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We employed idealized numerical experiments using Delft3D to investigate how the interactions of coalescing buoyant river plumes with mangrove vegetation affect sediment transport and deposition in the coastal zone. Our model system, based on the Firth of Thames in the North Island of New Zealand, is designed to represent two mangrove-lined rivers debouching into a coastal bay. Sediment transport patterns into the three adjacent mangrove forest regions (central, western, and eastern) were assessed by quantifying the sediment fluxes over a tidal cycle. The interactions between the buoyant river plumes resulted in a coalesced river plume that flowed along the central mangrove forest between the rivers. The dominant contributors to the total sediment transport into the mangrove forests varied with both the total and the relative flows through the two rivers. When both rivers were forced with their respective flows at the 50th percentile of observed flow, the analysis of sediment transport fluxes along the central forest revealed sediment deposition inside the mangrove forests lining the river mouth nearly four times greater than in the intermediate regions along the central forest; consistent with satellite imagery of the Firth of Thames. Winds of 5 m s−1 modify the overall sediment transport patterns by deflecting the river plume in the direction of the wind. The magnitude of sediment flux is the highest (nearly 80% compared to the no-wind scenario) in the case of an easterly wind scenario as the alongshore transport is enhanced by Coriolis deflection and wind direction aligned with the alongshore flows of the larger plume. • Plume-to-plume interactions result in a coalesced river plume. • Sediment plume bulge of one river inhibits along-shore sediment transport of the other. • The presence of winds altered sediment transport patterns. [ABSTRACT FROM AUTHOR]

Variability in supply of terrestrial sediments to the coastal zone impacts sediment transport processes and ecological dependencies. This study investigated surface sediment distribution and transportation within a 2020 km2 shallow embayment following a period of flood induced sediment supply. Extensive field sampling of 221 common sites between 2015 and the 2019 follow-up survey presented, identified significant reduction in fine sediment fractions, evident through a 367 km2 decrease in surface mud coverage. Wind-wave sediment transport processes controlled the spatial distribution of fine sediments, exporting 23 million tonnes of mud from the shallow sedimentation zones. In a broader context, sensitive benthic habitats such as seagrass meadows exhibited signs habitat recolonisation coinciding with areas of reduced mud content. Despite these promising short-term outlooks, the developed fine sediment budget suggests ongoing terrestrial sedimentation will exhaust key sediment sinks within Moreton Bay, effectively reducing the system's ability to buffer future sedimentation events. • Significant reduction in surface mud within subtropical embayment. • Rapid reduction in sediment supply consistent with climate oscillations. • Wind-wave resuspension controlling surface mud recovery. • Moreton Bayshows some level of resilience to extreme flood events. [ABSTRACT FROM AUTHOR]

A calibrated, coupled model of the Bohai and Yellow Seas is developed to investigate sediment transport and deposition processes around a distinct clinoform east of the Shandong Peninsula. On the clinoform, deposition patterns vary seasonally with the regional hydrography and the East Asian Monsoon. A local maximum in sediment deposition is located offshore from spring to autumn, whereas sediment deposition is greater near the coast and decreases seaward in winter. From spring to autumn, a thermal front develops around the Yellow Sea Cold Water Mass and limits sediment transport seaward, enhancing sediment deposition and leading to high deposition rate in a focused region offshore. Sediment is transported to the region by the southward basin-scale cyclonic circulation, and the cross-shelf gradient in stratification, sediment resuspension, and vertical mixing at the front lead to a convergence in sediment transport and deposition. Sediment deposition over the clinoform is the greatest during the transition from summer to autumn when the thermal front persists but sediment supply increases. This results from weakened stratification and greater resuspension near the coast due to stronger winds, waves, and sea-surface cooling. A seasonal seaward shift of the frontal position toward deeper water is consistent with the decreasing heat flux, and the corresponding shift in location of maximum sediment deposition shifts seaward. In winter, the thermocline disappears and vertical mixing increases throughout the region. SSC over the clinoform increases due to active resuspension and net deposition decreases, and instead sediment is transported seaward and deposits in the South Yellow Sea. Current velocity, SSC and sediment deposition rates decrease seaward, in contrast to the focused trapping at the thermal front from spring to autumn. The deposition rates around the thermal front from spring to autumn are greater than in the winter, so the high net sediment accumulation over an annual cycle occurs in the same region as the longer-term accumulation preserved in the omega-shaped clinoform. • Sediment deposition rates are increased at a thermal front from spring to autumn at east of the Shandong Peninsula. • Seasonal shifts of the position of thermal front and sediment depo-center broaden the sediment accumulation region. • The monsoonal climate plays a dominant role in the seasonally varying circulation and sediment transport in the Yellow Sea. [ABSTRACT FROM AUTHOR]

Abstract Geochemical studies are becoming more and more frequent in the context of the increasing pressure of human activities on marine coastal ecosystems and represent an appropriate tool to assess anthropogenic disturbances. Moreover, it is difficult to find discriminant markers. The eastern part of the Bay of Seine (English Channel) is highly impacted by the presence of harbour activities, fishing and sediment extraction. Dredged sediment from the Grand Port Maritime du Havre (GPMH) are deposited at the subtidal Octeville site, in the north-eastern part of the mouth of the Seine estuary (mixed sediment area). To understand natural and anthropogenic sedimentary mechanisms in this area, a geochemical and sedimentological study was conducted at the beginning of 2016. A dense sampling campaign including 179 stations was carried out between Cap de La Hève and Cap d'Antifer. For comparison, sampling was carried out in the harbour (13 samples in basins strongly or very weakly dredged), in the dredged grab itself and in the Seine estuary sediment (one station in the brackish zone and two stations in the river freshwater zone). Elemental compositions were determined by X-Ray Fluorescence spectrometry and infrared spectrometry. Using PCA (Principal Component Analysis), seven constituents were selected (Si, As, S, Pb, Rb, organic Br and TOC) to determine the area influenced by dumped spoil and the sediment transport directions. Sediment areas in dynamic equilibrium display a TOC gradient perpendicular to the coastline, linked to granulometric variations due to a combination of the swell and tidal currents. In the study area, dredged sediment are finer grained and have undergone changes due to the influence of diagenetic processes characteristic of the harbour environment. As a result, these sediment are enriched in sulphides, Pb, TOC and Rb, which allows us to highlight the in-situ deposited sediment spoil. Dumped sediment and the area subject to their influence are clearly identified since they locally disrupt the natural dynamic equilibrium state. Highlights • Dynamics of deposit sediment studied in the eastern part of the Bay of Seine (English Channel). • New methodology to study coastal sediment dynamic equilibrium using geochemical tracers. • Dynamical equilibrium is identified by two opposite organic matter gradients controlled by swell and tide. • Pattern of TOC gradients disrupted, delimit the area disturbed by the dumped dredge spoil. [ABSTRACT FROM AUTHOR]

The processes of sediment entrainment, transport, and deposition over bedforms are highly dynamic and temporally and spatially variable. Obtaining measurements to understand these processes has led to ongoing developments in instrumentation for studying near-bed sediment dynamics, with the outputs applied to the development and assessment of sediment transport modelling. In the present study results are reported from three acoustic systems deployed to make observations of bedforms, bedload, suspended concentration and horizontal and vertical velocity components. To evaluate the instruments a series of near-bed boundary layer measurements were collected in a large scale flume facility over a rippled bed of medium sand under regular waves. The observations were conducted as part of Joint Research Activities within the EU funded Hydralab project. The suite of acoustic instruments consisted of a Bedform And Suspended Sediment Imager, BASSI, a three dimensional acoustic ripple profiler, 3D-ARP, and three Acoustic Concentration and Velocity Profilers, ACVP's. Here results are reported from the deployment of the instruments, to illustrate the ongoing developments in acoustics and the expanding capability of the application of acoustics to the study of near-bed flow and sediment dynamics. [ABSTRACT FROM AUTHOR]

