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A numerical model is presented for ship-induced sediment transport, focusing on the fundamental role of propeller jet flow. The new module has been implemented in the open-source numerical model FUNWAVE in order to reproduce the effect of the propeller on sediment transport. Numerical simulations have been performed for both stationary and moving vessel cases, as well as for different values of propeller revolution speed. Numerical results are presented for the propeller-induced velocity field and the resulting morphological evolution of the seabed. Qualitative similarities are observed between the numerical results and literature experimental findings, showing the ability of the model to mimic complex morphodynamic processes induced by ship propellers. Compared to stationary vessel cases, smaller scour depths are generated in moving vessel cases. It is concluded that the effect of the propeller provides a major contribution to the mobilization and suspension of seabed sediment, and it should not be neglected in numerical models for ship-induced sediment transport. [ABSTRACT FROM AUTHOR]

Sandy beaches high in recreation value make up 16% of the over 4000 km long shoreline of Estonia. The shore processes associated with climate change have remarkably accelerated over recent decades. Many sandy shores have suffered from strong erosion, including an excellent former beach at Valgeranna. The jetties, which were built in the 1860s to protect the navigation channel of Port Pärnu from clogging, have prevented natural sediment transport along the coast from south to north. At the same time, the sandy beach in Pärnu is expanding, and part of the sand accumulates with strong storms also in between the jetties, reducing the width of the shipping channel. The channel needs regular dredging, but, so far, the dredged sediment has been taken far away to the open sea and accumulated on the seabed. The current paper addresses the possibilities of using that sand for beach restoration in destructed and eroded areas. An overview of the applied methods and measurements during field studies is given. The results of modelling the processes of wave activity and sediment transport are discussed. The recycling of shore sediments is an important measure in sustainable coastal zone management. Different options and scenarios are analysed in order to find the most reasonable ways to bring sand back onto beaches and stabilize natural processes. Support from the state by working out respective laws and regulations would be motivating as well. [ABSTRACT FROM AUTHOR]

The problem of shoaling on coastal structures is the result of an event that occurs as part of the natural cycle. In cases where shoaling cannot be detected or prevented, various economic and operational problems may arise and may cause disruptions. In this study, the complex coastal dynamic impact of shoaling on three sequential fishery coastal structures located within the borders of Rize province in the Eastern Black Sea region of Türkiye was examined in terms of bathymetric changes and sediment transport under the influence of the incident wave climate. The effects of these structures on each other were also investigated. With this aim, bathymetric measurements were carried out to examine the impact of waves on seabed erosion and deposition. A serious shoaling problem was identified at one of the harbor launches under investigation, where approximately 13,200 m3 of deposition occurs annually in a relatively small harbor launch area. Such physical problems are thought to be the result of shoaling, the selection of sites that are not viable for fishery-related coastal structures, or the wrong positioning of the breakwater. [ABSTRACT FROM AUTHOR]

Oyster farming may impact the estuarine tidal circulation with a series of effects on environmental conditions and cultures' growth. Hydrodynamic numerical models set up in estuaries integrated the presence of oyster structures by simply increasing the bottom friction coefficient over farming areas. However, for elevated oyster tables in tidal environments, such default calibration ignored the temporal variations of the friction coefficient between the conditions of submerged or unsubmerged structures. Thus, an original formulation of the Chézy coefficient was here proposed to integrate these modulations. Assessed against measured and predicted vertical velocity profiles on a 1/2 scaled model, this formulation was implemented in a simulation of the tidal circulation within the Aber Wrac'h estuary (Brittany, France). Particular attention was dedicated to the changes induced on the current magnitudes and sediment transport. Oyster tables were found to impact current magnitudes in the vicinity of elevated structures, with major differences at times of local peak flood and ebb. These modifications were characterised by (i) a reduction of current magnitudes over oyster farming areas and (ii) a tidal-flow acceleration on both sides of these structures. Increased sediment transport was, therefore, expected in the vicinity of these cultures, with potential implications on seabed morphology and water quality. [ABSTRACT FROM AUTHOR]

This study constructs a digital elevation model of the coastal waters of the Shandong Peninsula using GIS-based interpolation methods to investigate the 15-year characteristics of seabed scouring and siltation. It utilizes depth data of the Shandong Peninsula's offshore area between 2004 and 2006 and 2017 and 2019. The results indicated that the Shandong Peninsula's coastal-water seabed is characterized by integral siltation and local point scouring. In addition, the northwest Shandong Peninsula is dominated by siltation, mainly distributed between 2 and 5 m isobaths, with the largest siltation area located in Laizhou Bay. The net volume of siltation is 2.13 × 108 m3, the siltation rate is 0.152 × 108 m3.a−1, and the scouring is concentrated in the estuary and the west coast of Laizhou Bay. Scouring and siltation balance generally characterize the seabed in the northern coastal waters of the Shandong Peninsula, and the degree of scouring and siltation is weak. The siltation occurs primarily near the land area, while scouring occurs as the sea deepens. In the northeast of the Shandong Peninsula, the degree of scouring and siltation is extreme, and their distribution range is small, whereas in the south, siltation is distributed in large bays and straight coastal offshore waters, and its degree is relatively weak. At the southernmost end of the study area, the scouring and siltation in Jiaozhou Bay are spatially distributed and balanced. In reality, the key factors affecting the distribution of coastal-seabed scouring and siltation are materials transported by river sediment, coastal scouring, and human activities. [ABSTRACT FROM AUTHOR]

Numerical simulations were conducted to investigate steady current scour around rectangular and square subsea caissons. The caissons, which are representative of subsea structures, have a submerged height ranging from 0.5 to several times their longest base dimension. The flow model used is based on the Reynolds Averaged Navier-Stokes equations, and the scour model simulates bed load and suspended load transport to predict the evolution of the seabed profile. The aim of this study is to investigate the mechanisms of local scour, specifically the contribution of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour. The model is validated through comparisons with experimental measurements, indicating reasonable agreement. Based on the numerical model results, the mechanisms of local scour around square and rectangular caissons are found to be qualitatively similar. If the flow is directed perpendicular to one of the caisson faces, both the horseshoe vortex and the local streamline contraction contribute to scour. The maximum scour depth is located at the two upstream corners of the caisson due to the combined effects of the horseshoe vortex and streamline contraction. When the flow approaches the caisson with a diagonal attack angle of 45°, the two upstream faces act like a streamlined wedge that splits the incoming flow. The study found that the horseshoe vortex almost disappears and the scour is primarily caused by the local velocity amplification at the two side corners. If the flow attack angle is 45°, the scour does not initiate near the upstream corner without the contribution from the horseshoe vortex. These findings are expected to serve as a valuable theoretical foundation for the design of scour protection. • Scour around subsea caissons is studied by fully three-dimensional simulations. • The contributions of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour are identified. [ABSTRACT FROM AUTHOR]

Elevated concentrations of organic matter are found in sediments of hadal trenches relative to those found in the abyssal seabed, but the origin of such biological material remains elusive. Here, we report the composition and distribution of cell membrane intact polar lipids (IPLs) in surface sediments around the deepest points of the Atacama Trench and adjacent bathyal depths to assess and constrain the sources of labile organic matter in the hadal seabed. Multiscale bootstrap resampling of IPLs' structural diversity and abundance indicates distinct lipid signatures in the sediments of the Atacama Trench that are more closely related to those found in bathyal sediments than to those previously reported for the upper ocean water column in the region. While the overall number of unique IPL structures in hadal sediments is limited and they contribute a small fraction of the total IPL pool, they include a high contribution of phospholipids with mono- and di-unsaturated fatty acids that are not associated with photoautotrophic sources. The diversity of labile IPLs in hadal sediments of the Atacama Trench suggests the presence of in situ microbial production and biomass that resembles traits of physiological adaptation to high pressure and low temperature, and/or the transport of labile organic matter from shallower sediment. We argue that the export of the most labile lipid component of the organic matter pool from the euphotic zone and the overlying oxygen minimum zone into the hadal sediments is neglectable. Our results contribute to the understanding of the mechanisms that control the delivery of labile organic matter to this extreme deep-sea ecosystem, whereas they provide insights into some potential physiological adaptation of the in situ microbial community to high pressure and low temperature through lipid remodeling. [ABSTRACT FROM AUTHOR]

