BASIC CHARACTERISTICS OF SEICHES IN THE MAJOR COASTAL EMBAYMENTS OF BOHAI AND YELLOW SEAS

The water-level curves from 20 tide-gauge stations along the coasts of Bohai and Yellow Seas were collected.The seiche parameters were selected according to these water-level curves obtained.These data,especially the seiches with relatively large amplitudes,were examined by using statistics and natural period analysis.Basic chracteristics of seiches,such as monthly and annual occurrences,oscillation period, occurrence duration and occurrence time of the peak,were derived. Among these 20 tide-gauge stations, seiches with an amplitude of no less than 50 cm were found at 13 of them. The seiche occurs mostly in June and July.Among the total 147 seiche events, there are 33 sets(or events) with an amplitude greater than 80 cm, and the greatest amplitude is 293 cm. The occurrence of seiches has local and seasonal characters, and interannual variations. The seiches with a large amplitude occur mostly in summer.It basically does not occur in winter.Numbers of the annual occurrence of the seiches with a relatively large amplitudes are not constant and vary randomly in different years.The natural period of fundamental mode calculated by the improvement on Merians formula coincides well with the period estimated from the curve of observational water level except for one of the bays. Observations show that the greatest amplitude of each seiche event occurs mostly during high or low tide.This study shows that the fine coordination of rapid variation of atmospheric pressure,wind speed and direction are main causes of the formation of the seiches with large amplitudes.