rivers

River floods occur when river discharge exceeds bankfull capacity. Excess water then spills out of the channel and inundates the valley floor. They are caused by a number of physical and human factors.

Physical causes:

• Heavy precipitation. High intensity precipitation e.g. flashfloods (Boscastle 2004) or prolonged precipitation over several weeks or days causes slow floods. (Pakistan 2010)
• Rapid movement of surface and subsurface water into river channels, giving short duration, high peak flows. Factors that encourage this include steep slopes, impermeable rocks and lack of vegetation cover.
• Snowmelt e.g. sudden thaws that may melt weeks of accumulated snow.

Human causes:

• Land use changes that accelerate run-off e.g. urbanisation, deforestation.
• Siltation of river channels because of soil erosion and deposition that reduces channel capacity.

Hydraulic action is the force of air and water on the sides of rivers and in cracks.

Abrasion is the wearing away of the bank and bed by the load of the river. It is the impact of debris as it is picked up and moved by the force of the moving water on the bed and banks.

Solution is the removal of chemical ions, especially calcium. The key factors in the rate of erosion are the geology of the bedrock, solute concentration of the stream, discharge and velocity.

Attrition is the wearing away of the load carried by the river. It usually involves particles/pebbles banging into one another and knocking off any angularity as well as hitting the bed.

Suspension is where the surplus energy of the river