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Estuaries contain vital habitats and it is important to understand how these areas respond to human activities and natural processes such as sea-level rise and wave attack. As estuarine shorelines erode or become modified with hard structures, there is potential for significantly altering the availability of sediment and the filling of coastal systems. This study used a source-to-sink approach and quantified rates of shoreline erosion in the Tar-Pamlico sub-estuary, a tributary of the larger Albemarle-Pamlico Estuarine System (APES). The average shoreline change rate (SCR) determined using an end-point method was −0.5 ± 0.9 m yr −1 for the Tar-Pamlico. Incorporating bulk density estimates, this contributes 0.6 × 10 5 tons of fine sediment to the system annually, or after accounting for fluvial input, about 40% of the total sediment supply to the sub-estuary. The role of the Tar-Pamlico as a sink for these sediments was addressed using the radionuclide tracers 210 Pb and 137 Cs. Radionuclide activities and sediment accumulation rates identified several depositional regions, in particular in the middle of the estuary. Linear sediment accumulation rates ranged from 0.10 ± 0.02 to 0.38 ± 0.02 g cm −2 yr −1 , and total storage of fine sediment in the system was 1.6 × 10 5 t yr −1 . It was not possible to confidently discern a change in the rate of shoreline erosion or seabed accumulation. A preliminary budget for fine sediments (grain-size <63 μm) was then calculated to compare erosional sources with sedimentary sinks. Almost all (∼93.0%) of the fine sediment entering the system was accumulated and stored, while only about 7.0% was exported to Pamlico Sound. [ABSTRACT FROM AUTHOR]

Beach nourishment has been a strategy widely used to slow down coastal erosion in many beaches around the world. The dredging of sand at the borrow site, however, can have complicated physical, geological and ecological impacts. Our current knowledge is insufficient to make accurate predictions of sediment infilling in many dredging pits due to lack of detailed sediment data. Two sites in the sandy shoal southeast of Port Royal Sound (PRS) of South Carolina, USA, were sampled 8 times from April 2010 to March 2013; one site (defined as ‘borrow site’) was 2 km offshore and used as the dredging site for beach nourishment of nearby Hilton Head Island in Beaufort County, South Carolina, and the other site (defined as ‘reference site’) was 10 km offshore and not directly impacted by the dredging. A total of 184 surficial sediment samples were collected randomly at two sites during 8 sampling periods. Most sediments were fine sand, with an average grain size of 2.3 phi and an organic matter content less than 2%. After the dredging in December 2011–January 2012, sediments at the borrow site became finer, changing from 1.0 phi to 2.3 phi, and carbonate content decreased from 10% to 4%; changes in mud content and organic matter were small. Compared with the reference site, the borrow site experienced larger variations in mud and carbonate content. An additional 228 sub-samples were gathered from small cores collected at 5 fixed stations in the borrow site and 1 fixed station at the reference site 0, 3, 6, 9, and 12 months after the dredging; these down-core sub-samples were divided into 1-cm slices and analyzed using a laser diffraction particle size analyzer. Most cores were uniform vertically and consisted of fine sand with well to moderately well sorting and nearly symmetrical averaged skewness. Based on the analysis of grain size populations, 2 phi- and 3 phi-sized sediments were the most dynamic sand fractions in PRS. Mud deposition on shoals offshore of PRS presumably happens when offshore mud transport is prevalent and there is a following rapid sand accumulation to bury the mud. However, in this borrow site there was very little accumulation of mud. This will allow the site to be used in future nourishment projects presuming no accumulation of mud occurs in the future. [ABSTRACT FROM AUTHOR]

Erodibility is critical to the sediment resuspension process but has not been measured systematically in large river-dominated muddy continental shelves before. During early summer of 2011, the Mississippi River experienced a major flood event. This flood provided a unique opportunity to examine how shelf seabed erodibility responded to a large river flood, and the ultimate fate of flood deposition is important to geological and biogeochemical processes (e.g., stratal formation, carbon sequestration). A total of 106 sediment cores were collected on the Louisiana shelf during five cruises in 2010 and 2011, and a new dataset was used to evaluate the response of the seabed to the recent conditions. The localized flood deposit was mainly within tens of kilometers of river sources, and little sediment accumulated on the middle Louisiana shelf. Seabed erodibility was measured using a dual-core Gust Erosion Microcosm System. The erodibility of sediment collected in April 2011 exceeded that for August 2010 and August 2011. The springtime increase in erodibility seemed to be related to the recent presence of energetic waves that mobilized the seabed. Erodibility was highest on the inner shelf southwest of Atchafalaya Bay, intermediate on the middle shelf, lowest in the Mississippi Canyon, and highly variable on the Mississippi subaqueous delta. These spatial patterns were influenced by proximity to river sources, flood-deposit thicknesses, intensity of wave-driven bed stresses, and bioturbation. The flood-deposit thickness itself, however, was not sufficient to explain all the spatial variations of erodibility after the peak of the Mississippi flood. Comparing values to published data, the depth-varying erodibility on the Louisiana shelf was close to the “low erodibility” level for the York River of Virginia, and similar to the data collected from Baltimore Harbor in Maryland and the main stem of upper Chesapeake Bay. Our findings promote understanding of the resuspension of fluffy organic-rich layer at the water–sediment interface, which influences sediment oxygen demand on the Louisiana shelf. This dataset is also valuable to observational and modeling studies of large river sediment dispersal systems worldwide. [ABSTRACT FROM AUTHOR]

