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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]

Sediment transport and resuspension are essential processes in soft-sediment environments that mediate shellfish population dynamics, water column turbidity, and estuary geomorphology at multiple scales. Adult cockles, Austrovenus stutchburyi (in New Zealand), are key organisms on intertidal sand flats that influence sediment properties, and their removal could impact the stability of the seabed and affect crucial feedback processes in this system. We conducted a field experiment to explore our hypothesis that the loss of adult cockles will increase sediment transport and resuspension non-linearly. Changes in sediment transport regimes will then influence the properties of sediment such as grain size, organic content, and chlorophyll a content. We also expected that the direction and magnitude of this effect would depend on the varying environmental characteristics across the sites, such as hydrodynamic activity and sediment grain size. We excluded adult cockles from 3 × 3 m intertidal plots at 15 sites across a natural adult cockle density gradient (300–3500 ind/m2). After seven months, sediment traps were deployed in exclusion and control plots to measure the sediment transported over two tidal cycles associated with the bedload and water column with differing densities of adult cockles. Adult cockle density was a significant predictor in all models to explain sediment transport and accounted for the largest proportion of variability explained. Relationships between cockle density and sediment transport changed between sites and over time. In these situations, simple linear relationships were not always apparent due to the interacting effects of cockle density, hydrodynamic process, and sediment characteristics. The influence of adult cockles on sediment transport was also affected by the standing stock of microphytobenthos and interacted with the mud content in their surrounding environment. Our study demonstrates the existence of an important feedback between adult cockle density, sediment stability and sediment transport that is mediated by hydrodynamic processes and sediment characteristics. • We used traps to measure sediment transport across a cockle density gradient. • Changes in adult cockle densities alters sediment stability non-linearly. • Adult cockles interact with their environment to drive sediment transport processes. • Sediment transport is highly context-dependent, influenced by physical processes. [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]

The propeller jet generated by a ship's propeller during berthing and unberthing from quays initiates sediment transport and causes scouring around the quay structures. Investigating the flow area caused by ship propeller jets at quays and piers is important in terms of determining the local scour at the seabed. In this study, the water jet formed by ship's propellers and sediment movements on the seabed were investigated. While medium sand is typically used as the sediment type in literature, fine-grained sand and silt were used as base materials in this study. As a result of the experiments conducted with a three-dimensional flow velocity measuring device (ADV), the measured three-dimensional velocity components and the propeller efflux velocity coefficients were redefined. By examining the time dependent scour graphs of silt and sand, their scour rates were compared, and time scales were obtained from graphs. Silt was observed to have a significantly slower scour rate compared to sand. Contrary to the findings in the literature, despite silt having a smaller median particle diameter than sand, the maximum scour depths of silt were found to be less than those of sand under the same experimental conditions. Based on the results of this study and previous studies, two equations were proposed to quantify the scour depth for sand material, and silt and sand-clay mixtures. • Usage of silt as a seabed material. • Observation of time dependent scour development with bathymeter maps. • Investigation and comparison of scour time scales of silt and sand. • Calculation of propeller seabed-scour for sand, silt and clay-sand mixture. [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]

Although many studies have been performed in the northern Gulf of Mexico, there have been a paucity of high-resolution sediment data collected from seabed. New multicores were collected repeatedly at 4 fixed stations in the inner Louisiana shelf on March 12, 2021, July 7, 2021, November 20, 2021, and March 5, 2022, respectively, to study detailed spatial and temporal variations of surficial sediment. Being a very high sediment accumulation zone, Raccoon Island dredge pit and its nearby shelf were selected as our study area and additional six vibracores were collected inside and outside the pit for comparison. This study area is about 140 km west of the modern Mississippi Delta and 70 km southeast of the Atchafalaya Delta. A total of 624 samples from both vibracores and multicores were analyzed for this study. Spatially, our results show that sediment near Ship Shoal was the coarsest and sediment deposited in the pit was the finest. The organic matter was highest inside the pit and lowest near Ship Shoal. The carbonate content was the lowest near Ship Shoal. Temporally, surficial sediments were generally coarser in July and November of 2021 and finer in March of 2021 and March of 2022. Although the Mississippi and Atchafalaya Rivers supply a large amount of sediment during spring season, it takes time for the river-derived sediment to be delivered into the study area. Hurricanes Delta, Zeta and Ida passed this area and greatly impacted sediment transport in the years 2021 and 2022. The comparatively coarser sediment in July and November of 2021 were likely related to the resuspension of some coarse sediment by these hurricane events. Besides the river supply, Ship Shoal and barrier islands are nearby localized sandy sources and inner Louisiana shelf is a widespread mud source. This spatial heterogeneity of surficial sediment interplays with the timing of Mississippi and Atchafalaya River flood season and hurricanes and together dictates the sediment texture. This research on a filled-up dredge pit can benefit future evaluation of the safety of existing oil infrastructure and the feasibility of building new pipelines and platforms after dredging activities. Our results also shed light on the study of river- and hurricane-impacted sediment transport in muddy and mixed-texture passive continental margins around the world. • Pit sediment hasn't reached an equilibrium with ambient environment yet. • Surficial sediments were coarser in July and November and finer in March. • Sediment grain size spatial variations were categorized into four groups. • This research will benefit future coastal restoration and sediment management. [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]

