Sediment Exchange Research Articles

Sedimentary dynamic processes affected by tropical cyclones and anthropogenic forcing in a small-scale estuary: The Sheyang River Estuary, China

Amid the robust landward migration of tropical cyclones (TCs) in the context of global warming, the sedimentary dynamic processes in coastal areas are increasingly affected by TCs. Concurrently, coastal environments face tremendous stress from anthropogenic interventions, such as land reclamation and jetty construction. However, most research puts more emphasis on TC impacts on large-scale estuaries or marginal seas, leaving the sediment dynamics in small estuaries, particularly under the influence of TCs and man-made structures, largely unexplored due to the limited field data. To address this issue, this study took the Sheyang River Estuary (SRE) as an example and explored the sediment dynamic responses to TCs and the jetty construction during Typhoon Lekima in August 2019, utilizing the Finite Volume Coastal Ocean Model (FVCOM). The results showed that the rotating wind field of Lekima induced rapid variations in wave height, residual water level, and currents at different typhoon stages and strengthened the sediment resuspension and alongshore transport. Furthermore, by comparing the effects of Lekima with Typhoon Chan-hom in July 2015, we found that TCs with different tracks produced markedly different dynamic processes, yet all facilitated sediment exchange between the northern and southern areas within the SRE. The study also highlighted the impact of jetty construction on sediment transport during typhoons, noting that jetties can disrupt the typhoon-induced alongshore sediment flux and create eddies on either side by altering sediment advection. In the navigational channel area, the construction of jetties can strengthen the along-channel estuarine circulation during typhoon events, potentially leading to severe channel siltation by trapping riverine sediments and inducing net landward sediment transport. While site-specific, this study provides valuable insights into the combined effects of extreme events and human interventions on sedimentary environments in small estuaries.

Storm Dynamics Control Sedimentation and Shelf‐Bay‐Marsh Sediment Exchange Along the Louisiana Coast

AbstractHurricanes can benefit wetland accretion by augmenting the delivery of mineral sediment, an essential process allowing marshes to offset submergence during rising sea levels. Using Hurricane Gustav (2008, Louisiana) as a control, we examined eight synthetic storms with varying characteristics (track, speed, intensity, size) to evaluate sediment exchange between the inner shelf and bay and bay‐to‐marsh interfaces. All storms showed net landward sediment exchange from the inner shelf to the bay to the marsh—storms with closer proximity, higher intensity, and slower forward speed positively correlated with net sediment exchange; storm size had little impact. Except for slow‐moving storms (½ speed of Gustav), our analyses suggest that most hurricane scenarios cause net bay erosion, because more sediment is conveyed to landward wetlands than is replenished from erosion of the inner shelf. Our results suggest that the ongoing deepening of the bay will likely worsen because of rising sea levels.

Modeling of non-equilibrium suspended sediment transport process in open channel flows with vegetation

A mathematical model based on advection-diffusion theory is established to study the non-equilibrium sediment transport process in vegetated channels. The effects of vegetation on velocity distribution and sediment diffusion coefficients were considered, respectively. Validation against experimental data from flume studies confirms the model's ability to accurately predict the longitudinal sediment deposition rate and the vertical distribution of suspended sediment concentration (SSC). A comparative analysis of three sediment diffusion coefficient formulations indicates that the linear-exponential formula provides a more precise estimate of εsz, and the linear-exponential formula performs well in predicting the turbulent diffusion coefficients of both rigid and flexible vegetation when gently swaying. Moreover, the distance required for SSC to regain equilibrium is influenced by the submergence level of the vegetation canopy. At lower submergence levels, the canopy shear vortices significantly affect the vertical exchange of sediment, and the sediment diffusion coefficients exhibit pronounced stratification near the vegetation canopy. An increase in vegetation density at these lower submergence levels intensifies the shear vortices, thereby extending the distance needed for SSC to reach equilibrium. At higher submergence levels, the impact of canopy shear vortices is lessened, which reduces sediment diffusion coefficient stratification characteristics, and the flow is similar to rough boundary layer flow. An increase in vegetation density increases flow resistance, which shortens the distance required for SSC to attain equilibrium. However, further efforts are required to explore turbulent characteristics with highly flexible vegetation motion and the grain size distribution of non-uniform sediments in vegetated flows.

