Background behind 50 year flow passage standard (ODF)
The term x - year assigns a probability of failure for a culvert or bridge. The greater the number the lower the probability that a storm will come along that will destroy the installation.
The 50 year design for bridges includes a three foot freeboard requirement
The 50 year design for culverts only allows water to build to the top of the culvert (not the top of the fill and assumes a projecting inlet and inlet control conditions for flow capacity calculations)
The 50 year requirement is equivalent in most cases to 100 year requirements that allow for water to pool deeper than the top of the culvert inlet.