5. River Flooding and Management Issues

Question 0

Name the four main factors that encourage flooding:

Answer 0

• A steep-sided channel - a river channel surrounded by steep slopes causes fast surface run-off.
• A lack of vegetation or woodland - trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.
• A drainage basin, consisting of mainly impermeable (not allowing water to pass through) rock - this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
• A drainage basin in an urban area - these consist largely of impermeable (not allowing water to pass through) concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.

Question 1

What is Surface Run-off?

Answer 1

The water that runs over the surface of the land when the soil is unable to absorb it.

Question 2

What is a positive impact of flooding?

Answer 2

Flooding deposits alluvium onto the floodplain, making it very fertile and excellent for agriculture.

Question 3

How does constructing dams help a river?

Answer 3

Dams are often built along the course of a river in order to control the amount of discharge. Water is held back by the dam and released in a controlled way. This controls flooding.

Question 4

Name some river hard-engineering techniques:

Answer 4

• The river channel may be widened or deepened allowing it to carry more water.
• A river channel may be straightened so that water can travel faster along the course.
• The channel course of the river can be altered, diverting floodwaters away from settlements.

Question 5

What is Afforestation and how does it decrease flooding?

Answer 5

Afforestation is when trees are planted near to the river. This means greater interception of rainwater and lower river discharge.

This is a relatively low cost option, which enhances the environmental quality of the drainage basin.

Question 6

What is Managed Flooding?

Answer 6

Managed flooding is when the river is allowed to flood naturally in some places, to prevent flooding in other areas (towns, cities etc.).