"thin dams" in ROMS

[hlsimmons]

I'm wondering about parameterizing things like breakwaters or groynes. Delft3D (in regular, not unstructured grid formulation) has implemented what they call a "thin dam" which as far as I can tell is a no-normal-flow boundary condition along the edge of a grid cell. I guess folks have done it in FVCOM as well (http://fvcom.smast.umassd.edu/wp-conten ... l_2012.pdf)

Has anyone done anything like this in ROMS? In my case I am interested in implementing a breakwater so a full depth flow restriction. I do realize I could approximate this with a single cell mask.

The closest thing I have heard to this is that in MOM6 they have implemented the capability for subgridscale exchange point constriction so that your global model could have an open Strait of Gibraltar or Bosphorus or what have you, but the flow is constricted at the SGS level to limit the exchange to be realistic.

Harper Simmons
UW-APL

[jcwarner]

can you just set the u-mask to be zero at a point and that would prevent the flow across that interface (similar for v). ? .

[wilkin]

(There was another post about this a couple of years ago ... viewtopic.php?p=23078&hilit=thin#p23078)

Exercise some caution in simply setting mask_u(i)=0 to impose a barrier because the calculation at u(i+1) may use values at u(i-1) in addition to u(i) to complete the finite difference stencil.

mask_u(i-1) is not zero and there will be a non-zero velocity there. In fact, if the barrier is long, it is a totally separate body of water. If the barrier is one full grid cell wide, i.e. mask_rho(i-1) = 0, then mask_u at both i and i-1 are zero and this is safe.

[arango]

In ROMS, we have the GAMMA2 parameter to set the slip and no-slip boundary conditions. In the no-slip, the velocity is zero at the wall.

Code: Select all

! Slipperiness parameter: 1.0 (free slip) or -1.0 (no slip)

      GAMMA2 == 1.0d0

[hlsimmons]

Thanks folks. Regarding the other post about from several years ago, I would think having e.g. dzeta/dx across a u-mask is actually realistic as long as it can't develop a-physical normal flow or pressure gradient forces. As far as GAMMA2 goes, I *think* I DO want free-slip. So only a no-normal flow condition, no along-wall stress. My concern is that there might be a d(tracer)/dx across the "thin wall" that could result in unphysical mixing of salt or temp across the wall. It is possible that even if this is the case, advection will dominate given that this is a small region with a lot of flow.

At any rate, as was recommended, we will try it and see & report back.

Harper