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• Eight of thirteen typhoons occurred in 2016 impacted the Yangtze Delta wetland. • The impacting typhoons were 450–2000 km away from the delta. • The remote typhoons caused 2–5 times maximum increases in sediment dynamics. Intertidal wetlands are widely considered to be vulnerable to storm impacts. While much attention has been given to the effect of hurricane/typhoon landfall on local erosion/deposition in intertidal wetlands, less is known about how distant hurricanes/typhoons can affect an intertidal wetland or about the comprehensive effects of hurricanes/typhoons on hydrodynamics, sediment transport and seabed stability. Here, we investigate the possible impacts of thirteen typhoons that occurred in the western Pacific during summer–autumn 2016 on an intertidal wetland in the Yangtze Delta. We detected the impacts from eight of the typhoons during half of their duration when these typhoons were 450–2000 km away (nearest distance ever for each typhoon) from the delta. These remote typhoons caused 2–5 times maximum increases in hydrodynamics and suspended sediment concentration. Correspondingly, considerable net mudflat erosion and marsh deposition were observed. We therefore conclude that intertidal wetlands can be strongly affected by typhoons, even those passing by hundreds of kilometres away. These findings contribute to understand how far and in what extent hurricanes/typhoons can affect intertidal wetlands and to increase knowledge needed for coastal management. [ABSTRACT FROM AUTHOR]

A short-term intensive measurement campaign focused on flow, turbulence, suspended particle concentration, floc dynamics and settling velocities were carried out in a brackish intertidal creek draining into the main channel of the Scheldt estuary. We compare in situ estimates of settling velocities between a laser diffraction (LISST) and an acoustic Doppler technique (ADV) at 20 and 40 cm above bottom (cmab). The temporal variation in settling velocity estimated were compared over one tidal cycle, with a maximum flood velocity of 0.46 m s −1 , a maximum horizontal ebb velocity of 0.35 m s −1 and a maximum water depth at high water slack of 2.41 m. Results suggest that flocculation processes play an important role in controlling sediment transport processes in the measured intertidal creek. During high-water slack, particles flocculated to sizes up to 190 μm, whereas at maximum flood and maximum ebb tidal stage floc sizes only reached up to 55 μm and 71 μm respectively. These large differences indicate that flocculation processes are mainly governed by turbulence-induced shear rate. In this study, we specifically recognize the importance of along-channel gradients that places constraints on the application of the acoustic Doppler technique due to conflicts with the underlying assumptions. Along-channel gradients were assessed by additional measurements at a second location and scaling arguments which could be used as an indication whether the Reynolds-flux method is applicable. We further show the potential impact of along-channel advection of flocs out of equilibrium with local hydrodynamics influencing overall floc sizes. [ABSTRACT FROM AUTHOR]

Abstract: Sediment dynamics have an important influence on the morphological evolution of tidal wetlands, which consist of mudflats and salt marshes. To understand the nature of sediment behavior under combined current–wave action at an exposed tidal wetland, we measured the waves, currents, water depths, bed-level changes, and sediment properties at a mudflat–salt marsh transition on the Yangtze Delta, China, during five consecutive tides under onshore winds of ~8m/s, and calculated the bed shear stresses due to currents (τ c ), waves (τ w ), combined current–wave action (τ cw ), and the critical shear stress for erosion of the bottom sediment (τ ce ). The bed shear stresses under combined current–wave action (τ cw ) were approximately five times higher on the mudflat (up to 1.11N/m2; average 0.27N/m2) than on the salt marsh (up to 0.14N/m2; average, 0.06N/m2). On the mudflat, τ cw was larger than the critical erosion shear stress (τ ce =0.103N/m2) for 70% of the period of submergence, whereas τ cw was always lower than τ ce at the salt marsh site (τ ce =0.116N/m2). This result indicates that the sediment dynamics on the mudflat were dominated by erosion, whereas at the salt marsh they were governed by deposition, which is in agreement with the observed bed-level change during the study period (−3.3mm/tide on the mudflat and 3.0mm/tide on the salt marsh). A comparison of τ cw values calculated using the models for bed shear stresses under combined current–wave action indicates that both models are applicable to the present case and effectively predict the bottom shear stress under combined current–wave action. Overall, we conclude that τ cw in combination with τ ce is useful in assessing the hydrodynamic mechanisms that underlie the morphological evolution of exposed tidal wetlands. [Copyright &y& Elsevier]

Erosion of tidal flats is to a large degree determined by the erosion threshold of their cohesive sediments, i.e., the critical bottom shear stress identifying the onset of erosion. Given that the erodibility of tidal flats can vary strongly over both space and time, rapid in situ measuring methods for quantifying the critical bottom shear stress are needed. As the largest changes in tidal flat elevation are generally observed during storms when shear stress is wave dominated, we developed a mobile Oscillatory-Channel Resuspension flume, OsCaR, as a rapid method to measure the critical wave-generated shear stress on sediment cores directly taken from the field. We assessed the performance of the OsCaR-flume by conducting erodibility measurements on a range of artificial sand – mud mixtures, and along a natural longshore estuarine sediment type and benthic assemblage gradient. Measured erodibility of the artificial sediments matched well with sediment transport theory, indicating that the OsCaR-flume is able to reproduce wave-generated shear stress accurately. Moreover, the patterns observed in the influence of total benthic metabolic rate divided over functional groups on sediment erodibility show that the OsCaR-flume is able to capture the main benthic processes, as they corresponded well to previous laboratory based studies. • A new method for measuring natural sediment erodibility under waves is introduced. • Sediment resuspension rates can be used to quantify critical bed shear stress. • Benthic metabolic rate may explain erodibility patterns in a field context. [ABSTRACT FROM AUTHOR]