Answer:
Several types of data can be collected to assist
hydrologists predict when and where floods might occur.
The first and most important is monitoring the amount of
rainfall occurring on a realtime (actual) basis. Second,
monitoring the rate of change in river stage on a realtime
basis can help indicate the severity and immediacy of the
threat. Third, knowledge about the type of storm producing
the moisture, such as duration, intensity, areal extent,
etc., is valuable for determining possible severity of the
flooding. And fourth, knowledge about the characteristics
of a river's drainage basin, such as soil-moisture
conditions, ground temperature, snowpack, topography,
vegetation cover, impermeable land area, etc., can help to
predict how extensive and damaging an impending flood
might become.
Floods are a dangerous hazard throughout the world. On average in the
United States,
about 165 people are killed and about $2 billion of damage
occurs each year. Most people underestimate the power and
destructiveness of flood waters.
There are two basic kinds of floods, flash floods and the
more widespread river flooding. Flash floods generally
cause greater loss of life and river floods generally
cause greater loss of property. A flash flood occurs when
runoff from excessive rainfall causes a rapid rise in the
stage of a stream or normally dry channel. Flash floods
are more common in areas with a dry climate and rocky
terrain because lack of soil or vegetation allows
torrential rains (typically from summer thunderstorms) to
flow overland rather than infiltrate into the ground.
River flooding is generally more common for larger rivers
in areas with a wetter climate, when excessive runoff from
longer-lasting rainstorms (such as from a cold front) and
sometimes from melting snow causes a slower water-level
rise, but over a larger area. Floods also can be caused by
ice jams on a river, or high tides. Most floods can be
linked to a storm of some kind. The National Weather
Service collects and interprets rainfall data throughout
the United States and issues flood watches and warnings as
appropriate. The National Weather Service uses statistical
models and flood histories to try to predict the results
of expected storms. The USGS maintains a network of
streamflow-gaging stations throughout the country for
which the discharge and stage are monitored. Flood
estimation maps are generally produced by estimating a
flood with a certain recurrence interval or probability
and simulating the inundation levels based on flood plain
and channel characteristics.
