Civil Engineering Disasters – The St. Francis Dam Flooding

Amongst the history of civil engineering disasters, one of the least known is the collapse of the St. Francis Dam and subsequent flooding. Although this disaster claimed more than 500 lives and destroyed millions of dollars of property, very little is heard about it. Even so, it had a major impact on the history of civil engineering and civil engineering regulations today.
The St. Francis Dam was designed by William Mulholland, the self-taught chief engineer for the Los Angeles Department of Water and Power. He was an experienced dam engineer, having completed several smaller dams before this one. He was also responsible for the design and construction of the Los Angeles Aqueduct, which was the longest aqueduct in the world when it was built in 1913. Without this aqueduct, Los Angeles probably would not have grown to the size and prominence it has today.

 

The St. Frances Dam was a curved concrete gravity dam. By definition, a gravity dam is one where that depends upon the force of gravity to prevent it from being pushed aside by the water it contains. Essentially, the force of the water behind the dam is pushing the dam to tip forward upon it’s toe (the most downstream point of the dam). The weight of the dam is acting against that, causing it to rotate downward into the earth from the same point. As long as the weight of the dam structure itself is greater than the weight of the water behind the dam, it will not move.

Gravity dams are very common due to their high reliability and relative lower construction cost. They can be either solid or hollow, and can be made of only concrete or a combination of concrete and dirt. Other than the weight of the dam, the other major design criterion is to ensure that the toe of the dam is sunk deep enough into the earth to prevent it from sliding forward.

What caused the St. Francis Dam to fail wasn’t the design of the dam or its construction, but rather a limited understanding of the geological foundation of the dam. In the 1920s, when the dam was built, the technology wasn’t available to properly determine the strength of the paleo-mega-landslide rock formations that the left abutment rested upon. Two of the most famous geologists of the day examined the site and determined that it was suitable for the dam’s construction.
This was the major contributing factor to the failure of this dam. However, it was not the only factor. Twice during the construction of the dam, Mulholland added an additional ten feet to its height. While he compensated for this additional height in his design, those compensations didn’t bring the dam up to the standards practiced by other dam engineers of his day. According to modern standards, the design of this dam was subpar for a concrete dam of its size.