Sediment transport over Sandy Point dredge pit in the northern Gulf of Mexico during a cold front event in November 2014 was examined using a finely resolved numerical model. The Delft3D model was used to perform numerical experiments that simulate the effect of wind-generated waves, wind-driven currents, river discharge, and tides on sediment dynamics in Sandy Point dredge pit. The hydrodynamics and sediment models were validated and calibrated using field data of current, wave, water level, and suspended sediment concentration. Two potential sources of sediment were examined: fluvial sediment from the Mississippi River and resuspended sediments from the seabed. Results showed that during a cold front, shear stress from wave motions played a significant role in the resuspension of sediments in Sandy Point dredge pit. The maximum cold front-related wave impact on sediment resuspension could increase near-bed sediment concentration in Sandy Point dredge pit by 20–50 times. In addition, the results suggest that the primary source of sediment for Sandy Point dredge pit during a cold front was resuspension from the ambient seabed due to increased bottom shear stress by wind-induced waves and strong southward wind-driven currents. Currents dispersed sediments from the Mississippi River passes and inhibited riverine sediment supply from Sandy Point dredge pit. Results also showed that cold fronts contribute 16%–24% of the annual sedimentation in Sandy Point dredge pit. • A coupled wave-current-sediment model was used to study the sandy dredged pit. • The combined effect of waves and currents on sedimentation was assessed. • The reverse bottom current inside the pit was identified. • Bottom boundary layer was significantly affected by waves. • Sediment accumulation occurs mostly on the north of the pit. [ABSTRACT FROM AUTHOR]

A model is proposed to determine the sediment transport rate generated by the combined action of sea waves and tidal currents, the latter being modelled as a sequence of steady currents because of the large values of the Keulegan-Carpenter number of the tidal flows, which allow to neglect inertial effects in their modelling. The hydrodynamic module solves Reynolds-averaged momentum equations by introducing a two-equation turbulence model, which considers also the flow in the region closest to the sea bottom and enforces the no-slip condition and the vanishing of the turbulent kinetic energy at the bottom without the need to assume the existence of a logarithmic velocity profile and to fix the normal derivative of the turbulent kinetic energy close to the sea bed. Hence, the evaluation of the velocity profile in the near-bed region turns out to be accurate. The sediment transport module determines the sediment concentration and sediment transport for a mixture of non-cohesive and cohesive sediments. To validate the model, its predictions are compared with laboratory measurements and field data collected over rippled beds at two different tidal estuaries. Then, results are obtained for values of the parameters chosen to mimic a site close to Punta Umbría (Huelva, Spain) and considering different hydrodynamic conditions and different sediment mixtures. The influence of the waves on the suspended sediment concentration is lower for the cohesive fraction than for the non-cohesive fraction. For all the sediment mixtures, it turns out that the cohesive fraction is well mixed over the whole water depth, meanwhile the concentration of the non-cohesive fraction has significant values only near the bottom, thus showing that an accurate description of turbulence dynamics in the buffer layer and viscous sublayer (if it exists) is important to quantify the sediment transport rate. • Sediment transport module that reproduces the concentration of cohesive fraction is presented. • Tidal current is modelled as a sequence of steady-currents due to the large values of the Keulegan-Carpenter number. • Menter's turbulence model to predict the turbulence structure both in the wave bottom boundary layer and in the core region. • Turbulence dynamics description in the buffer layer and viscous sublayer is important to quantify the sediment transport. • Applicable to any type of sediment mixture. [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 High resolution bathymetry combined with structural modelling is used to estimate changes in the on-bottom stability of an offshore pipeline due to scour and sedimentation over an 11 year period. Detailed observations of post-lay embedment changes have been combined with the pipeline structural characteristics and an elastic-plastic model of soil resistance to estimate the vertical and horizontal stability of the pipeline using a finite difference solution to the beam bending equation. Application of the design approach indicates that post-lay increases to the critical (break-out) velocity of 1–2 m/s occur along the full 19 km of surveyed pipeline due to scour and sedimentation, which act to reduce load and increase soil resistance. The rate at which this increase in stability occurs with time is found to vary along the pipeline, and is dependent on the mechanism of pipeline lowering (i.e. whether the pipe lowered due to sagging into widely spaced scour holes, or by sinking into the shoulders between many closely spaced scour holes). By incorporating sediment transport into the pipeline design, the present results suggest potential for significant improvements in pipeline on-bottom stability and associated reductions in minimum required specific gravity and/or secondary stabilisation. Highlights • Seabed mobility driven changes to pipeline embedment are observed in the field. • The resulting changes to pipeline on-bottom stability are quantified. • Increases in critical velocity of 1–2 m/s are calculated for a steady current. • The amount of increase is dependent on the mechanism of pipeline lowering. • Increased soil resistance and hydrodynamic shielding are both shown to be important. [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]

The near-bottom nepheloid layer in the western margin of the Southern Adriatic Sea was monitored for 8-years by measurements acquired at two mooring sites. The two moorings, equipped with CTD probes and ADCPs, are located in the Bari Canyon and in an open slope sector along the Southern Adriatic Margin. These regions are of interest because affected by episodic dense shelf water cascading events whose dynamic has direct implications on deepwater morpho-dynamic, biogeochemical cycles and trophic networks. In this work, the sedimentation flux and its interdecadal dynamic is analysed examining in detail the sedimentary processes triggered by dense water flow through the analysis of the echo records of ADCPs. The integration of hydrodynamic, turbidity and particle grain-size data provided estimates of the sediment flux, separating phases when the flow actively erodes the seabed from phases when particles are transported to the mooring location through density flows. The frequency and velocity of dense-water cascading currents vary in time and space reflecting the capacity of sediment transport. Data analysis demonstrated that the hydrodynamic event that mostly accounts for sediment transfer to the deep basin is represented by current pulses induced by the passage of dense waters. The average annual sediment flux has been quantified and the Bari canyon shows transport more than five time larger than in the open slope sector, confirming that the canyon is the dominant pathway of sediment transfer to the deep basin. In contrast, in the open slope, albeit a minor lateral sediment advection, is impacted by currents that are able to trigger intense resuspension of seabed sediments, which can contribute over 80% of the total solid load. This study allows unravelling the role of cascading in the sediment resuspension and transport processes and is essential to support deciphering the sedimentary records in the study area. The long temporal extent of the dataset used for quantification provides a reliable contribution to the Quaternary sediment budget determination. • The integrated approach proposed in the study made it possible to determine sediment flux during dense water cascade events. • This study allows unravelling the role of cascading in the sediment resuspension and transport processes • The characterization of short-lived events can improve the high-resolution interpretation of the stratigraphic records. • The large temporal extension of the dataset used provides a reliable contribution to the Quaternary sediment budget [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan Tsunami caused severe morphological changes along the affected coastline on the northeast part of Japan. This study investigates the recovery of river mouths and breaching of sandy coast at the place used to be river mouth in Miyagi Prefecture, Japan after severe erosion induced by the tsunami through analyzing aerial photograph and investigating field data. After several years since the tsunami took place, the morphology of some river mouths has mostly recovered while others still have some damage. Results on the analysis of aerial photograph clearly indicate that the total length of sandy coast on both sides of river mouth plays important role in the recovery process. Moreover, the relevant phenomenon occurring during the morphological recovery process is also investigated. The findings demonstrated the intrusion of sand spit into river mouths due to the tsunami having scoured river mouth deeper than the depth of closure. Interestingly, the intrusion distance was found to be positively correlated with the width of river mouth. In addition, there is an important finding on the practical problem which is related to the intrusion upstream of sand spit. That intrusion shifts the boundary of fresh-sea water upstream, therefore external wave forces must be considered when implementing the design of river structures. [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]

