ETL 1110-2-367
31 Mar 95
against groundwater, and to simplify the ventilation of the
(2) Pressure conduits are pipes or closed conduits
operation areas. The consequence of exceeding pump design
designed to convey interior flood waters through the line-of-
stage must be evaluated. Pumping and gravity outlet effects on
protection under internal pressure. The inlet to the pressure
exterior stages and operation of other downstream gravity
conduit must be at a higher elevation than the river stage against
outlets should be considered in locating, sizing, and designing
which it functions. Some pressure conduits may serve as
the pumping station.
discharge lines for pumping facilities. The use of pressure
conduits reduces the contributing interior runoff area and the
(10) The pumping station capacity in urban areas is
magnitude and volume of flood waters that must be handled by
generally determined by the physical performance of the facility
other flood damage reduction measures.
and its effect on flood damage reduction, costs, and
environmental and social factors. Station capacities in rural
(3) Detention storage adjacent to the line-of-protection is
(agricultural type damage) areas are selected based on economic
defined in the POND module of HEC-IFH. The elevation-area
optimization.
relationship is entered and the corresponding storage values
calculated by the program. A ditch rating curve may be used to
d.
Detention areas adjacent to line-of-protection.
represent a channel link between the detention storage and
primary outlet at the line-of-protection and thus govern the
discharge to the outlet. Future conditions where the detention
(1) The use of detention areas can significantly reduce
storage is encroached and thus reduced are modeled by
gravity outlet and pumping station size and costs. A detention
adjusting the elevation-storage relationship appropriately.
basin may also increase the reliability of the system by providing
Sensitivity analysis of potential future development effects could
additional time for appropriate operation before damaging water
be performed in this manner. Similarly, enhanced flood
levels occur. A detention area may be natural or excavated
protection involving several excavation plans for the detention
sumps, or induced temporary ponding on vacant areas, streets,
storage area may be readily evaluated.
and parks. Only a few areas are typically available or selected.
An interceptor system to collect and convey runoff along the
f.
Physical measures remote from line-of-protection.
line-of-protection is generally required.
These measures are traditional structures such as channels,
(2) Topography, existing conveyance patterns, and land
diversions, interior levees, and storage reservoirs remote from
use usually govern the approximate locations of detention areas.
the line-of-protection. Their functional capability is therefore
Detention areas are normally located adjacent to the gravity
the same as with any other planning or design investigations
outlet or pumping station, but may be remote from these
involving flood loss reduction measures. Consequently, only the
facilities, connected by appropriately sized channels.
interrelationship with other specific interior measures will be
emphasized. For the most part, the evaluation of these measures
(3) In urban settings, application of nonstructural
is performed outside HEC-IFH with the resulting time series
measures to surrounding structures may be warranted. This is
hydrographs imported into HEC-IFH using the AUXFLOW
done to gain incremental storage versus increased capacity of
module. Conversely, the HEC-IFH ponding area stages may be
gravity outlet or pumping facilities. Detention basins can be
used as starting water surface profile elevations in the sizing
designed to be environmentally attractive and contribute to
studies of measures remote from the line-of-protection.
community social goals in urban areas when used as parks and
open spaces during periods not needed for runoff storage.
(1) Conveyance channels reduce flood losses for damage
Management of the functional integrity of the detention basin by
centers remote from the line-of-protection and collect and
preventing development encroachment and subsequent loss of
transport runoff and other interior waters to gravity outlets,
storage capacity is critically important.
Local agency
pumping stations, and pressure conduits. Where possible,
agreements should specify requirements for maintenance of
channels should follow natural drainage and conveyance routes.
detention basin functional integrity throughout the project life.
When this is not possible, consideration should be given to
locating channels near and parallel to the line-of-protection.
e.
Intercepting sewers and pressure conduits.
Channels may be required in combinations with detention basins
to connect with gravity outlets or pumping stations. Channels
may also be needed as exterior connections from the outlet
(1) These conveyance systems interconnect two or more
works of gravity or pressure conduits or pumping stations to the
existing sewers or channels within the line-of-protection for
river, lake, or ocean. The planning task is to approximately size
conveying their flows to gravity outlets, pumping stations, or
and locate the channel system. The design task is to perform
pressure conduits, for combined discharge through the line-of-
design in terms of size, location, gradient, and auxiliary control
protection. Interceptor systems are designed to minimize the
features of erosion protection and grade control.
number of gravity outlets, pumping stations, and pressure
conduits.
5-6
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