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Dear All,

I am trying to model the water exchange via a permeable rock structure between a closed basin and the sea (I have attached a small scheme).


I did a couple of tests including a high friction coefficient at the permeable rock structure to simulate the resistance to the flow. The model is showing something close from what I am expecting (I don't have data to calibrate this part of the model). However, I didn't manage to found a reference to justify the friction coefficient(Chezy coefficient) used at the permeable structure.

I wonder if anyone else had already done something similar and how they manage to consider a permeable structure inside their model.

Thanks in advance for your help.

Mathieu

Hello,

It would be better to find an equivalent with vertical structures and the subroutine dragfo.f (see example dragforce). It is closer to physics as the force increases when the depth increases, whereas the friction (relatively) decreases. Simple headlosses without reference to vertical structures would also work, but they will help to find orders of magnitude. You could also add porosity (see example porosite) to account for the obstacles. A porosity based on the fraction water/rocks in the porous structure sounds physical. Then you would have to tune only the drag coefficient of the vertical structures or the head losses.
We have already tested this approach for flows through cities (European project RESCDAM in 2001)

With best regards,

Jean-Michel Hervouet

Dear Jean-Michel,

Many thanks for your quick reply. I'll replace the high friction coefficient by the porosity and a drag coefficient. As you mentioned we will have to tune only the drag coefficient. Do you have any reference regarding the drag coefficient for a flow through a rubble mount rock structure?

Kind Regards,

Mathieu

Hello,

Actually not, maybe someone else ?

Regards,

JMH

Hello,

this is an interesting problem.

What pore-size has the material? I assume that it is coarse and that you neglect capillarity?

There are simple formulas to compute the flow velocities in fractures. Maybe this is a starting point for a drag coefficient.

I would recommend to have a look in the book "Dynamics of Fluids in Porous Media" from Jacob Bear. It is relatively old, but cheap and has many interesting solutions.

In the world of subsurface flow, the problem is usually to simulate the seepage line and the exit of water at the side of the closed bassin. The open sea is a variable Dirichlet boundary condition with a given water level. Depending on the water level of the open sea the seepage line will change. The boundary at the closed bassin is variable in time and dynamic boundary conditions are necessary to get the correct outflow.


Best regards,

Leo

Hi Leo,

Thank you for your reply and the reference. The porous structure is composed of 1-3T rock so I'll not consider the capillarity.

To start, I am preparing a couple of sensitivity test and I'll try to reproduce the seepage line.

Thank you for your support.

Kind Regards,

Mathieu