European Center for Risk Prevention Training at School
Level (CSLT), Sofia
EUR-OPA COUNCIL OF EUROPE
FLOODS
FLOODS
DISASTER
№1?
DISASTER№
1?
Assessment, prevention, mitigation, preparedness, response,
recovery, development, flood defenses, emergency management
2
1. What is a flood?
It is generally regarded that flooding takes place when the authorities say so,
then they ask for the evacuation of the island. Flooding takes place on a small
scale regularly, when the river can come up over the moorings on the high tide
on the full moon. (with a dog howling in the background.)
A flood is an overflow of an expanse of water that submerges land, a deluge.
In the sense of “flowing water”, the word may also be applied to the inflow of
the tide.
Flooding may result from the volume of water within a body of water, such as
a river or lake, exceeding the total capacity of its bounds, with the result that
some of the water flows or sits outside of the normal perimeter of the body. It
can also occur in rivers, when the strength of the river is so high it flows right
out of the river channel, particularly at corners or meanders.
The word comes from the Old English “flod”, a word common to Teutonic
languages (compare German Flut, Dutch vloed from the same root as is seen in
flow, float).
The term “The Flood”, capitalized, usually refers to the great Universal
Deluge described in Genesis and is treated at Deluge.
2. What types of floods are there?
Riverine flooding
Riverine flooding includes:
• overflow from river channel or river floods
• flash floods
• alluvial fan floods
• ice-jam floods
• dam-break floods
There is often no sharp distinction between river floods, flash floods, alluvial
fan floods, ice-jam floods, and dam-break floods that occur due to structural
failures or overtopping of embankments during flood (or other such as
landsliding, rockfalling, etc.) events. Nevertheless, these types of floods are
widely recognised and helpful in considering not only the range of flood risk but
also appropriate emergency preparedness and responses.
In general, the river floods are caused either by rainfall of extra-tropical or
frontal character, as experienced in temperate latitudes, or by large tropical
atmospheric depressions with moisture-laden winds, moving from a maritime
3
environment onto and across a land mass (for instance, seasonal monsoons in
Asia and line squalls on the west coast of Africa). Rainfall in these events is
generally widespread and can be heavy. The level of flooding can be high, and is
influenced by topographic features.
River floods
Overbank flooding of rivers and streams is the most common type of flood
event. River (riverine) flood plains (Fig. 1) range from narrow confined channels
in the steep valleys of hilly and mountainous areas, and wide, flat areas in the
plains and low-lying coastal regions. The amount of water in the floodplain is a
function of the size of the contributing watershed and topographic
characteristics such as watershed type and slope, and climatic and land-use
characteristics.
Consequently, the magnitude and extent of a river flood depends upon the size
of the catchment area of the river (contributing watershed), the topography, soil
conditions and vegetation, and the weather conditions involved. Size of
catchment area usually governs the character of flooding as well as the type of
meteorological event, or events, which are capable of inducing extreme floods.
For instance, river flow on very large rivers (such as the Nile, the Danube or
the Rhine) is relatively slow to change in the downstream reaches (Fig. 3a);
floodwaters will generally be a combination of many rainfall events occurring
over a wide area, sometimes augmented by melted snow. In large river basins
flooding is usually seasonal and peak discharges can be reached and maintained
over days or weeks.
Flooding in large rivers usually results from large-scale weather systems that
generate prolonged rainfall over wide areas. These same weather systems may
cause flooding in hundreds of smaller basins that drain to major rivers. Small
rivers and streams are susceptible to flooding from more localized weather
systems that cause intense rainfall over small areas.
The principal characteristics of river floods are their relatively slow build-up,
which in river systems is usually seasonal.
However, the shape of the catchment area has a considerable effect on the
peak water discharge in a river or stream (Fig. 3). The rounder the area and the
more uniform routes the water takes to the point in question (Fig. 3b), the more
the water tends to arrive simultaneously, increasing the possibility of an extreme
flood peak (hydrograph B of Fig. 3c). As a rule, round and small catchment
areas, which are commonly found in the upper reaches of rivers and in the
mountains produce a quickly rising hydrograph after intense (torrential)
rainfall. Thus, the flood peak at a given location is in general very pronounced.