Numerical Investigation of the Sediment Load Exchange between a Coastal Mud Bank and Its Neighbouring Estuary

Muddy coastlines cover much of the world’s shores, yet studies on the interaction between mud-affected coasts and estuaries are limited. This study focuses on the Mahury River estuary and its interaction with the muddy coast of the Guianas, primarily fed by the Amazon. A coupled wave–current–sediment transport model is developed to analyze the sediment exchange in an environment with strong interactions between the waves and the fluid mud. Simulations explore how seasonal changes in waves, mud availability, and tides affect sediment fluxes. The main processes influencing suspended particulate matter (SPM) and sediment transport are well emulated, notwithstanding the complexity of the ambient muddy environment. The results show that during the rainy season, strong wave damping and wave refraction zones cause high SPM resuspension in shallow waters (<5 m). In contrast, during the dry season, wave influence shifts to the estuary mouth. Erosion and sedimentation patterns indicate that ebb currents associated with neap tides during the rainy season represent the most favourable conditions for the alongshore migration of mud banks. Neaptide ebb currents also contribute to sedimentation during the dry season but only in the estuary mouth and the nearby coastal area. The abundance of mud leads to an extension of the estuary’s intertidal area during the dry season.

Numerical evaluation of the impact of sandbars on cross-shore sediment transport and shoreline evolution

Nearshore nourishments are a common intervention used to mitigate sediment deficits in coastal areas experiencing erosion problems. This coastal intervention involves placing nourished sediments in the submerged zone of the beach profile to form an artificial submerged sandbar which are then distributed by natural forces occurring in the coastal zone. Coastal processes that control the morphological evolution of coastal zones operate on several spatial and temporal scales (seconds to years) and can be divided into cross-shore and longshore components. Usually, the numerical models that simulate the evolution of cross-shore profiles due to sediment transport within the profiles, are related with short to medium-term events (days to months), and shoreline evolution models, that allow for long-term analysis (years to decades), are typically only considering longshore sediment transport. Describing all the processes in a single numerical model is complex and computationally demanding, and therefore, the numerical models are typically focused on specific processes, categorized by their temporal and spatial scales. This article presents a numerical study focused on medium to long-term numerical modelling of coastal zone evolution, examining the combined effects of cross-shore and longshore sediment transport processes of sediment transport. The model incorporates two simplified numerical models: LTC - Long Term Configuration, for shoreline evolution and CS-Model, for cross-shore sediment exchanges. Applied to nine different sandbar scenarios, model results revealed that disturbances in sandbar volume tend to return to equilibrium through cross-shore sediment transport processes. In situations that considered a constant sandbar volume alongshore, the longshore effects are null, because the volume entering in the coastal domain is equal to the volume leaving, and the sediment balance is only dependent of the cross-shore processes. Variable sandbar volume alongshore leads to impacts on shoreline position due to longshore sediment transport gradients generated by the sandbar disturbance. The results demonstrate the potential of the proposed combined model for medium to long-term projections, allowing for the interpretation of how physical aspects of nearshore nourishments evolve. These aspects include sandbar volume, shoreline position, berm width, and shoreline displacements induced by cross-shore and longshore sediment transport processes. These parameters aid in understanding sandbar-beach sediment dynamics, which is valuable for supporting coastal management, particularly in the development and design of shoreface sand nourishments, enabling the optimization of sand resources.

The importance of source data in river network connectivity modeling: A review

AbstractRiver network connectivity (RC) describes the hydrologic exchange of water, nutrients, sediments, and pollutants between the river channel and other “sites” via heterogenous flowpaths along the river corridor. As water moves downstream it carries these constituents, creating a stream‐to‐ocean continuum of connectivity that regulates global water, carbon, and nutrient cycling. River network connectivity models have developed over many decades, culminating in recent years with network‐scale RC models that explicitly simulate the transport and exchange of water and elements from headwaters to coasts, sometimes requiring models to contain tens of millions of river reaches. These advances provide transformative insights into the aggregate effects of RC on water and material transport across scales from local to global. Yet, recent reviews have pointed to several challenges that need to be overcome to continue advancing network‐scale RC modeling. In service of these goals, I summarize recent network‐scale RC maps and models to identify similarities and differences across the large‐scale RC modeling landscape. Although our computational and upscaling abilities have significantly improved and have revealed new insights, current models are still limited by the quantity, quality, resolution, and lack of standardization of the available in situ databases and source data maps necessary for the modeling. This suggests that we can extend recent advances if we keep improving these source datasets, while continuously revisiting our physics and theory to explain those new data. In doing so, we will continue to expand the role of network‐scale RC models in informing water quality modeling and management into the future.

