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Fifteen monthly field surveys were conducted from September 2012 to November 2013 at ten representative stations along a cross-shore profile, covering the entire tidal flat. Results indicate that tidal currents significantly affect bed level variations over bare flats, while subsurface processes (e.g., soil subsidence and expansion) are likely to play an important role in changing the bed level of the upper intertidal flat where salt marshes are present. The cross-shore profile shows a clear double-convex shape, and different geomorphic zones display distinctive variation. Above the mean high water level (MHWL), the bed level is generally stable. The region around the MHWL, where the upper convex point is present, is a location of high sedimentation due to the weaker hydrodynamic conditions and the settling and scour lag effects, it keeps growing with the increase of inundation frequency. A concave point occurs in the middle part of the intertidal flat, showing considerable erosion. Near the mean low water level (MLWL), the lower convex point is elevated due to the long-shore tidal current and associated sediment transport (the flood dominated transport during summer exceeds the ebb dominated transport during winter, hence the net effect favors sedimentation). Further seawards, the area below the MLWL is strongly eroded. The cross-shore profile follows a “stable-accretional-erosional-accretional-erosional” sequence. Overall, the measurements indicate that the interplay among vegetation, hydrodynamics and sediment transport is critical in shaping the cross-shore morphology of the intertidal flats along the Jiangsu coast of China. [ABSTRACT FROM AUTHOR]

Abstract: The Minas Basin, the eastern end of the Bay of Fundy, is well known for its high tide ranges and strong tidal currents, which can be exploited to extract electricity power. The properties of the tidally-induced sediment transport in the Minas Basin, where significant changes in tidal processes may occur due to a recently proposed tidal power project, have been studied with a three-dimensional hydrodynamic model, an empirical bed load sediment transport model and surface sediment concentrations derived from the remotely-sensed images. The hydrodynamic model was evaluated against independent observational data, which include tidal elevation, tidal current (in the full water column and bottom layer), residual current profile and tidal asymmetry indicators. The evaluation shows that the model is in good agreement with the observations. The sediment transport includes two components, bed load and suspended particulate load. The bed load is calculated using the modelled bottom shear stress and the observed grain size data. The estimated features of bed load transport roughly agree with the observed patterns of the erosion and deposition in the Minas Basin and Cobequid Bay. The transport of the suspended load is estimated using the modelled velocity fields and the surface sediment concentration derived from remote-sensing images. The comparisons between the modelled results and the limited observations illustrate that the observed directions of suspended sediment transport are basically reproduced by the model. The modelled net suspended sediment input into the Minas Basin through Minas Passage is 2.4×106 m3 yr−1, which is comparable to the observed value of 1.6×106 m3 yr−1. The variations of the bed load and the suspended load in space and time are also presented. The total net transport, defined as the mean value of the sum of bed and suspended load transports during the tidal cycle, shows strong spatial variability. The magnitude of the transport flux ranges from 0.1 to 0.2kgm−1 s−1 in Minas Channel and Minas Passage, 0.1kgm−1 s−1 in Cobequid Bay, to 0.01kgm−1 s−1 in the central Minas Basin and Southern Bight. In Minas Channel, the sediment transport follows the structure of the tidal residual circulation, which features a large anticlockwise gyre. The sediment in Minas Passage moves eastward and deposits into the central Minas Basin. However, the sediment from the eastern part of the Basin moves westward and deposits in the central Minas Basin as well. In the Cobequid Bay, sediment moves eastward and deposits in the upper bay. [Copyright &y& Elsevier]

Abstract: Linear sandbanks appear in the lee of coastal headlands where the hydrodynamics are dominated by strong tidal currents and the seabed is characterized by an abundance of sands. They may develop as symmetrical sandbanks on either sides of the headland or as an unique banner bank. The present study numerically investigates the combined effects of waves and tide on the initial development of headland-associated sandbanks. A morphological model based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) is applied to an idealized Gaussian shaped headland for waves conditions varying in heights and directions at the offshore boundary. The coupling considers the effects of the interactions between the wave and current bottom boundary layers, namely the enhanced levels of turbulence near the bottom and the increase of the total bottom shear stress. Waves substantially modify the initial development of sandbanks formed by suspension narrowing their width and reorienting them along the side of the headland. They weakly impact the morphogenesis of sandbanks by bedload favoring on a short-time scale the growth of symmetric circular-shaped features and a central depositional spit prolonging the headland tip. Waves of transverse directions toward the tip of the headland contribute to the initiation by suspension of a well-developed feature in the headland side of low energy limiting the seabed evolution in the exposed area. [Copyright &y& Elsevier]

