Once In Ten Thousand Years

The dams that surround Johnstown, stretching throughout the Conemaugh Valley, were unsuspecting accomplices in the Great Flood of 1977. They were duped by the instigator - the rain.

When they failed, six dams poured more than 128 million gallons of water into the Conemaugh Valley Twenty million gallons were unleashed on Johnstown when the South Fork Dam burst in 1889.

A phenomenal amount of rainfall - 11.82 inches In 10 hours was too much for both the dams and the sewers in the Conemaugh Valley The rainfall and the dam failures created the Great Flood of 1977.

The National Weather Service said a once-in-a-1,000 year flood in the valley could have resulted from 7.32 inches of rainfall in a 10 hour period. But an 11.82 Inch rainfall would be a once-in -a- 5,000 to 10,000 year occurrence.

The dams failed because of overtopping. The dams that burst were:

Laurel Run Dam on Laurel Run. The 10 year old earthen dam held 101 million gallons of water. It had a 42 foot high spillway. The dam Is owned by the Johnstown Water Authority. When it failed the water enveloped Tanneryville and caused deaths and heavy property losses.

Sandy Run Dam on Sandy Run. The dam held 18 million gallons of water. It Is owned by the Highland Water and Sewer Authority. The 63 year old dam had a spillway of 28 feet

Salix Water Dam on Otto Run. It held 2 million gallons of water. Its spillway height was under 25 feet. It is owned by the Adams Township Water Authority.

Cambria Slope Mine 33 on Sanders Run. It held 7 million gallons of water. The dam leased by Bethlehem Mines Corp. had a spillway of 32 feet

Unnamed Dam on Peggy’s Run, Franklin Borough. This dam was leased by Bethlehem Mines Corp. and held an undetermined amount of water at the time of the flood because it was used as a catch basin.

An unnamed impoundment dam east of Johnstown at St. Michael held less than 1,000 gallons. It was a reserve dam for Bethlehem Mines Corp.

The dams were simply overwhelmed. After overtopping, water eroded the earthen embankments. There was total failure of five dams. The sixth, Cambria Slope Mine 33, retained about two-thirds of its contents.

The dams had not shown any defects in past inspections and no trouble was reported in them by the Pennsylvania Dept. Of Environmental Resources (DER).

Col. Max Janairo Jr., district engineer for the Army Corps of Engineers - Pittsburgh office, said sufficient resources to control the amount of water that Johnstown received in rainfall July 19th and 20th are not available. No dams are constructed to contain such a quantity of water received in such a short period of time."

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