Types of Floods
Snow Melt Floods
Precipitation in the form of snow does not produce runoff or infiltrate
into the soil, but waits until the snow melts, which means that several
months of precipitation can accumulate above the soil surface. Rain
on the snow pack or water from the melting snow can be held within
the pack until the high liquid water content finally causes the pack
to collapse, releasing water catastrophically and causing a very
large runoff very rapidly. Warm rainwater a large cold spell may
cause the snow pack to melt while the underlying ground is still
frozen, which prevents any infiltration. Because of these reasons,
snowmelt floods can be very large.
Storm Surge
Floods in coastal areas and in river estuaries are usually due to
storm surges, which result from the sea being driven on to the land
by meteorological forces. Here to physical forces act together. A
storm with intense low pressure causes the level of sea to rise because
of barometric effects and strong winds associated with this storm,
if directed on shore, drive the sea on the land. Storm surges are
thus, commonly associated with tropical cyclones. The east coast
of India is particularly prone to storm surges. The storm that produces
the surge can also give rise to heavy rainfall inland so that the
estuary region can be subject simultaneously to river flooding and
storm surge.
Flash Floods
Flash floods are defined s floods of short duration with a relatively
high peak discharge. They arise from local precipitation of extremely
high intensity, typical of thunderstorms. The high concentration
of rainfall on a small area can have devastating effects as the river
flow can rise to several hundred times the normal flow in the space
of a few hours. Flash floods are common in arid and semi- arid areas.
In these areas, what little rainfall there is usually occurs in short,
intense storms. The intensity of the storms and the poor absorptive
capacity of arid zone soils lead to much to the annual runoff occurring
as flash floods. Which can also occur following thunderstorms in
more humid regions. Mountainous areas are prone to thunderstorms
and the steep terrain and thin soils in the mountains assure high
runoff with a short delay time.
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