Understanding estuarine hydrodynamics and sediment dynamics is of key importance to provide the foundation for sound management of these coastal systems. Turbidity maxima, which are zones of elevated suspended sediment concentration (SSC), are of particular interest as they control biogeochemical cycling and affect the overall environmental quality of the estuary. These turbidity maxima, however, are complex dynamic features that respond to changes in forcing conditions. In this study we use a 3D numerical model to investigate the response of hydrosedimentary dynamics to variations in river inflow and sea level rise in the Gironde estuary, which is one of the largest estuarine systems in Europe. Yearly simulations and comparisons with satellite data and measurements of salinity and SSC show that the model reproduces variations in salinity intrusion and the migration of the turbidity maximum driven by seasonal fluctuations in river inflow. Numerical experiments indicate that the formation of this dynamic turbidity maximum is mainly driven by tidal asymmetry. Density gradients play a secondary role by maintaining the stability of the suspended sediment mass. The model also simulates the presence of a secondary turbidity maximum which is more stable, consistent with observations. Evaluation of the sediment budget shows that sediment export mainly occurs during spring tides and when river discharge is high. Simulations including sea level rise suggest that salinity levels in the middle estuary will increase and rising water levels cause tidal amplification, strengthening of tidal currents and enhanced SSC levels in the upper estuary. On the other hand, the locations of the salinity front and the turbidity maximum remain relatively stable under rising water levels. Overall, our simulations suggest that decadal changes in river inflow can potentially have a larger effect on turbidity maximum dynamics than sea level rise. [ABSTRACT FROM AUTHOR]

Fluvial sediment is the major source for the formation and development of the Mekong Delta. This paper aims to analyse the dynamics of suspended sediment and to investigate the roles of different processes in order to explore flux pattern changes. We applied modelling on two scales, comprising a large-scale model (the whole delta) to consider the upstream characteristics, particularly the Tonle Sap Lake's flood regulation, and a smaller-scale model (tidal rivers and shelf) to understand the sediment processes on the subaqueous delta. A comprehensive comparison to in-situ measurements and remote sensing data demonstrated that the model is capable of qualitatively simulating sediment dynamics on the subaqueous delta. It estimates that the Mekong River supplied an amount of 41.5 mil tons from April 2014 to April 2015. A substantial amount of sediment delivered by the Mekong River is deposited in front of the river mouths in the high flow season and resuspended in the low flow season. A sensitivity analysis shows that waves, baroclinic effects and bed composition strongly influence suspended sediment distribution and transport on the shelf. Waves in particular play an essential role in sediment resuspension. The development of this model is an important step towards an operational model for scientific and engineering applications, since the model is capable of predicting tidal propagation and discharge distribution through the main branches, and in predicting the seasonal SSC and erosion/deposition patterns on the shelf, while it is forced by readily available inputs: discharge at Kratie (Cambodia), GFS winds, ERA40 reanalysis waves, and TPXO 8v1 HR tidal forcing. [ABSTRACT FROM AUTHOR]

This research examines whether or not mixes of clay and silt in a turbulent suspension act and settle independent of each other. More specifically, we examine the following three questions: (1) does the presence of silt in suspension alter the size of mud flocs relative to those from a pure clay suspension of equivalent concentration? (2) can silt particles become bound inside clay flocs?; and (3) if silt is bound within flocs, how does this change the settling properties of the clay and silt mixture? These questions are explored through a series of laboratory experiments in which: (a) images of flocs and silt particles within the turbulent suspension are used to measure their size distribution as a function of time; and (b) the settling velocity of the individual aggregates from each of the suspensions are measured in a settling column. The experiments use pure clay, pure silt, and two different mixture ratios of silt and clay. The results show three primary conclusions. First, the presence of silt has no significant impact on floc size. Second, most of the silt in suspension became bound up within the floc aggregates. And third, the bound silt within the flocs increased the floc settling velocity by at least 50%. These results have potential implications for the modeling of suspended mud mixtures in rivers, estuaries, and turbidity currents. [ABSTRACT FROM AUTHOR]

Sediment transport and depositional processes in the Southwest Atlantic Inner Shelf are strongly influenced by the contribution from Río de la Plata and Patos Lagoon fluvial systems. Understanding the behavior of erosive and depositional processes and bed evolution in the region is of utmost importance to address the impacts of anthropogenic activities, climate change and for navigation security. Therefore, this work aims to improve the understanding of such processes, and their temporal and spatial scales of variability. We employed the SISYPHE morphodynamic model coupled with the TELEMAC-3D hydrodynamic model to simulate the resulting depositional and erosional fluxes and the bottom evolution during a period of 8 years (2005–2012). Results indicate that during this period, depositional fluxes mostly exceeded erosional fluxes in the south of the Patos Lagoon mouth, resulting in predominantly positive bottom evolution. On the other hand, erosional fluxes exceeded depositional fluxes to the north of the Patos Lagoon mouth, resulting in predominantly negative bottom evolution. Widespread positive values of bottom evolution are observed along the northern and southern coasts off the Río de la Plata mouth, indicating a general dominance of sediment deposition over erosion. A wavelet analysis showed that fluvial discharge influences the variability of depositional process on an interannual time scale, whereas the synoptic variability is influenced by wind action. Moreover, positive ENSO periods (El Niño) are related to increased depositional fluxes, and decreased depositional fluxes during the negative phase (La Niña), with pronounced effects off Río de la Plata. • The study area presented predominantly positive bottom evolution. • Erosional fluxes exceeded depositional fluxes north of the Patos Lagoon mouth. • Positive values of bottom evolution are observed along coasts off the Río de la Plata mouth. • The fluvial discharge influences the deposition process on interannual time scale, while the wind action is related to the synoptic time scale variability. • The ENSO effect can increase the depositional fluxes during El Niño and decrease during the Niña. [ABSTRACT FROM AUTHOR]

Macro-tidal estuaries with mountain streams are typical estuarine systems in southeastern China. Suspended sediment transport and turbidity maxima are sensitive to changes in the tidal and fluvial regimes of estuaries. To improve our understanding of river-ocean interactions and thus improve estuarine forecasts for biogeochemical processes, we developed a high-resolution hydrodynamic and sediment transport coupled model to investigate the suspended sediment transport and estuarine turbidity maximum (ETM) in the Oujiang Estuary. This estuary is characterized as a macro-tidal estuarine system with a large tidal range and drastic fluctuations in freshwater and sediment discharge. The results led to the following conclusions: (1) the ETM occurred during spring tides due to strong tidal resuspension. In contrast, it was not formed during the neap tide because of weak bottom shear stress. The ETM moves upstream to Yanmen during low river discharge and migrates downstream to the river mouth during high river discharge, causing sediment deposition in the nearshore region. (2) Owing to the characteristics of mountain streams and upstream anthropogenic reservoirs, the ETM is strengthened during low river discharge (∼100 m3 s−1) even during the wet season, which supplements the fact that the ETM is more prevalent during the dry season and withers during the wet season. The suspended sediment forming the ETM mainly originates from shallow offshore shoals owing to tidal resuspension. Owing to the weak vertical exchange flow near the river mouth, the ETM is related to the exchange flow during the low water flow. (3) The ETM excursion shows semi-diurnal variability with different transport distance for high and low river discharges. Tidal pumping dominates the suspended sediment flux in the core of the ETM, which is mainly maintained by two components of the Euler and Stokes fluxes as the tidal energy depletes during the flood tide. • Estuarine modeling was conducted to study suspended sediment dynamics. • ETM excursion occurs for high and low river. • Decomposing sediment fluxes reveals ETM mechanisms. • River discharge and tidal pumping play roles on ETM formations. [ABSTRACT FROM AUTHOR]

