Shenliang Chen

Changes in progradation rate of the tidal flats at the mouth of the Changjiang (Yangtze) River, China

Progradation rates of tidal flats at the Chiangjiang River mouth, China, over different periods were examined, based on surveys and historical records. Rapid progradation after the Holocene sea-level rise began about 2000 years ago because of an increase in riverine sediment. The tidal flats at the river mouth have grown at a rate of about 5 km2/year and the southern mainland shoreline in front of the river mouth has advanced at a rate of 17 m/year. In recent decades, the lateral progradation rate at the frontal area has been from tens to hundreds of meters per year. The evolutionary process of the tidal flat is highly episodic under this general tendency of progradation. Spring–neap cyclicity shows clearly in calm seasons while the bare flat surface is sensitive to windy conditions. Different seasonal patterns occur between the marsh and the bare flat because of the influence of vegetation. Spatial changes in sedimentation rate are also striking, which could be attributed to differences in sediment distribution and hydrodynamics. The changeability of sediment surface shows a shoreward reduction from the subtidal area to the high marsh, likely due to the shoreward decrease in water energy and submergence time as well as the protective effect of marsh vegetation. It is predicted that the general progradation could be greatly slowed when the sharp reduction in riverine sediment caused by the Three Gorges project and the South-To-North Water Diversion will coincide with the rapid relative sea-level rise.

Spatial and temporal variability of water discharge in the Yellow River Basin over the past 60 years

Water discharge data of the Yellow River over the past 60 years was analyzed using the continuous wavelet transform (CWT) and Mann-Kendall (MK) test methods to identify spatial and temporal variation patterns. Potential connections between water discharge in the Yellow River Basin and El Niño/Southern Oscillation (ENSO) were also examined by the cross wavelet and wavelet coherence methods. CWT results show that the periodic oscillations in water discharges had occurred at the temporal scales of 1-, 2- to 4-, 6- to 8- and 10- to 22-year. It was also found that at the annual timescale (1-year) the phase relations between water discharge and ENSO were indistinct probably due to the strong influence by human disturbances. However, over the longer time scales, the phase relation becomes much clearer with an anti-phase relation being found mainly at inter-annual scale (2- to 8-year) and in-phase relation at decadal scale (16- to 22-year). According to the MK test results water discharge at most stations except Tangnaihai have decreased significantly and the abrupt change occurred in the mid-1980s or the early 1990s. The changes in water discharge were found to be influenced by both climate changes and human activities. Before 1970 the change in water discharge was positively related to precipitation variations in the river basin, but after 1970 the decrease in water discharge has been largely caused by various human activities including constructions of reservoirs, water abstraction and water-soil conservation with water abstraction being the main cause.

Seasonal, neap-spring variation of sediment concentration in the joint area between Yangtze Estuary and Hangzhou Bay

An analysis of variations in suspended sediment concentration (SSC) over one year is presented for the joint area between the Yangtze Estuary and the Hangzhou Bay. Results suggest that daily SSC presents periodic variations, which correspond with seasonal and neap-spring tidal cycle. SSCs are higher during the winter season and during spring tides when compared with summer and neap-tide conditions. Furthermore, data indicate that wave and tidal currents are the two dominant factors affecting SSC. Although the Yangtze River discharges abundant sediments to the study area, it does not directly affect concentration variations. But the changing path of the Yangtze River plume plays a certain role in the seasonal variations of SSC. Finally, the calculation of bottom stresses suggests that SSC is a function of both wave and tide-induced resuspension.

Partitioning of grain-size components of estuarine sediments and implications for sediment transport in southwestern Laizhou Bay, China

Sediment transport in estuarine systems has been of increasing interest for scientists during the past few decades. However, the mechanisms for sediment redistribution remain unclear. We characterized in detail sediment transport in the Xiaoqing River estuary using the mathematical Weibull function to partitiongrain-size components of surface sediments in the southwestern Laizhou Bay, Northeast China. Four partitioned components: finer than 4, 4.6–12.5, 23.4–63.3, and 67.1–132.6 μm were interpreted in terms of hydrodynamic conditions. During sediment transport, silt grains were suspended and moved seaward from three depositional centers, whereas fine-grained sands moved generally landward. Overall, sediments are transported clockwise in a generally NNE direction near shore and then turn eastward offshore. The mathematical partitioning method showed a great potential for future estuarine environmental studies.

