Flood Occurrence and Management

7/27/2019 Flood Occurrence and Management

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Introduction

A flood is usually caused by

rain, heavy thunderstorms,

and thawing of snow.

Its considered to be atemporary condition of two or

more acres of dry land either:

Overflowed with inland or

tidal waters

Rapid or runoff of surfacewaters

Mudflows


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CAUSES OF FLOODS

PRECIPITATION

INADEQUATE CAPACITY (WITHIN

BANKS)

BANK EROSION AND SILTING

LAND SLIDES

TIDAL AND BACK WATER EFFECTS

POOR DRAINAGE

SNOW MELT AND GLACIAL OUT

BURSTS


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How does Flooding Start and

end?

The shore or land by or surrounding a body of

water erodes and this erosion causes waves

currents that result in a flood.

Flood disasters have been increased because

of the expansion of settlements and growth in

floodplains.

Floods could be slow or fast but usually occurover a matter of days.

After the water eventually goes down or dries

up. On coastal floods, low tides and high tides

makes a change in heights.


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Why does it occur ?

A flood is too much water in the wrong place.

Sometimes a flood occurs from :

Sewer (drain) backup Collapse of land along the shore of a lake or

another body of water. This results in waves

or currents during a flood


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Flooding is the most worldwide natural disaster.It occurs in every country and wherever there israinfall or coastal hazards.

They are most likely to happen in tropical areasand tsunamis.

Most common floods happen around the worldslargest/greatest rivers.

Believe it or not, smaller rivers could causemore damage even though people dont pay asmuch attention to them.

Most Flooding occurs during the beginning ofspring.


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Types of floods:-

1. Flash Floods

2. River Floods3. Coastal Floods

4. Lakeshore Floods5. Urban and Ice Jams


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Flash Floods

Flash floods happen in a short

time, they have a great volume

of water, and are local floods.

The runoff of intense rain

results in high flood waves.

Flash floods result in failure of

dams and more.

It usually happens in desert

areas and mountain regions.

They are a threat in steep land,high runoff rates,

thunderstorms, and narrow

streams.

This is a picture of a town

after a Flash Flood


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River Floods

Riverine floods are caused by

melting of snow and precipitation

over large areas. They take place inrivers. Floods in large rivers take

hours to days.

The ground conditions effects the

runoff.


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Coastal Floods

Coastal floods are caused by

tides, storms, tropical cyclones,

or tsunamis.

They happen in the ocean andeffects the general public and

maritime interests along the

coastline.

They are caused by heavy surf,tidal piling, and storm surges,

Other factors are tidal cycles,

behaviors of the storm, river or

stream runoff, no offshore reefsor other barriers, and high

Venezuela in December of 1999.


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Urban Floods and Ice Jams

Urban floods are when the

land is turned from fields or

woods into roads and parking

lots. Since this happens it

cant absorb the rainfall.

During the urban floods all

the streets become rivers and

basements becomefull of

water, they are death traps. Ice Jam is floating ice that

adds up at a man-made or

natural area and stops the

flow of water. This causes the

area to flood.


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Lakeshore Floods

Lakeshore floods affect the

general public as well as some

areas of the Great Lakes.

These floods extend from the

beaches to rivers that flow intolakes. The extent of the flooding is

dependant on surrounding the

shore terrain. The causes of these

lakeshore floods are seiches in the

Great Lakes.

The Webster dictionarys definition

ofSeiches is:

The flow of periodically changing

direction of the surface of a

landlocked body of water (like a

lake) that varies from a few


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FLOOD MANAGEMENT


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Human response to Flooding:

1. Flood protection decreases risk of

bankfull capacity being exceeded

2. Flood abatement reduces stormflow and

reduces peak discharge levels

3. Behavioural responsessocieties adopt

different coping strategies


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Flood Protection

1) Modification to channel or banks:

Bank raising and dredging both increase bank fullcapacity

By increasing the hydraulic radius, channels alsobecome more efficient (velocity increases and sowater levels drop)

Widely used (e.g. Mississippi 3000 kms of raisedlevees up to 15 m high)


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Flood Protection

2. Artificial channel linings :-

Concrete lined channels createsmootherwetted perimeterand soincrease velocity

Thus water levels drop and flood risk is

reduced Expensive, and high maintenance

E.g. Los Angeles


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Flood Protection

3. Dam construction :-

Multi-purpose, but key tool for flood protection

Controlled release of water stored in reservoir throughsluice gates can spread discharge over a longerperiod (reducing peak flows)

Effectiveness depends on relative scaleof reservoirs

catchment area to that of the whole drainage basin Geo-politics can cause problems

e.g. India and Bangladesh (Ganges), Spain andPortugal (Tagus), Zimbabwe and Mozambique(Limpopo and Floods of 2000)


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Flood Protection

4. Flood relief channels:

It effectively increases bank full capacityand diverts flow away from high impactzones

It requires there to be space on floodplains

to skirt around high impact zones, so notalways possible

e.g. River Exe at Exeter, River Thames atWindsor


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Flood Protection

5. Spreading grounds:

Diverting flood water to low impact flood plainzones, for storage

Reduces downstream peak flows

Low impact zones can be recreational land use

Flood water will evaporate or eventually infiltrate,replenishing groundwater supplies

E.g. Los Angeles basin


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Flood Protection

6. Debris dams:

To trap sediment in upper catchments to prevent

downstream bed aggradation Maintains higher bank full capacities

downstream

Periodic need for emptying, but can be used for

construction materials Especially important in semi-arid, mountainous

catchments

E.g. Los Angeles Basin


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Flood Protection

7. Straightening of sinuous

rivers:

Increases gradient increases flow rates

Thus water levels drop and flood risk is

reduced Also reduces deposition and averts bed

aggradation

Also keeps channels navigable


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Behavioural responses

1. Accepting the loss Fatalism often only

option in countries like Haiti or Bangladesh

2. Public relief funds Emergency response

to hazard event requires funding, materials,

technical support, rebuilding. Sources vary

from UN agencies to governments andNGOs

3. Flood insurance A standard response in

flood prone communities in the North


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4. Monitoring and Prediction Data on rainfall

and stream discharge can be used to produce

accurate predictions of the timing of flood

surges Can be used for communities to prepare for

actual flood event or for authorities to organise

evacuations

Not always possibleflash floods have too

short a lag time (e.g. Boscastle, Devon, U.K.,

2004), lack of technical equipment / personnel

(Haiti, 2004), or communication systems


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5. Floodplain zoning

Planning authorities can prohibit certainland-uses in the more flood pronefloodplain zones

6. Flood proofing

Individuals bear responsibility for

reducing likely flood damage to property Techniques: water-proof garden walls,

windows and doors; sandbags; buildings

on stilts; removal of damageable goods


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THANK

YOU

Documents

Flood Occurrence and Management