Tom Jacobs introduced our speaker today, Ed Keller, professor of Earth Science and Environmental Studies at UCSB. Professor Keller provided the group with a geological perspective on the Montecito Debris Flow of January 9^th. He has been studying debris flows in the region for decades, and after this recent debris flow he has put together a team of researchers at UCSB to gather and analyze data from this event to allow for better understanding of how these debris flows occur and perhaps how to better predict their area of effect in the future.

Professor Keller described how a large debris flow is a relatively rare event that comes as a “one-two punch”. More specifically, there are three stages that need to occur for such an event to take place. First, there needs to be an accumulation of debris (i.e. boulders) in the canyon floor, which may take hundreds of years. Second, a wildfire needs to burn through the area (which occurs every 30-50 years) and form water-repellant soil. Lastly, intense precipitation (such as 0.5 inches in a 5-minute period, which occurs every few hundred years) must take place. After all three stages are met, a debris flow may occur only minutes later.

Debris flows have a viscosity about 200 times that of water. The unit weight of the debris flow mud is about 120 pounds per cubic foot, while the debris flow boulders have a unit weight of about 150 pounds per cubic foot (only a 30 pound difference!). As a result, boulders “bob like corks” and are carried along near the surface at the front or the sides of a debris flow. When the flow slows down, the mud moves out from under the boulders, leaving behind a boulder field.

Much of Santa Barbara and surrounding areas are built on old debris flows. For instance, Rocky Nook Park sits on just a small portion of the Mission debris flow (which occurred about 1,000 years ago), which in total is about 10 times larger than the Montecito debris flow.

For any further questions, you can contact Dr. Keller by email (keller@geol.ucsb.edu). He also was most kind and offered a PDF copy of his paper, “The Natural History of Misssion Creek A Story of Earthquakes and Debris Flows: A Walk Through History”, as well as a PDF copy of his 2011 book (currently sold out), “Santa Barbara Land of Dynamic Beauty: A Natural History” to any of those who are interested. If you would like a copy of either document, please email Mike Conte (mcontedo@gmail.com).

 

Visiting Rotarian Henry Siegel drew a silver marble from the bag, and Dr. Young won the fruit basket.