generally contain and dispose of all the runoff.
However, during the winter in the high mountains,
runoff is commonly interrupted by snow conditions;
that is, instead of running off, the potential runoff
accumulates in the form of snow. When this
accumulated mass melts in the spring, the runoff often
attains proportions that overwhelm the natural
channels, causing flooding of surrounding areas. In
the same fashion, unusually heavy rainfall may
overtax the natural channels.
Artificial Drainage.— When artificial structures
are introduced into an area, the natural drainage
arrangements of the area are upset. When, for
example, an area originally containing many hills and
ridges is graded off flat, the previously existing natural
drainage channels are removed, and much of the effect
of gravity on runoff is lost. When an area of natural
soil is covered by artificial paving, a quantity of water
that previously could have been absorbed will now
present drainage problems.
In short, when man-made structures, such as
bridges, buildings, and so forth, are erected in an area,
it is usually necessary to design and construct an
artificial drainage system to offset the extent to which
the natural drainage system has been upset. Storm
sewers are usually the primary feature of an artificial
Figure 10-1.—Working drawing for a typical curb inlet.
10-4
