Hydraulic Design of Highway Culverts - DOT On-Line Publications

It is also advantageous to consider approach and downstream velocities in the design of low
head installations. Equation (6) should be used instead of Equation (7) to calculate headwater
depth (HWo) in outlet control. In inlet control, the approach velocity head should be considered
to be a part of the available headwater when using the inlet control nomographs.
Sag culverts, called "inverted siphons," are often used to convey irrigation waters under
roadways (Figure VI-2). This type of culvert offers the advantage of providing adequate vertical
clearance for the pipe under the roadway pavement and subgrade. A possible disadvantage of
a sag culvert is clogging due to sediment. The design is not recommended for use on streams
with high sediment loads. Sag culverts require the use of bends and inclusion of their related
energy losses. Losses due to bends are covered in the next section.
3. Bends. A straight culvert alignment is desirable to avoid clogging, increased construction
costs, and reduced hydraulic efficiency. However, site conditions may dictate a change of
alignment, either in plan or in profile. A change of alignment in profile to avoid costly excavation
is generally referred to as a "broken back" culvert. (Figure VI-3). Horizontal bends may also be
used to avoid obstacles or realign the flow (Figure VI-4). When considering a nonlinear culvert
alignment, particular attention should be given to erosion, sedimentation, and debris control.
In designing a nonlinear culvert, the energy losses due to the bends must be considered. If the
culvert operates in inlet control, no increase in headwater occurs. If the culvert operates in
outlet control, a slight increase in energy losses and headwater will result due to the bend
losses. To minimize these losses, the culvert should be curved or have bends not exceeding
15-degrees at intervals of not less than 50 feet (15m) (30). Under these conditions, bend losses
can normally be ignored.
Figure VI-4--Culvert with a Horizontal Bend (Contech)
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