chapter 5: flood management Flashcards
Flood management is important because many people live in flood-prone areas ………………………………………………….. and a lot of investments in towns, plants and infrastructure have been made in such areas
the coast or on floodplains
Flood management can aim at
- a reduced risk of floods;
- a reduced vulnerability to floods;
- improved preparedness;
- streamlined emergency management once a flood damage has occurred; and
- improved knowledge (about cause-effect relationships, driving forces and management options);
The management can comprise activities such as
- forecasting services;
- education and awareness campaigns;
- flood proofing;
- land management (‘keeping people away from floods’);
- water management (storage operation);
- structural protection (dykes) (‘keeping floods away from people’); Need to protect population, agriculture, infrastructure, and environment Growing economic activity in flood-prone areas Flood management Interventions: Land use, infrastructure
- timely and appropriate response to events (such as disasters) and developments;
- studies of cause-effect relationships and of impacts of intervention; etc
It is the poor people and the children who are most vulnerable to
floods
It is the poor people and the children who are most vulnerable to floods. Therefore, a ……………………… must be extended to these parts of the population.
particular attention
Good flood management would take its starting point in a suitable knowledge about
- the flood risk (so that high-risk areas are delineated); and
- the flood vulnerability (so that the most important potential consequences are identified).
Good flood management would take its starting point in a suitable knowledge about
- the flood risk (so that high-risk areas are delineated); and
- the flood vulnerability (so that the most important potential consequences are identified).
If this knowledge is not adequate …………….. should be made
studies
Coastal floods can be caused by
storm surge
Coastal floods can be caused by storm surge, which is generated by
a combination of wind friction and low air pressure
Coastal floods can be caused by storm surge, which is generated by a combination of wind friction and low air pressure. …………………………….. will add to the water level
Local wind waves
Local wind waves will add to the water level, and the storm surge can be ……………………………..
amplified (or reduced)
. Local wind waves will add to the water level, and the storm surge can be amplified (or reduced) by
interference with the strictly regular astronomical tide.
Extreme floods can be related to
extreme storms - like cyclones -
Extreme floods can be related to extreme storms - like cyclones - that attack
attack the open coast
Extreme floods can be related to extreme storms - like cyclones - that attack the open coast. In areas that are otherwise more sheltered (like the…………………… )
Baltic Sea
Extreme floods can be related to extreme storms - like cyclones - that attack the open coast. In areas that are otherwise more sheltered (like the Baltic Sea), extreme floods can be generated by
by unusual sequences of wind set-up and air pressure variations.
Irrespective of the weather, flood waves can be generated by
distant, sub-sea earthquakes (such flood waves are called tsunamis), or, in arctic areas, by breaking glaciers.
Irrespective of the weather, flood waves can be generated by distant, sub-sea earthquakes (such flood waves are called tsunamis), or, in arctic areas, by breaking glaciers.
The effect depends on the
coastal profile
The effect depends on the coastal profile. A gently sloping profile causes
a high amplification of an approaching flood wave (and tidal wave) and a high set-up in response to local wind
A steep profile will tend to
reflect the flood wave (and tidal wave), rather than amplifying it
A steep profile will tend to reflect the flood wave (and tidal wave), rather than amplifying it, and the locally generated set-up will be small if
the water depth is large
Often, a shallow coastal profile occurs in places with
low-lying lands
. Often, a shallow coastal profile occurs in places with low-lying lands. If so, the flood risk will be
high
It is the weakest part of the poor population - rural women, elderly, and children - who are
most at risk
It is the weakest part of the poor population - rural women, elderly, and children - who are most at risk. Even those who escape unharmed in the first place may be victims to
to diseases in the days to follow
. Even those who escape unharmed in the first place may be victims to diseases in the days to follow, and may in any case have
lost whatever they owned.
