Floods

How can cities prepare for floods?

Types of floods

1. Coastal floods Storm surgehttps://www.youtube.com/watch?v=pNh-_SXdUgA

Tsunamihttps://www.youtube.com/watch?v=gmlixFM-Bn4&list=PLNspHwmzscYE2-cUkOfHo92ATHZUqqsPH&index=1&feature=iv&src_vid=j0YOXVlPUu4&annotation_id=annotation_1885845763

The huge waves cause tremendous destruction along coastal areas

2. River floods

Mississippi River

Normal river

channel

River overflows during 1997 flood

3. Flash floods

https://www.youtube.com/watch?v=vjVBbTuMXMg(till 3 mins)

Characteristics:Short durationHeavy rainQuick rise in water

Causes of floods1. Rainfall

Rainfall intensity (p 159) Rainfall duration Rainfall frequency

Tropical Storms – high intensity, long duration rainfalleg Hurricane Sandy

Causes of floods…2. Snowmelt

in temperate countries during springheat from volcanic eruptions

3. Storm surgeStrong winds during storms create huge waves

4. Failure of man-made structureseg levees, dams, floodwalls

Which cities are prone to floods?

Coastal citiesLocated on flat land / low reliefNear rivers and lakes

Read p 164 -165 and the identify reasons for flooding1. Calgary, Canada

In temperate area At foot of mountains

2. New Orleans, US In path of hurricane Coastal city Flat relief

3. Brazil Heavy rain Near river / dam (river bank and dam burst)

4. Newport, UK Coastal area Heavy rain Strong winds – possible storm surge

5. Dhaka, Bangladesh Path of seasonal wind ( receive heavy seasonal rain) Flat relief

Factors affecting risk of flooding

1) Relief (height above sea level)

2) Proximity to water bodies (sea, lakes, rivers)

3) Permeability of surface cover (ease of water seeping into the ground)

Natural vegetation in cities replaced with concrete Permeability decreases Causes greater surface runoff – resulting in floods

Factors affecting risk of flooding

4) Channel capacity (volume of water held in river channel without overflowing)

Cities have high surface runoff Network of canals to drain away reduce flood risk Wide, deep and well-maintained (dredged regularly) canals effective in reducing flood

riskIdentify 3 strategies to protect Singapore against floods. (see p. 170)

Widen and deepen Alexandra and Rochor Canals Construction of new canal to divert water from upstream end of Stamford Canal Install flood-forecasting system in Marina Catchment area

5) In path of storms, monsoon winds Some cities are located in the path of storms (cyclones, hurricanes, typhoons)–

eg Dhaka in Bangladesh, Manila in the Philippines Or in the path of seasonal winds, Kuantan in east coast of Malaysia Hence they regularly receive heavy rain at certain times of the year, increasing

the risk of flooding

Suggest reasons why New Orleans is flood prone

•Low relief

•Near water bodies (lake & river)

•Along hurricane path

How do floods affect people in cities?1. Social Impacts

a. Injuries and loss of lifeb. Spread of diseasesc. Homelessnessd. Disruption of clean water supply

2. Economic Impactsa. Damage to machinery and equipmentb. Damage to transport infrastructurec. Disruption of energy supply

Flood Mitigation Strategies 1. Regulation (law enforced by authorities)

a. Zoning How the government plans the physical development of the

land and its uses Flood prone areas are zoned as low-value developments and

used as parks and fields for recreation These low-value developments bring in less income, hence the

city will suffer less economic damage in times of floods Residential areas are zoned away from flood prone areas,

hence keeping homes safe from floods Advantage: economic and social damage caused by flood is

reduced Disadvantage:

less land available for economic use in the city In some cities, it may not be easy to enforce the regulations

Flood Mitigation Strategies….b. Elevated properties• Raise properties to a certain height • Minimum platform level (MPL) is 600 mm in

flood prone areas and 300 mm in other areas in Singapore

• MPL is imposed on existing and new buildings in Singapore

• Advantages: properties are protected against floods up to the MPL level

• Problems: increase cost of buildings

Flood Mitigation Strategies….

2. Investment in infrastructure (build and maintain)

a. Levees and floodwalls • Levees – raised river banks, made of mud or concrete• Floodwalls – made of concrete• Both are built higher than the floodplains to reduce

flooding • Advantages Cheaper than raising height of surroundingareas• ProblemsNeed for regular maintainance to ensure no cracks or vandalism.Unable to prevent floods above the level of the structure.

Flood Mitigation Strategies….b. Channel improvement• Widen and deepen channels to increase channel capacity• Straighten channels to increase water flow• Clear channel of rubbish and siltAdvantage:

Land next to rivers can be used for moderate and high-value developments, helping cities maximise the limited land in cities

Problem: It may cause flooding downstream if channel downstream is not

enlarged.

Flood Mitigation Strategies….3. Disaster preparedness (reduce harm caused by floods)

a. Forecasting and warning systems Cities monitor (collect data and analyse information on) :

water levels in canals and rivers Tide levels in seas Weather conditions

Monitoring done daily Data used to identify flood prone areas Flood warnings may be issued if necessary PUB has 170 water level sensors in various parts of Singapore There are also hundreds of mini-weather stations in Singapore All data is stored automatically in Central Data Depository Advantage: allows people time to prepared to reduce damage from

floods Problems: not all cities have the expertise and technology to collect

accurate data, especially cities in Developing Countries

Flood Mitigation Strategies….b. Evacuation drills People practise the steps to leave a place in a safe and

orderly manner during an emergency They are led to safe locations by trained personnel and

practise first aid This creates awareness in people It will reduce panic during a flood PUB has a public advisory on what to do during a flood Advantages: people are educated on what to do during

an emergency. In this way it can save lives and reduce damage to properties during a flood.

Such drills also allow cities to improve their flood evacuation plans

Problems: not all may participate, reducing the effectiveness of the drill

The End

Source Acknowledgment:

• http://i.telegraph.co.uk/multimedia/archive/02647/flood-philippines-_2647447k.jpg • http://img.timeinc.net/time/photoessays/2009/manila_flood/flood_01.jpg • http://4.bp.blogspot.com/_OBqPxn5JL7Y/TBjjE8Ei8gI/AAAAAAAAAqU/pWVcwNNaYcI/s1600/sgflood2.jpg• http://proceedings.esri.com/library/userconf/proc02/pap0490/P04901.JPG • http://workwithdrmarcus.com/wp-content/uploads/2011/08/dam-bursting-300x225.jpg• http://a.fastcompany.net/multisite_files/coexist/imagecache/inline-large/inline/2013/01/1681298-inline-

water-map-3.jpg

• http://media.washingtonpost.com/wp-dyn/content/graphic/2005/08/29/GR2005082900046.jpg • http://titanmaster.com/images/Dredge03.jpg