Lecture 3 Flood Management

7/30/2019 Lecture 3 Flood Management

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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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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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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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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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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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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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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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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

Lecture 3 Flood Management

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