Estuarine mixing regimes: the role of interfacial instabilities

In specific environments, two-layer systems can occur due to density differences between the upper and lower layer. At the interface between the two layers, perturbations can develop due to accelerating and decelerating flow in, either, the upper or lower layer, and the compensatory pressure adjustment. These perturbations, also known as instabilities, are thought to contribute to the overall mixing regime of the system.

In estuaries, density differences within a system can be brought on by such factors as suspended sediment, salinity, or temperature variation along the water column. In the case of sediment-related density variation, a typical structure has been observed to arise: a lower high concentration layer (in the range of 10 g/l and termed ‘fluid mud’, and an upper low concentration layer. The interface between these two layers is called the ‘lutocline’, the thickness of which is dependent on the stability of the water column. In a stable environment (laminar flow), the lutocline may be very thin; however, in an increasingly unstable environment (transitional flow), the width of the lutocline may increase and be characterized by an intermediate suspended sediment concentration. If the flow becomes fully turbulent (unstable), the lutocline may be seen to disappear at which point erosion and entrainment of the lower layer would lead to a transition from a two-layer system to a fully mixed system. It is thought that instabilities at the interface of the two layers, are the mechanism for the breakdown of the two layer system.

For my research, I am investigating the role of interfacial instabilities in an estuarine mixing regime. I aim to address the following questions:

- Are there critical instability characteristics that are indicators for the two-layer system breakdown?
- How does the stability of the water column relate to the type and persistence of interfacial instabilities?
- In an asymmetrical tidal estuary, are there inherent differences in how the instabilities contribute to the transition to a fully mixed system?

To answer these questions, I am analyzing a data set collected in 2010 in the Ems Estuary, Germany by Dr. Christian Winter (proponent, IC1).The data set includes high resolution current velocity profile data, sediment density echo sounder data, as well as suspended sediment, salinity, and temperature profile data.