乐清湾的潮位、潮流和余流特征

Features of the tidal level,tidal currents and residual currents in Yueqingwan Bay

2008年7月至2009年4月在乐清湾进行了代表春、夏、秋、冬4个季节的航次调查,设置了Y4、Y14、Y15和A共4个连续观测站位,共得到12组实测的海流流速和10组CTD数据。采用潮汐调和分析法分析了距江厦潮汐能试验电站3 km处的潮汐站位连续19个月的潮位资料,结合调查数据特性和乐清湾潮汐特点,引入M2与S2、O1与K1、M4与MS4、2MS6与M6分潮之间的差比关系,对连续观测站位的潮位和潮流进行准调和分析。潮位的统计和准调和分析结果显示:Y4、Y14、Y15站位和潮汐站位8个分潮振幅和的航次调查平均值为3.75,4.02,3.94和4.03 m,(HO1+HK1)/HM2的航次调查平均值为0.32,0.28,0.32和0.24。Y4、Y14、Y15和潮汐站位的M4、MS4、M6和2MS6浅水分潮振幅的航次调查平均值分别为0.20,0.31,0.35和0.25 m,M6和2MS6浅水分潮振幅的航次调查平均值分别为0.03,0.15,0.17和0.15 m。不同航次调查4个连续观测站位涨潮最大流速的平均值为81.5 cm/s,落潮最大流速的平均值为103.1 cm/s。Y4、Y14和Y15站位潮流的M2和S2分潮振幅百分比分别为86%,65%和68%,浅水分潮振幅百分比分别为11%,29%和25%。M4、MS4分潮振幅之和分别是M6、2MS6分潮振幅之和的2.1,1.2和1.7倍。由潮位和潮流的分析结果可知:从乐清湾湾口至湾顶,潮汐逐渐增强,半日潮比率逐渐增大,半日潮型的特性更为明显;浅水分潮强度逐渐增加,其中M6和2MS6分潮强度增强更为明显。位于湾口的Y4站位在秋季(2008年10月)航次调查时的日平均余流流向为西南偏南方向,冬季(2009年1月)和春季(2009年4月)航次调查时的余流流向为东南偏南方向。Y4站位余流受灵霓大坝影响,大坝建成后湾口余流改变方向,向南流出乐清湾。位于湾顶的Y14站位,余流流速变化不大,但方向变化明显,夏季(2008年7月)为西南方向,秋季为西南偏南方向,冬季为西南偏西方向,春季又为西南偏南方向。Y15站位余流流速较小,但方向变化明显。A站位两个航次调查时的余流流向均为东北方向。

During four seasons from the summer of 2008 to the spring of 2009,we took four cruises in each season.There were four 25-hours stations,in which Station Y4 was located at the bay mouth and the others at the head of Yueqingwan Bay.Station A was located in the fish garth,where only the tidal current measurements were taken,and 12 sets of observed data of tidal current and 10 sets of CTD data were collected.Continuous 19 months of tidal level data near the Jiangxia tidal power plant,are analysed with tide harmonic analysis and the ratio of tide type and the current difference between the main tidal constituents M2 and S2,O1 and K1;M4 and MS4,2MS6 and M6 were calculated,based on which quasi-harmonic analysis were made on the data of tidal level and tidal current.The results of quasi-harmonic analysis and tidal level statistics show that the mean sum of the oscillation amplitude of eight tidal constituents are 3.75,4.02,3.94 and 4.03 m at station Y4,Y14,Y15 and Jiangxia tidal power pilot plant respectively.The ratio of（HO1＋HK1）/HM2 are 0.32,0.28,0.32 and 0.24.The mean sum of oscillation amplitude of the shallow water components（M4,MS4,2MS6 and M6） are 0.20,0.31,0.35 and 0.25 m,and the proportions are 5.3%,7.7%,8.9% and 6% respectively.The mean sum of oscillation amplitude of shallow water components 2MS6 and M6 are 0.03,0.15,0.17 and 0.15 m,and the proportions are 0.8%,3.7%,4.3% and 3.7% respectively.The tide gradually strengthens from the bay mouth to the bay head,the proportion of semi-diurnal tide increases,and the characteristics of semi-diurnal bay becomes more evident.The strength of the shallow components also increases,in which that of M6 and 2MS6 increases more obviously.Station Y14 and A are located in a narrow passage,where the reciprocating feature of the tidal flow is more obvious than the other two stations.Mean maximum flow velocities of flood tide is 81.5 cm/s and that of ebb tide is 103.1 cm/s with a difference of 21.6 cm/s.The average velocities during flood tide and ebb tide are 50.3 cm/s and 51.1 cm/s respectively.Observations of all the 4 cruises show that the Maximum ebb tidal velocity is greater than that of flood tide,which results in stronger scouring on the seabed.The proportions of semidiurnal tidal currents are 86%,65% and 68% at station Y4,Y14 and Y15 respectively,and that of shallow water tidal components are 11%,29% and 25%.The sun of oscillation amplitude of M4 and MS4 is 2.1,1.2 and 1.7 times that of M6 and 2MS6.The tidal in Yueqingwan Bay is normal semidiurnal tide,and the tidal current is reciprocating flow.Obviously,semidiurnal tidal currents are the main tidal constituents,the proportion is more than 65%.Shallow water tidal currents intensify by more than twice from the bay mouth to the bay head.The observed direction of daily mean residual current at Station Y4 was south-southwest in October 2008 and south-southeast in January and April 2009.At Station Y14 at the head of the bay,the magnitude of residual currents varied slightly,but directions changed remarkably.It was southwest in July 2008,south-southwest in October 2008,west-southwest in January 2009 and south-southwest in April 2009.Daily mean residual currents at Station Y15 was tiny,while direction changed remarkably.Daily mean residual current direction at Station A was both northeast during two investigation cruises.The residual currents present apparent local features.At the mouth of the bay residual currents are interfered by Lingni Dam project.it changes its direction into southwards out of the bay.Residual currents in the bay are very slow,but directions change remarkably in different seasons.These observation results favor the verification of the numerical model used in Yueqingwan Bay,and the study of the water exchange there.