ETL 1110-2-367
31 Mar 95
Chapter 1
blocking natural flow paths or outlets. Protected interior areas,
formerly flooded from the exterior source by slowly rising flood
Introduction
waters generated from regional storms, may now flood from
rainfall events that are more localized, occur more suddenly, and
1-1.
Purpose
provide less warning. For example, flooding from the
Mississippi River can be forecast several days in advance, but
This ETL provides guidance for conducting hydrologic
flooding from a localized storm on a protected interior area may
engineering analyses for interior areas. The Hydrologic
occur in several hours or less. The flooding may be aggravated
Engineering Center Interior Flood Hydrology (HEC-IFH)
by coincident high exterior stages. The interior flooding that
program is used as the primary tool for analyzing interior
results usually may be of the nuisance variety (shallow,
flooding. This document is intended to assist with better
temporary flooding), but sometimes it can be more dangerous
scoping, planning, and analysis of interior flooding studies using
than the situation without the levee.
the HEC-IFH program. The information and analysis strategies
presented are consistent with present guidance, specifically, ER
c.
Interior flood waters are normally passed through the
1105-2-100, EM 1110-2-1413, EC 1105-2-205, and
line-of-protection by gravity outlets when the interior water
procedures described in the HEC-IFH Package User's Manual
levels are higher than water levels of the exterior. This is called
(USACE 1992).
a positive gravity condition. When exterior stages are higher
than the interior, flood waters are stored and/or diverted and
1-2.
Overview of Interior Flood Hydrology
pumped over or through the line-of-protection. This condition
Concepts
is known as a blocked gravity condition and is illustrated in
Figure 1-2.
a. An interior area is defined as the area protected by a
line-of-protection from direct river, lake, or tidal flooding.
d. Gravity outlets, pumping stations, interior detention
Interior areas may also include low depressions and natural
storage basins, diversions, and pressure conduits reduce flood
sinks. Figure 1-1 is a conceptual illustration of an interior area.
damage within interior areas. Other measures, such as hillside
The following paragraphs describing interior flooding are taken
reservoirs, channels, floodproofing, relocations, regulatory
from EM 1110-2-1413.
policies, and flood warning preparedness actions, may also be
integral elements of interior systems.
b. The levee, floodwall, or seawall associated with an
interior area is called the line-of-protection. The line-of-
e.
Interior areas are studied to determine the specific
protection excludes flood water originating from the exterior
nature of flooding and to formulate alternatives that reduce the
source but often aggravates the problem of interior flooding by
residual and/or induced flooding. The objectives are the same
Figure 1-1. Typical interior area
1-1
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