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Abstract: Groundwater responses to swash motions were observed on a natural gravel beach at Slapton Sands, UK. Detailed hydrodynamic measurements of swash depth, flow velocity and groundwater elevation were made using a cross-shore array of pressure transducers and current metres. The groundwater-level remained closely linked to the tidal elevation throughout the flood and ebb stages of the tide and prevented the formation of an elevated seepage face. The swash and groundwater spectra were dominated by a broad band of incident-frequency energy, but both displayed an increasing contribution of low-frequency energy moving landwards. A behavioural divergence was identified across the swash zone as groundwater motions were observed when no swash is overhead on the beach face. Fourier analysis and cross-correlations assisted to identify the role of two distinct pathways in the transferral of fluid from the swash to the groundwater. One that causes instantaneous swash-frequency groundwater-level rises via an energy flux through the saturated lower beachface. A second pathway, lagged by up to 4s, was caused by vertical infiltration from large swashes across the unsaturated upper beachface. Infiltration across the upper-swash zone causes the backwash discharge to be weakened by 50% compared to the uprush discharge, severely limiting the ability of the backwash to transport sediment. The lack of tidal-scale groundwater-level asymmetry ensures that the upper beachface remains unsaturated throughout the whole tidal cycle and allows continued infiltration. It is concluded that this persistent infiltration has important consequences for the morphological changes commonly observed on gravel beaches by allowing accretion in the upper-swash at all stages of the tide. [Copyright &y& Elsevier]

Abstract: The eastern Bay of the Seine (English Channel) was the subject in 1991 of a sampling survey of superficial sediments. Geostatistic tools were used to examine the complexity of the spatial distribution of the fine-grained fraction (<50μm). A central depocentre of fine sediments (i.e. content up to 50%) oriented in a NW–SE direction in a muddy coastal strip, in a very high energy hydrodynamical situation due to storm swells and its megatidal setting, is for the first time recognised and discussed. Within this sedimentary unit, the distribution of the fine fraction is very heterogeneous, with mud patches of less than 4000m diameter; the boundary between these mud patches and their substratum is very sharp. The distribution of this fine fraction appears to be controlled by an anticyclonic eddy located off the Pays de Caux. Under the influence of this, the suspended material expelled from the Seine estuary moves along the coast and swings off Antifer harbour, towards the NW. It is trapped within this eddy because of the settling of suspended particulate matter. Both at a general scale and a local scale the morphology (whether inherited or due to modern processes) has a strong influence on the spatial distribution of the fine fraction. At the general scale, the basin-like shape of the area facilitates the silting, and the presence of the submarine dunes, called “Ridins d’Antifer”, clearly determines the northern limit of the muddy zone. At a local scale, the same influence is obvious: paleovalleys trap the fine sediments, whereas isolated sand dunes and ripples limit the silting. This duality of role of the morphology is therefore one of the reasons why the muddy surface is extremely heterogeneous spatially. The presence of an important population of suspension feeding echinoderm, the brittle-star Ophiothrix fragilis Abildgaard, has led to a local increase in the silting, and to the modification of the physicochemical and sedimentological parameters. A complex relationship is shown to occur between the amount of fine fraction and the number of brittle-stars (ind.m−2). Classical statistical methods are not appropriate to study the spatial distribution of the mud fraction, because the spatial component of the percentage of the distribution is not integrated in the analysis. On the other hand, this is the main property of the geostatistic concepts. The use of geostatistic tools within a strict and clearly identified procedure enables the proposal of an accurate cartography. Further application of the proposed protocol (based on a semivariographic study and a conditional simulation interpolation) for surficial sediments mapping will help explain spatial and temporal variations of fine-grained fraction. Then assessments of sedimentation and erosion stages allow highlighting signature of environmental processes. [Copyright &y& Elsevier]

Abstract: Since their development for investigating sand transport in the surf zone more than 25 years ago, optical backscatter sensors (OBSs) have been in continual use for a wide variety of scientific, engineering, and environmental-monitoring applications. Many OBS designs have evolved in that time but they all work in the same simple way. A water sample is illuminated by a light source and photodetectors in the sensor convert the light scattered from the sample to photocurrent. The amount of photocurrent depends mainly on the illuminated area of the particles, and because particle volume is proportional to area, the photocurrent provides an indirect estimate of suspended-sediment concentration. The relationship between OBS signal and sediment concentration is almost linear for many suspended sediments and this is the most important “good” feature for OBS users. Many other factors influence OBS measurements, including: (1) particle size, shape, composition, and aggregation/flocculation; (2) dissolved light-absorbing matter; (3) bubbles; and (4) chemical and biological fouling. Some of these factors can be troublesome and can result in poor data quality or ambiguous results; these are “bad” ones. And a few of the same factors can have truly “ugly” consequences when their effects are ignored. Unknown time variation in particle size or aggregation/flocculation and fouling will nearly always result in inaccurate OBSs data. [Copyright &y& Elsevier]

Abstract: Instrumented bottom tripods have provided important data on sediment transport processes on continental shelves and in estuaries for four decades. Since the initial deployment in a tidal channel in Puget Sound, WA, in 1965 numerous tripods have been constructed to investigate bottom boundary layer and sediment dynamics worldwide. Tripod data have led to new understanding of near-bottom wave and current flows in the coastal ocean, and have been crucial to the development of shelf circulation and sediment transport models. Calculations of bottom stress, bottom roughness, and sediment flux that resulted directly from tripod data have been compared to bottom boundary layer model results. Where these have differed, new or revised model components have been developed to improve the skill of the models. The many discoveries that have been made from tripod experiments include dense, near-bottom fluid mud layers that transport large quantities of suspended sediment offshore into deeper regions of the continental shelf. This process has been linked to the seaward progradation of subaqueous deltas and to the boundaries of mid-shelf mud deposits off rivers with high fine-sediment discharge. [Copyright &y& Elsevier]

