FLOOD02514

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Design Guidelines for Flood Damage Reduction
• Date: 10/1/1981
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

marl' variable is the watershed, which is the natural drain-
age basin that conveys water runoff in the land-based
portion of the hydrologic cycle. Water that is not absorbed
by the soil and vegetation becomes surface water runof(
seeking the natural drainage lines according to local to-
jXlgraphy These lines merge to form a hierarchical sys-
tem of streams that includes rills, creeks, and rivers, each
of successively larger capacity
Streams have specific physiographic characteristics.
The primary element is the stream channel, which carries
the normal flow of water through the watershed system.
The area of flat or gently sloping land adjacent to the
channel is the floodplain. Flooding usually involves a
build-up of water in the channel, followed by overflow of
excessive quantities of water that inundate the floodplain.
Generall); this rise in water surface elev'dtion is quite slow
in large streams and more rapid in smaller ones.
Flooding is part of the natural renewal of the earth's
resources. Overflows playa positive role in the natural
system by replenishing soil moisture and depositing fer-
tile silt from the river channel onto the floodplain.
Hasb Flooding Flash flooding usually consists of a
quick rise in water surface elevation, with abnormally
high water velocity often creating a ~waIr of water mov-
ing down the channel and floodplain, FLLsh floods usually

Riverine systems evolve to
form distinct stream wan-
nels and floodplains, The
drawings at left illustrate
tbis gradual formation,
wbile tbe pboto at far left
shows a well-defined wan-
nel and floodplain

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