Environmental Engineering Reference
In-Depth Information
flood risk reduction measures at the regional
scale.
prerequisite to understanding and accurately
simulating the groundwater-surface water inter-
actions that drive the flooding system in the mod-
elled test area.
Regional modelling approach
Modelling at the regional scale required the use of
process-based mathematical models selected to
represent the relevant flooding mechanisms, with
a simplified degree of coupling. The models se-
lected were:
1 HYSIM (Manley 1993: a physically based,
lumped, rainfall-runoff model. HYSIM is a deter-
ministic, continuous simulation model that per-
forms a water balance on a lumped basis, taking
into account the most relevant hydrological pro-
cesses including interception storage, evapotrans-
piration, surface and subsurface runoff, soil
moisture content, recharge to groundwater and
discharges to watercourses.:
2 MODFLOW: a computer code that resolves the
equations for three-dimensional (3-D) flow in a
saturated porous medium, using a finite difference
scheme (McDonald and Harbaugh 1988). The pro-
gramdealswith recharge and evapotranspiration as
external fluxes, in the same way that it deals with
external leakage to and from surface water bodies.:
3 iSIS Flow: a one-dimensional hydrodynamic
model that solves the St-Venant equations for open
channel flow through a mixed system of channels
and hydraulic structures (Halcrow Group 2009).
The previous section described the conceptual
basis for the flood probability maps (FPMs). At a
broad scale, the regional groundwater model was
used to compute groundwater heads (from a time
series between 1963 and 1995) that, once com-
bined with the Digital Terrain Model, permitted
identification of areas of groundwater-induced
flooding and waterlogging. ISIS Flow was then
used in an uncoupled manner, to generate flood
extents for various return periods along the river
corridors. Fluvial and groundwater-based FPMs
were merged by choosing the lowest frequency of
occurrence for those pixels where the predicted
flood extents overlapped. A sample FPM, pro-
duced using the regional model, is shown in
Figure 22.10.
Modelling Groundwater and Surface Water
Interaction
This section focuses specifically on the flooding
mechanism that is dominant in lowland basins:
groundwater-surface water (GW-SW) interaction
(Mull 1984; Paoli and Giacosa 1984; Sacks
et al. 1991; Aradas 2001), as described in Box 22.3.
The importance of this mechanism goes beyond
flood risk management as it also has implications
for water resources management and the conser-
vation of key environmental assets such as
wetlands and marshes. Modelling GW-SW inter-
action poses very distinct challenges in relation to
issues of temporal and spatial scale. This is par-
ticularly the case when the model outputs are
required not only to simulate hydrological pro-
cesses per se but also to provide the information
needed to produce key decision-making tools such
as flood probability maps.
In the R´o Salado studies, two distinct model-
ling scales were adopted to support development
of flood probability maps. Both included appropri-
ate representation of GW-SW interaction:
. A regional model and a regional flood probability
map to support the broad-scale representation and
validation of flooding mechanisms and to identify
priority areas for flood risk management.
. A local model to test the specific (small-scale)
impacts of alternative flood risk management
measures and support engineering design of those
measures selected for feasibility study; for exam-
ple, the construction of drainage canals to reduce
flood probabilities and hence improve agricultural
productivity.
It was recognized from the outset that these
models would be used in tandem with, for exam-
ple, some of the results generated by the local
model (such as predicted discharges in proposed
drainage canals) being fed back into the regional
model to test the effectiveness of the local
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