INFLUENCE OF MODIFICATIONS TO THE SOUTH FORK DAM ON THE JOHNSTOWN FLOOD OF 1889

As part of an ongoing study on the collapse of the South Fork Dam that resulted in the catastrophic Johnstown Flood of 1889, we are working to determine the dimensions of the dam at the time of collapse. Although the existing dam remnants are maintained by the National Park Service and have been protected from major modifications, a detailed topographic survey is required for comparison against surveys conducted shortly after the dam failed in 1889. The updated survey will allow us to see where modifications or significant erosion has occurred and will allow us to pin elevations to the modern digital elevation model (DEM). The DEM is used to determine the volume of water held by the dam at the time of general failure. Our preliminary results indicate that the dam failed between 2:50 and 2:55 pm, which is about twenty minutes earlier than the time that is usally given. In addition to determining the volume of water and peak discharge rate due to the breach, we are also interested in evaluating the possibility of dam survival if modifications had not been made to the dam prior to the collapse. The dam was reconstructed in the 1880s to establish the South Fork Hunting and Fishing Club around the impounded reservoir. “Improvements” to the dam included: lowered height, widened crest, effective loss of a secondary spillway, and removal of drainage pipes and control valves. Additionally, the material used to reconstruct the dam was not the same as specified in the original plans and therefore, the dam was not of uniform construction material or techniques in 1889. Investigations conducted shortly after the 1889 flood concluded that the modifications made by the South Fork Hunting and Fishing Club were not the cause of the dam’s collapse. We seek to quantitatively evaluate whether the dam, as originally designed, could have withstood the flood inflow in late May, 1889.