31 Mar 95
Flood Damage Reduction Measures
(4) The type of inlet chosen defines the entrance loss
coefficient, which affects the design headwater elevation.
a. General. A range of potential flood damage reduction
Chapter 6 of the HEC-IFH user's manual lists these coefficients
measures and performance standards should be addressed in the
study of interior areas. These measures may be structural or
culverts. Inlet designs using a headwall and wingwall or a
nonstructural in nature. Emphasis here is on gravity outlets,
precast concrete or corrugated end section give lower loss
detention or ponding at or near the line-of-protection, and
coefficients and therefore greater flow capacity. Sometimes, in
pumping stations since they represent primary flood damage
locations where large debris can reach the inlet, a debris retarder
reduction measures for interior areas. A comprehensive array
or trashrack is needed.
of other measures combined into plans should also be
(5) The gatewell for the gravity outlet is normally located
on the riverside of the line-of-protection (see Figure 5-2). This
is done so that if problems in the gravity outlet under the line-of-
protection occur, the gate can be closed and exterior water
cannot enter the protected area. Hydrostatic pressure through a
(1) Gravity outlets are defined as culverts, conduits, or
break or separation in the outlet will not jeopardize the stability
other openings that permit discharge of interior waters through
of the earth levee or floodwall above it. Many Districts also
the line-of-protection. The size of the gravity outlet is based on
provide flap gates at the discharge end of the gravity outlet to
the economic, environmental, and social aspects associated with
prevent backflow into the interior area when the outlet is open.
the outfall ditch, gravity conduit, and ponding area analyzed as
Interior water could still flow into the exterior any time the
a collective system. The size selection must be based on the
interior ponding elevation exceeded the exterior.
and not on a single design event. Where possible, gravity
(6) Gravity outflow rating curves are normally required to
outlets should be located at or near where the line-of-protection
assess the outflow conditions of the major outlets. Rating curves
intersects the natural or existing conveyance system or detention
should be developed for primary gravity outlets but may be
area, called the primary location. It is normally more feasible to
combined for secondary outlets. Interior area discharge rating
provide one large gravity outlet than several smaller ones. This
curves for gravity outlets are determined for a range of low to
may require an interceptor system along the line-of-protection.
high tailwater conditions. Chapter 3 overviews the gravity
outlet input data for HEC-IFH and Chapter 6 of the HEC-IFH
(2) Most gravity outlets are corrugated metal or reinforced
user's manual describes the GRAVITY module concepts in
in EM 1110-2-3104 states that reinforced concrete pipe should
be used exclusively for urban levees and agricultural levees
(7) Existing gravity outlet operation criteria should be
where substantial loss of life and/or property can occur due to
obtained from the agency responsible for operating the interior
embankment failure at the outlet location. For agricultural
levees where no substantial loss of life and/or property can
plan formulation process. The normal operational procedure is
occur, corrugated pipe with a protective coating may be used.
to release water in an attempt to follow the lowering of the
In those cases, fill heights of levee embankment must be less
interior stages while maintaining a small positive head. The lag
than or equal to 3.66 m (12 ft) above the pipe invert, and pipe
time between interior and exterior peak stages may be a critical
diameters cannot exceed 0.914 m (36 in.). Corrugated pipe
factor in the operation specification. Detention storage near the
between 0.914 and 1.52 m (36 and 60 in.) may be used if
line-of-protection can reduce the capacity needed for outlets.
service conditions are investigated in detail and safety
Conveyance channels must be sized to assure that flows are
requirements are satisfied. Corrugated pipe with a diameter
conveyed to gravity outlets. The ditch rating curve option of the
greater than 1.52 m (60 in.) should never be used. Some new
POND module may be used to approximate controlled inflow to
gravity outlet pipes are made of reinforced fiberglass and
the gravity outlet at the primary location.
polyethylene that do not rust and have very low flow resistance.
Staff gauges are usually placed on both sides of the line-of-
(3) Gravity outlets should have a sufficient invert elevation
protection to effectively operate the gravity outlets. These
and slope to minimize siltation in the outlet. An exterior stage-
gauges show the water surface elevation on each side of the line-
exceedance duration table or plot can help pick an invert in
of-protection and thus give the differential head between the
which the exterior stage is below the invert most of the time.
inlet and outfall sides of the gravity outlet. When the exterior
HEC-IFH can determine and plot a stage-exceedance duration
stage reaches a specified staff gauge stage or elevation, the
table, if continuous simulation data are available. Likewise, the
gravity outlet gates are closed to prevent backwater flowing into
invert must be low enough to flow full before interior depth
the interior and to maintain the necessary storage in the ponding
reaches damage elevation.
area. This elevation is called the gate closing elevation.