The long-distance transport of Yellow River sediments has formed an omega-shaped ('Ω') distal muddy deposit in the Yellow Sea. Numerous studies have investigated the sediment dynamics and sedimentary evolution of the deposit, but several key questions remain unanswered due to insufficient observations. To reveal the mechanisms of sediment dynamics and quantify sediment transport and deposition processes, this study collected field datasets in the depocenter of the muddy deposit off the Shandong Peninsula. Bottom mounted tripods were deployed from August 18 to 27, 2017, and from February 23 to March 2, 2018, during which two strong wind events with speeds of over 10.8 m/s occurred. The results showed that near-bed suspended sediment concentration responded directly to varying hydrodynamic conditions, and significant differences in suspended sediment concentration between two wind events could be ascribed to a number of factors (e.g., tidal current, wave periods, wind stress, turbulence). The harmonic analysis demonstrated that suspended sediment variations during the spring tide were directed mainly by resuspension, while resuspension and advection contributed equally at neap tide. The variations in background suspended sediment concentration can be ascribed to the residual current. Furthermore, the sediment transport pattern is similar to the pattern of water circulation. Generally, the along-shore suspended sediment flux is about one order of magnitude higher than the cross-shore flux. In summer, the suspended sediment was transported mainly downcoast. In winter, sediment was transported downcoast during wavy periods and upcoast during calm periods. Advection dominated sediment flux in most cases, except during wavy periods when local resuspension increased significantly, and the contribution of resuspension term to along-shore flux was up to 87%. Meanwhile, cross-shore flux also increased significantly under strong winds, which was simultaneously modulated by downcoast and offshore winds. After statistical analysis, we found that offshore winds controlled the cross-shore sediment transport in winter. Using parameterized calculation and statistical analysis, the average accumulation rate of the study site was estimated to be 0.6 cm/yr. During calm periods, lower background sediment concentration in summer led to lower deposition rates than in winter. In addition, the frequent strong wind events in winter cause extremely high erosion rates. This study provides new perspectives on quantitative variations in suspended sediment transport and regional erosion/deposition from synoptic to seasonal variations in the distal mud deposit. • In-situ observations at the bottom boundary layer were conducted in summer and winter on the muddy deposit off the Shandong Peninsula. • The advection dominates sediment flux in most cases, except for during wind gusts when local resuspension increases significantly. • Combining parameterized method and observational data, the accumulation rate is estimated. [ABSTRACT FROM AUTHOR]

Episodic flood and storm events are important drivers of sediment dynamics in estuarine and marine environments. Event-driven sedimentation has been well-documented by field and modeling studies, though both techniques have inherent limitations. A unique opportunity to integrate field observations and model results was provided in late August/early September 2011 with the passage of Hurricane Irene and Tropical Storm Lee in the Chesapeake Bay region. Because these two storms occurred within a relatively short period of time, both are potentially represented in the sediment record obtained during rapid-response cruises in September and October 2011. Associated sediment deposits were recognized in cores using classic flood-sediment signatures (fine grain size, uniform 7 Be activity, physical stratification in x-radiographs) and were found to be <4 cm, thickest in the upper Bay. A coupled hydrodynamic-sediment transport model is used to simulate the sediment plume and sediment deposition onto the seabed. The predicted deposition thickness for TS Lee is in general agreement with the observational estimates. One exception with physical stratification but no 7 Be activity appears to be due to extreme wave activity during Hurricane Irene. Integration of observations and modeling in this case greatly improved understanding of the transport and fate of flood sediments in the Chesapeake Bay. [ABSTRACT FROM AUTHOR]

A one-year study examining water-column and near-bed time-series observations from benthic-boundary-layer tripods was undertaken in the Waipaoa Sedimentary System (WSS) on the Poverty Bay continental shelf, offshore of the Raukumara Ranges, New Zealand. Here, the nature of the adjacent post-glacial shelf deposits, fed by the Waipaoa River, and century-scale sediment accumulation is well established, but event-scale sediment dispersal and hydrodynamics had not been measured. Data from two tripods outfitted with acoustic and optical instrumentation, and x-radiograph images of seabed cores collected at 4-month intervals, were analyzed to quantify progressive sedimentological changes, and to document processes responsible for seabed strata formation, destruction, and preservation. At the Northern Tripod (NT) mid-shelf location 15 km southeast of the river mouth, we observed increases in near-bed suspended sediment concentration when shear velocities exceeded 0.03 m s −1 on 33 occasions over an 11.5-month period. Of these events, 9 were depositional, 10 were erosional, and 14 had no bed-elevation change at the 1-cm resolution of our instrumentation. Relatively high (2–5 m) and short-period (<11–12 s) waves, combined with current velocities <0.15 m s −1 , led to deposition on the seabed. Smaller-height (2–3 m) and longer-period (11–13 s) waves with stronger current velocities (>0.15 m s −1 ) typically induced erosion. Despite the short reach of this mountainous riverine dispersal system, we observed a consistent delay between river discharge and apparent deposition on the mid-shelf at the NT location. Following the three largest floods, ~120 h of strong, wave-driven bed shear stress preceded measurable deposition on the mid-shelf, suggesting that event duration and intensity are key for transporting material offshore from Poverty Bay. The largest flux event recorded over the study year was a 130-hour-long storm and associated river flood that resulted in near contemporaneous deposition at the NT site. Suspended-sediment concentration data suggest transport by a wave-supported fluid mud. X-radiographs from the NT location corroborate the instrumental record of 5 cm deposition, but only the two largest depositional events were preserved in the strata. On the southern shelf we observed no evidence of deposition in either the instrumental data or in the x-radiographs of cores. This study suggests that event duration, and spatial and inter-annual variability are critical considerations when evaluating along- and across-shelf sediment transport and seabed stratigraphic evolution associated with discharge from small mountainous rivers. [ABSTRACT FROM AUTHOR]

Abstract: In the hypertidal Bay of Fundy, environmental impacts in response to commercial-scale tidal power development remain to be fully understood. The extraction of tidal energy may impact sediment dynamics in far-field environments, such as the intertidal zone, through potential alterations to tidal amplitude in the Minas Basin. Tidal conditions (e.g. current velocity, turbulence, suspended sediment concentration) were monitored in a sheltered salt marsh creek over 18 tidal cycles in various stages of the spring-neap cycle. Samples of deposited and suspended sediments were collected and analyzed for grain size using a Beckman Coulter Multisizer III. Results suggest that the flocculated component of both deposited and suspended sediment is consistently high over a wide range of tidal conditions. A routinely high incoming concentration of highly-flocculated material results in large amounts of sediment deposition in tidal creeks in response to individual tidal cycles. Resuspension and removal of newly deposited material is shown to vary with over-marsh, bankfull and channel-restricted tides. Disruption of the tidal regime due to a reduction in Minas Basin tidal amplitude may lessen the cumulative export capacity of tidal channels over time, potentially leading to gradual infilling of tidal creeks. [Copyright &y& Elsevier]

