Interaction between the Yellow river and its estuary

Interaction between the Yellow river and its estuary

Kriele, H.O.

De Vries, M. (mentor)
Wang, Z.B. (mentor)
Fokkink, R.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1996-04-13

The 5,464 km long Yellow River (Chinese: Huang He) is the second longest river of the Peoples Republic of China. Its source is in Qinghai-Tibet Plateau and empties after serving an agricultural population of over 126 million people in the Bohai Bay. The main characteristic of the Yellow River is the extremely high sediment load which causes the river bed to rise up to 10 cm per year and the river mouth to propagate into the shallow Bohai Bay with an average velocity of 2 km per year. The rising bed level results in a rising water level causing dike-breaches, occurring with an average return period of 10 years. After a dike-breach the Yellow River changes its course, and a new flow path is formed. The length of the new flow path to the Bohai Bay is shorter than the previous one. This causes a drop in the water level at the place of the dike-breach. The bed level upstream of the dike-breach follows the drop of the water level. Meanwhile the river path becomes longer by deposition at the river mouth. Aggradation takes place and the water level rises again until the next dikebreach occurs. In chapter 1 this process and the Yellow River characteristicsis are described. The periodic rise and fall of water- and bed level, initiated in the delta region, is propagating in upstream direction causing a time dependent bed level. The amplitude of this process reduces in upstream direction. The research objective is to determine a length scale of the river reach under influence of this periodic rise and fall. The time depending variation of the riverbed can be estimated by a quasi-steady one-dimensional mathematical model, which is derived of the basic equations of motion and continuity of water and sand in chapter 2. To solve this model a numerical approach is applied, which is discussed in chapter 4. Linearising this model leads to a hyperbolic differential equation. I f more symplifications are made - not only quasi-steady, but also quasi-uniform - a parabolic differential equation is obtained. In chapter 3 both differential equations are studied more closely with for several boundary conditions and several parameters. The advantage of the analytical solution is a quickly obtained first estimation of the behaviour of the time depending bed level of the lower reach of the Yellow River. Both numerical- and analytical studies result in an estimation of the length scale, expressed as a relaxation length, of the river reach under influence of the development in the Yellow River delta. The results show that the diffussion coefficient, determined by the sediment transport formula and the slope of the bed level, is of great influence on the relaxation length. The relaxation length increases in case of: - increasing diffusion coefficient - increasing period of the boundary condition - using a hyperbolic model instead of a parabolic model - using a numerical model instead of an analytical model In case the boundary condition consists of a sum of sine functions, representing for example a sudden drop of the water level, the time depending bed level transforms into a sine function with increasing distance from the boundary condition. This is the result of the different periods of the sine functions. The smaller the period, the sooner it is damped. The diffusion coefficient for the Yellow River is approximately 400 m2/s, which results in a relaxation length of approximately 200 km in case of a periodic boundary condition with a sudden drop in water level and a period of 10 year.

Yellow river
siltation
estuary
estuarine morphology

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Student theses

master thesis

(c) 1996 Kriele , H.O.