In longer and wider catchments the run-off is spread better over time (Fig.
3a), as is mostly encountered in flat terrains at the lower reaches of rivers. The
hydrograph rises relatively slowly and then flattens out (hydrograph A of Fig.
3c). The water arrives at a given point gradually, even if rainfall is intense. The
characteristics of a catchment area and its hydrograph, such as hydrograph A of
Fig. 3c, can also result in the land being submerged for a long time.
4
However, if the rainstorm progresses over a long catchment area towards the
point in question in such a manner that it adds more and more water to the
flood peak, a situation can develop which is as precarious as the one seen in the
hydrograph B of Fig. 3c.
Flash floods
"Flash flood" \s a term widely used by flood experts and the general
population. However, there is no single definition, and a clear means to separate
flash floods from the rest of the spectrum of riverine floods does not exist. Floods
of this type are particularly dangerous because of the suddenness and speed with
which they occur. They develop in a basin following the occurrence of one or
more previously mentioned storm types, especially if the catchment slope is
conductive to acceleration of run-off rather than its attenuation.
Fig. 3
Flash floods are events with very little time occurring between the start of the
flood and the peak discharge (hydrograph B of Fig. 3c). They are often
associated with a short time between the storm incidence and the arrival of the
flood wave, which is not always the case; and are of short duration with
relatively high peak discharge.
Flash floods are characterized by a rapid rise in water level, high velocity, and
large amounts of debris. They are capable of tearing out trees, undermining
buildings and bridges, and scouring new channels. Major factors in flash
flooding are the intensity and duration of rainfall and the steepness of watershed
and stream gradients. The amount of watershed vegetation, the natural and
artificial flood storage areas, and the configuration of the streambed and
floodplain are also important.
Flash floods are often associated with isolated and localised intense rainfall. In
some regions, severe and destructive flash floods occur very infrequently in any
5
one of a large number of small catchments within a given region. Efficient
surveillance, warning and protection against the hazard are therefore difficult.
In other regions, flash floods occur annually on the same river; warning in these
cases is more a matter of timeliness. Because the warning time is invariably
limited, the flash floods are now the main cause of weather-related deaths.
Flash floods may result from the failure of a dam or the sudden break-up of
an ice jam. Both can cause the release of a large volume of water in a short
period of time. Flash flooding in urban areas is an increasingly serious problem
due to removal of vegetation, paving and replacement of ground cover by
impermeable surfaces that increase runoff, and construction of drainage systems
that increase the speed of runoff.
Alluvial fan floods
Alluvial fans are deposits of rock and soil that have eroded from
mountainsides and accumulated on valley floors in a fan-shaped pattern. The
deposits are narrow and steep at the head of the fan, broadening as they spread
out onto the valley floor. As rain runs off steep valley walls, it gains velocity,
carrying large boulders and other debris. When the debris fills channels on the
fan, floodwaters spill out and cut new channels. The process is then repeated,
resulting in shifting channels and combined erosion and flooding problems over
a large area.
Alluvial fan floods can cause greater damage than typical riverine flooding
because of the high velocity of flow, the amount of debris carried, and the broad
area affected. Floodwaters typically move at velocities of 5 to 10 metres per
second due to steep slopes and lack of vegetation.
Human activities often exacerbate flooding and erosion problems on alluvial
fans. Roads act as drainage channels, carrying high velocity flows to lower
portions of the fan, while fill, levelling, grading, and structures can alter flows
patterns.
Ice jam floods
Flooding caused by ice jams is similar to flash flooding. Ice jam formation
causes a rapid rise of water at the jam and extending upstream. Failure or
release of the jam causes sudden flooding downstream.
The formation of ice jams depends on the weather and physical conditions in
river channels. Ice jams are most likely to occur where the channel slope
naturally decreases, where culverts freeze solid, at headwaters of reservoirs, at
natural channel constructions such as bends and bridges, and long shallows
where channels may freeze solid.
