NBQ wave resolving surf-swash zone simulation for beaching of plastics

sediment
1

Hi,

I would like to setup a CROCO NBQ configuration of the surf - swash zone of a realistic beach similar to the @pmarchesiello Marchesiello et al 2021 (Redirecting) to simulate the 3D wave-current-turbulence dynamics that control the circulation and beaching of plastics e.g. wave drift, surface current, wet-dry swash action etc.

The question is how to simulate the plastics particles. Several options:

  1. Treat the plastic particles as a sediment particle (activate the sediment model) with appropriate density, particle size matching the plastics. Could this work if plastics are floating on the surface? Will it be deposited/accumulated on the beach within the swash zone due to the wave wet-dry action?
  2. Treat plastic particles by activating the “Lagrangian floats” model. I guess this is inherited from the ROMS code, but there is zero information in the CROCO docs.
  3. Use CROCO generated data (3D currents, stokes drift, 3D Diffusivity) to an external Lagrangian particle circulation model. Unfortunately, this option leads to loss of information such as wave action in the swash zone…

Any thought and suggestions?
Merci
Konstantinos

2

One more question. Does the morphodynamics model work in the swash zone? Can it predict deposition/resuspension of sediments/erosion of beach?

3

Hi Konstantinos

This is an interesting topic, and the ability to model plastic or any other buoyant material is becoming increasingly important in CROCO.

The simplest way to achieve your goal would be to use an external Lagrangian model. But, as you’ve guessed, you’ll need to use a high-resolution time output to include Lagrangian processes (swash, Stokes drift).

The online Lagrangian model might work, but it’s long out of use, undocumented and in the process of being replaced.

The last option (your first) of using an Eulerian tracer, for example from the sediment model, would also be appropriate. Note, however, that the sediment model’s sinking scheme is not suitable for upward velocities and should be modified (or may be replaced by a simpler upwind scheme for your purpose).

Patrick

4

Yes morphodynamics would essentially work in the swash zone. Some processes may be missing, such as percolation …

5

Thank you Patrick for your reply.

I guess my best option, taking into account the upwards velocity issue for sediments, would be the external Lagrangian model. I have also to check if the online Lagrangian still works.

I think it would be very useful to make the sediment model work with particles with positive buoyancy, not just upwind surface drift, but full 3D. That is because the breaking waves can entrain small buoyant particles which will spend some time in the water column and be subjected to turbulence and currents, before they resurface again. If the positive buoyant velocity is fixed in the sediment model, I would like to test it and compare the results with the external Lagrangian model.

Regards
Konstantinos

6

In the sediment model, sediment particles are transported using a 3D advection scheme, and the sinking scheme is an addition (not formulated in conservative form, as this velocity is not part of fluid dynamics and does not enter the continuity equation). All we need to do is adapt it to rising velocities.

7

That sounds very good idea. Then for the actual beaching when they reach the swash zone, would that require a separate function to estimate the amount deposited on the beach (some kind of function based on wave and beach characteristics?) since they have positive upwards buoyancy and sediment deposition would probably not work?

8

@pmarchesiello would it be easy to share the CROCO configuration files from your simulation with sediments from the paper linked above? I want to use it as a starting point to experiment with plastics. If it is easy you can send a link to my email konstantinos.kotzakoulakis@sintef.no
I will keep you in the loop of how this project is going if you are interested.
Thank you
Konstantinos