Field studies in the Song Hau distributary of the Mekong Delta in Vietnam conducted at high (Sept.-Oct 2014) and low (March 2015) Mekong River discharge are utilized to examine channel bottom morphology and links with sediment transport in the system. Multibeam bathymetric mapping surveys over the entire channel complex in the lower 80 km of the distributary channel, and over 12- to 24-h tidal periods at six transect locations in the reach are used to characterize bottom type and change on seasonal and tidal timescales, supplemented by bottom sampling. The results of this study indicate that the largest proportion of channel floor (up to 80% of the total area) is composed of substratum outcrops of relict sediment units deposited during the progradation of the delta in the last 3.5 ka. These take the form of outcrops that are either (1) steep-sided, tabular channel floor, (2) steep-sided sidewall, or (3) relatively flat channel floor. Flatter outcrops of channel floor substratum are identified by the presence of sedimentary furrows (<0.5 m deep) incised into the channel bottom that are exposed at high discharge and oriented along channel and laterally continuous for kilometers. These furrows are persistent in location and extent across tidal cycles and appear to be incised into relict units, sometimes with a thin surficial layer of modern sediment observable in bottom grabs. The extent of substratum exposure, greater than that observed previously in low tidal energy systems like the Mississippi River, may relate both to a relatively low sand supply from the catchment, and/or to an efficient transfer of both sand and mud through this tidally energetic channel. Sand bottom areas forming dunes, comprise about 19% of the channel floor over the study area and are generally less than a few meters thick except on bar extensions of mid-channel islands. Both sandy and substratum areas are mantled by soft muds 0.25–1 m thick during low discharge in the estuarine section of the study area. This mud mantling appears to be a key control on bottom sourcing of sand to suspension. An understanding of channel bottom morphology, particularly mobility and erodibility of sediments, is valuable for setting up morphodynamic models of channel evolution that can be used to test system response to anthropogenic alterations in the catchment and rising sea levels. [ABSTRACT FROM AUTHOR]

River gauging stations are often located upriver of tidal propagation where sediment transport processes and storage are impacted by widely varying ratios of marine to freshwater influence. These impacts are not yet thoroughly understood. Therefore, sediment fluxes measured at these stations may not be suitable for predicting changes to coastal morphology. To characterize sediment transport dynamics in this understudied zone, flow velocity, salinity, and suspended-sediment properties (concentration, size, and settling velocity) were measured within the tidal Sông Hậu distributary of the lower Mekong River, Vietnam. Fine-sediment aggregation, settling, and trapping rates were promoted by seasonal and tidal fluctuations in near-bed shear stress as well as the intermittent presence of a salt wedge and estuary turbidity maximum. Beginning in the tidal river, fine-grained particles were aggregated in freshwater. Then, in the interface zone between the tidal river and estuary, impeded near-bed shear stress and particle flux convergence promoted settling and trapping. Finally, in the estuary, sediment retention was further encouraged by stratification and estuarine circulation which protected the bed against particle resuspension and enhanced particle aggregation. These patterns promote mud export (~1.7 t s −1 ) from the entire study area in the high-discharge season when fluvial processes dominate and mud import (~0.25 t s −1 ) into the estuary and interface zone in the low-discharge season when estuarine processes dominate. Within the lower region of the distributaries, morphological change in the form of channel abandonment was found to be promoted within minor distributaries by feedbacks between channel depth, vertical mixing, and aggregate trapping. In effect, this field study sheds light on the sediment trapping capabilities of the tidal river – estuary interface zone, a relatively understudied region upstream of where traditional concepts place sites of deposition, and predicts how fine-sediment dynamics and morphology of large tropical deltas such as the Mekong will respond to changing fluvial and marine influences in the future. [ABSTRACT FROM AUTHOR]

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]