Abstract: An active upper-canyon system, Eel Canyon, was studied to determine its role as a conduit and/or sink for terrigenous material over decadal timescales and to assess the sedimentary record preserved by transport processes. These data are used to (1) link seasonal fluctuations in sediment transport and deposition to preserved stratigraphic signatures, and (2) assess sediment storage and removal in the upper Eel Canyon (100–850m water depth) over decadal timescales. Previous research has shown that upper thalwegs commonly experience gravity-driven flows during winter (November–March), due to increased sediment supply from Eel River flooding and intense storms that produce energetic wave/current conditions. Thick winter deposits composed of recently discharged fluvial sediment are formed in upper thalwegs, with distinct short- and long-lived radioisotopic and textural signatures (detectable 7Be and 234Thxs, lowered 210Pb activity, elevated clay content, and physical structures). Box and kasten cores were collected in the upper canyon (thalwegs and walls) to measure these signatures in recent and preserved winter deposits, and to calculate 100-yr accumulation rates. Non-bioturbated deposits (that have signatures indicative of rapid accretion by gravity-driven flows during the winter) are common in the upper canyon thalwegs. Short-lived radioisotopes (7Be and 234Th) show that sediment delivery to the upper thalweg varies temporally, sometimes beginning at the onset of river flooding, and at other times beginning during fall/early winter dry-storm events. In contrast, bioturbated deposits (which do not have signatures indicative of rapid deposition) are found on canyon walls. Non-bioturbated winter deposits are easily identified in the decadal record of thalwegs by decreases in 210Pb activity and increases in clay content. Stacking of multiple years of winter deposits (∼10cm preserved per winter) results in non-steady-state 210Pb profiles and high decadal accumulation rates. However, down-core changes in 210Pb profiles show that slope failures are actively redistributing these winter deposits. Partial or total removal of multiple winter deposits appears to happen periodically (every ∼13yr), which will inhibit preservation of the longer decadal record. 100-yr accumulation rates were calculated in the thalwegs from the resulting 210Pb profiles (i.e., the result of winter accretion and decadal removal by failures). Accumulation rates are much higher in thalwegs (1–6cm/yr) than walls (0.1–0.8cm/yr), which is likely the result of differing sediment delivery processes (via gravity-driven flows and nepheloid layers, respectively). At least 2.6±1.4% of the Eel River sediment budget is accumulating in the upper canyon over 100-yr timescales. However, this value greatly underestimates the total amount entering the canyon system because minimum accumulation rates were used in many areas (due to limited core length) and slope failures are moving sediment out of the budget area. [Copyright &y& Elsevier]

Abstract: Sediment delivered to coastal systems by rivers (15×109 tons) plays a key role in the global carbon and nutrient cycles, as deltas and continental shelves are considered to be the main repositories of organic matter in marine sediments. The Mississippi River, delivering more than 60% of the total dissolved and suspended materials from the conterminous US, dominates coastal and margin processes in the northern Gulf of Mexico. Draining approximately 41% of the conterminous US, the Mississippi and Atchafalaya river system deliver approximately 2×108 tons of suspended matter to the northern Gulf shelf each year. Unlike previous work, this study provides a comprehensive evaluation of sediment accumulation covering majority of the shelf (<150m water depth) west of the Mississippi Delta from 92 cores collected throughout the last 15 years. This provides a unique and invaluable data set of the spatial and modern temporal variations of the sediment accumulation in this dynamic coastal environment. Three types of 210Pb profiles were observed from short cores (15–45cm) collected on the shelf. Proximal to Southwest Pass in 30–100m water depths, non-steady-state profiles were observed indicating rapid accumulation. Sediment accumulation rates in this area are typically >2.5cmyr−1 (>1.8gcm−2 yr−1). Kasten cores (∼200cm in length) collected near Southwest Pass also indicate rapid deposition (>4cmyr−1; >3gcm−2 yr−1) on a longer timescale than that captured in the box cores. Near shore (<20m), profiles are dominated by sediments reworked by waves and currents with no accumulation (the exception is an area just south of Barataria Bay where accumulation occurs). The remainder of the shelf (distal of Southwest Pass) is dominated by steady-state accumulation beneath a ∼10-cm thick mixed layer. Sediment accumulation rates for the distal shelf are typically <0.7cmyr−1 (<0.5gcm−2 yr−1). A preliminary sediment budget based on the distribution of 210Pb accumulation rates indicates that 40–50% of the sediment delivered by the river is transported out of the study region. Sediment is moved to distal regions of the shelf/slope through two different mechanisms. Along-isobath sediment movement occurs by normal resuspension processes west of the delta, whereas delivery of sediments south and southwest of the delta may be also be influenced by mass movement events on varying timescales. [Copyright &y& Elsevier]

Abstract: A predominant sigmoidal clinoform deposit extends from the Yangtze River mouth southwards 800km along the Chinese coast. This clinoform is thickest (∼40m) between the 20 and 30m isobaths and progressively thins offshore, reaching water depths of 60 and 90m and distances up to 100km offshore. Clay mineral, heavy metal, geochemical and grain-size analyses indicate that the Yangtze River is the primary source for this longshore-transported clinoform deposit. 210Pb chronologies show the highest accumulation rates (>3cm/yr) occur immediately adjacent to the Yangtze subaqueous delta (north of 30°N), decreasing southward alongshore and eastward offshore. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in trapping most Yangtze-derived sediment on the inner shelf and transporting it to the south. [Copyright &y& Elsevier]

Abstract: A model for the emplacement, bioturbation, and preservation of fine-scaled sedimentary strata has been used to evaluate the impact of interacting physical and biological sedimentary processes on preserved sedimentary fabric in three depositional settings. The settings, which have contrasting hydrodynamic, geological, and biological characteristics, are Eckernförde Bay, German Baltic Sea; the Eel Shelf, northern California, USA; and the northern Gulf of Alaska continental shelf, USA. We have compared field measurements of sedimentation and bioturbation (from 7Be, 234Th, 210Pb, and 137Cs measurements), and observations of sedimentary fabric derived from core X-radiographs with model predictions of preserved sedimentary fabric. The one-dimensional model is forced by fluctuations in the instantaneous sedimentation rate, which deposits sediment with primary sedimentary fabric. Primary sedimentary fabric is then modified by bioturbation, which is represented as a first-order depth-limited reaction term, the rate of which is derived from radioisotopic and time-series estimates of bioturbation depth and intensity. Model output includes depth in seabed and corresponding preservation quotient, a measure of relative primary versus biogenic fabric. Although measured rates and depth scales of sedimentation and bioturbation vary across the three study areas by more than a factor of ten, model results strongly resemble actual sedimentary fabric in core X-radiographs in each case. Our results support and expand on concepts that invoke episodic sedimentation and depth-dependent bioturbation as important competing factors in the preservation or destruction of primary depositional fabric, and also suggest that radioisotopic estimates of bioturbation rates can adequately portray natural conditions. [Copyright &y& Elsevier]

Abstract: Recent field observations from several shelf environments show that gravity-driven transport within negatively buoyant layers is an important mode of fine sediment transport across continental shelves. Specifically, Dick Sternberg, along with his students and colleagues, stimulated a paradigm shift by reporting strong evidence from the Amazon and Eel shelves that hyperpycnal layers do not require autosuspension for sustenance but can be initiated by sediment flux convergence and supported by wave and current-induced suspension within relatively thin near-bed layers. As these layers move downslope under the influence of gravity, they may deposit sediment in response to decreases in bottom orbital velocities, near-bed current velocity, and/or bed slope. Direct or indirect evidence for wave or current supported sediment gravity flows has recently been reported off other high-load rivers including the Atchafalaya, Fly, Ganges–Brahmaputra, Klamath, Mad, Mississippi, Po, Rhone, Waiapu, Waipaoa, Yangtze, and Yellow among others. Growing evidence from observational and modeling studies suggests that flux convergence followed by wave and current supported gravity driven transport is a primary cause of across-shelf transport and emplacement of flood deposits on many muddy shelves and may be a major contributor to and control on the large-scale formation and morphology of subaqueous deltas and shelf clinoforms. Recent and ongoing studies on this subject are synthesized in this paper and recommendations are offered for further study. [Copyright &y& Elsevier]

