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We employed idealized numerical experiments using Delft3D to investigate how the interactions of coalescing buoyant river plumes with mangrove vegetation affect sediment transport and deposition in the coastal zone. Our model system, based on the Firth of Thames in the North Island of New Zealand, is designed to represent two mangrove-lined rivers debouching into a coastal bay. Sediment transport patterns into the three adjacent mangrove forest regions (central, western, and eastern) were assessed by quantifying the sediment fluxes over a tidal cycle. The interactions between the buoyant river plumes resulted in a coalesced river plume that flowed along the central mangrove forest between the rivers. The dominant contributors to the total sediment transport into the mangrove forests varied with both the total and the relative flows through the two rivers. When both rivers were forced with their respective flows at the 50th percentile of observed flow, the analysis of sediment transport fluxes along the central forest revealed sediment deposition inside the mangrove forests lining the river mouth nearly four times greater than in the intermediate regions along the central forest; consistent with satellite imagery of the Firth of Thames. Winds of 5 m s−1 modify the overall sediment transport patterns by deflecting the river plume in the direction of the wind. The magnitude of sediment flux is the highest (nearly 80% compared to the no-wind scenario) in the case of an easterly wind scenario as the alongshore transport is enhanced by Coriolis deflection and wind direction aligned with the alongshore flows of the larger plume. • Plume-to-plume interactions result in a coalesced river plume. • Sediment plume bulge of one river inhibits along-shore sediment transport of the other. • The presence of winds altered sediment transport patterns. [ABSTRACT FROM AUTHOR]

The estuaries and coastal zones of the Vietnamese Mekong Delta located at the end of the Mekong Basin are susceptible to a large variety of natural threats and human interventions such as upstream dam construction, climate change and sea level rise. In this study, we investigated sediment dynamics and morphodynamic changes in the Mekong estuaries and coastal zone, using a well-calibrated Delft3D-4 model for a 10-years period simulation. For the first time, the impacts of different drivers were distinguished to guide future adaptation strategies and policies. Our investigations pointed out that the influences of upstream sediment reduction and large-scale sand extraction would cause substantial modifications in the subaqueous delta region. Besides human-induced impacts, modeling results also showed that the sediment volume and spatial distribution changed through the simulated period according to monsoonal variation. The influence of each driver, however, not only varied in space but also time. In the flood season, the Mekong and Bassac Rivers provided a large amount of sediment (more than 90%) that was deposited on the delta front due to coastal processes. The sediment quantity transported along the coastline changed with the monsoon and was in a dominantly south-west direction due to the northeast monsoon, especially in November, December and January. As a natural process, in the estuarine region, erosion and deposition occurred alternately, but the south-west coast region (belonging to the Ca Mau peninsula) was dominated by erosion. This is because of the formation of sand bars in front of the Bassac estuaries, which influences the capacity of sediment transportation to the south-west. Consequently, navigation capacity will reduce significantly in the estuaries and coastline erosion in the Ca Mau peninsula leads to substantial loss of houses and assets of local people. Urgent actions such as stopping sand mining or modifying dam designs are needed to sustain the delta coastline. • A well-calibrated Delft3D model against observed and remote sensing data. • Sediment transport is strongly driven by both river discharges and monsoons. • Long-term sand bar formation diverted the direction of current sediment flows. • Human activities will lead to the severe recession of the delta. [ABSTRACT FROM AUTHOR]

This paper proposes the concept of a significant transport rate, in coastal environments that contains different spatial and temporal scales and multiple interacting forces (e.g., waves, tides, wave-current, and wind density currents) as well as, the complex physical processes of total-load sediment which is not easy to calculate for practical needs due to restricted range of applicability. The present study develops a unified classical formula for non-cohesive total-load sediment transport in marine coastal zones by using dimensional analysis and self-similarity concepts where a set of independent variables considered. A dataset of total-load collected at both field observation stations and from the laboratory flume conditions and the six well-known relevant formulas were used to evaluate the predictive capability of the proposed formula. Since the results show that, the new formula is in good agreement with both field and flume data sets measures, the authors are suggesting the use of it for the sediment-carrying capacity predictions of total-load sediment transport in marine coastal zones. [ABSTRACT FROM AUTHOR]

