EQ2 5.5 SURPLUSES IN THE HYDROLOGICAL SYSTEM LEAD TO FLOODING Flashcards
how can agriculture cause flooding
As demand for food increases, so does demand for agricultural land. Increasingly farms are made on deforestation sites, for example in the Amazon.
Agricultural practices increase surface runoff and the amount of soil exposed - leading to soil erosion. Sediment from the erosion is transported to the river.
The river’s water capacity is reduced by the sediment, increasing the likelihood that the river will burst its banks.
how does floodplain drainage had a negative impact
floodplain land is flat and fertile, making it desirable agricultural land but a target for floods.
In many countries, this land is drained to provide this. But the process of drying out the wetlands destroys habitats.
The drying out process leads to the area shrinking and lowering, which can actually increase the likelihood of the areas being subject to floods.
how urbanisation increases the risk of flooding
Urbanisation leads to the removal of vegetation resulting in an increase in surface runoff.
An increase in impermeable man-made surfaces - i.e. concrete, tarmac and tiles - lead to no infiltration and so all rainfall experiences runoff.
Runoff enters the drainage system which quickly takes the water to rivers and waterways. This significantly reduces the lag time and promotes reaching a high peak discharge.
how removal of vegetation increases the risk of flooding
Deforestation and removal of vegetation increase the risk of flooding in several ways:
The ground cover decreases, meaning that interception from the plants and their leaves is reduced. This water will now just reach the ground.
The removal of plants reduces infiltration. With less infiltration and interception, there is an increase in surface runoff.
how river management increases the risk of flooding
River management (especially that of hard engineering systems) is meant to provide protection from flooding. But poor river management can have the opposite effect. Channelisation (when the river is widened or deepened) and straightening the river (by cutting off meanders) has actually increased flood risk on the River Mississippi. The levees and man-made channels restrict the river, making it more likely to burst its banks.
how is natural flooding caused
Natural flooding is caused when a rivers discharge increases until the river bursts its banks. Flooding can be caused by an increase in rainfall or an increase in surface runoff water.
flash flooding
Flash floods happen quickly and frequently without warning.
Low air pressure systems can create intense tropical storms and thunderstorms that produce heavy rainfall.
prolonged and heavy rainfall
Prolonged and heavy rainfall can be caused by mid-latitude depressions (low air pressure systems that produce a lot of rain).
A depression happens when two air masses meet, one hot and one cold. The cold air pushes the hot air up, leading to its cooling, condensing and forming rain.
Heavy rains can lead to saturated soil, which stops infiltration and increases surface runoff. Water reaches the channel quicker and causes flooding.
extreme monsoon rainfall
A monsoon is a seasonal change in the direction of the prevailing winds as the ITCZ (Inter Tropical Convergence Zone) moves northwards.
The change results in wet and dry seasons in subtropical areas that are close to oceans. Because of this, India and South East Asia are at particular risk.
the summer monsoon
The summer monsoon (April-September) leads to extreme rainfall in India and South East Asia. During July 2015, the heavy monsoon rain resulted in 103 deaths in Myanmar.
snowmelt
When temperatures increase after winter, snow and ice in higher latitudes or altitudes will begin to melt.
Snowmelt is particularly evident in mountain environments and the increase in water results in increased surface runoff and consequential flash floods.
ENSO: La Niña
La Niña events exaggerate the normal weather patterns. The La Nina years bring warm water and low air pressure to Australasia so the rainfall increases, as does the risk of flooding.
what are the environmental impacts of flooding
Flooding leads to a mass of water in unusual places, which can be damaging for the environment because soils and ecosystems are impaired.
soils
When flooding takes place, sediment from places such as the banks and beds of the river are eroded.
This sediment can block sections of a river - for example, underground waterworks.
Flooding can also lead to soil erosion of fertile soil in places such as the Amazon.
ecosystems
Flooding can destroy plants so they are damaged beyond use or are uprooted and broken into pieces in the river channel.
When agricultural land is flooded, a farmer’s harvest may be destroyed and they will need to completely restart.
If plants, agriculture or habitats are damaged by flooding, the animals that are dependent on them for food and shelter are at risk. Animals have to compete for limited resources or migrate to other ecosystems, resulting in a decline in some species.