Ice jams floods can occur during fall freeze-up from the formation of frazil
ice, during midwinter periods when stream channels freeze solid forming anchor
ice, and during spring break-up when rising water levels from snowmelt or
rainfall break existing ice cover into large floating masses that lodge at bridges
6
and other constructions. Damage from ice jam flooding usually exceeds that
caused by open water flooding. Flood elevations are usually higher than
predicted for free-flow conditions and water levels may change rapidly. The
force of ice impacting buildings and other structures can cause additional
physical damage.
Dam break floods
Dam failures can occur as a result of structural failures, such as progressive
erosion of an embankment or overtopping and breaching by a severe flood.
Earthquakes may weaken dams. Disastrous floods caused by dam failures,
although not in the category of natural hazards, have caused great loss of life
and property damage, primarily due to their unexpected nature and high
velocity floodwater.
Local drainage or high groundwater levels
Local heavy precipitation may produce flooding in areas other than
delineated floodplains or along recognizable drainage channels. If local
conditions cannot accommodate intense precipitation through a combination of
infiltration and surface runoff, water may accumulate and cause flooding
problems. During winter and spring, frozen ground and accumulations of snow
may contribute to inadequate drainage and localized ponding. Flooding
problems of this nature generally occur in areas with flat gradients, and
generally increase with urbanisation which speeds the accumulation of
floodwaters because of impervious areas. Shallow sheet flooding may result
unless channels have been improved to account for increased flows.
High groundwater levels may be of concern and can cause problems even
where there is no surface flooding. Basements are susceptible to high
groundwater levels. Seasonally high groundwater is common in many areas,
while in others high groundwater occurs only after long periods of aboveaverage precipitation.
Fluctuating lake levels
Water levels in lakes can fluctuate on a short-term, seasonal basis, or on a
long-term basis over periods of months or years. Heavy seasonal rainfall can
cause high lake levels for short periods of time, and snowmelt can result in
higher spring levels. Long-term fluctuations are a less-recognised phenomenon
that can cause high water and subsequent flooding problems lasting for years or
even decades.
While all lakes may experience fluctuations, water levels tend to vary the most
7
in lakes that are completely landlocked or have inadequate outlets for
maintaining a balance between inflow and outflow. These lakes, commonly
referred to as closed-basin lakes, are particularly susceptible to dramatic
fluctuations in water levels over long periods of time, as much as 1 to 3 metres.
Fluctuations of lake water levels over a short period of time, initiated by local
atmospheric changes, tidal currents, or earthquakes, are known as "seiches".
These, free or standing wave oscillations of the surface of water in an enclosed
basin are similar to water sloshing in a bathtub.
Coastal flooding
Devastating floods can occur as a result of extreme wind storms (typhoons,
hurricanes or tropical cyclones). The Indian sub-continent (Bay of Bengal), and
countries in Asia and the Pacific are all typically subject to such events.
Catastrophic flooding from rainfall is often aggravated by wind-induced surge
and low atmospheric pressure along a coastline
(Fig. 4), which causes a rise in sea level and inundation of coastal and inland
areas. Rainfall intensities are high and the area of the storm is wide; the
combination of these factors can produce extreme flood discharge in both small
and large river basins, which can be maintained at high levels by a coastal
discharge.
Storm surges occur when the water level of a tidally influenced body of water
increases above the normal astronomical high tide. Storm surges commonly
occur with coastal storms caused by massive low-pressure systems with cyclonic
flows that are typical of tropical cyclones, northeasters, and severe
winterstorms. Other factors influencing storm surge intensity are:
• wind velocity
• storm surge height
• coastal shape
• storm centre velocity
• nature of coast
• previous storm damage
• human activity
Storm surges generated by coastal storms are controlled by the following four
factors:
• The more intense storms have higher wind speeds which drive greater
amounts of water across the shallow continental shelf, thereby increasing the
volume and elevation of water pushed up against the coast. In areas with mild
slopes and shallow depths, the resulting flooding can reach great heights.
• The low barometric pressure experienced during coastal storms can cause
the water surface to rise, increasing the height of storm surges.
• Storms landfalling during peak astronomical tides have higher surge heights
and more extensive flood inundation limits.
• Coastal shoreline configurations with concave features or narrowing bays
create a resonance within the area as a result of the winds forcing in water,
8
elevating the surface of the water higher than experienced along adjacent areas
of open coast.