The coupled Delft3D-object model has been developed to predict the mobility and burial of objects on sandy seafloors. The Delft3D model is used to predict seabed environmental factors such as currents, waves (peak wave period, significant wave height, wave direction), water level, sediment transport, and seabed change, which are taken as the forcing term to the object model consisting of three components: (a) physical parameters such as diameter, length, mass, and rolling moment; (b) dynamics of the rolling cylinder around its major axis; (c) an empirical sediment scour model with re-exposure parameterization. The model is compared with the observational data collected from a field experiment from 21 April to 13 May 2013 off the coast of Panama City, Florida. The experimental data contain both object mobility using sector scanning sonars and maintenance divers as well as simultaneous environmental time series data of the boundary layer hydrodynamics and sediment transport conditions. Comparison between modeled and observed data clearly shows the model’s capabilities and limitations. [ABSTRACT FROM AUTHOR]

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment. Plain Language Summary: In the Arctic Ocean deep basins, only a tiny fraction of the algae that grows in the surface layer sinks down to the sea floor. Most of the particles reaching the sea floor originate from the shallower regions closer to the coast. These particles have already settled on the sea floor once, and originate from rivers or algae that grew, died and sank down in shallow regions. Later, these particles are lifted off the ground again by strong turbulent motions and transported toward deeper regions in the middle of the water column. These lift‐off and transport events happen only occasionally, and have not been directly observed in the Eurasian part of the Arctic Ocean yet. Also, we present a new mechanism for the creation of turbulence, which is necessary to lift particles off the sea floor. This mechanism happens mostly during the summer season, when less sea ice is present. Based on this seasonality, it is likely that sediment transport events will become more frequent in the future, when the Arctic sea ice is further declining. Key Points: First direct observation of an intermediate nepheloid layer in the Eurasian part of the Arctic OceanCoinciding strong midwater turbulence is likely caused by a down‐slope current displacing isopycnalsSimilar downslope flow events exhibit a strong seasonality toward the ice free season [ABSTRACT FROM AUTHOR]

The problem of local scour around a submarine pipeline is vital for oil and gas projects. Conventional scour models around pipeline practically include the flow and sediment transport characteristics, but they ignore the interactions between flow and seabed soil in the scouring process. In this study, a new two-dimensional model for simulating the scouring process is established while considering the impact of the wave-induced soil response and the associated seepage. With the present model, the influence of seepage and soil response on the modified Shields number, repose angle and prediction of the local scour depth and profile are examined. The numerical results indicate that the influence of repose angle when modified by seepage in the whole scour process is small. Meanwhile, the wave-induced dynamic soil response promotes the development of the scour profile and eventually increases the maximum scour depth to some extent, regardless of the change in the soil characteristics. However, at smaller KC numbers, the variations in the maximum scour depth with seabed properties become more significant. [Display omitted] • A novel scour model considering soil response is numerically established in OpenFOAM. • The shape of the scour hole is asymmetrical due to the asymmetrical horizontal flow velocity. • The maximal scour depth with the soil response is larger than that without the soil response. • The difference in S / D with and without soil response would decrease with increasing in the KC number. • The soil response should be considered for a smaller shear modulus, lower soil permeability, and lower saturation. [ABSTRACT FROM AUTHOR]

A bolus is a vortex formed by an internal wave propagating upslope with the shoaling wave. The amount of sediment and biota transported by boluses travelling up the world's continental slopes and onto the continental shelves has not been established but could be vital for maintaining nutrient‐rich coastal ecosystems. Previous laboratory experiments and simulations of shoaling boluses have considered a two‐layer density stratification or a linear density profile. We present experiments on bolus formation and propagation in a stratification with a density profile ρ(z) represented by a tanh‐profile, which describes many ocean pycnoclines. Our laboratory experiments examine pycnoclines with thicknesses varying from nearly zero (a two‐layer system) to large thicknesses where the density varies almost linearly with depth. The bolus volume, displacement upslope, and available potential energy are measured as a function of pycnocline thickness. Maximum upslope displacement is found to be twice that observed for a two‐layer stratification. Plain Language Summary: Interfacial waves can be observed along the surface of the ocean, and as they approach the coastline the waves steepen, break, and ultimately surge up the beach. These waves are analogous to internal waves, which occur within the ocean at the horizontal interface of more and less dense water. These internal waves can be an order of magnitude taller than the waves seen at the beach, and the surge (called a bolus) can move kilometers along the ocean bottom. Previous efforts to model these boluses have approximated the ocean's density as two layers of different density water. However, the ocean has a more gradual transition between the layers, which can be over 100 meters thick. The impact of this transition layer, called a pycnocline, on bolus transport is poorly understood. Here, we perform laboratory experiments to demonstrate that the presence of a thick pycnocline can double the distance the bolus travels upslope and increase the amount of energy for mixing. This demonstrates the need for a better model of bolus propagation to understand how these breaking internal waves move material along the bottom of the ocean. Key Points: Boluses propagate further for pycnoclines of finite thickness than for a discontinuous stratification with two layers of uniform densityThe experiments reveal the existence of a pycnocline thickness that results in a maximum bolus propagation displacement upslopeThin pycnoclines yield large bolus volumes with high available potential energy despite short bolus propagation displacement [ABSTRACT FROM AUTHOR]

Double sandbar systems often characterize the surf zone of wave-dominated beaches and display a variety of poorly explained spatial configurations. Here, we explore the morphodynamic stability of double-barred beaches using a model based on linear stability analysis. Surf zone hydrodynamics are described by coupling depth and wave-averaged conservation of mass and momentum with the wave energy and phase equations, including roller dynamics. A simplified sediment transport formulation relates flow to seabed changes. Under normal wave incidence, an alongshore uniform coast with a cross-shore profile characterized by the presence of two sandbars can be unstable, thereby resulting in the development of crescentic and/or rip channel patterns. Our study demonstrates that sandbar coupling can be either in phase (highs and lows of both sandbars are at the same alongshore position) or out of phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). In line with observations, results of simulations show a large variability in the possible emerging bottom patterns. Our analysis indicates that modes of which the amplitude of the inner sandbar perturbation is larger than that of the outer sandbar are dominant for large height–depth differences between the two sandbar crests and small offshore wave heights. Patterns related to the outer sandbar dominate for small values of the difference in sandbar depth. For intermediate differences between the two sandbar depths, patterns on both longshore bars appear to be fully coupled (similar growth rates and strongly correlated pattern shapes). For relatively large waves and large depth over the outer sandbar, patterns can also develop close to the shoreline and/or inner surf zone together with patterns in the inner or outer sandbar. [ABSTRACT FROM AUTHOR]

