Search

Sandspits at river mouths are important socio-ecological but fragile coastal ecosystems. Due to their significant role, particularly in shielding areas behind them from wave impact, sandspits are crucial, albeit they also pose challenges such as increased fluvial flooding, which often necessitates substantial engineering interventions such as artificial breaching. Here, four wave-dominated sandspits are compared to determine their common behavior. The study sites at river mouths have in common a high control of waves on longshore sediment transport leading to sandspit development; the Langue de Barbarie at the Senegal River delta, the Bouche du Roi at the mouth of the Mono River in Benin, the Mataquito River mouth in Chile, and the Pomene River mouth in Mozambique, each characterized by unique hydrodynamic conditions and geomorphologic features that influence their evolution and susceptibility to natural and anthropogenic impacts. Their cycles of opening and development, typically spanning decades, are studied by collecting 37 years of shoreline and wave reanalysis data from the Landsat and Sentinel-2 missions. It is shown that part of the morphodynamics is truly intrinsic and that climatic wave evolution drives only a small part of their evolution. A simplified model is introduced to predict the cyclic behavior of spits. The proposed model for sandspit development is based on a cyclic behavior, where newly formed spits mature and eventually break, resetting the cycle. This cyclic perception is represented as a function of time, where the elongation of the spit relative to its maximum length is modeled as a function of time and a constant wave-driven longshore sediment transport component. The model is tested at several sites with similar environmental conditions and blindly at one site with reasonable results. The present work also highlights the importance of spit evolution and the associated variability they impose on coastal downdrift and updrift, especially when considering anticipating the implementation of engineering solutions. • Cyclic behavior of sand spit lengthening. • Predicting sand spit length with a conceptual model. • Updrift and downdrift coastal impact of a sand spit lengthening. [ABSTRACT FROM AUTHOR]

Tidal flats connect terrestrial and oceanic environments, and play an important role in the evolution of coastal ecosystems and geomorphology. Taking tidal flat on Zhairuoshan Island (ZRS) as an example, we studied the dynamic characteristics of suspended sediment concentration (SSC), sediment resuspension, and transportation in tidal flats. The field data of the waves, currents, and sediments were obtained from the tidal flat on the east of ZRS Island. The results show that current-induced bed shear stress is dominant. Under the combined action of waves, tides, and currents, SSC has two main periods (12.2 h, 6.2 h, 60−80%) and three minor periods (23.5 h, 8.2 h, and 5.1 h, 20−40%). Advection transport is dominant, at approximately 2–3 times the sediment resuspension, and the net sediment flux is onshore. Turbulent intermittent events play an important role in sediment resuspension, and their probability is affected by velocity. The vertical instantaneous transport of sediment depends on wave height. The vertical profile of SSC shows that, at high current velocity, the sedimentation formula that considers flocculation better fits the vertical profile of suspended sediment using the Rouse formula. The settling velocity of a single particle fits better at low current velocities, indicating that the flocculation of sediment is weak at low velocities. This study improves our understanding on near-bottom sediment characteristics and controlling mechanism in tidal flat in muddy coasts. • Suspended sediment concentration (SSC) in the tidal flat of Zhairuoshan Island is dominated by advection, followed by resuspension. • Intermittent turbulence is important for sediment resuspension, affected by wave and velocity. • Settling velocity with considering flocculation better fits the near-bottom sediment vertical profile at high velocity. • Harmonic analysis of SSC reveals the contribution of tides, waves, and turbulence events. [ABSTRACT FROM AUTHOR]

