潮汐应变对长江口北槽枯季湍流混合与层化的影响

Effect of tidal straining on turbulent mixing and stratification in the dry season within the North Passage of the Changjiang River estuary

利用TELEMAC-3D开展了长江口北槽2010年枯季条件下湍流混合与层化的有限元数学模拟研究。该模型在外海开边界设置了8个主要分潮,并在自由表面考虑了定常风的影响,利用北槽水域3个潮位站（横沙、北槽中、牛皮礁）和2个水文观测站（北槽中段CSW、北槽下段CS8）2010年枯季的潮位、流速及盐度观测资料对模型进行验证并获得了良好的精度,从而得到北槽水域纵向、平面流场和盐度场。模拟得到的流速、盐度被用来计算势能差异（φ）、势能差异变化率（φ/t）、Simpson数（Si）和梯度Richardson数（Ri）。结果显示：1）北槽水域大潮平均和小潮平均的势能差异的变化范围分别约为0～30 J/m3和0～90 J/m3,且较大的势能差异基本位于主航槽,这些表明北槽水体小潮的层化大约是大潮的3倍,主航槽的层化强于坝田区,而北槽中段往往具有更强的层化。2）落急时刻,就北槽下段而言,潮汐应变、潮汐与风共同搅动引起的势能差异变化率的范围分别约为-20×10-4～100×10-4W/m3、0～100×10-4W/m3,这些表明,从大潮至小潮,潮汐应变总体增强而潮汐与风共同搅动总体减弱。空间上,主航槽丁坝附近的潮汐应变明显强于坝田区,潮汐与风共同搅动的强度在坝田区内、外也存在差异,导堤和丁坝的影响明显。3）对于北槽下段CS8站,大潮至中潮的Si数在0.15～0.4之间（介于下临界值0.088和上临界值0.84之间）,表明潮汐与风共同搅动占优,属于应变致周期性层化（SIPS）。小潮的Si数在0.9～1.5之间（高于上临界值0.84）,表明潮汐应变显著增强并占优,属于持续性层化。4）北槽下段CS8站梯度Ri数的量级范围在混合较好的表层和底层约10-3～10-2,在层化较好的中间水层约100～101。该站湍动能耗散率的量级范围大潮为10-3～10-9W/kg,小潮为10-5～10-10W/kg,具有明显的M4周期性特征和涨、落潮不对称分布,且表层和底层分别由于风应力和底摩擦作用而具有较强的耗散,中间水层稳定层化区的耗散则显著减小,潮汐应变是造成湍动能耗散率在涨、落潮周期内不对称分布的重要因素。

A finite element mathematical model TELEMAC-3D is used to investigate turbulent mixing and stratification within the North Passage of the Changjiang River estuary.Eight tidal constituents,M2,S2,N2,K2,K1,O1,P1,and Q1,are taken into account.The model is calibrated against tidal elevations at three tidal stations,Hengsha,North Passage（M）,and Niupijiao,and current velocities and salinities at two hydrological gauging stations,CSW and CS8,in the dry season of 2010.Modeled current velocities and salinities are used to calculate the potential energy anomaly（φ）,the time derivative of the potential energy anomaly（φ/t） caused by tidal straining or combined tidal and wind stirring,the Simpson number（Si）,and the gradient Richardson number（Ri）.Results show that（i） tidal mean potential energy anomaly（φ） is in the range of 0 ～30 J/m3within the North Passage over a spring tide and 0 ～90 J/m3over a neap tide.Large tidal mean potential energy anomaly（φ） mainly occurs in the main curved Deepwater Navigational Channel than in adjoining dike fields.It is suggested that stratification of water column within the curved Deepwater Navigational Channel of the North Passage appears to be stronger during neap tide than during spring tide and also stronger in the main channel than within dike fields.The middle reach of the curved Deepwater Navigational Channel of the North Passage is always more stratified than the dike fields;（ii） within the lower reach of the curved Deepwater Navigational Channel of the North Passage at maximum ebb tide,the time derivatives of the potential energy anomalies（φ/t） caused by tidal straining and combined tidal and wind stirring are in the range of 0 ～ 100 × 10-4W/m3and-20 ～ 100 × 10-4W/m3,respectively.Tidal straining increases from spring to neap tides as combined tidal and wind stirring decreases.Tidal straining in the main channel near those groynes is much stronger than that within dike fields.Combined tidal and wind stirring is also quite different inside and outside dike fields;（iii） at hydrological gauging station CS8,the Simpson number（Si） is in the range of 0.15 ～ 0.40 from spring to moderate tides.This suggests that combined tidal and wind stirring is dominant and that Strain-Induced Periodic Stratification（SIPS） occurs.The Simpson number in the range of 0.90 ～1.50 during neap tide suggests that tidal straining becomes dominant and that persistent stratification is present;（iv） at hydrological gauging station CS8,the gradient Richardson number（Ri） is in the order of 10-3～ 10-2at the bottom and surface of the water column.Correspondingly,there is relatively high TKE dissipation rate（ε） because of bottom friction and wind stress,respectively.The gradient Richardson number（Ri） is in the order of 100～ 101in the stably stratified region of the middle part of the water column.Modeled TKE dissipation rate（ε） at hydrological gauging station CS8 is in the order of 10-3～ 10-9W/kg during spring tide and10-5～ 10-10W/kg during neap tide which follows a predominantly M4cycle with a flood/ebb tidal asymmetry.Tidal straining appears to be responsible for the flood/ebb tidal asymmetric distribution of TKE dissipation rate.Those preliminary results would enhance our understanding of vertical transport processes within the North Passage of the partially-mixed Changjiang River estuary.