Variation Character Of Tidal Dynamics Along Yangtze River Estuary For The Past Four Decades

The tide in the Yangtze River Estuary originates from the northwest Pacific Ocean.The runoff-tide interactions is the basic driving force for the processes of sediment and nutrient transportation,coastal evolution in the estuary,as well as the basic conditions for the construction of hydraulic engineering in the estuary.However,a series of reservoirs such as the Three Gorges Reservoir had been built along the Yangtze River basin since the 1950s.They have directly regulated the discharge.At the same time,the sediment deposition behind the dam resulted in a sharp decrease of the amount of sediment discharge.Therefore,river channel scour occurred in middle and lower Yangtze River.In addition,human activities such as channel dredging,land reclamation and sand mining change the boundary conditions of tidal propagation,led to the change of tidal characteristics.It is of great scientific significance and application value to study the tidal characteristic quantities in the Yangtze River Estuary for the development and utilization of estuarine resources,the maintenance and management of hydraulic engineering in the estuary.The calculation of tidal characteristic quantities as extreme tidal level,tidal range,amplitude of tidal constituents is made.It is done using methods such as the extreme value analysis,the wavelet analysis and harmonic analysis of water level at 11hydrography stations during 2008-2016 along Datong-Nanjing reach and estuarine section downstream the Xuliujing.It is also done for the data analysis in recent 40years.The driving forces of significant change of tidal characteristic quantities has been discussed in the present study.The main results are as follows:?1?The temporal and spatial variation of extreme tidal level.In the past 40 years,the annual maximum tide level in the Datong-Nanjing reach increased by 0.28-0.35 m,and the increase in the lower reach was greater than that in the upper reach.The annual minimum tide level increased by 0.19-0.56 m,and the increase range in the lower reach was smaller than that of the upper reach.Tidal power is mainly affected by runoff in the Datong-Nanjing reach,and the correlation coefficient between tidal level and flow reaches 0.98.The enhancement of tidal upstream ability of channel scour increased the maximum tidal level under the same runoff conditions.The annual maximum tide level in the estuary section decreased by 0.14-0.37 m,and the lower reach decreased less than the upper reach.The annual minimum tide level increased by 0.02-0.37 m in the estuary section,and the increase of the lower reach was greater than that of the upper reach.The tide-flow correlation coefficient was less than 0.50but the narrow topography increased the lowest tidal level at close discharge condition.?2?The temporal and spatial variation of tidal range.In the past 40 years,the monthly mean tidal range in the Datong-Nanjing reach increased by 5-24 cm,and it increased significantly from October to November,reaching 20 cm.The monthly mean tidal range was large in dry season and small in flood season,and the maximun value appeared in January to February.The average monthly tidal range increased by10 cm on average under the same runoff conditions and the increase in the upper reach was smaller than that in the lower reach.The monthly mean tidal range in the estuary increased by 0.12-0.28 m as a whole,but decreased by 0.26 m locally,and it was large in flood season and small in dry season.Its variable quantity was small in the upper reach of the South Channel upstream,while increased along the lower reach of the South Channel.When the riverbed was scoured,river width was shrunk or cross section area was increased,the average annual tidal range increased in the estuary section.Otherwise,average annual tidal range decreased.?3?The temporal and spatial variation of the amplitude of tidal constituent.In recent 40 years,the amplitude of the semi-diurnal tidal constituent in the estuary increased,in which the amplitude of the M₂ tidal constituent increased from 96-119cm to 111-123 cm,increasing by 1-12%,and the amplitude of the S₂ tidal constituent increased by 4-32%.The amplitude of diurnal and shallow-water tidal constituent decreased,of which MS₄ tidal amplitudes decreased the most significantly,decreasing by 16-32%.However,M₂ tidal amplitude decreased by 2.34-7.05 cm along the South Channel downstream for nearly a decade.As for the Datong-Nanjing reach,the amplitudes of M₂,S₂,M₄ and MS₄ tidal constituent increased and the amplitudes of K₁ and O₁ tidal constituent decreased.?4?The spatial and temporal variation of tidal characteristic coefficients.In the past 40 years,due to the changes in amplitude of tidal constituents,the ratios of them have been changed accordingly.It means that the diurnal inequality in the estuary is weakened,and the shallow water deformation is reduced.The tidal form number decreased from 0.38-0.42 to 0.35-0.37 in the South Channel upstream and decreased from 0.35 to 0.34 in the South Channel downstream.The tidal distortion coefficient A decreased within 0.02 in the estuary section.In recent 10 years,the F value and A value in the estuary section are basically stable,with the F value being 0.30-0.37 and A value being 0.10-0.20.However,under the influence of the reservoir engineering and bridge engineering construction in the basin,the diurnal inequality was weakened and the shallow water deformation increased after the tidal propagation to the near-mouth section.F value was greater than 0.50 with a decreasing trend and A value is greater than 0.10 with an increasing trend in Datong reach.In the lower flood season,F value is greater than 0.50,showing a decreasing trend.F value decreases from 0.61 to less than 0.50 in the dry season and A value of flood season and dry season remained unchanged at 0.22 and 0.28 respectively.?5?The driving forces of intensified tidal dynamics.Runoff has a controlling effect on the extreme tidal level,monthly mean tidal range,and tidal amplitude in Datong-Nanjing reach.The monthly mean tidal range in the dry season was 3-5 times of that in the flood season,and the tidal amplitude in the dry season was 2-3 times of that in the flood season.However,the change of topography may be the main influence factor of tidal dynamic enhancement.For example,the maximum brushing depth of the channel was 5-8 m in the reach from Datong-nanjing reach from 1998 to2013 and the monthly average tidal range increased by 10 cm under the same runoff condition during that time.In the same way,the topography had a significant influence on the average annual tidal range of the estuary section,which was mainly manifested in that the average annual tidal range increased with the scour depth of channel increased,the width of the river narrowed and the area of the section increased.From 1978 to 2011,the width of the river in the South Passage decreased significantly,and the area of Nanhui beach decreased by 91.8-95.5%,the average annual tidal range of the Zhongjun station increased by 0.15m and the average monthly tidal range increased by about 0.12m as a result.The maximum brushing depth of the channel was 5-15 m in the North Channel during 2008-2016,and the average annual tidal range of Baozhen station increased by 0.25 m.In addition,the wavelet analysis of the average annual tidal range showed that there were cycles of18-19 a,12 a,7-8 a and 4 a,which may be related to the lunar declination Angle regression cycle?18.6 a?,sunspot activity cycle?11 a?and El Nino cycle?3-7 a?.At the same time,sea level rise most likely leads to the counter-clockwise deflection of the same tidal time line,so the tide traveled faster in the estuary.With the redistribution of discharge and the change of channel topography,the tidal power of the Yangtze river estuary is generally enhanced.The Three Gorges project has a very significant effect among them.The runoff decreased in flood season,which leads to the relative enhancement of tidal power.The overall erosion of the riverbed in the middle and lower reaches is the main factor for the tidal dynamic enhancement of the estuary,which leads to the increase of tidal characteristic values under the same runoff conditions.Under the influence of reclamation project,deepwater channel project and Qingcaosha reservoir project,the river width shrank,the channel being scour and the intertidal zone area decreased,which are the main factors for the tidal dynamic enhancement of the estuary,especially near the mouth.In addition,the change of the tidal wave system caused by sea level rise on a 40-year time scale is also a important factor.When the enhanced tidal power and extreme hydrological conditions are superimposed on each other?such as extreme dry water situation,extreme flood situation,storm surge,etc.?,it will have adverse effects on the safety of estuarine navigation and bank slope stability.