FLOOD01659

• County: Statewide
• Title: Floodplain Management Handbook
• Date: 9/1/1981
• Prepared By: Flood Loss Reduction Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

Flooding sometimes occurs from
causes other than overflow of a stream.
A rela ti vely thin sheet of flooding (a
few inches to a couple of feet deep)
occurs in some places where slopes are
not steep enough to cause rapid runoff.
There are also instances of flooding
due to the rapid accumulation of
surface runoff in low lying areas. In
other cases, previously safe areas are
flooded when streams shift their
channels. Such shifts are common on
alluvial fans.

Flooding along lake shores ' may t
also be either short or long in
dura tion. Short-dura tion flooding is
usually caused by wind driven. waves. In
addition to flooding an area, waves
have great erosive powers and a batter-'
ing effect on structures. Very large
lakes sometimes experience a "seiche"
in which a combination of ,wind and

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changes in atmospheric pressure or an
earthquake causes the surface of the
lake to oscillate over a period of
minutes or hours. The water level in
lakes which have inadequate outlets can
also rise enough in response to large
inflows to temporarily flood the lake
shore.

Longer-term flooding occurs
around lakes and potholes which have no
outlet. During years with above normal
rainfall, these areas may fill to a
point that several years may be
.required for evaporation to lower the
water surface to its customary level.
Longer-term or even permanent flooding
of such areas may also occur if sedi-
ment fills the lake to the point that
,its capacity to store inflowing water
is reduced. Some usually dry areas may
also flood when groundwater levels rise
after heavy rains.

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Building close to the water's edge during a.dry period can result in large
losses when a lake's level rises in response to heavy rains.

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