Wave climate and Pacific basin coastal behaviour associated with El Niño Southern Oscillation (ENSO) is understood at a reconnaissance level, but the coastal response to different central Pacific (CP) versus eastern Pacific (EP) flavours of ENSO is unknown. We show that CP ENSO events produce different patterns of directional wave power to EP ENSO along the southeast Australian shelf and southwest Pacific region, because of significant variability in trade-wind wave generation. The modulation of the trade wind wave climate during CP ENSO has thus far been neglected in existing coastal process studies. We also show that coastal change between CP and EP ENSO cannot be inferred from shifts in the deepwater wave climate. This is because variability in trade wind wave generation is masked in deepwater by the persistence of high power extra-tropical waves that have reduced impact on nearshore processes due to high wave refraction. Morphodynamic modelling in a headland-bay beach indicates that CP ENSO leads to higher coastal erosion potential and slower post-storm recovery than EP ENSO during an El Niño/La Niña cycle. We show that the alongshore variability in beach morphological type can be used to model the static equilibrium planform response for each ENSO phase. Results indicate that shoreline response to ENSO in most headland-bay beach coasts is not as simple as the existing paradigm that (anti-) clockwise rotation occurs during El Niño (La Niña). Our methods provide a second-order approach to project coastal response and predict the discrete shoreline rotations for ENSO flavours. [ABSTRACT FROM AUTHOR]

The short, medium and long-term evolution of the sandy coastline of the Bight of Benin in the Gulf of Guinea, West Africa, has become a major regional focal point due to the rapid socio-economic development that is occurring in the region, including rapid urbanization and a sharp increase in harbor-based trade. Harbors have a significant impact on the present evolution of this coast, notably by affecting longshore sediment transport. However, little is known of the environmental drivers, notably the wave climate, that governs longshore sediment transport and the ensuing pattern of shoreline evolution of this coastal zone. This article aims to address this important knowledge gap by providing a general overview of coastal evolution in the Bight of Benin and the physical processes that control this evolution. Here, the 1979–2012 ERA-Interim hindcast is used to understand the temporal dynamics of longshore sediment transport. Oblique waves (annual average H s =1.36 m, T p =9.6 s, S-SW incidence) drive an eastward drift of approximately 500,000 m 3 /yr. The waves driving this large longshore transport can be separated into two components with distinct origins and behavior: wind waves generated locally in the Gulf of Guinea and swell waves generated in the southern hemisphere sub- (30–35°S), and extra-tropics (45–60°S). The analysis undertaken here shows that the contribution to the gross annual longshore transport from swell wave-driven longshore currents is an order of magnitude larger than the local wind wave-driven longshore currents. Swell waves are dominantly generated by westerlies in the 40–60°S zone and to a lesser extent by trade winds at 30–35°S. The longshore sediment drift decay (−5% over 1979–2012) is found to be linked with a decrease in the intensity of westerly winds associated with their southward shift, in addition to a strengthening of the trade winds, which reduces the eastward sediment transport potential. The equatorial fluctuation of the Inter-Tropical Convergence Zone (ITCZ) is found to explain most of the variability in transport induced by wind waves, while the Southern Annular Mode (SAM), an extra-tropical mode, has a predominant influence on transport induced by swell waves. The ITCZ and SAM have, respectively, a negative and positive trend over the period 1979–2012 that explains the decrease in both wind- and swell-wave-induced transport. For future scenarii, General Circulation Models (GCMs) predict a stabilization of the SAM, and, thus, a non-substantial or weak change in longshore sediment transport can be expected on the coast of the Bight of Benin. [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]

