You must enter the minimum water depth required for the fish to be able to swim when inside the culvert. When the water depth at any location within the culvert falls below this minimum depth, FishXing will identify the culvert as containing “insufficient depth” at that particular flow.  Additionally, it will report the overall length of the depth barrier in the rating tables.  The Water Depth for Swimming section discusses some of the issues to consider when choosing a minimum water depth.

See also: Defining Barriers, Water Depth for Swimming, Estimating Fish Body Depth