Tidal types and characteristics of the harbors along the Guangxi coast

Based on the tidal data collected in the hydrographic survey of the key harbors,according to two definition of morphological type and cycle type,tidal types along the Guangxi coast are discussed.According to morphological classification,the tidal type is regular diurnal tide at Zhenzhu harbor,Fangcheng harbor and Weizhou island,while that is irregular diurnal tide in Qinzhou and Tieshan harbor.Both of the two types of tide are consisted of semidiurnal tide and diurnal tide.The statistics of measured high and low tide number and mapping of the tide curve give the emerging time and transform process of diurnal and semidiurnal tide in the sea area of diurnal tide.The only difference of regular and irregular diurnal tide is the constituent proportion between diurnal and semidiurnal tide.The transform of the periodic tide at the harbors along the Guangxi coast is determined by the monthly declination.Diurnal tide occurs when the monthly declination is the biggest or nearly the biggest,while semidiurnal tide does when it is the smallest or approximately the smallest.Then the characteristics of the statistical values of the tide are studied,which shows that(1) monthly mean of day-to-day tidal range is different from the synchronous average tidal range of diurnal and semidiurnal tide,that is,diurnal tide rangemonthly mean tidal rangesemidiurnal tidal range in the studied area.Considering the emerging rule of semidiurnal and diurnal tide,it is practical to make a work plan at the sea according to the tidal range calculated with tidal type.The tidal range calculated day by day can reflect the temporal and spatial variation of the tidal range,which is bigger inshore than offshore and inside than outside harbor,and at the same latitude the tidal range increases from west to east.(2) Day-to-day calculated tidal hour shows that the flood tide hour is 1 h 03 min～2 h 12 min longer than that of the ebb tide,and that the flood tidal hour of diurnal tide is 3 h 35 min～4 h 35 min longer than ebb tide,while the ebb tidal hour of semidiurnal tide is 2 min～49 min longer than that of the flood tide.The tide hour calculated by time is different from that by tidal type.(3) The occurrence period and change pattern of spring tide is studied based on the tidal level data of Weizhou in 2008.The spring tide in this sea area belongs to tropic tide,emerging every 13.52 d.The emerging time is different in different season.The spring tide will turn up before midnight from spring equinox to summer solstice,in the afternoon from summer solstice to autumnal equinox,in the morning from autumnal equinox to winter solstice,after midnight from winter solstice to the next spring equinox.The low tide time of spring tide is later about 12 h than its high tide time.