Sigma coordinates.

Sigma coordinates are widely used in oceanography due to the differences in the elevation of the seabed.  The elevation can vary from several meters deep near the coast and several kilo-meters in the deep basins. The regular z coordinates are mostly compatible only with uniform basins.

z is the vertical length from water surface to any point. H is the depth of the total water column. η is the sea surface height that is affected by the tidal force. Sigma varies from 0 to -1 where -1 is at the  bottom earth surface where z=-H and 0 is at the top of water surface z=η.

From Wikipedia

 In sigma coordinates system the number of vertical levels in the water column is the same everywhere in the domain though the depth of water column is different from place to place. The Navier Stokes equation will then be represented in sigma coordinates.