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Due to the active sediment movement in sandy coasts, great changes may develop in the shoreline near the building after it is built into the sea. The downstream erosion problems and protective measures of buildings in sandy coast are reviewed and summarized based on the results of previous studies to facilitate further studies. On the basis of several typical cases such as that in the Friendship Harbor in Mauritania, the law of coastal changes in the downstream of sandy coastal buildings is analyzed from the aspects of coastline variation and sectional variation. The mechanism of water and sediment movement near protective measures such as groins, offshore breakwaters and offshore submerged breakwaters, is discussed, and soft protective measures such as artificial beach cultivation and by-pass sediment transport are summarized. Finally, the prospects for future research on coastal erosion of downstream sandy coastal structures and corresponding protective measures are given. [ABSTRACT FROM AUTHOR]

Storms and extremely energetic events may significantly impact the form and structure of beaches, and so cause erosive processes and coastal damages. Efficient management actions require an up-to-date and accurate knowledge of beach morphological changes, with the shoreline position being a good indicator of such changes. This work proposes the use of the open-source Shoreline Analysis and Extraction Tool (SAET) software for the definition of satellite-derived shorelines (SDSs) from L8 and Sentinel-2 imagery to reveal the shoreline position changes at the beaches of the Ebro Delta, NE Spain. Spatial-temporal models (STMs) of shoreline changes enable a characterisation of how the beaches responded to the storms of 2020. In conjunction with wave data, STMs enable an analysis of the erosive response to storm events, as well as a monitoring of subsequent beach recovery in the short and medium term (<1 year). Results show how Storm Gloria (January 2020, Hs max = 7.62 m) acted as a disruptive event and shifting point in the shoreline trend. As a response to that storm, major erosive processes occurred along the delta that caused an average shoreline retreat of 47 m. A progressive recovery during the spring and summer was mainly associated with periods of low wave energy. Nevertheless, by the end of the year a complete recovery had been achieved for about half of the coast, while the other half showed an average erosion of more than 10 m when compared to the pre-storm situation. Both the erosive and the recovery processes took place unevenly on different sections of the coast, probably dependent on factors such as the orientation of the beach and the pattern of longitudinal sediment transport along the coast. • Automatic shoreline extraction from Sentinel 2 and Landsat 8 imagery using SAET. • Pre and post-storm characterisation of SDS position in time and space using STMs. • Storm Gloria (January 2020) caused an unprecedented and heterogeneous impact on the Ebro Delta. • Slow recovery occurred in the months following the event conditioned by orientation and wave patterns. • About half of the delta showed a significant shoreline retreat by the end of 2020. [ABSTRACT FROM AUTHOR]

Whitley, A.E.; Figlus, J.; Valsamidis, A., and Reeve, D.E., 2021. One-line modeling of mega-nourishment evolution. Journal of Coastal Research, 37(6), 1224–1234. Coconut Creek (Florida), ISSN 0749-0208. As many coastal areas of the world suffer from chronic erosion, innovative solutions beyond traditional local nourishment must be explored. One such solution is the implementation of mega-nourishments (MNs), where a large sediment volume is deposited in a single location and redistributed via natural processes, thereby feeding adjacent beaches. Robust numerical modeling methods are needed to evaluate potential MN sites at coastal locations around the world. Two one-line numerical modeling approaches of MN evolution are presented using the Coastline Evolution Model (CEM) and GENESIS + Cascade (GenCade). Both models are parameterized for the Sand Motor (SM) that has been constructed on the Dutch coast to explore the feasibility of the MN approach. Here, a variety of K coefficients in the Coastal Engineering Research Center (CERC) sediment transport equation are tested since traditionally calibrated and used values may not apply for MN features. Both models are found capable of reproducing the shoreline morphological patterns observed at the SM over the first 3.5 years. GenCade shows a low difference between measured and modeled coastline position overall, showing accurate predictions of absolute shoreline position. CEM is able to capture feature tip migration due to its wave shadowing algorithm. The best results from both models can be obtained for higher K values than those commonly suggested in the literature. Given the size of a mega-nourishment relative to traditional nourishments, it is likely that a higher-than-usual K value may be a better fit when simulating mega-nourishments in a one-line model. [ABSTRACT FROM AUTHOR]

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]