Knowledge of sediment transport pathways is important for coastal management as well as for offshore infrastructure such as wind farms and other renewable energy installations. Here a three-way coupled model of tides, waves and sediment dynamics is presented, which extends the approach of Davies and Robins (2017) for the prediction of the locally-varying seabed roughness k s resulting from the interaction between the flow (tides and waves) and the bed sediment (grain size mixture) subject to wave-current interaction (WCI) at the seabed. The model was applied to the North Wales coastal area, with particular emphasis on an extensive shore-connected sand bank (Constable Bank), the stability of which was investigated through the study of residual sediment circulation patterns. Multi-beam (MBES) and seabed rig (AWAC) observations have allowed validation of the predicted k s and hydrodynamics, respectively. With due allowance for the supply of mobile sediment, the agreement between predicted k s and observed bedform height was generally good. The model has been used to determine residual velocities and sediment pathways, both with and without wave effects included. Wave influence and WCI are predominant in nearshore areas, affecting the magnitude and direction of the residual fluxes. A key modelling outcome offshore is a clockwise residual circulation pattern for water and sediment around the end of Constable Bank, set within a generally eastward net drift of sediment in the wider study area. • Sediment pathways predicted using a novel 3-way-coupled tide-wave-sediment model. • Coastal area flow modelled with temporally/spatially varying bed roughness. • Implementation in Telemac of wave-current interaction at the seabed. • Comparison between MBES-observed and Telemac-predicted physical bed roughness. • Residual flow and transport predicted highlighting a shore-connected sandbank. [ABSTRACT FROM AUTHOR]

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]

Estimating the erosion potential of seabed sediments and the magnitude of the resulting suspended load in relation to current dynamics near the shelf break is a key issue for better understanding shelf-slope sediment transport. On the outer Mackenzie Shelf (Canadian Beaufort Sea, Arctic Ocean), a thin and discontinuous veneer of recent surficial clays overlie old glaciomarine sediments that further pinch out at the shelf edge. Gas and fluid venting is known to underlie part of sediment instability in the area, but recent mooring-based measurements also indicate that sediments near the shelf break are recurrently remobilized by strong subsurface currents. Here, we relate storms to the development of current surges that resulted in the abrupt resuspension of sediments at two locations along the shelf break. Near-bottom concentrations of suspended sediments were estimated using the acoustic backscatter of high-frequency acoustic Doppler current profilers deployed from September 2011 to September 2013 as part of the Beaufort Regional Environmental Assessment (BREA) program. Near-bottom currents near the shelf edge (140 to 150 m isobaths) were characterized by recurring episodes of elevated velocities (instantaneous speeds up to ~ 40-50 cm s -1 ) that were extensions of current surges (~ 60-80 cm s -1 ) occurring in the core of the shelfbreak jet located at ca. 90-120 m. Sudden peaks in suspended sediments (above 100 g m -3 ) corresponded closely with current surges in the near-bottom boundary layer (< 10 m) implying the local erosion of surficial sediments and the rapid advection or redeposition of the resuspended sediments. A range of apparent threshold velocities from 18 to 36 cm s -1 was calculated based on the relationship between suspended sediment concentrations and near-bottom current speeds. Two meteorological scenarios were identified to explain the current surges underlying these erosion events at the shelf edge: (1) Pacific or Arctic-born low pressure systems that propagate into the southern Beaufort Sea and are associated with intense downwelling-favorable westerly winds. These amplify the eastward-setting shelfbreak jet along the upper slope, such as in September 2012 when successive cyclones developed during the record-low sea ice extent. (2) The persistence of a high-pressure system over the Beaufort Sea that causes strong upwelling-favorable easterly winds and a strengthening of anti-cyclonic ice motion. This induces a reversal of the shelfbreak jet to the west and a subsequent eastward-directed relaxation flow when the winds fade, such as during the major ice fracturing events of January to March 2013 that were associated with strong and persistent current speeds. [ABSTRACT FROM AUTHOR]

Compared to vertical piles, the position relationship between the inclined pile and seabed has some extra effects on the surrounding wave-current interaction and sediment transporting mechanism. A series of local scour experiments with wave-current flume are conducted for investigating scour characteristics and influencing factors around inclined piles and vertical piles at a certain inclination angles range. The experimental results indicates that there are substantial differences in scouring variation patterns between upstream inclined piles, downstream inclined piles and vertical piles, and the suppression impact of pile inclination angle on scouring was closely related to the wave-current conditions. The magnitude relationship of relative scour depth (S/D) between the inclined and vertical piles will change visibly when the sand transporting capacity under wave-current action reaches a certain level. The obtained multiple regression equation based on mixing training datasets could provide reasonable estimation for scour depth around inclined and vertical piles under wave-current and only current action. • A series of local scour tests around inclined piles with different inclination angles based on wave-current flume were conducted. • Local scour characteristics and influencing factors around upstream inclined piles, vertical piles, and downstream inclined piles were investigated. • A multivariate empirical equation of scour depth was obtained for inclined piles with 0°<.|α|<30° under combined wave and current action. [ABSTRACT FROM AUTHOR]

• 2D modelling of solute transport into deformable sediments driven by ocean waves. • The effects of wave-induced soil deformation on solute transport into sediments. • Parametric analysis on solute transport into a deformable seabed under ocean waves. • The effect of different wave types on solute transport into deformable sediments. Most existing studies do not consider the effect that the deformability of submarine sediments may have on solute transport under the action of ocean waves. In this study a two-dimensional numerical model based on Biot's poro-elastic consolidation theory and an advection–dispersion equation is established to investigate the transport of non-reactive solute into a deformable porous seabed under the influence of ocean waves. The numerical model is then validated against experimental and analytical results in order to demonstrate its accuracy and robustness. The simulation results indicate that ignoring the deformability of seabed soil and associated seepage force due to ocean waves may lead to a significant underestimation of the rate of solute transport in shallow layers of marine sediment, particularly fine sand or silt. The effects of parameters such as the shear modulus, non-dimensional mechanical dispersion coefficient (related to particle size and permeability), dimensionless sediment thickness, wave steepness and the relative water depth have on solute transport into deformable sediments were investigated. The results indicate that the ratio of the longitudinal coefficient of mechanical dispersion between deformable and non-deformable sediments that can represent the enhancement of solute migration due to soil deformation may reach up to 85. This becomes more pronounced as the shear modulus and permeability of sediments decrease and the depth of water increases when the sediment thickness is less than 0.8 times the wavelength. The rate of solute transport in deformable sediments reaches its maximum when the sediment thickness is about 0.2 times the wavelength. In fact, as the relative water depth increases from 0.1 to 0.3, the enhancement of solute migration due to wave-induced soil deformation increases by about 60%-80%. In comparison to linear waves, shallow water waves, e.g., the first-order cnoidal waves, can increase the transport of solute more due to its strong non-linear behaviour, particularly in deformable seabed. [ABSTRACT FROM AUTHOR]