Abstract: The eastern part of the chenier plain of the Louisiana coast has been prograding seaward over the last few decades while much of the rest of the Louisiana coast is experiencing high erosion rates. The source of sediment is the Atchafalaya River, which has been delivering sediment to the coastal ocean since the 1940s. Researchers have suggested that the repeated passage of cold fronts during winter and early spring plays an important role in delivering sediment to the coast. A sediment-transport study on the Atchafalaya coast was conducted between October 1997 and March 2001, which included several field experiments in early March, the period of high discharge from the Atchafalaya and frequent cold-front activity. A combination of shipboard profiling and time-series measurements from a bottom tripod and array of wave sensors on the inner shelf has resulted in a data set that illustrates the mechanism of onshore transport. For a cold-front passage sampled in 2001, during pre-front conditions, sediment is resuspended and mixed throughout the water column, with transport rates onshore and to the west of 53 and 184gs−1 m−1, respectively. Post-front conditions also result in onshore transport due to onshore flow (upwelling) in the lower meter of the water column and formation of a high-concentration bottom layer. Post-front onshore transport rates are 32gs−1 m−1 and most of the transport occurs in the bottom meter of the water column. The repeated cycling of cold-front passages leads to a positive feedback with transport onshore during both pre- and post-front conditions, and effective attenuation of wave energy over the muddy inner shelf inhibits erosion at the coast. Thus, the chenier-plain coast is experiencing high progradation rates (up to 29myr−1), while most of the Gulf coast is eroding. [Copyright &y& Elsevier]

Abstract: A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142dyr−1 over the total record, ranging between 89d in the Major Flood year (1996/97) and 171d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125cms−1. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30cmab, ranged from values close to 0–8gl−1. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods. [Copyright &y& Elsevier]

Abstract: Shear velocity was estimated from current measurements near the bottom off Grays Harbor, Washington between May 4 and June 6, 2001 under mostly wave-dominated conditions. A downward-looking pulse-coherent acoustic Doppler profiler (PCADP) and two acoustic-Doppler velocimeters (field version; ADVFs) were deployed on a tripod at 9-m water depth. Measurements from these instruments were used to estimate shear velocity with (1) a modified eddy-correlation (EC) technique, (2) the log-profile (LP) method, and (3) a dissipation-rate method. Although values produced by the three methods agreed reasonably well (within their broad ranges of uncertainty), there were important systematic differences. Estimates from the EC method were generally lowest, followed by those from the inertial-dissipation method. The LP method produced the highest values and the greatest scatter. We show that these results are consistent with boundary-layer theory when sediment-induced stratification is present. The EC method provides the most fundamental estimate of kinematic stress near the bottom, and stratification causes the LP method to overestimate bottom stress. These results remind us that the methods are not equivalent and that comparison among sites and with models should be made carefully. [Copyright &y& Elsevier]

Abstract: A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10min and wavelengths of 200–400m, and caused downward excursions of the thermocline of as much as 30m. At the 85-m site, the current measured 1m above bottom (mab) typically increased from near 0 to 0.2m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6mab increased from about 0.1 to 0.4–0.6m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves. [Copyright &y& Elsevier]

Abstract: The Aquitaine coast, the southern part of the French Atlantic coast, is exposed to very energetic waves, inducing large sediment transport rates and shoreline changes. Local wave characteristics are, however, not accurately defined. Neither is the longshore drift along the shore. In this paper, statistical parameters of directional waves are determined by using outputs over 8 years from numerical model WAVEWATCH III. Wave statistics obtained point out a very weak variability of peak directions, almost all events coming from the same 300° direction, highly energetic extreme events and a predominance of swells over wind sea states. Representative sea states are calculated using a classification process. They are set as boundary conditions for the numerical model SWAN to compute the wave parameters along the breaking line and the corresponding longshore drift with the Kamphuis formula. The net annual longshore drift is assessed by taking into account the occurrence of the wave classes. Results show a reversal of the longshore current at the town of Capbreton. To the north, the current is directed southwards while it is northwards but very weak to the south. Locally, the pattern is reversed due to the influence of the submarine canyon of Capbreton. This new result is supported by in situ current measurements carried out along the shore. The annual average amount of sediment transported longshore is about 40,000m3/year to the north of Capbreton and only 1000m3/year to the south. However, along this latter area, the drift shows larger variations than in the north. Energetic swells account for more than two-thirds of the net annual longshore drift while they only occur 25% of the year. Moderately energetic swells and sea states with higher steepness, though more frequent, contribute less to the total drift. Around the town of Capbreton, results are dependent on the frequency spectrum set at the boundary of the numerical model, highlighting the complex role played by the submarine canyon and suggesting the need for further investigations in that area. [Copyright &y& Elsevier]

Abstract: Pressure-gradient driven, advective flows through permeable sediments are recognized as an important mechanism by which dissolved and particulate materials are exchanged between the near-bottom water and the seabed. Here, we focus on the deposition of particulate organic matter in wave-formed, rippled beds representative of permeable coastal ocean sediments. In addition, we examine the release of previously deposited material when sediment transport is initiated. Laboratory experiments were conducted in an oscillatory water tunnel using cultured diatoms (4–6μm dia.) as a tracer for particulate organic matter. Using different ripple geometries on a coarse sand bed (median dia.=575μm) and following deposition under controlled flow conditions, we cored extensively to determine the distribution and flux of diatoms to the bed. Results showed greater diatom deposition (3.5×) and penetration depths (4–5cm) in ripple troughs in comparison to ripple crests (<2cm), qualitatively in agreement with past model predictions and experimental results. Both this pattern and the total diatom flux were consistent over a 3-fold variation in ripple wavelength (17–50cm) and a 4-fold variation in ripple height (1.4–7.7cm). The total diatom flux was only marginally enhanced by the presence of ripples (∼1.6×) compared to a flat bed. Our data suggest that the total diatom flux may be less controlled by rippled bed topography than solute exchanges, including but not limited to advective porewater flows. Repeatability between replicate depositional runs allowed us to conduct resuspension experiments in which rippled beds previously impregnated with diatoms were eroded and allowed to establish a new ripple configuration. Approximately half (45–60%) of the embedded diatoms were released during reconfiguration of the ripples, a process that lasted 20min. Remaining diatoms were buried comparatively deeper in isolated pockets, the location a function of ripple position both before and after sediment resuspension. Our results clearly indicate that small particle deposition to a rippled bed is a simple pattern predicted by models and prior experiments but that the total flux is not strongly determined by ripple geometry. However, the net depositional pattern in the real ocean will be determined by the combined effects of local sediment erosion and deposition, and thus these two processes act in tandem to lead to substantial, ripple-scale heterogeneity of organic matter input to shelf sediments in both the horizontal and vertical dimensions. [Copyright &y& Elsevier]

