Sedimentation in an estuarine mangrove system

摘要

Sediment dynamics were studied in a mangrove system in the Ba Lat estuary of the Red River, Vietnam. The study area was situated on a riverbank, which consisted of a bare mudflat containing a vegetation gradient from pioneering mangroves to 5- to 7-year-old mangroves. Measurements were conducted in the dry season (February/March 2000) and in the wet season (July/August 2000). Actual sedimentation rates were measured using sediment traps and the elevation of the area was surveyed in both periods. Two undisturbed sediment cores were taken in the densely vegetated area for ~(210)Pb-dating analysis and showed long-term sedimentation rates of 0.22 g cm~(-2)yr~(-1) and 0.36 g cm~(-2)yr~(-1). Sedimentation rates recorded by the sediment traps in the vegetated area were 2.94 g cm~(-2)yr~(-1) in the dry season and 3.46 g cm~(-2)yr~(-1) in the wet season. In both seasons, the sedimentation rates on the bare mudflat were five to ten times higher than in the densely vegetated area. However, the elevation of the mudflat was slightly lower in the wet season than in the dry season. This implies that erosion must have taken place between March and July. This study demonstrates that the bare mud bank of an estuary is highly dynamic until mangroves cover it. Sediment delivery to the vegetated zones is low but the protective effect of vegetation against erosion by waves and currents is strong. This results in small but steady actual sedimentation rates under mangroves and therefore sustained long term accretion. The study further identifies the need for similar measurements to be made in different seasons covering the effects of periodic storm events.