A series of experimental tests were performed to investigate the trench formation and development process of the mooring chain at touchdown zone in a sandy seabed. The surge, sway and heave motions with different motion cycles are imposed at the fairlead position of the mooring chain by an assembled multi-degree of freedom vibration device, respectively. The effects of the flow velocity and motion cycle on the trench formation and development are investigated. The results show that the seabed trench occurs at touchdown zone under different imposed motions, expands with the increase of the motion cycle and gradually tends to be stable. Simple formulas are proposed for the evaluation of three-dimensional trench scales such as length, width and depth for the imposed sway and heave motions. The seabed trench development mechanisms of mooring chain with different imposed motions are illustrated from the perspective of sediment transport. • A series of experiments are performed to investigate the trench development process of mooring chain at touchdown zone. • The seabed trench occurs at touchdown zone under different imposed motions, expands with the increase of the motion cycle. • The seabed trench development mechanisms of mooring chain are illustrated from the perspective of sediment transport. [ABSTRACT FROM AUTHOR]

Horizontal axial turbine (HAT) is widely used to extract kinetic energy from tidal current. The power absorption of HAT is largely decided by the surrounding flow. A two-phase, sediment-laden mixture model is coupled with the blade element momentum method (BEM) for the impact of HAT on surrounding flow over sediment seabed. The mixture model incorporates theoretical relative velocity, turbulence enhancement from particle wake flow and turbulence damping from high concentration sediment. A blade-induced turbulence (BIT) term is introduced into the mixture model to represent the turbulence production at blade edge. The computed velocity and turbulence are improved by the BIT term in the wake region of HAT. Three turbine hub heights are adopted to investigate the impact of the turbine on sediment seabed and the surrounding flow. Results show that with low turbine hub height, long wake region is generated after the turbine, force exerted on the turbine is asymmetric vertically and thrust and power coefficients decrease. The pattern of bed shear stress is highly dependent on the turbine hub height. The bed shear stress is enhanced under the turbine and strong scour and sediment transport tend to occur on the sediment seabed with low turbine hub height. • Mixture model is improved and coupled with blade element momentum theory. • Blade-induced turbulence term is introduced for turbulence model at blade edge. • Predicted velocity and turbulence are improved by blade-induced turbulence term. • Three turbine hub heights are adopted for impact of horizontal axial turbine. • Results perform blade-induced turbulence and impact of turbine on sediment seabed. [ABSTRACT FROM AUTHOR]

• We explore the behavior of tides in the mid-Archean era through simulations. • Mid-Archean ocean tides may have been larger than modern tides. • Tidal sea surface elevations impact photosynthetic processes on shelves. • Tidal currents impact sediment transport on shelves. • Tidal mixing fronts impact marine productivity on shelves. A variety of different controls have been proposed for Precambrian marine productivity ranging from environmental factors such as temperature to the flux rate of nutrients (e.g., phosphorus) derived from the continents. In addition to other first-order differences in the Earth system such as a less luminous young Sun, the Earth and Moon were substantially closer together, which would have resulted in a greater tidal forcing. The potential biogeochemical implications of this enhanced forcing have been little explored. Here we use idealized bathymetries and continental configurations to examine mid-Archean ocean tides. We find that, even in the absence of realistic coastal geometries, mid-Archean tidal velocities, sea-surface height, and mixing equaled or exceeded modern values, and that there would have been higher frequency of extreme values. We use a modern tidal model with accurate coastal geometry (HYbrid Coordinate Ocean Model; HYCOM) to scale the idealized results. We propose that mid-Archean water parcel velocities would have been at least 4.5 times greater than at present, that sea-surface height would have been at least 2.5 times greater than at present, and that tidal mixing fronts would have been more common. Each of these factors would result in greater flux and distribution of nutrients, both due to exposure of the sea beds/nascent continents and enhanced onshore-offshore transport, potentially strongly influencing marine productivity even in the absence of substantial permanently exposed subaerial continental landmass. [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 6-month deployment of instrumentation from April to October 2012 in 90 m water depth near the outer edge of the mid-shelf mud belt in southern Monterey Bay, California, reveals the importance regional upwelling on water column density structure, potentially accounting for the majority of the variability in internal tidal energy flux across the shelf. Observations consisted of time-series measurements of water-column currents, temperature and salinity, and near-bed currents and suspended matter. The internal tide accounted for 15–25% of the water-column current variance and the barotropic tide accounted for up to 35%. The subtidal flow showed remarkably little shear and was dominated by the 7–14 day band, which is associated with relaxations in the dominant equatorward winds typical of coastal California in the spring and summer. Upwelling and relaxation events resulted in strong near-bed flows and accounted for almost half of the current stress on the seafloor (not accounting for wave orbital velocities), and may have driven along-shelf geostrophic flow during steady state conditions. Several elevated suspended particulate matter (SPM) events occurred within 3 m of the bed and were generally associated with higher, long-period surface waves. However, these peaks in SPM did not coincide with the predicted resuspension events from the modeled combined wave–current shear stress, indicating that the observed SPM at our site was most likely resuspended elsewhere and advected along-isobath. Sediment flux was almost equal in magnitude in the alongshore and cross-shore directions. Instances of wave–current shear stress that exceeded the threshold of resuspension for the silty-clays common at these water depths only occurred when near-bed orbital velocities due to long-period surface waves coincided with vigorous near-bed currents associated with the internal tide or upwelling/relaxation events. Thus upwelling/relaxation dynamics are primarily responsible for variability in the internal tide, as well as transport of near-bottom sediment in the mid-self mud belt during the relatively quiescent summer months. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) ecosystem models of shelf environments should properly account for the biogeochemical cycling within the sea floor. However, a full and explicit representation of sediment biogeochemistry into 3D ocean models is computationally demanding. Here, we describe a simplified approach to include benthic processes in 3D ocean models, which includes a parameterization of the different pathways for organic matter mineralization and allows for organic matter remobilization by bottom currents and waves. This efficient approach enables decadal simulations that resolve the inertial contribution of the sea floor to the biogeochemical cycling in shelf environments. The model was implemented to analyze the benthic-pelagic coupling in the northwestern shelf of the Black Sea. Three distinct biogeochemical provinces were identified on the basis of fluxes and rates associated with benthic-pelagic coupling. Our model simulations suitably capture the seasonal variability of in situ flux data as well as their regional variation, which stresses the importance of incorporating temporally varying sediment biogeochemistry and resuspension/redeposition cycles in shelf ecosystem models. [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]

