During calculations, the culvert is divided into three zones:

·  Inlet Zone – The region within the culvert inlet where entering flows contract and then expand rapidly.  FishXing reports the inlet zone as the first two nodes (points) down the culvert. For culverts with a width or diameter of 9 feet or less the inlet zone is 2 feet in length, while for culverts larger than 9 feet the inlet zone is 3 feet in length (Belhke, 1992).  At this location within the culvert, FishXing calculates the Inlet Contraction Velocity. 

·  Barrel - All the nodes between the first two nodes and last two nodes in the culvert

·  Outlet Zone - The distance from the outlet to a distance equal to 4yc (four times critical depth) within the culvert.  This zone consists of the last two nodes of the culvert.  The area of rapidly varying flow associated with a full or partially free surface outlet will be contained within the Outlet Zone.

The zones also define where the inlet, barrel, and outlet Velocity Reduction Factors are applied.

For this application, the Barrel Velocity (VB) is the average cross-sectional flow in the barrel approximately one culvert diameter downstream from the inlet and represents the area immediately downstream of the inlet Contraction and Expansion zone.

See also: Inlet Head Loss Coefficient, Inlet Contraction Velocity, Velocity Reduction Factors