|Fig. 1: The Aftermath of the Johnstown Flood (Source: Wikimedia Commons.)|
During a rainstorm in May of 1889, the South Fork Dam collapsed and unleashed 20 millions tons of water on the valley below, killing over 2,200 people. In terms of lives lost, this event stands as the worst dam accident in the United States, revealing the true power of water stored in a dammed lake. 
In the 19th century Pennsylvania built a canal system to compete with the Erie Canal in New York. The Main Line of Public Works stretched from Philadelphia to Pittsburgh and its Western Division, which opened in 1831, connected Johnstown and Pittsburgh. After completion the operators discovered that during the summer months, this division of the canal system encountered water shortages, especially in Johnstown.  Therefore, the decision was made to build a dam in the mountains near Johnstown that could supply enough water to the canal during the dry summer months to keep it operational. Construction commenced in 1840 but due to the state's financial issues, it was not finished until 1852. 
The dam was designed and built as an earth-filled embankment, consisting of many layers of clay that were laid in successive horizontal lifts.  The layers of clay were soaked in water for days to make them watertight, and rocks were used to cover both the front and back slopes of the dam.  Due to the abundance of suitable earth near dam sites and lack of skilled labor needed to construct such structures, earth dams were the most prevalent and cost effective type of dam at the time.  An 85-foot-wide spillway, which was built to channel water over the dam in the event of a flood, was carved out of the rock on the eastern side wall. Spillways maintain the integrity of earth-filled dams by preventing water from running over the top and eroding the earth on the front face, which can lead to failure.  In order to release water to the river below the dam during normal conditions, five two-foot-diameter cast-iron pipes surrounded by a stone culvert were installed at the base of the embankment. Overall, the dam was 72 feet tall, 918 feet across, and covered an area of 407 acres. The base thickness was over 220 feet and the top thickness was 10 feet. 
The Pennsylvania Railroad, in direct competition with the canal system at the time of the dam's construction, ran its first train from Philadelphia to Pittsburgh just six months after the completion of the dam. Two years later the state decided to sell the canal system and three years later the Pennsylvania Railroad purchased it to use its right-of-ways. Naturally, the Railroad had no incentive to maintain the dam and the lake.  In 1862, after a series of rainstorms, a section of the culvert collapsed, although the valley below was not damaged significantly because the dam was less than 50% full at the time. After the incident, the dam remained no more than ten feet deep. 
In 1875 the dam was purchased by Congressman Reilly, who sold the cast iron pipes before reselling the dam in 1879.  The new owner had plans to transform the area into a lake resort for wealthy families, but before doing so, he needed to repair the dam and stock it with fish.  Instead of rebuilding the culvert to allow for normal discharge, he filled it, allowing the lake to rise and flow through the spillway. Furthermore, two feet of earth was removed from the top of the dam to widen the road that connected both sides of the valley, which decreased the amount of water the spillway could hold by 20%. Also, screens were placed across the width of the spillway to keep the fish inside the lake. 
In May of 1889, a large rainstorm occurred in Johnstown and the surrounding areas. The level of the lake behind the dam rose significantly, the spillway clogged, and the top of the dam sagged slightly in a previously repaired area. Water began pouring over the top of the embankment and the dam soon collapsed. It took only 45 minutes for the 20 million tons of water to pour out of the lake,  resulting in a disaster that took the lives of 2,200 of the valley's 30,000 inhabitants. 
The peak discharge rate from the dam is estimated at 424,000 cfs.  In comparison, the average discharge of the Mississippi River, the largest in the United States, is 593,000 cfs. The second largest river in the United States, the St. Lawrence River, has an average discharge of 348,000 cfs. 
When the wave of water reached Johnstown, it was moving at a speed 37 miles/hour.  If the entire body of water was moving at the same rate, the kinetic energy of the flood would have roughly equaled:
This is equivalent to approximately 70% of the energy released from a kiloton of TNT, which has an explosive energy of 4.2 x 1012 J.
The story of the Johnstown flood reveals the potential destructive power of water stored behind dams. It also calls into question the safety of earth and rock-filled dams, which are clearly susceptible to collapse caused by overtopping. Of the 10 dam accidents in U.S. history with more than 25 fatalities, eight involved earth and rock-filled dams and five of the eight were caused by overtopping.  As a result of a series of dam failures in the 1970's, the National Dam Inspection Act, the Reclamation Safety of Dams Act, the Federal Guidelines for Dam Safety, and the Association of State Dam Safety Officials were created. Also, a number of engineering improvements have reduced the risk of earth and rock-filled dams. 
© Bryan Fairbanks. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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