed potential sediment fluxes presented here has implications for coastal evolution at longer timescales (>10 years), especially along sediment-starved coastlines. • Delft3D was used to investigate nearshore sediment transport dynamics • Full picture of sediment bypassing rates and paths along an embayed coastline is provided • Sediment budgets and headland bypassing rates over multi-annual timescales are predicted • Hindcasted bypassing (103 – 105 m3 y-1) will affect coastal evolution over the decadal scale • Magnitude of sub-tidal bypassing indicates substantial interconnectivity between cells previously thought limited to cross-shore oscillations [ABSTRACT FROM AUTHOR]

• Analysis of the long-term morphological evolution of a muddy open-coast • Sediment transport suggests a growth of mudbanks migrating along the coast. • Cross-shore exchange of sediment is equally significant as longshore transport. • Mudbank and shoreline erosion/accretion trends are reversed. [ABSTRACT FROM AUTHOR]