General Rewision (Flooding)

Question 1

List out the factors causing flooding.

Answer 1

✮ Rainfall
✮ Snowmelt
✮ Storm surge
✮ Failure of man-made structures

Question 2

How does rainfall cause flooding?

Answer 2

✮ Condensation of atmospheric water vapour leads to the formation of water droplets, which form clouds.
✮ If the water droplets become too heavy, they fall as rain.
✮ The following three factors lead to high rainfall, which hence results in flooding:
✮ Rainfall intensity: This refers to the rate at which rain falls. A shower has a low rainfall intensity while a thunderstorm has a high rainfall intensity.
✮ Rainfall duration: This refers to the duration of the rain event.
✮ Rainfall frequency: This refers to the number of times a certain amount of rain falls within a period of time.
✮ Tropical storms have high intensities and long durations.
✮ They bring heavy rain and strong winds which can develop into tropical cyclones.

Question 3

How does snowmelt cause flooding?

Answer 3

✮ It is surface runoff that results from melting snow.
✮ During winter, ice and snow form on the ground and on the mountains. When spring arrives and temperature rises, the ice and snow melts.
✮ Volcanic eruption can also cause snowmelt.
✮ It increases the amount of water in the river and causes the river to overflow its banks and flood surrounding areas.

Question 4

How does storm surge cause flooding?

Answer 4

✮ A storm surge is a sudden rise of the sea level in which water is piled up against a coastline beyond normal high-tide conditions.
✮ During a storm, strong winds push water towards the coast and create huge waves, giving rise to a storm surge

Question 5

How does failure of man-made structures cause flooding?

Answer 5

✮ Dams are barriers built across the river to retain and store water. Besides generating hydropower, they help to regulate the amount of water flowing in the river.
✮ Dam failure will be disastrous to the people living downstream as it may cause massive floods.
✮ The failure of levees and floodwalls also has negative impacts on cities

Question 6

List out the factors that increase chances of flooding.

Answer 6

✮ Height of land above sea level
✮ Proximity to water bodies
✮ Permeability of surface cover
✮ Channel capacity

Question 7

How does height of land above sea level determine the likelihood that a place will flood?

Answer 7

✮ Sea level refers to mean sea level which is the average height of the sea surface at a given site.
✮ Cities located on low-lying land, especially those that are below sea level, are more prone to floods.

Question 8

How does proximity to water bodies determine the likelihood that a place will flood?

Answer 8

✮ Water bodies, such as oceans, seas, lakes, rivers and reservoirs, are significant accumulations of water on the earth’s surface.
✮ Cities which are located nearer to the water bodies, such as coastal cities or cities with a river running through them, are more prone to floods.

Question 9

How does permeability of surface cover determine the likelihood that a place will flood?

Answer 9

✮ Vegetated surfaces are most permeable and allow water to infiltrate into the ground more easily, but concrete surfaces are less permeable
✮ As cities expand, natural vegetation may be removed and replaced with concrete.
✮This leads to high surface runoff, which leads to a sudden accumulation of water in water bodies such as rivers.
✮ Water is unable to flow away quickly so floods are caused.

Question 10

How does channel capacity determine the likelihood that a place will flood?

Answer 10

✮ A city’s ability to drain water away quickly is very important because it has a high proportion of built-up areas
✮The concrete surfaces in the built up areas are impermeable, resulting in large amounts of surface runoff during a rain event
✮ Floods may occur when there is too much rain and the capacity of the channel is exceeded.
✮ Hence, a network of canals needs to be built to cope with removing high surface runoff from the area during rains.
✮ It is important to ensure that the canals are well-maintained to ensure that they can function at their best

Question 11

List out the impacts of flooding.