A major problem in this connection is that it is
is not possible to predict the route of a moving extreme depression like a cyclone
Cyclone shelters Built to be used only
once in a lifetime
Cyclone shelters Built to be used only once in a lifetime, cyclone shelters are
an expensive means of protection against the flood peril
Cyclone shelters Built to be used only once in a lifetime, cyclone shelters are an expensive means of protection against the flood peril. On the low islands in the Meghna Estuary, however, they are the only refuge within reach. After the 1991 cyclone, different donor organisations have funded the construction of some ……………. cyclone shelter in the …… area
1,200
Banglandesh
. After the 1991 cyclone, different donor organisations have funded the construction of some 1,200 cyclone shelters in the coastal areas of Bangladesh. They are impressive reinforced ………….. structures
concrete
After the 1991 cyclone, different donor organisations have funded the construction of some 1,200 cyclone shelters in the coastal areas of Bangladesh. They are impressive reinforced concrete structures, often built in places accessible by ……………………………….
narrow footpaths only
They are impressive reinforced concrete structures, often built in places accessible by narrow footpaths only. Apart from their primary objective, the cyclone shelters serve as
schools, mosques, storage, or clinics.
Cyclones are frequent in the Bay of Bengal. In Bangladesh, where around 2 mio. people live in the coastal areas and the islands of Meghna Estuary, a severe cyclone is expected to hit the coast (at one place or another) once in
5 years on the average
. During a severe cyclone, wind speeds range from
90 km/hour and upwards
During a severe cyclone, wind speeds range from 90 km/hour and upwards. In the northern Bay of Bengal, the associated storm surge can reach
5-10 m
In the northern Bay of Bengal, the associated storm surge can reach 5-10 m. This is well above
the land level of the coastal areas
In the northern Bay of Bengal, the associated storm surge can reach 5-10 m. This is well above the land level of the coastal areas, which is typically
only slightly above sea level
The cyclone that hit the coast of Bangladesh on April 1991 was among the most severe in recent times. It was forecast in due time, but the warnings were not heeded until the very last moment, or, in many cases, until it was too late. Furthermore, the protective coastal embankments were in
a poor state of maintenance
The cyclone that hit the coast of Bangladesh on April 1991 was among the most severe in recent times. It was forecast in due time, but the warnings were not heeded until the very last moment, or, in many cases, until it was too late. Furthermore, the protective coastal embankments were in a poor state of maintenance. As it turned out, in some places, the damaged embankments amplified the ………… rather than …………………….
impact
controlling it
The cyclone that hit the coast of Bangladesh on April 1991 was among the most severe in recent times. It was forecast in due time, but the warnings were not heeded until the very last moment, or, in many cases, until it was too late. Furthermore, the protective coastal embankments were in a poor state of maintenance. As it turned out, in some places, the damaged embankments amplified the impact, rather than controlling it. It is estimated that over
100, 000 people died
As it turned out, in some places, the damaged embankments amplified the impact, rather than controlling it. It is estimated that over 100,000 people died. Most of the livestock and all the poultry of the affected area was lost. ……………………………………… were gravely damagned
buildings, structures and crops
Inland, seasonal floods in the ………………. of a river ………………..
floodplains of river basins
Inland, seasonal floods in the floodplains of river basins are generated by the
seasonal melting of snow and ice, and/or seasonal rainfall variations
Inland, seasonal floods in the floodplains of river basins are generated by the seasonal melting of snow and ice, and/or seasonal rainfall variations, for example determined by
the monsoon weather system
The variation can be strong and regular in case of
large rivers that origin in mountainous areas with snow and glaciers
The variation can be strong and regular in case of large rivers that origin in mountainous areas with snow and glaciers (like the ……………………….)
