The Time-Varying Characteristics Of Tidal Duration Asymmetry And Its Response To Project

Tides are the common periodic vertical movement of sea surface.According to the motion of celestial bodies,tides can be divided into summation of finite symmetric cosine or sine funcions with different periods,thus the amplitudes and phases can be acquired through Fourier analysis,achieving better understanding and prediction of tidal motion.While,in coastal and estuarine areas,the shallow waters and complex coastlines will force tides to be distorted,resulting in difference between tidal rising duration and tidal falling duration,or difference between the distance from high water level to the mean sea level and the distance from low water level to the mean sea level,which both called tidal asymmetry.The more significant difference means stronger tidal asymmetry.In tide-dominant sea areas,tidal asymmetry is a crucial factor leading to the net salinity,sediment,nutrients and pollutant transport within a tidal cycle.Thus,it has vital significance of theory and practice to understanding the performance characteristics,spatial-temporal variations and the dynamic mechanism of tidal asymmetry.This thesis focuses on the difference between rising and falling duration,naming tidal duration asymmetry.It is quantified by the skewness of tidal elevation time derivative.Its three characters below are studied,from the spatial scale of global,estuarine,and local small sea area.1.The spring-neap(fortnightly)cycle of tidal duration asymmetry and its global distributionA large number of observations and theory studies have verified that tidal duration asymmetry characterized by spring-neap variation.But the behavior feature and the spatial distribution of it are still need to be studied comprehensively.In this study,the elevation time series is assumed to be the summation of different tidal constitutents owing different frequencies.Two lunar days is adopted to be the integral period to achieve the fortnightly variation.Then the skewness is expanded to describing the fortnightly variation in tidal duration asymmetry with tidal amplitudes and phases.This equation has been well validated by the water-level data from 481 global tidal gauges.And the global distribution of the fortnightly tidal duration asymmetry is also studied by applying this equation to the tidal constants from TPX08-atlas with a high-resolution of 1/30°..Results show that the fortnightly variation in tidal duration asymmetry is extremely common in the open oceans.It is primarily induced by three types of tidal interactions:interactions between long-period and principal tidal constituents(D0/D1),interactions within the principal tidal constituents(D1/D1),and interactions between high-frequency and principal tidal constituents(D1/D4).In the open oceans,D1/D1 interaction dominates 80%sea areas,mainly due to the deep topography;D1/D4 interaction dominates 18.7%sea areas,showing the shallow water effect in coastal waters;and D0/D1 interaction only dominates 0.3%sea areas,mainly in thepolar regions,may relating to the strongest fornight tide occurs in high latitudes,according to the equilibrium tidal theory.If denoting the fortnightly variation magnitudes induced by D0/D1,D1/D1 and D1/D4 interactions by A0,A1 and A4,respectively.Statistics basing on the water-level data at 481 stations worldwide show that A1 is nonlinearly related to the tidal form number F =(?K1+?O1)/(?M2+?S2).If denoting G =(?M4+?MS4)/(?M2+?s2)and H ?(?MSf+?Mf/(?M2+?S2),then there is a nonlinear relationship between the ratio of A4 and A1 and G/F2,and a nonlinear relationship between the ratio of A0 and A1 and H+4H/F2.These relationships can help us to estimate the dominant tidal interaction for the fortnightly tidal asymmetry rapidly using the three parameters F,G,H.The data also shows that if the fortnightly tidal duration asymmetry is dominated by D1/D1 interaction,then the asymmetry during neap tide can be stronger than that during spring tide.While if the fortnightly tidal duration asymmetry.is dominated by D1/D4 interaction,then the asymmetry during neap tide is always smaller than that during spring tide,due to the stronger nonlinear tidal interaction in spring.2.The spatial-temporal variation of tidal duration asymmetry under influence of land-sea interaction in an estuary.The one-year hourly water-level data at mutiple stations in the Changjiang Estuary,which is characterized by plentiful river discharge,are analyzed carefully.We only focus on the North and South Branchs,North and South Channals and North and South Passages,up to Xuliujing and down to Niupijiao,named Estuary Area in this study.River discharge can enhance the seaward barotropic gradient pressure and bottom friction in the river channels,blocking tides propagate upstream,thus decrease tidal amplitudes in the river channels.This mechanism leads to the M2 amplitude in the South Branch is larger during winter but smaller during summer.But at station Niupijiao which located at the export of North Passage,the M2 amplitude behaves seasonal variation of smaller during winter but larger during summer,and the varing magnitude can reach 0.10 m.A possible interpretion is that the stronger thermohaline stratification out of the Changjiang Estuary during summer can reduce the turbulent viscosity coefficient,decreacing the energy loss due to turbulence,and thus result in the stronger M2 amplitude in summer.In the North Branch,the special horn-like topography can amplificate the seasonal variation of M2 amplitude,leading to the summer M2 amplitde is 0.30 m larger than that in winer.Under the influence of river discharge,the falling duration in the Changjiang Estuary is longer than the rising duration.The larger discharge in summer lengthens the falling duration,enhancing tidal duration asymmetry.Since the discharge's influence is stronger in the upstream,tidal asymmetry and its seasonal and fortnightly variations are more significant in the upstream in overall.Observed water level data in 2009 shows that two peaks in the tidal duration asymmetry occurs during one year:The large peak in August is mainly influenced by the discharge directly;the other small peak in March is possiblely related to the low mean water level during March.The combination of shallow topography and strong river-tide interactions develops strong shallow tides in the Changjiang Estuary.Thus the D1/D4 interaction controls the tidal duration asymmetry and its fortnightly fluctuation.Additionally,the fortnightly-varying magnitude of tidal asymmetry.behaves a seasonal flocculation described as stronger in spring and fall.This is not only influenced by the D1/D4 interaction,and also related to the D1/D1 interaction.3.The mechanism that how dams in open,oceans affect the tidal duration asymmetryThe spatial distribution and time-varing characteristics of the tidal duration asymmetry along with the construction of Yangshan Deepwater Harbour are studied via analyzing the observated water-level timeseries and the FVCOM model results.The D1/D4 interaction is the main contribution for the duration asymmetry and its fortnightly variation.The long-time observation at station Xiaoyangshan shows that the amplification of shallow tides lead to stronger duration asymmetry.According to the FVCOM results,construction of first dam has significant contribution to it,and the shallower topography around this station induced by the constructions is also an important parameter.The observations also show that the tidal duration asymmetry in Yangshan Harbour owes intensive spatial dispcrepancy.Asymmetry increases gradiently from the East Entrance to the West Entrance after project,and then decreases westly.FVCOM model results reveal that the constructions have extremely enhanced the tidal choking in the East Entrance.An important nature of tidal choking is that it reduces the tidal amplitude,and reduces more on high frequency tides.As the duration asymmetry is dominated by M2/M4 and M2/S2/MS4 interactions,the stronger tidal choking then results in smaller amplitude ratio of M4 and M2 tide,i.e.,smaller duration asymmetry,which is different with the observations at station Xiaoyangshan.Meanwhile,the dams can decrease the amplitudes at its downstream(about the direction of tidal progation),thus weaken duration asymmetry,and converse for the upstream.In overall,this thesis shows that the tidal duration asymmetry behaves significant fortnightly variation under natural conditions,and it is always stronger during spring tide in a semi-diurnal tidal regime.In estuaries,both seasonal river discharge and tidal interactions can lead to seasonal variation of tidal duration asymmetry,and the seasonal variation is gradually smaller from upper estuary to lower estuary.Additionally,the increasing human activeties,such as the coastal enginnerings,also can alter the tidal dynamics,including tidal duration asymmetry of the local sea areas.