Slides, Floods & Debris Flows: Will the Coming Winter be Worse than the Fires?

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

3) Slopes

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.

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7 Responses to Slides, Floods & Debris Flows: Will the Coming Winter be Worse than the Fires?

  1. Tzila "Z" Duenzl says:

    I first thought of this when I saw how hot and hard the Basin fire was burning, and the results on the hillsides and in canyons and draws. Each resident must assess their property and decide whether there is a safety factor in the coming rains. Even ‘slight’ rain can cause flows in/on ‘moon-scaped’ land. I have two sets of friends thinking about the wisdom of not spending Winter on their land, as they have extensive damage in the area, and fear that the highway could be cut off for some time, trapping them.

    But…that is the glory and awe of Nature…the unknown danger, and all of us who do or have lived within a Wilderness area know this, and perhaps even get pleasure from it.

    Then, of course, the Sheriffs will come and make everyone stay off the road anyway…for our own safety.

  2. Annie Campbell says:

    There was a good article in the paper, a couple of weeks before the fire was even contained, all about the preparations to minimize winter damages through seeding and other tactics. They are definitely ‘on it’ and have been for a while. I was living in Big Sur in ’72 and saw what the floods did to the village. I’m pretty sure that fire didn’t ‘hold a candle’ to this one, as far as acreage burned. Yes, this winter could be devastating.

  3. Butch Kronlund says:

    I have been escorting 2 geologists/geomorphologists around Big Sur recently in an effort to jump start the assessment process which is pre-requisite for obtaining funds to effect remedies. One of these men, Barry Hecht, was prominent as a staff geologist during the 72 Molera fire and has over 40 years experience analysing post fire hydrological issues all over the world. When afforded the vantage point of both top of
    ridge and bottom of watershed as well as having the benefit of fire intensity mapping,
    it’s clear that he is very concerned about down stream mud flows of historic proportions.

    It is common knowledge that the last fire to burn this extensively on the western flank of the Santa Lucia as well as burn the majority of the back country affecting the Big Sur watershed was in 1907. The Highway was completed in 1939.

    The natural course of events would be that gravity and the lack of soil stability dictate
    that the drainages act as conduits for the transfer of large amounts of soil, rock, wood and ash to make their way to the ocean this winter. This is no bulletin. What is of note is that the amount and suddeness of mud flows after 100 years of relative stability on the slopes exceeding 45 degrees will certainly be unprecedented if we have anything close to a normal winter. Take a good hard look at your surroundings
    if you live anywhere near any of the more than 23 named watersheds and many more
    unnamed drainages from Molera to Dolan Canyon.

    Also, stay tuned for opportunities to participate in workshops that address erosion
    and watershed issues being held at the Grange, sponsored by the Coast Property Owners Association. This Organization of which I’m a Board member is actively
    trying to provide the best information and provide leadership and structure to a chaotic
    and disjointed response to the post fire issues facing us.

  4. Annie Campbell says:

    The voice of one crying in the wilderness: “Prepare!” Let us pray that God will be merciful and make this a low rainfall winter, giving the soil time to regrow and rejuvenate some erosion protection. Your comment on the timeline of the last Big Fire (1907) compared to the building of Highway 1 (1939) is duly noted. It has not really been tested. This highway washes out in places during the winter as it is. I can’t wrap my mind around this coming winter. When I was a kid we used to count the creeks on our daily drive from the Farmhouse (now Esalen) to Nepenthe. If memory serves me right, there were about a dozen? Now is the time to pray and prepare.

  5. bigsurkate says:

    The CPOA leadership in this area is striking and I, for one, am truly grateful for their energy and hard work. We all have to work together, and quickly, to avoid disaster this winter.

  6. Alex says:

    I found your site on technorati and read a few of your other posts. Keep up the good work. I just added your RSS feed to my Google News Reader. Looking forward to reading more from you down the road!

  7. Butch Kronlund says:

    As promised, an erosion control workshop sponsored by CPOA and the BSLT is scheduled for this Thursday 8-21-08 at 6 PM at the Lodge Conference Center
    at Pfeiffer State Park. Geologists, Hydrology experts and a Forrester will discuss
    remedies and be available for Q and A. We hope to see everyone with more than a passing interest in preparing for this winter attend.

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