On tidally-dominated shelf seas, hydrodynamic flow exerts shear stress on the seabed, inducing sediment transport and seabed evolution. Net sediment transport pathways and associated bed load partings zones (the 'head' of sediment transport pathways) are important elements of this sediment dispersal process. A robust comprehension of these dynamics on a local and regional scale are crucial for a range of offshore activities, including the rapidly growing offshore renewable energy industry. Previous studies infer a bedload parting zone (BLP) in the central Irish Sea, yet this predominantly bedform-derived BLP remains to be fully supported by underlying hydrodynamics. This study reviews how sediment transport pathways in the Irish Sea are defined to date, investigates whether a hydrodynamic regime capable of supporting this inferred BLP exists, and if proven, aims to elucidate its origin. Tidal asymmetry indicators, derived from the phase and amplitude relationships of numerically modelled M₂ and M₄ tidal constituents, reveal the existence of a symmetrical tidal zone in the proximity of the inferred BLP. New numerically modelled residual tidal current and bed shear stress vectors reinforce the presence of this distinct tidal character change and further reveal strong opposing tidal flows extending from this region, thus supporting the presence of a tidal flow capable of producing divergent bedload patterns. Analysis of tidal propagation through the Irish Sea Basin concludes that the origin of the BLP is mainly attributed to the intersection of the north and south tidal fronts at an inclined angle due to Coriolis Forcing and coastline interactions. Minor factors impacting the shape and location of the BLP are the change in tidal character at (a) abrupt bathymetry changes, (b) headlands and intricate coastline topography, and (c) large-scale constrictions. Further investigation into localised morphodynamics is needed. These findings are highly valuable to efficient marine spatial planning, underpinning a movement towards a climate resilient economy. The integrated approach used in this body of work can be applied to tidally-dominated continental shelves beyond the Irish Sea. [Display omitted] • Determine whether the bed load parting zone is fully supported by underlying hydrodynamics. • The origin of the BLP and factors impacting its shape and location are elucidated. • Strong divergence from previously defined net sediment transport pathways is revealed. [ABSTRACT FROM AUTHOR]

Abstract: Seasonal patterns of seabed erosion and deposition in a dynamic estuarine environment, the York River (USA), are examined through acoustic studies and seabed sampling to identify the dominant environmental controls on sediment mobility. Areas near Clay Bank in the York River were surveyed monthly using a dual-frequency sonar for a 12 month period. In addition, six stations within these areas were sampled during the surveys. Strong spatial and temporal variations of a soft mud layer were identified, providing new insight to recurring seasonal and spatial patterns in deposition and erosion in the middle reaches of the York River estuary. Sub-bottom data indicated maximum thickness (5–22 cm ± 4 cm) of the soft mud layer during spring and winter from April 2008 through March 2009. Minimum thicknesses (1–12 cm ± 4 cm) of the soft mud layer were observed during summer and fall. Maximum thickness coincided with the occurrence of the secondary turbidity maximum near Clay Bank in the York River. Spatial variations in the soft-mud layer thickness were also identified between and within the York River secondary channel, the inactive oyster reef area, the south west flank of the main channel, and the main channel. The upriver secondary channel, the south west channel flank, and the main channel areas were physically dominated. The downriver secondary channel area was biologically dominated and the inactive oyster reef area reflected both physical and biological processes. Water content trends showed no correlation with seabed stability in this area. Observed changes in soft mud thicknesses are interpreted to result from both biological and physical factors, which vary markedly between winter/spring and summer/fall seasons, and among the different environments of the estuary''s channel. [Copyright &y& Elsevier]