Do, J.D.; Chang, Y.S.; Kim, C.H.; Kim, W-.H.; Lee, B-.G.; Wie, G. J., and Jin, J.-W., 2020. Measurement of nearshore seabed bathymetry using airborne/mobile LiDAR and multibeam sonar at Hujeong Beach, Korea. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1067-1071. Coconut Creek (Florida), ISSN 0749-0208. Accurate bathymetric measurements are important, especially in nearshore areas, because coastal sediment transport and the resulting beach erosion/accretion processes are significantly affected by seabed topography. At Hujeong Beach in Korea, for example, local shorelines show severe spatiotemporal variations as a result of seabed topography including underwater rocks. The accuracy of bathymetry measurements are not guaranteed, however, because high-accuracy acoustic sensors—such as multi-beam echo sounders—have a limited measurement capability. In this study, we investigated the accuracy of the nearshore bathymetry at Hujeong Beach using airborne LiDAR (a-LiDAR) data from May, 2017. One of the benefits of a-LiDAR is its ability to conduct both onshore and offshore mapping by detecting laser pulses reflected from land, the sea surface, and seabeds. The a-LiDAR measurements were validated onshore in a sloped roadside area using data measured via ship-mounted mobile LiDAR (m-LiDAR). A high agreement was found between the two datasets, with R-squared values of 0.9999 and mean square errors less than 0.05 m2. In measurements of seabed bathymetry, a-LiDAR agreed well with measurements from a multi-beam echo sounder over a sandy seabed and showed an accuracy similar to the m-LiDAR validation results. However, the accuracy of a-LiDAR measurements decreased significantly over rocky seabed; the mean square error compared to the multi-beam data increased to 0.6–0.8 m2. The high a-LiDAR errors over this rocky area may be a consequence of the irregular shape of its surface. Return signals from underwater rocks of varying shapes may be spread over wider angles, causing signal interference and reducing the measurement resolution in the area. [ABSTRACT FROM AUTHOR]

Fang, K.; Wang, H.; Sun, J.; Zhang, J., and Liu, Z., 2020. Including wave diffraction in XBeach: Model extension and validation. Journal of Coastal Research, 36(1), 116–127. Coconut Creek (Florida), ISSN 0749-0208. An extended version of the XBeach model is presented in this paper to improve the accuracy of predicted wave and current fields and sandy beach evolution in scenarios in which wave diffraction takes effect. Following the approach successfully implemented in the Simulating Waves Nearshore (SWAN) spectral wave model, a diffraction parameter is introduced into the wave action balance equation in XBeach to take wave diffraction into account. An alternative wave breaking index and the formulation for the critical velocity are also tested in the extended model to achieve better model–data agreement. The extended model is validated by comparing the simulation results with those of physical tests for fixed and movable sea beds. The computed wave height, wave-induced current field, and beach evolution are found to be in satisfactory agreement with the measurements, and the numerical modeling capacity of the extended model to represent wave diffraction effect is thus demonstrated. [ABSTRACT FROM AUTHOR]

Numerical modeling using Surface-water Modelling System (SMS) is carried out to simulate the hydrodynamic and sediment transport pattern in Monano Bay, Gorontalo Province, Indonesia. The model applies scenarios of the existing bay before and after berthing structures development. Field data acquisition and secondary data collection are carried out to complete the model. The field measurement includes bathymetry, tide elevation, and current velocity measurements. Validation of the model and field measurement shows good agreement. The validated parameters are the tidal elevation and velocity data. The model shows that in flood and ebb condition, the current flows to the south-east and north-west direction, respectively. The velocity is in the range of 0.1 - 0.25 m/s. The sedimentation modeling shows that the seabed changes are not significant for model scenarios before and after the port development. It is found that the potential increment of sedimentation in Monano Bay with berthing structure being there is about 10 - 15 cm in a one-year simulation. [ABSTRACT FROM AUTHOR]

Local scour induced by tidal flow is a major hazard for sea-crossing bridge foundations in coastal areas. The irregular tidal flow and silt-muddy seabed environment generates a very complicated flow turbulence and sediment transport. This paper, for the first time, presents the field measured scour holes around the Hangzhou Bay Sea-Crossing Bridge (HBSB) under irregular tidal flow dominated water regions. The high-resolution seabed topography around 19 bridge foundations in the south waterway of HBSB is measured from 2015 to 2020 by using a multi-beam sonar system. The field measured data are analyzed to investigate the local scour characteristics under irregular tidal flows. Results indicate that under irregular tidal flows with stronger flood tide, the local scour hole around every bridge foundation is slightly asymmetric and the scour depth downstream is larger than that upstream. The slope angles at upstream, downstream and lateral sides of local scour hole are about 13°～18°, which are much smaller than the Angle of Repose (AoR = 35°) due to bidirectional scouring generated by tidal flows. During 2015～2020, the maximum scour depths around all bridge foundations range between 1.2 D e ～2.5 D e (where D e being the effective diameter of bridge foundation). The maximum scour depths vary frequently with a fluctuation within 0.6 De. Comparison between the field measured and predicted scour depth using three available scour depth predication equations show that all prediction equations overestimate the scour depth. As such, considering the discount effect of the varied tidal flow compared with steady current, an improved maximum scour depth prediction equation is proposed to estimate the scour depth under tidal flow on a silty seabed. Results show that the predicted scour depth using the proposed equation has a relative error of ±30%. • High quality field scour data around bridge foundations under irregular tidal flows are presented. • Slope angle of scour hole under tidal flows is smaller than the Angle of Repose. • An improved prediction equation for scour depth around bridge foundations under clear-water and live-bed tidal flow condition on silty seabed is proposed. [ABSTRACT FROM AUTHOR]

The world's shallow continental shelves are currently experiencing a rapid pace of development from the growth of offshore renewable energy. The emplacement of infrastructure on the seabed can change the morphology of the bed, the nature of the flow above it the transport of sediment, and so complicate the assessment of seabed stability for planning and designing offshore renewable infrastructure. To ascertain how much of an impact these natural processes have on the stability of cables, we present the first field observations made directly over a section of subsea cable, from two deployments in the Eastern Irish Sea at a location of current and planned offshore windfarms. Profiles of flow, turbulence and suspended sediment concentration were measured over a section of typical high voltage electricity cable. Upon deployment our observations showed that sediment was deposited around the cable and self-burial occurred. The rate of deposition varied between surveys dependent on forcing and local bed conditions. Turbulence generated from the cable itself reduced as the embedment depth increased, but the relationship between bed shear stress and suspended sediment concentration was not consistent between surveys. We discuss several processes potentially responsible for the prevalence of deposition around the cable, and the difference in seabed mobility between the surveys. • Field surveys quantified mean flow, turbulence and suspended sediments around a section of seafloor electricity cable. • Deposition around the cable occurred at all initial embedment depths, but rates of deposition varied between surveys. • Seabed mobility varied between surveys beyond any variation due to grain size and affected the rate of cable burial. [ABSTRACT FROM AUTHOR]

Abstract The seabed is sloped in shallow coastal regions, where strong nonlinear shoaling waves can generate sheet-flow sediment transport. Both wave nonlinearity and bottom slope can lead to a wave-averaged (net) sediment transport rate, but the bottom slope effect is often overlooked in existing sediment transport models. A full-scale experimental study of sheet-flow sediment transport under the oscillatory flows with nonlinear features (skewness or asymmetry) was conducted in an inclinable oscillatory water tunnel, so the bottom-slope-induced net transport rate in the context of nonlinear waves was experimentally investigated. The tests cover a variety of nonlinear wave shape, bottom slope (0 to 2. 5 ∘ ) and two sediment diameters (0.24 and 0.51 mm). Since the downslope direction is the "offshore" direction in our tests, it is found that bottom slope reduces the net "onshore" transport rate due to wave nonlinearity. The slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping-bed experiment and the measured net transport rate from the corresponding horizontal-bed experiment. The results suggest that slope-induced net transport rate is insensitive to wave nonlinearity, and its variation with bottom slope can be considered linear in the context of predicting net transport rate. Comparisons between this study and a preceding sinusoidal-flow study of Yuan et al. (2017b) suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate. Subsequently, an empirical model for predicting net sheet-flow transport rate due to bottom slope is established based on dimensional analysis. This model can be simply added to existing sheet-flow sediment transport models to improve their applicability in coastal regions. Highlights • Presented experiments on the sheet-flow sediment transport under skewed and asymmetric oscillatory flows above a sloping bed. • Test results show that the slope-induced net transport rate is insensitive to nonlinear wave shape. • Proposed an empirical model for slope's contribution in net sheet-flow sediment transport rate. [ABSTRACT FROM AUTHOR]