The other causes of coastal flooding are tsunamis, the large seismic sea waves,
impulsively generated by shallow-focus earthquakes.
- Estuarine floods
Commonly caused by a combination of sea tidal surges caused by storm-force
winds.
- Catastrophic floods
Caused by a significant and unexpected event e.g. dam breakage, or as a
result of another hazard (e.g. earthquake or volcanic eruption).
For example: Tropical Storm Alberto, the famous 1994 storm, produced
heavy flooding across Georgia, Alabama and northwest Florida and created
between 400-600 million dollars worth of damage in the Southeastern US in 1994
United States Dollars.
- Other types
Floods can occur if water accumulates across an impermeable surface (e.g.
from rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low
evaporation).
A series of storms moving over the same area.
Dam-building beavers can flood low-lying urban and rural areas, often
causing significant damage.
Regional Floods
Regional floods are caused by snow melt, and annual phenomena like the
Malaysian monsoons and the yearly Nile River overflow. The storms overload
the rivers. The floods can happen faster and be more serious if the ground is
frozen or already saturated with water.
- Storm Surge Floods
Storm Surges are huge waves that come onto the beaches and beyond,
flooding the shore. These are mostly caused by hurricanes and large storms.
9
3. Why do flood occur?
Flooding occurs most commonly from heavy rainfall when natural
watercourses do not have the capacity to convey excess water. However, floods
are not always caused by heavy rainfall. They can result from other
phenomenon, particularly in coastal areas where inundation can be caused by a
storm surge associated with a tropical cyclone, a tsunami or a high tide
coinciding with higher than normal river levels. Dam failure, triggered for
example by an earthquake, will result in flooding of the downstream area, even
in dry weather conditions.
In general, the factors which influence whether a flood will occur include:






Volume, spatial distribution, intensity and duration of rainfall over a
catchments;
the capacity of the watercourse or stream network to convey runoff;
catchments and weather conditions prior to a rainfall event;
ground cover;
topography; and
Tidal influences.
Flooding occurs in both natural and developed watersheds. When the rate of
rainfall or snowmelt exceeds the rate of infiltration to the ground, the excess
water, called runoff, moves across the ground surface toward the lowest section
of the watershed. As the surface runoff enters stream channels, stream levels
increase. If the rate of runoff is high enough, water in the stream overflows the
banks and flooding occurs. This area of over-bank flow is called the flood plain.
All natural watersheds have flood plains. Structures located in these flood plains
are subject to damage. In a natural watershed, flooding can be affected by ice
jams, the accumulation of debris at channel constrictions, and even the dambuilding activity of beavers.
Human activity has profound impacts on flooding. The two major activities
which impact flooding are land use change and the building of flood control
structures.
Land Use Change- Hundreds of years ago, the Delaware River Basin (USA) was
covered by forests. This maximized the infiltration of rainfall and slowed the
movement of runoff. As the land was cleared for agriculture, infiltration rates
were reduced and runoff rates increased. The increase in runoff rates widened
flood plains and stream channels in many of the basin's watersheds. With
1
0
gradual urbanization and the increasing use of asphalt and concrete paving, in
addition to densely spaced buildings, infiltration rates were further reduced
with corresponding increases in runoff rates. Because of these land use changes,
flood flow rates in many areas are much higher than they would naturally be for
a given rain storm. Although some land that was formerly in agricultural use
has been reforested, the runoff reduction benefits have been offset in many areas
by continued urbanization.
The transportation network associated with land use change also affects
flooding. In addition to the impacts of impervious paved surfaces, bridges and
culverts usually constrict stream channels and flood plains. This aggravates
upstream flooding, especially when the constrictions become clogged with ice or
debris.
Flood Control Structures- The purpose of flood control structures is to
physically constrain or to convey flood waters. Flood control structures include
dams, levees, lined stream channels, and storm sewers. Dams and levees have
been used for centuries to open flood plains to agriculture and settlement, and in
the case of dams, to detain flood waters for gradual release or for use for water
supply, recreation, and the generation of hydroelectric-power. Dams and levees
are highly effective in flood loss reduction.
