Hydraulic Design of Highway Culverts - DOT On-Line Publications

A popular method of providing for fish passage is to provide dual culverts, one culvert designed
for hydraulic capacity and one culvert designed for fish passage. The latter culvert would have
a flatter slope, higher roughness, and could contain fish baffles. In this case, the hydraulically
efficient barrel would convey most of the flow. To design parallel, dissimilar culverts, it is
necessary to construct separate performance curves (elevation versus discharge) for each
culvert. The two performance curves are added together at equal elevations to obtain the
combined performance curve. A similar technique is described later in this chapter for multiple
barrel culverts with unequal invert elevations.
The hydraulic design of culverts with fish baffles is accomplished by modifying the friction
resistance of the barrel in outlet control to account for the high resistance imposed by the
baffles. Reference (39) provides design curves and procedures for estimating the hydraulic loss
due to fish baffles using a modified version of Equation (4b). The remainder of the outlet control
calculations are the same as outlined in Chapter III. For inlet control, only the reduced area of
the entrance due to the baffles and any edge modifications need to be considered in the
procedure.
C. Erosion, Sedimentation, and Debris Control
Natural streams and manmade channels are subject to the forces of moving water. Pressure,
velocity, and centrifugal forces can be significant depending on the depth of flow, and the slope
and sinuosity of the water course. An evolutionary process is the result with the continuous
occurrence and dynamic interplay of erosion, sedimentation, and debris movement. This
process, referred to as fluvial geomorphology, is accelerated during storm events when stream
depths and velocities are high. Inserting a culvert into this dynamic environment requires
special attention to the effects of these natural phenomena on the culvert and the effects of the
culvert on the stream channel. Past experience has shown significant problems, including
erosion at the inlet and outlet, sediment buildup in the barrel, and clogging of the barrel with
debris.
1. Scour at Inlets. A culvert barrel normally constricts the natural channel, thereby forcing the
flow through a reduced opening. As the flow contracts, vortices and areas of high velocity flow
impinge against the upstream slopes of the fill and may tend to scour away the embankment
adjacent to the culvert. In many cases, a scour hole also forms upstream of the culvert floor as
a result of the acceleration of the flow as it leaves the natural channel and enters the culvert.
Upstream slope paving, channel paving, headwalls, wingwalls, and cutoff walls help to protect
the slopes and channel bed at the upstream end of the culvert. Figure VI-10 depicts a culvert
with a headwall and wingwall protecting the inlet against scour.
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