William Mulholland, the man who oversaw construction of the Los Angeles Aqueduct, occupies a huge place in the history of water use in California—not all of it good. The aqueduct, completed in 1913, conveyed much-needed water to Los Angeles from Owens Valley, but it virtually ended agricultural production there and dried up Owens Lake, which became a huge source of airborne dust.
William Mulholland, the man who oversaw construction of the Los Angeles Aqueduct, occupies a huge place in the history of water use in California—not all of it good. The aqueduct, completed in 1913, conveyed much-needed water to Los Angeles from Owens Valley, but it virtually ended agricultural production there and dried up Owens Lake, which became a huge source of airborne dust. [text_ad] Two new books about Mulholland have recently been published; both deal with his role in the incident of the St. Francis Dam, which was located 50 miles north of Los Angeles. On March 12, 1928, the dam collapsed, sending 12 billion gallons of water into San Francisquito Valley and killing more than 400 people. Earlier that day, an employee of Mulholland’s had noticed a crack in the dam. He summoned his boss to inspect it, and Mulholland declared the dam safe. When it collapsed hours later, that employee and his family were among the casualties. The St. Francis Dam was a huge structure, more than 200 feet tall, holding back enough water to create a devastating flood when it gave way; by some accounts the wall of water that surged through the valley was 125 feet high. Few stormwater structures are likely to hold nearly as much water. And yet, dam failures are something we do need to be aware of. Smaller dams associated with stormwater facilities can and do fail. And storm events themselves can lead to failures of dams that were perhaps already weakened, or of earthen barriers that had been slowly eroding before a large storm hit. As we saw recently during the flooding in South Carolina, many dams that had held fast for decades failed during the record storms. Although about 5,000 dams exist across the state, only “classified” dams—those on the radar of the state’s Department of Health and Environmental Control—are regularly inspected. That accounts for only about half the dams. Of the others, some are nearly a century old; many were built during the Great Depression by the Works Projects Administration. Some don’t conform to strict engineering standards, and some are on private property and not readily accessible. Many have no emergency spillways, making them more likely to fail. This article from the March/April issue of Stormwater magazine examines how designers can apply dam breach analysis to small stormwater management ponds—looking at the potential downstream impacts, calculating breach rates and release volumes, and determining how to lessen the risks. The article looks at both “sunny day” failures—those that happen without a storm, such as when the primary outlet of a facility is blocked and water can’t be released—as well as storm-event failures and overtopping without an actual dam failure occurring. What type of dam hazard classification does your local jurisdiction use—for example, FEMA’s standard low-, significant-, and high-hazard ratings, or a more complex numeric or alphabetic system? How many of the dams, levees, and other flood control structures would you say receive regular inspections?
Two new books about Mulholland have recently been published; both deal with his role in the incident of the St. Francis Dam, which was located 50 miles north of Los Angeles. On March 12, 1928, the dam collapsed, sending 12 billion gallons of water into San Francisquito Valley and killing more than 400 people. Earlier that day, an employee of Mulholland’s had noticed a crack in the dam. He summoned his boss to inspect it, and Mulholland declared the dam safe. When it collapsed hours later, that employee and his family were among the casualties.
The St. Francis Dam was a huge structure, more than 200 feet tall, holding back enough water to create a devastating flood when it gave way; by some accounts the wall of water that surged through the valley was 125 feet high. Few stormwater structures are likely to hold nearly as much water. And yet, dam failures are something we do need to be aware of. Smaller dams associated with stormwater facilities can and do fail. And storm events themselves can lead to failures of dams that were perhaps already weakened, or of earthen barriers that had been slowly eroding before a large storm hit.
As we saw recently during the flooding in South Carolina, many dams that had held fast for decades failed during the record storms. Although about 5,000 dams exist across the state, only “classified” dams—those on the radar of the state’s Department of Health and Environmental Control—are regularly inspected. That accounts for only about half the dams. Of the others, some are nearly a century old; many were built during the Great Depression by the Works Projects Administration. Some don’t conform to strict engineering standards, and some are on private property and not readily accessible. Many have no emergency spillways, making them more likely to fail.
This article from the March/April issue of Stormwater magazine examines how designers can apply dam breach analysis to small stormwater management ponds—looking at the potential downstream impacts, calculating breach rates and release volumes, and determining how to lessen the risks. The article looks at both “sunny day” failures—those that happen without a storm, such as when the primary outlet of a facility is blocked and water can’t be released—as well as storm-event failures and overtopping without an actual dam failure occurring.
What type of dam hazard classification does your local jurisdiction use—for example, FEMA’s standard low-, significant-, and high-hazard ratings, or a more complex numeric or alphabetic system? How many of the dams, levees, and other flood control structures would you say receive regular inspections?