Abstract: Sand transport models are often based on semi-empirical equilibrium transport formulae that relate sediment fluxes to physical properties such as velocity, depth and characteristic sediment grain sizes. In engineering applications, errors in these physical properties affect the accuracy of the sediment fluxes. The present analysis quantifies error propagation from the input physical properties to the sediment fluxes, determines which ones control the final errors, and provides insight into the relative strengths, weaknesses and limitations of four total load formulae (Ackers and White, Engelund and Hansen, van Rijn, and Karim and Kennedy) and one bed load formulation (van Rijn). The various sources of uncertainty are first investigated individually, in order to pinpoint the key physical properties that control the errors. Since the strong non-linearity of most sand transport formulae precludes analytical approaches, a Monte Carlo method is validated and used in the analysis. Results show that the accuracy in total sediment transport evaluations is mainly determined by errors in the current velocity and in the sediment median grain size. For the bed load transport using the van Rijn formula, errors in the current velocity alone control the final accuracy. In a final set of tests, all physical properties are allowed to vary simultaneously in order to analyze the combined effect of errors. The combined effect of errors in all the physical properties is then compared to an estimate of the errors due to the intrinsic limitations of the formulae. Results show that errors in the physical properties can be dominant for typical uncertainties associated with these properties, particularly for small depths. A comparison between the various formulae reveals that the van Rijn formula is more sensitive to basic physical properties. Hence, it should only be used when physical properties are known with precision. [Copyright &y& Elsevier]

Abstract: Headland-associated linear sandbanks are a specific type of sandbanks which are located in the lee of coastal topographic features such as headlands and islands. Interaction between tidal currents and topographic features generate complex three-dimensional circulation patterns that significantly influence the distribution of sediments in the vicinity of the feature. A morphological model, using a three-dimensional hydrodynamic model, has been developed in order to determine the mechanisms responsible for the formation of the sandbanks. The model has been applied to an idealised gaussian shaped headland and long-term simulation have been undertaken. Sandbanks are created on each side of the headland but are not located in the centre of the tidal residual eddy as suggested previously. The sandbanks modify the tidal current residual as they grow. Their growth is controlled by the tidal regime, the availability of sand, the sediment grain size and the presence of secondary flows. Sandbanks are formed due to the asymmetry in the sediment transport on each side of the bank, due to the reversing tidal flow. They initially develop as a circular shape and then evolve into a more elongated shape. [Copyright &y& Elsevier]

Abstract: This paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport and beach profile evolution can be successfully modelled. Beach groundwater hydrodynamics are a result of combined forcing from the tide and waves at a range of frequencies, and a large number of observations exist which describe the shape and elevation of the beach watertable in response to tidal forcing at diurnal, semi-diurnal and spring-neap tidal frequencies. Models of beach watertable response to tidal forcing have been successfully validated; however, models of watertable response to wave forcing are less well developed and require verification. Improved predictions of swash zone sediment transport and beach profile evolution cannot be achieved unless the complex fluid and sediment interactions between the surface flow and the beach groundwater are better understood, particularly the sensitivity of sediment transport processes to flow perpendicular to the permeable bed. The presence of a capillary fringe, particularly when it lies just below the sand surface, has influences on beach groundwater dynamics. The presence of a capillary fringe can have a significant effect on the exchange of water between the ocean and the coastal aquifer, particularly in terms of the storage capacity of the aquifer. Field and laboratory observations have also shown that natural groundwater waves usually propagate faster and decay more slowly in aquifers with a capillary fringe, and observations which suggest that horizontal flows may also occur in the capillary zone have been reported. The effects of infiltration and exfiltration are generally invoked to explain why beaches with a low watertable tend to accrete and beaches with a high watertable tend to erode. However, the relative importance of processes such as infiltration losses in the swash, changes in the effective weight of the sediment, and modified shear stress due to boundary layer thinning, are not yet clear. Experimental work on the influence of seepage flows within sediment beds provides conflicting results concerning the effect on bed stability. Both modelling and experimental work indicates that the hydraulic conductivity of the beach is a critical parameter. However, hydraulic conductivity varies both spatially and temporally on beaches, particularly on gravel and mixed sand and gravel beaches. Another important, but poorly understood, consideration in beach groundwater studies is the role of air encapsulation during the wetting of beach sand. [Copyright &y& Elsevier]

Abstract: Hydrodynamic forcing with respect to sediment transport and morphologic change, paying particular attention to relevant swash asymmetries, is reviewed. The hydrodynamics are categorized into their individual effects: high- and low-frequency motions, bores and turbulence, in/exfiltration, shear stresses and friction coefficients. Individual effects are then related to their potential for driving or influencing sediment transport and morphological change. Additional concepts such as settling/scour lag and sediment advection that have largely been ignored are also discussed. A simple framework is presented for the morphological response of the beachface under swash zone hydrodynamic processes. The framework acknowledges that the beachface cannot be considered in isolation from the surf zone and that the two zones are strongly linked through feedback processes. The swash zone itself is also a morphodynamic system and morphological response occurs as a result of disequilibrium between the beachface gradient and asymmetries in the swash hydrodynamics. Any beachface morphological development in response to such equilibrium will have direct and indirect effects on swash hydrodynamics and sediment transport processes. It is concluded that the two issues requiring most urgent research attention with regards to swash zone sediment transport processes are the roles of sediment advection and longshore swash motion. [Copyright &y& Elsevier]

Abstract: Field observations of instantaneous water surface slopes in the swash zone are presented. For free-surface flows with a hydrostatic pressure distribution the surface slope is equivalent to the horizontal pressure gradient. Observations were made using a novel technique which in its simplest form consists of a horizontal stringline extending seaward from the beach face. Visual observation, still photography or video photography is then sufficient to determine the surface slope where the free-surface cuts the line or between reference points in the image. The method resolves the mean surface gradient over a cross-shore distance of 5m or more to within ±0.001, or 1/20th −1/100th of typical beach gradients. In addition, at selected points and at any instant in time during the swash cycle, the water surface slope can be determined exactly to be dipping either seaward or landward. Close to the location of bore collapse landward dipping water surface slopes of order 0.05–0.1 occur over a very small region (order 0.5m) at the blunt or convex leading edge of the swash. In the middle and upper swash the water surface slope at this leading edge is usually very close to horizontal or slightly seaward. Behind the leading edge, the water surface slope was observed to be very close to horizontal or dipping seaward at all times throughout the swash uprush. During the backwash the water surface slope was observed to be always dipping seaward, approaching the beach slope, and remained seaward until a new uprush edge or incident bore passed any particular cross-shore location of interest. The observations strongly suggest that the swash boundary layer is subject to an adverse pressure gradient during uprush and a favourable pressure gradient during the backwash. Furthermore, assuming Euler''s equations are a good approximation in the swash, the observations also show that the total fluid acceleration is negative (offshore) for almost the whole of the uprush and for the entire backwash. The observations are contrary to recent work suggesting significant shoreward directed accelerations and pressure gradients occur in the swash (i.e., ∂u/∂t>0∼∂p/∂x<0), but consistent with analytical and numerical solutions for swash uprush and backwash. The results have important implications for sediment transport modelling in the swash zone. [Copyright &y& Elsevier]