Abstract: In this study we have performed a baseline characterisation of sediment properties in four depositional areas of the Ingøydjupet trough in order to investigate the sediment transport and deposition by ocean currents in the SW Barents Sea. Sedimentation rates, by the 210Pb method, were 1.0–2.4mm/yr and decreasing with distance offshore. Stronger and more variable bottom currents associated with the Norwegian coastal current operate nearshore based on the down-core distribution of sortable silt ( ) and sediment fractions (clay and silt). These nearshore stations exhibit a trend of decreasing fine sediment supplies and increasing sand content toward present day. The most stable bottom currents are associated with the farthest offshore station which is under the influence of the North Atlantic Current as indicated by the distribution of ( ) and also higher TOC contents (0.6–0.9%) compared to nearshore stations (0.4–0.8%). These baseline findings are of relevance for the petroleum industry in planning environmental assessment and monitoring programs connected to the future expansion of industry activities in this region. [Copyright &y& Elsevier]

Abstract: A monitoring effort to address the physical effects of bottom trawling was conducted on the Ebro prodeltaic mud belt during the RESPONSE project. The monitoring was carried out for 14 months covering periods of different trawling intensities and a close season for the trawling fleet. The seabed morphology was studied by side-scan sonar and sediment texture and organic carbon content were analysed. Suspended sediment vertical distribution was recorded by CTD+turbidity hydrographic profiles and sediment transport was computed using time series from moored turbidimeters and current meters. The results show that the seabed of the fishing ground is strongly affected by scraping and ploughing induced by bottom trawling. Part of the finer fraction of the prodeltaic mud resuspended by trawling is winnowed, increasing the silt content of the settling sediment and also near-bottom turbidity. Sediment resuspended by trawling is incorporated in the bottom nepheloid layer and transported across- and along-shelf, increasing sediment fluxes. Trawling also induces an increase in the organic carbon content in the bottom sediment. All these effects induced by trawling have occurred during the last few decades, changing natural conditions in the fishing ground. [Copyright &y& Elsevier]

Abstract: A Coupled Wave–Ocean–Sediment Transport Model was used to hindcast coastal circulation and fine sediment transport on the Mekong shelf in southeastern Asian in 2005. Comparisons with limited observations showed that the model simulation captured the regional patterns and temporal variability of surface wave, sea level, and suspended sediment concentration reasonably well. Significant seasonality in sediment transport was revealed. In summer, a large amount of fluvial sediments was delivered and deposited near the Mekong River mouth. In the following winter, strong ocean mixing, and coastal current lead to resuspension and southwestward dispersal of a small fraction of previously deposited sediments. Model sensitivity experiments (with reduced physics) were performed to investigate the impact of tides, waves, and remotely forced ambient currents on the transport and dispersal of the fluvial sediment. Strong wave mixing and downwelling-favorable coastal current associated with the more energetic northeast monsoon in the winter season are the main factors controlling the southwestward along-shelf transport. [Copyright &y& Elsevier]

Abstract: Typhoon Morakot, which first made landfall in central Taiwan on Aug. 7th and then in Fujian on Aug. 9th after crossing through the Taiwan Strait was the strongest typhoon to impact the East China Sea (ECS) in 2009. Two surveys were conducted, respectively on Aug. 1st and Aug. 12th, in the mud deposition center off the Zhejiang–Fujian coast to study the impacts of the typhoon on the marine environment and sediment transport. Continuous in-situ hydrographic data (water temperature, salinity, depth and turbidity) were recorded at 29 stations during the first survey and 23 stations during the second survey. The water and surface sediment samples were collected at the same time. A comparison of the measurements from the two surveys shows that significant variations in temperature, salinity and turbidity distributions occurred in the water column. The upper water layer was well-mixed after the passage of Morakot. The water temperature and salinity dropped approximately 0.2–1.4°C and 0.3–3.3psu, respectively, while the water turbidity increased from less than 1FTU to 1–30FTU. In the bottom layer, the temperature increased approximately 2.5–4.0°C, and the salinity dropped approximately 0.2–0.6psu due to the water mixing. The water turbidities generally increased to greater than 50FTU (greater than 150FTU in some locations), which were several times to more than tenfold greater than those in the first survey, which were generally between 10 and 60FTU. The thermocline, which existed at a depth of approximately 10m during the first survey, disappeared after the typhoon. A halocline appeared at approximately 10m depth in the near-shore area due to abundant fresh water supply, whereas the salinity was generally homogeneous during the first survey. Concurrently, the relatively warm and fresh water extended downward to the whole medial water layer due to the water mixing. The turbidity and volume of the bottom turbid layer was greatly increased at the second survey compared with the first survey. The intrusion of the low-temperature and high-salinity deep water (below 50m depth) was significantly reduced at the same time. [Copyright &y& Elsevier]

Abstract: A three-dimensional coupled hydrodynamic-sediment transport model for the Texas–Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed. At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20–40km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal–annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave–current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf. [Copyright &y& Elsevier]

Abstract: The source and transport mechanisms of land-derived Okinawa Trough sediments were studied using the field data of temperature, salinity and turbidity in the East China Seas. The results suggest that there are two primary sediments sources from the Chinese Mainland to the Okinawa Trough: one is the Old Huanghe River submarine delta, and the other is the Changjiang River sediments, which are distributed at the Changjiang River estuary and the off-coast of Zhejiang and Fujian provinces. It is difficult for the Huanghe River suspended sediments to arrive in the Okinawa Trough via the new estuary. Although the Taiwan warm current blocks the seaward terrigenous transportation to a certain extent, part of the coastal suspended sediments are transported to the outer shelf. Suspended particulate matter is unable to get through the barrier of the Kuroshio Current under normal conditions. However, episodic events, such as winter storms, internal-tidal waves and turbidity flows, are capable of transporting suspended particulate matter into the Okinawa Trough. The super typhoon “Ewiniar” induced strong waves and influenced the thermocline depth and suspended sediment concentration of the East China Seas. The typhoon-induced waves pushed the thermocline depth down to around 40m and caused the resuspension of large volumes of sediments in its path. In the other East China Seas regions, the typhoon-induced swells deepened the thermocline depth by about 5m and increased suspended sediment concentrations. The typhoon effect on suspended sediment concentration of the East China Seas disappeared within 2 weeks. [Copyright &y& Elsevier]

