html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> From the archives: Just like I promised you.

Thursday, October 26, 2006

Just like I promised you.

You people have no idea how good you have it. You are so spoiled, in fact, that you cannot even see the luxury you take for granted. If you live in a developed nation, you have likely never considered an alternative to the ways you receive water now. You expect that you should be able to take water any time you want it. You expect it to arrive immediately on opening your tap. You think that you should be able to open your own taps, rather than waiting for a water district employee to open it for you. You do not expect that you should have to notify the water district three days in advance if you want water. You do not think it is reasonable to get water deliveries once every two weeks. You expect water within a very small range of pressure (enough to get to the top of your shower, not so much that it blows your fixtures off the wall) and you think that water pressure should stay constant as long as you keep your tap open. You think you should be able to choose how much water you take. The type of flexibility and reliability you are used to is called on-demand water delivery. On-demand water is the goal of agricultural water district modernization.

The key to giving growers the ability to take water with the frequency, pressure and duration they want is to keep your canals full, at a constant water level. This is easier said than done, especially if you there are multiple turnouts on a canal reach and growers controlling their own turnouts. A grower opens one and sucks the water level down; three growers close theirs and the canal overtops.

A common solution is to use canal gates to divide the canal into a series of pools. Then you open and close the canal gates to maintain water levels in each pool. That takes a lot of fiddling, or a lot of (expensive) sensors and gear. But! There is another very neat trick that keeps the water level in canals within a narrow range, which lets growers get a constant water pressure for the duration of the irrigation event. You can install a combination of long-crested weirs and underflow gates, and it works like magic.

A weir is a structure that water flows over. Fallen tree in a stream, stacking water in a pond above it? That is a weir. Partially submerged dam, water cresting the top? That’s a weir. Here’s the trick. For a long crest length (the weir is diagonal in the canal, the water can fall over a relatively long length), the approximate weir equation for unsubmerged flow is:

Q = CLeH1.5
Q = flow (cfs)
C = discharge coefficient (3.33, for English units)
Le = effective length (feet)
H = head over the crest. You can think of this as height of the water over the weir. (feet)

I know you sharp-eyed people caught the important of that right away. The change in water height over the weir is two cube roots of the change in flow. You can change the flow a lot, and only get a small change in water level height in the canal.

Even more exciting is that you can pair your long-crested weirs with underflow gates! An underflow gate is essentially a gated hole in the side of the canal that is always underwater. As long as your turnout is taking water out of the canal below the water level, the discharge flow rate response to a change in head is:

Qnew = Qold * (Hnew/Hold).5

Did you get that?! The flow changes as a function of the square root of the change in canal water level height. So you have your long-crested weirs making sure the water level in the canal doesn’t change very much for a wide range of flows, and you have your underflow gates making sure that flow through a farmer’s turnout isn’t changing very much for a wide range of water levels in the canal! They work together like magic! And you don’t have to do anything to make it happen! I love canals.





Maybe, if you ask me very nicely, I’ll tell you all about upstream and downstream canal control. Or, if you would rather, I could tell you about Danaidean Gates. There are canal control gates currently in use in the San Joaquin Valley that use an ancient Roman design. So cool.

9 Comments:

Anonymous Anonymous said...

You know what M, I wish you had stuck to pies and your personal problems! :)

But okay, now that you've started: could you tell us what the per capita water consumption of Americans is vis-a-vis the rest of the world and just how big a problem water shortage is as a global problem?

12:44 PM  
Blogger Megan said...

Prolly not. I don't have more than a layperson's grasp of anything besides irrigation or California's water issues. But your question about New Orleans reminded me of something...

12:51 PM  
Anonymous Anonymous said...

Megan said, "love canals". Tee hee.

1:42 PM  
Blogger Megan said...

I also said "head".

1:44 PM  
Blogger LizardBreath said...

Damn, I miss learning about technical stuff. I didn't understand that in the slightest, but I recognize the "It is so cool seeing how this works," feeling, and I hardly ever get it anymore.

4:16 PM  
Blogger Megan said...

Careful, LB. If you express even a little more interest, I'll explain it again.

(I really did try to make it as clear as I could. Perhaps if I could draw you pictures.)

4:25 PM  
Anonymous Anonymous said...

Thanks, that was fascinating. I've always been interested in learning about things for which I'd previously known nothing. Can't explain why, it's probably just a quirk of mine. But thanks again.

Peter
Iron Rails & Iron Weights

6:54 PM  
Blogger ScottM said...

When we did weirs in school, we always did the boring perpendicular to the flow weirs. I like the way your long crest length weirs seem to work-- they certainly seem to be at least as simple as the cases we learned.

And yeah, fully submerged pipes are way better (for calculation, at least) than their partially full cousins.

When I talk to my friends about what we get for our taxes, civil engineering and sanitation benefits are hard to counter.

10:42 AM  
Blogger Megan said...

Scott M:
You are practically begging me to talk about upstream and downstream canal control. If Monday and Tuesday go as well as today did, I won't be able to help myself. Then you guys will know every neat thing that I know, and you'll have no more reason to ever read my blog.

5:57 PM  

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