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Living with Dams: Extreme Rainfall Events | 2015
What are the risks involved?
Most dams in the U.S. have spillway
systems capable of safely passing
small routine rainfall events. But,
when the rainfall event becomes
an extreme event, the dam may
experience extensive damage or
even failure. The dam may not be
capable of safely storing and/or
passing these floodwaters.
While many communities follow
minimum floodplain management
practices, if a dam is above or
upstream of a community there is
often still the potential risk for loss
of human life. Dam failure floods
from extreme rainfall events may
also cause unprecedented damage
to infrastructure including homes,
schools, small businesses, industrial
and commercial buildings,
recreational areas, agricultural
land, farm buildings, military
facilities, public utilities, roads,
power infrastructure, energy, and
communication systems.
Dam failures caused by extreme
events may also cause substantial
long term economic damage
to downstream communities.
Flooded homes and communities
become stigmatized. Jobs are
frequently lost when businesses,
industrial, and commercial facilities
are damaged and operations are
relocated. The property tax base
can be dramatically reduced
when structures and facilities are
damaged. The loss of the critical
impounded water resource, or the
flood reduction capability, may
also stigmatize the communities’
continuity as public and private
sector confidence in the
community suffers.
A large majority of dams were
not intended or designed to store
enough flood water to provide
significant flood protection to areas
downstream. Flood waters must
be allowed to safely pass through
designed spillways or risk the
water flowing over the vulnerable
embankment causing catastrophic
failure. Therefore, unusual and
substantial downstream flooding
risks may further exist for areas
below a dam even if a dam does
not fail during extreme rainfall
events.
While primary spillways are passing
their maximum amounts of flow,
a dam’s designed operation steps
often include planned releases
of substantial amounts of flood
water through secondary / auxiliary
spillway channels or gates. As
these auxiliary spillway features
are infrequently used and they are,
in most instances, situated away
from the main body of the dam,
these planned release flood flows
go to areas and elevations that
are likely not subject to local flood
plain zoning and development
restrictions. These extreme rainfall
events are not likely identified
on flood insurance maps since
insurance requirements are based
on minimal flood design standards.
What are the risks from
extreme rainfall events?
Loss of life and property
Impacts to community,
schools, economy,
transportation,
infrastructure, etc.
Small businesses and jobs
can be affected.
Loss of tax base
Loss of water resources
and/or or flood control
protection
Loss of community
confidence and continuity