Sedimentation on a Growing Intertidal Island in the Yangtze River Mouth

Abstract Based on surficial sediment samples, experiments of the trapping effect by vegetation on suspended particles, as well as measurements of changes in the sediment surface, this article deals mainly with general grain-size characteristics, the organic content of the sediments, the accretion rate in tidal marshes and the influence of the plant community along with the effect of benthic fauna on sedimentation. Conclusions that were reached included (a) mean grain-size M z (Φ) is between 3·0 and 8·5 under fair-weather conditions and can be as low as 2·5 in storm events, the grain-size composition is similar to the composition of the Yangtze River sediment; (b) a trend that the finer half of the sediment samples covers a wider grain-size range than the coarse half is shown by the fact that M z (Φ)>Φ50for 96% of the samples and SKI>0 for 97% of the samples; (c) positive correlation between M z and σ1supports the observation that sediment sorting is functionally related to grain-size with the best sorting occurring at a median grain-size of 2·5Φ–3·0Φ. This fact corresponds to the theory that less energy is needed for the transport of fine sands (compared with both coarser and finer grains); (d) the organic content in surficial sediments is less than 1·5%. This low content of organic matter largely results from a high accretion rate of mineral grains from the river; (e) the grain-size of surficial sediment varies both temporally and spatially. The finest sediments occur in the centre of the marsh, becoming coarser peripherally. Around the island, the coarsest grain-size occurs on the seaside because of a higher water energy there. Grain-size in the main marsh is also influenced by seasonal changes in the plant community while the water energy especially during the storms has a large effect on the grain-size of the bare flats as well as on the outer area of the marsh; (f) accretion rates in marsh areas can be up to several decimeters. In this type of environment, the trapping effects of plants, as well as the biodeposition by benthic fauna, contribute very little to the total sedimentation.

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