How to calculate Bottom & Surface elevation using 1 Contour file?

Hello All,
I want to set up a simple EFDC model for Lake Ontario to see the temperatures profiles under varying cloud cover.
I have a contour bathymetry (.xyz) for Lake Ontario. I could import that file as an Bottom Elevation in XYZ format and I got ELEV column populated in DXDY.INP file with Z values of the contour file.
But how do I get the Depth column in DXDY.INP file?

I tried importing the same contour data for Surface Elevation. It doesn’t allow me to use Point Measurements/Gridded Data option, so I used Constant option and I got Depth column populated in DXDY.INP with an arbitraty constant value of +0.09.

I have just one bathymetry contour file. I want to set 3 vertical layers for the model.
How do I calculate Bottom Elevation and Depth in DXDY.INP file from one contour file?
How do I put water column values for 3 vertical layers?

Thanks for your help.