Evolutions of hydrodynamics and sediment transport pattern in the Qiantang Estuary (China) in response to multidecadal embankment constructions

Since the late 1960s, large-scale coastal embankments have been constructed along the Qiantang Estuary (QE, China) covering both the Qiantang River (QR) and the Hangzhou Bay (HZB) to reduce the tidal prism and thus stabilize the channel. While most previous studies have reported their relevant impacts on hydrodynamic and sediment transport in the HZB, a more comprehensive investigation covering the whole QR + HZB system remains lacking, so as a quantitative analysis of the tidal prism and the channel stability. Therefore, numerical case studies have been conducted to examine evolutions of hydrodynamics and sediment transport of the QR + HZB system in response to different coastal embankments (i.e., no embankment as in 1958; QR embankment as in 1984; QR embankment + part of HZB embankment as in 2004; both QR and HZB embankment as in 2016). It is shown that the flood tidal asymmetry in the QR and the tidal range in the HZB are significantly increased, implying an intensified tidal distortion and thus a strengthened river role after embankments. While this intensification does not change the direction (from HZB to QR) of the net sediment transport, all of the sediment exchange rates [e.g., the landward (during flood tides) and seaward (during ebb tides) sediment transport volumes, as well as the net sediment transport rate between them] exhibits obvious decreasing trends. In addition, strong deposition is observed in the Jianshan (JS) reach, which has led to further increase of the underwater giant sandbar. The constructions of embankments have facilitated the initial goal of reducing tidal prism in most part of the QR + HZB system, except some local parts of the HZB, where increased tidal prism can be observed as the effect of the enlarged tidal range overwhelms that of embankments. Moreover, the channel stability was greatly improved as the mainstream has been mostly converged to the reach centerline and the width-depth ratio of the channel has been considerably reduced. The present investigation of the QE regulation could be indicative for the management of other estuaries in the world.

Impact of seasonal variations in fronts on suspended sediments transport off the coastal area of Fujian Province

The Zhejiang-Fujian (Zhe-Min) coastal muddy area plays a crucial role in facilitating sediment exchange through a cross-front. The mud depocenter off the Zhe-Min coastal area is a source of suspended sediment that can be transported to the continental shelf of the East China Sea (ECS). Although the front of the inner shelf of the ECS has been extensively reported, the cross-front material transport off the coastal area of Zhe-Min in summer has not been well studied, especially using measured data. To reveal how the front controls the transport of suspended sediment, this study focuses on the impact of fronts on the dispersion of suspended sediment off the coastal area of Fujian Province in different seasons. The results indicate that the front acts as a barrier, inhibiting the dispersion of suspended sediment into the sea. The high-concentration suspended sediment is mainly found to the northwest of the front, with an average SSC of 8.5 mg/L in winter and 3.1 mg/L in summer. The suspended sediment concentration (SSC) follows a V-shaped distribution along the cross-isobath transects, with lower SSC observed at the 50-m isobath compared with the shallow water area and the deep water area. The SSC at the front was the lowest, with an average concentration of 2.3 mg/L in winter and 1.9 mg/L in summer. The front is crucial for the development of the Zhe-Min coastal muddy area. The winter monsoon is strong, resulting in a sufficient supply of suspended sediments in the muddy area and a high transport flux of suspended sediments in the nearshore. The front hinders the dispersion of high-concentrated sediment from the nearshore to the offshore, resulting in the deposition of fine-grained sediments in the nearshore and the formation of an inner shelf muddy sedimentary zone. The findings of this study will help improve our understanding of the sediment source-to-sink processes in the ECS and land–sea interactions.

