2000, Volume 2, Issue 4
Strategic Study of CAE >> 2000, Volume 2, Issue 4
Diverting Seawater to Scour the Estuary and to Regulate the Lower Yellow River
1. China Institute of Water Resources and Hydropower Research , Beijing 100044
2. Tsinghua University, Beijing 100084
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Abstract
The writers propose to divert seawater from Guanli Harbor on the coast of Laizhou Bay with transversal canals and have it discharged into the Lower Yellow River at Lijin, about 110 km upstream of the river mouth (route B, Fig. 1). The flow diverted ranges from 500 to 1 500 m3/s, to be stipulated later in the feasibility study. This flow of “clear” seawater would strongly scour the channel downstream of Lijin and reduce its surface slope so much that for a flood of 3 000 m3/s an abrupt drop of 3.3 to 7.9m in the water surface would occur at Lijin. This drop would bring about retrogressive erosion of the riverbed upstream of the point of injection of seawater. The extent of this erosion is estimated to be 270 km upstream from Lijin. Thus a total of about 380 km of river channel would be scoured down. The reach downstream of Lij in would be transformed from an above - the - ground river to a down - in - the - ground river and the reach above Lijin would possess a deeper main channel. Safety in the defense against flood would thus be much enhanced.
For rational use of the Yellow River water resources, the State Council has allocated definite water right to every riparian province. A runoff of about 20 km3 is, however, to be reserved for sediment transport, in order to prevent the channel bed from excessive silting. With the facility available for the injection of seawater at Lijin, however, in the region down stream of Lijin one may redeem a portion of fresh water so reserved and restored the channel afterwards by scouring with seawater. Depending on capacities of pumping and reservoir storage available, the amount of fresh water redeemed may well run up to 1 km3 annually. This would be very valuable to the development of the estuarine region.
Since 1972, the Lower Yellow River has experienced a dry spell and has dried up from time to time. This has upset the ecological balance of the Bohai Sea because of interruption in the discharge of nutrient - carrying turbid water from the river. Scouring the estuary by diverting seawater will maintain a year round discharge of turbid water to the sea and help improve the ecological conditions as well as checking the receding of the coastline.
Turbid seawater being of greater specific weight than the clear seawater would form density current when discharged to the Bohai Sea. Sediment would then be conveyed to points far from the river mouth, thus delaying or even stopping the extension of the Yellow River delta. Moreover, when the river dries up, the river mouth would most likely recede under the scour of the discharging seawater because of lack of sediment supply from the drainage basin. Taking all the factors into account, one may expect little delta extension when seawater is diverted for scouring. Lijin, where seawater is injected, may then be taken as the effective location of the river mouth where the longitudinal equilibrium profile of the Lower Yellow River would terminate. This equilibrium profile would be lower than the original one by 4～12 m, depending on the estimated values of equilibrium slope for the final reach of the estuary. The hazard of levee breaching is thus much reduced.
Keywords
diverting seawater ; scouring ; the Lower Yellow River ; above-the-ground river ; water resources
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References
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