ETL 1110-2-367
31 Mar 95
assuming unblocked gravity outlet conditions. Each plan
evaluated. All HEC-IFH data entry modules will be the same
evaluated would include the same data or PRECIP, RUNOFF,
except the GRAVITY module will change for each plan. Using
POND, EXSTAGE, and AUXFLOW modules as the without
local storm HEA, compare the results of each plan using the
line-of-protection condition plus the GRAVITY module.
program's plan comparison capability. The plan comparison
assessment should be for the with line-of-protection and no
d. Minimum facility sizing analyses. The following
outlets (Section 4-3) condition and each gravity outlet plan
analyzed by HEC-IFH. They should then be compared to the
paragraphs describe the strategy for sizing the minimum gravity
targeted local stream frequency that is not computed in HEC-
outlet facility using HEC-IFH.
IFH.
(1) Define three or four gravity outlet configurations
(4) Select the minimum facility which is the gravity outlet
(different GRAVITY modules) of increasing capacity. Outlet
capacity or plan that essentially makes the stage-frequency and
sizes should envelop the largest storm sewer size or ditch
associated flood damage to the interior area no worse than
capacity at the line-of-protection.
flooding to the area from the local stream without the line-of-
protection in place. Rarer events, which exceed the local storm
(2) Enter the gravity outlet data requirements into HEC-
sewer design, may be greater with the minimum facility in place.
IFH. Both the CSA and HEA methods have the same data
See Figure 4-3.
requirements. For interior analyses, the outlet headwater is the
interior ponding elevation and the tailwater is the exterior stage.
4-5.
Summary
The following two items of information are required for each
gravity outlet:
The minimum facility is justified as part of the line-of-
protection. It is almost always gravity outlets. Minimum facility
(a) A gravity outlet rating table that lists the headwater
analysis involves both the base year conditions and at least one
depth required for a range of outlet flow rates and tailwater
future condition analysis, if it is likely to change and impact the
depths. This table may be entered by the user or computed by
analysis. Interior stage-frequency relationships for these
HEC-IFH for circular or box culverts. Generally, the user will
conditions may be needed to select a minimum facility. The
choose the option that allows the program to compute the outlet
minimum facility provides interior flood protection during
rating tables.
unblocked or low exterior conditions such that the local storm
sewer system functions essentially the same as without the levee
(b) HEC-IFH allows the user to adjust the exterior stage
in place for floods up to the storm system design. The
or tailwater condition to match the actual location of each
subsequent without-project condition is used to formulate and
gravity outlet.
evaluate interior flood damage reduction measures assuming the
minimum facility in place.
(3)
Define a new plan for each gravity outlet capacity to be
4-6
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