长江口北支上段异重流潜入过程及其对河槽冲淤的影响

The Upper North branch of the Yangtze estuary is of morphologically importance to the evolution of the entire North branch. But it is still unclear how the Upper North branch evolves in terms of the channel erosion and sedimentation processes. Based on the previous studies, we find this reach is characterized by the plunging dynamics of gravity currents which developed during the saltwater and sediment intrusion of the North branch in the dry season. Considering the plunging dynamics can be correlated with the channel morphodynamics as suggested by some recent studies, we hypothesize that such plunging processes are responsible for the channel morphodynamics of the Upper North branch. ..To verify such idea, we propose to examine the plunging dynamics of gravity currents in the Upper North branch during the intrusion of the North branch. A series of high-resolution field instruments (ADCP, OBS, LISST, echosounder, RTK GPS etc.) will be used in both on-track and/or vessel-anchored manners to measure the hydro-morphological elements which include the flow velocity, suspended sediment concentration (SSC), salinity, partial size and depth. Obtained results will allow us to reveal the characteristics of plunging dynamics, and correlate these processes with the channel erosion and sedimentation with the consideration of tidal influence in the Upper North branch. Ultimately our findings will be helpful to examine the mechanism of channel evolution of the Upper North branch.

长江口北支上段河槽冲淤，对北支地貌演化非常关键；但其冲淤机理，仍需进一步研究。北支上段又以“枯季北支倒灌初期形成的异重流潜入过程”为主要特征。本项目参照单向流条件下异重流“潜入－冲淤”对应关系，认为异重流潜入过程与北支上段河槽冲淤密切相关，拟对其进行研究。为此，本项目计划采用高精度物理仪器（ADCP、OBS、LISST、测深仪等），针对北支上段异重流潜入过程的水文、泥沙、地貌参数，进行定点、走航相结合的野外观测；揭示其潜入的动力地貌演化特征，建立双向流（潮汐）影响下的潜入过程与河槽冲淤对应模式，阐明异重流对北支上段河槽地貌的影响。

长江口北支上段以枯季倒灌密度流为特征，但密度流的地貌效应仍需深入研究。对此，本项目基于野外观测与历史资料，利用水文、泥沙、地貌等多学科手段，针对北支上段密度流水沙动力特征、河槽冲淤变化、潜入与层化模式进行了研究。..结果表明，北支上段倒灌密度流发生时，长江入海径流量在~12000m3s-1，北支上段涨潮量在~10000 m3s-1，涨落潮差超过3m。北支上段涨潮流与南支咸淡水之间的密度差导致了密度流的发生。其中，口外涨潮流的盐度密度梯度占据主导，最高可达~16kgm-3；悬沙密度梯度相对贡献较少，以絮凝泥沙为主，并包括潮汐再悬浮泥沙。密度流发生后底床粒径向南支岔口趋于变细，显示了密度流向南支的沿程淤积特征。在密度流潜入区附近略微冲刷，逐渐向北支入口过渡为淤积，最大可达10cm。这与涨潮流速的沿程分布，以及近底摩阻流速等变化基本一致；表明密度流存在短时间尺度的“潜入-冲淤”对应模式。大量泥沙被携带淤积于南北支岔口，发育局部三角洲。通过模拟不同涨潮量下的密度流“潜入-冲淤”对应模式，发现当北支上段涨潮量超过14000m3s-1，密度流潜入冲刷趋于扩大；当模拟涨潮量超过23000m3s-1，冲刷可延伸到崇头附近，影响南北支岔口三角洲发育。通过“长江入海径流-北支倒灌”相关关系，结合北支上段潮汐变化，重建了1950年以来北支上段密度流发生频率，估算了其河道冲淤影响。对比基于海图的河槽冲淤变化发现，1990-1998、2000-2013北支上段多呈现“洪淤枯冲”模式，与重建的密度流发生情况一致，表明枯季密度流是调节河槽季节性冲淤的重要过程之一；而1950-1980时期冲淤并不完全一致，推测涂围垦等人类活动是其主导因素。此外，北支上段密度流显著增强了局部河口环流，其层化主要由重力环流所导致，比潮汐应变大1-2个数量级。与之相反，北港口外盐水楔主要由潮汐应变所主导，远大于（2-3个数量级）重力环流的作用。口外盐水楔可向崇明、横沙等湿地输送泥沙（~1-3 kgm2s-1），是河口滩涂自我维护的重要过程之一。..本项目研究可为长江河口地貌演变、航道疏浚、湿地保护等提供参考。