长江口南支南港的北槽枯季水体中混合、层化与潮汐应变*

2010年1月1日至10日在长江口南支南港北槽航道弯道段内3个水文测站位CS1、CSW和CS8，观测得到大、中、 小潮的潮位、流速、盐度和含沙量的时间序列。这些资料揭示了由盐度和含沙量引起的垂向层化的大、小潮和涨、落潮的 潮周期变化特性。为定量了解航道弯道段水体的垂向混合程度，采用考虑含沙量后的水体密度来估算其梯度Richardson数 (Ri)。在转流时刻，CS1和CSW站位的量级为101 ~ 102，水体呈现层化状态；在涨急、落急时，Ri量级为10-2 ~ 10-1，水体呈 现强混合状态。CS8站位涨潮时的Ri在0.25~5，落潮时平均为10-2量级。3个水文测站位，涨潮时的层化均强于落潮时；大潮 时的层化程度最强，而小潮时的层化持续时间最长; 均存在潮汐应变的现象，且以非持久性的SIPS层化为主。采用Simpson 等2]的公式，估算了长江口北槽航道弯道段内水体由河口环流、潮汐应变和潮汐搅动引起的势能变化率。潮汐应变是水体 层化的主要动力机制，而河口环流引起的势能变化率比潮汐应变和潮汐搅动引起的小102 ~ 103量级。

Mixing, stratification and tidal straining in dry season within the north passage of the south branch/south channel of the Changjiang River estuary

Field measurements are made of time series of tidal elevation, current velocity, salinity and suspended sediment concentration at three hydrological gauging stations CS1, CSW and CS8, within the curved three hydrological gauging stations CS1, CSW and CS8. Strain induced periodical stratification is predominant at these locations. It is found that the tidal straining is the key mechanism for the stratification within the waters of the curved channel. The rate of change in the potential energy dut to estuarine circulation is smaller than that dut to tidal straining and tidal stirring by the order of 102 ~ 103. navigational channel of the north passage of the south branch/south channel of the Changjiang River estuary, on spring, moderate and neap tides, on 1 to 10 January 2010. Those data display the spring/neap and flood/ebb tidal variability in the vertical stratification caused by salinity and suspended sediment concentration.To quantitatively evaluate the potential of vertical mixing within the curved channel, we estimate the gradient Richardson number (Ri) using the density of water after taking suspended sediment concentration into account. Ri at stations CS1 and CSW can be in the order of 101 ~ 102 at the turning of the tide. The strongest mixing with the order of 10-2 ~ 10-1 occurs at the maximum flood and ebb tides. Ri at station CS8 is between 0.25 and 5 at the flood tide and of the order of 10-2 at the ebb tide. At the three hydrological gauging stations, stratification appears to be stronger at the flood tide than at the ebb tide. Stronger stratification is present on the spring tide, while stratification lasts longer on the neap tide.Following Simpson, we estimate the rate of change in the potential energy of the water column within the curved channel caused by tidal straining, estuarine circulation, and tidal stirring. Tidal straining is present at the