Hydrologic Modeling of The May 1889 South Fork Dam Failure

On May 30th, 1889, a festive Memorial Day in Johnstown, Pennsylvania was cut short by showers in the late afternoon. The rain continued all night and was bringing local rivers to the top of their banks when the sun rose on May 31st. On May 31st the rains did not relent, and the rivers continued to rise reaching the highest ever recorded stage in the City of Johnstown. Fifteen miles upstream, past a few small towns located at stops along the Pennsylvania Railroad (PRR), was the South Fork Dam. As the rivers crested, the spillway capacity was greatly exceeded, and the dam overtopped. While Johnstown was hopeful the worst had passed, the dam was beginning to unravel. Despite human intervention to dig a new emergency spillway and raise the crest, the downstream slope eroded through the dam crest eventually resulting in a breach and emptying the lake in about 45 minutes. The flood wave covered the 15 miles to Johnstown in about an hour. It collapsed the PRR Viaduct bridge downstream, as well as many smaller bridges, on its way to Johnstown. However, the recently completed stone PRR bridge at Johnstown resisted the flood wave creating a massive pile of debris that covered several acres. It included most of the town’s wood framed buildings, rail cars, and much more. By evening the debris caught fire resulting in an estimated 80 additional fatalities. Overall, roughly 30,000 people were impacted resulting in at least 2,200 fatalities. The authors discuss how they developed and calibrated a 2D HEC-RAS model of the dam breach and flooding prior to the breach. Data collection included: terrain, hydrology, high water marks, and flood arrival times gathered from historic photos, interviews and other published works. Development and calibration of the 2D HEC-RAS model will be discussed in detail. Historic observations are compared to the model results. A brief history of the flood and consequences is also covered.