Burned slopes, like the ones in this photo, are ripe for landslides – in fact rivers of gravel and small stones are already flowing down them. They can also generate floods by shedding rain much more quickly than vegetated slopes. But for pure shock and awe, slides and floods alone can’t hold a candle to the mayhem caused by the landslide/flood hybrids known as debris flows. And debris flows, remember, did enormous damage in Big Sur following the 1972 fire.
Here’s a taste of what a debris flow looks like in action:
Those weren’t exactly pebbles flowing down that hill were they?
Debris flows have the power to move everything from gravel and sticks to huge boulders. And they can move all this stuff a lot further from the slopes where they started than you might think.
So what does it take to start a debris flow?
1) A large quantity of loose material
2) A lack of vegetation to hold that material in place
4) A lot of water
So how does Big Sur stack up?
Well, as can be seen in the photo at the top of this page, there’s no lack of loose gravel and rocks in the Santa Lucias. The slopes are more than steep enough. And the fire has removed large swaths of the vegetation that holds the loose material in place. So the only missing ingredient is water. If it rains hard enough before new growth stabilizes the slopes, debris flows will almost certainly result. Not much anyone can do about that besides hope for gentle rain.
Need more convincing that this is a serious threat? Here’s a helicopter view of a debris flow that happened only a couple of weeks ago in Lake Isabella, California, when a thunderstorm dropped heavy rain on a recent burn:
The bottom line? If you live anywhere near a stream channel draining burned slopes, move your family treasures somewhere else before the rains start – and consider staying somewhere else yourself when the first rains of the season begin. The first warning of a debris flow is more likely to be boulders knocking down your door than a friendly deputy knocking on it to politely inform you of the mandatory evacuation.