Hydraulic Design of Highway Culverts - DOT On-Line Publications

3. Headwater. Energy is required to force flow through a culvert. This energy takes the form of
an increased water surface elevation on the upstream side of the culvert. The depth of the
upstream water surface measured from the invert at the culvert entrance is generally referred to
as headwater depth (Figures I-13 and I-14).
A considerable volume of water may be ponded upstream of a culvert installation under high fills
or in areas with flat ground slopes. The pond which is created may attenuate flood peaks under
such conditions. This peak discharge attenuation may justify a reduction in the required culvert
size.
4. Tailwater. Tailwater is defined as the depth of water downstream of the culvert measured
from the outlet invert. Figure I-14 It is an important factor in determining culvert capacity under
outlet control conditions. Tailwater may be caused by an obstruction in the downstream channel
or by the hydraulic resistance of the channel. In either case, backwater calculations from the
downstream control point are required to precisely define tailwater. When appropriate, normal
depth approximations may be used instead of backwater calculations.
5. Outlet Velocity. Since a culvert usually constricts the available channel area, flow velocities
in the culvert are likely to be higher than in the channel. These increased velocities can cause
streambed scour and bank erosion in the vicinity of the culvert outlet. Minor problems can
Figure I-15--Culvert Performance
Curve
occasionally be avoided by increasing the barrel
roughness. Energy dissipaters and outlet protection
devices are sometimes required to avoid excessive
scour at the culvert outlet. When a culvert is
operating under inlet control and the culvert barrel is
not operating at capacity, it is often beneficial to
flatten the barrel slope or add a roughened section
to reduce outlet velocities.
6. Performance Curves. A performance curve is a
plot of headwater depth or elevation versus flow
rate. The resulting graphical depiction of culvert
operation is useful in evaluating the hydraulic
capacity of a culvert for various headwaters. Among
its uses, the performance curve displays the
consequences of higher flow rates at the site and the
benefits of inlet improvements.
In developing a culvert performance curve, both inlet
and outlet control curves must be plotted. This is
necessary because the dominant control at a given
headwater is hard to predict. Also, control may shift
from the inlet to the outlet, or vice-versa over a
range of flow rates. Figure I-15 illustrates a typical
culvert performance curve. At the design headwater,
the culvert operates under inlet control. With inlet
improvement the culvert performance can be
increased to take better advantage of the culvert
barrel capacity.
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