I’ve seen news reports for the past few years about a long drought. It’s caused lots of wildfires.
It seems like when it does rain we hear about mud slides. The news has been talking about them for the past week.
If the ground is that dry from years of drought, then why doesn’t it soak up the water? Here in my state heavy rains go into the ground like a sponge. It takes weeks of heavy rain to saturate the ground and flood.
Six months from now we’ll hear about dry conditions,drought and wild fires in Calif. It’s like the current rain they’re getting now, never happened.
Why is California so different from other places? I don’t understand why rains don’t end their drought.
Because hard, dry ground is hard for water to penetrate. It just flows away.
Also, quite a bit of California is basically desert that we are artificially keeping wet with irrigation, sometimes to the ridiculous degree of creating rice paddies. That sort of waste eats up a lot of water.
Except it does penetrate; that’s why the mudslides are happening.
There are areas in the desert southwest where the top layer is either rock or hardened soil that is indeed hard for water to soak into in a short amount of time, and these are the situations that create flash floods: a violent thunderstorm in the mountains flows downhill (instead of being absorbed) and gets channeled into riverbeds and slot canyons, where water levels can rise from nothing to dangerous torrents in a matter of minutes.
In other parts (e.g. near LA and San Diego), the topsoil is plenty absorbent. The rain is a problem for three reasons:
They’re getting a shitload of it in an extremely short time.
The fires have burned away all of the vegetation, leaving nothing to hold the soil together.
The presence of water in the interstices of the soil increases its weight, which increases the stress on the bottom layers of soil, increasing the propensity for a slide.
The result of #2 and #3 together is that when the soil becomes saturated with water, it loses its stability. Without roots to hold it all together into a coherent mass, it’s prone to collapse under its own weight plus the added weight of the absorbed water.
The Los Angeles basin is mostly semi-desert/dry-land. It has not evolved to handle much more than the 15 inches average of precipitation it gets per year. The river system (Los Angeles, San Gabriel, Santa Ana) drains a combined watershed of approximately 4000 sq miles with an annual average discharge of 650 cu ft/s. The underground aquifers absorb relatively little of that drainage.
As a contrast, the Columbia River in the Pacific Northwest drains 258,000 sq miles with an annual average discharge of 265,000 cu ft/s, more than a couple of orders of magnitude larger (more than 400x) than the L.A. basin rivers. Seattle experiences an annual average rainfall of about 52 inches, about 3.5x the rainfall Los Angeles gets. The entire ground system has developed over the millennia to absorb vast amounts of water.
For about 3.5x the annual rainfall, Seattle gets 400x the drainage capacity. If Seattle gets a rainier year, the ground can easily capture the extra water. Los Angeles? Not so much. Which is why we get mudslides with relatively little rainfall, we experience chronic drought conditions, and we have to import most of our water from Owens Valley and the Colorado River.
The problem is that that absorbent topsoil is all covered with asphalt and concrete!
Once upon a time, water could penetrate into the ground and percolate to the aquifers. A lot of water was held in the ground. Paving the L.A. Basin prevents this. The L.A. River was lined with concrete to turn it into a flood control channel. Now all of that water that could be held in the soil gets shot right out into the Pacific – with all of the pollutants it picks up along the way.
Not that that has anything to do with mudslides, but it’s a general problem with rain in the area.
EDIT: Scooped by mikews99!
One of the problems with the current weather is that the LA area has received something like half theirannual average rainfall in a single week. Links are to historical averages at the airport (12.01 inches) and the 7 day history from NOAA (8.15 inches). Just imagine Seattle receiving 30 inches in a single week.
There are lots of places, New York City and Boston come to mind, that get more rain than Seattle. A few weeks ago all it took was a couple inches of rain and a good chunk of Western Washington was under water. 30 inches of rain and towns located near the mountains will become beachside resorts. The Seattle are also suffered a number of mudslides during the recent rains. The ground was so saturated around here there was a lot of damage caused by falling trees during what should have been a not very strong windstorm a few days after the rains.
Sorry, that should be about 37 inches. Seattle has had a much wetter than normal year and we are currently at 45 inches for the year.