Though effective, one drawback to the use of dams and levees for flood loss
reduction is that they are very expensive. Secondly, local cost sharing
requirements and environmental issues have slowed construction of new
facilities in recent years. Flood control dams and levees are not necessary where
there is no flood plain development.
4. Where do flood occur?
Riverine flooding occurs in relatively low-lying areas adjacent to streams and
rivers. In the extensive flat inland regions of Australia, floods may spread over
thousands of square kilometers and last several weeks, with flood warnings
sometimes issued months in advance. In the mountain and coastal regions of
Australia flooding can happen rapidly with a warning of only a few hours in
some cases.
The Great Dividing Range which extends along the length of eastern Australia
provides a natural separation between the longer and slower westerly flowing
rivers and the shorter, faster easterly flowing coastal rivers. In some cases
natural blockages at river mouths, including storm surge and high tides, also
may cause localized flooding of estuaries and coastal lake systems.
Flash floods can occur almost anywhere there is a relatively short intense burst
of rainfall such as during a thunderstorm. As a result of these events the
1
1
drainage system has insufficient capacity or time to cope with the downpour.
Although flash floods are generally localized, they pose a significant threat
because of their unpredictability and normally short duration.
A flood typically occurs when a river (or other body of water) overflows its
banks. As you can read Physical Geography: The Global Environment, third
edition, annual floods can even be a normal part of a floodplain’s development.
These floods deposit sediments that build a river’s natural levees, broad ridges
that run along both sides of the channel. Figure F-3 shows the relationship
between floods and natural levee development. As the river spills out of its
channel, the coarsest material it is carrying is depostied closest to the overflow,
hence along the levees. When the river contracts after the flood, it stays within
its self-generated levees.
Image from Physical Geography However, not all floods are so regular and
productive. Infrequently—perhaps once in a century—a river may experience a
flood of such magnitude that its floodplain is greatly modified. Water up to
several meters deep may inundate the entire floodplain, destroying submerged
1
2
levees,
eroding
bluffs,
and
disrupting
the entire
system.
These
sorts of
floods
have cost
millions of
lives in
the
densely
populated
floodplains of Asia’s major rivers. They also occur in the Mississippi Basin of
the central United States, where the damage, too, can be enormous. No
reinforcement of natural levees or construction of artificial levees can withstand
the impact of such a powerful “100-year” flood.
- What
were the largest floods in the world?
- What were the largest floods in the world and in the Europe?
1
3
London is protected from flooding by a huge mechanical barrier across the
River Thames, which is raised when the water level reaches a certain point (see
Thames Barrier).
Venice has a similar arrangement, although it is already unable to cope with
very high tides. The defenses of both London and Venice will be rendered
inadequate if sea levels continue to rise.
The largest and most elaborate flood defenses can be found in the
Netherlands, where they are referred to as Delta Works with the Oosterschelde
dam as its crowning achievement. These works were built in response to the
North Sea flood of 1953 of the southwestern part of the Netherlands. The Dutch
had already built one of the world’s largest dams in the north of the country: the
Afsluitdijk (closing occurred in 1932).
Flood blocking the road in JerusalemCurrently the Saint Petersburg Flood
Prevention Facility Complex is to be finished by 2008, in Russia, to protect Saint
Petersburg from storm surges. It also has a main traffic function, as it completes
a ring road around Saint Petersburg. Eleven dams extend for 25.4 kilometres
and stand eight metres above water level.
The New Orleans Metropolitan Area, 35% of which sits below sea level, is
protected by hundreds of miles of levees and flood gates. This system failed
catastrophically during Hurricane Katrina in the City Proper and in eastern
sections of the Metro Area, resulting in the inundation of approximately 50% of
the Metropolitan area, ranging from a few inches to twenty feet in coastal
communities.
In an act of successful flood prevention, the Federal Government of the
United States offered to buy out flood-prone properties in the United States in
order to prevent repeated disasters after the 1993 flood across the Midwest.
Several communities accepted and the government, in partnership with the
state, bought 25,000 properties which they converted into wetlands. These
wetlands act as a sponge in storms and in 1995, when the floods returned, the
government didn’t have to expend resources in those areas.
