Environmental Engineering Reference
In-Depth Information
should be adapted as more knowledge is acquired
and as systems change.
But there are institutional implications here.
Implementing this approach involves the collec-
tive action of a range of different government
authorities and stakeholders from outside govern-
ment. This places an increasing emphasis upon
effective communication and mechanisms to
reach consensus. In this portfolio-based approach,
risk estimates and assessments of changes in risk
provide a vital common currency for comparing
and choosing between alternatives that might
contribute to flood risk reduction (Dawson
et al. 2008).
The principles of flood risk assessment have
become well established (CUR/TAW 1990;
Vrijling 1993; USACE 1996; Goldman 1997) and
are dealt with in more detail later in this volume.
It is worth reviewing here how the risk-based
approach addresses some of the main challenges of
analysing flooding in systems (Sayers et al. 2002):
1 Loading is naturally variable: The loads such as
rainfall and marine waves and surges on flood
defence systems cannot be forecast beyond a few
days into the future. For design purposes, loads
have to be described in statistical terms. Extreme
loads that may never have been observed in
practice have to be accounted for in design and
risk assessment. Extrapolating loads to these
extremes is uncertain, particularly when based
on limited historical data and in a changing
climate.
2 Load and response combinations are important:
The severity of flooding is usually a consequence
of a combination of conditions. So, for example,
overtopping or breach of a sea defence is usually a
consequence of a combination of high waves and
surge water levels, rather than either of these two
effects in isolation. In complex river network sys-
tems the timing of rainfall and runoff at different
locations in the catchment determines the sever-
ity of the flood peak. The severity of any resultant
flooding will typically be governed by the number
of defences breached or overtopped, as well as the
vulnerability of the assets and preparedness of the
people within the flood plain. Therefore, analysis
of loads and system response is based on an
protected area, noting urban areas could expect
higher levels of protection than sparsely populated
rural areas (Johnson 2005).
However, the practical process of flood defence
design, whilst having probabilistic content, was
not fundamentally risk based, proceeding some-
what as follows:
1 establishing the appropriate standard for the
defence (e.g. the '100-year return period' river
level), based on land use of the area protected,
consistency and tradition;
2 estimating the design load, such as the water
level or wave height with the specified return
period;
3 designing (i.e. determining the primary physical
characteristics such as crest level or revetment
thickness) to withstand that load;
4 incorporating safety factors, such as a freeboard
allowance, based on individual circumstances.
Meanwhile, as flood warning systems were pro-
gressively introduced and refined in the decades
since the 1950s, the decision-making process was
also essentially deterministic, based on comparing
water level forecasts with levels that would
trigger the need for and the dissemination of a
warning.
Over the last two decades the limitations of
such an approach in delivering efficient and
sustainable flood risk management have become
clear. Because ad hoc methods for decision-
making have evolved in different ways in the
various domains of flood risk management (flood
warning, flood defence design, land use plan-
ning, urban drainage, etc.), they inhibit the in-
tegrated systems-based approach that is now
promoted.
That systems approach is motivated by the
recognition that there is no single universally
effective response to flood risk (Proverbs 2008).
Instead, portfolios of flood risk management mea-
sures - be they 'hard' structural measures such as
construction of dikes, or 'soft' instruments such as
land use planning and flood warning systems - are
assembled in order to reduce risk in an efficient
and sustainable way. The makeup of flood risk
management portfolios is matched to the func-
tioning and needs of particular localities and
