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?