Abstract: Field observations and numerical model simulations are used to investigate mechanisms of sediment transport in the inner-surf and swash zones. Both the detailed two-phase and (dilute) turbulent suspension model results suggest that sediment transport is in phase with the bottom stress and can be parameterized by a Meyer–Peter-type power law for typical sandy-beach grain sizes ( diameter ) and wave conditions (wave period ). However, comparison of bottom stress (and the resulting sediment transport) predicted from observed flows by the detailed models with that predicted by a quasi-steady model suggests that the phase lag between the bottom stress and the fluid forcing may be important under strongly pitched-forward, saw-tooth-shaped swell and sea waves. Bottom stress predicted by a boundary-layer model that accounts for flow turbulence, but not particle interactions, is similar to that from the two-phase model if a large roughness is used to compensate for neglected intergranular and fluid–sediment interactions. Preliminary analysis of field observations in the swash zone suggests that breaking-wave (surface) generated turbulence affects the near-bed flow during passage of the breaking wave (bore) front and may have significant effects on sediment transport. [Copyright &y& Elsevier]

Abstract: Laboratory data of free surface elevation and fluid velocities (cross-shore and vertical velocities) were obtained using a laser-Doppler velocimeter (LDV) for the case of a periodic wave plunging over an impermeable, steep (1:10) slope with a fixed bottom roughness (nominal diameter=2.2mm). The measurements were conducted over 14 cross-shore locations from the outer surf zone to the swash zone with approximately 10 vertical points at each location. The vertical resolution included measurements in the bottom boundary layer and some points above the trough level. Various hydrodynamic quantities, including ensemble-averaged turbulent kinetic energy (TKE) (〈k〉) and turbulence intensity (〈u′2〉 and 〈w′2〉), were estimated to better understand inner surf and swash zone hydrodynamics induced by strongly plunging waves. In the outer region of the bottom boundary layer, the ratio of the time-averaged horizontal turbulence intensity to the time-averaged vertical turbulence intensity agreed well with the case of plane wake turbulence . The average ratio near the bottom was larger which is closer to flows typified by the inner region of a boundary layer . In the surf zone, the ensemble-averaged horizontal fluid velocity (〈u〉) near the bottom leads in phase compared to 〈u〉 in the upper layer, and 〈u〉 at the bore front exceeds the theoretical wave celerity. At the impinge point, a strong return flow occurred which was greater in magnitude than the onshore directed velocity . In the surf zone, 〈k〉 was largest just below trough level with a forward shift in phase of the peak intensity. The turbulence energy was mostly dissipated after the passage of the crest at this location. 〈u〉 near the bottom was leading in phase with a sharp vertical gradient in 〈u〉 indicating that boundary layer processes may have been important. In the swash zone, the vertical gradient of 〈u〉 was relatively small compared to the vertical gradient of 〈u〉 in the surf zone and may be due to the effect that the strong downrush had on turbulent mixing. Finally, the time-averaged estimate of TKE was vertically uniform over the inner surf and swash zone. [Copyright &y& Elsevier]

Abstract: A volume-of-fluid Navier–Stokes solver (RIPPLE) was used to simulate inner surf and swash zone flow with a 3s wave period and wave height of 0.14m on a planar, 1:10 sloping beach (Iribarren number of 1.0). In addition to other hydrodynamic information, RIPPLE was used to provide high-resolution predictions of the pressure gradient and fluid velocity in the horizontal and vertical dimensions that served as forcing to a discrete particle model (DPM). Sediment transport processes in the inner surf and swash zones were simulated for a thin veneer of sediment particles over a 5m test section in the DPM. Coupling between RIPPLE and the DPM was one-way such that particle–particle and fluid–particle interactions in the DPM did not provide feedback to alter the flow predicted by RIPPLE. The numerical simulation showed strong sediment suspension localized under vortices that reach the bed. Interestingly, the bulk of the sediment located in the small-scale vortex originated from locations nearly 0.2m landward. These findings suggest that (1) sediment motion for a single swash event can be significant, (2) that sediment measured in suspension likely originates from locations other than the bed directly below the suspension plume suggesting the importance of sediment advection and (3) that sparse cross-shore measurements in the field will only sporadically capture localized suspension events. [Copyright &y& Elsevier]

Abstract: In the present work, the sediment transport rate in the swash zone is estimated using a Boussinesq model coupled with a porous flow model in order to incorporate the infiltration–exfiltration effects. The transport rate is estimated from a modified Meyer–Peter and Müller formula using different values the multiplier C for uprush and backwash to incorporate the observed higher transporting efficiency of the uprush due to the presence of pre-suspended sediment from the bore collapse. Alternatively, the depth-integrated transport equation for suspended sediment is solved. The comparison between model results and experimental data shows that both approaches are able to give satisfactory results. [Copyright &y& Elsevier]

Abstract: On the open coast of the Changjiang Delta, waves temporally play their dominant roles in shaping the tidal-flat profile, especially during typhoons. Detailed analyses are presented of the variations in grain sizes of surface sediments and bed level, measured in the summer of 1999 at Nanhui mudflats, south flank of the Changjiang Delta, China. Cross-shore variations in bed level are distinctly site-specific in response to waves. The site-specific erosion rates are related to local water depth, sediment properties, vegetation, and exposure time per semidiurnal tidal cycle. A great difference exists between the higher and lower intertidal mudflats bordered at the mean sea level (MSL): the higher section is dominated by continuous accretion, while the lower section is characteristic of dynamic changes in erosion and accretion phases. Swells play their more important roles in shaping the profile than local wind-driven waves at the study mudflat, where swells propagate onshore without great barriers’ damping and local winds are not highly strengthened by distant typhoons. Storm processes are greatly modulated by tides. The magnitude of erosion is greater by a weak storm during spring tides than a strong storm during neap tides. Significant changes in entrainment capacity of tidal currents from neap to spring tides account for the different erosion and accretion models of the intertidal mudflat. [Copyright &y& Elsevier]

Abstract: The complex three-dimensional tidal flow measured in the vicinity of a headland-associated linear sandbank near Cape Levillain and the Levillain Shoal (Shark Bay, Western Australia) is described. The measurements included moored instruments (current meters, ADCP), shipborne ADCP transects, and drifters. Eulerian and Lagrangian current measurements provided a detailed description of the hydrodynamics. Results showed the presence of strong currents near the headland, reversal of currents veering towards the bank during the flood and ebb, and the formation of a transient eddy around the bank at slack water. Throughout the tidal cycle, secondary currents were present at the headland, with secondary flow near the seabed directed towards the headland and surface secondary flow directed away from the headland. Near the bank, near-bed secondary flows were directed towards the centre of the bank on both flanks at both the flood and ebb phases of the tide. Based on the detailed hydrodynamic data, a conceptual model for sediment transport pathways near the sandbank is proposed. The sandbank appears to be part of a sand circulation cell where the sand circulates around the bank with exchanges between the sandbank and the headland. [Copyright &y& Elsevier]

