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Flood and Drought Management
Lecture 3 Flood Management
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Flood Management Nature of Flood
Damage
The extent of flood damage
Physical
Economical
Social
Environmental
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Flood Management Damages due
to Flooding
Every year, nearly 196 million individualsfrom over 90 countries are exposed to a
calamitous flood event. In sum, between1980 and 2000 over 170,000 deaths wereattributable to major flooding. Over time,the techniques to minimize detrimental
effects and amplify some of the benefits, hasgrown and become more complex (Andreaand Karthick. 2004)
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Flood Management - Definition
Flood management is defined as the set of allmeasures , physical or otherwise , that enable
the communities that inhabit the flood plain
to live in harmony with the extreme natural
events, minimizing undue hardship to the
extent practicable.
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Flood Management Building Blocks
Flood management includes :
Preventive,
Engineering,
Social,
Economic, and
Administrative measures
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Flood Management core philosophy
An adjustment of the river flow to suit mansconvenience
An adjustment of mans activities to suit therivers convenience.
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Flood Management - percepts
Reducing the scale of floods (i.e. better catchmentmanagement, controlling runoff, detention basins,dams, protecting wetlands);
Isolating the threat of floods (i.e. flood embankments,flood proofing, limiting floodplain development);
Increasing peoples coping capacity (emergencyplanning, forecasting, warning, evacuation,compensation, insurance)
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Flood Management
A number of conceptualization of flood managementmeasures cited in literature : Arey and Bauman(1971) ,Kate(1962), White(18),Yevjevich ( 1974), etc
Yevjevich ( 1974) Flood prevention
Flood prediction
Flood proofing
Physical control
Insurance
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Flood Prevention
Defined as those designed to prevent or mitigatea flood hazard by controlling its causes.
Causal factors : Primary
The natural process ( rainfall, snow melt, breach ofimpoundments)
Secondary Man made developments in the watershed which tamper
with the natural process
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Flood Prediction
The measures of flood prediction depends asthose techniques that allow the competent
authority to forecast the intensity and durationof a flood event.
Important questions
How accurate are the forecasts and how far inadvance can they be made ? and
What is the economic worth of the forecasts ?
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Flood Prediction
The flood forecast or flood warning alerts thecommunity to prepare for an eventual flood
defense
Prediction can be considered as an input to
warning systems
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Flood Plain Proofing
Is defined as those measures designed to minimize
damage to life and property in the event of inundation
, whether by calculated risk or by the failure of some
other means to perform as expected.
Measures
Land use planning
Land elevation Flood proofing of buildings
Community awareness of the extent of flooding
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Physical Control
Physical control measures are defined as thosemeasures by which man engineers thecharacteristics of the watershed and watercourse inorder to prevent the flood event from causing
damage to life and property.
Can be
intensive or
Those of structural type by which the flood waters are confinedto the main channel or otherwise channeled so that they do notrun uncontrolled
Extensive
Relate to the practices in the overall basin, such as soil
conservation measures and control of vegetative cover
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Flood Insurance
Is a means of spreading the individual losses
due to flood among the wider sector of thesociety.
The public disaster approach
Public flood insurance
Mixed public private flood insurance
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Flood Management - Methods
Structural measures
which are engineering measures to minimize the risk
of a water course overtopping its bank
Non- structural measures are varied in nature and are aimed at minimizing the
social and economic loss to individuals in the eventthat the watercourse does overtop its bank and runsuncontrolled over the flood plain..
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Structural Measures
Physical measures
Types
Intensive ( in the river channel)
Extensive ( in the watershed)
Intensive measures are of three types :
those that accelerate the flow
those that retard the flow, and
those that divert the flow
Include : floodways and diversion channels
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Structural Measures
Extensive measures
Are those that aim at modifying the rainfall runoffrelation for the watershed.
Changing the rate of overland flow
Controlling sediment yield
Snow melt management
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Extensive Measures Source Control
Measures
The assessment of the likely effectiveness of sourcecontrol also considers pre-flood conditions such as thestate of saturation of the soil, and whether or not theground is frozen
Thus, a potential drawback with some forms of sourcecontrol, and other forms of land-use modification suchas afforestation, is that the capacity to absorb or storerainfall depends on the antecedent
conditions of the catchment.
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Extensive Measure Vegetative
Control
Its beneficial effect lies in decreasing the rate of
overland flow, thus hindering runoff and soil erosion.
Effect dependent on the type and nature of the land
cover.
Is a watershed scale measure and might turnout to be
an expensive undertaking.
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Hydrologic Cycle
Runoff, streamflow,
Q(t)
Precipitation, P(t)
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Extensive Measure Soil Loss Control
Aimed primarily at minimizing soil erosion
from the watershed.
Erosion control and soil control at the
watershed level are intimately connected.
Categories of soil control measures
Ve etative
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Intensive Measure Dikes and Levees
Levees are small earthen dams placed on thefloodplain at a certain distance from the banks
of a stream to serve as artificial banks during
flood periods when the stream overflows itsnatural banks.
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Intensive Measure Dikes and Levees
Hydraulic effects of confining flood waters of a
river in between levees are : To increase the rate at which the flood event
travels downstream,
To increase the river stage for a particular floodevent,
To increase the velocity and the scouring potential
through the leveed section.
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Intensive Measure Dikes and Levees
Provide the most direct means of flood
protection.
Can be constructed where needed, affording high
degree of protection to a portion of the flood
plain.
Draw backs :
The increase in flood stages brought upon by the
construction of a levee system
The uncertainty in the calculation of the maximum
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Intensive Measure Channel
Improvement
The purpose of channel improvement is to
increase the discharge of a stream in order to
enable the flood waters to flow off faster and
thus decrease the flood stages and reduce the
frequency of flood damage.
