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

Aerial photographs
dramatize the potential for
flood damage. The picture
on the left shows the nor-
mal flow of a river through
an urbanized region. A1
right is the same area dur-
ing aflood.

12

Hash flood.ing can occur
in small, usually shallow
or dryl stream,',,~ .'iuch as ar-
royos in tbe southwestern
part of the country.

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u.s_ Geological Survey

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result from some combination of intense precipitation,
steep slopes, a small drainage basin, and a high propor-
tion of impervious ground surfaces. They are evident in
many parts of the country and often occur in small
streams that are otherwise shallow or df); such as arroyos.
Shallow Flooding Shallow flexlding of several types
occurs commonly throughout the cQumr): Included in
this category are unconfined flows over broad, relatively
low areas such as alluvial plains; intermittent flows in arid
or semi~arid regions that have not developed a system of
well-defined channels; minor overbank flows that remain
unconfined; overland flow of runoff in dense urban areas;
and flows where heavy debris deposits cause constantly
shifting channels, such as in alluvial fans. These types of
flooding are also referred to as sheet flow, ponding, shal-
low overflo\\; and alluvial fan flo\\: It is verv difficult to
determine shallow flooding depths, the extent of such

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