Abstract The purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinders – such as pipelines, cables or idealised chains – close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (K C number). For K C ≲ 4 sediment motion results in bedload transport that is symmetric about the cylinder centreline. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4 ≲ K C ≲ 9 sediment motion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9 ≲ K C ≲ 12 the sediment motion is characterised by localised transport events and asymmetric bedload transport driven by overall vortex dynamics. In terms of trench size, the maximum (equilibrium) depth is found to increase with K C and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with K C number and ψ , with an additional dependency on the cylinder β number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the K C number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue. Highlight • Experiments investigate sediment transport and trench formation beneath a cylinder oscillating normal to a sandy bed. • Transport mechanisms and trench geometry linked to pumping of fluid beneath the cylinder and to vortex dynamics. • Control volume arguments combined with an oscillatory boundary layer assumption predict initiation of trenching for small oscillation amplitudes close to the bed. • Rate of trench development and maximum trench depth increase with oscillation amplitude and maximum cylinder velocity. [ABSTRACT FROM AUTHOR]

We describe and demonstrate algorithms for treating cohesive and mixed sediment that have been added to the Regional Ocean Modeling System (ROMS version 3.6), as implemented in the Coupled Ocean-Atmosphere-Wave-Sediment Transport Modeling System (COAWST Subversion repository revision 1234). These include the following: floc dynamics (aggregation and disaggregation in the water column); changes in floc characteristics in the seabed; erosion and deposition of cohesive and mixed (combination of cohesive and non-cohesive) sediment; and biodiffusive mixing of bed sediment. These routines supplement existing noncohesive sediment modules, thereby increasing our ability to model fine-grained and mixed-sediment environments. Additionally, we describe changes to the sediment bed layering scheme that improve the fidelity of the modeled stratigraphic record. Finally, we provide examples of these modules implemented in idealized test cases and a realistic application. [ABSTRACT FROM AUTHOR]

On the basis of advanced concentration and velocity profiles above a mobile seabed, an instantaneous analytical model is derived for sediment transport in asymmetric oscillatory flow. The applied concentration profile is obtained from the classical exponential law based on mass conservation, and asymmetric velocity profile is developed following the turbulent boundary layer theory and the asymmetric wave theory. The proposed model includes two parts: the basic part that consists of erosion depth and free stream velocity, and can be simplified to the total Shields parameter power 3/2 in accordance with the classical empirical models, and the extra vital part that consists of phase-lead, boundary layer thickness and erosion depth. The effects of suspended sediment, phase-lag and asymmetric boundary layer development are considered particularly in the model. The observed instantaneous transport rate proportional to different velocity exponents due to phase-lag is unified and summarised by the proposed model. Both instantaneous and half period empirical formulas are compared with the developed model, using extensive data on a wide range of flow and sediment conditions. The synchronous variation in instantaneous transport rate with free stream velocity and its decrement caused by increased sediment size are predicted correctly. Net transport rates, especially offshore transport rates with large phase-lag under velocity skewed flows, which existing instantaneous type formulas failed to predict, are predicted correctly in both direction and magnitude by the proposed model. Net sediment transport rates are affected not only by suspended sediment and phase-lag, but also by the boundary layer difference between onshore and offshore. [ABSTRACT FROM AUTHOR]

The frictional influence of the seabed on the tidal flow in shelf seas and estuaries is usually modelled via a prescribed, spatially/temporally invariant drag coefficient. In practice, the seabed exhibits considerable variability, particularly spatially, that should in principle be included in simulations. Local variations in the seabed roughness (k s ) alter the flow strength and, hence, local sediment transport rates. The effect of using a spatially/temporally varying k s is assessed here with reference to a tidal channel (Menai Strait, N. Wales) in which the variability of the bedforms has been monitored using multi-beam surveying. The channel not only exhibits strong tidal flow, but also a residual induced flow that is used here as diagnostic to assess various bed roughness formulations tested in a Telemac model. Tidal simulations have been carried out with both constant and temporally/spatially variable k s , and the predicted residual flow is shown to be sensitive to these representations. For a mean spring-neap (SN) cycle with variable k s , the average residual flow is calculated to be 525 m 3 s − 1 , consistent with observations. This residual flow can be recovered using imposed, constant values of k s in the range 0.15 m to 0.3 m. The results suggest that the overall, effective roughness of the seabed is less than half of the maximum local roughness due to the dunes in mid-channel, but more than the spatially-averaged k s value in the channel as a whole by about 50%. Simulations carried out with an M 2 -alone tide using variable k s produce a somewhat smaller (by 7%) residual flow of 491 m 3 s − 1 . The use of an ‘equivalent M 2 ’ tide of amplitude enhanced by 7.3% reconciles these estimates. The main contribution to k s is made by dunes which are modelled using Van Rijn's (2007) formulation subject to an additional ‘history effect’. The modelled k s is found to equal approximately the observed height of the dunes along mid-channel transects rather than half the height as expected. This is attributed to the non-equilibrium nature of the bedforms in the reversing tidal flow, which exhibited shorter wavelength and more symmetrical profiles than dunes in steady flow. [ABSTRACT FROM AUTHOR]

