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4 |

Living with Dams: Extreme Rainfall Events | 2015

Extreme rainfall events can

severely damage dams and or

cause them to fail completely.

There are more than 87,000 dams

in the United States with various

shapes, sizes, age and uses. From

Hoover and Grand Coulee Dams

out West to the small New England

stone and masonry dams, they all

have potential to cause damage

and loss of life. For that reason, it is

imperative that they are properly

designed and maintained.

Modern dams are built to withstand

earthquakes and floods, seepage

and slope instabilities. Many older

dams were not designed to modern

standards and are showing signs of


Many are in need of maintenance,

upgrading and repair. Dam

engineering and hydrological

science have improved over

the past 50 years as has the

understanding of the risk and

liabilities associated with the

storage of water. Much of this

science is intuitive, understandable

and accepted by dam owners.

Some of it is not and is more

mysterious, such as the size of

potential extreme rainfall events

and the resulting flooding that


Often the combined effect of a

series of storms repeatedly moving

over the same area, dumping heavy

rains over several days, can cause

rainfall totals similar to a single

extreme rainfall. Meteorologists

refer to this as “storm training.”

Why should I care about

extreme rainfall events?

Data taken from the National Weather Service rain gage stations, State record 24-Hour

Precipitation (most recent data from 2006). Higher amounts have unofficially been recorded

in many states. For example, in Pennsylvania in 1942, 34.5 inches were recorded in a 12

hour period. For more information go to:

Dataunder review

Extreme Rainfall Events By State

5 - 10”

10 - 15”

15 - 25”


than 25”

Maximum 24 hour precipitation

per state (inches)