Suspended sediment exchange between the Changjiang Estuary and Hangzhou Bay and their key driving mechanisms

Nowadays estuarine deltas are threatened by factors such as insufficient terrigenous sediment supply and rising sea levels. The Changjiang Estuary is adjacent to Hangzhou Bay, characterized by high turbidity waters, and the suspended sediment exchange between the two will inevitably affect the erosion and deposition of subaqueous delta in the future. Based on in situ observation data and numerical model, we found that suspended sediment is transported from the Changjiang Estuary into Hangzhou Bay during spring tides, while from Hangzhou Bay into the Changjiang Estuary during neap tides. The key driving mechanisms for the difference during spring-neap tides were expounded by momentum analysis and numerical sensitivity experiments. The barotropic pressure gradient force during spring tides is the key force that drives the suspended sediment from the Changjiang Estuary into Hangzhou Bay. Conversely, the baroclinic pressure gradient force during neap tides is the key force that drives the suspended sediment from Hangzhou Bay into the Changjiang Estuary. This study can provide valuable scientific evidence for erosion and deposition in estuaries and adjacent seas, as well as the sustainable utilization of regional coastal sediment resources.

Wind-driven sediment exchange between the Indian marginal seas over the last 18 000 years

The Indian Coastal Current is the only channel for material exchange between the two largest marginal seas in the northern Indian Ocean: the Bay of Bengal and the Arabian Sea. However, its past history is poorly known, limiting accurate predictions of its future changes. Here, we present a new clay mineral record from south of India supported by interpretations of model simulations to trace its variability over the last 18 000 years. Decreased smectite/(illite + chlorite) ratios during the cold intervals suggest that a stronger northeasterly wind led to a mean southward flow of the Indian Coastal Current in the Bay of Bengal. In contrast, increased smectite/(illite + chlorite) ratios during the warm intervals suggest the opposite scenario. Combining the proxy record with model simulations, we infer that atmospheric circulation changes were the main driver of the changes. Moreover, a possible link is observed between a positive Indian Ocean Dipole (IOD) and weakened southward flow of the Indian Coastal Current in the Bay of Bengal during the Holocene. These findings imply that future warming scenarios, if associated with more intense positive IOD events as proposed, may lead to a reduction in fresh water transport from the Bay of Bengal to the Arabian Sea.

Copper Distribution and Binding Affinity to Size-Fractioned Dissolved and Particulate Organic Matter in River Sediment

This study investigated the distribution of copper in sediment dissolved and particulate organic matter (DOM and POM) based on their size. The DOM and alkaline extracted POM (AEOM) were separated into five size fractions using a cross-flow ultrafiltration (CFUF) system. The results showed that Cu mass was mainly distributed in the low molecular weight (<1 kDa, LMW) fraction of the DOM with an average range of 78.1–83.1%. Conversely, the high molecular weight (1 kDa–0.45 μm, HMW) AEOM fraction had a higher distribution of Cu mass with an average range of 92.6–93.3%. The Cu and AEOM binding affinity ratios (CuBAAEOM) ranged from 17.0 to 149.6 μmol/g-C in site-1 and from 20.6 to 143.7 μmol/g-C in site-2. The HMW CuBAAEOM ratios were significantly higher than the LMW ratios. The Cu and DOM binding affinity ratios (CuBADOM) ranged from 5.6 to 358.6 μmol/g-C and 17.2 to 126.6 μmol/g-C in site-1 and site-2, respectively. However, the LMW CuBADOM ratios were significantly higher than the HMW ratios. Optical indices suggested that the AEOM had more aromaticity and terrestrial and allochthonous contributions than the DOM. The optical indices were significantly correlated with the CuBAAEOM ratios but weakly correlated with the CuBADOM ratios. Sediment exchange between POM and DOM may affect copper distribution. DOM has a low-molecular-weight composition, while POM retains high-molecular-weight organic matter.

Insights into nearshore sandbar dynamics through process-based numerical and logistic regression modeling

Process-based nearshore morphodynamic models are commonly used tools by coastal engineers and planners to predict the nearshore morphology change of sandy beaches across various spatiotemporal scales. Accurate modeling of the morphological response on medium and long time scales is imperative for quantitative assessments of coastal infrastructure over a project’s intended life-span. However, most previous modeling applications have focused on single/sub-seasonal storm events and are often limited to an assessment of the subaerial beach (i.e. berm and dune). This not only leaves uncertainty concerning the quality of morphology predictions on extended (> weeks) time scales, but also the capacity of process-based models to emulate realistic nearshore sandbar dynamics and the corresponding exchange of sediment between the nearshore-beach system. To shed light on these meso-scale dynamics, CSHORE, a 1D phase-averaged, process-based nearshore morphodynamic model, was applied on an annual scale to a multi-barred, dissipative beach in Oysterville, WA, USA. Thousands of unique sediment transport and hydrodynamic parameter combinations were executed during model calibration. A large portion of these simulations displayed physically realistic sandbar dynamics, including the growth, decay, and migration of intertidal and subtidal sandbars. To explore the model mechanisms enabling realistic bar behavior, the binary and multinomial logistic regression model were used to quantify the relationship between model parameter selection and the probability of various categorical bar configurations occurring in the final predicted profile. The results indicate the most sensitive parameters associated with barred morphology, in this study, and support the use of separate sediment transport parameters for low and high wave energy conditions. The co-utilization of numerical and statistical modeling outlined in this publication is generalizable to future exploratory modeling and/or calibration routines concerned with categorical outcomes.

