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).
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.
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