关键词: 河漫滩, 粒度特征, 长江下游, 沉积环境

Abstract:

River floodplains are constructional features of the fluvial landscape, and the overbank deposition on floodplain represent the geomorphological and hydrological conditions of a certain river. This article analyzed grain-size characteristics of three 100 centimeters long overbank cores sampled at Nanjing-Zhenjiang Reach in the downstream of the Changjiang River and its environment implication. The results indicate that the grain size results of overbank cores are not only controlled by the floods but also genesisly related to sedimentary environment. Both the floods intensity and the depositional environment of the overbank should be considered during the research of the overbank sediments feature and its influence factors. Along the Nanjing-Zhenjiang Reach, the grain size analysis of the three cores indicated the hydrodynamics on the overbank during the floods was weak and the whole cores were mainly composed of the fine sediments. The relatively stable part of the sediments was clay, varying between 11.43%-17.65%. Among the three cores, the ZR Core was located at the highest depositional hydrodynamics condition, the coarser grain size was obvious and the highest sand content was 66.73%. The vertical distributions of the grain size parameters of the GB Core were varied in the same way, and it was also the same in the ZR Core, furthermore, the sand contents increased upward in the ZR Core. The sorting of the ZH core had two obvious stages, the upper part with the much better sorting than the lower part of the core, which was different from the vertical distributions of the other parameters. Along the Nanjing-Zhenjiang Reach, the outcrop of the overbank sediments was composed of the thin layers of fine and coarser silts, corresponding with the alternation of weak and strong hydrodynamics during the flooding on the overbank. The obvious fine sand layers of the outcrop indicated the strongest period of flooding. The other factors influenced the grain size features included the following factors: the river channel gradient, river channel regimes, the evolution of the braided river and the vegetation conditions on the overbanks. As Nanjing-Zhenjiang Reach is located in the downstream of the Changjiang River with a small channel gradient, the water velocity on the overbank is slow resulting in the fine overbank sediments mostly composed of silts. The ZR Core is located at the junction of two channels where it is scoured. During the flooding time the hydrodynamics was strong on the sampling site and the deposition were composed of coarser sand, or sandy silts. The ZH Core is located in the downstream of the approach channel of the Zhenjiang Port. As the approach channel diverting the water, the runoff of the ZH Core is decreased, thus the depositional hydrodynamic is weak corresponding with the fine deposits. The sand contents increase upward in the ZR Core with the evolution of the braided river, while the grain size composition of ZH Core is little influenced for it is located in the downstream right bank 4 km from the braided river. In addition, as located at a stable straight reach, the grain size composition of the GB Core is stable. The ZH Core site is surrounded by the reed plant expanding rapidly in the past decade. In front of the reed, the deposition is quick trapped the coarser and fine sediments and in the central part of the reed covered overbank the deposition is mainly composed of fine sediments. So the variation of the sorting parameter of the ZH Core indicated the changes of the reed cover evolution in the past decade. Further more, thin-bedded strata is developed in the overbank located in the estuary of Changjiang River. During flood deposit research, the sampling interval should be designed in accordance with the deposit thickness of the thin bed.

Key words: overbank, grain size features, lower reaches of the Changjiang River, depositional environment