Dune erosion during storm surges can lead to excessive damage to the dune system with devastating floods as a potential consequence. A risk assessment of areas protected by dunes can be facilitated by an understanding and description of the physical processes that take place. Field measurements, knowledge of underlying processes and numerical modelling have developed with time, which enabled a more comprehensive description and new predictive techniques. This review concerns dune erosion in the collision regime, and summarises relevant observations, describes underlying processes and explains existing models predicting dune erosion. Observations of dune erosion consist of field observations, laboratory experiments and manipulative field campaigns. The underlying physical processes that contribute to dune erosion are divided into processes that contribute to sediment transport due to hydrodynamic forcing, which occurs in the surf and swash zone, and sediment transport due to avalanching, which occurs in the swash zone, on the dune face and on the dune crest. The existing dune erosion models that are discussed here contain (empirical) equilibrium profile models and process-based models, which can both be a valuable tool for the risk assessment of storm surges. However, model uncertainties still remain, as specific processes are not yet fully understood and described. Examples are the influences of wave obliquity, sediment grain size, and vegetation on the dune face. By improving our knowledge through research and reducing these uncertainties, we can further improve our predictive models. This could eventually lead to more accurate predictions, more complete risk assessments, and sandy coastlines which are more resilient to excessive dune erosion and possible floods. • Reviews dune erosion during storm surges in the collision regime. • Summarises relevant field and laboratory observations • Summarises underlying hydrodynamic and morphodynamic processes • Summarises existing dune erosion models. [ABSTRACT FROM AUTHOR]

Bedforms like ripples are widely distributed in the coastal zone. They influence the bed roughness thus the estimation of bed shear stress and associated sediment transport. An echo sounder was mounted on a bottom-supported tripod intended to measure the erosion and deposition of seabed in the subaqueous Yellow River Delta, China. However, variations in bed elevation are found to be not the net erosion or deposition at the observation site, but the migration of silt ripples which were generated by waves and pushed back and forth or flattened off by the tidal currents. Ripple heights were observed to be within 0.1–0.7 cm and were used for testing the model of Nielsen (1981). The model overestimated the height of silt ripples as it was developed for sands, but the deviation can be well addressed by incorporating a linear modification (R 2 = 0.79). Alternatively, a new model specifically for silt ripple height was regressed from the field data with R 2 up to 0.78. The existence of silt ripples increases the bed shear stress by 4 times due to the additional bed roughness. A "fluffy layer" overlies the consolidated seabed, therefore, net seabed erosion occurs after the "fluffy layer" is resuspended. A representative critical bed shear stress for net seabed erosion in the study area was found to be 0.8 Pa. The echo sounder can be an alternative tool for observing silt ripples in coastal regions like the Yellow River Delta where the water is too turbid for underwater videos. The proposed model infers silt-ripple features from bed grain size and flow condition and provides a quick estimate for bed roughness improving the understanding on sediment transport. • A new model for silt ripple height is proposed based on field observation. • Bed shear stress is better estimated by incorporating bed roughness due to silt ripple. • Critical shear stress for erosion is inferred from field observations. • Echo sounder is used for observing silt ripples in turbid waters. • Ripple height is inferred from field-measured bed elevations. [ABSTRACT FROM AUTHOR]

Tak, W.J.; Jun, K.W., Kin, S.D., and Lee, H.J., 2020. Using drone and LiDAR to assess coastal erosion and shoreline change due to the construction of coastal structures. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 674-678. Coconut Creek (Florida), ISSN 0749-0208. Recently, The construction of structures on the coastal zone near estuaries, which are the supply sources of sediments, causes a variation in the waterway; this induces a change in the surrounding coasts, and the resulting coastal erosion can adversely affect the lives of the local residents. In the case of a balanced sediment flow, the quantity of the sediment deposited on the coast is equal to the quantity of the released sediment. However, an increase in either of the quantities causes an imbalance in the sediment transport; consequently, the sand is either deposited or eroded in the coastal or proximate shore regions. In this study, the changes in the shoreline and Wolcheon Beach region located on the east coast of Korea was monitored between 2010 to 2019 investigate the phenomena of coastal erosion and beach deformation. The shorelines, beach width, and beach profile were primarily monitored by employing a drone and a terrestrial LiDAR. The results showed that the sediments were transported northward in summer and southward in winter until 2010 in Wolcheon Beach; however, this seasonal sediment transport was blocked during 2011–2012 by the construction of coastal structures such as breakwaters, leading to partial coastal erosion and sedimentation. Wolcheon Beach originally had the coastal characteristics of the Korean East Coast region (2010), but the sediment transport changed due to the variations in the waterway by the construction of coastal structures, leading to a significant change in the coast (2011). In addition, the northern beach volume increased with respect to estuary, while the southern beach volume decreased. When the offshore breakwater was constructed, the northward sediment occurred dominantly in the shield area. In particular, the planar layout of the harbor facilities concentrated the waves, which increased the quantity of the northward sediment. [ABSTRACT FROM AUTHOR]

