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.