Typhoon Mangkhut, in 2018, was one of the worst typhoons in recent history that has made landfall in the Pearl River Estuary (PRE) China. It swept along the coast of Guangdong Province, causing severe damage to many areas and affecting more than one million people. To study the hydrodynamics and sediment dynamics in different sub-regions of the PRE during the typhoon, we implemented a 3D wave-current-sediment coupled ocean model based on the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM). A series of numerical experiments were conducted to study the variation in suspended sediment concentration (SSC) under different external forcing mechanisms, including tides, waves, river discharges and strong winds. Moderate Resolution Imaging Spectroradiometer (MODIS) images, collected after the typhoon landed, were used to assess the simulation results. We found that in the area where the water depth was shallower than 20 m, the increased total suspended sediment concentration was dominated by the regional erosion of fine sediment which was caused by near-bottom wave orbital velocity and bottom shear stress, especially at the four western outlets and along the western coast of the PRE. For the deeper coastal zone, at depths between 20 and 30 m offshore of the western coast, the high total SSC resulted from the contribution of fine sediment transported from the upper and middle estuaries by southwestward advection. A small amount of sand eroded and migrated locally, owing to the corresponding increase in the bottom shear stress. The variation in SSC during the typhoon had a time lag compared with other dynamic conditions, resulting in a maximum 1 h delayed response after the typhoon made landfall. Based on the results of the sensitivity experiments under the same typhoon conditions, we found that, as a result of wave-enhanced bottom shear stress, the SSC values associated with wave-current interaction were significantly higher, by as much as 1.5 and 1.3 times those of simulations without the waves at the surface and near-bottom layers, respectively. This indicated that the sediment resuspension induced by waves was important during typhoon landfall. However, the strong wind-induced current was weakened owing to the joint effect of wave radiation stress and enhanced bottom stress; thus, considering wave effects, the magnitude of the resulting sediment transport rate was smaller than that without considering wave effects, especially in shallow waters. During the approach, landfall and retreat of Typhoon Mangkhut, Lingding Bay experienced two transitions from a slight state of sediment loss to a relatively high state of sediment gain, followed by an even greater state of sediment loss. During an M 2 tidal cycle after Typhoon Mangkhut made landfall, the net seaward suspended load sediment transport in Lingding Bay reached 0.37 megatonnes. For the typhoon period covering three M 2 tidal cycles, the overall state of suspended load sediment transport in Lingding Bay was sediment loss, with the magnitude of sediment loss on the west side being larger than that of sediment gain on the east side. The magnitude of the overall sediment loss in Lingding Bay would be enlarged unrealistically by approximately 73% if the wave effects were excluded. In each period, the extent of sediment transport between the two sides of the bay was strengthened by the wave-current interaction under typhoon conditions. We determined that the net effect of wave-current interaction and strong wind-induced currents under typhoon conditions was a key factor for erosion, which resulted in SSC-influenced areas being approximately 8.7 and 4.4 times as large as those under normal weather conditions for the surface and near-bottom layers, respectively. The turbidity maximum moved from the West Shoal of Lingding Bay under normal weather to the Modaomen Estuary, which was the most affected area during the typhoon when wave effects were considered. We also showed that the relative location of the typhoon path with respect to the estuary had a more significant effect on erosion than the intensity of the typhoon. ● A 3D wave-current-sediment coupled ocean model was set up and validated to simulate sediment transport during Typhoon Mangkhut. ● Waves were primarily responsible for resuspension of both fine sediment and sand during the event. ● Strong wind-induced currents were weakened owing to the joint effect of wave radiation stress and enhanced bottom stress. ● Turbidity maximum moved westward along the path of the typhoon. [ABSTRACT FROM AUTHOR]

Abstract: Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55ma−1. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget. [ABSTRACT FROM AUTHOR]

Abstract: The present study offers a two-dimensional horizontal wave propagation and morphodynamic model for muddy coasts. The model can be applied on a general three-dimensional bathymetry of a soft muddy coast to calculate wave damping, fluid mud mass transport and resulting bathymetry change under wave actions. The wave propagation model is based on time-dependent mild slope equations including the wave energy dissipation due to the wave-mud interaction of bottom mud layers as well as the combined effects of the wave refraction, diffraction and breaking. The constitutive equations of the visco-elastic–plastic model are adopted for the rheological behavior of fluid mud. The mass transport velocity within the fluid mud layer is calculated combining the Stokes’ drift, the mean Eulerian velocity and the gravity-driven mud flow. The results of the numerical model are compared against a series of conducted wave basin experiments, wave flume experiments and field observations. Comparisons between the computed results with both the field and laboratory data reveal the capability of the proposed model to predict the wave transformation and mud mass transport. [Copyright &y& Elsevier]

Abstract: Tidal flats with fluvial input can have ample fine-grained sediment supply, and yet the Skagit River tidal flats remain mostly sandy. Therefore, hydrodynamic mechanisms must efficiently export mud off the flats. This study evaluates short-term deposition and long-term burial of mud on an intertidal flat and links seabed deposits to sediment-transport processes. To investigate the fine-grained sediment-transport mechanisms within the sandy Skagit River tidal flats, seabed and water-column observations were collected seasonally from 2008 to 2009 under varying river discharge conditions. Little mud was observed deposited on the seabed. Spatially, mud was most common near channels and on the seaward flat edge. Its presence was temporary, and was typically observed following high river discharge. Strongest current velocities and high suspended-sediment concentrations were measured during spring tidal cycles with peaks in the late ebb. Maximum velocities were observed within channels, permitting for rapid transport of fine-grained particles to the seaward edge of the flat and export beyond. These physical processes also act to mobilize the mostly sandy seabed, which is reworked at various frequencies: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1–2cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1–2m, making the limited mud deposits available for export. Overall, channel networks on sandy tidal flats with fluvial input not only deliver fine-grained sediment to the flat, but also aid in the export of suspended sediment at ebb tides and in the release of muddy deposits through reworking of the seabed. [Copyright &y& Elsevier]