Actual Depths of Sigma Layers in Delft3D FM

2022-07-21T15:56:45Z

Hello Everyone,

I am working with a Delft3d FM output file (FlowFM_map.nc) with sigma layers, where I need to calculate the actual depths of each sigma layer for each element of the unstructured mesh. Not sure if there are any instructions on how to do this, but I made some assumptions and took the following steps to get the actual depths of sigma layers -

a. In the output file, there is a variable called mesh2d_layer_sigma which I believe represents the sigma layers as a cumulative percentage of the total depth (which is represented by a variable called mesh2d_flowelem_bl) for each element. I multiplied these two variables (mesh2d_layer_sigma X mesh2d_flowelem_bl) to get the actual sigma layer depths for each elements. Is it the right way to calculate actual sigma layer depths using these variables?

b. The mesh2d_layer_sigma has values in the following order: -0.83, -0.50, -0.17. Does that mean the first layer is located at a depth of (0.83 x total depth) below the mean water level (surface layer)?

c. If assumption b. is correct, does that mean in addition to the sigma layers, and water velocity components u (mesh2d_ucx) and v (mesh2d_ucy), the bottom layer is the first index, and the surface layer is the last index along the depth dimension of the array?

d. The depth variable (mesh2d_flowelem_bl) mostly shows negative values. Does the negative sign mean, the depth measurement is below the surface level (e.g. -7 m means 7 m below the surface level)?

I would really appreciate it if you share your thoughts on this

Thanks,

Mahmud