THE INFLUENCE OF STORM TIDE ON MUD PLAIN COAST——WITH SPECIAL REFERENCE TO JIANGSU PROVINCE
Ren Mei'e Zhang Renshun Yang Juhai Zhang Dachu (Nanjing University)
1)Storms, including typhoon and winter storms, can exert a great effect on the evolution of mud plain coast of Jiangsu. The result produced by one strong typhoon often surpasses the changes occurred in one whole season under ordinary weather. As the frequency of winter storms is greater than that of typhoon, their influence should be carefully studied and measured. 2)The typhoon 8114 caused a great rise of water level along the coast of whole Jiangsu Province in 1981. This extraordinary high water level and huge waves were just coordinate with a large spring tide. As a result, the velocity of flood current as well as the unsymmetry of tidal current on mud flat were greatly increased. Moreover, wave became an important agent in shaping the coast and mud flat. Wave, as a leading role on the mud flat of a receding coast, can form a concave profile on the upper part of mud flat. On prograding and accreting mud flats, the increased importance of wave action was also evident in the formation of miniature cheniers near the supratidal zone. 3)On profiles of mud flat along receding coast, the lowering of the upper part and the accretion of lower part are quickened during the typhoon. The amount of scouring in the typhoon 8114 exceeded that of the whole scouring season. The amount of the lowering of upper part of mud flat depends on the altitude and width of the mud flat, and the direction and exposure of the coast. The disapp-earance of cheniers and the formation of depressions parallel to coastline give evidence to the action of increased erosion during typhoon. 4)The typhoon 8114 changed the usual process of development on prograding mud flat. Contrary to the mud flat along receding coast, aggradation in the upper part and scouring in the lower part took place during the typhoon. Therefore, the gradient of mud flat was increased and its sediments coarsened. The most severe erosion occurred in the central and lower parts of intertidal zone. But in supratidal zone, only accretion could be seen. 5)During the typhoon, the intertidal zone was scoured first and then aggraded. Burrowing anirnals were often suffocated and died due to rapid burial by typhoon deposits. A scour surface can usually be seen between the typhoon deposits and the underlying sediments, therefore, the appearance of a scour surface and the changes of lithology and fossils in marine strata do not necessarily indicate a change of sea-level, but may be resulted from catastrophic events, like storms. Typhoon deposits have distinct sedimentary structures, such as cross beddings and soon. 6)During the typhoon, local changes in the netwerk of tidal channels also occurred. Owing to the increased lateral erosion, tidal channels were widened. Silted channels may be opened again during the typhoon. Changes in tidal creeks are chiefly the silting of their upper course and the widening of their lower course. 7)The effects of winter storm on tidal flat and tidal channel are similar to that of typhoon. Winter storms often in crease the lateral erosion in tidal channels and even form a series of scouring depressions on lower supratidal flat. Tidal flats near to the large tidal channels were often eroded while that farther from the tidal channels aggraded. When low temperature comes with winter storm, groove marks are often formed by ice on the surface of intertidal flat and low mud mounds formed by sudden freezing of water in crab burrows on supratidal flat.