Los Angeles geyser on Sunset Boulevard!

Back in the 1970's I used to run on the UCLA track near Sunset Boulevard. Two days ago, a 93-year-old water pipe and a 58-year-old pipe broke under Sunset Boulevard near the track, sending a pulsating geyser of water high into the air.  You can view a video of it here (the video symbol in the center of the photo doesn't work because it's just a frame grabbed from the CNN video).

The track was flooded, as well as newly rennovated ($136 million)Pauley Pavilion, the home of UCLA basketball named in honor of the famous coach of winning teams back in the 1970's. At its peak, the broken pipes were sending 35,000 gallons of water per minute onto the streets, with estimates of 20 million gallons released before the flood was brought under control. Maybe the tartan track will survive, the basketball court is questionable. Firefighters had to rescue some people trapped in a parking structure

Flooded track and athletic field at UCLA

If you watch the video, you'll see that the jet is strongly pulsating. This is likely due to an effect known as a "water hammer." The pipeline was a high pressure line, and these lines are subject to very destructive forces due to the water hammer effect (sometimes called a hydraulic shock). These are pressure surges that arise when the water changes direction or momentum.  In the news, you'll see reports that the pipeline had to be shut down gradually--that's because they had to minimize the potential for water hammers. If a pipe is shut off suddenly at the downstream end (where the vent is on Sunset Boulevard), the mass of water upstream is still moving and therefore can build up high pressure.  Such shocks can cause further breakage in the pipelines. (This is common in noisy old water/steam heaters in buildings.)

Photo of Pauley Pavillion
basketball court by Jason McIntyre

I found an interesting set of numbers on Wiki about this effect: "In hydroelectric generating stations, the water travelling along the tunnel or pipeline may be prevented from entering a turbine by closing a valve. However, if, for example, there is 14 km of tunnel of 7.7 m diameter, full of water travelling at 3.75 m/s,^[3] that represents approximately 8000 Megajoules of kinetic energy that must be arrested. This arresting is frequently achieved by a surge shaft^[4] open at the top, into which the water flows; as the water rises up the shaft, its kinetic energy is converted into potential energy, which decelerates the water in the tunnel."

See the Wiki article for more on water hammers.