Himalaya plateau
The variation can be strong and regular in case of large rivers that origin in mountainous areas with snow and glaciers (like the Himalaya Plateau) and proceed through areas with …………………….. (like …………………………… )
a monsoon climate (like South and Southeast Asia)
In such areas, in the course of time, the environment and the economy have adapted to the
occurrence of floods
In such areas, in the course of time, the environment and the economy have adapted to the occurrence of floods, to an extent that some livelihoods (and ecosystems)
depend on it
In such areas, in the course of time, the environment and the economy have adapted to the occurrence of floods, to an extent that some livelihoods (and ecosystems) depend on it, while others are
strongly supported by it
In such areas, in the course of time, the environment and the economy have adapted to the occurrence of floods, to an extent that some livelihoods (and ecosystems) depend on it, while others are strongly supported by it.
Problems occur in case of
deviations from the normal pattern, with floods being higher than usual, or if they take place at an unusual time of the year
Problems occur in case of deviations from the normal pattern, with floods being higher than usual, or if they take place at an unusual time of the year. This can be for
natural reasons, or due to human intervention, or both
Also, problems can occur when the development takes a course that does not conform well with
the occurrence of floods
Also, problems can occur when the development takes a course that does not conform well with the occurrence of floods. This can relate to
land use, investment in physical infrastructure, or new ways of natural resources utilisation.
The flood risk is influenced by
- the climate (which can fluctuate over small, medium and long periods of time);
- morphological changes of the river system, whether of natural or human origin; and
- any human intervention in the river system that changes the runoff, the flow resistance, or the wet season flow itself.
Comprehensive deforestation can occur due to
forest fires and human intervention
Deforestation can increase
the flood risk
Deforestation can increase the flood risk, both by changing the
runoff pattern and by increasing the sediment yield
Deforestation can increase the flood risk, both by changing the runoff pattern and by increasing the sediment yield and hereby increase the
flow resistance in the main river channels
………………………………………………………………………. can at the same time increase and reduce the flood risk at different places
Dredging of navigation channels and regulation of bends
Likewise, a flood protection scheme can
reduce the flood risk at one place while increasing it at other places
Likewise, a flood protection scheme can reduce the flood risk at one place while increasing it at other places. Earthquakes can change the ……………… by
flood risk by changing the planform of the river, and by releasing large pulses of sediments
Apart from the climate, the flood risk depends on
- the runoff
- the flow resistance
- the storage capacity
- structural protection
Damage caused by floods can comprise
- loss of life, property, crops and livestock;
- diseases related to interrupted water supply and sanitation;
- damage to buildings and different types of infrastructure;
- pollution caused by flooding of waste deposits, storage facilities, and waste disposal facilities; and
- disruption of livelihoods and education. As it is the case with coastal floods, it is the poorest part of the population that is most vulnerable.
In broad general, floods tend to affect the …………….. population to a higher extent than the ……………….. population
rural
urban
In broad general, floods tend to affect the rural population to a higher extent than the urban population, and the ………………………….. population to a higher extent than …………………………………… groups
poor part of the
middle- and high-income
In broad general, floods tend to affect the rural population to a higher extent than the urban population, and the poor part of the population to a higher extent than middle- and high-income groups. This can be due to
inadequate information, knowledge and awareness, but also due to a lack of capacity to sustain one or two failed crops, or to sustain a disruption of paid employment
Benefits of floods comprise
- water being retained for (paddy) cultivation;
- improved soil fertility;
- maintenance of primary production on the flood plains;
- maintenance of the fish yields; and
- preservation of wetlands ecosystems.
Flash floods are caused by
local rainfall in mountainous catchments
Flash floods are caused by local rainfall in mountainous catchments. They got their name because they occur
unexpectedly and develop rapidly
Flash floods are caused by local rainfall in mountainous catchments. They got their name because they occur unexpectedly and develop rapidly. They can cause
loss of human lives and comprehensive damage to buildings, infrastructure and crops
Flash floods are caused by local rainfall in mountainous catchments. They got their name because they occur unexpectedly and develop rapidly. They can cause loss of human lives and comprehensive damage to buildings, infrastructure and crops. A related type of disaster is
dam breaks
They can cause loss of human lives and comprehensive damage to buildings, infrastructure and crops. A related type of disaster is dam breaks. In …………………. Asia, there are indications that flash floods have become
Southeast
more frequent and/or more serious in recent years
A related type of disaster is dam breaks. In Southeast Asia, there are indications that flash floods have become more frequent and/or more serious in recent years. In some cases it may be speculated that deforestation
plays a role
In Southeast Asia, there are indications that flash floods have become more frequent and/or more serious in recent years. In some cases it may be speculated that deforestation plays a role. In other cases, the observation may be due to
new settlements in areas that have hitherto been undeveloped
In Southeast Asia, there are indications that flash floods have become more frequent and/or more serious in recent years. In some cases it may be speculated that deforestation plays a role. In other cases, the observation may be due to new settlements in areas that have hitherto been undeveloped. Another development is that today, due to effective ………………………, any serious flash flood will become a spectacular ……………….. item.
