There’s been some rain in the mountains here in southern California, and as a result, the washes/arroyos/whatever you want to call them have running water in them.
My boyfriend and I noticed with curiosity that there are periodic rushes of water, a few inches higher than the water already running in the wash, that surge periodically like waves in the ocean. What causes them? I would think the water would flow at an even rate, or that increases would not be so sudden. Why are they so sudden?
My guess would be in that the side draws upstreams, there may be natural dams such as rock falls or accumulated dead vegetation that finally breaks away giving rise to a short term increase in water flow.
Also, these draws may drain very dry, rocky, hard, arid regions with minuscule ability to actually absorb water–it’s like water running off a parking lot. Also, the area the wash/arroyo actually drains may be quite small and thus very subject to present rate of rainfall. Rain rarely falls at the same rate for any period of time. Considering that you are 10 min. down stream of the foothills. At 3:00 PM, it is drizzling (0.05"/hour). At 3:10 PM, a T-storm is moving across the foothills (1.0"/hr), but only last 5 minutes before moving on and the drizzle returns. You would expect to see the water flow increase at 3:20 PM where you are standing and last maybe a 10 minutes before subsiding again. Normally, flowrate should be “dampened” the more you are downstream of a river (during the great Mississippi flood, St. Louis nearly flooded, but New Orleans only noticed a small rise in river level). Hills and mountains can funnel a lot of water into a small arroyo in a heavy-rainfall, especially in an arid region. There is little vegetation or soil to absorb the runoff, and disastrous flash-floods can result. The Texas Hill Country is known for these flash floods. Most of the year, the weather is dry. The land is broken limestone with steep cliffs and dotted with juniper and oak with shallow soil. But during the spring, the area can tap the moisture from the Gulf of Mexico and lead to torrential downpours. I remember seeing Barton Creek in Austin during one of these episodes. During the summer, there might not even be any water in the creek. But this day it was like looking at the Ocoee or Nantahala. Someone told me the best whitewater in Texas is Barton Creek in a flood. I don’t know if I would want to paddle it because of the amount of stringers I saw though. Looks real dangerous.
The surges could also be due to normal turbulent flow in an irregular channel without any real change in inflow. I recall seeing similar phenomena in my long-ago basic fluid mechanics course. The flow would be relatively smooth and regular most of the time but it would periodically “hiccup” and a big rush of water would come barrelling down the trough. This was in a smooth channel and was just a function of the water itself, not from any obstructions or (visible) disturbances.
I can’t recall, if I ever knew, the detailed reason why this happened, but in the vague mists of my memory it was because the flow was on the edge of a transition – it wanted to flow faster or deeper or something but it didn’t quite have the right stuff. Every so often it would get bumped over the edge into the fast regime but the surge would change the conditions and it would then reset itself into the slower regime, where it would start to build up again, ad infinitum.
I’m a vegetarian once removed. I only eat meat from animals that are vegetarians.
Pluto, I’m surprised at you. That answer is not up to your usual standards. How can water just “want to” flow faster or deeper without some external force being involved (such as increased inflow or slope of the streambed, etc.)? “The edge of transition” sounds like newspeak for “it’s magic.”
Excuse me for anthropomorphizing. It was late at night.
Stated more objectively: There are different flow regimes based on conditions such as the shape and smoothness of the channel, the depth of the water, the amount of water in the channel, how fast it’s currently flowing, etc. The most important of these regimes are laminar flow, where everything flows along smoothly and Bernoulli’s Principle reigns supreme, and turbulent flow, which is characterized by unpredictable surface effects, bubbles, waves, etc. (I don’t recall if there are different regimes within the turbulent regime. Like I said, it was a long time ago.)
What I was suggesting is that if the conditions in the channel are at their maximum values for laminar flow then any small disturbance could trigger a transition to turbulent flow. This transition tends to be dramatic and sudden, as if a gate were opened. A large rush of water follows, creating a new set of conditions in the remaining water, typically back in the laminar flow regime again.
So the flow in the channel is in an unstable state – too much water for laminar flow, but as soon as it goes turbulent, there’s not enough water to sustain turbulent flow. It oscillates back and forth between the two regimes. Downstream it looks like periods of “normal” flow (while laminar) punctuated by short surges of higher flow (turbulent) which quickly reduce to normal flow again.
My point was that this is possible without any obstructions or other periodic disturbances, and is created by operating at conditions which are just slightly outside of the laminar regime. The force necessary to trigger a surge could be any small increase in flow, or just an accumulation of water which can’t flow through the channel fast enough while laminar. It’s like a superheated state – a very small trigger leads to a large response.
Whew! I hope that’s more clear. It certainly required more words.
p.s. If you look very closely you can see the turbulence elves opening and closing their magic gates. They do this whenever the water wants to go faster.
I’m a vegetarian once removed. I only eat meat from animals that are vegetarians.
What you describe are most likely “bores”. Tidal bores are relatively well-known, but sudden steps in water depth are also common in runoffs, especially where the water can occasionally spread out over flat silted washouts.
Long waves in shallow regular channels travel much faster in deeper water, so the water behind a bore tends to catch up to it, and reinforce it. So, any small flucuation that develops tends to end up as a discrete step in water depth.