An ability to estimate the large-scale spatial variability of seabed sediment type in the absence of extensive observational data is valuable for many applications. In some physical (e.g., morphodynamic) models, knowledge of seabed sediment type is important for inputting spatially-varying bed roughness, and in biological studies, an ability to estimate the distribution of seabed sediment benefits habitat mapping (e.g., scallop dredging). Although shelf sea sediment motion is complex, driven by a combination of tidal currents, waves, and wind-driven currents, in many tidally energetic seas, such as the Irish Sea, long-term seabed sediment transport is dominated by tidal currents. We compare observations of seabed sediment grain size from 242 Irish Sea seabed samples with simulated tidal-induced bed shear stress from a three-dimensional tidal model (ROMS) to quantitatively define the relationship between observed grain size and simulated bed shear stress. With focus on the median grain size of well-sorted seabed sediment samples, we present predictive maps of the distribution of seabed sediment classes in the Irish Sea, ranging from mud to gravel. When compared with the distribution of well-sorted sediment classifications (mud, sand and gravel) from the British Geological Survey digital seabed sediment map of Irish Sea sediments (DigSBS250), this ‘grain size tidal current proxy’ (GSTCP) correctly estimates the observed seabed sediment classification in over 73% of the area. [ABSTRACT FROM AUTHOR]

At least 26 genera of macroalgae have been observed attached to and carrying sedimentary materials (rock, veneer, or carbonate) to beaches and the sea floor. Most studies have been descriptive, with few quantifying flux rates. This study reviews and quantifies the mainly-biological literature on macroalgae-mediated sediment transport in coastal and marine environments. Dislodgement of macroalgae from a hard substrate can result in erosion up to 0.01 mm y − 1 . The proportion of buoyant macroalgae carrying rock material (usually a thin veneer, but sometimes boulders up to 365 kg) varies by location, but can reach as high as 35%. Macroalgae carrying attached carbonate material (e.g., crustose coralline algae, shells) are more common: up to 85% of holdfasts on a beach may contain up to 116 g CaCO 3 each. Sand and carbonate trapped in haptera, along with encrusting carbonate-producing organisms, have a mass of up to tens of grammes. While most dislodged macroalgae land close to where they came from, others may be stranded many thousands of km from their place of origin. Sediment deposited by macroalgal transport depends largely on the characteristics of local source rocks. While hard rocks may only provide a few g m − 2 y − 1 , highly fractured basalt has been shown to provide up to 78 g m − 2 y − 1 . Rock transport by macroalgae is less than carbonate flux, which may reach over 8000 g m − 2 y − 1 . Based on the projected abundance of macroalgal rafts, some 350 tonnes of sediment (both carbonate and clastic) may be afloat in the Southern Ocean at any one time. Macroalgal transport may explain the presence of exotic rocks and shells found far from their source environment. While cold-climate rafting of sediment is dominated by glacial processes, in warm periods rafting may be limited to biological transport. Overall the transport of rock and carbonate by macroalgae has the potential in some environments of being a significant source of sediment. [ABSTRACT FROM AUTHOR]

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]

Wave and/or current supported gravity-driven sediment flows (GDSFs) cause substantial sediment movement across the continental shelf, contributing to morphological evolution in many regions worldwide. However, they appear to occur episodically and ephemerally and, therefore, it remains a challenge to document them in detail with in situ measurements. Here we present solid evidence of frequent generation of such flows over the shallow sea floor of a muddy open coast. They were triggered by wave resuspension and/or sediment settling from the overlying water column, maintained by wave- and/or current-induced bed stress, and terminated due to upward spreading of bottom sediment within the high concentration layer. Randomly selected GDSF events were analyzed to realize parameterization with a buoyancy-friction model; the resultant bed drag coefficient for two of the cases is higher than the value of 0.003, which is attributed to the additional drag at the interface between the overlying flow and the moving GDSFs. Although short-lived, the observed GDSF events in the three different seasons indicate that they occur more frequently than previously thought. • Multiple events of short-lived gravity-driven sediment flows (GDSFs) were directly observed. • GDSFs were neatly parameterized with a simple but elegant buoyancy-friction model. • GDSFs may occur more frequently than previously believed. [ABSTRACT FROM AUTHOR]

