Category Archives: Basic stuff

January 30, 2009 · :

Culverts

I got a question from a reader who wants to know what makes a good culvert.  She has some land, and an impaired stream crossing, and a mother with a bulldozer, who is not afraid to use it.  She has been offered a new culvert for Christmas.  I love this.

Caveats:
I don’t know anything about this stream or crossing, and am surely not offering engineering advice here.
A project like this could well require permits, from the Army Corps or from the state Fish and Wildlife department.

But, I can tell you in general what you want from a culvert. What you want from a culvert is that it doesn’t get in the stream’s way. From the upstream side, the culvert should be able to pass a flood’s worth of water. If is too small, or choked with debris, water will back up behind it. You’ll get a big pool behind it, if flood pressure doesn’t wash out the obstruction.

On the downstream side, the worry is whether fish can swim upstream. A too-small culvert is bad news for that. Water constrained through it will act like a fire hose; during wet periods, it can be a velocity barrier, too fast for fish to swim through. There are lots of studies and specs on fish flows. If you want to get precise, you can find out exact velocities for different species at different life stages. If you don’t want to be precise, you can look at nearby reaches with fish. They have small pools upstream of rocks, and eddies near the edges where fish can rest between bursts of swimming against the current. Stillwater refugia. You want that.

Fortunately, the solution to both these problems is the same: big wide culverts, with floors that look like the natural stream. (Here.) As a rough recipe, put in a big section of pipe as wide as the stream, buried about a third deep. “As wide as the stream” is wider than just the water, incidentally. You should look for bankfull width, where the sides start to slope down.

***
There is another potential problem, one that I hope is not plaguing the reader. It is possible that the streambed below the culvert has been eroding, and the culvert is now the most hardened point along the stream. (The nick point.) The perched culvert is the only thing stopping that waterfall from marching upstream. This is a big problem for fish (because it is hard to leap into a firehose and take off swimming against the flow), and a big problem for fixing the stream. Basically what you have to do then is elevate the downstream section of the stream with a series of weirs. That’s more project than I hope the reader faces.

So, this site is great, with lots of pictures. Another guide, with pictures of failures. Basically, make your culvert really big, as transparent to the stream as you can. 

Good luck!  Send pictures!

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December 29, 2008 · :

I will never explain the unit “acre-foot” on this blog.

I’ve been slow to finish up my review of the Pacific Institute report, because that will mean that I have to get into the concepts of field and basin efficiency and I dread that.    I don’t mean to be a tease; I know how badly you’ve been craving more of this series.  I’ll put some peripheral thoughts here,  so we can have a clean discussion of field and basin efficiency and the core conclusion of the Pacific Institute report without them buzzing around and plaguing us.

A thought:

I agreed with the technical critiques in the irrigation professors’ report, which isn’t too surprising, since one of them trained me.  If you have any technical questions about anything in those reports, I’d be happy to take my best shot at answering it.  As much as those critiques cast doubt on whether the Pacific Institute report is identifying real potential for water savings, I’m also a doubter.

Another thought:

I said that I’d critique the Ag Water Management Council’s report.  Here goes.  This is self-reporting by a self-selected group of the most progressive ag water districts.  I don’t think the claims are very impressive; there’s a lot of hedging about how far along the districts are in adopting Efficient Water Management Practices.  That said, these leading districts are alert to the practices and implementing them at some rate unspecified by the report.  I don’t take this report as proof that ag is doing everything it can.  It is more like the best likely spin you could put on ag’s water use practices.  Also, man.  Love the pictures. 

Yet more thought:

For your sense of scale, here are some recent rough numbers.

3.5 million acre-feet/year – the Pacific Institute report thinks this is the amount ag could yield without hurting, or maybe even while doing better.

31.5 million acre-feet/year – this is roughly how much ag applies overall.  Am I really doubtful that irrigated ag could give up 10% of its applied water without hurting?  Are they really so tight that there isn’t 10% slack in the system?  Yeah, I really am doubtful, and I’ll tell you why when I tackle the field and basin efficiency talk.  But OH LOOK!

6 million acre-feet/year – this is how much warmer winters will cost CA in snow storage of water by mid-century.  That’s twice the amount we’re wrangling about in the Pacific Institute report, so as much as the Pacific Institute report seemed to be making bold claims, this should be twice as dramatic.

I’ll give you three more numbers that I find really handy.

10 million acres – the rounded-up area of irrigated ag in California.  It is closer to 9 million acres these days, but 10 million acres is easier to use.

California is about 100 million acres. 

As a very, very rough estimate, one irrigated acre uses about three acre-feet of irrigation water a year.  (This goes neatly with the 10 million acres and 31.5 million acre-feet/year.)

These are the numbers I use to get a gut sense of things*, decide whether a claim is big or little or ridiculous.  You may use them too.

 

 

 

 

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December 12, 2008 · :

Mitigation and adaptation

In the broadest sense, people talk about two aspects of addressing climate change, mitigation and adaptation.

Mitigation is preventative, trying to shape our society so we put out as little greenhouse gas as possible and avert as much climate change as we can. Mitigation tends towards energy, sprawl and transportation policy. In California, mitigation is being handled by the AB 32 Scoping Plan. The other state agencies pretty much think that the Air Board has got mitigation covered and are now thinking about adaptation.

Adaptation is the acknowledgement that we will suffer from the effects of climate change already underway, so we need to get ready for that. Adaptation will heavily influence water resources, disaster preparedness and response, coastal anything, natural systems like forests and deserts, biodiversity and agriculture. The agencies that address those are now looking fifty years out and guessing what we need to do to be ready for a harsher world. They’re starting to issue reports, saying what they’ll do and what they expect everyone else to do. I’ll tell you about those as they come out.

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