ETL 1110-2-367
31 Mar 95
Chapter 4
hydrologic engineering approach used and the corresponding
flood damage computations. Is the flooding between the two
Line-of-Protection and Minimum Facility
systems coincident? Are the events dependent? The assessment
Analysis Concepts
of the study area to determine the coincidence and dependence
of flooding from the main stem and local stream is often a
complex but necessary step in flood damage analyses. Section
4-1.
Overview
2-3 and Table 2-1 describe coincidence and dependence for
interior studies, and are relevant for line-of-protection feasibility
a. This chapter discusses the hydrologic engineering
studies. Figure 4-1 illustrates how a damage center can be
analysis for studies where the line-of-protection is analyzed as
flooded by both the main stem and the local stream runoff.
part of the study prior to analysis of the interior system. It fo-
cuses on hydrologic engineering study requirements and associ-
(2) The dependence of events causing the flooding of the
ated HEC-IFH analysis capabilities for implementing a mini-
two systems can influence the type of hydrologic analysis.
mum interior facility as part of a line-of-protection project.
Analysis of observed or historical events should always be used
for validation and calibration of the assumptions and results. If
b. The study strategy assumes that the interior facilities
the main stem and local stream are highly dependent, such as for
(which will become part of the recommended plan) are planned
a main stem drainage area that is relatively small (e.g., 259 sq
and evaluated separately and incrementally from the line-of-
km or 100 sq miles) in comparison to the local stream (e.g.,
protection project. The major project (levee/floodwall) is
25.9 sq km or 10 sq miles), the same storm events would likely
conceptually divided from the planned interior facilities by
affect each system. Analyses would normally include evaluation
initially evaluating a minimum facility considered integral to the
of balanced hypothetical storms over both systems. For
line-of-protection. If a levee/floodwall exists, the minimum
thunderstorms, the evaluation may also include storms centering
interior facility is that which is presently in place. If the
over the interior area. Continuous record analysis could also be
levee/floodwall is being planned, the minimum facility must be
used, if sufficient data are available.
formulated and the evaluation of the line-of-protection benefits
performed with the facility in place. The residual interior
(3) For studies with no or little dependence, such as a
flooding is the target of the interior planning efforts; benefits
25.9-sq-km (10-sq-mile) local stream flowing into the
attributed to the increased interior facilities will be the reduction
Mississippi River main stem, a different approach is normally
in the residual damage.
required. The events causing flooding are likely independent
and may be highly noncoincident. Again, assessment of historic
c.
The following sections assume that the line-of-
data and other information is required to assure this assumption
protection does not exist and is being planned as the initial part
is valid. Assuming it is, the two systems could be analyzed
of the investigation. The minimum facility analysis is therefore
using the coincident frequency method or continuous record
part of the study.
analyses described in Chapter 2.
4-2.
Without Line-of-Protection Condition
(4) For most studies, the degree of dependence and coinci-
Analysis
dence will not be at the two extremes. The hydrologic engi-
neering analysis may include continuous records, hypothetical
a. Overview. The without line-of-protection condition
event type studies, or combinations of both. As applicable, all
assumes no protection is in place. HEC-IFH cannot directly
other information and analyses should be used to provide data
analyze the without-project condition. Traditional analytical
and insights as to the reasonableness of the results.
procedures and programs, beyond the scope of this document,
are used. It is briefly discussed here because the hydrologic
4-3.
With Line-of-Protection and No Interior
runoff analyses of the main stem (exterior) and local stream
Facilities
(interior) and their coincidence and dependence may be
applicable in subsequent interior analyses involving HEC-IFH
a. General. The formulation and evaluation of the size
analysis.
and configuration of the line-of-protection are separate problems
beyond the focus of this document. Required analysis
b.
Hydrologic engineering analysis concepts.
procedures are described in the following documents:
(1) The without line-of-protection analysis is often
Guidance for Conducting Civil Works Planning
complicated by the coincident and dependent nature of flooding
Studies, ER 1105-2-100.
from the main stem and local stream. The nature of flooding
between the main stem and local stream is critical to the type of
4-1
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