Future sediment transport to the Dutch Wadden Sea under severe sea level rise and tidal range change

Future sediment transport from the North Sea coasts to the Dutch Wadden Sea for various future sea level scenarios has been studied because it influences the future sand nourishment demand for the maintenance of the coastline and because it determines bio-geomorphological development of the Wadden Sea. The present study focuses on two questions which have not yet been considered in the previous modelling studies using ASMITA: How will the transport develop around drowning of the intertidal flats in the Wadden Sea? How will tidal range change influence the future sediment exchange? By using SLR scenarios with faster acceleration and running the simulations for longer periods of time some inlets exhibited drowning, i.e., where the tidal flat volume vanishes. When drowning occurs, the sediment import rate approaches a maximum or a minimum, depending on the initial morphological state of the tidal inlet system. This maximum or minimum rate for a certain tidal inlet system depends on the SLR scenario. Theoretical analysis as well as modelling results show that tidal range change will influence the sediment import to the Wadden Sea. A tidal range increase will cause a decrease of the sediment demand in the Wadden Sea resulting into less sediment import to the Wadden Sea. It is thus important to study the tidal range development in the Wadden Sea by considering the interaction between SLR, tidal range change and morphological development in the system. It is further concluded that the empirical relation used in the previous studies is not representative of conditions in a tidal basin with fixed basin area, even though this relation has been derived from field observations in many tidal inlet systems worldwide. The equilibrium channel volume should be proportional to the tidal prism instead of to its 1.5th power.

Mechanism of rapid accretion-erosion transition in a complex hydrodynamic environment based on refined in-situ data

Morphological changes in estuaries are typically discussed on decadal time scales, with reported changes often exceed several decimeters. In our study, we utilized refined in-situ data to investigate bed-level changes and rapid accretion-erosion transitions during tidal cycles off the Changjiang Estuary. A field study employing a high-resolution bottom tripod system was conducted, measuring various parameters such as water level, waves, velocity profiles, near-bottom suspended sediment concentration (SSC), and bed level changes over ten tidal cycles. Our findings reveal significant and intense bed level changes occurring at the scale of tidal cycles. The data suggests that downward sediment transport plays a dominant role in the overall accretion tendency throughout the study period. Additionally, our research introduces a dynamic critical bed shear stress that increases during tidal cycles, deviating from fixed values. We also present a new quadrant analysis method to elucidate the primary processes controlling near-bed sediment transport. The results highlight the importance of horizontal advective sediment transport in frequent accretion-erosion transitions at tidal-cycle timescales. This implies a frequent exchange of sediment between adjacent shoals and channels in the seaward region of the North Channel in the Changjiang Estuary, which holds significance for future navigation and construction management.

Spatial distribution, source identification, ecological risk assessment, and air–seawater/seawater– Sediment exchange rates of polycyclic aromatic hydrocarbons in Pars special economic energy zone, Iran

Polycyclic aromatic hydrocarbons (PAHs) have been introduced as major pollutants released by the oil, gas, and petrochemical industries. Against this background, the present study examined the coastal air, seawater, and sediments in the Pars Special Economic Energy Zone (PSEEZ), Iran, in terms of pollution caused by such compounds. Considering the industrial facilities and residential areas in this region, the total suspended particulate (TSP), seawater, and sediments were sampled (n = 10) and then analyzed over the course of two seasons via gas chromatography-mass spectrometry (GC–MS). The total concentrations in the TSP, seawater, and sediments were found to be in the range of 346-739ngPAH/gTSP, ND-84.87 µg/L, and ND–306.7 ngg−1(dw), respectively. The positive correlation coefficients between sediment organic matter (OM) and concentrations did not show a significant relationship. Crude oil and its derivatives and biomass-coal combustion were identified as the main sources of pollution. Carcinogenic PAHs (CPAHs) were further detected at much higher concentrations near industrial facilities. Sediment qualityguidelines (SQGs) and risk quotients (RQs) showed that coastal seawater and sediments posed acceptable and moderate ecological risks. According to the fugacity diagrams, the seabed and seawater served as secondary sources of pollution. The total concentration and values obtained in earlier investigations were also comparable. Nonetheless, most maximum points could provide a point-form of the pollutant concentration. Therefore, preventive measures are required to ensure the protection of the environment.