Abstract: This paper presents a method to investigate the relative influence of waves, wind-induced currents and tidal currents on sediment mobility in coastal environment. It is based on the assumption that, knowing high uncertainties linked to sediment dynamics studies, simple sediment mobility indicators are sufficient tools to provide useful information. This method is based on five main steps: (1) assessing hydrodynamic conditions and related errors spatially and temporally for an area; (2) evaluating the bottom shear stress induced by these hydrodynamic conditions; (3) providing simple sediment mobility indicators; (4) analysing the sensitivity of these indicators to the existing hydrodynamic uncertainties; (5) interpreting indicators knowing their limits (identified in the sensitivity analysis). This methodology is applied to a test site on the inner shelf, seaward of the Pertuis Charentais region in France. Using wave (SWAN) and sea-level and currents (MARS-2DH) modelling, we study the spatial distribution of the relative contribution of waves, tides and wind-induced current in the non-cohesive sediment mobility under storm conditions. The deduced sediment dynamics indicators reveal the dominant action of waves in sediment initiation in the offshore area, in water depths of up to 60m. The sensitivity study, performed in order to evaluate the validity of results, shows that the sediment mobility indicators still provide relevant information, even for hydrodynamic uncertainties of ±20% or furthermore when only a qualitative sedimentological knowledge of the area is available. [Copyright &y& Elsevier]

Strong tidal currents, shallow water, waves and mobile bed sediments present in many tidal inlets prevent deployment of instrumentation using benthic frames and restrict measurements that can be obtained safely from a boat and thus data from these environments remain scarce. Recent deployments of the new POL Instrument Package (PIP) from a jack-up barge have provided a solution to this problem. Here the performance of the various sensors on the PIP is assessed critically and the degree to which near-bed processes are affected by the PIP is investigated by comparing measured and predicted flows, bedforms and rates of sediment transport. [Copyright &y& Elsevier]

Offshore gasmining is an example of a human intervention with a morphological impact. On land, it is usually attended with a dish-like bed depression. We show that, if located at sea, such a bed depression can become morphodynamically active by triggering mechanisms related to tidal sand bank formation. To that end, a simple morphological model is considered which describes an erodible bed subject to a tidal wave in a shallow sea. The continuous subsidence is modelled by a sink term in the sediment balance. Then, a linear approximation is carried out to describe the bed evolution after the onset of subsidence. The results, presented in physical space, show that the subsidence triggers the formation of a sand bank pattern that gradually spreads around the centre of subsidence, at a rate that may go up to 160 m year−1, depending on the tidal transport rate and the tidal eccentricity. The dimension of the depression does not affect the spreading rate nor the orientation of the sand banks, but it does influence their spacing. The main conclusion is that the horizontal extent of the area influenced by the bed depression by far exceeds that of the direct subsidence, thus showing that bed depressions on land and at sea indeed behave in fundamentally different ways. The results suggest that nonlinear effects are worthwhile to be investigated in order to describe finite amplitude development of sand banks as well as the interaction between subsidence and bed forms. [Copyright &y& Elsevier]

The growing interest in tidal energy has led to a need to better understand the interaction between energetic tidal flows and submarine bathymetry. The Gulf of Corryvreckan (GoC) is a highly energetic tidal channel, ∼3.2 km long and ∼1.1 km wide with a maximum depth of 220 m occurring between the islands of Scarba and Jura, western Scotland, UK. A high-resolution bathymetric survey of the region, groundtruthed by sediment samples, has enabled the bathymetry to be related to Lagrangian flow data from GPS-tracked surface drifter buoys. The maximum recorded drifter velocity was 4.75m/s within the GoC during the ebb (east-flowing) tide. Exiting the GoC into open water to the west on the flood tide, the 'Great Race' decelerates and subsequently flows in a curve to the north. East and west of the GoC, sedimentary bedforms lie on the flanks of the main flow paths. The northern flank of the Great Race aligns with a 6 km elongate bedform ('banner bank') extending from the GoC. East of the GoC, complex tidal bottom flows are suspected to result in uncommon sediment morphology, including suspected sediment wave interference patterns expressed by the mobile bedforms. The novelty of these findings indicates the importance of further research into sediment pathways in energetic tidal zones. A greater understanding of how sediment transport may be modified during offshore construction is an important consideration for offshore installations and the feasibility of marine renewables. • Constraining bathymetry significantly accelerates tidal flow speeds. • Surface hydrodynamics inferred by GPS tracked drifters can reflect the local bathymetry, even in deeper (>100 m) shelf waters. • The nature of submarine bedforms may be indicative of dominant bottom flow paths and residual tidal currents. • Complex coastlines promote intricate residual tidal currents and sediment transport pathways, producing structurally uncommon submarine bedforms. [ABSTRACT FROM AUTHOR]