Between 1938 and 1943 the Army Corps of Engineers lined nine miles of river through downtown Johnstown, Pennsylvania with eight inches of concrete. Engineers in Vicksburg built a scale model first and ran the 1936 St. Patrick's Day flood through it until the new channel held. The design standard was the maximum flood of record — the biggest one anyone had measured. The Corps also priced a larger channel, sized for the worst storm the region could plausibly produce, and declined it.
1936 was slow water off a big basin, snowmelt and days of rain. On July 19, 1977, thunderstorm cells stalled over the hills and dropped up to twelve inches on steep little hollows the channel had never been asked about. One five-square-mile catchment shed water fifteen times faster per acre than the main river did. The concrete was rated for 81,500 cubic feet per second. It got 115,000.
Meteorologists put the region's physical ceiling far above that. 1977 spent under 40 percent of it.
The scope decisions, in order
- 1939 — Corps documents fix the design target at the 1936 peak. Building to the larger "standard project flood" would mean channelizing every small stream in the county. Ruled not economically feasible.
- 1943 — Project complete. Second-largest flood control works in the country at the time.
- 1972 — Pittsburgh District studies extending the channel upstream. Same conclusion: not economically feasible. Recommends local floodplain management instead.
- 1974 — Corps publishes photos of Johnstown landmarks with dotted lines marking a worst-case flood. City Hall: twelve feet.
- Jan 1977 — Sixty flood insurance policies in force citywide.
- July 1977 — Seven earthen dams fail on the small tributaries. Laurel Run alone releases 101 million gallons.
- After — Corps estimates the water would have stood 11 feet higher without the channel. The project performed as designed.