Culverts
This refers to a system involving a headwall, pipes and an outlet that is used to convey water under earthworks or structures that fill a waterway, such as a road embankment.
The headwall on the upstream side of the culvert usually has side walls or wingwalls that direct flows into the pipes.
If flows exceed the culvert capacity, flows will pour across the roadway, which acts as a weir control.
The way to specify culverts in DRAINS is to set up a culvert using a headwall, a pipe and an overflow route, similar to the specification of a detention basin. This has the advantage of allowing users to examine details of pipe and overflow characteristics.
The pipe for a culvert is specified in the same way as a pipe laid between pits, but it may use multiple pipes with the same upstream and downstream invert levels. These can be rectangular sections, as shown below, or circular pipes.
The pipes must be designated as 'fixed', with invert levels and sizes specified.
The overflows routes from headwalls are the same components as those used for pits. As well as a flow cross-section, a weir coefficient Cw (in the equation Q = Cw. w. h1.5) and crest length, w must be supplied. Since the overflow lengths are small, and their longitudinal profiles contain only one prismatic reach, the calculated depths will be approximate. The weir controls the amount of overflow.
It is possible to have more that one overflow route with the Full unsteady hydraulic model. To model culverts more accurately, they can be defined as detention basins. The storages can be defined by the water volume in the upstream channel, or by nominal storages, with sensitivity checks to ensure that they work in a stable way.
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