Page 287 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

SITE UTILITY SYSTEMS 6.9

storm sewer selected, and work started on a trial location of storm water inlets. A trial layout of
the site piping should be developed at this time and coordinated with water mains, electrical or
telephone duct banks, sanitary sewers, gas mains, and other piping to ensure proper clearance
and allow for proper and reasonable slope for the storm sewer lines. Obstacles to be avoided,
such as trees, underground structures, and rock, should be located. Recommendations for final
grading could be made to the landscape architect to possibly reduce the cost of storm water
sewers by advantageous placement of inlets to minimize piping runs.
In general, the following should be considered when locating and designing inlets:
1. Where streets or roads intersect, inlets must be located upstream of traffic flow.
2. Where a series of inlets are located in a road with continuous slope, each intermediate
inlet should be designed for approximately 75 to 90 percent of design flow, passing
10 to 25 percent on to the next downstream inlet. If the slope is shallow and flow is
not great, 100 percent capacity should be considered. More than one inlet should be
provided at the bottom to accept the additional flow.
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3. Inlet capacity should be limited to approximately 5 cfs (0.15 m /s) of water.
4. All site low points must be provided with an inlet if off-site gravity flow is not possible.
Combination inlets are used for streets or roadways. Parking lot inlets should be flat
grates only, even when located at a curb.
5. Distance between drainage inlets should be a maximum of about 300 ft.
6. In a roadway with a sag vertical curve, more than one inlet should be considered if a large
quantity of runoff is anticipated. Additional inlets should be located on either side of the
low point inlet to minimize flooding and sediment buildup resulting from large flows.
7. Manholes should be used at all changes of direction and slope of pipe and at multiple
pipe connections. Drain structures also may serve the same purpose. Distances between
manholes should be 250 to 600 ft (75 to 188 m), depending on sewer size.
8. Steep slopes should have inlets located closer together than normally required to limit inflow.
9. A hooded catch outlet should be used as a final structure before connection to a com-
bined sewer to prevent the passage of unwanted gases.
10. The efficiency of inlets in roadways depends on gutter flow, water velocity, roadway
slope, and inlet depression, if any. In many cases, freedom from clogging or interfer-
ence with traffic may take precedence over hydraulic considerations.
In general, the following should be considered:
• The road crown should be as steep as possible.
• Where traffic will not travel close to curb and clogging is not a problem, use a depressed
gutter inlet. Use a depressed curb or combination-type inlet where clogging may occur.
• Where traffic moves close to the curb, use an undepressed inlet and gutter grate with
longitudinal bars only.

STORM WATER INLET SELECTION

Drainage Structures

A drainage structure is an assembly of elements outside of piping built or installed below
ground. Inlet structures are intended to collect surface storm water and route the effluent
into a piping system.

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