Professor Paddle: Sky Gauge Recalibrated??
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arnobarno
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  Quote arnobarno Replybullet Topic: Sky Gauge Recalibrated??
    Posted: 23 Oct 2010 at 7:07pm
Looks like they've made some changes to the calibration of the Sky gauge. The entire series seems like it has shifted down by hundreds of cfs.

Two examples: On Oct 12, I paddled the Sky and noted that the flow was 3700 at 4:30PM. Now, the data is showing it at 3170. On Oct 15, I paddled the Sky and noted the flow around 2300 and it is now showing 1900.

This morning, the flow looked to be around 1400, now it is showing it around 1100.

Anyone know what is going on??
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fiddleyak
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  Quote fiddleyak Replybullet Posted: 23 Oct 2010 at 7:27pm
After noticing this phenomenon on another run, I emailed USGS. Basically, every once in a while the actual CFS is measured and compared to the foot gauge. If the foot gauge is reading differently, the "conversion" from feet to CFS is altered. You can track these changes at http://waterdata.usgs.gov/nwis/measurements/?site_no=12134500
On Oct 13th the "GH Change" was "-0.4". This means that the foot guage was reading high and so previously reported CFS ratings were too high also. They are retroactively changed to the lower, more accurate values. Confusing, I know.
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  Quote jP Replybullet Posted: 23 Oct 2010 at 8:00pm
Right on, well thanks for shedding light on that, Ben.
 
Looked as if Robe could be in the zone for next weekend. Wanna paddle it  if it is? I think I'll be in Seattle then, I think. My only issue is that my boat may or mat not be available to me, along with all of my gear.
 
But I got the Diablo, and some spare gear I could wear...
Anyway keep it in mind. I miss Robe!
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arnobarno
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  Quote arnobarno Replybullet Posted: 23 Oct 2010 at 8:58pm
Getting back the original topic...

Thanks for that information and it explains some of their recalibration methodology but not the magnitude of the specific change I see in the data effective today.

The "GH change" you mention was only .04 feet. That change in gauge height corresponds to substantially less than 100 cfs (at the levels I mentioned) yet it got restated 400-600 cfs lower than previously.

Thoughts??
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  Quote huckin harms Replybullet Posted: 24 Oct 2010 at 8:15am

yeah, I know noticed the change as well... but not sure how to answer the question posed.  Thanks Ben for the insight... 

 

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Courtney
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  Quote Courtney Replybullet Posted: 24 Oct 2010 at 8:39am
Well I'm glad to hear it's finally back to normal.  Some of us knew that they gauge still wasn't right and that's just about how many cfs we thought they were off.  Don't know why they did it but at least now I don't have to fugure the math in my head before heading out.
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Courtney
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  Quote Courtney Replybullet Posted: 24 Oct 2010 at 8:41am
Whit a second.  My brain must not be awake yet.  It's just the opposite so now I'm going to have to double my math.  Damn!
 
Courtney
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  Quote mcruni2000 Replybullet Posted: 25 Oct 2010 at 7:32am
The magnitude of the change in discharge is related to the amount of fill or scour of the channel. For example, if the channel has scoured, the cross sectional area of the channel will increase while the Gage height remains the same. This leads to an erroneous discharge (too low) for that Gage height. Conversely, if the channel fills, there is less cross sectional area at the same Gage height, which is also erroneous (too high). So the shift (correction) in the discharge rating may be larger or smaller depending on the amount of change in the channel. Additionally, the stage-discharge relationship used to interpolated discharge between measurement points is not linear. This means that changes in Gage height at higher flows will correspond to much larger increases in discharge than those a lower flows (e.g.: a 0.10 change in Gage height at a stage of 1.00 ft may only be a difference of 100cfs, while a change of 0.10 at a stage of 5.00 ft could be 400cfs). I hope I made sense.......
It looks like it goes, but I'm not the guy who's gonna prove it!
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  Quote mokelumnekid Replybullet Posted: 25 Oct 2010 at 8:50am
Makes sense but how does one then estimate the change in channel bed morphology without a dry-bed cross section survey at a number of points? Perhaps by doing a precise estimate of discharge at a given gage height? But how is that done? Lazy, shiftless minds are curious...I'm sure it is obvious if I gave it like a few seconds, but it is easier to just be lazy.
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  Quote arnobarno Replybullet Posted: 25 Oct 2010 at 11:48am
Good explanation.

But still...

The examples in the original post and later were specifically cited because I was aware that the relationship between gauge height and flow is non-linear. The delta on the gauge height - around the flows I mentioned - could only account for less than a 100 cfs change in the flow.

For sure, an overall change in the channel bed near the gauge could cause the gauge height difference to be relatively small but the overall effect larger. In fact, it wouldn't really be related to gauge height at all - thus making this type of field measurement reporting pretty incomplete in terms of understanding changes. Still, you'd think on something like the Sky (where there are lots of potential measurement points), the measurement point would be chosen where the bottom effects disappear relatively quickly in their model such that they are only dealing with additional surface area spread for each delta on the gauge height as opposed to obstructions under the water.

