# Velocity

Hydrodynamic models simulate velocity and transport fields, elevation of the free water surface; and bottom stress. The state variables of EFDC include: stage height or free water surface elevation; salinity, water temperature and velocity. A three-dimensional application of EFDC simulates velocity in three-dimensions (x,y,z) as the 'u' and 'v' horizontal (x,y) components and the 'w' vertical (z) component.

Velocities in EFDC are computed on the boundaries between cells (temperature, salinity, and density are computed at the center of each cell). In the internal mode, associated with vertical shear of the horizontal velocity components is solved using a fractional step scheme combining an implicit step for the vertical shear terms, with an explicit step for all other terms.