Comprehensive Risk Assessment for Natural Hazards - Planat

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resolution digital terrain models (computerized topographic
data) are currently available for the world, and new
satellite data are likely to improve the resolution.
It is important that maps be up to date and field
checked in critical locations, because new highway and railway
embankments and other man-made topographic
changes can alter drainage areas and drainage patterns.
If topographic data are lacking or the available resolution
is completely inadequate, sketch maps can be drawn
from mosaics of aerial photographs or satellite images. Such
maps cannot be used for flood routing, but can be used to
display areas of known or suspected flood hazard.
(c) For locations downstream of major reservoirs, information
on the dam operating policy, reservoir
flood-control policy, etc.
(d) Data on drainage alterations and degree of urbanization.
Up-to-date aerial photographs can be excellent
sources of information. Drainage alterations include
levees, breaks in levees for roads and railways, small and
large dams, etc.
(e) Historical flood data, that is, oral or written accounts of
floods occurring before systematic streamflow measurements
begin or after they end. Data sources include
floodmarks on buildings; old newspaper reports;
records and archives of road and rail authorities and
other businesses; municipalities; and churches, temples,
or other religions institutions that have recorded flood
damage.
(f) Maps showing the aerial extent of a flood based on
aerial photographs or ground information gathered
after the event.
(g) Specialized geologic, geomorphic or soils studies
designed to provide data on previous floods. Unless
scholarly studies have already been performed, it will be
necessary to engage specialists to conduct these studies.
(h) Streamflow measurements (annual peaks) from rivers
in the same region as the river for which hazard assessment
is desired; see item (a) above.
(i) Rainfall frequency data for the region of interest or
rainfall data from exceptional storms. These data can be
used as inputs to a rainfall-runoff model; see section
3.5.4.2.
(j) Envelope curves showing the largest observed floods as
a function of drainage area. Curves are available for the
world (Rodier and Roche, 1984) and selected regions.
3.8 ANTHROPOGENIC FACTORS AND
CLIMATE CHANGE
3.8.1 Anthropogenic contributions to flooding
In the case of rainfall or snowmelt flooding, natural processes
can be exacerbated by watershed changes that enhance runoff
production, cause flows to move more rapidly into the channel
or cause flows to move more slowly or more quickly within
the channel. Thus, deforestation, overgrazing, forest or bush
fires, urbanization and obstruction or modification of
drainage channels can be so extensive or severe as to have a
significant effect on flooding. Deforestation, in particular, has
been credited with causing important increases in the
frequency and severity of flooding.
Typically flood plain risk management is directed to
waterways and neighbouring lands within large basins,
while local urban drainage systems are geared to smaller
basins. Flooding in an urban environment may result from
runoff of local precipitation and melting snow or may result
from vulnerable urban areas located in floodplains of nearby
streams and rivers. The majority of flooding depicted in
this report is directed towards the latter cause of flooding.
However, aspects of local municipal drainage systems must
be considered in a comprehensive assessment of the vulnerability
and risk of urban centres from extreme precipitation
events. The hydrological characterization of urban and rural
or natural basins is exceedingly different. Urbanization
tends to increase the responsiveness of an area to a rainfall
event, usually leading to flash flooding and increased maximum
rates of streamflow. Infrastructure planning and
implementation, usually as part of a regional master plan for
the development of an urban area, contribute greatly to the
mitigation of damages from such events.
Some observers note that while dams reduce flood
damage from small and medium floods, they are less likely
to affect catastrophic floods and likely to produce a false
sense of security. Dam failures can cause catastrophic flooding;
average failure rates are about one failure per 1 000
dam-years (Cheng, 1993). Levees also can cause a false sense
of security on the part of the public. As noted by Eiker and
Davis (1996) for flood-mitigation projects, the question is
not if the capacity will be exceeded, but what are the impacts
when the capacity is exceeded. Thus, land-management
planners and the public must be fully informed of the consequences
when the levee or dam fails.
In the next several decades, it is expected that landuse
changes will exacerbate flood hazards in a great many
watersheds. Deforestation, overgrazing, desertification,
urbanization and drainage /channel alterations will continue
to a greater or lesser degree all over the world.
3.8.2 Climate change and variability
Chapter 3 — Hydrological hazards
Scientific recognition is increasing that, in general, climate is
not constant but fluctuates over time scales ranging from
decades to millions of years. For example, modern flood
regimes are undoubtedly different than they were 18 000
years ago during glacial conditions. Some regions have
experienced discernible climate shifts during the last 1 000
years. European examples are the medieval warm period
and the little ice age. In terms of flood-hazard assessment,
however, it has usually been considered impractical to take
climate fluctuations into account. It is generally difficult to
describe the exact effect of climate on the flood-frequency
relation of a given river, and even more difficult to predict
future climate fluctuations.
There is a growing body of scientific opinion that a significant
possibility exists that hydrological regimes
throughout the world will be altered over the next several
centuries by climate warming associated with increased levels
of anthropogenically produced greenhouse gases. Global