1
4
In China, flood diversion areas are rural areas that are deliberately flooded in
emergencies in order to protect cities.
Autumn Mediterranean flooding in Alicante (Spain), 1997.In western
countries, rivers prone to floods are often carefully managed. Defences such as
levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting
their banks. Coastal flooding has been addressed in Europe with coastal
defences, such as sea walls and beach nourishment.
5. What could be the consequences of the floods?
. Human loss
. Socio – economic
. Environmental
. Cultural heritage
. Others
Typical effects
Primary effects
Physical damage: Can range anywhere from bridges, cars, buildings, sewer
systems, roadways, canals and any other type of structure.
Casualties: People and livestock die due to drowning. It can also lead to
epidemics and diseases.
Secondary effects
Water supplies: Contamination of water. Clean drinking water becomes
scarce.
Diseases: Unhygienic conditions. Spread of water-borne diseases
Crops and food supplies: Shortage of food crops can be caused due to loss of
entire harvest.
Trees: Non-tolerant species can die from suffocation.
Tertiary/long-term effects
Economic: Economic hardship, due to: temporary decline in tourism,
rebuilding costs, food shortage leading to price increase etc.
1
5
Flood clean-up safety
Clean-up activities following floods often pose hazards to workers and
volunteers involved in the effort. Potential dangers include electrical hazards,
carbon monoxide exposure, musculoskeletal hazards, heat or cold stress, motor
vehicle-related dangers, fire, drowning, and exposure to hazardous materials.
Because flooded disaster sites are unstable, clean-up workers might encounter
sharp jagged debris, biological hazards in the flood water, exposed electrical
lines, blood or other body fluids, and animal and human remains. In planning
for and reacting to flood disasters, managers provide workers with hard hats,
goggles, heavy work gloves, life jackets, and watertight boots with steel toes and
insoles.
Benefits of flooding
There are many disruptive effects of flooding on human settlements and
economic activities. However, flooding can bring benefits, such as making soil
more fertile and providing nutrients in which it is deficient. Periodic flooding
was essential to the well-being of ancient communities along the TigrisEuphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow
River, among others. The viability for hydrological based renewable sources of
energy is higher in flood prone regions.
6. Can the causes of floods be influences by human behavior?
7. Can the consequences of floods be influenced by human
behaviour?
Flooding is defined as the accumulation of water within a water body and the
overflow of excess water onto adjacent floodplain lands. The floodplain is the
land adjoining the channel of a river (Fig. 2), stream, ocean, lake, or other
watercourse or water body that is susceptible to flooding.
1
6
Riverine floodplain and causes of flooding
Fig.1
Flooding is the most common environmental hazard. It regularly claims over
20,000 lives per year and adversely affects around 75 million people worldwide.
The reason lies in the widespread geographical distribution of river flood plains
and low-lying coasts, together with their long-standing attractions for human
settlement.
Several types of flood
hazards
confront
the
physical planner, urban
planner and emergency
manager:
• riverine flooding
• fluctuating lake levels
• local drainage or high
groundwater levels
• coastal flooding
Fig. 2
(or inundation) including storm surges and tsunamis
The appearance of flood hazard is dominantly limited to the prevailing
weather system and geomorphological and topographical features of a given
area.
Inland flooding, as distinct from coastal flooding, is generally caused by the
overflow of watercourses as a result of intense rainfall or of a reduction in
waterway area by landslide or debris damming (which themselves may be
triggered by natural events such as earthquakes).
Coastal flooding can, in addition, be caused by extreme winds leading to
storm surges, by off-shore earthquake induced tidal waves (known as tsunamis)
1
7
or the subsidence of coastal land. Human manipulation of watersheds, drainage
basins, floodplains and the effects of deforestation, soil erosion, silt carriage have
increased volume and speed of runoff.
Landscape susceptible to flood development
The most flood-susceptible landscape settings are:
• Low-laying parts of active floodplains and river estuaries. In their natural
state, these settings will suffer the most frequent inundation - coastal flooding.