Methods for increasing discharge :
Increase the size of the channel
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Intensive Measure Channel
Improvement
Increasing the size of the channel :
Widen the river channel Economic factors severely limit its use as a
management measure.
Increasing the velocity Decreasing the roughness of the channel;
Increasing the hydraulic radius, and
Increasing the slope of the water surface
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Intensive Measure Channel
Improvement
Decreasing roughness
Channel clearing
Increasing the hydraulic radius :
Can be increased by dredging the channel bed.
Building levees
Slope modification
Straightening the channel alignment
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Intensive Measures - Reservoirs
Flood control reservoirs provide for temporary
storage of the flood waters , limiting the flow
downstream to a quantity which the channel
can safely carry, thereby preventing floods.
Philosophy :
Dampen the peak of the flood by artificial storage
and release after the flood event has subsided.
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Intensive Measures - Reservoirs
Storage has to be used in an appropriate
combination with other structural and non-
structural measures.
Releases from reservoirs can create risks, and
the careful operation of reservoirs can
minimize the loss of human life and property
due to such releases. In this context
transboundary cooperation is indispensable.
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Intensive Measures - Reservoirs
The storage capacity is dictated by the
difference between the rate of inflow and themaximum rate of outflow below the dam.
Reservoirs
Regulated type (storage reservoirs)
Unregulated type( detention reservoirs)
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Intensive Measures - Reservoirs
Regulated type
Outlet is equipped with power/manually/automatically operated gates
Controlled release of water and excess water is
stored until the inflow flood recedes.
Detention reservoir
Has fixed outlet works.
Sufficient size to pass freely all normal flow.
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Blue Marsh Reservoir near Reading, PA/Photo By
U.S. Army Corps of Engineers, Philadelphia District
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Intensive Measures Floodways and
Channel Diversions
Floodways and channel diversions are
designed to alleviate the passage of flood
wave through the main channel , by
providing for the release of some of the
excess water to be carried by improving
natural channels or artificial channels
especially constructed for this purpose.
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Intensive Measures Floodways and
Channel Diversions
Flood ways :
Water returns to the channel
Channel diversion
Taking flood waters out of the main channel and
conveying them into inland lakes, off-channelreservoirs , or directly into the lake/sea.
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A comparison of the non
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Non Structural Measures
Flood proofing
Emergency measures
Land use regulation
Loss Bearing
Public relief
Flood insurance
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Flood Proofing
Refers to sets of measures designed to reduce
the damage to buildings located in the
floodplain, in the event of inundation by flood
waters.
Three classes : Permanent
Contingency, and
Emergency
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Flood Proofing
Permanent measures are those that provide
protection against the flood for which they
were designed, independent of any humanjudgment, forecast or action.
Contingency measures are those that become
effective after the receipt of a warning orforecast and involve human action to some
degree.
Emergency measures are either improved in
the event of a flood or carried out accordin
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Flood Proofing Measures - Permanent
Seepage control
Sewer adjustments
Permanente closure
Openings protected
Interiors protected
Elevation
Timber Treatment
Deliberate Flooding
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Flood Proofing Emergency Measures
Refer to all those actions by individuals and
the community , whether preplanned or
contingent upon the onset of a flood,
designed to avert damage to life and property,
ad minimize material losses.
Types
Removal,
Flood fighting, and
Rescheduling
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Flood Proofing Emergency Measures
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Emergency Measures - Rescheduling
Comprises shifts in public service and other
production schedules which aim at
maintaining service during the flood.
Some measures
Rail and highway traffic
Supplying bottled gas for hospitals and other key
institutions
Emergency repair of eroding highway beds.
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Land Use Regulation
Land use management programs cannot prevent
all flooding or reduce all flood losses.
Reduce flood losses to acceptable levels however,
by altering the course of flood waters and the
type of land use patterns.
Flood damage are related to both the flood
potential at a site and the nature of land use.
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Land Use Regulation
Some general classes of land use are moresusceptible to flood damage than others.Damage to forestry , agricultural and openspaces is less than to residential or commercialuses. Damage can effectively be reduced if
open spaces.
Land use planning
Land use regulation must be used incombination wit other measures to achieve
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Land Use Regulation
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Loss Bearing
Los bearing refers to a series of adjustments
through which a community rationalizes its
occupancy of the flood plain.
The dwellers of the flood plain bear the lossesindividually.
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Public Relief
Refers to the economic measures taken by local,
state and federal governments to alleviate the
financial hardship of flood plain occupants that
have been subjected to a flood.
This is an alternative and a substitute for
individual losses, where upon the general public
shares in the losses of the les fortunate
communities.
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Flood Insurance
It entails the mechanism by which floodplain
occupants purchase institutional protection
against undue damage, and become the
beneficiaries of private or public insurance in
the event of economic loss due to floods
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Flood Management
Traditional flood management interventionsare listed below:
Source control to reduce runoff (permeable
pavements, afforestation, artificial recharge); Storage of runoff (wetlands, detention basins,
reservoirs);
Capacity enhancement of rivers (bypass channels,
channel deepening or widening);
Separation of rivers and populations (landuse
control, dikes, flood proofing, zoning, house
raising);
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Flood Management
Emergency management during floods (flood
warnings, emergency works to raise or strengthendikes, flood proofing, evacuation); and
Flood recovery (counseling, compensation or
insurance).
Flood Management Source Control
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Flood Management Source Control
Measures
Source controls intervene in the process of the
formation of runoff from rainfall or snowmelt,
and take the form of storage in the soil or viathe soil.
The use of this strategy normally considers theconsequential effects on the erosion process,the time of concentration in the soil and the