A 1-dimensional-two-layer transient drift-flux model for hydraulic transport pipelines: modelling and experiments of bed layer erosion and density wave amplification

Abstract Hydraulic transport pipelines in the dredging, mining and deep sea mining are designed using steady-state methods. However, these methods cannot predict density wave formation. Density waves form a risk for pipeline blockages, therefore there is a need to understand and preferably be able to model the process. The density waves studied in this research are caused by a stationary sediment deposit in the pipeline. This article explores the development of a new transient design model, based on 1-dimensional-two-layer Driftflux CFD. The two layers model the exchange of sediment between the turbulent suspension, and a stationary bed layer, and can therefore model density wave amplification. An empirical erosion-sedimentation closure relationship is applied to model the sediment exchange between the two layers, and is calibrated using experiments. The final model is also validated against a second experiment, specifically for density wave amplification. The experiments and the model show good agreement on the erosion of a stationary bed layer and the growth rate of a density wave and the amplitude of the density wave.

Storm and tidal interactions control sediment exchange in mixed-energy coastal systems.

Storms can have devasting effects on shorelines, causing flooding and the destruction of property and infrastructure. As global warming and the frequency and magnitude of tropical storms increase, barrier islands comprising 10% of the world's coast may undergo significant change caused by beach erosion, loss of dunes, and formation of washovers and tidal inlets. Understanding how storms affect sediment transport at tidal inlets is an understudied subject that directly influences barrier island erosional-depositional processes and long-term sediment budgets. This study models hydrodynamics and sediment transport at a conceptualized mixed-energy, mesotidal inlet system using 10 synthetic storm tracks. We investigate the provenance and the role of various storm characteristics and timing between the peak storm surge and high tide on sediment fluxes for different grain sizes. We find that most storms (38 of 40) cause a net import of sediment into the basin that is sourced primarily from the updrift and downdrift nearshore and secondly from the ebb-delta. Very little sediment comes from inlet channel scour. Cumulative (net) transport correlates well with peak significant wave height because wave height influences bottom shear stresses and sediment suspension on the ebb-tidal delta and in the nearshore. The duration of the storm surge also correlates with net transport because it controls the period of flood-directed currents. Our findings help explain the formation of flood deltas inside tidal inlets and the formation of sand shoals in backbarrier regions. Storm-induced enlargement of these deposits represents a permanent long-term loss of sand from barrier islands that will lead to erosion.

Morphodynamic Response of Open and Embayed Beaches to Winter Conditions: Two Case Studies from the North Atlantic Iberian Coast

The morphological responses of two mesotidal beaches located in different coastal settings (embayed and open sandy beaches) on the northwestern Iberian coast were monitored during the winter of 2018/19. The offshore wave time series analysis is related to high-resolution topo-bathymetric measurements to explore spatial-temporal morphological variability at monthly to seasonal scales. Both locations are subjected to the North Atlantic wave climate which exhibits a pronounced seasonality. Throughout the last decade (2010–2020), significant wave heights reached values of up to Hs~9 m during winters and up to Hs~6 m during summers. On average, approximately 12 storms occurred annually in this region. The results clearly reveal divergent morphological responses and sediment transport behaviors at the upper beach and the intertidal zone during the winter for each location. In the embayed beach (Patos), sediment transport in the nearshore is governed by cross-shore processes between the beach berm and a submerged sandbar. In contrast, the open beach (Mira) showed dynamic sediment exchanges and three-dimensional morphologies alternating between accumulation and erosion zones. Overall, both beaches exhibited an erosional trend after the winter, particularly concerning berm erosion and the subaerial beach volume/shoreline retreat. This study highlights the contrasting morphodynamic response on open and embayed beaches to winter conditions, integrating both the subaerial and submerged zones. Local geological and environmental factors, as well as the coastal management strategies applied, will influence how the beach responds to winter wave events. Monitoring and understanding these responses are essential for effective coastal management and adaptation to changing climate.

