Environmental Engineering Reference
In-Depth Information
Particularly important processes include: exten-
sive evapotranspiration from the subsurface satu-
rated zone; widespread soil saturation with
exfiltration and large-scale ponding of water on
the surface; highly variable contributing areas
with respect to saturation overland flow and sur-
face runoff; slowmovement of underground water
leading to saline and brackish conditions; and very
low base flows in drainage channels. As will be
demonstrated throughout this chapter, groundwa-
ter and surface water interaction is a key process
responsible for explaining floodingmechanisms in
the R´o Salado, as well as many other large, low-
lying basins.
5 Thresholds: In relation to points 3 and 4 above,
geomorphic features may generate threshold be-
haviour in surface water-shallow groundwater
interactions, triggering significant step changes in
the depth and extent of surface flooding. See
Figure 22.7 for an example of this phenomenon.
6 Engineering interventions: Finally, engineering
interventions in large, low-lying basins require par-
ticularly careful and thoughtful planning supported
by broad-scale scientific studies. This is the case
because the flat relief and slow lateralmovement of
water in large, lowland areas means that the terres-
trial hydrological system responds slowly to
changes in precipitation and the operation of struc-
tural flood management structures, making it nec-
essary to anticipate the natural, autonomous
evolution of a flood event in order to intervene and
manage it effectively. For example, in the Salado
basin, floods develop over months rather than days
or weeks, and forward planning is essential to oper-
ate sustainable land drainage infrastructure (regu-
lated drainage canals) to mitigate groundwater-
induced surface flooding while simultaneously
avoiding unacceptable damage to the environment.
Given the complexities inherent to the consid-
erations outlined above, risk management pro-
vides the logical framework within which to
assess current and possible future flooding scenar-
ios that account for various options for water
resource management and different trajectories of
socioeconomic evolution.
Flood risk depends on the probability of flooding
and the consequences should a flood actually
baseline studies of hydrology, geomorphology and
the environment, through complex computer
modelling and multi-criteria analyses of alterna-
tives supported byaneconomic evaluation.Models
constituted the key tools employed to gain an
understanding of the main flooding and waterlog-
ging mechanisms and, consequently, to assess the
potential impacts of a range of proposed strategies
for flood risk reduction.
Flooding Mechanisms and Risks in Large
Lowland Basins
General concepts
The first step in any flood risk analysis is to
characterize the flood probability by understand-
ing the mechanisms responsible for flooding.
Some of the prime considerations in large lowland
basins include:
1 Scale: The physical size and complexity of
large basins presents challenges to modelling
and management strategies in terms of data needs
and availability, the resolution required and
the appropriateness of different analytical tools
(bearing in mind modelling costs and run time
constraints).
2 Catchment and study area boundaries: When
dealing with large national or trans-national
catchments more than one administrative body
will usually be responsible for catchment and
flood risk management. This poses challenges in
terms of procuring information and dealing with
multiple institutional arrangements and, hence,
the possibility of multiple preferred policies and
options for managing flood risk.
3 Geomorphology: In large, low-lying catch-
ments the importance of geomorphic processes
and features is enhanced as complex interactions
between the hydrology and basin geology, soils,
vegetation and landscape control not only the
generation of runoff but also how, when andwhere
waterlogging and groundwater flooding occur.
4 Enhanced role of interaction between surface
water and shallowgroundwater processes: In low-
land catchments vertical fluxes and shallow pro-
cesses predominate due to the lack of relief.