Information about seabed stability and sediment dynamics is part of the fundamental geoscience knowledge required for the extraction of tidal energy in the Bay of Fundy and for the integrated management of the Bay. Waves, tidal currents, and wind-driven and circulation currents were obtained from oceanographic models to assess the wave and current processes for the broader Bay of Fundy. The wave and current outputs were coupled with observed grain size in a sediment transport model to predict, for the first time, the seabed shear stresses, sediment mobility, and sediment transport patterns for the entire Bay. The root mean square tidal current, highest in the upper Bay (>1.4 m x [s.sup.-1]), is reduced to moderate in the central Bay (0.50.8 m x [s.sup.-1]) and decreases further in the outer Bay (0.2-0.5 m x [s.sup.-1]). The maximum tidal current occurs in the Minas Passage and is >5 m x [s.sup.-1]. The mean significant wave height, in contrast, is the greatest in the outer Bay (~1.3 m) and gradually decreases to the northeast in the central and upper Bay (30% of the time over most of the Bay and reaches 100% of the time in some areas. The maximum total-load sediment transport rate under spring tide can reach ~5 kgx[m.sup.-1] x [s.sup.-1] and is to the northeast during flood and to the southwest during ebb. Net sediment transport flux, however, is dominantly to the northeast and reaches 2 kg x [m.sup.-1] x [s.sup.-1]. Eddies of net transport are found to occur around headlands and at the narrows of Grand Manan Island, largely due to the occurrence of eddies of residual tidal flows. The regional distribution of substrate types and bedform fields and patterns of seabed erosion and deposition are well correlated with tidal current strength and sediment transport patterns. Des donnees concernant la stabilite du plancher oceanique et la dynamique des sediments constituent une partie fondamentale des connaissances geologiques requises pour l'extraction de l'energie maremotrice dans la baie de Fundy et pour une gestion integree de la baie. Les vagues, les courants de maree, les courants pousses par les vents et les courants de circulation ont ete obtenus de modeles oceanographiques afin d'evaluer les processus des vagues et des courants pour l'ensemble de la baie de Fundy. Les donnees de sortie sur les vagues et les courants ont ete jumelees aux granulometries observees dans un modele de transport de sediments afin de predire, pour la premiere fois, les contraintes de cisaillement sur le plancher oceanique, la mobilite des sediments et les patrons de transport pour toute la baie. Le courant de maree moyen quadratique, plus eleve dans le fond de la baie (>1,4 m x [s.sup.-1]), est reduit a modere dans le centre de la baie (0,5-0,8 m x [s.sup.-1]) et il decroit en encore plus au large de la baie (0,2-0,5 m x [s.sup.-1]). Le courant de maree maximal se trouve dans le Passage Minas, ou il est >5 m x [s.sup.-1]. La hauteur significative moyenne de vague est par contre la plus elevee au large de la baie (~1,3 m) et elle decroit ensuite vers le nord-est dans la partie centrale et dans le fond de la baie (30 % du temps a travers la plus grande partie de la baie et elle atteint meme 100 % du temps dans certains secteurs. Le taux maximal de transport de la charge totale des sediments peut atteindre ~5kg x [m.sup.-1]x[s.sup.-1]lors des marees de printemps et cela se produit vers le nord-est durant la maree montante et vers le sud-ouest durant la maree descendante. Le flux net de transport des sediments est toutefois dominant vers le nord-est ou il atteint 2 kg x [m.sup.-1]x[s.sup.-1]. Des contre-courants de transport net se retrouvent autour des promontoires et au passage de l'ile de Grand Manan, grandement en raison de l'occurrence de contre-courants de debits de maree residuels. La distribution regionale des types de substrat et des morphologies de fond ainsi que les patrons d'erosion du plancher oceanique et de deposition sont bien correles avec la force des courants de maree et les patrons de transport des sediments. [Traduit par la Redaction], Introduction The Bay of Fundy, located on the east coast of Canada (Fig. 1), has been of great economic, ecological, and scientific significance because of its world-renowned tides, unique seabed [...]

The understanding of cross-shore sediment sorting is of primary importance for the design of sand nourishments and for assessing the suitability of the seabed to different ecological species. In this paper, sediment sorting processes were investigated by using a combination of physical and detailed numerical modelling. Data from large-scale wave flume experiments were used to validate a 2DV cross-shore Delft3D model. The model solves coupled short-wave averaged equations for flow, sediment transport, bed composition and bed level change. The infra-gravity wave motions were explicitly resolved. In order to investigate sorting processes, eight sediment fractions were used as well as a layered bed stratigraphy. The effects of different wave conditions (high energetic and more moderate energetic waves) on the morphodynamic profile development and sorting processes were investigated. The Delft3D model reproduced the profile development and bar position very accurately. Additionally, model predictions of sediment sorting across the profile fitted very well with the available observations. The numerical model simulations showed the importance of including short-wave grouping and infragravity wave effects in order to reproduce the cross-shore profile development, especially the breaker bar dynamics and sediment sorting processes. Infragravity waves contribute to larger sediment entrainment and more offshore bar development. Besides leading to a better prediction of the bed profile, infragravity waves also lead to a better prediction of the bed composition. Model results are in agreement with experimental data, showing its capabilities in functioning as a tool to predict sorting processes. [ABSTRACT FROM AUTHOR]

The interaction between a forced vibrating pipeline and an erodible seabed in unidirectional flows is experimentally investigated. The experiments were conducted under clear-water scour conditions with a water depth of 0.3m and averaged approach velocity of 0.261 m/s. The pipeline model with a diameter (D) of 3.5 cm, was subjected to a vertical sinusoidal motion of varying amplitudes (A 0 = 2∼6 cm) and frequencies (f 0 = 0.1–0.6Hz). The initial gap (G 0) between the lower pipe surface and the undisturbed flatbed level was fixed at 1 D. The results show that the maximum scour depth increases significantly with both vibration amplitude and frequency, with the former having a more dominant influence. However, frequency tends to exacerbate the scour hole development more in the early scour stage. Distinct scour mechanisms are observed based on the different combinations of vibration amplitude and frequency. In the high A 0 conditions (A 0 > G 0), the combined effect of pounding (when the pipe hits the seabed) and piston actions (rising and falling of the pipe) during the pipe descending period dominates the earlier stages of scouring, and the pick-up of sediment particles during its rising period controls the subsequent scour process. In the low A 0 conditions (A 0 < G 0) when pounding does not occur, vortex shedding during the pipe-falling stage controls the development of scour hole when the frequency is low. When the frequency is high, however, the flow field associated with the pipe-rising period dominates the scour development. Different empirical formulas for the prediction of the maximum scour depth and width are proposed and compared in the present study. • 2 and 3 scour stages with different scour mechanisms are identified for the low and high amplitude conditions, respectively. • The pounding effect, which is related to pipeline-scour subjected to forced vibration, is discussed. • Vibration amplitude has a dominant influence on the maximum scour depth while frequency influences the early scour stages. [ABSTRACT FROM AUTHOR]

A series of field observation of turbidity and current velocity etc. were carried out at the sea bottom off a river mouth during flood periods. The measurement location was 1.2 km off the mouth of Tenryu River, Japan, where water depth was about 18m. Four flood events of which the maximum mean hourly discharge ranged from 1,570m 3 /s to 4,620 m 3 /s were selected and the time series of river discharge, turbidity, water temperature, salinity and current velocity were compared with each other and the relationships between them were discussed. Furthermore, vertical profiles of turbidity and salinity observed after a flood were used in the discussion. Bathymetric changes surveyed off the river mouth were also taken into account. [ABSTRACT FROM AUTHOR]

Information about seabed stability and sediment dynamics is part of the fundamental geoscience knowledge required for the extraction of tidal energy in the Bay of Fundy and for the integrated management of the Bay. Waves, tidal currents, and wind-driven and circulation currents were obtained from oceanographic models to assess the wave and current processes for the broader Bay of Fundy. The wave and current outputs were coupled with observed grain size in a sediment transport model to predict, for the first time, the seabed shear stresses, sediment mobility, and sediment transport patterns for the entire Bay. The root mean square tidal current, highest in the upper Bay (>1.4 m·s−1), is reduced to moderate in the central Bay (0.5-0.8 m·s−1) and decreases further in the outer Bay (0.2-0.5 m·s−1). The maximum tidal current occurs in the Minas Passage and is >5 m·s−1. The mean significant wave height, in contrast, is the greatest in the outer Bay (∼1.3 m) and gradually decreases to the northeast in the central and upper Bay (<0.5 m). Seabed shear in the Bay of Fundy is mostly due to tides, and wave effects are only important in coastal areas. The strongest mean shear velocity of 10 cm·s−1 occurs in the Minas Passage area. Strong shear velocity of 4-5 cm·s−1 also occurs in Minas Basin, in the central Bay, and in the narrows around Grand Manan Island. Sediment mobilization in the Bay of Fundy is predominantly by tidal current. Mobilization frequency is >30% of the time over most of the Bay and reaches 100% of the time in some areas. The maximum total-load sediment transport rate under spring tide can reach ∼5 kg·m−1·s−1 and is to the northeast during flood and to the southwest during ebb. Net sediment transport flux, however, is dominantly to the northeast and reaches 2 kg·m−1·s−1. Eddies of net transport are found to occur around headlands and at the narrows of Grand Manan Island, largely due to the occurrence of eddies of residual tidal flows. The regional distribution of substrate types and bedform fields and patterns of seabed erosion and deposition are well correlated with tidal current strength and sediment transport patterns. [ABSTRACT FROM AUTHOR]