ABSTRACT Kim, M.; Lim, H.S.; Do, J.-D.; Kim, S.-S.; Lee, H.J., and Shim, J.-S., 2018. Variations of nearshore currents induced by seasonal waves in Haeundae. 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. 1551–1555. Coconut Creek (Florida), ISSN 0749-0208. Haeundae, located on the southeastern coast of the Korean Peninsula, is a famous sand beach with a 1.6 km long, 70 m wide coastline. It suffers from wave-induced beach erosion in both summer and winter. For beach restoration, the Korean government has conducted beach nourishment (620,000 m3), and installed submerged breakwaters at both ends of the beach. Despite these efforts, the beach is still vulnerable to erosion caused by high waves. To understand beach erosion processes and sediment transport mechanisms, the government initiated an R&D project for the development of coastal erosion control technology since 2013. As part of the project, we have measured over three years of AWAC measurements at a water depth of 22 m and 1.8 km from the beach. Intensive measurements of wave, current, sediment transport near the surf-zone in summer and winter have been made using AWAC, VECTOR, and ADV with OBS. Analyses of nearshore currents induced by seasonal waves in the surf-zone have shown that cross-shore currents in summer are mainly induced by strong waves from the SSW or S that propagate to the beach due to wave breaking and shoaling effects on the reef near the beach. However, longshore currents in winter are induced by high waves from the E or ESE that propagate to the shoreline by diffraction at the east coastline. Eroded sand in summer returns along the beach by cross-shore and weak longshore currents induced by waves from the SSW or S. However, suspended sand in winter is moved to the west by strong longshore currents induced by waves from the E or ESE. Due to its pocket beach shape, beach erosion and sediment transport are caused by variations of nearshore currents induced by seasonal waves. [ABSTRACT FROM AUTHOR]

Coastal dunes provide many ecosystem services including protection of infrastructure from wave overtopping and habitat for native species. Foredunes grow at different rates and assume different forms (i.e., short and wide to tall and narrow) depending on a range of factors including pre-existing beach and dune morphology, wind, wave, and water levels, sediment grain size, and vegetation characteristics, yet the relative importance of these factors on foredune growth is understudied. Here, we quantify foredune evolution (2016–2019) and explore the relative influence of a suite of metocean, sedimentary, and ecological factors for dune growth on three barrier islands in the U.S. North Carolina Outer Banks (Cape Lookout National Seashore). We incorporate observed and hindcast wind, wave, and water level data into the process-based, coupled beach-dune evolution model, Windsurf, to explore the relative contribution of factors likely to influence foredune growth at the annual timescale (2016–2017). Our cross-shore topographic profile observations show varied interannual foredune change rates and characteristics, including horizontal retreat and progradation at the dune toe and vertical erosion and accretion at the dune crest. Model results indicate that, of the factors explored, pre-existing morphology had the greatest influence on the type of foredune growth that occurred (i.e., incipient dune development, widening of the dune, and/or vertical accretion), which dramatically altered the final shape of the dune. Variations in wind and wave climates were associated with the relative contribution of marine- and aeolian-driven bed elevation changes and were particularly influential during storms. In addition, increases in the minimum elevation of vegetation on the dune profile (analogous to the cross-shore distance between the shoreline and established vegetation line, i.e., the vegetation limit, as coined by Duran and Moore, 2013) increased dune crest height and dune volume. Moreover, variations in the minimum vegetation elevation resulted in a larger range of dune crest elevations and dune volumes than differences in the median sand grain size. We suggest that insights into the relative influence of metocean, sedimentary, and ecological factors to dune growth can assist in the development of practical coastal management strategies. • Pre-existing beach-dune morphology had the largest influence on dune growth type. • Exposure to the dominant wave direction led to increased dune volume. • Higher minimum vegetation elevation and finer sand resulted in taller dunes. • Minimum vegetation elevation influenced dune growth more than sand grain size. [ABSTRACT FROM AUTHOR]