Abstract: Coarse-grained ripples (CGRs) develop within the seafloor depression–sand ridge systems along the Shimizu coast, in the northwestern part of Suruga Bay facing the Pacific Ocean, central Japan. In this paper, we analyze data from high-resolution bathymetry, seafloor observation by an underwater video camera, and high-resolution seismic surveys and seafloor trenching of shallow strata, obtained between 2005 and 2007, to describe their geomorphological and sedimentological characteristics in detail, and to examine the formative processes and maintenance of the systems. In water depths of 5–30m, CGRs occur in seafloor depressions that are approximately 40–70cm deeper than the surrounding sandy seafloor. In many cases, the ripples develop in depressions between shore-normal sand ridges composed mainly of fine sand (<70cm high, 100–400m long) on a relatively narrow shelf. The alongshore width of the sand ridges is 10–100m, and that of the depressions is 10–150m. The CGRs (wavelength<3.5m, height<30cm) have almost shore-parallel crests with a symmetrical or slightly asymmetrical shape, suggesting wave-generated oscillating flows. The crests of the lager ripples are composed of gravel (maximum long axis: 5–8cm) and the troughs consist of fine to very fine sand. The CGR–depression–sand ridge systems develop widely during the passage of typhoons in the summer–autumn seasons. At the narrower shelf, the surface fine-grained sediments and the surface and subsurface coarse-grained sediments, which form the sand ridges and the CGRs respectively, are repeatedly eroded and redeposited during typhoon-related storms. In contrast, active sediments are currently limited to the surface fine deposits on the wider shelf areas. The sediments that make up the CGRs appear to be derived from both modern beach sediments and relict sediments. We postulate that the CGR–depression–sand ridge system appears to be controlled mainly by both seafloor erosion and offshore-directed sediment transport during storms, and these processes may depend on the shelf width. In fact, the sand ridges (covering the CGRs) that imply significant offshore-directed transport only develop on the narrower shelf. The distribution and seabed expression of the CGRs is constrained by the thickness and distribution of surface-covering sandy sediments, such as the sand ridges. This represents a new interpretation of the distribution of CGRs within seafloor depressions on the shelf of an active margin that receives a large supply of sandy and gravelly fluvial sediments, and differs from the processes seen on the sediment-starved shelves of passive margins. [Copyright &y& Elsevier]

Abstract: The investigation of hydrodynamics near seabed is an initial input to study marine current turbine-induced seabed scour. The authors investigated the slipstream between the seabed and the marine current turbine via OpenFOAM. The axial component of velocity is the dominating velocity of flow below marine current turbine. The maximum axial velocity under the turbine blades is around 1.07 times of the initial incoming flow. The maximum radial and tangential velocity components of the investigated layer are approximately 4.12% and 0.22% of the maximum axial velocity. The slipstream varies in direct proportion to the incoming velocity. A schematic diagram to describe the flow pattern under the marine current turbine has been proposed based on the study. The turbine adopted in current simulation has three blades. The acceleration of flow under the marine current turbine changes seabed boundary layer profile. The height of tip clearance and turbine geometry are the two principal parameters in scour design of the marine current turbine. [Copyright &y& Elsevier]

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]

Abstract: In this paper, a well-developed numerical model based on the immersed boundary (IB) method is used to study oscillatory flows over a bed with large-amplitude ripples in a systematic manner. The work shows that the complex flow over the rippled bed can be numerically dealt with in Cartesian coordinate by the IB method and that the IB method is able to provide main features of the flows near the ripples. An accurate simulation of vortices generation as a result of flow separation at the rippled bed is obtained. It is found that the oscillatory flows start to separate during the flow deceleration when the Keulegan–Carpenter (KC) number is small. The steady streaming for various ripple steepness is simulated and the criterion for separating the single and double structure streaming is also discussed. Moreover, a new type of steady streaming which consists of a pair of embedded recirculations in the vicinity of the ripple trough is obtained for relatively steep ripples in this work. The numerical results, including the steady streaming in particular, may be helpful to improve the understanding of the sediment transport and the seabed evolution with natural ripples under sea waves. [Copyright &y& Elsevier]

Abstract: The effect of streaming on the sea bed boundary layer flow beneath combined waves and current has been investigated for waves following and opposing a current, taking both linear waves and second order Stokes waves into account. This flow results from an interaction between the classical wave–current seabed boundary layer mechanism and two different streaming mechanisms. The classical wave–current seabed boundary layer mechanism leads to a reduced mean velocity relative to the current alone; the two streaming mechanisms are streaming caused by turbulence asymmetry in successive wave half-cycles (beneath asymmetric forcing) and streaming caused by the presence of a vertical wave velocity within the seabed boundary layer as earlier explained by Longuet-Higgins. The effect of wave asymmetry, wave length to water depth ratio, magnitude of current and bottom roughness have been investigated by numerical simulations for realistic physical situations. Mean Eulerian quantities as well as the mass transport (wave-averaged Lagrangian velocity) are presented. The resulting sediment dynamics near the ocean bottom has been investigated; results for both suspended load and bedload are presented. For wave dominated situations, the velocity profiles beneath opposing waves and current are substantially different from those beneath following waves and current, both for linear and second order Stokes waves. As the flow becomes current dominated, the velocity profiles beneath opposing and following waves and current approach the velocity profile beneath horizontally uniform sinusoidal forcing. The net transport of suspended sediments and bedload is in the direction of the current both for waves following and opposing the current. For wave dominated situations the net sediment transport is strongly affected by the streaming mechanisms, both for linear and second order Stokes waves and current. [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]

Summary: Turbidity currents play a critical role in effective sediment and reservoir management in alluvial rivers. Yet previous estimations of reservoir turbidity currents are mostly based on simple empirical relations and it remains unclear how and to what extent physically based numerical models can resolve current evolution as compared against field observations. This paper presents a physically-based,process-resolved computational study of turbidity currents in the Xiaolangdi reservoir in the lower Yellow River, China. A coupled layer-averaged 2Dnumerical model is applied, which explicitly incorporates the interactions between the current, sediment transport and morphological evolution, and features a new well-balanced numerical scheme dealing with irregular topography. Two turbidity current events in July 2004 are numerically simulated to calibrate and validate the model. The current advance and the sediment transport rate computed by the model compare favourably with field measurements. These suggest the present model is a viable tool for determining the timing for operating the bottom outlets, which is critical for effective reservoir sediment management. The sediment entrainment flux and bed resistance are key factors dictating the evolution of turbidity current and warrant further investigations. [ABSTRACT FROM AUTHOR]