It looks like the USGS is doing this recalibration about every 2 months. It would be pretty fascinating (at least to me) to participate in one of their surveys to see exactly what they are doing.
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  Quote fiddleyak Replybullet Posted: 25 Oct 2010 at 12:15pm
Yeah, it seems to me that the gauge height change doesn't always account for wide changes in CFS. EG, on the NF Snoq there was a change from 950 to 550 CFS, and the corresponding gauge height change was fairly small. (Sorry no date/data to back this up).
I like this topic and it would be really interesting to get some more information on this gauge recalibration process. I'd also be interested to have a breakdown of how CFS is measured in a river.

JP: Hope to see you next weekend. I've been wielding the plastic welder lately which means I've got some usable creekboats. Jefe?
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GHannam
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  Quote GHannam Replybullet Posted: 25 Oct 2010 at 7:54pm
Thanks for the insight! Just like all forms of science, it needs a little recalibration every now and then-- glad to know they're on it
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  Quote mcruni2000 Replybullet Posted: 27 Oct 2010 at 7:21am
In order to get the correct discharge to be recorded, a "shift" in the rating curve was applied throughout the rated Gage heights. This means that throughout the entire range of stage, a correction is applied to get each interval of Gage height to correspond to the correct discharge. This shift gets prorated back to the last measurement that was made. That's why the USGS puts a disclaimer on their web page that states that the data are provivional until approved and published. Discharge is measured roughly every two months to ensure quality data. As you can imagine, the river is a dynamic environment, and is constantly changing. The discharge is measured by determining cross sectional area (square feet) times the average velocity (feet per second) of the river along that cross section. Area times velocity yields discharge (cubic feet per second). The area and velocity are measured using a tagline and a current meter. The tagline is essentially a tape measurer (or cableway) that indicates how far accross the channel on is. The current meter simply measures he velocity at each subsection of the cross section. On larger rivers like the Columbia, Acoustic Doppler Current Profilers (ADCP) are used to measure the river. They use the Doppler effect and gps, and thus do not require a tagline. I actually work for the USGS doim exactly this. If anyone ever has any questions regarding how this stuff works, your local water science center (Tacoma or Sedro Wooley) loves to answer questions for the public (part of the USGS mission). If what I said in this reply doesn't make sense, feel free to contact me as well.
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  Quote mcruni2000 Replybullet Posted: 27 Oct 2010 at 7:23am
Sorry for the typos. My iPhone is hard to type with.
It looks like it goes, but I'm not the guy who's gonna prove it!
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Courtney
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  Quote Courtney Replybullet Posted: 27 Oct 2010 at 8:15am
Well now since the gauge has been recalabrated, instead of the minimun being 500 cfs, we can run it down to 200 cfs.  I remember when the lowest I could run it was 850 cfs.
 
Courtney
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  Quote arnobarno Replybullet Posted: 27 Oct 2010 at 8:53am
Ryan,
Thanks for the explanations and thoughtful reply - pretty interesting stuff.

I'm wondering though what the durability is of the "rating curve." That is, what causes you to have to generate a new curve. In looking at the streamflow measurements page, it looks like we are on curve #15 for the Sky - last updated in 2003. I assume that switching to a new curve is more event driven (e.g. streamflow measurement parameters out of some bounds) than time driven - still, my casual empiricism of flow (based on paddling the Sky over the past four years) and the latest changes to flow estimates make me wonder about the integrity of the current curve.
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  Quote doggievacation Replybullet Posted: 27 Oct 2010 at 9:09am
Just think, Courtney, by the time we get to curve #16, you'll be able to run it down to -200cfs!  Think of the possibilities!!!  
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arnobarno
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  Quote arnobarno Replybullet Posted: 27 Oct 2010 at 11:19am
Actually, another simple explanation of the huge change is just an error (either in measurement or in the curve shift).

I have to say that I'm still struggling to understand the data that is presented on the USGS page. Is it the case that with a "shift adjust" and GH change of the magnitudes we saw in October, one would expect a 500 cfs change in the reported flow (at flows around 3000 cfs - I realize it is nonlinear). Over the past few years, there have been shift adjustments of greater magnitude but I don't remember a flow adjustment of this magnitude - at least since the winter of 2006/2007 when we had the epic floods.

Thoughts?
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Courtney
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  Quote Courtney Replybullet Posted: 27 Oct 2010 at 9:56pm
All I know is that I have a WKC trip to lead in November on the Sky and I sure don't want any first timers with me when I don't actually know what the flow is.  I can see it right now.  The first timer asks, "so what would you say a good first time run flow should be?"  I'm really going to sound like an idiot when I say, "beats the hell out of me."
Courtney
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  Quote mokelumnekid Replybullet Posted: 27 Oct 2010 at 10:51pm
Darn tooting there Courtney- as I hope to be joining you that day, and I'll need all the help I can get 
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  Quote jP Replybullet Posted: 28 Oct 2010 at 1:26pm
Thanks Ben- give ya a hollar if I can make it up there, saddly it isn't looking likely at the moment. Too many hoops to jump through down in Hoody McRiver
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  Quote JoesKayak Replybullet Posted: 28 Oct 2010 at 1:58pm
Originally posted by Courtney

All I know is that I have a WKC trip to lead in November on the Sky and I sure don't want any first timers with me when I don't actually know what the flow is.  I can see it right now.  The first timer asks, "so what would you say a good first time run flow should be?"  I'm really going to sound like an idiot when I say, "beats the hell out of me."
Courtney


Aw you're in the valley.. just go visual. It cleans right up when all the rocks in boulder drop are covered.
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