Hydraulic Design of Highway Culverts - DOT On-Line Publications

2. Hydraulic Considerations. Long span culverts generally are hydraulically short (low length
to equivalent diameter ratio) and flow partly full at the design discharge. The same hydraulic
principles apply to the design of long span culverts as to other culverts. However, due to their
large size and variety of shapes, it is very possible that design nomographs are not available for
the barrel shape of interest. For these cases, dimensionless inlet control design curves have
been prepared (Chapter III). For outlet control, backwater calculations are usually appropriate,
since design headwaters exceeding the crowns of these conduits are rare. The bridge design
techniques of HDS No. 1 are appropriate for the hydraulic design of most long span culverts, but
the long span shapes are not included in that publication (21).
I. Culvert Durability
Culvert material longevity is as important a consideration to a culvert installation as proper
hydraulic and structural design. At most locations, the commonly used culvert materials are very
durable. However, there are hostile environmental conditions which will deteriorate all culvert
materials. The two problems affecting the longevity of culverts due to adverse environmental
conditions are abrasion and corrosion (Figure VI-38). Proper attention must be given to these
problems in the design phase. Field inspection of existing culverts on the same or similar
streams will prove invaluable in assessing potential problems.
The annual cost of a culvert installation is very dependent on its service life. All other conditions
being equal, the most durable culvert material should be selected to minimize annual
costs. Measures are available to increase the service life of a culvert, such as lining the barrel
with a more durable material. When considered, these measures should be included in an
economic analysis comparing other culvert materials or other alternatives, including periodic
replacement. Periodic replacement of culverts under low fills on secondary roads with light
traffic may prove cost effective.
1. Abrasion. Abrasion is defined as the erosion of culvert material due primarily to the natural
movement of bedload in the stream. The characteristics of the bedload material and the
frequency, velocity, and quantities which can be expected are factors to be considered in the
design phase. The resistance of various culvert materials to the expected abrasion is then
analyzed. Most materials are subject to abrasion when exposed to high velocity, rock laden
flows over a period of time. Performance data on other installations in the vicinity may prove to
be the most reliable indicator of abrasion potential and culvert material durability.
When abrasion problems are expected, several options are available to the designer. Debris
control structures can often be used to advantage, although they require periodic
maintenance. A liner or bottom reinforcement utilizing excess structural material is another
option. Concrete or bituminous lining of the invert of corrugated metal pipe is a commonly
employed method to minimize abrasion. Concrete culverts may require additional cover over
reinforcing bars or high strength concrete mixes. The use of metal or wooden planks attached to
the culvert bottom normal to the flow will trap and hold bedload materials, thereby providing
invert protection. Oversized culvert barrels which are partially buried accomplish the same
purpose.
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