Building a new housing estate on the floodplain increases the risk of flooding because the surface becomes impermeable due to the construction of houses, tarmac roads and concrete pavements. Surface run-off increases leading to a rapid increase in a river’s discharge.

Find out more about human and physical causes of flooding.

It becomes narrow and steep-sided in the upper course due to vertical erosion.

Find out more about the long profile of a river.

Vertical erosion involves the wearing away and deepening of the river bed. This is mostly by hydraulic action. It is most common in the upper course of the river.

Find out more about river erosion.

Water flows slowly over shallow areas (riffles) in the riverbed and faster through pools, deeper sections of the river. This leads to helicoidal flow that corkscrews from one bank to another. Fast flowing water on the outside bank causes lateral erosion through hydraulic action and abrasion which undercuts the bank, creating a river cliff. The eroded material is then deposited on the inside of the next meander where friction slows the flow, creating a slip-off slope.

Find out more about meanders and oxbow lakes.

Levees are naturally raised riverbanks found along the sides of the river channel that has experienced flooding.

Find out more about landforms in the lower course of the river.