珠江磨刀门水道咸潮上溯加剧的原因

Study on the causes of intensified saline water intrusion into Modaomen Estuary of the Zhujiang River in recent years

对磨刀门水道不同年份地形资料和多年枯季潮位资料进行了对比分析,结果表明：大规模的河床采砂,引起磨刀门上游河段河床大幅下切,河道深泓平均加深0.59~2.25 m,河槽容积平均增加8.91×106~12.11×106m3,使该河段枯季潮差增大,进潮量增加,潮汐动力增强,导致咸水界向上游河段推移;口门围垦整治后,磨刀门口门区由众多岛屿环抱而成的内海区变成人工导堤控制的＂一主一支＂两条水道,水域面积减少了80%,河槽容积减少了40%,深槽趋于顺直加深,使口门区的＂调淡＂作用消失,这是咸潮上溯的重要原因;根据多年水文频率分析,发生严重咸潮灾害时,西、北江枯季流量并非都是枯水流量,因此枯季上游来水量变化并不是近年来咸潮灾害加剧的主要原因。

Modaomen Estuary is one of the main outlets of the Zhujiang River,where the four cities of Zhongshan,Jiangmen,Zhuhai and Macao are located.Since mid-1990s,saline water intruded frequently into Modaomen waterway,resulting in great impact on the production of agriculture and industry and the life of local residents during dry season.Large-scale engineering projects and human activities such as riverbed sand dredging and estuary regulation have resulted in some changes of channel topography and hydrodynamic conditions since 1980s.These changes play important roles in saline water intrusion.Based on the topographic data of Modaomen water course（1∶5000 topographic map of the water course of 1977 and 1999） and Modaomen outlet（1∶5000 nautical chart of 1983 and 2003）,data of tidal level at Dahengqin,Denglongshan and Zhuyin stations in dry season（Feb.and Dec.） from 1980 to 2004,annual runoff at Gaoyao,Makou,Xijiang and Sanshui Stations from 1960 to 2004,and tidal influx at Denglongshan Station in dry season（January,February and November） in 1964,1967,1976,1983 and 2001）,comparative analysis shows that as a result of large-scale sand dredging,the upriver bed of Modaomen waterway was greatly deepened,with the river valley deepened by about 0.59~2.25 m in average,and the channel capacity increased by about 8.91×106~12.11×106 m3.Because of polder and regulation at the Estuary,the outlet area at Modaomen Estuary became ＂one main one branch＂ water channels,Henzhou waterway（main） and Hongwan waterway（branch） controlled by artificial guide work.As a result,the water area reduced by 80%,and channel capacity by 40%.Riverbed of the two water courses was tending towards deepening and straight.During dry season in recent years,topographic change at Modaomen waterway has led to remarkable tidal dynamic enhancement at Zhuyin station on the upper reaches of Modaomen waterway,that is,the rise of high-tide level,drop of low-tide level and increase of tidal range influx.At the outlet area,both the high and low-tide level increased and the tidal range decreased,and the Henzhou waterway became the main tidal inlet in dry season.The phase difference between Dahengqin and Zhuyin reduced and the tidal wave velocity increased.Riverbed sand dredging at Modaomen waterway led to the tidal dynamic enhancement during dry season and the upward movement of the saline water boundary,which is the basic cause of saline water intrusion in recent years.Estuary regulation resulted in the vanishing of the ＂diluting function＂ of the outlet area,which is another important reason for the saline water intrusion.Hydrological frequency analysis shows that Modaomen runoff regime did not change since mid-1990s in dry season,which means that runoff change is not the main cause for saline water intrusion.