# CMS-Flow Coordinate System

## Coordinate System

Before presenting the depth-integrated and wave-averaged governing equations, it is useful to define the coordinate system and basic variables. Variables are defined spatially in a Cartesian coordinate system , where *x* and *y* are the horizontal coordinates, and *z* is the vertical coordinate (positive is upwards). A schematic of several of the main variables in the vertical direction is provided in Figure 2-1. The vertical coordinate datum is usually the Still Water Level (SWL). The bed elevation *(z _{b})* is measured from the vertical datum (i.e., negative downwards).

**Vertical conventions used for the bed and mean water surface elevation.**