1A.3 Sustainable Management Of Rivers

Question 1

Flooding

Answer 1

A temporary covering by water of land which is normally dry.

Question 2

3 physical causes of flooding

Answer 2

▫️Precipitation - flooding occurs when there are periods of heavy rainfall over a few days. The more water that falls onto a drainage basin over a short period of time, the more rapidly the soil will become saturated and the more surface sun off there will be into the river. Flooding can also happen following periods of dry weather and drought. In the summer, the soil can become baked by the sun, making it hard and rainwater can struggle to infiltrate into the soil. The water washes over the surface as surface runoff and creates a flash flood which is where the water levels in the river rise very quickly following a rain event.
▫️ Soil and underlying rock - different types of soil will influence how quickly or slowly rainwater will infiltrate into the soil. Sandy soils are permeable and allow water to pass through them quickly, so this reduces the flood threat. However clay soils are less permeable so increase rapid surface runoff to the river and may lead to flooding.
▫️ Land use/ vegetation - any drainage basin that has very little vegetation will be more likely to experience flooding that areas with much vegetation and forests. This is because trees intercept water through their leaves and root system and water stored in this way will reduce the amount of water reaching the river system.

Question 3

3 human causes of flooding

Answer 3

▫️ Deforestation - the removal of vegetation causes an increase in surface runoff therefore increases the risk of flooding. The removal of trees and roots (which hold the soil structure together) increases the risk of soil erosion.
▫️ Urban growth - the world is urbanising at a rapid rate and farmland is being converted into urban/concrete jungle. This means less water is stored within the drainage basin. It also means water falling onto buildings and roads is drained very quickly (due to less infiltration and through flow) which rapidly returns the excess water to rivers, increasing surface runoff.
▫️ Global warming - more water is released from ice stores (glaciers and polar ice caps) due to global warming. Therefore more rain is available to the drainage basin. This means the river has to cope with more water than in the past.

Question 4

2 POSITIVE impacts of flooding on PEOPLE

Answer 4

▫️ It replenishes drinking water supplies, especially wells which may be the only sources of drinking water for some communities, especially in LEDCs such as Bangladesh.
▫️ Flooding flushes out water snails and parasites that cause harmful diseases such as bilharzia in LEDCs.

Question 5

2 NEGATIVE impacts of flooding on PEOPLE

Answer 5

▫️ Loss of life - people may drown in the floodwaters, especially if they can’t swim or warning systems are poor.
▫️ Property - homes and businesses may be destroyed as floodwaters damage premises.

Question 6

2 POSITIVE impacts of flooding on the ENVIRONMENT

Answer 6

▫️ Fish benefit as they can breed in the standing floodwater.
▫️ In dry areas, floods bring relief from drought, providing drinking water for wild animals.

Question 7

2 NEGATIVE impacts of flooding in the ENVIRONMENT

Answer 7

▫️ Pollution - flooding can wash chemicals or sewage into the local rivers and so pollute them, killing fish species.
▫️ Wildlife - habitats on the river banks can be destroyed and animals may drown in a flooded floodplain.

Question 8

Soft engineering strategies

Answer 8

Strategies that do not involve major river alterations. Instead, they are sympathetic to the natural landscape, manage rather than prevent flooding, making them more sustainable measures.

Question 9

Hard engineering strategies

Answer 9

Strategies that involve making large man-made structures to control the river, or require major alterations to the river channel. These measures are generally not sustainable in the long term.

Question 10

Dam

Answer 10

A concrete barrier built across a river channel or gorge to control river flow.
+ When built in the upper course, it reduces the flood risk downstream because it controls the discharge by holding the river back
+ The resulting reservoir can be used for leisure and HEP (Hydro-Energy Production)
- Extremely expensive to build
- Will flood good farmland, displacing local people and destroying habitats for animals

Question 11

Levee

Answer 11

A concrete or raised ridge running along the river bank.
+ Increases the capacity of the river channel by raising the banks, therefore prevents flooding
+ When constructed, they don’t have too much disruptive impact on the river itself
- Expensive to build
- Unsightly as they are made of concrete
- Can speed up water flow in the river (due to less friction), so flooding can be worse
- Can prevent silt being deposited on the floodplain

Question 12

Flood wall/ embankments

Answer 12

Normally concrete walls built beside river channels to protect settlements and important factories or roads from flooding.
+ Increases the capacity of the river channel by raising the banks, therefore prevents flooding
+ When constructed, they don’t have too much disruptive impact on the river itself
- Expensive to build
- Unsightly as they are usually made of concrete
- Can speed up water flow in the river (due to less friction), so flooding can be worse
- Can prevent silt being deposited on the floodplain

Question 13

River channel straightening

Answer 13

+ When meanders are removed, water flows faster in a straighter channel. This lessons the flood risk in some areas by allowing the excess water to leave the section of the river more quickly. This is more preferable for farmers and property developers.

• Floodwaters may simply be carried downstream to cause problems there
• The removal of river meanders destroys natural habitats for fish to lay eggs

Question 14

Deepening the river bed (dredging)

Answer 14

+ Increases the capacity of the river channel by making it deeper and wider as sediment is removed. This reduces the flood risk as more more water can be held.
+ The appearance of the river is not changed drastically and usually continues to protect natural habitats
- Needs to be repeated regularly to remove the new buildup of silt
- The use of heavy machinery can disrupt bank habitats
- May speed up river flow which carries water downstream, making flooding worse there

Question 15

Storage areas

Answer 15

Lakes into which the river’s water can be pumped and temporarily stored.
+ Reduces the amount of water in the river at peak times, lessening the threat of flooding
+ Water can be pumped back to the river when needed
- Requires a large area of spare land to capture excess water

Question 16

Washlands

Answer 16

Parts of the river floodplain in the lower course, into which the river can flood temporarily.
+ Usually used for sports pitches or nature reserves so that there is not major threat to property
- It can be difficult to find enough spare land, especially close to cities

Question 17

Land use zoning

Answer 17

Floodplain land has different building controls depending on how much flood risk there is.
+ Land beside the river cannot be built on and may only be used for grazing animals. The next zone may be used for other farming activities. Zones furthest away may be used for residential purposes and other urban activities. This protects more valuable property from flooding while exposing less valuable land to potential floods.
+ Cheap and easy to carry out
- Unrealistic to apply land use zoning to areas that have already been built up as people would need to be relocated

Question 18

Afforestation

Answer 18

Covering part of the drainage basin (usually the upper course) with trees which will intercept precipitation and trap more water through their roots.
+ Reduces surface runoff into the river and delays processes such as throughflow, therefore lowering the flood risk downstream
+ New woodlands can provide animal habitats and a source of timber
- Requires a large area of spare land with the land owners’ permission
- Takes tens of years for the trees to mature and be effective at trapping water