A study of suspended sediment concentration in Yangshan deep-water port in Shanghai, China

Abstract Suspended sediment concentration (SSC) plays an important role in the estuarine environment. Its spatial or temporal variations in coastal zones and estuaries indicate that sediments are suspended, transported and deposited under various actions, such as the river discharge, waves, currents and local topography. Yangshan deep-water port is located in the Qiqu Archipelago with many islands and tidal channels. Taking Yangshan deep-water port sea area as an example, an analysis of distributions and variations in the SSC based on data measured in Oct. 1996, Jan. 1999, May 2006 and May 2007 and the surface SSC data measured during the period from 1998 to 2008, has been undertaken. Results from the study suggest that: (1) the yearly-averaged SSC decreased from 1998 to 2008; (2) the SSC has strong seasonal variations, the monthly-averaged SSC values are higher from November to April of the next year (the winter half year) than from May to October (the summer half year); (3) the periodic nature of the changes are obvious, which is consistent with a tidal semimonthly cycle of approximately 14.8 days; (4) the vertical distribution which is an e exponential distribution increase from the surface to the bottom. Furthermore, the data indicate that tidal currents are the most dominant factor affecting the SSC. Although the Yangtze River discharges abundant sediments to the study area, it does not directly affect concentration variations in seasonal time scales. But in yearly variation or long time scales, the SSC varying trend is coincident with sediment discharge change of the Yangtze Estuary. In addition, the construction projects, or dumping of sediments, have obvious effects on SSC and sedimentation for part of the area. “Strait-channel effects” have some degree of influence to the SSC distribution around islands.

Spatial and Temporal Variations in Grain Size of Surface Sediments in the Littoral Area of Yellow River Delta

Abstract The impacts of human activities on the sediment load of the Yellow River and the long-term evolution of the delta have been extensively investigated, but less is known of the variation in sediment grain size in the littoral zone. In this study, the data of surface sediment samples collected from the littoral area of the Yellow River Delta in 2000 and 2007 are used to investigate the spatial distribution and transport pathway of the sediment, as well as the grain size response to the drastic decrease in riverine sediment discharge. By applying the geostatistics analysis method to grain size parameters in determining the characteristic distance in the sediment trend analysis method, an effective way to understand the movement and gradation of surface sediments around the delta has been proposed. The results show that as a whole the sediment in the inshore erosion area is coarser than that in the offshore accretion area. The grain size trends obtained reveal two converging zones of sediment movement. We find a coarsening trend in sediment from 2000 to 2007. Possible mechanisms for this trend are discussed.

Progress of estuarine research in China over last 50 years

It is well known that the estuarine research in China has got great achievements since the past 50 years. The development of estuarine research in China and progress in fundamental study are presented firstly in this paper. Especially in the aspect of theoretic studies, a distinctive feature is the combination studies of dynamics, sedimentology and geomorphology, which obtain a series of achievements, such as estuarine diluted water, estuarine fronts, turbidity maximum, sediment movement, formation of mouth bar, estuarine discontinuity phenomena, estuarine development, etc. Then some successful applications are introduced taking the Qiantang River Estuary and Yangtze Estuary as examples. Finally, main directions of estuarine research are discussed on fundamental study, the impacts of human activities on estuaries, estuarine material flux, sediment movement, prediction of estuarine erosion and accretion, estuarine discontinuity, artificial estuaries, land reclamation and wetland protection.