Seamless bathymetric and topographic digital elevation models (SBT-DEM) of coastal zones are important for modeling tsunami propagation and coastal inundation, monitoring coastal erosion and deposition, and modeling hydrodynamics and sediment transport. Fine-scale SBT-DEM construction has been elusive due to the difficulty of accurately measuring shallow water depths with conventional techniques. We attempt to perform coastal bathymetric and topographic surveys using an SBES-equipped unoccupied surface vehicle (USV) and a LiDAR-equipped unoccupied aerial vehicle (UAV), and construct a fine-scale SBT-DEM using GIS (Geographic Information System) techniques. The results show that the proposed spatiotemporal divisional measurement method can seamlessly collect coastal topographic and bathymetric data using tidal range. The improved Blend mosaic algorithm not only seamlessly and smoothly stitches the land and seabed DEMs, but also minimizes the loss of elevation accuracy. The fine-scale SBT-DEM constructed in this paper has the advantages of high resolution (Pixel size = 1 m) and high accuracy (RMSE = 6.2 cm). Fine-scale SBT-DEM can provide integrated topographic and bathymetric data for coastal dynamic monitoring and environmental management. • A novel method for fine-scale SBT-DEM construction was proposed. • The first combined UAV LiDAR and USV SBES for coastal SBT-DEM construction. • This method can quickly and accurately generate seamless and high-resolution SBT-DEM. • Fine-scale SBT-DEM provides new data support for coastal dynamics studies. [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]

Silty coast, as a transitional state between muddy coast and sandy coast, possesses a considerable percentage of the overall coast in China. Huanghua-Binzhou coast in the southwest Bohai Bay in the northeastern China is a typical silty coast which is vulnerable to cold fronts when its seabed is prone to suffer suddenly drastic erosion and deposition due to the sediment there with relatively low incipient velocity and high settling velocity. In this study, a multi-sediment fraction model was established and two-way coupled with the wave-tide-surge model to investigate the responses of the sediment transport and morphodynamics of Huanghua-Binzhou coast to a cold front event during 10–13th Oct. 2003. The model results demonstrate that three sediment fractions, i.e. mud, silt and sand, showed different responses to the storm due to their different spatial percentages and distinct physical properties. The silt fraction, as the primary fraction, played a leading role in the suspension and transport of the total sediment, and showed a pattern of "deposition-erosion-deposition" in the cross-shore direction like the total morphological change. The fine sediment was transported westward and offshore during the storm, which indicated the sediment coarsening in the study area. The depth-induced wave modulation considerably enhanced the resuspension ability of the nearshore waves for sediment and thus played a critical role in intensifying the erosion. The Huanghua waterway siltation mainly resulted from the resuspension of the offshore sediment by the strong wind waves and its westward transport by the wind-induced currents during the storm. The construction of two long jetties in 2006 can significantly reduce the deposition in the waterway by 75%, and the additional large-scale construction in 2016 can further lessen the waterway siltation by intercepting the westward sediment transport and reduce the suspended sediment concentration by 30% by the land reclamation in the study area. • Silt played a leading role in sediment transport and morphological evolution during the cold front event. • Depth-induced wave modulation considerably enhanced nearshore sediment resuspension. • Offshore sediment resuspension and westward transport by strong wind-induced waves and currents caused waterway siltation. • The extended jetties in 2006 reduce deposition in the waterway by 75% under the action of the cold front event in 2003. [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]

Erosion rates along Alaska's Beaufort Sea coast, among the highest in the world, are negatively impacting communities, industrial and military infrastructure, and wildlife habitat. Decreasing maximal winter ice extent and increasing summer open water duration and extent in the Beaufort Sea may be making the coast more vulnerable to destructive storm waves than during recent, colder, icier decades. Previous studies of Beaufort Sea coastal change have been limited to subaerial analyses of the shoreline. Here we describe nearshore seafloor change by comparing post-World War II (WWII) (1945-53) bathymetry data to recently acquired (1985–2018) bathymetry data and relate the observed seafloor change to adjacent shoreline change near Utqiagvik, within Stefansson Sound, and immediately west of Barter Island and Kaktovik. Within the Utqiagvik region, seabed erosion was generally highest (>1.0 m of loss) offshore of Point Barrow and along the eastern end of the Tapkaluk Islands, while there were lesser amounts of deposition (<0.5 m of gain) within the protected waters of Elson Lagoon. Sedimentation was generally highest offshore of Point Barrow, in a region of converging currents, and on the landward side of the barrier islands and spits fronting Elson Lagoon, which is likely related to a regional trend of westerly sediment transport and landward migration of the barrier islands. Within Stefansson Sound, perhaps the most notable changes from post-WWII bathymetry data compared to recent data are a switch from mixed, low erosion and deposition in 1997 to low deposition (<0.5 m) in 2018 east of the Boulder Patch, a switch from low erosion in 1997 to neutral depth change in 2018 in the channel between the north and south Boulder Patch areas, and higher deposition from 1997 to 2018 landward of the rapidly retreating barrier islands along the Sound's northern border. At Barter Island, high erosion near north-facing shorelines and high deposition near west-facing shorelines generally matched shoreline changes. One of our goals is to identify possible processes responsible for the depth changes we quantified. Using simple metrics that relate sediment characteristics with modeled waves and non-wave induced currents, we show that sediment resuspension and transport by both wave and non-wave driven currents likely contribute to the overall patterns of change within the ∼13 m isobath along the open coast, and that the influence of wave action affecting sediment transport is expanding seaward. • Patterns of decadal-scale changes in shoreline positions and bathymetry indicate possible linkages within the nearshore. • Depth changes are additionally driven by sediment resuspension and transport by both wave and non-wave driven currents. • Wave action affecting sediment transport expanded seaward and deepened by as much as 15 cm/yr between 1979 and 2019. [ABSTRACT FROM AUTHOR]