Sediment leakage on the beach and upper shoreface due to extreme storms

The quantification of sediment exchanges between the beach and the lower shoreface, although being still poorly understood, is required to adequately forecast long-term coastal evolution. The effect of extreme storms on the morphodynamics of the nearshore and the sediment budget of the coastal zone seems to be strongly conditioned by local parameters, which should be incorporated into studies of medium- and long-term coastal evolution. In the upper shoreface, the dynamics of shore-parallel bars in sandy coasts have been extensively studied, but their implications for the sediment budget remain controversial as it is unclear whether their evolution simply represents a morphological change or denotes an escape of sediment from the upper shoreface. In order to provide new knowledge to address these questions, fifteen topo-bathymetric surveys that extend from the emerged beach to 15 m water depth were carried out along 1 km of Castelldefels beach (Llobregat Delta, Barcelona, western Mediterranean Sea) during the period 2011–2022. Morphological variations on the shoreface are studied, volumetric changes are estimated and implications on sedimentary exchanges in the littoral profile are inferred. The largest morphological (volumetric) changes are caused by cross-shore displacements of the nearshore bars, usually onshore during fair-weather conditions and offshore during storms. Volumetric changes suggest that shoreline retreat and the erosion of the shallow section of the profile during the last decade provides sediment to both the distal upper shoreface and the lower shoreface. The resulting sediment budget in the profile is almost zero in absence of extreme storms, but becomes highly negative when including extreme storms events that cause a net offshore transfer of sediment from the upper to the lower shoreface and, probably, the inner shelf. The characteristics of the study area (morphology, sediments, waves, tides) and in-situ hydrodynamic measurements at the lower shoreface suggest that the main potential onshore transport mechanisms (velocity and acceleration skewness during low-mid energy event) are small, making a significant transfer of sediment from the lower shoreface towards the shallow beach area unlikely at interannual scales. Thus, the study area is an example of low-lying coastal stretch in which the potential long-term shoreline retreat caused by sea level rise cannot be fully compensated by the sediment supplied from the lower shoreface. Other mechanisms, such as sediment supplied by longshore transport, are needed to counteract this trend.

Influence of a small submarine canyon on biogenic matter export flux in the lower St. Lawrence Estuary, eastern Canada

Abstract. Submarine canyons enhance shelf–slope sediment exchange and influence hydrodynamic processes, with consequences for biogeochemical cycles. This work documents variations in the vertical export of biogenic matter on the northern shore of the lower St. Lawrence Estuary (LSLE, Quebec, eastern Canada), which is characterized by the presence of an active submarine canyon system. A total of three moorings were deployed from November 2020 to September 2021. One nearshore mooring (PDMc) was deployed in the main axis of the Pointe-des-Monts (PDM) canyon system and was equipped with an acoustic Doppler current profiler (ADCP), and two moorings equipped with sediment traps were deployed in the distal PDM canyon system (PDM-154, PDM-224) and offshore Baie-Comeau (BC-133). The ADCP data revealed the occurrence of a minor sediment remobilization event (December 2020) and a small turbidity current (February 2021) in the canyon. Concurrent elevated fluxes of total particulate matter, particulate organic carbon, particulate nitrogen, and chloropigments showed that these events left a signature in sediment traps PDM-154 and PDM-224 located > 2.6 km further offshore by enhancing lateral dispersion of resuspended sediments. The composition of diatom and dinoflagellate assemblages was similar in the canyon system and offshore BC, but the diatom bloom occurred 2 weeks earlier (in mid-April) at the PDM site. A bloom of the potentially toxic diatom Pseudo-nitzschia seriata was also observed during the second half of September 2021 at the BC site. Annual diatom and dinoflagellate fluxes were almost 2 times lower at the PDM site than at the BC site, possibly due to differences in riverine input and the structure of the water column, as well as increased sediment input and resuspension at the PDM site, leading to limited light availability. This study notably helps identify the relationship between near-bed canyon processes and biogenic matter export in the water column, thereby directly influencing the ecosystem offshore PDM. The study period further covered an anomalously nearly ice-free winter, and thus, in the context of climate change, it provides valuable insight into future trends of biogenic matter export in the LSLE.