Side-bank and bottom scour can be controlled by laying erosion-resistant material at the affected points. The use
of downstream walls will assist in the reduction of side-bank scour.
Remember-the last step in designing a culvert is to check the velocity to see if some form of erosion control will
be necessary at the outlet. Erosion velocity of the soil is obtained from table 4-1, page 4-4.
If headwalls and wingwalls are not used on the downstream end, the culvert should project a minimum distance of
two feet beyond the toe of the fill. The area under the projecting culvert should be well protected by erosion-
resistant material, such as sandbags or rock riprap. Rock riprap should be of rock not less than a 6-inch minimum
dimension. On the sides of the channel, riprap should be two layers deep; under the culvert outfall, the depth of
riprap should be at least three layers.
Changing the slope of the culvert is another way to decrease velocity and thus reduce the chance of erosion. The
slope(s) of the culvert will generally be established by the stream bed. In the event the grade is steep, generating
high outlet velocity, the slope of the culvert can be set by the designer to lower the outlet velocity.
Often erosion of drainage structures occurs as a result of a very large storm. Military drainage structures are
seldom, if ever, designed to discharge the worst storm on record. For this reason it may be assumed that a storm
more severe than the design storm may occur and overload the drainage system. Since most military drainage
structures will be overloaded at one time or another during their useful life, it is usually a good procedure to
design the area around a culvert or drain inlet to take care of a certain amount of ponding. In some cases, for
economy, a drainage engineer may deliberately specify a system which cannot immediately take care of even the
design storm. When this procedure is followed, sufficient ponding areas must be included in the overall plan so
that inundation of vital areas does not occur. In this case, the excess water is merely stored until the intensity of
the storm decreases to the extent that the drains can handle the water. As a general rule, drainage systems on
military installations are designed to take care of the runoff from the design storm without ponding. For reasons
previously mentioned, however, some provisions should be made for ponding to take place in those areas where
inundations for a period of time will not affect the operational duties of the installation.
The following specifications are generally followed in the design of ponding areas for military installations:
The edge of ponding areas must be at least 75 feet from the pavement edge.
The pond must be drained before damaging infiltration of the subgrade can occur. The actual time during
which ponding is allowable depends upon the condition and type of soils found in the ponding area.
Generally speaking this period will be about four hours after the storm begins for most of the soils
encountered in practice.
Table 6-1 lists the various erosion problems and their recommended solutions.
EN5465
6-4
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