Debris Flow Consequences

The May 19 report by Melinda Burns about a study of past debris flows in our area is a wake-up call. Debris flows are not rare, once in a millennium events, but they occur on an astoundingly frequent basis.

Geological and historical evidence shows that the area between Gaviota and Carpinteria has experienced 36 of those flows in the last 100 years! When they do occur, they destroy homes, property, and often take lives, like the dreadful event in Montecito on January 9, 2018. Most of the flows occur after wildfires that denude steep mountainsides and are triggered by winter rains.

Even though these disasters occur regularly, they are infrequent enough in any one specific spot that our short-term memories tend to forget and discount them. That is unfortunate. It is a grievous mistake in the age of climate change. Wildfires continue to become more frequent, burn hotter, often spread explosively, and consume larger and larger areas. At the same time, while we are experiencing a megadrought, the rain that does fall tends to occur in shorter, more concentrated bursts that greatly heighten the possibility of triggering a flow. Looking ahead, we should expect that debris floods like the one in Montecito will become more frequent and ever more devastating.

My home at the bottom of Mission Canyon sits on an ancient debris flow that rerouted Mission Creek, and sculpted the picturesque landscape of Rocky Nook Park. Mission Canyon is a prime candidate for another debris flow. We don’t know when it will occur, but it almost certainly will come, and it could be calamitous, threatening homes, lives, and infrastructure. One of our essential pieces of infrastructure, and a beloved historical icon, is the stone bridge over Mission Creek behind the Old Mission. Now 130 years old, the bridge has withstood various floods, but it almost certainly would not be able to handle a debris flow of the magnitude that we should prepare for in the future.

A few years ago, the City of Santa Barbara commissioned a respected engineering firm to conduct a study of the Mission Bridge and make recommendations for improvements that would enhance the safety of pedestrians, bicyclists, and motorized traffic, and strengthen the bridge to serve effectively for another 100 years. The consultants’ final report recommended options that would protect the bridge and the neighborhood around it from future floods and debris flows. In the end, though, the city decided last year to greatly limit the scope of the project to the possibility of adding a new pedestrian passage, while leaving the bridge itself and the roadway untouched.

In the light of the new insights, this was a grave mistake as it discounts the near certainty of future debris flows and the likelihood that climate change might make them monster. Hardening the bridge and its surrounding against the inevitable and ever worsening impacts of climate change should be a high priority for the city and county.