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II |

Living with Dams: Extreme Rainfall Events | 2015

“Fact or Fiction” – Common Beliefs about Dams


Many manmade structures including dams, bridges and buildings were not

built to withstand the extreme rainfall events happening today.

Advancing age makes dams more susceptible to failure.

The average age of dams in the U.S. is more than 50 years old.

As dams get older, deterioration increases and construction costs rise. Some

common problems of older dams are:

Deteriorating metal pipes and structural components—after 50 years. It is

not unusual that metal rusts and loses its structural integrity.

Subdivisions and businesses built upstream—roofs and paved streets and

sidewalks increase the volume of runoff to the dam.

Most dams are privately owned. Dam owners are responsible for

maintenance and upgrades.

Private dam owners are responsible for more than 65% of the nation’s dams.

Incidents and emergencies at the dam are handled by the dam owner and local

emergency management officials.

A 100-year flood has a 1% chance of occurring each year or a 26% chance of

experiencing a flood of that magnitude or greater during the life of a 30-year

mortgage. There are storms that occur in the U.S. every year that are many

times larger than the 100-year storms.

The PMP is possible. Extreme rainfall events have many labels. Storms now

have names and probabilities; 100 year, Design Storm, Non Exceedance Event,

PMP, Worst Case Event.

Extreme rainfall events do occur. Storms happen every year, if not here then

somewhere. There are normal storms and extreme storms such as 100-year

storms and probable maximum precipitation (PMP) events.


That dam has been here for years

– it’s not going anywhere. It can

handle any storm.

Dams are like roads. The

government takes care of them.

The 100-year flood is the biggest

storm that can happen, and it can

only happen once every 100 years.

Probable maximum precipitation is

an engineering calculation that is

not real. It can never happen.

It never rains that much here?