communication
news
In Southeast Asia, there are indications that flash floods have become more frequent and/or more serious in recent years. In some cases it may be speculated that deforestation plays a role. In other cases, the observation may be due to new settlements in areas that have hitherto been undeveloped. Another development is that today, due to effective communication, any serious flash flood will become a spectacular news item. Flash floods cannot be predicted with any …………………………
reasonable lead time.
In Southeast Asia, there are indications that flash floods have become more frequent and/or more serious in recent years. In some cases it may be speculated that deforestation plays a role. In other cases, the observation may be due to new settlements in areas that have hitherto been undeveloped. Another development is that today, due to effective communication, any serious flash flood will become a spectacular news item. Flash floods cannot be predicted with any reasonable lead time. Only, a warning can be issued once a flash flood is
imminent or in progress
Flash floods cannot be predicted with any reasonable lead time. Only, a warning can be issued once a flash flood is imminent or in progress. A simple and inexpensive warning system based on dissemination by mobile telephone has been developed in
Thailand
the floods are ranked according to
magnitude
the floods are ranked according to magnitude, the largest flood is assigned a rank
1
The flood statistics are estimated graphically by
plotting the logarithm of discharge for each flood in the annual series against the fraction of floods greater than or equal to that flood
- The flood statistics are estimated graphically by plotting the logarithm of discharge for each flood in the annual series against the fraction of floods greater than or equal to that flood; this fraction is given by
r/(n+1)
- The flood statistics are estimated graphically by plotting the logarithm of discharge for each flood in the annual series against the fraction of floods greater than or equal to that flood; this fraction is given by
r/(n+1),
where r is …………………………………. and n is ………………………………………
r is the rank of the particular flood and n is the number of observations (years)
The return period:
the average span of time between any flood and one equaling or exceeding it
- The return period, the average span of time between any flood and one equaling or exceeding it, is calculated as
Treturn = 1/(exceedance probability)
The 100 years flood can then be estimated from
the graph
Normal distribution works often well with
precipitation data and peak discharge
Problems: not deterministic, based usually on
non-adequate data, climate and terrestrial environment is variable
One of the things we want to know most about rivers is “what’s the probability that a flood of size x will happen this year? In 100 years?”
