To examine the spatial and temporal variability of sediment grain size in exposed tidal wetlands with ample sediment supply, we sampled sediments and measured hydrodynamics, accretion/erosion rates, and vegetation characteristics in the Yangtze Delta. Sediment grain size exhibited a landward/upward decreasing trend. This trend is mainly attributed to attenuation of hydrodynamics. A 630-day series of daily surface sediment sampling at a fixed site on an unvegetated intertidal flat revealed significant seasonal and storm-cyclic changes in grain size. This temporal variability was related to alternating accretion/erosion events, with erosion associated with coarser grain size. Such temporal dynamics were not present in vegetation, where sediment remained fine grained throughout the year. In the marsh, vegetation cover enables the trapping of fine-grained sediments in the following ways: (a) adherence of suspended sediments onto plants; (b) deposition of suspended sediments stimulated by attenuation of hydrodynamics through plant obstruction; and (c) prevention of resuspension of fine-grained deposits due to the protection of the plant canopy. The influence of vegetation on sediment grain size was clearly seen when comparing sediment trapped by different vegetation types and seasonal patterns of trapped sediment on different vegetation canopy densities. The relatively high plant biomass of the recently introduced Spartina alterniflora enhanced the trapping effect, whereas plant degradation due to buffalo grazing reduced the trapping effect. We conclude that for exposed tidal wetlands with ample sediment supply such as the Yangtze Delta, the spatial and temporal variability of sediment grain size is governed predominantly by physical controls on the unvegetated flat and predominantly by biophysical interaction of hydrodynamics and vegetation in the salt marsh, rather than by sediment supply.

Spatial and temporal variations in sediment grain size in tidal wetlands, Yangtze Delta: On the role of physical and biotic controls

http://www.vliz.be/imisdocs/publications/235246.pdf

Relating accretion and erosion at an exposed tidal wetland to the bottom shear stress of combined current-wave action

Intra-tidal sedimentary processes associated with combined wave-current action on an exposed, erosional mudflat, southeastern Yangtze River Delta, China

Sub-estuaries, i.e. tidal creeks and also larger estuaries that branch off the stem of their main estuary, are commonplace in many estuarine systems. Their physical behaviour is affected not only by tributary inflows, winds and tides, but also by the properties and behaviour of their main estuary. Measurements extending over more than an annual cycle are presented for the Tavy Estuary, a sub-estuary of the Tamar Estuary, UK. Generally, waves are small in the Tavy because of the short wind fetch. A several-hour period of up-estuary winds, blowing at speeds of between 7 and 10 m s −1 , generates waves with significant wave heights of 0.25 m and a wave periodicity of 1.7 s that are capable of eroding the bed over the shallow, ca. 1.5 m-deep mudflats. Waves also influence sedimentation within and near salt marsh areas. An estuarine turbidity maximum (ETM) occurs in the Tavy's main channel, close to the limit of salt intrusion at HW. Suspended particulate matter (SPM) concentrations typically are less than 40 mg l −1 at HW, although concentrations can exceed 80 mg l −1 when tides and winds are strong. Flood-tide SPM inputs to the Tavy from the Tamar are greater during high runoff events in the River Tamar and also at spring tides, when the Tamar has a high-concentration ETM. Higher SPM concentrations are experienced on the mudflats following initial inundation. Without wave resuspension, this is followed by a rapid decrease in SPM for most of the tide, indicating that the mudflats are depositional at those times. SPM concentrations on the mudflats again increase sharply prior to uncovering. Peak ebb tidal speeds at 0.15 m above the mudflat bed can exceed 0.26 m s −1 at spring tides and 0.4 m s −1 following high runoff events, which are sufficient to cause resuspension. Time-series measurements of sediment bed levels show strong seasonal variability. Higher and lower freshwater flows are associated with estimated, monthly-mean sediment transport that is directed out of, or into, the upper sub-estuary, respectively. Seasonal sediment transfers between the estuary and its sub-estuary are discussed.

Turbidity and sediment transport in a muddy sub-estuary

Controls on suspended sediment concentration profiles in the shallow and turbid Yangtze Estuary

During the spring-neap period of 17–24 August 2004, turbidity, horizontal and vertical current velocities and echo intensity were measured using OBS-3A and ADP-XR instruments over an intertidal flat within the semi-enclosed Jiaozhou Bay, China, to examine patterns in suspended sediment concentration (SSC) and possible control factors. SSC was found to be lower than 30 mg l−1 in most of the water column and for most of the tidal cycle. This is attributed mainly to the low hydrodynamic energy, in particular weak currents (near-bottom maximum 1- and 8-min-interval velocities were only 26.1 and 14.2 cm s−1, respectively), and limited fine-grained sediment supply by rivers. However, high SSC values ranging from 100 to >1,000 mg l−1 occurred over short periods at the beginning and the end of inundation. This phenomenon is attributed to the shoaling effect of frequent wind-generated waves, as a result of which near-bottom SSC fluctuations display a U-shaped trend during each tidal cycle.

Cyclical variability of suspended sediment concentration over a low-energy tidal flat in Jiaozhou Bay, China: effect of shoaling on wave impact

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Cyclical variability of suspended sediment concentration over a low-energy tidal flat in Jiaozhou Bay, China: effect of shoaling on wave impact