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8 |

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


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,


infrastructure, etc.

Small businesses and jobs

can be affected.

Loss of tax base

Loss of water resources

and/or or flood control


Loss of community

confidence and continuity