Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
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Figure 3.1 — Flooding in
Guilin, China, 1995
(photo: W. Kron)
in level, sometimes historically unprecedented, results in
damage and potential loss of life. Whereas, with tsunamis,
the damages are related both to the height of the water column
and with the energy of the tsunami, the latter more
closely conveying the destructive force of the event.
Data since 1960 (Wijkman and Timberlake, 1984;
Wasseff, 1993) indicate that each passing decade sees an
increase in the number of flood disasters and the number of
people affected by flooding. However, Yen and Yen (1996)
have shown that relative flood damages in the USA
expressed as a fraction of annual Gross National Product
show a declining trend from 1929–1993. The increases in
flood damage have been attributed to increased occupancy
of floodplains and larger floods due to deforestation and
urbanization. Deforestation and urbanization increase
flooding because they decrease the capacity of the land to
absorb rainfall. It is widely agreed that disasters, including
flood disasters, affect developing countries more severely
than developed countries, and that the poor suffer disproportionately.
It has been argued (Maskrey, 1993) that human
vulnerability to flooding is increasing because disadvantaged
individuals and communities do not have the
resources to recover from sudden misfortunes. It is likely
that future economic and population pressures will further
increase human vulnerability to flooding.
Assessment of flood hazard is extremely important in the
design and siting of engineering facilities and in zoning for
land management. For example, construction of buildings and
residences is often restricted in high flood hazard areas.
Critical facilities (e.g., hospitals) may only be constructed in
low-hazard areas. Extremely vulnerable facilities, such as
nuclear power plants, must be located in areas where the flood
hazard is essentially zero (WMO, 1981c). Care should also be
exercised with the design and siting of sewage treatment as
well as land and buildings having industrial materials of a toxic
or dangerous nature, due to the potential widespread
conveyance of contaminants during floods resulting in contaminant
exposure to people and the environment. Finally, for
locations where dam failure may result in massive flooding
and numerous fatalities, dam spillways must be sized to pass
extremely large floods without dam failure.
3.3 CAUSES OF FLOODING AND FLOOD
HAZARDS
3.3.1 Introduction
The causes of floods and flood hazards are a complex mixture
of meteorological, hydrological and human factors. It
must be emphasized that human exposure to flood hazards
is largely the result of people working and living in areas that
are naturally — albeit rarely — subject to flooding.
River floods can be caused by heavy or prolonged rainfall,rapid
snowmelt,ice jams or ice break-up,damming of
river valleys by landslide or avalanche, and failure of natural
or man-made dams. Natural dams may be composed of
landslide materials or glacial ice. High tides or storm surge
can exacerbate river flooding near the coast. Most floods
result from rainstorms or rain/snowmelt events, and, thus,
rainfall/snowmelt induced flooding is the focus of this
chapter.
Under most circumstances, damaging floods are those
that exceed the capacity of the main conveyance of the river
channel. The main conveyance may be the primary channel
of a river without levees or the area between the levees for a
river with levees. The capacity may be exceeded as a result of
excessive flows or blockages to flow such as ice or debris
jams. There are a number of unusual circumstances when
flows that do not overtop the river banks or levees might be
considered floods. These are: flows in channels that are usually
dry, high flows in deep bedrock canyons and flows
causing significant erosion. Some regions, such as inland
Australia, contain extensive flatlands that are normally dry
but are occasionally subject to shallow lake-like flooding
after exceptional rainfall. Similarly, local areas of poor
drainage may be subject to shallow flooding. Flood hazards
can also result from rivers suddenly changing course as a
result of bank erosion. If sediment loads are very high or the
watershed is small and steep, mudflows (or debris flows or
hyperconcentrated flows) may occur instead of water
floods. Hazard mapping and mitigation measures are different
for water floods and debris flows (Pierson, 1989; Costa,
1988).