There are two ways to do this
empirically, and parametrically
There are two ways to do this—empirically, and parametrically. First, empiricism
Let’s take a bunch of data. For now, we’ll take flood data—the maximum flood for each year for some number of years
To plot this data empirically, we need
to order these according to rank. That is, the highest flow comes first, and then the next highest, on down
Incidentally, you can get Excel to do this for you. Select the data, then go to Data|Sort, and it will order all the data! From here, use the formula
Incidentally, you can get Excel to do this for you. Select the data, then go to Data|Sort, and it will order all the data! From here, use the formula:
where T, n , m represent
Where T is the recurrence interval, n is the number of years in the record, and m is the rank
All that’s left is to plot …………………. on the horizontal axis and ……………………….on the vertical
T
Flow
What is actually meant by “100-year flood,” by the way, is that it has a 1% chance of happening
every year
. Here’s a nifty formula for determining frequency or probability rather than recurrence
PMP stands for
Probable Maximum Precipitation
PMP is defined as
the theoretically greatest depth of precipitation for a given duration that is physically possible over a particular drainage area at a certain time of year
PMP is defined as “the theoretically greatest depth of precipitation for a given duration that is physically possible over a particular drainage area at a certain time of year,”
In consideration of our
limited knowledge of the complicated processes and interrelationships in storms
In consideration of our limited knowledge of the complicated processes and interrelationships in storms, PMP values are identified as
estimates
Another definition of PMP more operational in concept is
the steps followed by hydrometeorologists in arriving at the answers supplied to engineers for hydrological design purposes
Another definition of PMP more operational in concept is “the steps followed by hydrometeorologists in arriving at the answers supplied to engineers for hydrological design purposes”. This definition leads to answers deemed adequate by
competent meteorologists and engineers and judged as meeting the requirements of a design criterion
PMP is the estimated limiting value of
precipitation
PMP is defined as the estimated
greatest depth of the precipitation for a given duration that is possible physically and reasonably characteristic over a particular geographic region at a certain time of year.
PMP cannot be exactly estimated as its probability of occurrence is
not known
PMP is useful in
operational applications (e.g., design of large dams)
Any allowance should not be made in the estimation of PMP for
long term climate change
Any allowance should not be made in the estimation of PMP for long term climate change. ……………….Probable Maximum Precipitation (PMP)
43%
PMS involves
temporal distribution of rainfall
……………………………………….. is required to develop hyetograph of a PMS
Spacial and temporal distribution of rainfall
PMS values are given as
maximum accumulated depths for any specified duration
PMS values are given as maximum accumulated depths for any specified duration. For example, given depths for 4h, 8h,…24h represent the
total depth for each duration & not the time sequence in which precipitation occurs
PMS values are given as maximum accumulated depths for any specified duration. For example, given depths for 4h, 8h,…24h represent the total depth for each duration & not the time sequence in which precipitation occurs. ……………. Probable Maximum Storm (PMS)
45%
PMS stands for
probable maximum storm
PMF stands for
probable maximum flood
The PMF is the
largest flood that could conceivably occur at a particular location, usually estimated from probable maximum precipitation, and where applicable, snow melt, coupled with the worst flood producing catchment conditions
The PMF is the largest flood that could conceivably occur at a particular location, usually estimated from probable maximum precipitation, and where applicable, snow melt, coupled with the worst flood producing catchment conditions. Generally, it is not physically or economically possible to provide complete
protection against this event
The PMF defines the extent of
flood prone land, that is, the floodplain
The PMF defines the extent of flood prone land, that is, the floodplain. The extent, nature and potential consequences of flooding associated with
a range of events rarer than the flood used for designing mitigation works and controlling development, up to and including the PMF event should be addressed in a floodplain risk management study
PMF is the greatest flood expected considering
complete coincidence of all factors that produce the heaviest rainfall and maximum runoff.
PMF is derived from
PMP
PMF is used only for
selected designs in view of economy
PMF is used only for selected designs in view of economy For example,
large spillways whose failure could lead to excessive damage and loss of life
A realistic approach is to scale downwards by certain percentage depending on ………………………………………… .
………..% Probable Maximum Flood (PMF)
type of structure and the hazard if it fails
46%
PMF is also termed as
Standard Project Flood (SPF)
SPF is estimated from
Standard project storm (rainfall-runoff modeling).
SPF is estimated from Standard project storm (rainfall-runoff modeling). ……………….. % Probable Maximum Flood (PMF)
47%
Water quality refers to
the chemical, physical, biological, and radiological characteristics of water
Water quality refers to the chemical, physical, biological, and radiological characteristics of water. It is a measure of
the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose
Water quality refers to the chemical, physical, biological, and radiological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of
standards against which compliance can be assessed
Water quality refers to the chemical, physical, biological, and radiological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which compliance can be assessed. The most common standards used to assess water quality relate to
to health of ecosystems, safety of human contact, and drinking water.