Abstract: The ability to predict the presence and physical properties of small-scale sand ripples on the sea bed is important for determining the bed roughness felt by currents and waves, for sediment transport applications, and for acoustic applications. A fully time-evolving model is presented for predicting the height, length and orientation of sand ripples generated by currents, waves or both, which includes processes dealing with threshold of motion, ripple wash-out and biological degradation. We believe that this is currently the only predictor to feature all these properties, and is thus more widely applicable to shelf seas than other more sophisticated, but less comprehensive, methods. Tests against a large data-set of observations using an Acoustic Ripple Profiler at a site on the east coast of England, where ripples are generated by both tidal currents and waves, showed that the predictor captures most of the main features observed. A modified criterion for the wave/current dominance in the prediction equations improved some aspects of the agreement. The properties of current-generated ripples are predicted accurately, but the wavelength, and to a lesser extent the height, of wave-generated ripples show some under-prediction. Ripple orientations are modelled in an approximate way, but the main features of the observed orientations are captured. The bio-degradation feature of the predictor could not be tested with this data-set. [Copyright &y& Elsevier]

Abstract: The observation and prediction of the mobility of the seabed are in high demand with rapidly expanding offshore engineering developments and the increasing urgency to start mitigation against changing coastlines. From repeatedly surveyed sediment wave fields in the Irish Sea, average sediment wave migrations have been mapped, from which broad-scale net sediment transport directions can be derived. The proximity of bathymetric highs and lows seems to redirect sediment migration pathways and increases migration rates by a factor 5 on average via current deflection and amplification. A few hundred sediment waves were analysed geometrically in this study and compared to their migration behaviour, revealing a lack of statistical correlation. Sediment wave lee slope directions, asymmetry indices and other geometrical parameters appear poorly indicative of their degree of mobility in the Irish Sea. A considerable amount of sediment waves counter-intuitively migrate in the direction of their gentle stoss slope, i.e. opposite to the expected sense of migration. Even when considering the limitations of the predominantly 2-dimensional sampling method, these results fundamentally challenge established assumptions on the indicative meaning of sediment wave geometry. They question the validity and robustness of sediment wave migratory predictions based on single geometrical readings that fail to represent the complexity of local hydrodynamics and partial sediment wave modifications. Until predictive tools for sediment wave migration are developed that can pass the test in all aquatic environments, repeat surveying with sonar systems provides the only reliable way to record true natural variability. [Copyright &y& Elsevier]

Abstract: In this paper, transport pathways for coarse sediments were studied on an artificial pebble beach located at Marina di Pisa (Tuscany, Italy). Radio frequency identification (RFID) technology was used to trace the pebbles. The RFID technique is an inexpensive and reliable methodology that is used to address sediment displacement along the coastline. The use of low-frequency radio signals (125kHz) enabled the detection of the marked pebbles within a range of 40cm on the underwater and subaerial portions of the beach. About one hundred pebbles were sampled on the beach and paired with passive transponders. The tracers were released along a series of cross-shore transects (March 2009). The pebbles were injected on the fair-weather berm, on the swash zone, and on the step crest along each transect. The recovery campaign was carried out in May 2009. Fifty-three pebbles were detected; this number is rather high given the intensity of the three storms that hit the coast during the period of the experiment and the significant reworking that this beach is subjected to even under fair-weather conditions. The analysis of the tracer pathways highlighted twofold convergent longshore movement on the beach. The trajectories indicated that the longshore movement dominated the cross-shore movement, but there was no prevailing transport direction. To explore the morphology of the sea bottom fronting the beach, a topographic survey was carried out using an echo sounder. The survey showed the presence of two distinct shoals 20m off the coastline that are responsible for the resulting coarse sediment transport. These results confirm that even mild sea bottom irregularities may significantly affect sediment transport dynamics along a beach, suggesting that accurate planning for defense schemes such as artificial coarse replenishment must begin with an in-depth knowledge of the morphology of the sea floor. Such information may increase the durability and/or the efficiency of the intervention. [Copyright &y& Elsevier]

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]

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: Coherent structures develop due to an instability process on smooth beds under turbulent flow conditions and they are relevant for the incipient motion of fine sand sediment. Similar flow structures were found to be relevant for the sediment transport under rough wall conditions, but an instability process could not explain them. In this article we show that rough walls produce a new class of Λ-vortices, these vortices develop from separations on the grains, and are new relevant flow structures. [Copyright &y& Elsevier]

Abstract: This study examines the hydrodynamic performance of a wave absorbing double curtain-wall breakwater. The breakwater consists of a seaward perforated wall and a shoreward impermeable wall. Both walls extend from above the seawater to some distance above the seabed. Then the below gap allows the seawater exchange, the sediment transport and the fish passage. By means of the eigenfunction expansion method and a least square approach, a linear analytical solution is developed for the interaction of water waves with the breakwater. Then the reflection coefficient, the transmission coefficient and the wave forces acting on the walls are calculated. The numerical results obtained for limiting cases agree very well with previous predictions for a single partially immersed impermeable wall, the double partially immersed impermeable walls and the bottom-standing Jarlan-type breakwater. The predicted reflection coefficients for the present breakwater also agree reasonable with previous experimental results. Numerical results show that with appropriate structure parameters, the reflection and transmission coefficients of the breakwater may be both below 0.5 at a wide range of the relative water depth. At the same time, the magnitude of wave force acting on each wall is small. This is significant for practical engineering. [Copyright &y& Elsevier]

Abstract: Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale. [Copyright &y& Elsevier]