The estuary of the Rio Jaguaribe (NE Brazil) is filled by a sediment sequence of more than 23 m and characterised by estuarine plains that – today - extend 30 km upstream. We investigated the sedimentary dynamics in the Rio Jaguaribe estuary since the early Holocene by means of clay mineralogical analyses, as well as sediment analyses from 11 sediment cores. The clay mineralogical approach proves to provide the most robust results. A transition of the illite/kaolinite peak area ratio from 0.3 to 1.8 within the vertical depositional record suggests a change of sediment transport direction over time. Whereas in the lower sequence marine influence dominates, characterised by the relatively high kaolinite content originating from the weathered Barreira formation at the coast, in the upper sequence sediments of fluvial origin dominate, which are characterised by the relatively high illite content originating from the crystalline rocks in the source area of the Jaguaribe river. The depositional architecture of the Holocene sediments of the Jaguaribe estuary shows a seawards prograding infill, assumed to have occurred since the mid-Holocene and supports studies suggesting a stagnating or slightly falling relative sea level (RSL) since ∼7000 BP. • Within the estuary sediment infill a transition from low illite/kaolinite to high illite/kaolinite ratios is documented. • Clay mineralogy is a robust tool for defining changes of dominant sediment pathways in an estuary. • The change from marine ingression to progradation of the estuary is assumed to correlate with RSL change around 7000 BP. [ABSTRACT FROM AUTHOR]

Sediment transport is a key driver of reef zonation and biodiversity, where an understanding of sediment dynamics gives insights into past reef processes and allows the prediction of geomorphic responses to changing environmental conditions. However, modal conditions within the back-reef seldom promote sediment transport, hence direct observation is inherently difficult. Large benthic foraminifera (LBF) have previously been employed as ‘tracers’ to infer sediment transport pathways on coral reefs, as their habitat is largely restricted to the algal flat and post-mortem, their calcium carbonate test is susceptible to sediment transport forces into the back-reef. Foraminiferal test abundance and post-depositional test alteration have been used as proxies for sediment transport, although the resolution of these measures becomes limited by low test abundance and the lack of variation within test alteration. Here we propose the novel use of elemental ratios as a proxy for sediment transport. Two species, Baculogypsina sphaerulata and Calcarina capricornia , were analysed using a taphonomic index within One Tree and Lady Musgrave reefs, Great Barrier Reef (Australia). Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to determine Mg/Ca and Sr/Ca and these ratios were compared with taphonomic data. Decreases in test Mg/Ca accompany increases in Sr/Ca in specimens from algal-flat to lagoonal samples in both species, mirroring trends indicated by taphonomic values, therefore indicating a relationship with test alteration. To delineate mechanisms driving changes in elemental ratios, back-scattered electron (BSE) images, elemental mapping and in situ quantitative spot analyses by electron microprobe microanalysis (EPMA) using wavelength dispersive X-ray spectrometers (WDS) were performed on un-altered algal flat and heavily abraded tests for both species. EPMA analyses reveal heterogeneity in Mg/Ca between spines and the test wall, implying the loss of appendages results in a decrease in Mg/Ca. BSE imaging and WDS elemental mapping provided evidence for cementation, facilitated by microbial-boring as the primary cause of increasing Sr/Ca. These novel proxies hold advantages over taphonomic measures and further provide a rapid method to infer sediment transport pathways within back-reef environments. [ABSTRACT FROM AUTHOR]

This paper provides an overview of the geomorphological characteristics of UK estuaries and the factors which control them. Many of the features included in previous classifications of UK estuaries are not true estuaries since they do not possess significant river influence. The features considered in this paper to be ‘true’ estuaries are divided into ‘restricted entrance’ and ‘unrestricted entrance’ types on the grounds that the size and geometry of the estuary mouth exerts a critical influence on water levels, tidal currents, wave action, sediment transport and morphological evolution. An estuary which has a wide mouth, narrows and becomes shallower towards the head is likely to be flood dominated, especially if it has a large tidal range, whereas an estuary which has a narrow mouth and widens and/or becomes deeper towards the head is more likely to display ebb dominance, especially if it has a relatively small tidal range. Wide-mouthed estuaries are influenced to a greater degree by wave processes than estuaries with a narrow mouth. Previous authors have hypothesised that estuaries may maintain a state of dynamic equilibrium through alternating periods of flood and ebb dominance, but it is concluded that there is presently no substantive evidence to support this hypothesis. UK estuaries have been affected to varying degrees by embanking, land claim, dredging, sea wall breaching and managed realignment. Some estuaries have adjusted quickly to such perturbations, but others continue to show progressive change, either sedimentary infilling or erosion and sediment loss. The quantification of estuary morphometry, identification of change over time, and testing of hypotheses regarding the morphodynamics and stability of estuaries requires adequate bathymetric/topographic, hydrodynamic and sediment data. At present, such data are available for relatively few UK estuaries. [ABSTRACT FROM AUTHOR]

