Processes Impacting on Estuarine Turbidity Zones in tidal estuaries

河口最大浑浊度(ETM)是潮汐河口普遍存在的一种现象。ETM的一阶动力学早在几十年前就已被认识：在纵向盐度梯度区，SPM净输运指向上游，而在淡水范围，净SPM输运一般指向下游，导致SPM输运汇聚，从而导致盐入侵的陆端积累，这是经典ETM所在的位置。在涨潮强度较强但涨潮时间较短的强动力潮汐情况下，淡水范围内也产生了上游的SPM输送，导致(无盐)感潮河段也出现了ETM。最近对倡导者和同事进行的最新审查揭示了ETM动力学方面的重大知识差距，这些差距限制了对与ETM相关的自然和人为过程及其生态后果的理解，并阻碍了河口的可持续管理。拟议项目的一个重点是将总交换流量(TEF)分析框架扩展到每个盐度类别(Q1)中的SPM输送。利用这一分析工具和其他分析工具，将调查以下主要研究问题。问2：水体中SPM的快速动态和底池的慢动态如何相互作用来确定ETM的位置和变异性？问题3：随着水动力条件的变化，SPM动力学和ETM形成的调整时间是什么？问题4：河流和海洋SPM类别在ETM中所占的比例是多少？研究这些问题的方法是建立一个由理想的三维收敛河口模型组成的数值模型实验室。将对模型设置进行校准，以重现易北河口和沙尔德河的纵向盐度和SPM剖面的多十年观测数据。数值实验室将首先用来系统地研究ETM动力学对实际潮汐和淡水强迫条件的敏感性，就像几十年来在易北河口和沙尔德特河观测到的那样。之后，将研究覆盖多维参数空间的理想化强迫条件的情景，以系统地回答上述研究问题。

Estuarine turbidity maxima (ETMs) due to accumulation of suspended particulate matter (SPM) in the water column as well as at the bottom are an ubiquitous phenomenon in tidal estuaries. The first-order dynamics of ETMs have been understood already a few decades ago: in the region of longitudinal salt gradients, net SPM transport is directed upstream whereas in the freshwater range, net SPM transport is generally directed downstream, leading SPM transport convergence and consequently accumulation of the landward end of the salt intrusion, which the classical ETM location. In the case of strongly dynamic tides with the flood being stronger but shorter than the ebb, an upstream SPM transport is also generated in the freshwater range, leading to ETMs in the (salt-free) tidal river as well. A recent state-of-the-art review of the proponent and co-workers revealed substantial knowledge gaps in ETM dynamics which limit the understanding of natural and anthropogenic processes related to ETMs and their ecological consequences and which hamper the sustainable management of estuaries. One focus of the proposed project is the extension of the Total Exchange Flow (TEF) analysis framework towards SPM transport in per salinity class (Q1). Using this and other analysis tools, the following major research questions will be investigated. Q2: How do the fast dynamics of SPM in the water column and the slow dynamics of the bottom pool interact to determine ETM locations and variability? Q3: What are the adjustment timescales of SPM dynamics and ETM formation in response to changing hydrodynamic conditions? Q4: What are the fractions of fluvial and marine SPM classes in ETMs? The method to investigate these questions is a numerical model laboratory consisting of an idealised model setup of a three-dimensional convergent estuary. The model setup will be calibrated to reproduce multi-decadal observational data of longitudinal salinity and SPM profiles from the Elbe estuary and the Scheldt. The numerical laboratory will first be used to systematically study sensitivities of ETM dynamics to realistic tidal and freshwater forcing conditions as observed for the Elbe estuary and the Scheldt over several decades. Afterwards, scenarios with idealised forcing conditions covering a multi-dimensional parameter space will be studied to systematically answer the above research questions.