Abstract: A comparison of 1927, 1970 and 2002 bathymetric surveys in the Lagoon of Venice was used to reconstruct historical changes in sedimentation. A detailed GIS-based analysis of the charts revealed the timing and pattern of geomorphic changes and allowed calculation of sediment deposition and erosion for the entire lagoon and each of its four sub-basins: Treporti, Lido, Malamocco and Chioggia. Two main developments are discernible from comparative observation of the areal distribution of the main elevation ranges: the diminution in area of the saltmarshes, which decreased by more than 50%, from 68km2 in 1927 to 32km2 in 2002, and the progressive deepening of the lagoon, with a huge increase in the area of subtidal flats (between −0.75 and −2.00m depth), from 88 to 206km2 during the same period. Generally, the lagoon showed a clear-cut change in the most frequent depths (modal depth) from a value of −0.62m in 1927 to −0.88m in 2002. The deepening of the lagoon affected mostly the lagoonal sub-basins south of the town of Venice, where modal depth increased from −0.65 to −1.12m in Lido, from −0.64 to −1.75m in Malamocco and from −0.39 to −0.88m in Chioggia. Large changes in lagoonal morphology were caused by human-induced subsidence, the dredging of navigation channels between 1927 and 1970, and intense natural erosion enhanced by sediment re-suspension due to Manila clam fishing between 1970 and 2002. There was a net loss of about 110Mm3 of sediment from the lagoon, most of which (73Mm3, ca.70%) was in the earlier period. A significant amount was lost by dredging and direct disposal outside the system, either on land or at sea, and there was a net loss of 39Mm3 from the lagoon to the sea through the inlets, at an annual rate of 0.5Mm3. Comparison of erosion rates in the two periods revealed an alarming acceleration, from a net sediment loss of 0.3Mm3 yr−1 in the period 1927–1970 to 0.8Mm3 yr−1 in 1970–2002. Deterioration caused a shift from a highly differentiated lagoon morphology in the 1930s to a sediment-starved and subsidence-dominated structure in the 1970s, and from there to the high-energy and more open (bay-like) lagoon of today. The results demonstrate the potential application of GIS to reconstruct the recent chronology of sediment distribution and to improve the understanding of the geomorphic processes shaping the seafloor, whilst providing an insight into the possible impacts of environmental changes induced by natural and anthropogenic forcing. [Copyright &y& Elsevier]

Abstract: Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia''s coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits. [Copyright &y& Elsevier]

Abstract: This contribution aims to model the dominant processes that control sedimentation within the ocean inlet to intermittently open–closed coastal lagoons; focussing on the role of infilling, backfilling and morphodynamic feedback. The key elements that have been included in the present model are: (1) the delivery of sediment to the mouth of the estuary by littoral processes; (2) sediment transport processes within the inlet due to non-linear tidal flow; and (3) the down-slope diffusion of sediment. The model discussed here includes a simple one-dimensional (1-D), non-linear flow parameterisation that predicts the spatial variability in the magnitude and non-linearity of the tidal flow. The predicted third and fourth velocity moments are used to drive a morphodynamic module. Down-slope diffusion of sediment is dealt with in a separate diffusion term in the bed-evolution equation. Feedback between the evolving morphology and the flow field are integral to the model. Numerical simulations are used to investigate different modes of evolution for this type of inlet system. Inlet closure due to infilling is critically controlled by the balance between sediment addition at the mouth due to littoral processes and the removal of sediment by non-linear tidal flow. Rapid widening or deepening of the inlet at its landward margin leads to the deposition of a flood shoal. Under conditions of high sedimentation (and low diffusion) the flood shoal can become sufficiently well developed to present a physical barrier to sediment entering the lagoon. Under these circumstances backfilling can become significant. The infilling and backfilling processes are ameliorated by efficient down-slope diffusion which is controlled in the present model by a diffusion parameter, D. High diffusion coefficients slow inlet closure and allow sediment to propagate further into the lagoon. [Copyright &y& Elsevier]

Abstract: As a part of the ANR-Forclim experiment, particle mass fluxes and sedimentation processes were investigated on the slope of Aquitanian margin of the Bay of Biscay, between the canyons of Cap-Breton and Cap-Ferret. Interface sediments were collected along a depth transect from 145 to 2000m; simultaneously a mooring line was deployed at the deepest station (WH, 2000m) with two traps (800 and 1700m) for a 16-month period (June 2006–November 2007). 210Pb activities of settling particles and of interface sediments were determined to study transport processes of particles. Sediment and mass accumulation rates, calculated from excess 210Pb profiles in the sediment column, show the expected decreasing trend with depth, as usually observed on margins. Mean particulate mass fluxes at 800 and 1700-m depth at site WH are, respectively, 27 and 70gm−2 a−1. The 210Pb budget points out events of temporary high lateral input of particles. The comparison of mass and 210Pb fluxes between the water column and the seabed indicates that lateral transport plays an important role in particle accumulation on the Aquitanian margin. Regarding the objectives of the ANR-Forclim program, which aims to improve significantly the interpretation of fossil foraminifera signals, as a proxy for hydrological changes in the North Atlantic ocean, these results highlight advection processes must be considered when interpreting fluxes of foraminifers on the Aquitanian margin. [Copyright &y& Elsevier]

Although many studies have been carried out on the mud wedge in the inner shelf of the East China Sea (ECS), the sediment sources and seismic textures of the mud wedge are still unclear and remain controversial. Therefore, this study utilizes grain size and clay mineral analyses of surficial sediment samples and high-resolution sub-bottom seismic profile interpretation to identify the provenance and texture of these mud wedge sediments. The results show that sediment sources on the inner shelf of the East China Sea are mixed by the Yangtze River, small mountainous rivers along the Zhejiang-Fujian, and western Taiwan coasts. The sediments derived from the Ou and Min rivers are mainly concentrated in the shallowest shelf area within a water depth of 20 m which is located north of the Min River Estuary. Their contribution to the estuaries and coastal areas reflects the different sediment transport patterns of the two small mountainous rivers. The thickness of the Holocene mud wedge between Hangzhou Bay and Taizhou Bay is greater than 20 m at a water depth of approximately 10 m. The cause of this phenomenon is possibly related to the presence of the Zhoushan Islands, which are distributed along the Zhejiang coast and appear to effectively trap sediments from the Yangtze River, facilitating sediment accumulation. The findings of this study will help improve our understanding of the sediment transport and deposition mechanism of deposition on the inner shelf of the East China Sea. • We produced grain size and clay mineral analyses and high-resolution sub-bottom seismic profile interpretation. • Sediments derived from small mountainous rivers are mainly concentrated in the shallowest shelf area. • The thickness of the Holocene mud wedge is locally greater than 20 m in the shallowest shelf area. [ABSTRACT FROM AUTHOR]

Abstract: Understanding the impact of marine sand mining operations in a complex coastal environment requires a combined observational and modeling approach. Here, we use field measurements collected during mining operations in Kyunggi Bay, Korea to develop sediment parameters and source conditions for a three-dimensional (3D) sediment transport model built on the Regional Ocean Modeling System (ROMS). The model is run with realistic forcing obtained from a 9km meteorological model, tides, and river discharges. The resulting vertical and horizontal distributions of sediment show encouraging agreement with the field data, demonstrating markedly different dispersal patterns due largely to the differential settling of the various sand classes. The resulting depositional patterns suggest that only the coarser size classes (500 and 250μm) particles remain close to the mined site, while finer size classes are widely dispersed. These results suggest that this new methodology of multi-size class, 3D sediment transport modeling is quite promising, and further work is ongoing to include more realistic representation of sediment resuspension processes. [Copyright &y& Elsevier]

