flooding - causes & control

When does flooding occur

When the capacity of the river channel is exceeded

What is flooding usually the result of

Heavy or prolonged rainfall

What is the key factor in flooding

Lag time - the time taken from the precipitation to reach the river from where it falls

Shorter laugh time = higher risk of flooding as discharge increases faster

How do human factors increase flood risk (4)

Deforestation

• a lack of trees reduces interception and infiltration, increasing surface run off

Urbanisation

• impermeable materials (e.g concrete) increase surface run off

• Water flows into the drains which are designed to take water to a river rapidly

Agriculture

• bare soil and ploughing increase surface run off

Climate change

• rising global temperatures may increase storm frequency and intensity which leads to heavy and prolonged rain fall

How do physical factors increase flood risk (6)

Relief

• steep slopes reduce infiltration and increase surface run off

Rock & soil

• impermeable rocks and frozen, saturated or sump acted soil reduces infiltration and percolation and increase surface run off

Weather

• heavy or prolonged rainfall means that the rate at which water reached the surface exceeds the infiltration rate leading to increases overland flow

Seasons

• in autumn, winter and monsoon seasons rainfall is much more frequent

• In warmer seasons there are higher temperatures leading to snow melt

High drainage density

• where drainage density is high there are many tributaries taking water to the main channel causing a rapid increase in discharge

Lack of vegetation

• where there is little natural vegetation there is reduced interception leading to increased overland flow

What do flood forecasts take into account

• rainfall amounts

• Hazard impact mapping

• Past storm hydrographs

• Draining basin characteristics

Define hard and soft engineering

Hard - involves building structures or changing the river channel

Soft - works with natural processes of the river and surrounding environment

Give 4 examples of hard engineering

Dams and reservoirs - controls downstream discharge

Man made levees - increases capacity of river

Straightened channels - avoids vulnerable areas

Flood relief channels - allows some water to flow out of main channel

Give 4 examples of soft engineering

• river restoration - supports the river by restoring its original regime

• Wetland conservation - provides somewhere for excess water to go

• Catchment management plans - assess the risk of flooding in an area and outline how this will be managed

• Floodplain zoning - means that only certain land uses are allowed on the flood plain reducing the risk

How to make a building flood resistant

• home raised on stilts

• Replacing carpet with tiles

• Power sockets above the likely level of flood