Page 295 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.17

where L = minimum length of opening, ft
V = velocity of water in gutter, fps
Y = depth of water in curb, ft
D = thickness of grate, ft (from a manufacturer’s catalog)
Discussion:
1. A minimum length of 3 ft (1 m) and a minimum width of 2 ft (1.3 m) are recom-
mended.
2. Net opening of the grate should be 50 percent of width or greater.
3. The nomograph in Fig. 6.14 should be used to determine water velocity in a gutter or
channel.
Curb Grate. This type of inlet is very inefficient, and a long length is usually required
to collect the entire design flow. The capacity for complete interception is determined from
the following formula:
+
.
Q = 07 L A Y) 15 (6.4)
(
.
where Q = capacity of grate, cfs
L = length of clear opening, ft
A = depth of depression at inlet, ft (below roadway level)
Y = depth of water in gutter, ft (above roadway level)
Combination Grate. The capacity of combination inlets has been determined by a
series of complex equations. Since actual experiments have proven that the capacity of
combination inlets is not much greater than that of flat inlets, it is much easier to design the
grate as if it were flat. Since some allowance can be made for the combination grate, the
size of the flat grate portion can be reduced if the calculated size is larger than that of the
standard size produced by a manufacturer.

SYSTEM DESIGN CRITERIA

General

When large areas require storm drainage, such as commercial and industrial sites and park-
ing lots, the methods and tables found in most codes for interior roofs are not applicable.
The solutions obtained using those methods would result in systems that are oversized for
the flows, involved, and far too large to be economically feasible.
The two most common methods used at this time are the unit hydrograph for very large
sites and the rational method for smaller sites. The rational method, which is the only
method to be discussed, uses a basic equation [Eq. (6.5)] to identify the flow behavior and
collection of surface runoff.

Q = A I × R
×
AI×× R (6.5)
Q = (SI units)
100
where Q = quantity of storm water runoff, cfs (1cfs = 448 gpm)
2
2
A = area to be drained, acres, 43,560 ft (4047 m )
I = imperviousness factor of surface comprising drainage area, dimensionless
R = rate of rainfall, in/h (cm/h)

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