bend will not affect the structure, then the culvert can be laid in the stream bed. If the bend is relatively
close to the structure, it is preferable to recut the stream, as shown, and lay the culvert in the new stream
bed. Remember to fill in the old stream and place a dam faced with erosion-resistant material at the
junction between the old and new channels.
A bad situation exists when the stream flows close to and parallel to the structure before the crossing as
shown in figure 5-5(d), page 5-7. The problems in this situation are manifold. Erosion at the bends can
cause problems, as noted above. The stream may develop new meander loops and attack the structure.
Further, if any ponding occurs, the subgrade may become saturated over a large area, causing the
structure to fail under loads. The best solution is to rechannel the stream as shown. Again, care must be
taken to fill in the old stream bed and to provide a faced dam with erosion-resistant material at the
Normally, culverts are placed on the same slope as the material and artificial drainage channel. However, for
culverts to be self-cleaning the velocity of flow should be at least three fps. To achieve this velocity, culverts
should not, if practicable, be set on a slope less than 0.4 percent. To prevent excessive velocity, which could
cause outlet erosion, the culvert grade should not exceed the critical slope for the particular pipe size being used.
Critical slope is the minimum slope of the hydraulic gradient that will permit the maximum Q (flow or discharge).
SPACING OF RELIEF CULVERTS
Culverts should be located wherever natural drainage channels are large enough to require cross drainage. On
side-hill roads or where in-place roads intercept surface water, either in cut or in fill, the water is drained to the
low side of the road, and, if possible, away from the road by ditch-relief culverts. On 8-percent grades, ditch-
relief culverts should be placed about 300 feet apart; on 5-percent grades, 500 feet apart. The distance between
the pipes in multiple-pipe culverts should be at least one-half the diameter of the pipe. See figure 5-6.
Figure 5-6. Maximum culvert specifications for CMP for
vehicular traffic on roadways only