Hydrological and Slope Hazards: Floods and Landslides
Floods and landslides are hydrological and slope hazards: a flood is the overflow of water onto land that is usually dry, and a landslide is the rapid downslope movement of rock, soil or debris, often triggered by heavy rain, earthquakes or human activity. In brief: floods are the most common and widespread natural hazard and include river floods, flash floods, coastal floods and urban (surface-water) floods. Landslides occur when the driving forces on a slope exceed its resisting forces, which water usually helps to bring about. Both hazards are increased by deforestation, building on floodplains and steep slopes, and by heavier rainfall in a warmer climate.
These hazards affect more people worldwide than almost any other natural hazard. This guide explains the types and causes of floods and landslides, their impacts, how risk is reduced, compares the hazards in a table, works through discharge and slope stability calculations, outlines the history of notable events and corrects common mistakes.
What causes floods?
A flood happens when the water supply to an area exceeds the capacity of rivers, drains and soils to carry it away. Physical causes include prolonged or intense rainfall, rapid snowmelt, storm surges and tsunamis, saturated or frozen ground, steep and impermeable catchments, and the failure of dams or levees. Human factors add to the risk: removal of vegetation, which speeds runoff, paved surfaces that stop infiltration, building on floodplains and the straightening of rivers. A river’s response to rain is described by a storm hydrograph, in which a steep rise and a high peak show a fast, dangerous response, typically in a small, steep, urban catchment.
What types of flood are there?
- River (fluvial) floods: rivers overflow their banks after heavy rain upstream or snowmelt. They can last days to weeks, as in the Mississippi in 1993 or the Yangtze in 1998.
- Flash floods: sudden floods from intense rain in small catchments or dry valleys, with little warning, a major cause of deaths.
- Coastal floods: sea water driven inland by storm surge, tides, tsunamis and rising sea levels.
- Pluvial (surface water) floods: heavy rain overwhelms urban drainage, even away from rivers.
- Groundwater and dam-break floods: the water table rises above the surface, or a dam, levee or glacial lake fails.
The impacts include deaths by drowning, destruction of homes and crops, contamination of water and disease, damage to roads and power, and long-lasting economic loss. Floods also bring benefits: they deposit fertile silt, as the Nile floods did for ancient Egypt, and recharge groundwater.
What causes landslides?
A slope is stable when its resisting forces (the strength of the soil or rock and its friction and cohesion) exceed the driving forces (the weight of the material and gravity pulling it downhill). The ratio of these is the factor of safety. Landslides happen when this falls below 1.
- Water: heavy rain or snowmelt increases the weight of the material and builds up pore-water pressure, which reduces friction. Most landslides follow intense or prolonged rain.
- Earthquakes and volcanic activity: shaking triggers thousands of landslides at a time, as in the 2008 Sichuan earthquake.
- Steepening and undercutting: rivers, waves, road cuts and quarries cut away the base of a slope.
- Loss of vegetation: deforestation, fires and overgrazing remove roots that bind the soil.
- Loading and weak materials: buildings and spoil heaps add weight, and clay, weathered rock and loose volcanic ash are weak.
Types include falls (rock falling from cliffs), slides (rotational slumps and translational slides), flows (debris flows and mudflows that may travel at tens of km/h and carry boulders), and lahars (volcanic mudflows). Debris flows are especially deadly, because they come suddenly and move fast, often at night.
How can the risks be reduced?
For floods, measures include engineered defenses (levees, dams, flood walls and storage basins), natural flood management (restoring wetlands, floodplains and woodland to slow runoff), sustainable urban drainage, land-use planning to keep homes off floodplains, flood forecasting and warning, evacuation plans and flood insurance. For landslides, measures include mapping hazardous slopes, controlling land use, planting vegetation, retaining walls, drainage of slopes, rock bolts and nets, and monitoring and rainfall-threshold warnings. Community education about warning signs, such as cracks in the ground, tilting trees and unusual sounds, saves lives.
Floods and landslides compared
| Feature | Floods | Landslides |
|---|---|---|
| Main trigger | Heavy rain, snowmelt, surge | Heavy rain, earthquakes, undercutting |
| Human contribution | Building on floodplains, sealed surfaces | Deforestation, steep construction |
| Warning time | Hours to days (river), minutes (flash) | Minutes to hours |
| Typical damage | Wide, slow-onset damage, drowning | Local, destructive, burial |
| Main response | Defenses, forecasting, planning | Slope mapping, drainage, vegetation |
Worked example: discharge and slope stability
- River discharge: discharge Q = cross-sectional area × velocity. A river channel is 40 m wide and 3 m deep (area 120 m2), with a flow speed of 2.5 m/s. Q = 120 × 2.5 = 300 m3/s.
- Flood stage: in flood, the depth rises to 5 m, and the flow spreads to 60 m width (area about 300 m2) at 3.5 m/s. Q = 300 × 3.5 = 1,050 m3/s, 3.5 times the normal value.
- Water volume: a flow of 1,050 m3/s for 12 hours (43,200 s) delivers 1,050 × 43,200 = 45,360,000 m3, or about 45 million m3.
- Rain on a catchment: 120 mm of rain on a catchment of 200 km2 (200,000,000 m2) is 0.12 × 200,000,000 = 24,000,000 m3. If 50% runs off, the volume in the river is 12,000,000 m3. If this runs off over 10 hours (36,000 s), the average discharge is 12,000,000 ÷ 36,000 = 333 m3/s.
- Factor of safety: a slope has resisting forces of 800 kN and driving forces of 600 kN along a potential slip surface. FS = 800 ÷ 600 = 1.33, so it is stable. After heavy rain, the water pressure reduces the resisting force to 540 kN, and the driving force increases to 640 kN because of the added weight. FS = 540 ÷ 640 = 0.84, which is less than 1, so the slope fails.
- Debris flow arrival: a debris flow moving at 40 km/h reaches a village 6 km down the valley in 6 ÷ 40 = 0.15 hours, which is 9 minutes.
What are notable events?
The Yellow River in China has caused some of the deadliest floods in history, with the floods of 1931 and 1887 estimated to have killed hundreds of thousands to millions of people, though estimates vary widely. The North Sea flood of 1953 led to major flood defenses in the Netherlands and the UK. The 2022 floods in Pakistan, after extreme monsoon rain, killed over 1,700 people and affected about 33 million. The Aberfan disaster in Wales in 1966, when a collapsing coal spoil tip buried a school, killed 144 people, 116 of them children, and led to new laws on mining waste. In 2008, the Wenchuan (Sichuan) earthquake triggered tens of thousands of landslides, and landslides blocked rivers and created dangerous lakes. Studies suggest that heavy rainfall events have become more intense in many regions with warming.
Which mistakes are common?
Mistake 1: Underestimating shallow, fast-moving water
About 15 cm of fast water can knock a person over, and 60 cm can float a car. Driving through flooded roads is a major cause of flood deaths.
Mistake 2: Believing that flood defenses remove the risk
Defenses reduce the probability of flooding but can be overtopped or fail, and they may encourage building in areas that remain at risk.
Mistake 3: Thinking that landslides happen only on very steep slopes
Weak soils, saturated ground and undercut slopes can fail on moderate slopes of 15 to 20°, or even less in clay.
Mistake 4: Seeing floods and landslides as separate problems
Heavy rain often triggers both together, and landslides can block rivers and cause further floods.