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The formation and geomorphic evolution of the Jiuduansha Shoal are investigated over the past 50 years in this paper. The sedimentary processes, formation, and geomorphic evolution of the Jiuduansha Shoal in the Yangtze River Estuary, eastern China, are analyzed based on digitized sea maps from 1945 to 2001, satellite images from 1975 to 2001, and field survey data in the spring-neap tidal cycle in the dry and flood seasons in 2003. The suspended sediment concentration and hydrodynamics of the North Passage and South Passage during the dry and flood seasons in the spring-neap tidal cycle in 2003 were investigated, and relations between the North Passage and South Passage of the Jiuduansha Shoal are analyzed. Results show that seasonal and spring-neap tidal cycle variations in sediment and water discharge from the drainage basin correspond to erosion and deposition on the Jiuduansha Shoal. The results and data can provide useful information for the management of the Yangtze River Estuary and restoration of the estuarine tidal flat ecology.

This paper presents a three-dimensional (3D) mathematical model for suspended load transport in turbulent flows. Based on the stochastic theory of turbulent flow proposed by Dou, numerical schemes of Reynolds stresses for anisotropic turbulent flows are obtained. Instead of a logarithmic law, a specific wall function is used to describe the velocity profile close to wall boundaries. The equations for two-dimensional suspended load motion and sorting of bed material have been improved for a 3D case. Numerical results are in good agreement with the measured data of the Gezhouba Project. The present method has been employed to simulate sediment erosion and deposition in the vicinity of the Three Gorges Dam. The size distribution of the deposits and bed material, and flow and sediment concentration at different times and elevations, are predicted. The results agree well with the observations in physical experiments. Thus, a new method is established for 3D simulation of sediment motion in the vicinity of dams.

The ecological and hydraulic features of step-pool systems are studied by field investigation, measurement, sampling, and analysis. The study is done on Shengou and Jiuzhai Creeks, where step-pool systems have developed, Fork Gully, where a step-pool system is developing, and the Jinsha River and Jiangjia and Xiaobaini Ravines, where there is no step-pool system. Boulders, cobbles, and gravel tightly interlock and form the steps having an inherent stability that only extreme floods are likely to disturb. Gravel and sand deposit in the pools behind the steps. These steps and pools provide high diversity of habitat for the stream biocommunity. The density of benthic macroinvertebrates in streams with step-pool systems is several 100 times higher than neighboring streams without step-pool systems. A new habitat diversity index is proposed considering the spatial distribution of various substrates, velocity, and water depth. The study reveals that the biodiversity of benthic macroinvertebrates increases with habitat diversity. Measurements with a specially designed instrument were done to study the development of step-pool systems and its effects on resistance to the flow and stream bed stability. A step-pool system maximizes the flow resistance and protects the bed sediment from erosion. Thus, the riverbed and bank slope are stabilized. The development degree of step pools is proportional to the streambed slope. The bed resistance increases with the development degree of step pools. Riverbed inertia represents the stability of the streambed. The development of step-pool systems greatly increases the riverbed inertia, and, therefore, maximizes streambed stability.

Vegetation and erosion are a pair of competing and interactive factors that affect the quality of watershed ecosystems. The objective of this study is to develop an innovative approach for conceptualizing and simulating the vegetation-erosion dynamics. Differential equations of vegetation-erosion dynamics have been developed to describe the relevant vegetation processes, with the relevant solution methods being provided. Based on the developed model, a vegetation-erosion chart can be produced for predicting the tendencies of vegetation and erosion under different land-use conditions. Thus decision supports in terms of desired measures to improve the system conditions can be provided. In general, vegetation of a watershed may exist in three states, including (1) vegetation-developing and erosion-reducing; (2) vegetation-deteriorating and erosion-increasing; and (3) transitional state between states (1) and (2). Humans may change a watershed system from one state into another. The effort needed for such a change depends on the distance between the present position and the destination one as shown on the vegetation-erosion chart. The developed model has been applied to three regions, including the Xiaojiang, Heishui, and Shengou Watersheds in China. The results demonstrate that the proposed vegetation-erosion dynamics is a powerful tool for simulating and predicting vegetation evolutions in the watersheds. Generally, reforestation and erosion-control measures would improve vegetation coverage slowly in the first 10 years, but become much faster in the second 10 years; this implies that a long-term strategy is needed. The results also indicate that, for revegetating hilly areas, erosion control is critical; merely planting trees and shrubs is insufficient for greening the exposed land.

Drag reduction in flows with clay mineral particles was experimentally studied. Experiments with clay suspensions flowing over smooth, gravel, and stone beds proved that resistance of the flow may be considerably reduced if the flow boundary is rough. The friction factor of flows of high clay concentrations over the gravel bed was less than half the clear-water flow at the same discharge and and energy slope. At low concentrations, a floc of clay particles having a long-chain structure may damp turbulence and result in drag reduction. At high concentrations, all flocs in clay suspension connect together and form a three-dimensional net structure, which affects the resistance in two ways: (1) By damping turbulence; (2) by increasing the viscous resistance. For flows over rough boundaries, turbulent shear dominates the resistance, and high clay concentrations cause drag reduction. For flows over a smooth bed, the effect of damping turbulence was counterbalanced by the effect of increasing viscous resistance; therefore, no drag reduction occurred.

The effect on turbulence of adding coarse sand, carried as bed load, in flows of water and clay suspensions was studied. The results showed that the mean velocity of the flows carrying bed load was smaller and the turbulence intensity was greater than those in the flows without bed load. The presence of bed load resulted in higher Reynolds stress and larger velocity gradient, and thus greatly enhanced the energy transmitted from the mean motion to turbulence. In flows of clay suspension, turbulence developed in the lower flow with quite high turbulence intensity, while laminar flow occurred in the upper flow. However, with bed load the entire flow became turbulent. The position of maximum turbulence intensity was more distant from the bed. The average turbulence intensity increased with rate of bed‐load transport. The spectrum of turbulent energy shifted to higher frequencies in bed‐load‐carrying flows.