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A 'catchment' is any device or structure that captures water.

Contents

General description

Catchment factors

Topography

Shape

Size

Soil type

Land use

Watershed

See also

General description

Catchment area or drainage basin is the term used to describe the area which is drained by a river. It is sometimes called the river basin or watershed (see below). The catchment is the most significant factor determining the amount or likelihood of flooding.

Catchment factors

Catchment factors are: topography, shape, size, soil type and land use (paved or roofed areas). Catchment topography and shape determine the time taken for rain to reach the river, Catchment size, soil type and development determine the amount of water to reach the river.

Topography

Topography determines the speed with which the runoff will reach a river, clearly rain that falls in steep mountainous areas will reach the river faster than flat or gently sloping areas.

Shape

Shape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment.

Size

Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding.

Soil type

Soil type will help determine how much water reaches the river. Certain soil types such as sandy soils are very free draining and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground.

Land use

Land use will contribute to the volume of water reaching the river, in a similar way to clay soils, only faster. Rainfall on roofs, pavements and roads will be collected by rivers with almost no absorption into the groundwater.

Watershed

Another common term for catchment is ''watershed''. This link leads to a website showing a flash animation of how rain falling on to the landscape will drain into a river depending on the terrain.

See also

★ Catchment area

★ Drainage basin