The tidal channels and mudflats of estuarine wetlands have vital ecological functions for the habitat of aquatic organisms and the exchange of nutrients and sediment. Understanding the spatiotemporal dynamics of their morphology helps to formulate an appropriate management plan for mangrove swamps. The water levels, wetland topography, suspended sediment concentration, sediment particle size, and mangrove stand characteristics were investigated in the Tanshui River estuary to verify the geomorphological model established in this study. The decoupled model includes a hydrodynamic module, a sediment module, and a mangrove module to integrate the effects of tidal hydroperiods and river currents. The simulation results revealed that the model could effectively predict the tidal channel morphodynamics, but the siltation of tidal mudflats was overestimated. Sensitivity analysis results showed that the soil properties and erosion rate had the highest impacts. The flow velocities and bed shear stress of the tidal channels were several times those of the mudflats. The hydrodynamics of ebb tide were more extensive than those of flood tide. The asymmetry between ebb and flood tides contributes to maintaining a self-sustaining tidal channel. The deposition rate during the flood period was 20–80 times that of the normal period. The results indicated that river floods entrained large amounts of sediment to accelerate wetland siltation. The reduced tidal range scenarios and the corresponding suspended sediment input suggested an alternative for preventing channel deposition in the wetland. This study offers a quantitative tool for delivering effective maintenance of tidal channels and tidal flats that may enhance the integrated and adaptive management of mangrove swamps. • The geomorphological model contains hydrodynamic property, sediment transport process, and mangrove characteristics. • Model simulation results agree with field observations and reveal the prediction capability. • River floods induce tens of times to average siltation and cause tidal channels to alter into flats. • Ebb tides are critical in removing larger particles of suspended sediments in mangrove swamps. • Reduced tidal range and suspended sediment inputs are significant in sustaining wetland geomorphology. [ABSTRACT FROM AUTHOR]

Abstract: The effects of 2 functional groups of bioturbators have been predicted in terms of long-term impact on erodability: (1) one superficial mobile deposit-feeder, the gastropod Hydrobia ulvae; and (2) one endobenthic deposit-feeder, the bivalve Scrobicularia plana. Different scenarios of morphodynamical cross-shore 1DH/1DV model were performed to simulate the equilibrium profile of an intertidal mudflat under tide and wave forcings. This process-based model for erosion is able to simulate multiphasic sequential resuspension, by discriminating various erosion behaviour like benthos-generated fluff-layer erosion (BGFL) and general bed loosening and burrowing activity in deep layers. The results were analysed and compared to examine the long-term effect of macrofauna after 14 years. It reveals that the impact of the bivalve S. plana is very significant after only 4 years of simulation while the effect of the gastropod H. ulvae is negligible in terms of sediment transport even after 14 years. More generally, this reveals the strong impact of stationary endobenthic bioturbators that induces a high downward shift of the upper shore while the effects of superficial motile bioturbators remain very low. This impact is mainly due to the effect of endobenthic species in deep layers associated to burrowing activities and their consequences on the bed erosion, but the production of a fluff layer by surface grazer like H. ulvae at the sediment surface can be neglected. The importance of macrofauna mediation of bed erodability is discussed in this study by comparing the activities of the two functional groups of bioturbation on the general functioning of intertidal mudflats. The model outcomes (transferred in a 1DV framework) were in close agreement with the measured results of flume data at 3 different bathymetric levels of the mudflat over the cross-shore profile. This validation step revealed that model of sediment transport under influence of biota effects does not need further refinements at the upper shore where S. plana dominates the species assemblage, whereas there is still a need to include further formulations of biota effects to simulate the erosion experimental results at the lower shores where other molluscs and Annelids significantly contribute to the species assemblage. [Copyright &y& Elsevier]