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Seven years of field survey measurements of a subsea pipeline obtained using sonar profilers and underwater video indicate significant lowering of the pipeline into the seabed due to sediment mobility and scour. The majority of the lowering occurs within 2 years of pipeline laying and appears to result from sustained ambient tidal and soliton currents as opposed to large storms. The lowering results in an increase in pipeline embedment relative to the far field seabed of up to 0.8 times the pipe diameter (referenced at a distance ± 8 D from the pipeline). At most locations along the pipeline, this increase in far-field embedment is uniform and occurs after the formation of many closely spaced scour holes. This suggests that the pipeline lowered mainly through sinking into the seabed at span shoulders, rather than sagging into widely spaced scour holes, for much of the pipeline length. A beam bending analysis confirmed the dominance of sinking, but did show some evidence of pipeline sagging, calculating deflections of up to 0.3 pipeline diameters at the time of surveying. In contrast to the traditional conception of span growth and self-burial, which conceives of complete pipeline burial as an endpoint, this pipeline primarily appears to exhibit ‘self-lowering’ towards a mature state that consists of a pseudo-static profile of alternating spanning and embedded sections that are distributed at regular intervals. The observed changes appear to be predictable given sufficient pipeline setting data, which suggests that they can be quantified in the stability design of new pipelines. This opens up the possibility of more efficient on-bottom stability design, as the beneficial shielding and support provided by the self-lowering process is not usually accounted for. [ABSTRACT FROM AUTHOR]

Marine nepheloid layer is widely distributed in the oceans and marginal seas. The concentration of suspended particles in the nepheloid layer is significantly higher than that of the adjacent layers. Marine nepheloid layers include the surface nepheloid layer (SNL), intermediate nepheloid layer (INL), and bottom nepheloid layer (BNL). As a transport pathway for the particulate matter in the oceans, nepheloid layer is important to the carbon cycle and the source–sink system at the continental margin. This paper focused on the characteristics of the marine nepheloid layer and the mechanism of internal solitary waves that form INL and BNL, providing a reference for further research on the marine ecological environment dynamic process and source–sink system. BNL is formed by suspended seabed sediments with strong currents near the bottom, but the fate of BNL remains unknown. Marine nepheloid layer significantly affects the transportation of materials within the ocean. The transport of sediments by ISWs dramatically affects the formation of deep seabed sedimentary landform. However, the formation process and the transport contribution of INL and BNL are still unclear. The systematic and long-term research on the marine nepheloid layer in the world oceans is still limited. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a unique new physical testing facility for studying ocean-structure–seabed interactions, and in particular pipeline on-bottom stability on erodible seabed under hydrodynamic loading. The facility, named the O-tube due to its shape, is a fully enclosed flume in which ambient and storm-induced near-seabed flows are generated by a computer-controlled flow pump. Combined steady and oscillatory flow can be generated by alternating pump flow directions in a controlled manner, and computer control also allows irregular flow to be generated. The design of the O-tube combines the capabilities of a conventional open channel flume (which provides steady current) with a U-tube (which provides oscillatory flow). The facility is designed to physically model severe storm conditions, as well as ambient or tidal flows. When studying pipeline stability on erodible seabed under severe hydrodynamic loading conditions, tests can be performed at a relatively large scale (typically 1/5) for large diameter pipelines (e.g. 40in. gas trunklines) and at full scale for small diameter pipelines (<8in.) to minimize potential scaling effects associated with movable bed model tests. The specifications of the O-tube, a model pipe and an actuator system that supports the model pipe are given in detail. Preliminary model testing results show that the facility has met its design expectations. [Copyright &y& Elsevier]

In this work, a new numerical model for cross-shore beach profile evolution, IH2VOF-SED, is developed. It consists in the bidirectional coupling of a 2D RANS hydrodynamic solver and a sediment transport module. The resulting model is extensively validated against three benchmark cases at different scales, attending to the hydrodynamics, bottom shear stress and bathymetry evolution. Comparisons between experimental and numerical results show a good agreement for both the flow variables and the seabed evolution in all the validation cases without making use of calibration parameters. Additionally, the qualitative analysis of the results is in accordance with previous experimental observations of sediment transport induced by breaking waves. The computational cost is greatly reduced to about 1/10 of other available RANS models. As a novel aspect regarding RANS models, the model is able to simulate the swash zone and changes in the position of the coastline. A good compromise between precision and computational cost is achieved, allowing for an in-depth analysis of the processes leading to the cross-shore profile evolution. • A new, efficient RANS model for cross-shore beach morphodynamics is presented. • It includes bedload and suspended sediment transport and the effect of landslides. • The hydrodynamic and sediment transport features are validated at different scales. • The model is able to reproduce the evolution of the beach profile accurately. • The computational cost is 10 times lower than that associated with other RANS models. [ABSTRACT FROM AUTHOR]

The main aim of this study was to determine the size of the load carried by the Błądzikowski Stream and discharged into the Puck Lagoon, and the role of the river load in the formation of the sea bed deposits at the stream mouth. The Błądzikowski Stream, with the length of only 10 km, discharges about 610 tons of sediment into the Puck Lagoon per year, including 89.8% (548 tons) of the bed load and 10.2% (62 tons) of the suspended load. The amount is 4.9% of the river load discharged into the Puck Lagoon. Mineral particles are more common in the composition of the river load and account for 94.6% of the suspended load and 98.7% of the bed load. It is mainly the bed load deposited in the estuarine area that creates a contemporary underwater accumulation form in the Puck Lagoon, which is corroborated by the granulometric composition of the channel and surface sediments; the suspended load is carried further into the Lagoon. [ABSTRACT FROM AUTHOR]

This paper presents the development of a sediment transport model to predict the bed evolution processes in estuarine and coastal waters. The model is based on an existing hydrodynamic and sediment transport model, with significant refinements being made to enhance its capability for simulating the transport of graded sediments under nonequilibrium conditions. A multifraction sediment transport model has been developed to replace the existing single-fraction model. The sediment mixture is divided into several fractions according to the grain size. For each fraction, the particle size is considered to be uniform, and its transport form, either as suspended load or bed load, is determined by the magnitude of a suspension index. A bed evolution model is developed to simulate the processes of bed level change and sediment grain size sorting. The model is applied to a laboratory model harbour, for which measurements of tidal currents and bed level inside the harbour are available. The water level, velocity distributions, and bed level changes predicted by the numerical model are compared with the laboratory data. Comparisons are also made between predictions made by the fractional model and the single-size model. The effect of bed sediment size change on the erosion process has been investigated. [ABSTRACT FROM AUTHOR]

Abstract: Understanding of sea bottom boundary layer characteristics, especially bottom shear stress acting on the sea bed, is an important step needed in sediment transport modeling for practical application purposes. In the present study, a new generation system for bottom boundary layer under solitary wave is proposed. Applicability of this system is examined by comparing measured and numerical solution velocities. Moreover, transitional behavior from laminar to turbulence was investigated. It is concluded that the critical Reynolds number in the experiments shows good agreement with DNS result of Vittori and Blondeaux (2008) and laboratory data of Sumer et al. (2010), indicating validity of the generation system. Since the present generation system enables continuous measurement to obtain ensemble averaged quantities, it can be effectively utilized for future experimental studies on solitary wave boundary layers, including sediment transport experiments with movable bed. [Copyright &y& Elsevier]

