THE JOHNSTOWN FLOOD – AN ENGINEERING FAILURE

THE JOHNSTOWN FLOOD – AN ENGINEERING FAILURE

The small town of Johnstown is located about 50 miles east-southeast of Pittsburgh, in the Allegheny Mountains, in southeastern Pennsylvania.  Prior to May 31, 1889 Johnstown would be all but unremarkable in nearly every way. But on May 31, 1889 events in Johnstown Pennsylvania would forever change the landscape of the United States, as a “natural disaster” that would have otherwise affected many people in a larger area, would be amplified by poor engineering and inadequate maintenance that would ultimately claim the lives of 2,209 people.

Introduction and Background

Johnstown Pennsylvania situated at the confluence of the Stony Creek and Little Conemaugh River, forming the Conemaugh River.  About 14 miles upstream from Jonestown, the Little Conemaugh River divides into the North Fork and South Fork.  In the 1830’s, the Commonwealth of Pennsylvania built the South Fork Dam along the South Fork of the Little Conemaugh River to hold back water in a reservoir as a local water supply for a series of canals (Figure 1).

Figure 1: Relationship between Johnstown, PA and South Fork Dam

With increased rail traffic in the area in the 1850s, accompanied by a period of unusually low rainfall, there was apparently little need for such a series of canals.  As a result, the South Fork Dam and reservoir was sold in 1881 to a local hunting and fishing club with members made up primarily of steel, coal, and railroad executives.  With the operation of the reservoir under the “South Fork Fishing and Hunting Club”, little was done to maintain the South Fork Dam.  There were evidently reports of numerous leaks through the South Fork Dam when the Club owned the structure and reservoir.  There were also reports where the steel pipe used in the overflow system had been sold for scrap prior to purchase by the Club.

Construction

The dam measured about 72 feet high by about 931 feet long.  The reservoir (Lake Conemaugh) behind the earthen South Fork Dam was about 2 miles long, and about 60 feet deep at its deepest point.  These measurements would allow for about 12 feet of “freeboard” before the water level in the reservoir would overtop the South Fork Dam.  Modern-day measurements put the water level in the reservoir about 450 feet above street level in Johnstown.

Running under the center of the dam was a huge stone culvert used to discharge water into the South Fork Creek to be fed to the canal via the Little Conemaugh River.  The water into the culvert was controlled by five sets of valves and cast iron pipes, each about 2 feet in diameter.  In the event that the discharge culvert could not handle water flows during heavy rains, an 85 foot-wide spill-way was cut through the solid rock of the hillside near the eastern end of the dam.

The dam suffered a major break on June 10, 1862, when the up-stream portion of the stone culvert (control tower) running under the dam collapsed.  Although there was little damage to property downstream, a large section of the dam over the damaged portion of the culvert collapsed and was washed away.

In the original specifications, there was to be a 10-foot deep spillway at the southern edge of the dam.  When the crest of the dam was lowered by about 2 feet to widen the roadway, the capacity of the spillway was reduced by one-fifth (20%).  Furthermore, because of the way in which the dam was reconstructed in 1880 and 1881, the repaired section settled until it was at least six inches lower than the ends of the dam.

It is not uncommon for the best earth dams to settle, especially at their centers, which is also the weakest point and where the water pressure is the greatest.  But with proper maintenance earthen dams can be built back up.  At the South Fork Dam the part of the embankment which should have been the highest (center), if only by inches, was the lowest.

South Fork Dam – Before (see headland indicated by arrow for reference)

Flood Event

In late May 1889 a strong storm system produced about 6 to 10 inches of rain within a 24-hour period throughout the Allegheny Mountains, causing rivers throughout the area to expand to near-capacity.  By 11 a.m. the water was even with the sagging center of the dam and started to eat at the small mound that had been thrown up by a plow. There are also several reports of a thick layer of debris that was blocking the spillway.  Workmen frantically tried to keep the water from breaking through the crest of the dam.  The water through the trench the plow had cut was running almost knee-deep and its force had widened the trench but had not cut much deeper as hoped.  The tremendous weight of the water in the lake forced several serious leaks to develop on the outer face of the reconstructed section (center).  Some of the workmen also reportedly refused to venture out on the dam.  Around 11:30 a.m. the small mound of earth thrown up by the plow suddenly gave way and the water started over the dam, quickly widening out to about 50 feet.

At about 3:10 in the afternoon of May 31, 1889 the South Fork Dam ultimately failed, releasing about 20 million tons of water down the Valley.  In previous years, many “alarms” had been sounded regarding the imminent failure of the dam.  Under the misguided belief that this final alarm was just another “false alarm”, many people in Johnstown did not seek higher ground.  By the time the floodwater reached Johnstown, it was no longer water, but rather included much of the debris from the 14-mile long Valley between the South Fork Dam and Jonestown.  The debris flow was reportedly up to 1/2-mile wide and may have been as tall as 40 feet above ground in places.  There was no safe refuge in town.

South Fork Dam – After (see headland indicated by arrow for reference)
Note construction details exposed in slump blocks in right-center of photo

Aftermath

In Johnstown, there was an industrial steelworks plant against the river.  By most accounts, and as is evident in several photos taken the day after the flood, the steelworks severely damaged.  Also destroyed were nearly every house in town along with nearly every business, bridge, civil structure and building.  Little was left standing.  Nothing was left unaffected.  Communications of the day were carried primarily via telegraph that followed rail lines.  With the demolition and destruction of the bridges and rail lines, telegraph lines were also clipped.  Investigators subsequently concluded that “the failure was due to the flow of water over the top of the earthen embankment caused by the insufficiency of the waste-way [spillway] to discharge the flood water.

Images showing devastation due to flooding caused by collapse of the South Fork Dam

On June 5, 1889 Ms. Clara Barton, the founder and then-president of the American Red Cross (ARC) arrived in Jonestown in what would be the ARC’s first domestic disaster relief effort.  Donations poured into the area from every state in the nation, with more than $3.7 million collected from local, domestic, and international resources.  It would be five years later when an observer remarked that he “would’ve been hard-pressed to imagine the destruction in the Valley on May 31, 1889.Jonestown residents blamed and ultimately sue members of the South Fork Fishing and Hunting Club.  Ultimately however, neither the Club nor individual members were held legally accountable or responsible for the disaster.  Rather than recognize inadequate maintenance programs and unauthorized modifications to the dam, local courts deemed the disaster as an “Act of God”.  Furthermore, it was also reported that individual Club members effectively hid their personal assets behind the Club’s financial structure.

In the time since, many smaller “nuisance” floods have come and gone in Johnstown.  The first physical improvements to reduce the impact floods would have on Johnstown would not be made until 1936.  To this day, high-water marks from present-day floods are recorded at City Hall.

References

Encyclopedia Britannica, 11th edition, Cambridge University Press (common access)

Gibson, C., 2010, “Our 10 Greatest Natural Disasters”, American Heritage

Smoter, F. W., 2004, “The Cause of the Johnstown Flood”, Civil Engineering, pp. 63-66, May 1988