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Living with Dams: Extreme Rainfall Events | 2015
Georgia Case Study
1. 1994 Tropical Storm Alberto
-
More than 280 dams—
including two Category I (high-
hazard-potential) structures—
failed.
2. September 2009 Flood
Some areas recorded more
than 20 inches of rain in a 24-
hour period. 20 stream gauges
destroyed by floodwaters; one
gauge overtopped by at least
12 feet.
Rainfall amounts went well
beyond the 500-year storm.
National Weather Service
official: “…the chance of an
event like this occurring is 1 in
10,000.” (USGS Press Release,
11/4/2009)
About 10% of state’s high-
hazard potential dams affected
by storm: 4 overtopped; 46
auxiliary spillways activated.
96 high-hazard potential dams
were inspected soon after the
event.
Flood control dams helped
mitigate flooding (14 Gwinnett
County dams held back
billions of gallons of potential
floodwater.)
Iowa Case Study
Lake Delhi, Iowa – July 2010
Lake Delhi Dam experienced
what is believed to be a
record inflow of water that
exposed long dormant design
deficiencies and unrepaired
maintenance problems.
48-hour rainfall totals up to 13”.
150-ft-long breach formed
when 10 inches of rain swelled
the 448-acre lake to 9,920 acre-
ft, or 3.2 billion gallons, from
its normal 3,790 acre-ft, or 1.2
billion gallons of water.
Lake Delhi’s flood level is 15 ft.
The level on July 24 reached
nearly 25 ft.
Extensive property damage
occurred in the reservoir
above the dam and in the
communities downstream of
the dam; no loss of life.
1100 homes flooded.
What are the risks involved?