The Truth about the Great 2010 La Canada Mud Flood! Part 2: The Perfect Storm!

In Part 1: The Lay of the Land, Rusty explained the formation and geography of the greater Los Angeles basin.  Rusty’s lived in the northern part of that basin his whole life and has heard about and seen personally what can happen with the Perfect Storm of events.   Here is the whole process laid out for you.

Remember that Rusty described those transverse mountains that run east to west rather than north to south because the two tectonic plates merging along the San Andreas Fault make a sharp turn just north of Los Angeles.  Those plates are moving still.  That’s why LA has earthquakes and how those mountains get pushed up so steeply from the flat plains.  Those mountains are covered with a high chaparral they call the sagebrush area, where the plants grow up to a couple dozen feet tall.  They don’t look like a forest because they’re much lower to the ground and much thicker than a forest.

The perfect storm starts with that high chaparral getting thicker and thicker every year.  The plants have adapted to the somewhat arid climate, so that with just a little rain and the absence of any fires, they will grow very dense.  Several of the species of high chaparral have seeds that will not germinate unless they have been through a forest fire.  They have adapted to a familiar pattern of fires over the years.  Inevitably, during the long hot summers those plants dry out and become the perfect dry kindling, just waiting for a spark.  Pretty much anything can light them up; it can be a power line that sparks, a bolt of lightning, or unfortunately even unbalanced individuals who like to watch it burn.  Whatever the cause, once the fire starts; it is no simple matter to extinguish it.

There are two things that make a Southern California sagebrush fire unique.  The first is that the high chaparral, while not as tall as a forest, actually has much more fuel per acre.  Since it is dense and low down, that fuel is going to burn hot.  Usually, it is much hotter and faster burning than a forest fire and made worse by the second unique factor.

The second factor is the wind.  Not just any wind; it is the infamous Santa Ana winds.  These are not the normal gentle ocean breezes that slide in over the Pacific Ocean and keep the LA basin temperate.  No, these are special winds that come from the opposite direction.  They’re coming from the Mojave Desert on the other side of those transverse mountains.  When the right weather conditions exist, usually in the fall and winter, the wind direction shifts and the winds blow from north to south over the mountains and down the canyons at up to 80 to 100 mph.  Can you just imagine what that does with that hot sagebrush fire?

Rusty’s seen it from the valley below the mountains many times.  Sometimes just a wisp of smoke is visible over the mountains, and a few hours later the hillsides are ablaze, with flames running for miles along the face of the San Gabriel Mountains.

Those winds are one reason why these fires can be incredibly difficult to control.  Constantly shifting high winds carrying the fire over those steep, crumbling, slippery canyon sides does not make an easy job for any firefighter crew.  That’s why the big super scooper airplanes are so valuable in trying to keep these fires somewhat in line as they ravage hillsides.  That’s where words like “controlled” or “contained” with some sort of percentage in front of them become so significant to residents with homes bordering the sagebrush area.

To give you some idea of what it’s like during one of these so-called “firestorms”, Rusty remembers when fire trucks would wait at the top of every road that dead ended into the hills.  They would be there to protect the homes that bordered the sagebrush.  Inevitably, huge burning embers borne by the 80 MPH winds would alight on a wood shingled roof, even a few blocks below the fire trucks.  It was possible to lose the house to that windswept fire in just a few minutes.  Wood shingled roofs are largely outlawed or uninsurable in the area nowadays, but they accounted for many lost homes over the years.

Propelled by winds, the fire races along the hillsides, denuding acre after acre of “watershed” until the mountains are laid bare.  It’s bad enough to lose those dense bushes, trees, grasses and weeds that hold that crumbly, boulder-filled soil together, but the perfect storm gets worse.  Now the earth itself becomes complicit in the set up for more devastation.

You read it right; the earth itself.  These fires, with all that extra tonnage of fuel and wind, burn so hot that they actually cause a chemical change in the soil itself!  This change makes the soil somewhat waterproof so that when you pour water onto the surface it is not absorbed; it does not run into the soil but rather beads up and begins to roll down the hillside.  You can see now where all this is going, can’t you?

There is only one ingredient left for the perfect storm after the fire, winds and soil have done their thing  -  RAIN.  Although Southern California is not known as a rainy area, in general when it does rain there is a significant difference in the rainfall totals from the coast to downtown Los Angeles and up to the mountains.  As the weathermen always say “we’re expecting this much rain in Los Angeles and the mountainous “upslope” areas will get significantly more rain.”  That’s because those steep tall mountains hold up the weather pattern right on their slopes.

Not only do temperatures vary as much as 20° between the coast and inland areas; rainfall also varies dramatically.  Los Angeles averages about 15 inches of rain per year, with records as low as 3.8 inches in 1953 and as high as 34 inches in 1983.  But that is downtown Los Angeles.  The mountains can get more than double that.  In fact the San Gabriel Mountains sometimes get more density of rain over a day or two than almost anywhere else in the contiguous 48 states.

In the next installment, you’ll see what happens when the wind pushes one of the worst fires ever across the face of the foothills and south facing slopes of the San Gabriel Mountains.  That set up the ground for the perfect storm of mud in Southern California.  We’ll detail exactly what happened when the mountains were hit with 7.31 inches of rain in just 24 hours the very same year as one of the worst fires.  No one was prepared.  Life and property was lost.