31 Mar 95
except now the dependence and coincidence of interior and
exterior flooding must be considered. This is instead of just the
Analysis of Interior System Flood Damage
unblocked outlet condition used to size the minimum facility for
most studies. Input data and analysis would essentially be the
same as described in Chapter 3. The existing without-project
conditions plan is described in HEC-IFH by the PRECIP,
RUNOFF, POND, EXSTAGE, GRAVITY (minimum facility)
modules, and perhaps the AUXFLOW and the PUMP modules.
a. This chapter describes the formulation and evaluation
of a set of flood damage reduction plans for interior areas. The
(2) The HEC-IFH analysis results should be validated
condition with the line-of-protection and the selected minimum
from several perspectives. Historic events (stage-frequency,
facility becomes the without-project condition for evaluating
durations, coincidences, etc.) may be analyzed and the model
additional features. If the line-of-protection is in place, the
calibrated to observed and reasonable results. The percent run-
existing system is the without condition for analysis of enhanced
interior facilities. The enhanced facilities may include additional
recovery rates for continuous record analysis must be reason-
gravity outlet capacity, pumping stations, ponding area storage,
able, as should other factors such as gravity flow, seepage and
and nonstructural measures. Figure 5-1 conceptually shows an
general operation and performance. The results should be care-
interior system with gravity outlets and pumps.
fully inspected and the flood hazard (stage-frequency, depth and
extent of flooding, duration, warning time, etc.), performance,
b. The criteria specified in the "Planning Guidance
and operation of the system clearly defined. Performance in-
Notebook" (ER 1105-2-100) and EM 1110-2-1413, are
cludes how the interior system responds for a range of events
principal references for analysis of interior systems. The
and conditions. Operation should closely approximate that pres-
application of continuous record and hypothetical event
ently used in a physical and institutional sense. This normally
analytical procedures using HEC-IFH is detailed in this chapter.
is the gravity outlet but includes pumps if they presently exist.
More detailed examples of its application are provided in the
case example studies described in Appendices D and E.
Future without-project conditions.
(1) Hydrologic engineering analysis of future without-
project conditions typically involves urbanization effects on
Existing and future without-project
watershed runoff. The process includes identification of areas
conditions analyses are required to determine the value of
for the most likely future urbanization or intensification of
implementing flood damage reduction measures. The initial
existing urbanization from future land use planning information
hydrologic engineering goal is to define the flood hazard,
obtained during the preliminary investigation phase. This
performance, and operation procedures of the existing without-
includes types of land use and conveyance system changes.
project condition. Observed event information is important to
Conveyance system changes refer to the storm drainage and
define these characteristics and validate the analysis results.
authorized flood control projects likely to be implemented by
The continuous simulation and/or hypothetical event options of
locals. Other future alternate land use conditions may be
HEC-IFH may be used in the analyses depending on the
assessed if necessary. The future years in which to determine
information available and the nature and complexity of the
project hydrology are normally specified by the study manager.
interior and exterior system.
Generally, the start of project operation or base year (existing
conditions may be appropriate), and some year during the
b. Storm sewer design and configuration. If the levee
project life (often the year when land use planning information
and minimum facilities are in place, the layout, planned changes,
is available) are selected.
design discharges, and invert elevations of existing and potential
future storm sewer systems must be considered as part of the
(2) The HEC-IFH plan for future without-project
with- and without-project conditions for the interior analysis.
conditions normally consists of the existing conditions plan with
See section 4-4b.
changes only to the runoff and perhaps routing characteristics
defined in the RUNOFF module. Runoff would relate to
Existing without-project conditions.
urbanization effects on the unit hydrograph and loss rates.
Routing changes might be related to alterations in the
(1) The existing without-project condition used in the
conveyance channel prior to entering the lower ponding area or
evaluation of interior flood damage reduction measures is the
encroachment into the natural storage remote from the line-of-
initial focus. The line-of-protection and minimum facilities are
protection. Other changes could also occur depending on the
assumed in place, as described in Chapter 4 and EM 1110-2-
study area and any projected flood damage reduction measure
1413. The analysis is the same as that for the minimum facility