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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]

The process of flocculation in dredge plumes was investigated by field measurements using Laser In-Situ Scattering and Transmissometry (LISST), Optical Back Scatter (OBS) sensors and an Acoustic Current Doppler Profiler (ADCP) during dredging campaigns in Port Curtis, Queensland (Australia). Suspended sediment characteristics differed significantly between natural ambient suspended sediment and sediment suspended by dredging as well as between sediment suspended by mechanical and hydraulic dredgers. LISST measurements enabled distinction of the suspended sediment by origin and showed that the majority of sediment particles naturally in suspension were aggregated flocs made up of multiple smaller particles. Primary particles in natural suspended sediment had a median particle size (d 50) of 8 to 14 μm, while in-situ flocs had a d 50 of 52 to 99 μm. There were both microflocs and macroflocs present in the natural suspended sediment, with the microflocs being dominant (representing 67%–80% of the flocs). Mechanical dredging resulted in the release of individual particles, microflocs and macroflocs, with the potential for a comparable percentage of macroflocs as in natural suspended sediment (11%–27% compared to 20%–33% in natural suspended sediment). Macroflocs released by mechanical dredging were likely to have originated from the seabed. During overflow from the hopper, the suspended sediment released from the hydraulic dredger had smaller flocs compared to the natural suspended sediment, with a reduction in the d 50 of in-situ flocs from 91 to 67 μm and a reduction in the proportion of macroflocs from 32% to 24%. These results indicate that larger macroflocs present on the seabed had been broken up by the hydraulic dredging, with most of the suspended sediment remaining in the form of smaller microflocs, suggesting that the flocs were not completely disaggregated by the shear-stress generated during hydraulic dredging. Ongoing aggregation of suspended sediment was shown to occur in the sediment suspended by both dredging methods within 20–25 min of the plumes being generated. This study demonstrates the importance of considering flocculation in predictive dredge plume modelling studies to prevent significantly underestimating the settling velocity and substantially over-estimating the extent and intensity of dredge plumes if flocculation is not accounted for. • Flocculation occurs in plumes generated by both mechanical and hydraulic dredging • Flocs released by mechanical dredging were comparable to natural flocs • Flocs from hydraulic dredging were smaller than natural flocs • Ongoing aggregation of flocs occurred in the plume for both dredge types [ABSTRACT FROM AUTHOR]

Cross-front transport is a vital process in the offshore transport of terrestrial materials. The factors influencing cross-front transport are multifaceted and exhibit regional variability. As important factors regulating material transport patterns in the shelf seas of eastern China, the East Asian Winter Monsoon (EAWM) and Kuroshio current (including its branches) vary on multiple time scales, yet their role in controlling offshore transport is not fully understood. In this paper, cross-front offshore transport in the North Yellow Sea is chosen to investigate the aforementioned issue using numerical simulation. By excluding other potential processes for offshore transport, frontal instability is identified as the primary mechanism for offshore transport, which is triggered by winter storms (bursts of the EAWM); hence, the intensity of offshore transport is highly associated with the strength of winter storms on synoptic scales. However, on the inter-annual scale, the intensity of offshore transport is more likely regulated by the strength of the Kuroshio current, and a warming Kuroshio facilitates offshore transport. Although the role of each factor in controlling the intensity of offshore transport is delineated, the interactive effects of the EAWM and Kuroshio current on offshore transport are quite intricate, and how to quantitatively estimate their roles on multiple time scales remains a challenge using modelling. The results obtained from this study can be applied to analyse cross-front transport throughout the eastern China seas and bear significant implications for future studies on pollutant diffusion, nutrient distribution, and sediment transport in the coastal area. • Cross-front transport around the Shandong Peninsula was revealed using a numerical model. • EAWM and Kuroshio affect the strength of cross-front transport on different time scales. • With global warming, the temperature of Kuroshio might gradually become the primary control of cross-front transport. [ABSTRACT FROM AUTHOR]

Estuarine dams are constructed in estuaries for reasons such as freshwater, flood control, and navigation. By changing tidal and river properties, estuarine dams can change the circulation and sediment transport in estuaries. Previous studies have investigated the along-channel changes in flow and sediment transport, however across-channel changes due to an estuarine dam are not well understood. To increase our understanding, this study analyzed an idealized estuary using the COAWST numerical modeling system. Models of strongly stratified, partially mixed, periodically stratified, and well-mixed estuaries were run for one year. Then, the models were subject to the construction of an estuarine dam and run for another year. For each estuarine type, scenarios with an estuarine dam at x = 20, 55, and 90 km from the mouth and freshwater discharge intervals of Δt = 0.5, 3, and 7 days were investigated, and the scenarios were compared. The results indicated that the river-dominated and tide-dominated estuaries behaved differently. In river-dominated estuaries, the residual circulation tended to be inflow in the channel and outflow over the shoals due to the estuarine exchange flow, and the secondary circulation was bottom divergent due to differential advection. The exchange flow and secondary circulation were found to weaken with longer discharge interval. The sediment fluxes in the channel were dominant, and, despite the weaker exchange flow, were found to be greater and directed seaward due to strong dam discharge for long discharge intervals. In tide-dominated estuaries, the residual circulation tended to be inflow over the shoals and outflow in the channel due to the tide-induced circulation, and the secondary flow was bottom convergent due to differential advection resulting from Stokes return flow. As the estuarine dam was located nearer to the mouth, the tide-induced circulation was replaced with exchange flow, and the secondary flow became bottom divergent. The sediment fluxes in the channel were dominant in this case as well, and were determined by tidal asymmetry from the interaction of the tides with the residual current, becoming directed landward as the seaward tide-induced circulation decreased with the estuarine dam near the mouth. With respect to bed level change, gradients in along-channel sediment fluxes were found to contribute most to bed level change near the mouth and estuarine dam, whereas gradients in across-channel sediment fluxes contributed most to deposition on the estuarine shoals. This study demonstrates that estuarine dam location and discharge interval can alter estuarine transverse variability and improves our understanding compared to previous analyses based solely on along-channel processes. • River-dominated estuaries exhibited inflow in channel, outflow over shoals. • Tide-dominated estuaries exhibited inflow over shoals, outflow in channel. • Estuarine dams near mouth with large discharge interval reduced these circulations. • Tide-dominated estuaries switched from tide-induced circulation to exchange flow. • Lateral sediment fluxes contributed to shoal deposition. [ABSTRACT FROM AUTHOR]