Culverts are crossing structures in running waters and allow the passage of water. They may be pipes that are laid under a road, allowing the flume to cross. The culvert may be flowed through partially or in full, depending on the discharge occurring. A partially filled culvert with free surface is treated in the same way as an open channel. By contrast, a full flow through culvert and a culvert in which the inlet is completely submerged are classed as control structures. These result in a limiting of the discharge. There may also be a combination of these two states, so that the culvert is sometimes fully flowed through and sometimes partially filled. For various reasons, culverts are not ideal from a hydraulic point of view: they cause flow losses, are vulnerable to blockages (rubbish, sediment), can cause scour at the inlet and outlet and – in the event of floods – are often too small. Furthermore, they are difficult for aquatic creatures to pass through. Bridges are a much better alternative from a hydraulic point of view, but of course much more expensive.

Discharge type 1

full flow through culvert, upstream and downstream of culvert Fr < 1; hu upstream water discharge depth, hc critical depth, Q discharge, d culvert diameter, hd downstream water discharge depth

Discharge type 2

full flow through culvert, upstream of culvert Fr < 1, immediately downstream of culvert Fr > 1

Discharge type 3

partially filled culvert, here with flow transition in the inlet and downstream of culvert; also possible: continuous discharge with Fr < 1 or Fr > 1

Discharge type 4

submerged culvert inlet with discharge control; flow transition also possible in culvert, so that culvert is partially filled