One of the most serious floods in US history was that of the St. Francis Dam in 1928. The dam was built a few years earlier, between 1924 and 1926, as a concrete gravity dam which curved and held a reservoir in the San Francisquito Canyon, just outside of LA. It was designed by the Los Angeles department of Water and Power, which at that time was run by William Mulholland.
Rather than a natural disaster, this flood was a failing of man-made civil engineering. At two minutes to midnight on the 12th March, 1928, the St. Francis Dam leaked, collapsed, and flooded the surrounding areas. Nearly six hundred people were killed, making the disaster California’s second worst next to the Earthquake and fires of 1906.
The descent began on the 1st March 1926 when the first water was pumped in to the reservoir. This was relatively uneventful, even when a few cracks appeared in the dam. Mulholland inspected the cracks and but they were not of note. Over the next two years, the dam worked well, even when a section of the San Francisco Aqueduct exploded and the dam was the only source for water.
Water levels began to rise in the six months before the disaster, and at one point levels were measured at one foot short of the spillway. But it continued, and a few days before the disaster, on the 7th March 1928, there were only three inches left. Mulholland said that no more water should be let into the reservoir.
On the morning of the disaster, and new leak was found during an inspection. Unlike other leaks, this one was a different colour of water. Investigation indicated that the foundation of the dam was being worn away. Up to twenty two gallons of water was being leaked every second. Mulholland decided that it could be fixed later, and staff went home for the day.
That night, the first indication of the failure was a drop in voltage on power lines, recorded at 11.57 pm. There were no witnesses when the dam collapsed, but as it did, 12.4 billion gallons of water flowed in a wall down the canyon and into the surrounding areas. The first to die were the dam keeper and his family, as their house was a mere quarter mile from the site. The wave was measured at forty three meters high when it hit their home, and the dam keeper’s body was never found.
The wave continued to surge forward at eighteen miles an hour, and with momentum, the wave reached thirty seven metres in height when it hit a local powerhouse, killing sixty four workmen and their families. The dam water joined the local rivers, causing them to overflow and flood local towns. It completely demolished local substations, so there was no power.
In five and a half hours, the flood waters had travelled fifty four miles. In a few cases, the flood victims were found as far away as Mexico.