ETL 1110-2-367
31 Mar 95
Figure 5-2. Gravity outlet concepts
Anytime the interior elevation is higher than the exterior, the
(3) Pump stations are typically located adjacent to the line-
gate could be opened to allow flow out of the interior until the
of-protection. Normally a larger capacity station is more
differential becomes zero.
desirable than several smaller ones. Gravity outlets may be
offset if located near pump stations where significant direct flow
c.
Pump stations.
access to both the pump and gravity outlets is unavailable.
(4) As with gravity outlets, pump stations should have staff
(1) Pumps are designed to lift storm water and other
gauges on both sides of the line-of-protection, unless the gravity
interior flows over or through the line-of-protection to the
outlet already has staff gauges. Pump start elevation should be
exterior river, lake, or coastal areas as shown in Figure 5-3.
set such that all pumps are in operation before the start of
Pump stations operate to reduce peak stages and duration of
interior damage. The sequencing of the pumps is dependent on
ponding when flow through gravity outlets is precluded or
the approach channel's ability to deliver adequate water;
impeded by high exterior stages. Consideration should be given
therefore, an approach channel rating curve is required. The
to setting these elevations so that the pumps may be operated at
pump stop elevation is set below the damage elevation and
least once or twice annually for maintenance and testing
although not necessarily tied to the channel rating curve,
purposes. Pumps may be used for storm runoff, groundwater
pumping should not continue if the capacity is not delivered by
and seepage, water accumulated from overtopping waves, and
the channel. If the pump stop elevation is set too low, the sump
mixed flows with sanitary sewage.
would have to be lowered to maintain sufficient water depth
over the impeller. A significant cost increase would occur in
(2) Pump stations are generally considered after analysis
this situation.
of gravity outlets and detention storage, since the initial and
continuous operations, maintenance, and power costs of the
(5) The pumping station should be aligned to allow direct
stations are commonly significantly greater than other measures.
flow patterns into the forebay from the conveyance channel or
For areas where interior and exterior flooding are highly
detention areas. The key, therefore, is to design the station with
dependent (high likelihood of blocked gravity outlets coincident
an evenly balanced flow distribution in the approach channel or
with interior flooding), pumping may be the only means to
pipe. A long straight approach of about 100 m (several hundred
significantly reduce interior flood stages. For areas with inde-
feet) is recommended as well as a straight approach through the
pendent interior and exterior flood conditions, where coincident
station inlet into the sump area. A trashrack is located at the
flooding is not likely, pumping facilities may not be required.
5-4
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