Geological investigations conducted over a vast seabed offshore the giant Saemangeum Dike on the west coast of Korea indicate that the surface area of fine to very fine sands has rapidly expanded southward in recent years (2002-2008). To unravel the processes responsible for this phenomenon, two field campaigns of hydrodynamic measurements were carried out at the same station with an instrumented tripod during May 2007 and January 2008. The measurements show the definite occurrence of wind-generated residual currents that were more distinctive and consistent during winter compared with the relatively weak, inconsistent analogues of the late-spring season. The simple algebraic calculations for bedload transport based on a cubic relationship between the bedload transport rate and steady currents suggest that the southward movements of surface sands result from interplay of wintertime residual currents and macrotidal currents. In addition, numerical model experiments illuminate that the dike construction has increased the N-S component of tidal currents over the study area. The sediment transport evaluations further suggest that this artificial increase in tidal currents in the N-S direction could considerably enhance southward sand transport. [ABSTRACT FROM AUTHOR]

Grain-size trend analysis is applied to the determination of sediment transport pathways over the Kwinte Bank, southern North Sea; which had been subjected to intensive dredging, within the context of the environmental impact of dredging activities. On the basis of the results of grain-size trend analysis, focused mainly upon the transportation of the non-biogenic sedimentary material (<2 mm), it appears that: (i) there is a main sediment pathway over the western (bank crest) and central (dredged area) part of the bank directed toward the NE; whilst a secondary pathway is established over its eastern gently-sloping flank, having a SE direction. Further, the present analysis shows that the area of the central (dredged) depression acts more as a `by-passing' zone rather than as a depo-centre for the non- biogenic sediments. Comparison undertaken with the results of an earlier investigation, for a non-dredged area at the northern end of the same bank, reveals that the depression due to dredging modifies significantly the sediment transport pathways; this may be attributed to a change in the seabed morphology which, in turn, modifies the near- bed hydrodynamics (related to tide and/or storm events). [ABSTRACT FROM AUTHOR]

Numerical modeling of dredged pits is conducted to investigate the hydrodynamic and morphodynamic interaction in offshore sand extractions. Based on an analytical formulation, a semianalytical numerical model (MEMPITS) has been developed to study the morphodynamic evolution of offshore (ho . 20 m) sand borrow areas. The numerical model has been applied to study the morphodynamic evolution of two offshore sand borrow areas in the Balearic Islands (Spain). Field data allowed a detailed characterization of the evolution of the sandpits. Time series of local hydrodynamics have been obtained using generation models (hindcast) combined with local wave and flow models. A verification of the simple model has been carried out using relatively slight adjustments to the calibration factors. The simple model provides good estimates of the infill rate and migration velocities of the offshore pits on the scale of years. This semianalytical tool allows a quick systematic investigation of the physical mechanisms as well as a etailed sensibility analysis regarding the pit design parameters. These parameters include location (water depth), pit length, width, depth, and orientation with respect to the mean flow. A nondimensional analysis based on the model is also carried out to explore the role of the different variables involved in the evolution of offshore sandpits. Based on the field data and the nondimensional analysis, some basic design recommendations for offshore sandpits are proposed. [ABSTRACT FROM AUTHOR]

Although seagrass canopies are known to enhance particle sedimentation, there is still limited knowledge about how seagrasses modify the vertical distribution of sediment particles; especially when particles come from allochthonous sources. This study determined the volume of particles trapped by the seagrass leaves, the amount that remains in suspension both within and above the canopy, and the amount deposited onto the seabed. A set of laboratory experiments were conducted in which hydrodynamic conditions and canopy densities were varied to mimic real field conditions. This study demonstrated and quantified previously recorded observations concerning the fate of sediment in seagrass meadows. Seagrass meadows decreased the amount of suspended sediment by capturing the sediment on the blades of the seagrass and by enhancing particle sedimentation on the seabed. However, particles trapped by the blades of seagrass in the whole canopy increased with canopy density and reduced the number of particles in suspension within the canopy. The ecological implications were significant, since a seabed covered by vegetation, when compared to a bare seabed, produced a reduction in the suspended sediment particles within the canopy, improving water clarity. Furthermore, canopies (compared to bare substrates) enhanced seabed sedimentation and the denser the canopy was, the greater the amount of sediment deposited on the seabed. • Seagrasses modify the amount of sediment deposited on the seabed. • Canopy density controls the amount of both suspended and leaves deposited sediment. • Trapping of sediment by leaves depends on the particle size for covers above 50%. [ABSTRACT FROM AUTHOR]

Beach profiles surveyed over a period of 18 y (1982 to 2000) at 41 sites were analyzed to assess beach changes, seabed scour, and grain sorting pattern fronting the 5 km long seawall built in 1991 to protect the Rosetta promontory from beach erosion, which had been retreating at a rate of -106 m/y. Although the seawall has succeeded in halting the recession, the shoreline shows adverse erosion at the leeside of the seawall ends (-14.4 m/y) associated with depth scour at a maximum of -0.50 m/y. The erosion along the promontory tip progressively decreases with longshore distance both to the east and to the southwest along the promontory flanks, reverting to accretion on both sides within the promontory saddles and yielding two nodal areas that represents zones where the sediment regime changes from erosion to deposition. Moreover, the seawalls have slightly altered the mean grain sizes of the beach sediment and seabed slope of the surf zone. The overall pattern of beach erosion and seabed scour results from wave refraction- induced longshore sediment transport along the promontory tip. The geographic correspondence between patterns of shoreline and seabed depth changes serve to refine boundaries of littoral subcells of the Rosetta promontory, including sediment paths, sources, sinks, nodal points, and zones of sediment transport convergence and divergence. The morphologic changes along this promontory reflect a combination of factors, including sediment availability, transport pathways from source areas, as well as the impact of protective structures at the Rosetta estuary. [ABSTRACT FROM AUTHOR]

Six Mile Reef, a sandy, 22-m-high shoal trending east-west and located about 7.8 km off the Connecticut coast, has a core of postglacial marine deltaic deposits mantled by tidally reworked modern sediments. Sedimentary environments off the eastern end of the shoal are characterized by processes associated with long-term erosion or nondeposition, a mobile-sediment-limited seafloor armored by gravelly sand, and scattered elongate fields of barchanoid sand waves. The barchanoid waves reach amplitudes of 20 m, are concave westward, and occur in individual and coalesced forms that become progressively more complex westward. The seafloor on and adjacent to the shoal is characterized by processes associated with coarse bedload transport and covered primarily with asymmetrical transverse sand waves. The transverse waves exceed 8 m in amplitude, have slip faces predominantly oriented to the west and southwest, and have straight, slightly sinuous, and curved crests. Megaripples, which mimic the asymmetry of the sand waves, are commonly present on stoss slopes and in troughs; current ripples are ubiquitous. The amplitude and abundance of large bedforms decrease markedly westward of Six Mile Reef. The seabed there is covered with small, degraded ripples, reflecting lower-energy environments and processes associated with sorting and reworking of sea- floor sediments. Megaripples and current ripples on the sand waves suggest that transport is active and that the bedforms are propagating under the present hydraulic regime. Net bedload sediment transport is primarily to the west, as evidenced by textural trends of surficial sediments, orientation of the barchanoid waves, and asymmetry of the transverse waves and of the scour marks around bedrock outcrops, boulders, and shipwrecks. One exception occurs at the western tip of the shoal, where sand-wave morphology indicates long-term eastward transport, suggesting that countercurrents in this area shape the shoal and are important to its maintenance. [ABSTRACT FROM AUTHOR]

Abstract: A simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented. This formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape. Predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k–ε turbulence closure. The bed shear stress formulation is combined with a Meyer-Peter and Müller-type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed. The predictions agree with oscillatory water tunnel measurements from the literature. [Copyright &y& Elsevier]