Abstract: The boundary layer flow and the net sediment transport beneath asymmetrical waves have been examined by a turbulence boundary layer model in conjunction with a diffusion model for suspended sediments based on the linearized boundary layer equations with horizontally uniform forcing. The turbulence closure is provided by a high Reynolds number k–ε model. The flow model and sediment transport model have been verified against existing laboratory measurements for sheet flow conditions. Overall, these comparisons show that the mean (wave-averaged) velocity profile, sediment flux and total sediment transport (including mean bedload transport) are well predicted by the present model. A further investigation of the boundary layer structure shows the mechanisms causing the near-bed backflow (opposing the drift direction of the outer wave velocity). The effect of roughness and asymmetry on the mean boundary layer flow and sediment transport beneath asymmetrical waves have been investigated. The wave friction factor, the mean bedload transport and the mean velocity profiles are given for a wide range of near-bed wave excursion to bed roughness ratios. Moreover, for asymmetrical waves with the same mean drift velocity, but with different asymmetry, the wave with strongest asymmetry induces the largest mean sediment transport. This demonstrates the importance of wave asymmetry. [Copyright &y& Elsevier]

Abstract: Dating techniques based on the natural and anthropogenic radionuclides 210Pb and 137Cs were applied to the study of the sedimentation regime in Cienfuegos Bay, Cuba. Core samples were collected from different locations in the bay and analysed to evalute the accumulation rate. Results evidenced significant changes in the sedimentation rate during the last 40 years: the recent sediment accumulation rates (0.47–0.50gcm−2 yr−1) in the Northen basin are almost double those estimated before 1965 (0.30gcm−2 yr−1). The 210Pb profiles show significant deviations from a simple exponential decline and abrupt variations between 1966 and 1970. These irregularities match closely periods of changes in land use (intense de-forestation and regimentation of the Arimao and Caonao rivers in the late 1960s and early 1970s) and exceptional natural events (Hurricane “Camille” in 1969 and the intense rainfall of 1988) which occurred in Cienfuegos. The 137Cs and chlorite minerals profiles validate the results obtained from 210Pb dating and confirm the effect of exceptional events and changes in the natural hydrological regime of the bay during the past 40 years. [Copyright &y& Elsevier]

Abstract: This paper presents field investigation and numerical modelling of waves and wave-induced currents on a wave-dominated and non-alongshore uniform multiple barred beach. This study aims at establishing the first analysis of the dynamics of horizontal flows on the French Aquitanian coast. The spectral wave program SWAN is coupled with the time- and depth-averaged (2DH) coastal area model MORPHODYN. This coupled-model is applied to Truc Vert Beach, and results are compared with field data. From the 14th to the 19th of October 2001, a field experiment was carried out in order to characterize hydrodynamics and sediment transport over a complex bathymetry in the presence of oceanic wave conditions. From this data we calibrated three parameters: the bottom friction for wave propagation from the Aquitanian continental shelf to the nearshore zone, the spatially constant bottom friction coefficient due to waves and currents, and lateral mixing. Despite model approximations and the fact that the offshore wave boundary condition was located 15km off the coast, the model is in good agreement with measurements. During weak wind conditions, computed waves and longshore currents fit well with field data on the ridge and runnel system. The strong tidal modulation of surf zone processes over this system is revealed. Hydrodynamics are strongly controlled by the beach morphology. For near-normally incident swells, the ridge and runnel system is responsible for a strong rip current located at the runnel outlet, associated with a circulation cell. Prediction of the tidal modulation and the sensitivity of the rip current to offshore wave conditions are in agreement with observations. Maximum rip current flow velocities occur approximately at mid-tide, which differs from what most researchers have found in other environments. [Copyright &y& Elsevier]

Abstract: Biogeochemical data from four sequential sediment traps deployed for one full year (April 1987–March 1988) at the shelf edge (200m isobath) of the Canadian Beaufort Sea are presented. In addition, multi-traps and Kenney traps, which collect material suspended near the bottom, provided data from the inner shelf for short intervals in spring and summer. A novel aspect of this work is the discrimination of biogenic and terrigenous contributions to the total carbon using a relationship between terrigenous carbon and aluminum (Al). The vertical flux in the Beaufort Sea derives primarily from three sources: marine biological production, the Mackenzie River Plume, and coastal and seabed erosion. The material trapped at all shelf edge sites was predominantly biogenic and the seasonal fluxes of both biogenic and terrigenous matter varied with geographical location. In open-water conditions, deposition of terrigenous material can be attributed at various times and places to (1) transport by the Mackenzie Plume to the shelf edge during freshet; (2) resuspension of sediment during north-westerly gales; and (3) erosion of steep coastlines during south-easterly gales followed by northward transport. On the west side of the shelf, the biogenic and terrigenous fluxes were highest in summer and were primarily influenced by the Mackenzie River freshet, increased solar flux, available nutrients, sea ice break-up, and water column stability. On the east side of the shelf, biogenic and terrigenous fluxes were highest in the fall and the most important controlling factors were the extensive open water, intense resuspension events accompanying storms from the northwest, and the flow of the fresh, warm, turbid waters of Mackenzie River to the northeast. There was strong evidence of sediment transport in mid-water and bottom layers, but the mechanisms involved cannot be inferred from this study. Stable isotopes of carbon and nitrogen, C/N ratios, biogenic silica, and chlorophyll a measurements help to distinguish between marine autotrophic production and heterotrophic production, of which the latter appears to dominate in the fall and continue beneath the ice into the polar night. Al and iron correlate strongly and are primarily associated with the terrigenous fraction whereas calcium and phosphorus have both terrestrial and biogenic associations. [Copyright &y& Elsevier]

Abstract: A LISST-100 in situ laser particle sizer was deployed together with a digital floc camera during field work in the Newark Bay area (USA) and along the Apennine margin (the Adriatic Sea, Italy). The purpose of these simultaneous deployments was to investigate how well in situ particle (floc) sizes and volume concentrations from the two different instruments compared. In the Adriatic Sea the two instruments displayed the same temporal variation, but the LISST provided lower estimates of floc size by a factor of 2–3, compared to the DFC. In the Newark Bay area, the LISST provided higher values of floc size by up to a factor of 2. When floc size was computed using only the overlapping size bins from the two instruments the discrepancy disappeared. The reason for the discrepancy in size was found to be related to several issues: First, the LISST measured particles in the 2.5–500μm range, whereas the camera measured particles in the 135–9900μm range, so generally the LISST should provide lower estimates of floc size, as it measures the smaller particles. Second, in the Newark Bay area scattering from particles >500μm generally caused the LISST to overestimate the volume of particles in its largest size bin, thereby increasing apparent floc size. Relative to the camera, the LISST generally provided estimates of total floc volume that were lower by a factor of 3. Factors that could explain this discrepancy are errors arising from the accuracy of the LISST volume conversion coefficient and image processing. Regardless of these discrepancies, the shapes of the size spectra from the instruments were similar in the regions of overlap and could be matched by multiplying with an appropriate correction coefficient. This facilitated merging of the size spectra from the LISST and the DFC, yielding size spectra in the 2.5–9900μm range. The merged size spectra generally had one or more peaks in the coarse end of the spectrum, presumably due to the presence of flocs. The fine end (<100μm) of the spectrum displayed a flat tail with equal concentration of particles in all size classes. Size spectra with this shape indicate that the classical Junge model for description of in situ particle size spectra is reasonable for particles smaller than 100μm but not for larger particles. Floc fraction was computed for the merged spectra by using a diameter-to-mass conversion and found to vary between 0.34 and 0.95, within the range reported by other authors. [Copyright &y& Elsevier]

