Menu

Experiments by Invited Researchers

 

The dynamics of bi-directional exchange flows: implication for morphodynamic change within estuaries and sea straits

Project acronym: H+-CNRS-01-Adduce
Name of Group Leader: Claudia Adduce
User-Project Title: The dynamics of bi-directional exchange flows: implication for morphodynamic change within estuaries and sea straits
Facility: Coriolis platform
Publications:

click to view Video
click to view video

The aim of this project is to model bi-directional, density stratified, exchange flows through non-erodible and erodible channel topographies, as occurring in an estuary. The experiments were expected to provide new datasets to fill existing knowledge gaps associated with (i) improved physical understanding of exchange flow bathymetry feedback mechanisms in channels with deformable (i.e. erodible) boundaries; (ii) better representation of interfacial mixing, secondary flow circulations and near-bed boundary dynamics for numerical modelling simulations of exchange flow processes in estuaries and sea straits; and (iii) enhanced understanding of other external parametric influences (i.e. tidal/freshwater forcing; density differences; bed friction and Coriolis effects) on the nature of these exchange flow-bathymetry interactions. As such, the specific research questions that the proposal addressed are:

 

  • How does channel bathymetry (i.e. channel cross-sectional shape) influence flow distribution, interfacial mixing and entrainment, secondary circulations, and near-bed dynamics in bi-directional, stratified, exchange flows?
  • How is the exchange flow behaviour influenced by other external forcing characteristics within sea straits and estuaries (i.e. tidal fluxes; freshwater flows; density differences; Coriolis effects)?
  • What are the reciprocal feedback mechanisms generated between the bi-directional exchange flow and the channel topography with a deformable (i.e. erodible) bottom boundary?
  • Does the erodible channel adjust to quasi-equilibrium profile under specific external forcing conditions?

Experimental measurements were conducted in order to obtain high temporal and spatial resolution density and velocity fields using both flow visualisation (i.e. PIV and LIF) and probe profiling (i.e. ADV, micro-conductivity) measurements. Moreover, a laser bed scan method to measure changes of the bed morphology during and after each experimental run was implemented.

A total of 83 experiments divided in four laboratory configurations was planned and performed:

1) Bi-directional exchange flows in a non-rotating, rigid-bottom channel
2) Bi-directional exchange flows in a rotating, rigid-bottom channel
3) Bi-directional exchange flows in a non-rotating channel with an erodible bed
4) Bi-directional exchange flows in a rotating channel with an erodible bed

The experimental datasets, consisting in detailed velocity and density fields, was re-organised and preprocessed. It will be analysed to characterise the exchange flow dynamics at the interface (i.e. lateral variations in layer thicknesses, interfacial mixing and entrainment, cross-channel pycnocline tilt, etc.), bi-directional flow distribution across the channel (e.g. maximum velocity thalwegs in lower/upper layers), and secondary flow circulations generated in the fixed and erodible trapezoidal channels under both rotating and non-rotating conditions. These exchange flow characteristics will be correlated with measured bed deformations (configuration 3 & 4 only) to investigate key drivers of morphodynamic change within the erodible channel.

Due to the recent end of the project, the experimental datasets are still under analysis.  However, preliminary results suggest that the increase of the upper fresh water flow causes a reduction of the thickness of the lower saline layer. The rotation causes a tilt in the interface between lower salty and upper fresh water flow.  For increasing rates of rotation the tilt of the interface increases, up to generate a meandering pattern within the salty layer. Moreover, the measured bed morphologies indicate that both the rotation and the velocity in the salty water layer affect the bottom boundary. A more detailed analysis needs to be conducted to investigate how the channel cross-sectional shape influences the flow distribution, the interfacial mixing and entrainment, the occurrence of secondary circulations, and near-bed dynamics in bi-directional, stratified, exchange flows; study the reciprocal feedback mechanisms generated between the bi-directional exchange flow and the channel topography with an erodible bottom boundary.

Back