A spatial multi-risk hazard assessment and vulnerability study of ...

worth of property are at risk of flooding in the United States. Thousands of floods occur
worldwide each year, making it one of the most common and damaging of all hazards
(Federal Emergency Management Agency, 1997).
A flood is a normal event for any river or stream that could occur over a period of time
varying from several times a year to once every few hundred years. Floods are caused
when excess water from heavy rainfall, snowmelt or storm surge accumulates and
overflows the river or stream’s normal path onto its banks and adjacent floodplains
(Miller, 1997).
Floodplains are lowlands adjacent to rivers, streams, lakes and oceans that are subject to
recurring floods. Floods can occur anywhere in a country, any time of the year, day or
night. The potential volume of water that could reach the floodplain is a function of the
size of the contributing watershed and topographic characteristics such as watershed
slope and shape, and climatic and land use characteristics (Coch, 1992).
Several factors determine the severity of floods, including rainfall intensity and duration.
A large amount of rainfall in a short time span can cause flash flooding. A small amount
of rain can also cause flooding if the soil is saturated from a previous wet period, or if the
rain is concentrated in areas where the surface is impermeable, such as in developed areas
where most of the surface is covered with concrete, tar and other building materials
(Federal Emergency Management Agency, 1997).
Topography and groundcover are also contributing factors for floods. Water runoff is
higher in areas with a steep slope and low vegetation density. Urbanization of floodplains
and manipulation of stream channels have increased both the frequency and magnitude
of floods in many areas. Floods are most common in the season of highest precipitation
(Miller, 1997).
The intensity of floods in South Africa is measured in cubic metres per second, and the
risk of flooding is defined as the probability of occurrence in a year in percentage or
return period (years). Therefore a 50 year flood has a 2% (1/50x100) chance of occurring
in any specific year. The probability of a 50 year flood occurring once in a 50 year time
span is thus calculated at 66% (RAVA, 2002).
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