Abstract: Rippled scour depressions (RSDs) are distinct morphologic and textural features that have been observed in numerous nearshore environments as depressions containing coarse rippled sediments. The nature and significance of these features can be better understood with the use of multibeam sonar systems that acquire high-resolution bathymetry and co-registered backscatter intensity data and reveal both the small-scale characteristics of RSDs and larger-scale seabed patterns associated with them. In this paper, we compare RSDs identified in high-frequency (300kHz) multibeam sonar surveys on the inner shelves of southwest Washington, northern California and Long Island, New York. RSDs at these sites are evident as distinct features that are generally up to 1m deeper than the adjacent seabed and exhibit high acoustic backscatter intensity due to the presence of coarse sediments, typically gravel. Shore parallel oscillatory ripples (typically 1m wavelength) exist within the coarse material associated with RSDs and are attributed to wave-orbital motion. We propose a classification of RSDs based on their morphology and textural characteristics as well as the larger-scale seabed patterns associated with them, which together provide insight about the processes that affect their development and evolution. The RSDs described here are apparently the result of localized flow that can redistribute significant volumes of sediment in the nearshore zone under a range of energetic regimes over relatively short time scales. One type of linear shore-normal RSD always corresponds to larger-scale low-amplitude transverse bedforms that appear to be related to longshore currents. Some of these low-amplitude bedforms migrated as much as 50–150m over 2 years. Another type of elongate RSD, appears to be relatively stable over time, is characterized by bathymetric and textural asymmetry with one distinct upcurrent edge and one diffuse downcurrent edge, and is apparently a result of transverse flow. Other irregularly shaped nearshore RSDs that can be created and destroyed on time scales of less than 1 year, are the result of localized erosion and accretion of up to 1m, and are distributed rhythmically along-shore in water depths of 8–16m suggesting that they are related to hydrodynamics associated with the breaker zone. The characteristics of RSDs elucidated by high-frequency multibeam sonar data provide evidence of a variety of physical processes associated with RSDs and emphasize the importance of these features with respect to sediment transport on the inner shelf. [Copyright &y& Elsevier]

Abstract: Predictions of nearshore depth evolution using process-based numerical simulation models contain inherent uncertainties owing to model structural deficiencies, measurement errors, and parameter uncertainty. This paper quantifies the parameter-induced predictive uncertainty of the cross-shore depth evolution model Unibest-TC by applying the Bayesian Generalised Likelihood Uncertainty Estimation methodology to modelling depth evolution at Egmond aan Zee (Netherlands). This methodology works with multiple sets of parameter values sampled uniformly in feasible parameter space and assigns a likelihood value to each parameter set. Acceptable simulations (i.e., based on parameter sets with a nonzero likelihood) were found for a wide range of parameter values owing to parameter interdependence and insensitivity. The 95% uncertainty prediction interval of bed levels after the 33 days prediction period was largest (0.5–1m) near the sandbar crests that characterize the Egmond depth profile, reducing to near-zero values in the sandbar troughs and the offshore area. The prediction interval built up during storms (when sediment transport rates are largest) and remained the same or even reduced slightly during less-energetic conditions. The prediction uncertainty ranges bracket the observations near the inner-bar crest, its seaward flank, and at the seaward flank of the outer bar, suggesting that elsewhere model structural errors (and, potentially, measurement errors) dominate over parameter errors. The interdependence and the non-Gaussian marginal posterior distribution functions of the free model parameters cast doubt on the ability of commonly applied multivariate normal distribution functions to estimate parameter uncertainty. [Copyright &y& Elsevier]

Abstract: A numerical modelling study on the effects of sediment-induced stratification on tidal currents is conducted for an idealized tidal estuary with a muddy seabed. The numerical model is a three-dimensional oceanic sediment transport model capable of predicting dynamics of Newtonian fluid mud layers due to coupling of the sea water density and suspended sediment concentration. The model uses a reparameterized bottom drag coefficient that incorporates a linear stability function of flux Richardson number Rf . The study has shown that the sediment-induced stratification in the bottom boundary layer (BBL) reduces the vertical eddy viscosity and bottom shear stress in comparison with the model prediction in a neutrally stratified BBL. In response to these apparent reductions, the tidal current shear in the water column is increased in the estuary. [Copyright &y& Elsevier]

Abstract: The Red River, annually transporting 100 million tons of sediment, flows into a shallow shelf sea where it rapidly deposits most of its sediment on a prograding delta front. Oceanographic cruises were carried out in February–March and July–August 2000 to determine the vertical structure of the Ba Lat river plume and sediment transport patterns on the delta front. The surface waters in the coastal zone were strongly stratified with a low density and high sediment concentration during the larger part of the wet season, caused by low mixing rates of river plumes with ambient water. The river plume is advected to the south by a well-developed coastal current which originates from the river plumes that enter the Gulf of Tonkin North of the Ba Lat and are deflected southward by the Coriolis force. Sediment predominantly leaves the surface plume by settling from suspension and less by mixing of fresh and marine water. A one-dimensional model for plume deposition valid for fair weather conditions indicates that most sediment is deposited within 10km and southward of the river mouth. Of prime importance for this depositional pattern is the phase relation between river outflow and tidal currents, in combination with the southward surface flow; alongshore advection is very low during outflow of the turbid river plume. The agreement of modeled plume sedimentation patterns with long-term bathymetric changes strongly suggests that fair weather depositional processes determine delta front development. This may be related to the fact that reworking of sediment mainly occurs several months after the peak deposition period; in the meantime sediment compaction and consolidation have increased the shear strength of deposited sediments. [Copyright &y& Elsevier]

Abstract: Concurrent video images of sand ripples and current meter measurements of directional wave spectra are analyzed to study the relations between waves and wave-generated sand ripples. The data were collected on the inner shelf off Huntington Beach, California, at 15m water depth, where the sea floor is comprised of well-sorted very fine sands (D 50=92μm), during the winter of 2002. The wave climate, which was controlled by southerly swells (12–18s period) and westerly wind waves (5–10s period), included three wave types: (A) uni-modal, swells only; (B) bi-modal, swells dominant; and (C) bi-modal, wind-wave dominant. Each wave type has distinct relations with the plan-view shapes of ripples that are classified into five types: (1) sharp-crested, two-dimensional (2-D) ripples; (2) sharp-crested, brick-pattern, 3-D ripples; (3) bifurcated, 3-D ripples; (4) round-crested, shallow, 3-D ripples; and (5) flat bed. The ripple spacing is very small and varies between 4.5 and 7.5cm. These ripples are anorbital as ripples in many field studies. Ripple orientation is only correlated with wave directions during strong storms (wave type C). In a poly-modal, multi-directional spectral wave environment, the use of the peak parameters (frequency, direction), a common practice when spectral wave measurements are unavailable, may lead to significant errors in boundary layer and sediment transport calculations. [Copyright &y& Elsevier]