If the rain would stop after it gets done flooding California, that would be bad enough. But noooo. It has to come east and give us more damn snow. We’ve had snow on the ground since early this month. We don’t need more.
W/regard to San Diego, the problem hasn’t been so much with mudslides as with lowland flooding. The San Diego River, with its terminus at the ocean extends several east and north of SD. Its located in a valley that runs up toward the mountains. As you would imagine, it collects all the run off from the miles of hillsides it flows through. In the summer/dry months it’s a lazy stream, but when rains hard it overflows its banks until it drains into the ocean.
In SD itself this valley (Mission Valley), being wide and flat, was used for an E-W Interstate. And either side of that much development occurred (housing, businesses, a stadium, etc.). Nobody could resist the flat land that was once farmland. Remember that large tracts of flat, easy to build on land are are at a premium here. That the river flows over it banks and floods the valley during these storms surprises no one. Unfortunately, there is nothing anyone can do about it. Chalk another one up for Mother Nature .
A lot of California falls under different varieties of “mediterranean climate”; situations similar to those being described are common on the Mediterranean basin: people building housing on ramblas(1), selling them to outsiders, then the outsiders being surprised to wake up during gota fría(2) to find the lower floor completely underwater; people parking in a dry rambla on August 31 and gota fría dragging their car away on September 1; landslides, specially bad if there have been droughts and in areas close to too-tamed rivers or where a hill has been carved due to roadworks…
Rambla: one of the many Spanish names for a place that’s a flat, wide area 11 months out of 12 and a raging river during the other one.
Gota fría (“cold drop”): the phenomenon whereby cold air runs under humid, hot air leading to ridiculous amounts of rain in a very short time. Many areas in the eastern coast of Spain get the immense majority of their rainfall in a single month’s time (usually within September/October).
California has a Mediterranean climate, which in this case means one rainy season in the winter and that’s it. It does not rain in the summer in CA outside of freak occurrences. ALL of the state’s water for the year must be accumulated in that single season ( mostly as snow pack, but also in reservoirs ). It is perfectly possible for a heavy storms to hit for a solid week, dropping feet of rain that results in flooding and mudslides, but for the season as a whole to erratic or abbreviated, resulting in overall water shortages. In addition several years in a row of weak winters can create a “water debt”, which means that even a single good year may not be enough to bring reservoirs back up to normal.
Also as to mudslides and fire, CA is a fire-adapted climate ( due to the very dry summers ) and fire is built into the natural system. Indeed is necessary in some areas for healthy plant propagation. Chapparral burns - it is what is “designed” to do. And when it does it can cause polymerized sheets of compounds a few molecules thick below the surface of the ground that are largely impermeable to water. As a result water saturated soil will eventually wash right off the top of that layer and come sheeting down. A paper on the topic.
Last year, 2009, we had a record breaking wet year. Ended the year 10 inches above normal. Nearly all of it absorbed into the ground. We had some mild flooding, in a few areas, towards the end because it was pretty saturated.
It sounds like Seattle gets double the rain we ever get.
I only visited Calif. once and didn’t learn much except it’s cold. I froze my ass off during a trip with no coat.
Also, keep in mind that California is an enormous state with a huge number of climates. Having a drought in northern California and a flood in southern California is like having a drought in D.C. and a flood in Savannah.
Let me guess…San Francisco?
Anyway…yes, what everyone said.
Los Angeles is incredibly overdeveloped - pretty much the only areas that haven’t got houses, malls, etc on them are some of the hills.
When it rains in the winter/early spring, this causes any and all wild plants (remember, they’re mostly on the hills) to grow like…well, like weeds. They know this is their only shot at growing and reproducing, and they get right to it.
When the summer comes, the abundant growth dries out.
When it dries out, the abundant dead plants easily catch fire.
When the dried plants catch fire, the roots lose their hold in the soil.
When the soil isn’t held together by roots, the next rain causes the saturated earth to become slidey, slidey mud.
And thus…heavy rains = 1) mudslides in the short term and 2) a much higher risk of fire in the coming seasons.
Add to this the fact that many of the houses that are on the hills are multimillion dollar movie star homes, which are put there to take advantage of the beautiful views…and which then become threatened by the mud/fire…and it causes some Serious Concern.
