DAM-BREACH HYDROLOGY OF THE JOHNSTOWN FLOOD OF 1889 – CHALLENGING THE FINDINGS OF THE 1891 INVESTIGATION REPORT

In 1891 a report was published by an American Society of Civil Engineers committee to investigate the cause of the 1889 Johnstown flood. They concluded that changes made to the dam by the South Fork Fishing and Hunting Club did not cause the disaster because the embankment would have been overflowed and the breach formed if the changes had not been made. We dispute that conclusion based on hydraulic analyses of the dam as originally built, estimates of the time of concentration and time to peak for the South Fork watershed, and reports of conditions at the dam and in the watershed.

We present a LiDAR-based volume of Lake Conemaugh at the time of dam failure (1.455 × 10⁷ m³) and hydrographs of flood discharge and lake stage decline. Our analytical approach incorporates the complex shape of this dam breach. More than 65 minutes would have been needed to drain most of the lake, not the 45 minutes cited by most sources. Peak flood discharges were likely in the range of 7200 to 8970 m³ s^-1. The original dam design, with a crest ~0.9 m higher and the added capacity of an auxiliary spillway and 5 discharge pipes, had a discharge capacity at overtopping more than twicethat of the reconstructed dam. A properly rebuilt dam would not have overtopped and would likely have survived the runoff event, saving thousands of lives.

The ASCE report represented state-of-the-art for 1891, with review of the embankment design and history, careful surveys and photographic documentation of the dam site, hydrologic analyses, and the inclusion of alternate views of the dam breach and site characteristics. However, the report contains discrepancies and lapses in key observations, and most importantly relied too heavily on reservoir inflow estimates that were excessively large. The confidence they expressed that dam failure was inevitable was inconsistent with information available to the committee. Our research also shows that, from 1889 to 2012, a new soil layer up to half a meter deep has accumulated on the main spillway. Hydrodynamic erosion was a likely culprit in the 1862 failure of the dam that seriously damaged the embankment. The Club’s substandard repair of this earlier breach sowed the seeds of the dam’s eventual destruction.

Our paper in the journal Heliyon is open access (CC BY-NC-ND license) and can be obtained at http://www.heliyon.com/article/e00120/.