[LMB] [OT] More McPhee...on southern CA mudslides
sdean2003 at gmail.com
Wed Jan 10 13:01:12 GMT 2018
...in the news now. You think you could conceive of them and you've seen the pictures. It's so much worse and McPhee
lays it out like the end-of-the-world Act of God they are...a riveting read...note he wrote this in 1988....nothing has
> The water was now spreading over the street. It descended in heavy sheets. As the young Genofiles and their mother
> glimpsed it in the all but total darkness, the scene was suddenly illuminated by a blue electrical flash. In the blue
> light they saw a massive blackness, moving. It was not a landslide, not a mudslide, not a rock avalanche; nor by any
> means was it the front of a conventional flood. In Jackie’s words, “It was just one big black thing coming at us,
> rolling, rolling with a lot of water in front of it, pushing the water, this big black thing. It was just one big
> black hill coming toward us.”
> In geology, it would be known as a debris flow. Debris flows amass in stream valleys and more or less resemble fresh
> concrete. They consist of water mixed with a good deal of solid material, most of which is above sand size. Some of it
> is Chevrolet size. Boulders bigger than cars ride long distances in debris flows. Boulders grouped like fish eggs pour
> downhill in debris flows. The dark material coming toward the Genofiles was not only full of boulders; it was so full
> of automobiles it was like bread dough mixed with raisins. On its way down Pine Cone Road, it plucked up cars from
> driveways and the street. When it crashed into the Genofiles’ house, the shattering of safety glass made terrific
> explosive sounds. A door burst open. Mud and boulders poured into the hall. We’re going to go, Jackie thought. Oh, my
> God, what a hell of a way for the four of us to die together.
> The parents’ bedroom was on the far side of the house. Bob Genofile was in there kicking through white satin draperies
> at the panelled glass, smashing it to provide an outlet for water, when the three others ran in to join him. The walls
> of the house neither moved nor shook. As a general contractor, Bob had built dams, department stores, hospitals, six
> schools, seven churches, and this house. It was made of concrete block with steel reinforcement, sixteen inches on
> center. His wife had said it was stronger than any dam in California. His crew had called it “the fort.”
...after, no one came for them, because no one thought they could have survived
> the neighborhoods of northern Los Angeles assume a macabre resemblance to New England villages under deep snow: the
> cleared paths, the vehicular rights-of-way, the parking meters buried within the high banks, the half-covered
> drift-girt homes. A street that is lined with palms will have debris berms ten feet up the palms. In the Genofiles’
> front yard, the drift was twelve feet deep. A person, without climbing, could walk onto the roof. Scott’s bedroom had
> a few inches of space left at the top. Kimberlee’s had mud on the ceiling. On the terrace, the crushed vehicles, the
> detached erratic wheels suggested bomb damage, artillery hits, the track of the Fifth Army. The place looked like a
> destroyed pillbox. No wonder people assumed that no one had survived inside.There was a white sedan under the house
> eaves crushed to half its height, with two large boulders resting on top of it. Near the pool, a Volkswagen bug lay
> squashed. Another car was literally wrapped around a tree, like a C-clamp, its front and rear bumpers pointing in the
> same direction. A crushed pickup had boulders all over it, each a good deal heavier than anything a pickup could
> carry. One of the cars in the swimming pool was upside down, its tires in the air. A Volkswagen was on top of it.
James Mc Phee is a name to conjure with; /nobody/ writes about the world with such facility, such catholic interest, in
such easy, powerful and yet precise and detailed language. Here is explaining one of the plagues of the paradise that
is Los Angeles. It is said that Nature has two settings in California: off, with weeks or months of halcyon sunny days
in paradise, and on, when there's earthquake, flood, fire, you name it. The mountains that form the eastern wall of LA
are some of the newest, steepest and most unstable anywhere on earth....and then fire and rain combine to make the
debris flow above...and bring down boulders as big as house down from the mountains and out far into the plains of LA.
We've seen the images of fire ringing LA, where it hasn't burned in 70 years
> The chaparral fires are considerably more potent than the forest fires Wade Wells saw when he was an undergraduate at
> the University of Idaho or when he worked as a firefighter in the Pacific Northwest. “Fires in the Pacific Northwest
> are nothing compared with these chaparral fires,” he remarked. “Chaparral fires are almost vicious by comparison.
> They’re so intense. Chaparral is one of the most flammable vegetation complexes there are.”It burns as if it were
> soaked with gasoline. Chaparral plants typically have multiple stems emerging from a single root crown, and this
> contributes not only to the density of the thickets but, ultimately, to the surface area of combustible material that
> stands prepared for flame. Hundreds of acres can be burned clean in minutes. In thick black smoke there is wild orange
> flame, rising through the canyons like explosion crowns. The canyons serve as chimneys, and in minutes whole mountains
> are aflame, resembling volcanoes, emitting high columns of fire and smoke. The smoke can rise twenty thousand feet. A
> force of two thousand people may fight the fire, plus dozens of machines, including squadrons in the air. But Santa
> Ana firestorms are so violent that they are really beyond all effort at control. From the edge of the city upward,
> sixteen miles of mountain front have burned to the ridgeline in a single day.
...and all of that fire is part of a chain that will lead to the next step...
> Fine material tumbles downslope and collects in the waterless beds of streams. It forms large and bulky cones there,
> to some extent filling the canyons. Under green chaparral, the gravitational movement of bits of soil, particles of
> sand, and other loose debris goes on month after month, year after year, especially in oversteepened environments,
> where it can represent more than half of all erosion. After a burn, though, it increases exponentially. It may
> increase twentyfold, fortyfold, even sixtyfold. This steady tumbling descent of unconsolidated mountain crumbs is
> known as dry ravel. After a burn, so much dry ravel and other debris becomes piled up and ready to go that to live
> under one of those canyons is (as many have said) to look up the barrel of a gun.
