chennai floods

CHENNAI FLOODS

- N.Karthick, Head, Dept of Mechanical Engg, Veltech Hightech Engg College.

Prevention is better than Cure

A Presentation on Causes and Prevention

Reference: http://www.hindustantimes.com/analysis/chennai-floods-present-a-lesson-in-urban-planning/story-QOQlkG76xthZcNSqmQ5icL.html

Introduction A flood is usually caused by rain, heavy thunderstorms, and thawing of snow. Its considered to be a temporary condition of two or more acres of dry land

either:• Overflowed with inland or tidal waters• Rapid or runoff of surface waters• Mudflows

CAUSES:• PRECIPITATION • INADEQUATE CAPACITY (WITHIN BANKS)• BANK EROSION AND SILTING• LAND SLIDES• TIDAL AND BACK WATER EFFECTS• POOR DRAINAGE• SNOW MELT AND GLACIAL OUT BURSTS

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 occur over 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.

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

Where does flooding occur ?

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

They are most likely to happen in tropical areas and tsunamis.

Most common floods happen around the world’s largest/greatest rivers.

Believe it or not, smaller rivers could cause more damage even though people don’t pay as much attention to them.

Most Flooding occurs during the beginning of spring.

Types of floods:-

1. Flash Floods 2. River Floods3. Coastal Floods4. Lakeshore Floods5. Urban and Ice Jams

FLOOD MANAGEMENT

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 responses – societies adopt different coping strategies

Flood Protection

1) Modification to channel or banks:

Bank raising and dredging both increase bank full capacity

By increasing the hydraulic radius, channels also become more efficient (velocity increases and so water levels drop)

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

Flood Protection

2. Artificial channel linings :-

• Concrete lined channels create smoother wetted perimeter and so increase velocity

• Thus water levels drop and flood risk is reduced

• Expensive, and high maintenance• E.g. Los Angeles

Flood Protection3. Dam construction :-

• Multi-purpose, but key tool for flood protection• Controlled release of water stored in reservoir

through sluice gates can spread discharge over a longer period (reducing peak flows)

• Effectiveness depends on relative scale of reservoir’s catchment area to that of the whole drainage basin

• Geo-politics can cause problems – e.g. India and Bangladesh (Ganges), Spain and Portugal (Tagus), Zimbabwe and Mozambique (Limpopo and Floods of 2000)

Flood Protection4. Flood relief channels:

• It effectively increases bank full capacity and diverts flow away from high impact zones

• It requires there to be space on floodplains to skirt around high impact zones, so not always possible

Flood Protection5. Spreading grounds:

• Diverting flood water to low impact flood plain zones, for storage

• Reduces downstream peak flows• Low impact zones can be recreational land use• Flood water will evaporate or eventually

infiltrate, replenishing groundwater supplies

Flood Protection6. 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

Flood Protection7. 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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