Temporal Change in Bed Level of a River Mouth Channel, Yangtze River Mouth: with Emphasis on the Response to River Discharge and Storm
Keywords: Channel bed-level, river discharge, storm, river mouth, the Yangtze River, China.
AbstractBased on 23 bathymetric surveys from January 1988 to December 1990 in the South Passage, Yangtze River mouth, along with corresponding data on river discharge and coastal storm, this paper examines the different time-scales of bed level change, with emphasis on the response of bed level to river water discharge and coastal storm. Results are summarized as follows: (a) Under normal conditions, the range of change in bed level was on the order of a few centimeters during a period of half a month, 10 to 50 centimeters in the different seasons and 40 to 60 centimeters in the comparison between flood season and dry season, on average of the along-channel survey sites. A maximum change could be several times the average change. (b) The average along-channel depth was negatively correlated with the river water discharge. (c) The response of bed-level of the South Passage lagged changes in river discharge at the Datong Station by 1 to 2 months, which reflects the influence of the 600 km distance between the hydrological station and the river mouth. (d) In taking into account this lag effect, a correlative coefficient of 0.90 was found. Based on these data, the range of annual change in the channel bed level was forecasted to be the order of 50 cm for normal discharges, 20 cm for minimal difference in seasonal discharge and 100 cm for maximum difference in seasonal discharge. (e) A ten year storm event was found to result in an average accretion of 60 cm, which superimposed upon the river-induced seasonal change. (f) A good spatial unanimity of erosion and accretion events was found for the main part of the channel. Conclusions include the following: (a) The channel bed-level of the Yangtze River mouth is sensitive to both seasonal changes in river discharge and to storm events. Under conditions of normal river water discharge and severe storms, the combined annual difference in average bed level can reach the order of 1.0 m. It is probable that accretion in flood season could exceed 1.5 m in the case of co-occurrence of extreme high water discharge and extreme severe storms. (b) In view of a good positive correlation between river water discharge and riverine sediment discharge, the response to water discharge suggests a response of bed-level to the sediment discharge. (c) Due to its regulative function in annual distribution of the river discharge and its role in reducing sediment supply, the Three Gorge Dam will perhaps reduce the range of annual change in the bed level of the river mouth channels and increase the depth of the navigational channels.
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