Abstract: A series of sand bed experiments was carried out in the Large Wave Flume in Hannover, Germany as a component of the SISTEX99 experiment. The experiments focussed on the dynamic sediment response due to wave group forcing over a flat sand bed in order to improve understanding of cross-shore sediment transport mechanisms and determine sediment concentrations, fluxes and net transport rates under these conditions. Sediment concentrations were measured within the sheet flow layer (thickness in the order of 10 grain diameters) and in the suspension region (thickness in the order of centimetres). Within the sheet flow layer, the concentrations are highly coherent with the instantaneous near-bed velocities due to each wave within the wave group. However, in the suspension layer concentrations respond much more slowly to changes in near-bed velocity. At several centimetres above the bed, the suspended sediment concentrations vary on the time scale of the wave group, with a time delay relative to the peak wave within the wave group. The thickness of the sheet flow changes with time. It is strongly coherent with the wave forcing, and is not influenced by the history or sequence of the waves within the group. The velocity of the sediment was also measured within the sheet flow layer some of the time (during the larger wave crests of the group), and the velocity of the fluid was measured at several cm above the sheet flow layer. The grain velocity and concentration estimates can be combined to estimate the sediment flux. The estimates were found to be consistent with previous measurements under monochromatic waves. Under these conditions, without any significant mean current, the sediment flux within the sheet flow layer was found to greatly exceed the sediment flux in the suspension layer. As a result, net transport rates under wave groups are similar to those under monochromatic waves. [Copyright &y& Elsevier]

Abstract: Geological studies offshore of Wrightsville Beach, North Carolina reveal subtle large-scale regions of coarse sand with gravel and shell hash (widths between 100 and 200m and negative relief of ∼1m) that trend obliquely to the coast. It was previously suggested that these regions serve as conduits for sand exchange between the shoreface and inner shelf during storm-associated downwelling. Consequently they were classified as rippled scour depressions. More recently, the role of alongshore flows and self-organization as a result of inhibited settling of fine sand has been discussed. In this study, 45 days of near-bed current measurements were analyzed using benthic boundary layer and sediment transport models to examine the role of along- and cross-shore flows in driving sediment transport at this site. The wind climate was found to be a dominant influence on near-bed flows. Six distinct sediment transport events were recognized. During these events, sediment transport models show that bedload transport is directed mainly in the cross-shore direction, while suspended sediments are directed alongshore to the southwest. Current observations during these sediment transport events provide no evidence of cross-shore sediment transport caused by steady downwelling currents. Instead, benthic boundary layer model results are used to show that differences in bed roughness between the coarse areas of the seabed within the “rippled scour depressions” and the finer areas of the inner shelf are more pronounced during increasingly energetic wave and current conditions. The enhanced difference in roughness results in increased turbulence intensities over coarse regions inhibiting the deposition of the fine sand that is resuspended over the shelf during these events relative to finer areas over the shelf. Findings from this study contribute to explaining the observed long-term persistence of these features. [Copyright &y& Elsevier]

Abstract: Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10gl−1 from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100cm above the bed) drop off from around 100cms−1 within the distributary channels to &#60;50cms−1 on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35h to traverse the 20-km-wide, low-gradient (2×10−3 degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates. [Copyright &y& Elsevier]

Abstract: Sedimentological parameters of 206 box-core tops and 387 van Veen grab samples were analysed so as to understand the distribution, source and sedimentation rates in the shelf and upper slope of the southern Southeastern Brazilian margin. Most of the parameters (bulk organic matter constituents, δ13C, and ɛNd) can be grouped on northern and southern sectors, separated by a sharp boundary. This suggests that sediment sources and sedimentary processes are different in both regions. The southern sector of the study area is more influenced by cold waters coming from the southern portion of the South American shelf and the organic fraction revealed an important role played by the primary productivity in the sedimentation. Also, ɛNd values indicate that part of the inorganic fraction of the southern sector sediments are allochtonous, probably coming from younger rocks of the Andean chain probably reaching the shelf through the La Plata river runoff. In the northern part of the study area the sedimentation is controlled almost exclusively by the meandering of Brazil Current. Compositional and isotope bulk organic parameters showed a more complex mixing of terrigenous and pelagic fractions. Also, ɛNd values are associated with the precambrian rocks of the Brazilian shield. The whole area exhibits sedimentation rates, varying from 5 to 660mmkyr-1. [Copyright &y& Elsevier]

Abstract: This study looks at the complex interactions between sedimentology, morphology and hydrodynamics near a kink in an offshore tidal sandbank (Westhinder) and combines all these data to understand the kink''s characteristics. The Westhinder sandbank lies in the northern part of the Belgian continental shelf, where the main hydrodynamical agents are the tidal currents. The data sets include two multibeam surveys, 59 surficial Van Veen grab samples and a hydrodynamical and sediment transport model (set up by MUMM). The different data sets point to a triple division of the kink region. The parts north and south of the kink are covered with larger dunes, culminating at the bank''s crest into a symmetrical very-large dune. Here, the bank is actively maintained by a net sediment transport up both bank flanks and by higher residual water transport strengths. The kink part of the bank lies deeper, is characterized by a steeper eastern flank and by a faster and eastward movement of the very-large dunes over both bank flanks. Coarse sediments west of the kink reveal that the peak flood current is hindered by the changing orientation of the bank northward into a more rectilinear N–S position. The finest sediments are found on the lee slope and might be washed out from the kink''s stoss slope. A combination of all the results points to a possible breakthrough in the kink, a hypothesis which is worked out into a scheme. [Copyright &y& Elsevier]

The beach and nearshore area of Teignmouth (UK) has been the object of a multidisciplinary research project, COAST3D, carried out by 11 European institutions. This paper emphasises the sedimentological and morphological investigations of the intertidal and nearshore area using a variety of techniques. From the results presented, it is clear that the Teignmouth site is very dynamic. Sediment mobility is high during fair-weather conditions owing to the strong jet-like current flows associated with the presence of the narrow estuary mouth. The beach at Teignmouth can be categorised as composite with a dissipative low-tide terrace and a reflective upper beach. From surficial sediment sampling, selective transport processes were identified, imposed by the high current velocities at the estuary mouth. This process also leads to the formation of characteristic depositional lobes with poorly sorted sediment. From digital side-scan sonar mosaicing, cross-shore and longshore-oriented bedforms could be observed, with their characteristics being dependent on the pre-survey hydro-meteo conditions. Although the nearshore system is dominated by the jet-outflow, littoral drift processes can also be important. They proved to be important for beach morphological changes leading to a rythmic alongshore beach behaviour which was attributed to a reversal of the littoral drift imposed by a period of larger waves. Moreover, the influence of waves and associated longshore currents cannot be neglected as they form the link between different sedimentary environments. From combined side-scan sonar imagery and sediment trend analysis, residual sediment transport directions were identified. These showed that fair-weather and storm-dominated processes are needed to explain the textural and morphological differentiation at this site. Further analysis will be required to identify the different time-scales involved in producing these changes. [Copyright &y& Elsevier]