# Cartesian/Curvilinear grid

Below are statements, where i am a bit confused:

Regarding the two ways to generate a new model - Cartesian and Curvilinear:

“The vertically hydrostatic momentum and continuity equations of EFDC model for turbulent flow are solved in a coordinate system which may be curvilinear and orthogonal in the horizontal and stretched or topography-free surface in vertical direction. [4] [2]”

Does the statement applies to both Cartesian and Curvilinear Grid generation? Meaning to say, the momentum and continuity equations in EFDC model being used are the same whether cartesian or curvilinear grid will be used?

“EFDC uses stretch or sigma vertical coordinates and cartesian or curvilinear, orthogonal horizontal coordinates to represent the physical characteristic of a waterbody.” - to clarify, whether I use Cartesian or curvilinear grid generation, sigma vertical coordinates is going to be used - for the vertical; and for the horizontal, that will be either cartesian or curvilinear will be used?

thank you for those who will reply.

Armi

Hello Armi,

Thanks for this interesting question. In EFDC you are allowed to use both the cartesian and curvilinear grids. The equations are designed such that it will solve for both cartesian and curvilinear grids.

For example the momentum equation in X direction is given by:

mx, my are the metric coefficients that would be calculated from dx and dy of the cells. However, if the cells are cartesian, then mx and my would be 1 and the equation would be transformed to :

In the vertical, sigma coordinate will be used in both as you said. I hope it helps.

Best,
Janesh Devkota