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10 |

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—


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,


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


Flood control dams helped

mitigate flooding (14 Gwinnett

County dams held back

billions of gallons of potential


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?