Abstract:

The sedimentary environments of the Yangtze Estuary riverbed have changed a lot with the construction of water conservancy projects in the drainage basin and the estuary, which have significantly influenced the river regime evolution and accretion/erosion of the estuary riverbed. However, less has been reported on detailed sedimentary characteristics of the current estuary riverbed, which restrains further evaluation on the impact on both hydrology and sedimentology of the Yangtze Estuary. Therefore, based on the large-scale surface sediment samples and associated flow-sediment data collected during September 2015, we analyzed the sedimentary characteristics in the seven riverbeds within the estuary and discussed the influencing factors and mechanisms. The particle sizes of the sediment samples were analyzed by Model LS-13320 Coulter laser particle size analyzer and the types of the surface sediments were classified by Folk’s classification. The results are shown as follows. The riverbed sediments of the whole estuary is dominated by sandy silt and silty sand. The coarsest surface sediments are observed in the outer estuary of north branch/north channel above 10 m and upper north/south branch, while the finest surface sediments are found in the north passage, lower south passage and the outer estuary of north/south passage deeper than 5 m. Generally, the particle sizes of the river sediments are coarser in the main channels and finer on the shoals, and decrease from west to east and from north to south. The lateral sedimentation distribution characteristics are affected by the sediment transportation and deposition in ebb and flow process. The longitudinal sedimentation distribution reflects the differences of the sedimentary environment strengthened by the wind wave. The sediments in the north branch are coarsened remarkably due to the intense north wind wave action. The difference of sediment origins is the factor leading to deviation of the sedimentary environment. The sediment in the south branch and upper south channel show a characteristic of riverine sediments, while the sediments in the north channel, north passage and south passage show a mixture of riverine and marine sediments. Sediments in the outer estuary mainly provide fine-grained particles to the inner estuary.

Key words: Yangtze Estuary, riverbed, sediment characteristics, sediment classification, impact factors