Answer 11

✮ Injuries, spread of diseases, loss of lives
✮ Homelessness
✮ Disruption to clean water supply
✮ Damage to machinery and equipment
✮ Damage to transport infrastructure
✮ Disruption to energy supply

Question 12

Explain how flooding causes injuries, spread of diseases and loss of lives. (Social)

Answer 12

✮ People may be injured by objects and debris washed along by fast-moving floodwaters.
✮ Floodwaters may cause buildings to collapse and injure/kill people
✮ When people are caught in the flood for a long time, they may suffer from hypothermia with symptoms such as constant shivering, confusion and loss of coordination
✮ Water supplies may be contaminated by chemical spills from factories and sewerage.
✮ People will be exposed to contaminated drinking water and diseases such as cholera and dysentery will spread.
✮ Stagnant pools of floodwater may also encourage mosquitoes to breed, resulting in more cases of malaria and dengue fever
✮ People may suffer from emotional distress due to loss of homes and loved ones.

Question 13

Explain how flooding causes homelessness. (Social)

Answer 13

✮ Floodwaters can cause damage to property and even sweep houses away.
✮ During floods, people may have to evacuate to emergency shelters and stay there for long periods of time while trying to rebuild their houses.
✮ Flooding in cities tend to affect many people due to high population densities.

Question 14

Explain how flooding causes a disruption to clean water supply. (Social)

Answer 14

✮ The supply of clean water may be disrupted due to damaged water pipes.
✮ Sewage pipes may also have burst, causing sewage to flow and contaminate water supplies.

Question 15

Explain how flooding causes damage to machinery and equipment. (Economic)

Answer 15

✮ Drilling machines, cranes and computers can be damaged by floods.
✮ Motors and other electrical components may be damaged after being submerged in water.
✮ As such, factories are unable to operate at full capacity and suffer economic losses. Companies may lose profits and and have to spend money buying new equipment.
✮ People may lose jobs.

Question 16

Explain how flooding causes damage to transport infrastructure. (Economic)

Answer 16

✮ During a flood, transport infrastructure can become impassable because they are submerged or damaged.
✮ The force of the floodwater may be so powerful that it weakens transport infrastructure.
✮ Cities may need to spend lots of time and money to repair the damaged infrastructure.
✮ When roads are submerged/damaged, people are unable to travel to work and tourists are discouraged from visiting the city.
✮ This leads to an overall reduction in productivity, employment and profits.

Question 17

Explain how flooding causes disruption to energy supply. (Economic)

Answer 17

✮ Floods may affect the distribution of energy.
✮ Trucks will not be able to transport gasoline from terminals to gas stations due to flooded road conditions.
✮ The disruption of electricity supply can affect the ability of factories and companies to produce goods and services.
✮ Power lines may be damaged so power outages may occur which could last for weeks.
✮ This may result in a loss of employment and income for individuals and companies.

Question 18

List out the strategies that can be used to mitigate floods.

Answer 18

✮ Zoning
✮ Elevation of properties
✮ Levees and floodwalls
✮ Channel improvement
✮ Forecasting and warning systems
✮ Evacuation drills

Question 19

Explain how zoning helps to reduce the impact of flooding. Evaluate its effectiveness.

Answer 19

✮ It is a way in which the government plans the physical development of the land and its uses.
✮ Zoning laws often specify the areas where residential, industrial, commercial or recreational activities may take place.
✮ A low-value development does not cost much to develop and has activities that bring in less income so they can be built in flood prone areas.
✮ Government would usually zone flood-prone areas as low-value developments (e.g. parks, fields for recreational activities) and higher-value developments on areas which are unlikely to flood
✮ This ensures that cities suffer less economic losses during floods.

👍 Zoning creates green spaces such as parks and fields which help to reduce the impact of flood damage.
👍 Lesser people would suffer a decreased monetary loss
👍 (Cities suffer less economic losses during floods) [Mentioned Abv]
👎 However, it may limit the development of available land because only certain land uses are permitted.
👎 Zoning is difficult to carry out because of competing land uses and people may not abide by the recommended land use.

All in all, in the long run, zoning helps to reduce the social and economic impacts of flooding as buffer zones will be created to cushion the flood. However, zoning may not be possible in certain cities with space constraints.

Question 20

Explain how elevation of properties prevents flooding from taking place. Evaluate its effectiveness.