Abstract: The transport of fine-grained particles in estuarine and coastal waters is influenced by flocculation processes (aggregation and floc breakup). As a consequence, the particle size varies with time in the water column, and can be orders of magnitude larger than those of primary particles. In this study the variations in floc size is simulated using a size-resolved method, which approximates the real size distribution of particles by a range of size bins and solves a mass balance equation for each bin. To predict the size distribution both aggregation and breakup processes are included. The conventional rectilinear aggregation kernel is used which considers both turbulent shear and differential settling. The breakup kernel accounts for the fractal dimension of the flocs. A flocculation simulation is compared to the settling column lab experiments of Winterwerp [1998. A simple model for turbulence induced flocculation of cohesive sediment, Journal of Hydraulic Research, 36, 309–326], and a one-dimensional sediment transport model is verified with the observed variations in floc size and concentration over tidal cycles in a laboratory flume experiment of Bale et al. [2002. Direct observation of the formation and break-up of aggregates in an annular flume using laser reflectance particle sizing. In: Winterwerp, J.C., Kranenburg, C. (Eds.), Fine Sediment Dynamics in the Marine Environment. Elsevier, pp. 189–201]. The numerical simulations compare qualitatively and quantitatively well with the laboratory measurements, and the analysis of the two simulation results indicates that the median floc size can be correlated to the sediment concentration and Kolmogorov microscale. Sensitivity studies are conducted to explore the role of settling velocity and erosion rate. The results are not sensitive towards the formulation of settling velocity, but the parameterization of erosion flux is important. The studies show that for predicting the sediment deposition flux it is crucial to include flocculation processes. [Copyright &y& Elsevier]

Abstract: Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5mg/l. In Cap de Creus canyon head (located ∼14km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists. [Copyright &y& Elsevier]

Abstract: To investigate the processes by which sediment is transported through a submarine canyon incised in a continental margin affected by recurrent dense shelf water cascading events, several instrumented moorings were deployed in the Cap de Creus Canyon from September 2004 to September 2005. This was done as part of the EuroSTRATAFORM Program that investigated sediment transport and accumulation processes in the Gulf of Lions. Results obtained in this observational study confirm that major cascading events can effectively contribute to the rapid export of sediment from the shelf and upper slope to deeper environments, and suggest that the associated strong currents carrying coarse particles are able to erode the canyon floor and generate sedimentary furrows. During winter 2004–2005, persistent northerly winds and the absence of river floods contributed to decrease the buoyancy of coastal waters and to dramatically enhance the intensity of dense shelf water cascades in the Gulf of Lions. Under such conditions, cascading continuously affected the entire Cap de Creus upper canyon section for more than a month and sustained cold temperatures and down-canyon steady currents >60cm/s (up to 100cm/s), showing periodic fluctuations that lasted between 3 and 6 days. Increases in suspended sediment concentrations were associated with dense shelf water cascading outbursts, but the magnitude of the concentration peaks decreased with time, suggesting a progressive exhaustion of the resuspendable sediments from the shelf and canyon floor. Grain size analyses of the particles caught by a near-bottom sediment trap show that dense shelf water cascades are able to transport coarse sediments (up to 65% sand) in suspension (and presumably as bed load), which have the potential to abrade the seafloor and generate erosive bed forms. The orientation of a large field of “wide” (i.e., widths about 1/2 spacing indicative of erosive formation) sedimentary furrows recently observed in the Cap de Creus Canyon clearly coincides with the preferential direction of highest velocities measured by the moored current meters, indicating a causative relationship between contemporary dense shelf water cascades and furrow formation. [Copyright &y& Elsevier]

Abstract: In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a hydrodynamic–sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1Mt during the study period. [Copyright &y& Elsevier]

Abstract: The effects of a giant offshore dyke, Saemangeum, have been of special interest on both scientific and environmental aspects. Without any rival worldwide with regard to the ample scale and estuarine setting, the Saemangeum dyke and its vicinity (Saemangeum Area) should be an unprecedented experimental site for monitoring the consequence of the estuarine developments with dykes. In order to unravel geologic changes caused by the Saemangeum dyke, we collected a number of surface sediment samples and bathymetric profiles from a full coverage of the Saemangeum Area, and compared topography and sediment phase on the inter-annual (2002–2006, post-construction) and decadal (1982–2002, between pre- and post-construction) timescales. The dyke construction induced vigorous erosion along the dyke, especially across a few open gaps between dyke segments. These open gaps led to forming deep erosional troughs and the associated channels to the river mouth. At the same time, mild accumulation of sediments occurred on the broad seafloor offshore the erosional zone of dyke sector IV and on shoals between the erosional troughs off dyke sectors I–II. Inside the dyke, tidal flats have become expanded and aggraded with continuously discharged riverine sediments. Sands have been newly introduced into the offshore area of sector IV owing to tidal currents with an orientation forced to be parallel to sector IV (NE–SW), reinforced by high waves during winter. By contrast, the expanded sandy area off sectors I–II resulted from redistribution of pre-existing sands without any new input from offshore. As a whole, the concentrated erosion and sandy-seafloor expansion offshore represent the man-made influences triggered by the Saemangeum dyke. [Copyright &y& Elsevier]

Abstract: An important element within the Defra funded Estuary Process Research project “EstProc” was the implementation of the new or refined algorithms, produced under EstProc, into cohesive sediment numerical models. The implementation stage was important as any extension in the understanding of estuarine processes from EstProc was required to be suitable for dissemination into the wider research community with a level of robustness for general applications demonstrated. This report describes work undertaken to implement the new Manning Floc Settling Velocity Model, developed during EstProc. All Manning component algorithms could be combined to provide estimates of mass settling flux. The algorithms are initially assessed in a number of 1-D scenarios, where the Manning model output is compared against both real observations and the output from alternative settling parameterisations. The Manning model is then implemented into a fully 3-D computational model (TELEMAC3D) of estuarine hydraulics and sediment transport of the Lower Thames estuary. The 3-D model results with the Manning algorithm included were compared to runs with a constant settling velocity of 0.5mms−1 and settling velocity based on a simple linear multiplier of concentration and with the above mentioned observations of suspended concentration. The findings of the 1-D case studies found the Manning empirical settling model could reproduce 93% of the total mass settling flux observed over a spring tidal cycle. The floc model fit was even better within the turbidity maximum (TM) zone. A constant 0.5mms−1 only estimated 15% of the TM mass flux, whereas the fixed 5mms−1 settling rate over-predicted the TM mass flux by 47%. Both settling velocity as a simple linear function of concentration, and van Leussen''s method, did not fare much better estimating less than half the observed flux during the various tidal and sub-tidal cycle periods. When the Manning-settling model was applied to a layer with suspended concentrations approaching 6gl−1, it calculated 96% of the observed mass flux. The main conclusions of the implementation exercise were that it was feasible to implement a complex relationship between settling velocity and concentration in a 3-D computational model of estuarine hydraulics, without producing any significant increase in model run times or reducing model stability. The use of the Manning algorithm greatly improved the reproduction of the observed distribution of suspended concentration both in the vertical and horizontal directions compared to the other simulations. During the 1-D assessments, the Manning-settling model demonstrated flexibility in adapting to a wide range of estuarine environmental conditions (i.e. shear stress and concentration), specifically for applied modelling purposes. [Copyright &y& Elsevier]

Abstract: Across a limited depth range (5–10m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20m water depth while the SMT associated with the Pescara River varies between 15 and 25m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT. [Copyright &y& Elsevier]