Given the right geometry of the channell, bores can be self-sustaining and reinforcing. The result is that farther downstream, you will see surges of water. People have followed bores in canal waterways for miles, without the bore dissipating.
An analogous thing happens in sound: the air behind a shock wave is compressed and heated, so the speed of sound behind the wave is faster than in front of the wave, so sound “catches up” to the wave front, and reinforces the shock wave front, making the jump in air pressure very sharp.
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rocks</font>
Just to support pluto, the region is called the transition region - transition from laminar to turbulent. There are pretty good math models for flow behavior at laminar and turbulent levels, but they break down in the transition region.
Think of it as a flash flood. Traditionally, people think of flash floods as happening in a dry flood plain/arroyo/stream bed etc. But there’s no reason it couldn’t happen when there is already a flow. Basically, it may be bright and sunny where you are, but raining weasels in the nearby mountains. The water follows its natural path and voila! Flash flood.
Feelgood posted a more technical version of the same thing.
I hate to disagree, (heh) but I can’t see a transition from laminar to turbulent flow being responsible for a wave crest of “several inches.” Besides, since when would water flowing down an arroyo be anything but turbulent?
Did anyone look for a big fat broad doing cannonballs into the swimming hole upstream?
A “wash” is to the southwest what a creek or small stream is to the northeast, except the vegetation is different.
Washes also tend to be a little less wandering and meandering than the northeastern streams, probably due to the difference in vegetation mentioned above.
FixedBack
“When learned men begin to use their reason, then I generally discover that they haven’t got any.”~~*G.K.Chesterton 1908 *
But that’s not the way it’s used in the OP, who identifies it with “arroyo” and says that they actually have running water in them. From American Heritage: “11. (Western U.S.) The dry bed of a stream.”
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I’m from the West, but here in North Carolina, I think that the topic at hand is not the same as “wash tide,” either.
I believe I live just below the same mountain range Ruffian does (the San Gabriel Mountains). The arroyos are omnipresent and are almost always dry unless it is raining. Most are just gullies or ravines that have been carved by relatively recent floods. An arroyo is not a long lasting geological feature like a river or stream. I doubt many of them are much older than 100 years. They are not fed by any sort of upstream reservoir. All the water that runs in them is the collective runoff from a rainstorm. At the base of the mountains are flood control reservoirs that collect the runoff from the arroyos and directs the water to the Los Angeles and San Gabriel rivers.
As for the effect mentioned in the OP, I think Johnny LA had it right when he said “think of it as a small flash flood”. When hiking on the south side of the mountains there is only one thing that outnumbers the arroyos and that is the number of “Danger: Flash Flood Area” signs.
Well, ruffian did say “periodic rushes of water.” What’s the period? Once a day, or every couple minutes? If the latter, they are probably bores.
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When I saw “wash,” I am referring to the man made cement “rivers” leading down from mountains designed to prevent flooding in the lower lands. They are not comparable to a creek, which are (usual) natural and at the very least are not made of concrete. I never saw one of these prior to my move to California, where they are common.
These washes are about 20 feet deep and 10-20 feet wide. The only vegetation in them is the algae that has grown on the cement, or small plants that have grown up from cracks in the cement. Typically there is a very shallow (say, 1 inch) stream of water that is rarely more than a couple feet wide.
What we observed on Saturday was typical after the rain; the wash has about a foot of very fast moving water in it. (Several people drown every year when they try to cross these.) The periodic rushes occurred very frequently, say anywhere from 20 seconds to two minutes inbetween each. These were roughly 6 inches taller than the rest of the running water.
Note: I saw the same effect in a (man-made) creek that drained into the ocean.
Thanks for all the info! We’re still scratching our heads.
GermBoy, the link explains what I saw at the creek draining into the ocean, but I’m not sure it explains what I saw in the river/wash/arroyo/barranca (I’ll get to definition trouble in a moment.) The phenomenon explained at the hydrology link discusses the build up of sand; there is minute–if not no–amounts of sand or other material in these man-made structures.
Definitions: I’m having a hard time defining these things at all. When I first moved here and asked what they were, the most common response was the “LA River” or just “river.” However, these do not match my picture of the Ohio, Wabash, White or other natural rivers I’ve observed before. It seems a misnomer. I was also told it was a “wash;” this is an accurate description for when they are dry, as they frequently were. I’ve heard arroyo, and now barranca. Looking up barranca, it seems synonymous with arroyo. I didn’t realize these things had such an ambiguous name. Other names I’ve heard: creek, drain, stream…
I’d also like to say this is post #100 for me…whoo hoo! I think I can lose the “LauraRae” tag on my signature now. I’ll do that after posting this.
Ruffian ,
I’ve been to the Sand Dunes and asked the very question you pose. Explanation proffered: not so much the sand as the irregularity of the surface. Water “piles up” and gets released in rhythmic flows. I think the part about the sand giving way is not the whole answer. This creek is very shallow and, consequently, very warm in the summer. It is intoxicating to sit in it and let the water massage you.
(congrats on the 100 a milestone in any dimension).