Submarine landslides can generate sediment-laden flows whose scale is impressive. Individual flow deposits have been mapped that extend for 1,500 km offshore from northwest Africa. These are the longest run-out sediment density flow deposits yet documented on Earth. This contribution analyses one of these deposits, which contains ten times the mass of sediment transported annually by all of the world’s rivers. Understanding how this type of submarine flow evolves is a significant problem, because they are extremely difficult to monitor directly. Previous work has shown how progressive disintegration of landslide blocks can generate debris flow, the deposit of which extends downslope from the original landslide. We provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide, encased within deposits of a more dilute flow type called turbidity current. Very little sediment was deposited across the intervening large expanse of sea floor, where the flow was locally very erosive. Sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea-floor gradient from 0.05° to 0.01°. This debris flow was probably generated by flow transformation from the decelerating turbidity current. The alternative is that non-channelized debris flow left almost no trace of its passage across one hundred kilometres of flat (0.2° to 0.05°) sea floor. Our work shows that initially well-mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean. [ABSTRACT FROM AUTHOR]

Abstract: The nearshore region exhibits many striking morphological patterns with a variety of spatial and temporal scales. The formation of these rhythmic features has been initially ascribed, depending on the pattern in question, to spatial structures in the flow or geological constrains. These forcing templates have been hypothesized to provide the spatial structure that becomes imprinted on the shoreline or seabed morphology. More recently, new explanations for rhythmic patterns have involved interactions between fluid flow and sediment transport that create morphological feedbacks and lead to pattern self-organization. While forcing-template models do not explicitly treat transport of the sediment that makes up the pattern, self-organization models focus on the strong couplings within flow/sediment systems, and on interactions between emergent structures. We illustrate the sweeping shift from template explanations to self-organization by discussing four nearshore patterns: beach cusps, surfzone crescentic sandbars, inner-shelf sorted bedforms, and large-scale cuspate shorelines. Models involving self-organization show that local interactions between flow and sediment transport can collectively give rise to patterns with large-scale coherence and that the driving feedbacks can be associated with either topographical or grain-size composition instabilities. An approach based on self-organization also allows researchers to establish limitations in the predictability of the occurrence of rhythmic patterns and characteristics as well as to study mechanisms leading to the observed variability of pattern or lack of regularity. [Copyright &y& Elsevier]

Abstract: Existing, easily applicable methods to calculate the depth and height of breaking waves are hampered by two obstacles. First, the breaker depth is usually required to compute its height, and vice versa. Second, the equations take into account either the deepwater height to wavelength ratio or the sea floor slope, but not both. A simple iterative procedure is therefore proposed which incorporates both elements. For fully developed waves breaking over a nearly horizontal bottom, the breaker height and depth are also direct functions of the deepwater wavelength. [Copyright &y& Elsevier]

Abstract: This paper analyses the morphological response induced by low-crested structures on the adjacent seabed, with particular interest in the erosion patterns that frequently develop at gaps and roundheads. The mechanisms responsible for erosion processes are examined by means of morphodynamic simulations with the numerical suite MIKE 21 CAMS developed by DHI Water & Environment. The main purpose of the paper is to verify how and how far a commercial code can predict bed evolution, in vicinity of defence structures in a real case, in order to get information that may be very useful for structure design and possible maintenance of existing works. The code is applied to a long-term simulation on a study site that is characterized by a composite intervention and suffers from severe erosion, Lido di Dante (Italy). The simulation covers the period (one year and a half) between two available multi-beam surveys, in order to have a detailed real bathymetry as starting point and another one, as accurate as the first, to compare numerical with surveyed results. All the other input data for the model, as waves, tide, wind and sediment characteristics, are derived from measurements in the area; moreover, the code is calibrated using wave and current data acquired during a field campaign. The bed evolution derived from simulations shows a good agreement with the survey both in the locations and in the intensity of erosive and depositional areas. A sensitivity analysis of results to some selected modelling parameters is performed on a shorter simulation period (one month), showing that accounting for bed slope in sediment transport modelling has greater effects in bathymetry evolution than the use of a complex sediment bathymetry or the representation of wave diffraction. [Copyright &y& Elsevier]

Sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences (basin area and relief), geography (temperature, runoff), geology (lithology, ice cover), and human activities (reservoir trapping, soil erosion). A new model, termed "BQART" in recognition of those factors, accounts for these varied influences. When applied to a database of 488 rivers, the BQART model showed no ensemble over- or underprediction, had a bias of just 3% across six orders of magnitude in observational values, and accounted for 96% of the between-river variation in the long-term (±30 years) sediment load or yield of these rivers. The geographical range of the 488 rivers covers 63% of the global land surface and is highly representative of global geology, climate, and socioeconomic conditions. Based strictly on geological parameters (basin area, relief, lithology, ice erosion), 65% of the between-river sediment load is explained. Climatic factors (precipitation and temperature) account for an additional 14% of the variability in global patterns in load. Anthropogenic factors account for an additional 16% of the between-river loads, although with ever more dams being constructed or decommissioned and socioeconomic conditions and infrastructure in flux, this contribution is temporally variable. The glacial factor currently contributes only 1% of the signal represented by our globally distributed database, but it would be much more important during and just after major glaciations. The BQART model makes possible the quantification of the influencing factors (e.g., climate, basin area, ice cover) within individual basins, to better interpret the terrestrial signal in marine sedimentary records. The BQART model predicts the long-term flux of sediment delivered by rivers; it does not predict the episodicity (e.g., typhoons, earthquakes) of this delivery. [ABSTRACT FROM AUTHOR]

Grain-bed collision is a key component of saltation, but substantial uncertainty remains regarding many aspects of this process. Previous empirical work is used to develop a conceptual model of this process that accounts for the partitioning of impact energy between rebounding grains and the bed. The model envisions two distinct collision regimes: (i) a quasi-elastic regime at low impact speeds and (ii) an inelastic regime at high impact speeds. In the quasi-elastic regime, colliding grains retain a constant proportion of their kinetic energy, and thus rebound at speeds that are proportional to the impact speed and shear velocity. In the inelastic regime, colliding grains retain a fixed and limited amount of kinetic energy, transferring any excess above this limit to the bed. Rebound speeds are thus constant and independent of both impact speed and shear velocity. Impact energy transferred to the bed is expended in ‘deformation’ (i.e., ejection, rearrangement, etc. of bed grains). These two collision regimes are separated by a critical impact speed/kinetic energy level that is controlled by bed texture (more directly, by the inertia or resistance to motion of the bed grains). Field measurements under near-threshold conditions are found to correspond to the higher speed inelastic regime, suggesting that inelastic collisions predominate in aeolian saltation on loose, unconsolidated sand beds. The quasi-elastic regime might therefore only be pertinent for reptating grains, or in situations in which interparticle bonding (from crusts, surface moisture, etc.) increases the ‘effective’ inertia of bed grains and thus the critical impact speed separating the two regimes. It is also shown that rebound speed in the inelastic regime varies in inverse proportion to grain size (mass), so that particles rebound with an approximately constant level of kinetic energy regardless of size. Thus, larger grains will rebound at smaller speeds and follow lower, shorter trajectories, providing a mechanism that accounts for the commonly observed size sorting of grains subjected to aeolian saltation. [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on the analysis of the morphological response induced by low-crested structures on the adjacent seabed, with emphasis on the far-field erosion that is frequently observed close to roundheads and at the gaps between structures. The analysis is based on observations of far-field erosion, both at prototype scale and in laboratory experiments. Mechanisms responsible for the observed erosion are highlighted using coastal area numerical modelling tools to simulate the flow patterns induced by schemes involving low-crested structures. Finally, an example of the use of a coastal area morphological modelling system to investigate the dependence of the far-field erosion processes on the freeboard of the coastal protection structures is presented and discussed. [Copyright &y& Elsevier]