Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
is shown as a function of the discount rate in Figure 8.3
(Ang and De Leon, 1996, 1997). Lee (1996) varied the discount
rate between 2 and 8 per cent and found that although
the total expected life-cycle costs decrease significantly as
the discount rate decreases, the optimal design levels and
target reliabilities are much less sensitive to changes in discount
rate.
8.4 ALTERNATIVE APPROACHES FOR
RISK-BASED FLOOD MANAGEMENT
When considering flood-management issues, the question
that must be answered is not if the capacity of a floodreduction
project will be exceeded, but what are the impacts
when the capacity is exceeded, in terms of economics and
threat to human life (Eiker and Davis, 1996). Therefore, risk
must be considered in flood management. Flood risk results
from incompatibility between hazard and acceptable risk
levels measured in commensurate units on the same plot of
land (Gilard, 1996). However, in traditional flood management
in many countries, an implicit acceptable-risk level is
assumed, and only the hazard level is studied in detail.
In the USA, the implicit acceptable-risk level for floodplain
delineation and other flood-management activities is
defined by the requirement to protect the public from the
flood exceeded once on average in 100 years (the 100-year
flood). Linsley (1986) indicated that the logic for this fixed
level of flood hazard (implicit acceptable risk) is that everyone
should have the same level of protection. However, he
noted that because of the uncertainties in hydrological and
hydraulic analyses all affected persons do not receive equal
protection. He advocated that the design level for flood hazard
should be selected on the basis of assessment of hazard
and vulnerability. In this section, two approaches for risk
assessment for flood management are described. These are
the risk-based approach developed by the US Army Corps
of Engineers (USACE, 1996) and the Inondabilité method
developed in France (Gilard et al., 1994). These approaches
offer contrasting views of flood-risk management. The riskbased
approach seeks to define optimal flood protection
through an economic evaluation of damages including consideration
of the uncertainties in the hydrologic, hydraulic
and economic analyses; whereas the Inondabilité method
seeks to determine optimal land use via a comparison of
flood hazard and acceptable risks determined through
negotiation among interested parties.
8.4.1 Risk-based analysis for flood-damage-reduction
projects
A flood-damage-reduction plan includes measures that
decrease damage by reducing discharge, stage and/or damage
susceptibility (USACE, 1996). For Federal projects in
the USA, the objective of the plan is to solve the problem
under consideration in a manner that will “... contribute to
national economic development (NED) consistent with
protecting the Nation’s environment, pursuant to national
environmental statutes, applicable executive orders and
other Federal planning requirements (USA Water Resources
Council, 1983).” In the flood-damage-reduction planning
traditionally done by the USACE the level of protection provided
by the project was the primary performance indicator
(Eiker and Davis, 1996). Only projects that provided a set
level of protection (typically from the 100-year flood)
would be evaluated to determine their contribution to NED,
effect on the environment and other issues. The level of protection
was set without regard to the vulnerability level of
the land to be protected. In order to account for uncertainties
in the hydrological and hydraulic analyses applied in the
traditional method, safety factors, such as freeboard, are
applied in project design in addition to achieving the specified
level of protection. These safety factors were selected
from experience-based rules and not from a detailed analysis
of the uncertainties for the project under consideration.
The USACE now requires risk-based analysis in the formulation
of flood-damage-reduction projects (Eiker and
Davis, 1996). In this risk-based analysis, each of the alternative
solutions for the flooding problem is evaluated to
determine the expected net economic benefit (benefit
minus cost), expected level of protection on an annual basis
and over the project life, and other decision criteria. These
expected values are computed with explicit consideration of
the uncertainties in the hydrologic, hydraulic, and economic
analyses utilized in plan formulation. The risk-based analysis
is used to formulate the type and size of the optimal plan
that will meet the study objectives. The USACE policy
requires that this plan be identified in every flood-damagereduction
study. This plan may or may not be the
recommended plan based on “additional considerations”
(Eiker and Davis, 1996). These “additional considerations”
include environmental impacts, potential for fatalities and
acceptability to the local population.
In the traditional approach to planning flood-damagereduction
projects, a discharge-frequency relation for the
project site can be obtained through a variety of methods
(see Chapter 3). These include a frequency analysis of data
at the site or from a nearby gauge through frequency transposition
or regional frequency relations. Rainfall-runoff
models or other methods described by the USACE (1996)
can be used to estimate flow for a specific ungauged site or
site with sparse record. If a continuous hydrological simulation
model is applied, the model output is then subjected to
a frequency analysis; otherwise flood frequency is determined
on the basis of the frequency of the design storm.
Hydraulic models are used to develop stage-discharge relations
for the project location, if such relations have not been
derived from observations. Typically, one-dimensional,
steady flows are analysed with a standard step-backwater
model, but in some cases, streams with complex hydraulics
are simulated using an unsteady-flow model or a twodimensional
flow model. Stage-damage relations are
developed from detailed economic evaluations of primary
land uses in the flood plain as described in Chapter 7.
Through integration of the discharge-frequency, stagedischarge
and stage-damage relations, a damage-frequency
relation is obtained. By integration of the damage-frequency
relations for without-project and various with-project conditions,
the damages avoided by implementing the various
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