Answer 20

✮ Government sets a minimum height level for all developers to comply with above a recorded flood level when building properties there
✮ Applies to both new and existing developments
✮ The MPL (Minimum Platform [height] Level) would be revised when there are changes to the the highest recorded flood level
✮ When buildings are raised to a higher height, floods are less likely to reach them.

👍 Developments are protected against floods up to the height of the MPL.
👎 However, it is challenging to implement MPL, especially in countries in which the highest flood level repeatedly increases, because of the increased cost of developers for the private owners

All in all, elevation of property may be beneficial in the short term, but in the long run, if floods become a more serious issue, property will need to be repeatedly raised which may not be cost-effective.

Question 21

Explain how levees and floodwalls prevent flooding from taking place. Evaluate their effectiveness.

Answer 21

✮ Levees are raised river banks which can be natural (made of mud) or man-made.
✮ Floodwalls are man-made structures and made of concrete.
✮ They are built higher than floodplains and rivers to keep out floodwater.

👍 It is cheaper to build levees and floodwalls to mitigate and prevent floods as compared to raising the height of the surrounding land.
👎 However, regular maintenance is required to ensure that levees and floodwalls are strong enough to hold back floodwater.
👎 Levees and floodwalls are designed to hold back floodwater up to a certain height. They are unable to prevent floods beyond that level.

All in all, levees and floodwalls are quite cost-effective, but they need to be continually upgraded to ensure that they are still effective against floods.

Question 22

Explain how channel improvement reduces chances of flooding. Evaluate its effectiveness.

Answer 22

✮ River channels and canals can be widened and deepened so that their capacity increases so they can carry more storm water away.
✮ They can also be straightened so that water can be carried away quickly.
✮ This will help to prevent water in river channels and canals from overflowing.

👍 Channel improvement makes it possible for moderate-value and high-value developments to take place on the land adjacent to river channels and canals.
👎 However, channel improvement may create problems for areas upstream or downstream of the improved area.
👎 There is a limit to the changes that can be made to channels as land next to the channels are already being used.

All in all, in the long run, channel improvement is effective in ensuring that floods never happen but it must be done hand-in-hand with the improvement of the network river channels.

Question 23

Explain how forecasting and warning systems reduce the impact of flooding. Evaluate their effectiveness.

Answer 23

✮ For forecasting to be accurate, there must be monitoring, which is the systematic process of collecting and analysing information to identify changes.
✮ Cities monitor water levels in river channels, canals, tide levels in seas and surrounding weather conditions.
✮ Based on the information collected, flood forecasting can be made to identify potential flood areas affected by a flood event. If the situation is serious, a warning needs to be issued.
✮ Cities can use manual monitoring systems and technology such as remote sensing techniques and geographic information systems (GIS) to make more accurate flood forecasts.
✮ This makes it possible to map the flood-prone areas for close monitoring.
✮ A forecast then can be issued; if the flooding situation is serious, a warning will be issued to affected residents of the area through a flood warning system
✮ Flood warnings can be communicated through radio, social media, television, text messages and sirens.

👍 A forecasting and warning system provides useful information so that there is time for people to take action to minimise flood damage.

This is essential in all flood-prone areas, but it merely reduces the impact of floods without doing anything to prevent them from happening.

Question 24

Explain how evacuation drills reduce the impact of flooding. Evaluate their effectiveness.

Answer 24

✮ Cities have large populations, so during emergencies, there has to be a plan to evacuate a large number of people in the shortest possible time.
✮ Evacuation drills are a form of preparedness measure where people practise the steps to leave a place in a safe and orderly way during an emergency.
✮ The people also have to follow instructions from trained personnel and practise first aid.
✮ They create awareness among the people and reduce panic during a flood event.

👍 People are more educated on what to do when a flood warning is received.
👍 Evacuation drills will enable cities to improve their flood evacuation plans.
👎 However, there may be limited participation in the drill because many of them would have work to attend to in the day → Hinder effectiveness of evacuation drill

All in all, the impact of a potential flood will be greatly lessened but such drills do not do anything to deter floods.