Abstract: A mooring and tripod array was deployed from the fall of 2002 through the spring of 2003 on the Po prodelta to measure sediment transport processes associated with sediment delivered from the Po River. Observations on the prodelta revealed wave-supported gravity flows of high concentration mud suspensions that are dynamically and kinematically similar to those observed on the Eel shelf [Traykovski, P., Geyer, W.R., Irish, J.D., Lynch, J.F., 2000. The role of wave-induced density-driven fluid mud flows for cross-shelf transport on the Eel River continental shelf. Continental Shelf Research 20, 2113–2140]. Due to the dynamic similarity between the two sites, a simple one-dimensional (1D) across-shelf model with the appropriate bottom boundary condition was used to examine fluxes associated with this transport mechanism at both locations. To calculate the sediment concentrations associated with the wave-dominated and wave-current resuspension, a bottom boundary condition using a reference concentration was combined with an “active layer” formulation to limit the amount of sediment in suspension. Whereas the wave-supported gravity flow mechanism dominated the transport on the Eel shelf, on the Po prodelta flux due to this mechanism is equal in magnitude to transport due to wave resuspension and wind-forced mean currents in the cross-shore direction. Southward transport due to wave resuspension and wind forced mean currents move an order of magnitude more sediment along-shore than the down-slope flux associated wave-supported gravity flows. [Copyright &y& Elsevier]

Abstract: A simple model for wave-supported gravity flows is applied to sediment deposition off the mouth of the Po River at time scales ranging from a single major flood to steady-state clinoform progradation. Wave-supported gravity flows are a newly appreciated class of turbidity currents, which rely on the velocity shear produced by waves near the seabed to keep sediment in suspension. The modeling approach used here, which limits the gravity flow''s sediment load via a critical Richardson number, is applicable to fine sediment transport near river mouths wherever wave energy is available to move abundant sediment offshore during floods. Results suggest this phenomenon can account for the majority of the fall 2000 flood deposit mapped by EuroSTRATAFORM investigators in the vicinity of the Po River prodelta and also for the rate of prodelta progradation observed off the dominant Pila outlet of the Po over a century time-scale. Model results predict that convergence of down-slope sediment transport by wave-supported gravity flows increases with bed slope but decreases with slope gradient, such that greatest deposition occurs near where steep slopes first stop increasing with distance offshore. Thus on profiles which reach maximum steepness near shore, like those off Tolle–Gnocca–Goro mouths today or off the Pila mouth 150y ago, gravity-driven deposition occurs in shallower water. Over time, if deposition overwhelms subsidence, the prodelta becomes less steep near shore and steeper offshore, and the locus of deposition moves progressively into deeper water. If the prodelta is prograding across a relatively flat shelf, the shape of the prodelta eventually reaches a stable form which progrades seaward as a unit. This has occurred off the Pila; but subsidence has likely overwhelmed deposition off the Tolle–Gnocca–Goro, keeping steepest slopes and maximum deposition in shallower water. [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: 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: 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]

A string of four time-series sediment traps was deployed at each of the two mooring sites located in the deep basin of the northern South China Sea for 6 months. Each of these traps was equipped with 12 sampling cups and each cup collected the settling particulates for 15 days. Large temporal and spatial variations in particulate mass flux were observed and the temporal variations were generally not synchronous at each site. The time-series particulate flux varies between about 50 and 2000 mg/m2/d for all the traps and generally increases with depth. The mean particulate flux of each trap varies from about 200 to 550 mg/m2/d. The total organic matter in the trapped particulates as indicated by the loss on ignition decreases from 25% to 35% at the shallow traps to about 7–12% at the deep traps. These values are much higher than those observed in the southern East China Sea. 210Pb activity in the trapped particulates varies from about 40 to 370 dpm/g, and the mean activity increases with depth from about 70 to 300 dpm/g. Although 210Pb activity at each depth varies inversely with the particulate flux, the 210Pb flux correlates with the particulate flux positively and the slope increases with depth.The mean fluxes of particulates and 210Pb obtained from the traps are compared with the fluxes determined from the box cores taken in the same deep basin environment. The deepest particulate flux measured at 3240 m depth averages about 260 mg/m2/d while the mass accumulation rate (MAR) estimated from the box cores is at least over 3 g/m2/d, an order of magnitude higher. The high MAR derives from the very high sedimentation rate which is based on the excess 210Pb distribution assuming no mixing effect on the sediments. The sedimentation rate can be reduced by an order of magnitude if a reasonable mixing rate in the sediments is adopted. Thus, the MAR of the sediments can be quite variable and made comparable to the mean mass flux as observed from the deep traps. However, the 210Pb inventory and so the 210Pb flux at steady state can be obtained by integrating the excess 210Pb over the core depth regardless of the mixing effect. The 210Pb flux determined this way varies between 230 and 310 dpm/m2/d although the cored sites are far from each other. The 210Pb flux measured from the deepest sediment trap at M1 and M2 sites (over 300 km apart) averaging over 6 months is about 130 and 80 dpm/m2/d, respectively. Thus, the 210Pb flux determined on the cored sediments is significantly higher than that measured from the trapped particulates, suggesting that a large 210Pb input into the sediments is required. This additional input may be derived from lateral transport, probably via benthic nepheloid layers which may exist below deepest traps. [Copyright &y& Elsevier]

During the 1960s and 1970s, the coastline of the Bay of Nice receded. As remedial measures, breakwaters were built in the western part of the Bay, and a beach replenishment policy was adopted. This resulted in the beach broadening from 10 or 15 to about 25 m. The main aim of this study was to determine the impact of these works and other urban changes on the sedimentation in the circalittoral zone along the Bay, using a geochemical approach.The concentrations of Pb, Zn and Cu and the activity of 210Pb were analysed in 8 cores collected at a water depth of 30 m along the coast, and in another one collected more offshore, at a depth of 756 m. The isotopic ratio 206Pb/207Pb was measured in 5 cores; the ratios in surface sediments range from 1.163 to 1.195. The isotopic ratios indicated that most of the lead contamination is from industrial and domestic sources, rather than gasoline. The average sediment accumulation rates, calculated assuming constant 210Pb supply to bottom sediment, range from 0.2 to 1.5 g/cm2/yr.Metal concentration profiles as well as the 210Pb profiles at the 30 m sites reflect the evolution of Nice since 1920, in particular the expansion of the town to the West on the Plio-Quaternary hills. At that time, expansion works furnished the materials for beach replenishments with uncontaminated sediments, whereas from 1985 sediments used were those of the Paillon bed (the small river that crosses the town), characterised nowadays by a moderate metal pollution.The high sediment accumulation rate in the centre of the Bay suggests the convergence of drifting from the western beaches toward the east, and from the Paillon embouchure toward the west. The sediment accumulation rate appears lowered during the last 10 yr in some 30 m sites. This can be related to the fact that, in the recent years, the sedimentary material discharged on the beaches for replenishment purpose originated chiefly from the Paillon bed, but such material would anyway have been transported to the Bay, and its use for the replenishment seems therefore unable to compensate the sedimentary load loss due to the Nice airport expansion on the Var prodelta area. [Copyright &y& Elsevier]

While the role of shore-parallel transport is extensively documented, its contribution to source-to-sink and provenance studies is often overshadowed by tectonic variations, sediment influx, and sea-level change. To isolate the sedimentary contribution of shore-parallel transport, this study examines the central Levant continental margin (easternmost Mediterranean), where the other three factors are well constrained. New high-resolution multibeam and seismic reflection data collected in a dedicated cruise aboard the RV Bat Galim are used for analyzing current-indicating morphologies formed since the Last Glacial Maximum (LGM). Results show six different current indicators, which attest to the ongoing shore-parallel Levant Jet System (LJS) flow. Throughout the post-LGM transgression and coastal retreat, the LJS shifted landward to flow over the submerged shelf and upper slope. This shift, along with periodic/seasonal alterations in the flow intensity, regulated the sediment transport, distribution and erosion, along and across the margin, during the lowstand, transgression, and highstand conditions. This case study emphasizes the central role of shore-parallel currents in regulating the sedimentary budget. It provides a reference case for continental margins where the shore-parallel role is obscured by more dominant tectonic-eustatic-sedimentation factors. • New high-resolution multibeam and Chirp data from the central Levant continental margin (easternmost Mediterranean). • The tectonic, eustatic, and sedimentation factors are well constrained in the study area. • The study examines the shore-parallel sediment transport role in shelf build-up. • Shore-parallel currents regulate sediment transport, distribution, and erosion, along and across the margin. [ABSTRACT FROM AUTHOR]