Coastal zones are vulnerable to both erosion and flood risk, which can be assessed using coupled hydro-morphodynamic models. However, the use of such models as decision support tools suffers from a high degree of uncertainty, due to both incomplete knowledge and natural variability in the system. In this work, we show for the first time how the multilevel Monte Carlo method (MLMC) can be applied in hydro-morphodynamic coastal ocean modelling, here using the popular model XBeach, to quantify uncertainty by computing statistics of key output variables given uncertain input parameters. MLMC accelerates the Monte Carlo approach through the use of a hierarchy of models with different levels of resolution. Several theoretical and real-world coastal zone case studies are considered here, for which output variables that are key to the assessment of flood and erosion risk, such as wave run-up height and total eroded volume, are estimated. We show that MLMC can significantly reduce computational cost, resulting in speed up factors of 40 or greater compared to a standard Monte Carlo approach, whilst keeping the same level of accuracy. Furthermore, a sophisticated ensemble generating technique is used to estimate the cumulative distribution of output variables from the MLMC output. This allows for the probability of a variable exceeding a certain value to be estimated, such as the probability of a wave run-up height exceeding the height of a seawall. This is a valuable capability that can be used to inform decision-making under uncertainty. • First use of MLMC (multilevel Monte Carlo) with a coupled hydro-morphodynamic model. • Large drop in computational cost for same accuracy using MLMC instead of Monte Carlo. • Determine the distribution of variables of interest in coastal zone cases using MLMC. [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]

Coelho, C.; Ferreira, M., and Marinho, B., 2020. Numerical modelling of artificial sediment nourishment impacts. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 209–213. Coconut Creek (Florida), ISSN 0749-0208. In general, coastal erosion problems are related to significant sediments deficits. A possible coastal erosion mitigation strategy involves restoring the sediments balance through artificial nourishments. However, the complexity of the physical processes in the coastal zones challenges the numerical tools prediction capacity. This is usually overcome through numerical modelling of the shoreline evolution and the cross-shore profile along time, as an attempt to anticipate the performance of nourishments operations. The coastal morphology depends on the sediments dynamics and the incident wave climate is considered the main modelling agent responsible for the potential sediment transport capacity. The cross-shore sediment transport is usually associated to the shortterm behaviour of the morphological evolution of the beach (seasonal changes) and the longshore sediment transport is related to the long-term changes (towards an equilibrium state). Typically, these distinct sediment transport components are studied and modelled separately due to the incompatibility of their time scales of interest. This work was developed to numerically model the impact of artificial nourishments. LTC (Long-Term Configuration, Coelho, 2005) and CS-model (Larson et al., 2016) were both applied to analyse the spatial and temporal distribution of sediments induced by artificial nourishments along and across the shore, considering different intervention scenarios. LTC was applied to evaluate the nourishments impact in the shoreline evolution and to quantify the volume of nourished sediments in different longshore locations along time. The CS-model was used to analyse the performance of multiple intervention scenarios, varying the cross-shore location, frequency and volume of the artificial nourishments. The performance of this type of intervention generally represents a smaller shoreline retreat and an increase of the cross-shore profile volumes during a limited period of time. The project results aim to increase numerical modelling capabilities, helping on the selection of optimal artificial nourishment schemes and the establishment of more efficient coastal management policies. [ABSTRACT FROM AUTHOR]