Seasonal changes in coastal dynamics and morphological behavior of the central and southern Changjiang River delta

Seasonal changes in sea level, tidal range, wind, riverine discharges, nearshore SSC (suspended sediment concentration) and bed-level of intertidal flat at 4 different sites were shown. In addition, the statistical relationships between the dynamics and the behavior of the sediment surface were examined. The average intertidal elevation seems negatively correlated to sea level while positively correlated to nearshore SSC. The effect of wind on seasonal cycle of average intertidal elevation is not evident although wind is an important factor governing short-term erosion/accretion events. The influence of riverine discharges on seasonal cycle of deltaic intertidal flats is masked by other factors. It is concluded that seasonality on mudflats is more complicated than on beaches.

1500-year cycle dominated Holocene dynamics of the Yellow River delta, China

The world’s mega-deltas are extremely important from a human perspective and attract considerable effort to reveal their evolution, growth-related driving forces, and human impacts. Here, we report a case study on the Holocene deltaic evolution of the Yellow River, through development of a conceptual model, which is compared with paleo-proxy to analyze the forcing acting on the delta. The main conclusion is that superlobe switching was modulated by the 1500-year cycle. Cooling in Mongolia in response to strong Bond IRD events, which is coincident with warming in eastern China due to a strong Kuroshio Current, enhances the meridional temperature gradient, which then increases cyclone frequency and activates dust storms and terrestrial erosion throughout the catchment. Enhanced erosion supplies great amounts of material to the Yellow River and causes channel evulsion and superlobe development, expressed as dominant 1500-year cycle. At the same time, summer monsoon and solar forcing are uncorrected with deltaic evolution on these timescales. Therefore, we conclude that Holocene dynamics of the delta on a millennial timescale was dominated by winter cyclone activity across northern China and Mongolia.

AN EXPERIMENT STUDY OF EROSION CHARACTERISTICS OF SEDIMENT BED AT THE YELLOW RIVER DELTA

The modern Yellow River Delta is formed by fine-grained sediments through fast transportation and accumulation. Its coast is characterized by partially consolidated bed structure, high water content and weak erosion resistance. Despite recent advances in numerical morphodynamic modeling techniques, the current ability to predict long-term coastal changes at the Yellow River Delta is hampered by the lack of reliable data on the erosion resistance of the sediments deep in the bed. This paper describes a laboratory-based erosion study using a 30 m sediment core collected from the tidal flat on the northern coast of the Yellow River Delta in April 2004. The influence of various sediment properties such as bed stratification, grain composition, wet bulk density and water content on the measured erosion resistance has been analyzed in detail. The results reveal that there exist a definite negative relationship between the critical erosion shear stress and water content or median diameter, and also a clear posit...The modern Yellow River Delta is formed by fine-grained sediments through fast transportation and accumulation. Its coast is characterized by partially consolidated bed structure, high water content and weak erosion resistance. Despite recent advances in numerical morphodynamic modeling techniques, the current ability to predict long-term coastal changes at the Yellow River Delta is hampered by the lack of reliable data on the erosion resistance of the sediments deep in the bed. This paper describes a laboratory-based erosion study using a 30 m sediment core collected from the tidal flat on the northern coast of the Yellow River Delta in April 2004. The influence of various sediment properties such as bed stratification, grain composition, wet bulk density and water content on the measured erosion resistance has been analyzed in detail. The results reveal that there exist a definite negative relationship between the critical erosion shear stress and water content or median diameter, and also a clear positive relationship between the critical erosion shear stress and clay content. For the sediments of the same type, there was a positive relationship between the critical erosion shear stress and wet bulk density. It was also found that the erosion resistance changes considerably through depth with the lowest being near the surface and the highest at a depth of 9.1-13.5 m. The relationship between erosion rate and shear stress for erosion was shown to be dependent in a complex way on the grain distribution, sedimentation structure, wet bulk density and water content of the sediments. A number of distinct erosion behavior have been identified and although in most cases the erosion rate (ε) can be related linearly to the net bed shear stress (τ – τc), further study is needed to establish the erosion-stress relationship for the case were the erosion seems to show undulating behavior.