Environmental Engineering Reference
In-Depth Information
control, accept or redistribute risks of flooding'.
Schanze (2006) defines it as 'the holistic and con-
tinuous societal analysis, assessment and reduc-
tion of flood risk'. These definitions touch upon
several salient aspects of flood risk management:
.
a reliance upon rational analysis of risks;
.
a process that leads to acts intended to reduce
flood risk;
.
an acceptance that there is a variety of ways in
which flood risk might be reduced;
.
a recognition that the decisions in flood risk
management include societal choices about the
acceptability of risk and the desirability of differ-
ent options;
.
a sense that the process is continuous, with
decisions being periodically reviewed and modi-
fied in order to achieve an acceptable level of risk
in the light of changing circumstances and
preferences.
Table 1.1 summarizes the range of flood risk
management actions that flood risk analysis
might seek to inform. It summarizes attributes of
the information that is required to inform choice.
So, for example, national policy analysis requires
only approximate analysis of risks, though at suf-
ficient resolution to allow the ranking of alterna-
tive national-level policies.
So, we do not need to know everything at every
scale. Indeed, one of the principles of risk-based
decision-making is that the amount of data col-
lection and analysis should be proportionate to the
importance of the decision (DETR et al. 2000). In
selecting appropriate analysis methods, the apti-
tude of decision-makers to make appropriate use
of the information provided is also a key consid-
eration: so, for example, for flood warning deci-
sions, timeliness is of paramount importance
(Parker et al. 2007a, 2007b); for insurance compa-
nies, themagnitude ofmaximumpossible losses is
of central concern (Treby et al. 2006). The outputs
of analysis therefore need to be customized to the
needs and aptitudes of the different categories of
decision-makers.
In Table 1.1 there is an approximate ordering
of decisions on the basis of the spatial scale at
which they operate. National policy decisions and
prioritization of expenditure require broad scale
knowledge of recent flood events and availability
and affordability of mitigation measures).
The consequences of flooding include the direct
damage caused by flooding and the indirect dis-
ruption to society, infrastructure and the economy.
Whilst the primary metric of the consequences is
economic, the social, health and environmental
effects of flooding are well recognized (Smith and
Ward 1998). Thus, full descriptions of flood risk
will be expressed in multi-attribute terms. More-
over, flood risk analysis problems invariably look
into the future, so risk analysis involves weighing
up streams of benefits and costs, which introduces
problems of time-preferences. Whilst this is rou-
tinely dealt with by discounting of risks that are
expressed in economic terms, the limitations,
particularly for intergenerational issues, are well
known (Shackle 1961; French 1988).
The benefit of a risk-based approach - and per-
haps what above all distinguishes it from other
approaches to design or decision-making - is that
it deals with outcomes. Thus in the context of
flooding it enables intervention options to be
compared on the basis of the impact that they are
expected to have on the frequency and severity of
flooding in a specified area at some future date.
A risk-based approach therefore enables informed
choices to be made based on comparison of the
expected outcomes and costs of alternative
courses of action. This is distinct from, for exam-
ple, a standards-based approach that focuses on
the severity of the load that a particular flood
defence is expected to withstand and the design
of schemes to match that load.
Flood Risk Management Decisions
Flood risk management is a process of decision-
making under uncertainty. It involves the pur-
poseful choice of flood risk management plans,
strategies and measures that are intended to re-
duce flood risk.
Hall et al. (2003c) define flood riskmanagement
as 'the process of data and information gathering,
risk assessment, appraisal of options, andmaking,
implementing and reviewing decisions to reduce,