• Low-laying inland shorelines, that become unprotected due to erosion of
barrier islands, sand dunes or bluffs, so the wind-driven wave attacks may
inundate and cause damage to buildings and other immovable shoreline
facilities.
• Small basins subjected to flash floods. Flash floods are formed in arid and
semi-arid zones where there is a combination of steep topology, little vegetation
and high intensity, short-duration connective rainstorms. They can also occur in
narrow valleys and heavily developed urban settings.
Alluvial fens can create a special type of flash flood threat, especially in semiarid areas where the fens support urban development. The braided drainage
channels
can
meander
unpredictably
across the relatively
steep
slopes,
bringing very high
velocity flows of 510
metres
per
second, which are
highly charged with
sediment
and
produce significant
hydrodynamic
forces capable of
destroying
built
structures. Velocity
as low as 0.5 metres
per second are
capable of sweeping
victims off their
feet.
• Areas below
unsafe
or
1
8
inadequate dams. When the foundations of the Malpasset Dam (France, 1959)
failed, 421 people died. In 1963 a landslide created a major flood surge behind
the Vaiont Dam (Italy). Although the structure withstood soundly, the
overtopped water surge killed 3,000 people downstream. In the coal mining
valley of Buffalo Creak (West Virginia, USA, 1972), 125 people were killed and
4000-5000 were made homeless when a poorly maintained dam burst.
Flood forecasting is based on seasonal patterns, capacity of the drainage
basin, flood mapping, and surveys by air and land. Warnings can be issued far
in advance for seasonal floods, but only shortly before onset in cases of storm
surge, flash flood and tsunamis.
In its simplest form flood hazard is depicted in plan by lines that represent the
areal extent of water-surface (Fig. 1). The longitudinal profiles of water-surface
should also be shown (Fig. 2). Those water-surface profiles are determined both
for floods of special frequency of occurrence and for historically recorded or
estimated events of catastrophic magnitude. When provided with sufficient and
relevant annotation, flood hazard plans serve to indicate the potential severity of
inundation, thus provide relevant data for planning the organisation and
resources for effective and efficient emergency response.
8. Can the flood be predicted?
New World Meteorology System has possibility to predict all types of the
floods.
Monitoring and control systems of the water.
GPS and satellite pictures in real time.
9. Is there any way to prevent floods?
Responsibilities of :
- Regions;
- Countries;
- Municipalities;
- Peoples.
10.
Is there any way to mitigate consequences?
-
Paining and Realization of the engineering works.
Organization of the Crisis Management;
1
9
-
Creation of the Specific structures.
Although floods are initially caused by nature, there are things that we
humans can do to help prevent and make sure that they are not devastating. If
you are planning on building a new home, you should take into to consideration
not building on an active flood plain. If there is a big storm or flood warning for
some apparent reason, make sure that all your storm drains are clear of leaves
and any other substances blocking the storm drain. If all else fails you should
put down sandbags in front of your house to prevent the water from coming in.
Last but not least if there is a possibility of flood from the river or a lake, it is
possible to build up the bank so that it will hold more water.
The goverment is attempting to reduce flooding by building dams and levees
in flood-prone areas. Levees are artificially raised riverbanks, and dams are
walls to block water. Levees can only go so high, and are easily overflowed in
large floods. Dams are very effective in preventing floods but change river
direction and take water away from some areas. In addition to controlling
floods, dams can provide hydroelectric power. One great
example of all this is the Hoover dam in Nevada . The dam controls floods,
regulates the direction of the Colorado River, and provides power to nearby Las
Vegas (Department of the Interior).
City Maintenance commissions continue to improve the drainage in towns,
especially in the roads. Road flooding is one of the first thing that happens in
most floods, and obliviously cuts off the major mode of transportation for most
people. Places below sea level need to be especially vigilant and prepared for
floods, because they will happen. people should be careful about driving in
floods because it is easy for your engine to flood and strand you on a flooded
highway. If you live in a low area you should make sure your house is
reasonably flood-proof, and no matter where you live you should have flood
insurance.
Organisation and implementation of operations
Since ancient times floods have been seen as the most terrible calamity. In
many world religions they have been described as "God's punishment". Among
all natural calamities, flooding heads the list in sheer number of catastrophes, its
wide coverage of territory and the most economically destructive.
