31 Mar 95
minimizing the risk of loss of life, flood damage, and social
disruption if other methods are not feasible.
b. Without-project conditions. Analyses of the existing
(2) Flood forecasting-emergency preparedness plans are
and future without-project conditions are performed as
generally compatible with other structural and nonstructural
previously described in Section 5-2.
flood reduction measures. Implementation is more frequent in
HEC-IFH gravity outlet analysis. The following is a
urban interior areas than in agricultural interior areas. Flood
forecasting-emergency preparedness actions are usually feasible
series of steps that may be used as a guide to tailor the gravity
even if other structural and nonstructural measures are not.
outlet analysis to a specific study. The goal is to determine the
appropriate size and configuration of gravity outlets.
(3) The HEC-IFH analysis results provide information on
the flood hazard (frequency, stage, duration, and extent of
(1) Define new plans for evaluating gravity outlets using
flooding) that may be used directly in evaluation of flood
modules for CSA or HEA with the minimum facility in place.
warning-preparedness systems or in conjunction with other
Existing condition rainfall (PRECIP module), runoff and routing
programs such as those used to compute water surface profiles.
parameters (RUNOFF module), ponding area characteristics
The implementation of flood warning-preparedness programs
(POND module), minimum facility (GRAVITY module), and
for interior systems may enhance the operation for large and
seepage (AUXFLOW module) are from the CSA analysis of the
complex systems, but will primarily improve the response so
selected minimum facility.
that more damage reduction may occur. The potential for loss
of life is normally not a factor for interior systems due to
(2) Assemble outlet characteristics for several standard
typically shallow flooding, but would be for design exceedances
size outlets and develop composite rating curves for each using
for the line-of-protection.
HEC-IFH. Alinement, invert elevations, number and size of
outlets, and entrance and exit configurations are important
Interior Analysis Using HEC-IFH
(3) Develop three to six gravity outlet configurations
(plans with different GRAVITY modules) with one or more
(1) The formulation and evaluation process of interior
gravity outlets in addition to the minimum facility outlet, with
flood damage reduction measures must be conducted within the
each module representing an incremental increase in total outlet
framework of Corps guidance and regulations. The details of
the hydrologic engineering and other analyses are study
dependent. There is, however, an analysis progression that is
(4) Run HEC-IFH using the CSA option and develop
applicable for most interior studies.
several plans that incorporate the gravity outlet modules and
determine interior stage-frequency relationship for each plan.
(2) The initial step is to determine the existing and future
A maximum annual interior elevation versus frequency plot
without-project conditions. The second step is to determine the
comparing plans is illustrated in Figure 5-4.
configuration and feasibility for additional gravity outlet capacity
assuming the minimum facility is in place. The third step
(5) Test the additional capacity with the HEA-generated
determines the design and configuration of additional pump
balanced storms over the interior and exterior basins for selected
capacity, assuming that the minimum facility and the gravity
frequencies and determine the interior stage-frequency
outlets are in place. The next step explores tradeoffs of
relationship for each plan if interior and exterior flooding can be
pumping capacity versus ponding area storage and includes
highly coincident. The relationships help determine if rare
evaluation of nonstructural measures to increase nondamaging
ponding area storage. For studies with large and complex
relationships will also help establish the upper end of the
systems, such as many urban settings, traditional evaluation of
graphical stage-frequency relationship.
flood damage reduction measures remote from the line-of-
protection is often necessary. Finally, the feasibility of other
(6) If the interior and exterior flooding can be independent
flood damage reduction actions such as flood warning-
and noncoincident, define additional plans using HEA and local
preparedness and institutional arrangements would be evaluated.
storm depth-duration-frequency data for a range of exceedance
The following paragraphs describe the procedures and how both
frequency events occurring over the interior area for unblocked
the continuous simulation and the hypothetical event analyses
gravity outlet conditions. Determine the corresponding stage-
capabilities of HEC-IFH can be applied. Chapter 3 overviews
frequency relationships for each plan. This relationship helps
the data entry and the general procedures for HEC-IFH
determine if rare local events are being captured by the CSA and
applications. Appendices C, D, and E present a detailed
helps define the frequency relationships.
strategy, and two case examples detailing the HEC-IFH analysis