Environmental Engineering Reference
In-Depth Information
of ecosystems, or intentional and unintentional
human interventions in the flooding system, such
as floodplain development. Social and economic
change will have a profound influence on the
potential impacts of flooding and the way they
are valued, which will be different in different
countries owing to cultural factors or institutional
differences.
To add further complexity, the term'flood risk'
is used today in a number of different ways. A
range of meanings derived from either common
language or the technical terminology of risk
analysis are in use (Sayers et al. 2002). These
different meanings often reflect the needs of par-
ticular decision-makers - there is no unique
specific definition for flood risk and any attempt
to develop one would inevitably satisfy only a
proportion of risk managers. Indeed, this very
adaptability of the concept of risk may be one of
its strengths.
In all of these instances, however, risk is
thought of as a combination of the chance of a
particular event and the impact that the event
would cause if it occurred. Risk therefore has two
components - the chance (or probability)ofan
event occurring and the impact (or consequence)
associated with that event. Intuitively it may be
assumed that risks with the same numerical value
have equal 'significance' but this is often not
the case. In some cases the significance of a risk
can be assessed by multiplying the probability by
the consequences. In other cases it is important
to understand the nature of the risk, distinguish-
ing between rare, catastrophic events and more
frequent less severe events. For example, risk
methods adopted to support the targeting and
management of flood warnings represent risk in
terms of probability and consequence, but low
probability/high consequence events are treated
very differently to high probability/low conse-
quence events. The former can be catastrophic
leading to substantial loss of life, whereas the
latter are frequent 'nuisances'. But numerical risk
values are not the end of the story: other factors
affecting risk and response include how society
or individuals perceive that risk (a perception that
is influenced by many factors including, e.g., the
understanding of the probability of combinations
of random loading conditions and the system's
responses, including the human dimension. Im-
proved understanding of system behaviour has
illustrated the importance of increasingly large
combinations of variables.
3 Spatial interactions are important: River and
coastal systems show a great deal of spatial inter-
activity. It is well recognized that construction of
flood defences or urbanization of the catchment
upstream may increase the water levels down-
streamin a severe flood event. Similarly, construc-
tion of coastal structures to trap sediment and
improve the resistance of coasts to erosion and
breaching in one area may deplete beaches down-
drift (Dickson et al. 2007; Dawson 2009) and
exacerbate erosion or flooding there, leading to
economic damage or environmental harm. These
interactions can be represented in systemmodels,
but engineering understanding of the relevant
processes, particularly sedimentary processes
over long timescales, is limited. Even where we
have a detailed understanding of the physical
processes, there may be fundamental limits to our
ability to predict behaviour due to the chaotic
nature of some of the relevant processes and
loading.
4 Complex and uncertain responses must be ac-
commodated: Models of catchment processes are
known to be highly uncertain due to the complex-
ity of the processes involved and the scarcity of
measurements at appropriate scales (Beven 2006).
The response of river, coast and man-made de-
fences to loading is highly uncertain. The direct
and indirect impacts of flooding depend upon un-
predictable or perverse human behaviours for
which relevant measurements are scarce (Egorova
et al. 2008).
5 Flooding systems are dynamic over a range of
timescales: Potential for long-term change in
flooding systems, due to climate and socioeco-
nomic changes, adds further uncertainty as one
looks to the future. Change may impact upon
the loads on the system, the response to loads or
the potential impacts of flooding. It may be due to
natural environmental processes, for example,
long-term geomorphological processes, dynamics
