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10.4 Collection of Stormwater 341
or about 0.5 ft/s (0.15 m/s) more than for sanitary sewers. The following factors determine
the capacity of storm drains:
1. Intensity and duration of local rainstorms
2. Size and runoff characteristics of tributary areas
3. Economy of design, determined largely by the opportunity for quick discharge of
collected stormwaters into natural water courses.
Rate of storm runoff is ordinarily the governing factor in the hydraulic design of storm
drains. To prevent inundation of streets, walks, and yards and flooding of basements and
other low-lying structures, together with attendant inconvenience, traffic disruption, and
damage to property, storm sewers are made large enough to drain away—rapidly and without
becoming surcharged—the runoff from storms shown by experience to be of such intensity
and frequency as to be objectionable. The heavier the storm, the greater but less frequent is
the potential inconvenience or damage; the higher the property values, the more sizable is the
possible damage. In a well-balanced system of storm drains, these factors will have received
proper recognition for the kind of areas served: residential, mercantile, industrial, and mixed.
For example, in high-value mercantile districts with basement stores and stock rooms, storm
drains may be made large enough to carry away surface runoff from all but unusual storms,
estimated to occur only once in 5, 10, 20, 50, or even 100 years, whereas the drains in subur-
ban residential districts are allowed to be surcharged by all but the 1- or 2-year storm.
Until storm drains have been constructed in a given area and the area itself is devel-
oped to its ultimate use, runoff measurements are neither possible nor meaningful.
Accordingly, the design of storm sewers is normally based not on analysis of recorded
runoff but on (a) analysis of storm rainfalls—their intensity or rate of precipitation, dura-
tion, and frequency of occurrence—and (b) estimation of runoff resulting from these rain-
falls in the planned development.
Storm sewers are occasionally surcharged and subjected to pressures, but usually no
more than their depth below street level. Nevertheless, they are designed for open-channel
flow and equipped with manholes in much the same way as sanitary sewers. In North
American practice, the minimum size of storm sewers is 12 in. (300 mm) to prevent clog-
ging by trash of one kind or another. Their minimum depth is set by structural require-
ments rather than basement elevations. Surface runoff enters from street gutters through
street inlets or catch basins (Fig. 10.6) and property drains. Size, number, and placement

Street inlet
1

Sidewalk
2 Street
Sewer
Pedestrian crossing 3
Sidewalk

Figure 10.6 Street Inlets (1 and 2) and
Plan of inlets Their Connection to a Manhole (3)