闽江上游支流九龙溪永安段的演化过程及其工程地质意义

Evolutionary Process of the Yong'an Section in Jiulong River of the Upper Minjiang River Tributary and its Engineering Geological Significance

闽江上游支流九龙溪永安段的两处特殊的地质现象:九龙溪在流至永安霞鹤时流向由南东向转北东向,形成近90°的转弯;文川溪汇入九龙溪后,河曲异常发育。本次研究通过对该区域内地质构造、岸线形态、河岸地貌和岩性条件等的对比分析,认为构造特征是九龙溪永安段形成近90°转弯的主要原因,岩溶塌陷是九龙溪永安段河曲发育异常的主要原因。九龙溪永安段的河曲发育过程属于岩溶塌陷改造河道的模式,其演化分为三个阶段:第一阶段,由于河流南北地块的差异性抬升形成曲流;第二阶段,北岸发生岩溶塌陷,河道开始向北岸方向出现弯曲;第三阶段,因矶头的挑流作用,迫使河流再次改向,最终形成了河曲建造。九龙溪永安段的演化模式说明了岩溶塌陷对河流改道的重要影响,在今后的护岸建设及地质灾害防御方面应充分考虑这一因素的影响。对于沙溪流经的两处岩溶区的岸崩预防区,九龙溪永安段应将重点放在其北岸,三明市台江一带的沙溪两岸都是重点。

The main objectives of this work are to explain the current approximately 90° change of Jiulong River at Xiahe,Yong’an and to illuminate the abnormal development of Jiulong River’s meander after being into the Wenchuan River. Through comparative analysis of geological structures,shoreline morphology,river bank geomorphology and lithological conditions in this region,it is indicated that structure features are a dominating factor of the previous phenomenon and karst collapse is a dominating factor of the latter one. Karst collapse affected the development of Jiulong River in Yong’an in three phases. In the first phase,differential uplifting caused the formation of meander. In the second phase,intense erosion in the south and karst collapse in the north occurred. In the thrid phase,modern meander formed due to the impact of rock spurs. The study shows that karst collapse is an important factor in development of river and is also significant in engineering geology. Two karst areas in the study area and the nearby Taijing of Sanming City should be prevented from bank collapse;the first one should pay more attention on the north of the river and the last one should on both the north and south.