To identify the drivers of sediment deposition within a mangrove-lined river delta, we developed a three-dimensional numerical model in an idealized domain. Our model system is based on the Firth of Thames, a modern flat-fronted, bay-head deltaic system, in the Hauraki Gulf, in the North Island of New Zealand. The interactions of the buoyant river plume with mangroves are examined, including sediment deposition and erosion patterns. The morphological study presented here is restricted to cohesive sediments, whose patterns are observed after 4 months of morphological evolution. For the non-vegetated case, represented by a spatially uniform roughness, sediment deposited throughout the central model domain with a formation of a cuspate delta at the river mouth. However, in the vegetated case (with vegetated areas represented by an enhanced and spatially varying roughness coefficient), sediment deposition occurred in the regions of forests and flats, whereas the fringe areas experienced erosion. The sediment deposition patterns along cross-sections were qualitatively consistent with field observations from the Firth of Thames. Analysis of the across-transect and along-transect momentum balances demonstrates that the predominant balance between the bottom shear stress and baroclinic pressure gradient largely controls the sediment deposition in the riverine sections of the domain. However, close to the center of the plume, vertical advection promotes suspension of the sediments in the fringe region during the flood tide. • The presence of mangroves altered sediment deposition patterns. • Sediment deposition occurred in the forests and over the mudflats. • The fringe area between vegetation and bare mudflat experienced erosion. • Sedimentation patterns are explained by across- and along-shore momentum balances. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of erodibility, porosity and sediment size were made three times over the course of a year at sites within a muddy, mesotidal flat-channel complex in southern Willapa Bay, WA, to examine spatial and seasonal variations in sediment properties and transport potential. Average critical shear stress profiles, the metric we used for erodibility, were quantified using a power-law fit to cumulative eroded mass vs. shear stress for the flats and channel. Laboratory erosion measurements of deposits made from slurries of flat and channel sediment were used to quantify erodibility over consolidation time scales ranging from 6 to 96h. Erodibility of the tidal flats was consistently low, with spatial variability comparable to seasonal variability despite seasonal changes in biological activity. In contrast, channel-bed erodibility underwent large seasonal variations, with mobile sediment present in the channel thalweg during winter that was absent in the spring and summer, when channel-bed erodibility was low and comparable to that of the tidal flats. Sediment on the northern (left) channel flank was mobile in summer and winter, whereas sediment on the southern flank was not. Seasonal changes in channel-bed erodibility are sufficient to produce order-of-magnitude changes in suspended sediment concentrations during peak tidal flows. Porosity just below the sediment surface was the best predictor of erodibility in our study area. [Copyright &y& Elsevier]

Abstract: It is well known that landward transport of fine sediments on tidal flats is caused by lag effects, of which the scour lag and the settling lag are the best known. These lag effects result from a combination of sediment properties and hydrodynamic asymmetries. However, it is not well-understood how, in a quantitative way, these lag effects depend on the sediment properties and the hydrodynamic forcing, and what the relative importance of these sediment properties and hydrodynamics is. As a result, it is not known which lag effect is more important under which conditions. We therefore set out to explore the relative importance of hydrodynamics and sediment properties on tidal-flat sedimentation using a schematized 2DV cross-shore profile model with a tidal range of 6m and an intertidal width of ∼5km. The effect of hydrodynamics is parameterized through a varying offshore tidal asymmetry and four different cross-shore profiles (a horizontal, convex, linear and concave profile). The effect of sediment properties is examined by evaluating a range of settling velocities w s and critical bed shear stresses for erosion τ cr . The main findings of this work are that (1) conditions of maximum sediment deposition rates exist for w s ∼0.5mm/s and τ cr ∼0.1Pa, (2) deposition rates due to slack tide asymmetry are comparable to symmetric tides, while peak flow asymmetry produces the greatest deposition rates, (3) tidal flat deposition rates are greatest for concave profiles and least for convex profiles mainly due to the horizontal velocity gradient, and (4) the type of lag effect dominating landward transport varies spatially. [Copyright &y& Elsevier]

Abstract: Tidal flats with fluvial input can have ample fine-grained sediment supply, and yet the Skagit River tidal flats remain mostly sandy. Therefore, hydrodynamic mechanisms must efficiently export mud off the flats. This study evaluates short-term deposition and long-term burial of mud on an intertidal flat and links seabed deposits to sediment-transport processes. To investigate the fine-grained sediment-transport mechanisms within the sandy Skagit River tidal flats, seabed and water-column observations were collected seasonally from 2008 to 2009 under varying river discharge conditions. Little mud was observed deposited on the seabed. Spatially, mud was most common near channels and on the seaward flat edge. Its presence was temporary, and was typically observed following high river discharge. Strongest current velocities and high suspended-sediment concentrations were measured during spring tidal cycles with peaks in the late ebb. Maximum velocities were observed within channels, permitting for rapid transport of fine-grained particles to the seaward edge of the flat and export beyond. These physical processes also act to mobilize the mostly sandy seabed, which is reworked at various frequencies: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1–2cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1–2m, making the limited mud deposits available for export. Overall, channel networks on sandy tidal flats with fluvial input not only deliver fine-grained sediment to the flat, but also aid in the export of suspended sediment at ebb tides and in the release of muddy deposits through reworking of the seabed. [Copyright &y& Elsevier]

Abstract: A field study of a temperate mudflat in Willapa Bay, Washington was conducted to determine whether seasonal variation in physical forcings (e.g., temperature, rainfall, insolation) coupled with variation in key biological properties (e.g., seagrass and microphytobenthos abundance) would result in measurable changes in seabed porosity. On two mudflats and in a nearby secondary channel, replicate samples for surficial grain size, chlorophyll a content, and seagrass (Zostera japonica) shoot density were obtained at roughly 7–wk intervals for one year starting in November 2009. In addition, high-resolution profiles of porosity in the upper ∼9cm of the seabed were measured using an in situ resistivity profiler. Ancillary meteorological and river discharge data were obtained from nearby stations operated by Washington State University and the U. S. Geological Survey, respectively. Despite significant seasonal variation in seagrass abundance (0 to >3000 shoots m−2), a less dramatic, but still significant variation in chlorophyll a content, as well as seasonal changes in physical forcings, the near-surface porosity of the flats was a normally consolidated bed that remained steady throughout the year. In contrast, porosity of the channel often departed from a normally consolidated profile, with evidence for both erosion and deposition of several centimeters of sediment. These departures did not display a clear seasonal pattern (i.e., deposition in winter, erosion in summer), however, suggesting that the provision and removal of sediment to/from the channels was governed by processes (e.g., channel slumps, rainfall events) occurring on shorter time scales or that there was significant, unresolved small-scale variability. [Copyright &y& Elsevier]