Floods are caused by spills of rivers in high water, heavy rains, ice blocks on
rivers, heavy melting of ice, failure of dams due to earthquakes, bombing or
technological catastrophes at hydro facilities and diversions of rivers.
Floods cause rapid inundations of vast territories, where people are injured
2
0
and lost, agricultural and wild animals are killed, dwelling, industrial buildings
and other structures, utility plants, roads, electrical and communication lines
are damaged or destroyed.
Agricultural produce is destroyed, the structure of the soil and the relief of the
land is changed, productivity is interrupted and storage of raw material fuel,
food, forage, fertilizers and construction materials is either destroyed or
becomes unusable.
If a basement or underground floors are inundated, the water may cause
malfunction of equipment, which in turn, will cause electric accidents and short
circuits in electric systems.
In a number of cases floods may result in landslides and mudflows.
The basic characteristic features of floods are water expenditure, its volume
and the level to which it rises, the area covered, its duration, the speed and
composition of water flow.
During such accidents, people can be affected by the kinetic energy produced
by the burst waves. Mechanical injuries of varying severity could be the result
of:
o direct dynamic impact on human body by a burst wave
o traumatic effects caused by fragments of building and other structures
destroyed by the burst waves
o different items, involved in the motion by the burst waves
Magnitude and structure of population losses vary depending on a density of
population in a flooded area, time of a day, velocity of movement and height of a
burst wave, temperature of water and others.
At accidents in hydro dynamically hazardous objects, the total losses of
population in a burst wave area, can reach 90% at nighttime and 60% in
daytime. The irretrievable losses could be 75% at nighttime and 40% in
daytime, while the sanitary loses 25% and 60%, respectively.
Frequently, secondary flood effects could cause greater disaster than a flood
itself.
Prevention and minimization of adverse flood consequences includes adequate
organisational and engineering-technical measures such as: reinforcement of the
hydro-technical facilities, construction of additional dams and banks to hold up
water flows, accumulation of emergency material (soil) to fill up holes, increase
of height of existing dikes and dams, training in emergency swimming, etc. A
permanent hydrological forecast is necessary including the estimates on
potential and possible water levels in water storages. Transport means has to be
allocated and on disposal for organisation of possible evacuation of population
and of some significant values (valuable paints, movable historic heritage,
archives, etc.). Training of population and special units to operate efficiently
under flood condition should be organised.
11. What to do in case of flood?
(good reactions, personal protective measures, school drills, …)
During a Flood
2
1
If a flood is likely in your area, you should:



Listen to the radio or television for information.
Be aware that flash flooding can occur. If there is any possibility of a flash
flood, move immediately to higher ground. Do not wait for instructions to
move.
Be aware of streams, drainage channels, canyons, and other areas known to
flood suddenly. Flash floods can occur in these areas with or without such
typical warnings as rain clouds or heavy rain.
If you must prepare to evacuate, you should do the following:


Secure your home. If you have time, bring in outdoor furniture. Move
essential items to an upper floor.
Turn off utilities at the main switches or valves if instructed to do so.
Disconnect electrical appliances. Do not touch electrical equipment if you are
wet or standing in water.
If you have to leave your home, remember these evacuation tips:


Do not walk through moving water. Six inches of moving water can make you
fall. If you have to walk in water, walk where the water is not moving. Use a
stick to check the firmness of the ground in front of you.
Do not drive into flooded areas. If floodwaters rise around your car, abandon
the car and move to higher ground if you can do so safely. You and the
vehicle can be quickly swept away.
Driving Flood Facts
The following are important points to remember when driving in flood conditions:



Six inches of water will reach the bottom of most passenger cars causing
loss of control and possible stalling.
A foot of water will float many vehicles.
Two feet of rushing water can carry away most vehicles including sport utility
vehicles and pick-ups.
12. WHAT TYPE OF MAPS ON FLOOD EXIST ?
. WHAT ARE THEY USED FOR ?
. CAN I GET THESE MAPS AND FROM WHERE ?
2
2