Environmental Engineering Reference
In-Depth Information
1 Groundwater-induced surface flooding which
occurs when the groundwater level (phreatic sur-
face) rises to an elevationabove the ground surface,
causing water ponding and inundation, or ground-
water-induced waterlogging which occurs when
the phreatic surface is located within 50-70 cm of
the ground surface, but does not rise above it.
2 Ponding of rainwater which occurs whenwater
accumulates on the surface due to low infiltration
capacity and/or flat terrain.
3 Fluvial flooding which occurs when the dis-
charge of water from upstream exceeds the con-
veyance capacity of the natural or engineered
channel and, consequently, a significant area of
the surrounding floodplain is inundated.
Box 22.1 describes how a conceptual SPRmodel
can be applied to characterize the resulting risk in
a basin dominated by groundwater-induced flood-
ing mechanisms.
It is necessary to understand flood risk concep-
tually, and define adequate risk models, prior to
the delineation of the strategy for model develop-
ment. This is the case because it enables identifi-
cation of the analytical components that must be
included in themodelling effort, together with the
degree of coupling required between models to
adequately capture interactions between the var-
ious flooding sources and pathways that together
control the conditional probability that a given
area may be flooded. In this respect, the interac-
tions likely to occur in large, lowland catchments
operate at a wide range of scales and this must be
reflected in their analysis. Figure 22.4 illustrates
possible interrelationships at local and regional
scales. For instance, groundwater-induced flood-
ing can interact with rainfall to cause a further
increase of water ponding on the surface and,
hence, an increase of overland flow that promotes
fluvial flooding.
To flesh out themodelling strategy presented in
Figure 22.4, it is next necessary to conceptualize
howthethreefloodingmechanismsoutlinedabove
act and interact physically. Figure 22.5 presents a
conceptual representation of the response of the
basin to each floodingmechanism, illustrating the
importance of thresholds in generating non-linear
behaviour in the flooding system.
occur. Hence, when assessing flood risk it should
be noted that the consequences of a flood do not
share the boundaries defined by the duration
and extent of the inundation. For example, with
respect to agricultural and livestock activities,
adverse effects extend beyond the inundated area
because disruption of the transportation network
may prevent the movement of animals or their
feedstuffs, while the duration of adverse impacts
usually lasts much longer than the time necessary
for soilmoisture to return to normal levels. Under-
representing the extent or duration of the adverse
consequences of flooding leads to under-estima-
tion of flood losses, which may result in faulty
decisions on flood risk management and, hence,
poor use of resources.
To understand how flood probabilities, hazards
and consequences are linked, and so assess the
potential for different mitigation measures to re-
duce flood risk, conceptual models of risk such as
'Source-Pathway-Receptor' (SPR) have proven
useful (Fleming 2002). Their application provides
the basis for representations of flood probabilities,
hazards, exposures and, ultimately, risks that are
accurate and consistent across regions that are
both large and complex. However, to produce
reliable estimates of current and future flood risks,
models such as SPR require a thorough under-
standing of the physical sources of floodwater, the
pathways by which floodwaters move through the
basin, and the socioeconomic factors that influ-
ence the sensitivity or resilience of a flood-prone
area to suffering losses. Once developed and val-
idated, conceptual models of flood risk such as
SPR can also be useful to inform decision-making
on when and where to deploy flood risk manage-
mentmeasures. While little can usually be done to
manage the sources of floodwaters, flood defences
may be positioned to block key flooding pathways,
while non-structural measures such as floodplain
zoning and land use planning to reduce exposure
are powerful responses aimed at reducing the
adverse consequences of flooding.
Taking into consideration all of the issues high-
lighted above, three types of hazardous flood
events can be conceptualized in large, lowland
basins:
