Environmental Engineering Reference
In-Depth Information
plied by the magnitude of the consequences
should it occur. From a modelling perspective,
mapping flood risk therefore demands the ability
to characterize first the extent of flooding associ-
ated with an event of a given return period and,
second, the exposure to flood hazard (conse-
quences) of the population, property and infra-
structure. The first requirement is usually
addressed through the generation of floodwater
surface elevations in the fluvial system and their
expression as flood maps using a Geographical
Information System (GIS). The widely adopted
assumption that flood frequency estimates can be
an adequate surrogate for the actual frequency of
floodplain inundation then allows conversion of
these maps of flood extent into maps of flood
probability. The accuracy of flood probability
mapping has recently been markedly improved
through application of two-dimensional (2-D)
hydrodynamic models and wider availability of
digital elevation models (DEMs) based on LiDAR
(light detection and ranging) surveys (Horrit and
Pender 2008).
However, mapping the probability of inunda-
tion presents new modelling challenges in large,
lowland catchments where fluvial processes are
less dominant and interaction between groundwa-
ter and surface water is in itself an important
flooding mechanism. This challenge centres on
merging data on both the extent and the probabil-
ity of flooding associated with flooding mechan-
isms that drive spatially and temporally distinct
flooding systems.
Inundation modelling that can reliably repre-
sent both the relevant physical processes and the
impacts of proposed engineering interventions is
vital to the production of useful flood probability
maps. Consequently, this chapter emphasizes the
generation of flood probability maps that reliably
represent the coincident risks of groundwater and
fluvial flooding to support accurate appraisal of
alternative strategies for reducing flood risk. How-
ever, inundation modelling is particularly chal-
lenging when dealing with large basins, complex
hydrological processes and a wide array of options
for flood risk management. This reinforces the
importance of developing a modelling strategy
caused by the increase in mean annual rainfall
since the 1970s coupledwith a landscape inherited
from a former geomorphic regime. The work pre-
sented here addresses these issues and illustrates
the practical advantages gained by integrating
project components concerned with hydrology,
geomorphology, remote sensing and modelling to
develop the capability to produce broad-scale flood
hazard and risk maps, as recommended by
UNESCO (1984, 1993).
The need to conserve environmental capital
when managing flood risk in lowland catchments
is increasingly accepted, and in 2001 the American
Society of Civil Engineers (Panigrahi 2001)
published a review of groundwater and surface
water interaction in lowlands that highlighted the
multiple functions of wetlands in attenuating
floods, supporting water resources and providing
ecological habitats (Carter 1984; Hollis and Acre-
man 1994; Acreman and Adams 1999). Mortellaro
et al. (1995) define wetlands as 'those areas that are
inundated or saturated by surface and groundwater
at a frequency and duration sufficient to support a
prevalence of vegetation typically adapted for life
in saturated soils.' The Everglades in South Florida,
USA, is one of the largest 'regional' wetlands in the
world. The area features a flat topography, highly
permeable sandy soils, high water tables and a
dense network of engineered drainage channels.
Due to soil permeability and the shallow aquifer
conditions, aquifer levels are highly influenced by
rainfall, direct evapotranspiration from the satu-
rated zone and seepage to and from the channels.
The Everglades is undergoing a major restoration
scheme, designed to promote natural processes and
reverse the environmental decline caused by the
excessive land drainage works and, as part of this
project, significant effort has been devoted to de-
veloping coupled models of groundwater and sur-
face water interaction. This chapter acknowledges
that effort, but it goes further in exploring which
processes need to be coupled and the degree of
coupling necessary as functions of the scale and
objectives of the investigation, with the FCMS
providing a platform to analyse these issues.
Flood risk may be defined as the product of the
probability of occurrence of a flood event multi-