Abstract: Simulations of estuarine bathymetric change over decadal timescales require methods for idealization and reduction of forcing data and boundary conditions. Continuous simulations are hampered by computational and data limitations and results are rarely evaluated with observed bathymetric change data. Bathymetric change data for Suisun Bay, California span the 1867–1990 period with five bathymetric surveys during that period. The four periods of bathymetric change were modeled using a coupled hydrodynamic-sediment transport model operated at the tidal-timescale. The efficacy of idealization techniques was investigated by discontinuously simulating the four periods. The 1867–1887 period, used for calibration of wave energy and sediment parameters, was modeled with an average error of 37% while the remaining periods were modeled with error ranging from 23% to 121%. Variation in post-calibration performance is attributed to temporally variable sediment parameters and lack of bathymetric and configuration data for portions of Suisun Bay and the Delta. Modifying seaward sediment delivery and bed composition resulted in large performance increases for post-calibration periods suggesting that continuous simulation with constant parameters is unrealistic. Idealization techniques which accelerate morphological change should therefore be used with caution in estuaries where parameters may change on sub-decadal timescales. This study highlights the utility and shortcomings of estuarine geomorphic models for estimating past changes in forcing mechanisms such as sediment supply and bed composition. The results further stress the inherent difficulty of simulating estuarine changes over decadal timescales due to changes in configuration, benthic composition, and anthropogenic forcing such as dredging and channelization. [Copyright &y& Elsevier]

The results of 2DH numerical models of the Frisian Inlet (located in the Dutch Wadden Sea) are discussed to gain further knowledge about the physical mechanisms causing the presence of both ebb-tidal deltas and of channels and shoals in tide-dominated inlet systems. A hydrodynamic model, extended with sediment transport formulations, was used to verify earlier conceptual models that deal with ebb-tidal delta characteristics. The model does not confirm their hypothesis concerning the observed spatial asymmetry of ebb-tidal deltas and suggests that long-term morphological simulations are needed to understand this aspect. Furthermore, the model indicates that the initial formation of the ebb-tidal delta is mainly due to convergence of the tidally averaged sediment flux related to residual currents, whilst the net sediment transport in the basin is mainly caused by tidal asymmetry. A second model (accounting for feedbacks between tidal motion and the erodible bottom) was used to simulate the long-term bathymetric evolution of the Frisian Inlet under fair weather conditions. This model reproduces the gross characteristics of the observed morphology: the presence of a double-inlet system with two distinct ebb-tidal deltas having different sizes and the presence of channels and shoals. The role of the ‘Engelsmanplaat’, a consolidated shoal in the middle of the Frisian Inlet, was not found to be crucial for the morphodynamic stability of this inlet system. [Copyright &y& Elsevier]

Flow dynamics are a key part of the physical processes that drive the sediment transport in an aquatic system, affecting depositional processes and leading to landform evolution. The current asymmetry (CA) noted in Vembanad Lake (VL), India, have a particularly strong influence on the sediment budget and thus shape its geomorphology. In the present study the Coastal Ocean Model - Finite Volume Coastal Ocean Model (FVCOM) was used to compute the lateral current asymmetries and their implications for the long-term morphological changes in VL during different tidal phases. Model results were utilised to calculate the CA using two criteria: (1) Peak flood/ebb velocities with duration, and (2) Asymmetry Index (AI DV). AI DV values suggest the prevalence of ebb dominance over the entire system except at Cherai and Vallarpadam. However, several isolated pockets were noted on the right/eastern bank (shallow depth) of the system where the peak flood velocity exceeded the ebb, favouring accretion. Contrary to the concept of ebb asymmetry, mean velocity and duration asymmetries were prevalent in the asymmetry index (AI DV); hence we adopted peak flood/ebb velocity and its duration asymmetries to improve our understanding of the sediment transport dynamics. Typical flood asymmetry was identified on the right bank and at many of the lateral stations in VL, which switched to the modified ebb asymmetry during substantial river discharge. Analysis of CA enables understanding of the process that influences selective morphological changes, as explained by the model results. This conclusion was well corroborated by results obtained over the past three decades (1973–2008) from analysis of Landsat satellite imagery. Flood/ebb dominance in VL is explicitly substantiated with regions of accretion/erosion derived from satellite imageries. A holistic approach encompassing modelling, observations, remote sensing, and GIS techniques were used to address the erosion-accretion process in VL. •Vembanad Lake (VL) experiences ebb-flood asymmetry laterally. •Several isolated pockets in the right bank of the VL where the peak flood velocity exceeds the ebb favoring accretion. •Modified asymmetries for longer period alter sediment dynamics and geomorphology of the VL. •Long-term erosion and accretion zones identified using Landsat imageries well corroborated with modelling results. [ABSTRACT FROM AUTHOR]

The objective of the present work was to analyze the bottom morphology, the genesis of the bedforms, and the patterns of current and sediment transport of the internal portion of the south mouth in Paranaguá Bay, Paraná - Brazil. This region was used as a laboratory field to study the geomorphology of a dune field on a tide channel, with features of flood delta in the inner portion and ebb delta in the direction of the continental shelf. A sediment hydrodynamic model was used to calculate the direction and magnitude of the sediment transport and the residual current flows. A map of apparent roughness was used in the numerical model, obtained through the soundings of a sidescan sonar (Starfish Sonar) and estimated from the classification of the dune field, so it is possible to obtain simulations with higher correspondence with reality. It was observed two vortices of residual circulations near the shores of Pontal do Sul and Ilha do Mel, with directions towards the continental shelf. The residual circulation vortex is responsible for the triangular shape of the large dunes near the Pontal do Sul margin. In the flood, the sediment material is conducted inward forming small ripples and accumulating sediment material on large dunes. During the ebb, these ripples are supposedly destroyed and attached to the large dunes or to the water column. On the other hand, the bedforms of Saco do Limoeiro are formed due to the effects of the tide when entering the mouth and the confinement of the water mass in the region until the next moment of padding. • The bedforms and the sediment transport of the south mouth of the Paranaguá Bay were investigated. • The residual current vortices are responsible for the superposition of small ripples in the dunes and also by its shape. • At Saco do Limoeiro was demonstrated how the bedforms could lead to a false interpretation of geomorphologic evolution. [ABSTRACT FROM AUTHOR]

The purpose of the present work is to study the effect of an isolated depression and a related rocky outcrop on the flow field in a mesotidal channel of the Bahía Blanca Estuary, Argentina. To this end, measurements of current velocity (ADCP), swath bathymetry (multibeam), and seismostratigraphic surveys (3.5 kHz) were made in the study area. In general, the channel bed studied (Cabeza de Buey Channel) is flat and mainly composed of clayey silts. In the inner sector, there is a depression 6 m-deep and 50–60 m in diameter, and on either side (in a streamwise sense) there are two isolated rocky outcrops that extend up to 2 m in height relative to the prevailing bed level. The two features differ in that the up-estuary outcrop is longer and extends over the whole width of the hole, while the down-estuary outcrop is shorter and only extends over part of the diameter of the hole. Seismostratigraphic analysis, in correlation with bathymetric and lithological data, indicates that these high points correspond to the outcropping of relict deltaic materials that formed part of a regional Late Pleistocene drainage system. Fine-grained estuarine sediments deposited above the old deltaic materials show evidence of the present-day sedimentological conditions related to the hydrodynamics of the channel. The flow patterns within the depression observed over a complete tidal cycle demonstrate that the flow behaves differently depending on the stage of the tide. The depth-averaged flow shows higher velocities during the ebb tide, generating a net flow directed towards the sea. Results reveal the generation of return flows (turbulent eddies) at the bottom of the hole, which expand immediately towards the water surface. During the ebb tide, intense vertical currents develop inside the hole. The eddy formation is attributed to the unequal size of the outcrops. Hence, the larger upstream channel outcrop plays an important role in eddy generation, just as downwelling-upwelling interaction produces erosion and transport of sediments, which exerts a strong influence on the deepening of the hole. • The behavior of the tidal flow in a scour hole between rocky outcrops. • The outcrops consist of Pleistocene material that underlies marine fine sediments. • The flow in the hole shows different behavior at each tidal stage. • Eddies and high vertical velocities only occur in the hole during ebb tide. • The interaction between the downwelling-upwelling flows causes erosion in the hole. [ABSTRACT FROM AUTHOR]