The role of Scirpus marsh in attenuation of hydrodynamics and retention of fine sediment in the Yangtze Estuary

In order to examine the influence of marshes on processes of hydrodynamics and sedimentation, comparative pairs of measurements were conducted respectively in marsh environments (natural or artificial) and their adjacent bare flats (also natural or artificial) for data on current velocity, wave height, suspended sediment concentration, grain-size of deposits and rate of accretion. The ratios of mean current velocity (the average of a tidal cycle) over the marsh to its adjacent bare flat was 0.18-0.84, and those for mean wavelength, mean wave height and mean suspended sediment concentration at the beginning of submergence were 1.4, 0.43 and 0.71, respectively. Waves over the marshes travelled in rows while those over bare flats were solitary. The amount of sediment trapped by the stems and leaves of Scirpus mariqueter reached 298 g m(-2). Due to the attenuation of water energy and the sediment-trapping effect, it was easier for fine sediments to deposit in the marsh. Marsh sediments were much finer tian those on the adjacent flat with mean grain-size (phi) of 8.27 in Spartina alterniflora marsh, 7.45 in S. mariqueter marsh and 5.83 on thr bare fiat. Consequently, preferential accretion occurred in the marsh areas. The influence of marshes on hydrodynamics and sedimentation was related to plants and the location of the measurement. Marshes with S. mariqueter, S. alterniflora and reeds were similar in attenuation of hydrodynamics and retention of fine sediments. (C) 1998 Academic Press.