Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure VI-6--Subatmospheric Pressure in Culverts
Jens also provides additional equations for losses in the lateral conduit under full flow
conditions (33). Laboratory modeling may be necessary for particularly sensitive installations.
Partly full flow is analyzed by using backwater calculations. If supercritical flow is possible, the
flow at the junction is very complex, and an appropriate reference on the subject should be
consulted (31, 34). However, in this case the culvert will operate in inlet control, and the losses
are not needed to calculate the headwater.
Erosion may be a problem at the junctions of culverts with natural bottoms. In this case,
protection of culvert foundations and anchorage is very important. This can be accomplished by
proper alignment, selective invert paving, and strategically placed energy dissipaters within the
culvert.
5. Siphons. A siphon is a water conveyance conduit which operates at subatmospheric
pressure over part of its length. Some culverts act as true siphons under certain headwater and
tailwater conditions, but culverts are rarely designed with that intention. Figure VI-6 shows two
culverts acting as true siphons.
149