Floods: causes, effects and mitigation

Definition

A flood is a high flow of water in a river, or an overflow onto land normally dry, when discharge exceeds the carrying capacity of the channel and causes damage or threatens life and property. Statistically it is a peak discharge of large magnitude above a chosen threshold (e.g. the annual maximum).

Causes

Natural

• Heavy, intense or prolonged rainfall, especially the monsoon (June to September in Nepal) and cloudbursts
• Snow and glacier melt, and glacial lake outburst floods (GLOF) in the Himalaya
• Landslides and debris flows blocking rivers, and their sudden breaching
• Steep, young, geologically unstable (Siwalik/Chure) catchments with high runoff
• Backwater, high tides or river-bed aggradation reducing channel capacity

Man-made

• Deforestation and overgrazing, which reduce infiltration and raise runoff
• Urbanisation and paved surfaces, encroachment on floodplains and rivers, blocked drains
• Poor design or failure of embankments, dams and weirs
• Unplanned roads and sand/gravel extraction that change river morphology

Effects

• Loss of life and livestock, injuries, and water-borne diseases (cholera, typhoid)
• Damage to houses, roads, bridges, irrigation canals, hydropower and drinking water infrastructure
• Destruction of standing crops, loss of fertile topsoil, and sand deposition on farmland, causing food shortage
• Displacement of people, loss of livelihood, and economic loss to the nation
• Erosion of river banks and change of river courses; environmental pollution

Mitigation

Structural

• Dams and reservoirs with flood storage; detention basins
• Embankments (levees/spurs), flood walls, channel improvement and dredging
• Diversion channels, bypass channels, proper culverts and bridges
• Check dams, bio-engineering and afforestation in the catchment (watershed management)

Non-structural

• Flood forecasting and early warning systems, flood-plain zoning and land-use control
• Flood-proofing of buildings, insurance, community awareness and evacuation plans
• Emergency preparedness, relief and rehabilitation; a legal and institutional framework (e.g. Water Resources Act and disaster management acts in Nepal)