A DETERMINATION OF PEAK DISCHARGE RATE AND WATER VOLUME FROM THE 1889 JOHNSTOWN FLOOD

The South Fork Dam was breached in 1889, releasing a large volume of water into the Little Conemaugh River valley and causing devastation in the downstream communities, including Johnstown, PA. Despite the notoriety of the 1889 Johnstown flood, relatively little scientific research has been conducted at this site. Since the breach did not cause complete obliteration of the dam, remnants of the breach, the spillway, and dam flanks remain in place with minimal modification. Using a combination of high-resolution GPS at the dam remains, contemporary photographs, local DEMs, field measurements, and hydrologic models, we have been able to constrain the peak discharge rate and maximum water volume of this catastrophic flood. Estimates of floodwater volume and discharge rate are complicated by the series of notches and breaches that occurred prior to the final and largest dam breach. Eye-witness accounts provide information regarding the timing of the breaches and collapse, but contain inherent inaccuracies and approximations. Field evidence and historic photographs indicate the location and cross-sectional area of the breaches, which have been used to constrain the flow of water through the opening as well as the remaining volume of water impounded after each breach. We have found that the maximum volume of water impounded at any time behind the South Fork Dam was 1.8 × 10⁷ m³ and that the peak discharge rate was approximately 9000 m³/s. This release of water, however, was later exacerbated by a second, temporary impoundment behind a railroad viaduct six km downstream. The collapse of this second impoundment released flood waters into the city of Johnstown at a rate that likely exceeded 12,000 m³/s. It is this second release of water that caused vast destruction when it reached Johnstown and other communities in the Conemaugh Valley (see companion abstract by Coleman et al.).