Comprehensive Risk Assessment for Natural Hazards - Planat

Chapter 3
HYDROLOGICAL HAZARDS
3.1 INTRODUCTION
This chapter provides an overview of flood hazards, the
causes of flooding, methods for assessing flood hazards and
the data required for these analyses. These topics have been
covered in depth elsewhere; the purpose here is to provide a
summary that will allow a comparison between assessment
techniques for floods and assessment techniques for other
types of natural hazards. In terms of techniques, the emphasis
is on practical methods, ranging from standard methods
used in more developed counties to methods that can be
used when minimal data and resources are available. It is
assumed that the motivation for such analyses is to understand
and predict hazards so that steps can be taken to
reduce the resulting human suffering and economic losses.
It should be noted that flood-related disasters do not
confine themselves exclusively or even primarily to riverine
floods. Tropical cyclones, for example, produce hazards
from storm surge, wind and river flooding. Earthquakes and
volcanic eruptions can produce landslides that cause flooding
by damming rivers. Volcanic eruptions are associated
with hazardous mudflows, and volcanic ash may cause
flooding by choking river channels. From a natural hazard
perspective, there are important similarities between river
flooding; lake flooding; flooding resulting from poor
drainage in areas of low relief; and flooding caused by storm
surges (storm-induced high tides), tsumani, avalanches,
landslides and mudflows.All are hazards controlled, to some
extent, by the local topography, and to varying degrees it is
possible to determine hazard-prone locations. Mitigation
and relief efforts are also similar. Nonetheless, this chapter
will focus on riverine flooding, with some discussion of
storm surges and tsunami, and so, unless otherwise noted,
the term “flood” will refer to riverine floods.
3.2 DESCRIPTION OF THE HAZARD
The natural flow of a river is sometimes low and sometimes
high. The level at which high flows become floods is a matter
of perspective. From a purely ecologic perspective,
floods are overbank flows that provide moisture and nutrients
to the floodplain. From a purely geomorphic
perspective, high flows become floods when they transport
large amounts of sediment or alter the morphology of the
river channel and floodplain. From a human perspective,
high flows become floods when they injure or kill people, or
when they damage real estate, possessions or means of livelihood.
Small floods produce relatively minor damage, but
the cumulative cost can be large because small floods are
frequent and occur in many locations. Larger, rarer floods
have the potential to cause heavy loss of life and economic
damage. A disaster occurs when a flood causes “widespread
human, material, or environmental losses that exceed the
ability of the affected society to cope using only its own
resources” (UNDHA, 1992). The physical manifestations of
floods are discussed in section 3.4; the following paragraphs
describe the human consequences.
The human consequences of flooding vary with the
physical hazard, human exposure and the sturdiness of
structures. Primary consequences may include:
(a) death and injury of people;
(b) damage or destruction of residences, commercial and
industrial facilities, schools and medical facilities,
transportation networks and utilities;
(c) loss or damage of building contents such as household
goods, food and commercial inventories;
(d) loss of livestock and damage or destruction of crops,
soil and irrigation works; and
(e) interruption of service from and pollution of watersupply
systems;
Secondary consequences may include:
(f) homelessness;
(g) hunger;
(h) loss of livelihood and disruption of economic markets;
(i) disease due to contaminated water supply; and
(j) social disruption and trauma.
Floods are among the most common, most costly and
most deadly of natural hazards. For comparison of flood
disaster to other disasters, see Aysan (1993). Wasseff (1993)
also discusses the geographical distribution of disasters.
Davis (1992) lists 118 major flood disasters from the
biblical deluge to the present, and Wasseff (1993) lists 87
floods during 1947–1991 that resulted in homelessness of at
least 50 000 people. The worst recorded flood disaster
occurred in 1887 along the Yellow River in China. This flood
caused at least 1.5 million deaths and left as many as ten million
homeless (Davis, 1992; UN, 1976). More recently, floods
during 1982–1991 caused approximately 21 thousand
deaths per year and affected 73 million persons per year
(Aysan, 1993). Annual crop losses from flooding have been
estimated to be on the order of 10 million acres in Asia alone
(UN, 1976). Figure 3.1 shows an all too typical scene of
damages and hardship caused by flooding.
Storm surge and tsunami can also be very destructive.
On at least three occasions (in China, Japan and
Bangladesh) storm surges have killed at least a quarter of a
million people. There have been a number of tsunami that
individually resulted in tens of thousands of deaths. The
tsunami caused by the Santorini eruption is reputed to have
destroyed the Minoan civilization (Bryant, 1991). As well,
landslides and ice-jams can result in flooding. Rapid mass
movements of materials into lakes or reservoirs can result in
overtopping of structures and flooding of inhabited lands,
such as in the case of Vajont dam in Italy where a landslide
into a reservoir resulted in the death of approximately 2 000.
The formation of ice jams can result in the rapid rise of
water levels that can exceed historically high open water levels.
Various characteristics of water, such as its stage or
height, velocity, sediment concentration, and chemical and
biological properties reflect the amount of danger and damages
associated with an event. In the case of ice-jams, the rise