Abstract: Reefs are one of the marine habitats listed in Annex I of the European Union''s Habitats Directive, which aims to establish a coherent European ecological network of Special Areas of Conservation. EU Member States are required to prepare and propose a national list of sites for evaluation under the scheme, but currently the occurrence of reefs in the United Kingdom''s nearshore and offshore areas is not well documented. Here we report on our search for rocky reefs in the central English Channel, which unexpectedly revealed an extensive reef system covering an area of 1100km2. Prior to our work, it was generally perceived that the seabed in this area comprised mostly gravel, with a few isolated rock outcrops. Our approach to determining the location, extent and character of these reefs incorporated broad, medium and fine-scale analyses over a 3200km2 area of seabed, using single- and multi-beam acoustic data, ground-truthed by underwater video and stills imagery. A benthic terrain model was developed in ArcGIS to map topographic features at the broad and medium scales. Biotope assignments were made at the fine scale through detailed analysis of video footage obtained from 30 sampling stations. The study area has a complex geological history and lies at the centre of a major bedload-parting zone. Together, these strongly influence the seabed character and the distribution of biotopes. An integrated assessment of the physical and biological features was used to map the study area to level 4 of the EUNIS habitat classification system. Similar physical conditions exist in other areas of the UK continental shelf, raising the prospect of predicting where other rocky reef systems might occur. In the absence of a co-ordinated national seabed survey programme, such predictions, coupled with interpretation of existing single-beam bathymetry data, can help prioritise areas where limited survey resources could be most effectively deployed. [Copyright &y& Elsevier]

Abstract: Biological activity on the bottom of the seabed is known to have significant influence on the dynamics of cohesive sediment on a small spatial and temporal scale. In this study, we aim to understand the large-scale effects of small-scale biological activity. Hereto, effects of biology are quantitatively incorporated into the process-based sediment transport module of Delft3D. This Bio-mud model is used to study cohesive sediment transport and deposition patterns in the Western Wadden Sea for a period of 1 year to capture seasonal changes. The modeling results suggest that the seasonal variation in the sediment concentration is caused by the combined effect of the suspended sediment concentrations at the North Sea, wind and biological activity. Moreover, the stabilizing organisms are mainly responsible for the seasonal variation in suspended sediment concentrations, while the destabilizing organisms are mainly responsible for the spatial variation in fine sediment on the bed. [Copyright &y& Elsevier]

Waves are fundamentally important for sediment re-suspension in estuary and coastal areas, especially for silty sediments, which can be easily suspended by waves, but the differential effects of swell and wind waves are still unclear. Integrated field observations were made from November 2012 to March 2013 including waves, currents, and suspending sediments on the offshore seabed of the Huanghe Delta to explore the mechanism of sediment re-suspension in silty coastal zones. During the five months of observation, there were more than 30 winter wind events that affected the study area and induced sediment re-suspension with varying suspended sediment concentration. The observed wave composition was separated into swell and wind waves using a bandpass filter. Results show that large swell (with significant height > 1.0 m) coming from the offshore direction (NE in our study area) dominated sediment resuspension in the coastal seabed due to the fact that this wind direction had the longest average fetch. Winds from the onshore direction usually had smaller swell due to their short fetches and caused limited sediment re-suspension. The residual currents caused by NE winds also transport larger sediment. An individual NE wind event could transport sediment 8–13.6 t/m2 and 5.1–8.2 t/m2 in directions parallel and perpendicular, respectively, to the isobaths, which is much higher than the sediment transportation during an individual NW wind event, which could transport 1.5–4 t/m2 and 0.6–5 t/m2 parallel and perpendicular, respectively, to the isobaths. Our research shows that large swell and the accompanying residual currents caused by NE winds (from offshore direction) are a vital driving force for sediment resuspension and transportation in the offshore zone. • The height of swell strongly affected sediment resuspension in coastal seabed. • Large swell coming from offshore (NE in our study area) dominated sediment resuspension events in offshore zone (water depth >10 m). • During and after swell, residual currents are the major dynamics that transport sediment from coastal areas. • Abstract. [ABSTRACT FROM AUTHOR]

Seabed erosion is a vital process for sediment transport in estuaries and coasts. In this study, we investigated the seabed erosion process and its spatiotemporal variability in the Pearl River Estuary, China, via in-situ observations and laboratory experiments. It is found that the seabed was more erodible in the channel than at the shoal in most cases due to the difference in sediment size. In a neap-spring-neap tidal cycle, a laminated seabed with different sediment ages was inferred to form due to deposition on flood tides and erosion on ebb tides. The seabed was more erodible during spring tides than neap tides because of the difference in sediment availability at the unconsolidated fresh layer. Furthermore, the erosion curve showed evident flood-ebb asymmetry, which was found to be related to the variations of sediment availability and bottom stress. Besides, erosion experiments with different sediment consolidations revealed that sediment erodibility decreased with the growth of consolidation. This study promotes our understanding on the spatiotemporal variability of sediment erodibility, and has significant implications for simulation and prediction of estuarine sediment transport and morphological evolution. • The seabed was found more resistant to erode with the growth of sediment age. • The flood-ebb asymmetry in erodibility in the Pearl River Estuary was noted. • Seabed experiencing frequent erosion-deposition processes was prevented from further consolidation and became more erodible. [ABSTRACT FROM AUTHOR]