Existing comment:
The Spillway:
"... there was no doubt that if the spillway had been of sufficient capacity, the disaster would not have occurred."
-- American Society of Civil Engineers, June 22, 1889
When properly designed and maintained, dams made of earth can hold back huge bodies of water. However, water must never be allowed to run over the top of an earth dam. Running water can rapidly erode an embankment, causing a dam to give way. It was this "overtopping" that doomed the South Fork Dam.
The dry, rocky channel is front of you was the dam's spillway. Designed to prevent overtopping, it should have allowed excess water to escape during floods. Why didn't it save the dam?
(1) The spillway was not wide enough or deep enough to handle the flow which increased dramatically after the heavy rains.
(2) The spillway was the only outlet. Pipes and valves originally installed at the base of the dam to release water had been removed.
(3) Heavy metal screens had been fixed to the bottom of the spillway bridge to keep fish from escaping. The screen impeded the water.
Before the flood, the spillway resembled a picturesque mountain stream. The dam's builders carved the spillway through bedrock that floodwaters would not erode. This 1887 photo was taken just downstream of this point.
This floating, V-shaped structure made of timbers armed with spikes was attached to the spillway bridge. It was intended to catch debris before it could clog the spillway, but may actually have impeded water during the flood.
Map of the dam and spillway before the flood. When the lake level rose on May 31, 1889, water that could not escape through the spillway washed over the dam.
A bridge across the spillway was equipped with screens to prevent fish from escaping downsteam. The modern bridge you see today was built by the National Park Service. |