> One wouldimagine that the first rain would set the whole thing off, but it doesn’t. The early-winter rains—and
> sometimes the rains of a whole season—are not enough to make the great bulk move. Actually, they add to it.
> If you walk in a rainstorm on a freshly burned chaparral slope, you notice as you step on the wet ground that the
> tracks you are making are prints of dry dust. In the course of a conflagration, chaparral soil, which is not much for
> soaking up water in the first place, experiences a chemical change and, a little below its surface, becomes
> waterproof. In a Forest Service building at the foot of the mountains Wade Wells keeps some petri dishes and soil
> samples in order to demonstrate this phenomenon to passing unbelievers. In one dish he puts unburned chaparral soil.
> It is golden brown. He drips water on it from an eyedropper. The water beads up, stands there for a while, then
> collapses and spreads into the soil. Why the water hesitates is not well understood but is a great deal more credible
> than what happens next. Wells fills a dish with a dark soil from burned chaparral. He fills the eyedropper and empties
> it onto the soil. The water stands up in one large dome. Five minutes later, the dome is still there. Ten minutes
> later, the dome is still there. Sparkling, tumescent, mycophane, the big bead of water just stands there indefinitely,
> on top of the impermeable soil. Further demonstrating how waterproof this burned soil really is, Wells pours half a
> pound of it, like loose brown sugar, into a beaker of water. The soil instantly forms a homuncular blob-integral,
> immiscible—suspended in the water.
> In the slow progression of normal decay, chaparral litter seems to give up to the soil what have been vaguely
> described as “waxlike complexes of long-chain aliphatic hydrocarbons.” These waxy substances are what make unburned
> chaparral soil somewhat resistant to water, or “slightly nonwettable,” as Wells and his colleagues are wont to
> describe it. When the wildfires burn, and temperatures at the surface of the ground are six or seven hundred
> centigrade degrees, the soil is so effective as an insulator that the temperature one centimetre below the surface may
> not be hot enough to boil water. The heavy waxlike substances vaporize at the surface and recondense in the cooler
> temperatures below. Acting like oil, they coat soil particles and establish the hydrophobic layer—one to six
> centimetres down. Above that layer, where the waxlike substances are gone, the veneer of burned soil is “wettable.”
> When Wells drips water on a dishful of that, the water soaks in as if the dish were full of Kleenex. When rain falls
> on burned and denuded ground, it soaks the very thin upper layer but can penetrate no farther. Hiking boots strike
> hard enough to break through into the dust, but the rain is repelled and goes down the slope. Of all the assembling
> factors that eventually send debris flows rumbling down the canyons, none is more detonative than the waterproof soil.
> In the first rains after a fire, water quickly saturates the thin permeable layer, and liquefied soil drips downhill
> like runs of excess paint. These miniature debris flows stripe the mountainsides with miniature streambeds—countless
> scarlike rills that are soon the predominant characteristic of the burned terrain. As more rain comes, each rill is
> going to deliver a little more debris to the accumulating load in the canyon below. But, more to the point, each
> rill—its natural levees framing its impermeable bed—will increase the speed of the surface water. As rain sheds off a
> mountainside like water off a tin roof, the rill network, as it is called, may actually cube the speed, and therefore
> the power, of the runoff. The transport capacity of the watershed—how much bulk it can move—may increase a
> thousandfold. The rill network is prepared to deliver water with enough force and volume to mobilize the deposits
> lying in the canyons below. With the appearance of the rills, almost all prerequisites have now sequentially occurred.
> The muzzle-loader is charged. For a full-scale flat-out debris flow to burst forth from the mountains, the final
> requirement is a special-intensity storm.
In LA this time there was nearly an inch of rain in 15 minutes. That did it. And afterwards...
> When you walk in the stream valleys of the San Gabriels, you will see rocks the size of heads wedged among the
> branches of trees. In a small tight valley called Trail Canyon, I saw two boulders that were a good deal wider than
> the bed of the brook that had carried and rounded them. They were bigger than school buses. Surrounded by lesser
> debris, they had moved a long distance in its company. At a guess—from their dimensions and specific gravity—the
> aggregate weight of the two rocks was a hundred and sixty tons. In February, 1978, a boulder weighing three hundred
> and fifteen tons ended up on a residential street about a third of a mile inside the Los Angeles city limits. Through
> some neighborhoods, boulders in great numbers advance like Chinese checkers. People pile them up against fences, use
> them in retaining walls. When Dan Davis was working for Flood, he found debris—on an urban thoroughfare after a
> storm—a mile and a half from the nearest debris basin. (“When I saw that, I knew we had a real problem.”) In 1938, a
> restaurant on the main street of Sierra Madre was destroyed by invading boulders. Two-foot boulders rumbled through
> Claremont, coming to a stop three miles from the mountain front. Five miles from the front you can see boulders a foot
> in diameter. If you ask people how the rocks got there, they assume it was by a process that is no longer functioning.
> If you suggest that the rocks may have come from the mountains, people say, “No way.” Off the eastern end of the San
> Gabriels, rocks the size of soccer balls are eight miles south of the front.
Like I say, a riveting read. The local government builds empty reservoirs that will never hold water but are meant to
try to catch and hold much of the debris slides. When they fill up, a contractor mines them until they are empty again.
Mother Nature laughs at us.....
// Stewart Dean == Kingston, NY
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