Floods and water hazards

Mechanism of destruction

Inundation and flow of water with mechanical pressures of rapidly flowing water. Currents of moving or turbulent water can knock down and drown people and animals in relatively shallow depths. Debris carried by the water is also destructive and injurious. Structures are damaged by undermining of foundations and abutments. Mud, oil and other pollutants carried by the water is deposited and ruins crops and building contents. Flooding destroys sewerage systems, pollutes water supplies and may spread disease. Saturation of soils may cause landslides or ground failure.

Parameters of severity

Area flooded (km²), depth or height of flood, velocity of water flow, amount of mud deposited or held in suspension. Duration of inundation. Tsunamis or tidal waves measured in height (meters).

Causes

River flooding results from abnormally high precipitation rates or rapid snow melt in catchment areas, bringing more water into the hydrological system than can be adequately drained within existing river channels. Sedimentation of river beds and deforestation of catchment areas can exacerbate conditions leading to floods. High tides may flood coastal areas, or seas be driven inland by windstorms. Extensive precipitation in urban areas or drainage failures may lead to flooding in towns as hard urban surfaces increase run-off loads. Tsunamis are caused by underwater earthquakes or eruptions. Dam failures or collapse of water retaining walls (sea walls, dikes, levees).

Hazard assessment and mapping techniques

Historical records give first indication of flood return periods and extent. Topographic mapping and height contouring around river systems, together with estimates of capacity of hydrology system and catchment area. Precipitation and snow-melt records to estimate probability of overload. Coastal areas: tidal records, storm frequency, topography and beach section characteristics. Bay, coastal geography and breakwater characteristics.

Potential for reducing hazard

Retaining walls and levees along rivers, sea walls along coasts may keep high water levels out of flood plains. Water regulation (slowing up the rate at which water is discharged from catchment areas) can be achieved through construction of reservoirs, increasing vegetation cover to slow down run-off, and building sluice systems. Dredging deeper river channels and constructing alternative drainage routes (new river channels, pipe systems) may prevent river overload. Storm drains in towns assist drainage rate. Beaches, dune belts, breakwaters also reduce power of tidal surges.

Onset and warning

Flooding may happen gradually, building up depth over several hours, or suddenly with the breach of retaining walls. Heavy prolonged precipitation may warn of coming river flood or urban drainage overload. High tides with high winds may indicate chance of coastal flooding some hours before it occurs. Evacuation may be possible with suitable monitoring and warning system in place. Tsunamis arrive hours or minutes after earthquake.

Elements most at risk

Anything sited in flood plains. Earth buildings or masonry with water-soluble mortar. Buildings with shallow foundations or weak resistance to lateral loads or impact. Basements or underground buildings. Utilities: sewerage, power, water supply. Machinery and electronics including industry and communications equipment. Food stocks. Cultural artifacts. Confined/penned livestock and agriculture. Fishing boats and other maritime industries.

Main mitigation strategies

Land-use control and locations planning to avoid potential flood plain being the site of vulnerable elements. Engineering of structures in floodplain to withstand flood forces and design for elevated floor levels. Seepage-resistance infrastructure.

Community participation

Sedimentation clearance, dike construction. Awareness of flood plain. Houses constructed to be flood resistant (water-resistant materials, strong foundations). Farming practices to be flood-compatible. Awareness of deforestation. Living practices reflect awareness: storage and sleeping areas high off ground. Flood evacuation preparedness, boats and rescue equipment.