ETL 1110-2-367
31 Mar 95
Risk-Based Analysis for Evaluation of Hydrology/
reduction measures. The residual damage with the minimum
Hydraulics and Economics in Flood Damage Reduction Studies,
facility in place is thus the target for damage reduction of
EC 1105-2-205.
additional flood reduction measures.
b.
Storm sewer design and configuration.
Hydrologic Analysis of Interior Areas, EM 1110-2-
1413.
(1) The layout, planned changes, design discharges, and
Hydraulic Design for Local Flood Protection Projects,
invert elevations of existing and potential future storm sewer
ER 1110-2-1405, and other documents.
systems must be considered as part of the minimum facility
analysis. These data are used to define contributing drainage
For the interior area analysis, the line-of-protection is assumed
areas, invert elevations of major conveyance channels, gravity
in place and local stream conveyance to the main stem or
outlet inverts, pump on-off elevations, and local design criteria
exterior is cut off by the line-of-protection as shown in Figure 4-
for inlet and outlet works. Data collection and analysis of storm
2. The runoff and contributing area of the existing and potential
sewer systems, which include the existing and future system
storm sewer system must be considered. Flooding from the
layout, design, and operation information, are generally provided
exterior is blocked by the line-of-protection up to the
by the local public works department or city engineer. The
overtopping event. This is the without-project condition for the
proper delineation of drainage areas that contribute to the
minimum facility analysis and represents an upper bound for the
interior ponding adjacent to the line-of-protection is important
stage-frequency relationship with the minimum facility in place.
to the interior analysis. The natural topography should be used
The goal is to subsequently reduce the stage-frequency
for initial boundaries. The storm sewer layout often crosses
relationship for the local stream without the line-of-protection
topographic boundaries and thus may affect the amount of runoff
in place by implementing the minimum facility discussed in the
into or out of the system.
following section.
(2) The location of flow concentration at the line-of-
b. HEC-IFH analysis. HEC-IFH may be used to
protection often affects where gravity outlets or pumps may be
determine the stage-frequency relationship for the ponding area
located and the layout of the collector/conveyance system
associated with the line-of-protection in place and no interior
adjacent to the line-of-protection. The potential of combining
facilities. The runoff procedures and hydrographs generated for
flows into a collector system should be evaluated. Finally, if a
the local stream are often event-based since this condition only
storm sewer system does not exist, one may need to be designed
represents an upper limit for the minimum facility analysis and
to assure the interior system is compatible with contributing
has no outlets to enable evacuation of water from the interior
flow areas and invert elevations of any planned interior flood
area. The analysis will normally be HEA but could be discrete
damage reduction system.
observed events using HEC-IFH analysis that includes a plan
consisting of the PRECIP, RUNOFF, POND, EXSTAGE, and
(3) The effect of storm sewers may be analyzed using
perhaps AUXFLOW modules. Gravity outlets and pumps are
HEC-IFH by modifying the unit hydrograph for events affected
not analyzed. Stage-frequency relationships may be developed
by storm sewers in the RUNOFF module of HEC-IFH. The
for each interior ponding area using HEC-IFH. The local stream
contributing drainage areas may also be adjusted in the
runoff analysis may be the same as described for the without
RUNOFF module or the AUXFLOW diversion option can be
line-of-protection condition including, if applicable, future
used to adjust storm sewer flows into or out of the subbasin.
without-project conditions. The difference, however, is that
The time series of runoff hydrographs, including storm sewer
local stream runoff will pond behind the line-of-protection and
flows, may be imported into HEC-IFH (AUXFLOW module)
main stem (exterior) flooding will be blocked to the top of the
instead of directly calculating the runoff. This is appropriate for
line-of-protection.
complex systems and those requiring more sophisticated runoff
computations such as for situations when pressure storm sewer
4-4.
Minimum Facility Analysis
flows are a significant issue.
a. General. The minimum facility of the interior area is
c.
Evaluate range of minimum facilities. The minimum
justified as an integral part of the line-of-protection as shown in
facility will almost always consist of gravity outlets, but may
Figure 4-3. The minimum facility should provide interior flood
include pumps if the coincidence of flooding between the
protection during gravity (unblocked or low exterior) conditions
interior and exterior is high for very prolonged periods such as
such that the local storm sewer system functions essentially the
for lakes or new upstream storage projects. The physical
same as it did without a levee in place for floods up to the storm
characteristics of the minimum facility gravity outlets should be
sewer design. The stage-frequency relationship for the with-
established prior to the analysis and refined as the analysis
minimum-facility-in-place condition becomes the without-
proceeds. The analysis should be performed for the range of
project condition for evaluating additional interior flood damage
hypothetical frequency events. The analysis is performed
4-3
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