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[VAGAbond]

I live a long way from the river so I have not yet seen the changes from the flood but I notice that the river level as reported by the gauge seems to be staying very high and not coming down with the reduced rainfall. It seems a full meter higher than I expect.   Has gravel movement filled the river bed near the guage so that there is a new typical gauge level? 

[DanL]

FWIW, the current flow ~130 m3/s is quite a bit of water coming down. I would assume that with somewhat rising temps and the recent rain, there might be a lot of that Xmas snow melting and contributing? Just a guess. I see your point though, there's been sustained high flow for 10+ days now

It seems a full meter higher than I expect.   Has gravel movement filled the river bed near the gauge so that there is a new typical gauge level?

If you compare from one year to the next, the height vs flow relationship for the gauge definitely does change each year, sometimes by quite a bit (but not by a meter).  The relationship is not linear, but at least for the current flow, the corresponding height does not seem to be too much different than prior to the Nov flood event so far.
 
Typically the biggest differences tend to be at lower flows. Only compared a few values but it seems that once it drops, the 'new' gauge might actually show a bit less flow than previously. ie. it may read slightly higher now, but I'm guessing it's not really too far 'out of whack' compared to historical.

FWIW height vs flow tends to be pretty consistent within a season (massive floods notwithstanding), but if comparing year to year, flow is the more useful (and constant) value. No question that height is more intuitive, but as you suggest, it can and does change.

[geomorphin]

I’m almost certain you guys are right. What you’re talking about is something called “Specific Gauge Analysis”. It uses the stage-discharge relation to infer bed level change.

I haven’t done this yet- I will when I have a few hours to kill. But the idea is that, before the flood a certain discharge gave a certain water level. Now, after the flood, that same discharge shows a higher water level. So, probably a bunch is sediment from above the crossing dumped below the crossing. Now there’s less space in the channel for water to go at the crossing, so the water level has to rise. (If you have erosion, it goes the other way: water level lowers because there’s more room for the water). Given all the channel change and erosion above the crossing- it’s not hard to imagine a ton of sediment depositing in that run below the crossing.

In addition to learning alll new runs, we’ll also have to learn our new favorite water levels!

[Roderick]

Measuring water level is easy, measuring flow is hard.  Usually the level is the direct measurement, and the flow is mathematically derived from the level. How they do the calculation I don't know but I'm sure changes in the width and depth of the river have an effect.  On the graph there are sometimes those large coloured dots which are, I think, when they do the assessment of flow vs. level. 

[clarki]

Measuring water level is easy, measuring flow is hard.  Usually the level is the direct measurement, and the flow is mathematically derived from the level. How they do the calculation I don't know but I'm sure changes in the width and depth of the river have an effect.  On the graph there are sometimes those large coloured dots which are, I think, when they do the assessment of flow vs. level. 

Not a hydrologist, but I am Google smart

Here's a link describing how river flow is calculated using the "Manning Equation" https://www.who.int/water_sanitation_health/resourcesquality/wqmchap12.pdf

Significant flood events that move large amounts debris/sediment could affect affect the profile of the river at the measuring station and thus affect the accuracy of flow calculation. If I understand correctly, they would need re-measure the bottom profile of the river to recalibrate the flow calculations.     

[CohoJake]

Not a hydrologist, but I am Google smart

Here's a link describing how river flow is calculated using the "Manning Equation" https://www.who.int/water_sanitation_health/resourcesquality/wqmchap12.pdf

Significant flood events that move large amounts debris/sediment could affect affect the profile of the river at the measuring station and thus affect the accuracy of flow calculation. If I understand correctly, they would need re-measure the bottom profile of the river to recalibrate the flow calculations.   

I find this discussing to be very reassuring. I thought my memory was failing me, as I would come back in a new season at a particular level and see the river not as high or low as I expected it. It isn't me, the numbers change.

[geomorphin]

Y'all are mostly spot on again.

Measuring water level is easy, measuring flow is hard.  Usually the level is the direct measurement, and the flow is mathematically derived from the level. How they do the calculation I don't know but I'm sure changes in the width and depth of the river have an effect.  On the graph there are sometimes those large coloured dots which are, I think, when they do the assessment of flow vs. level. 

At gauges, Water Survey of Canada puts effort in to manually measuring discharge and the water level. They then build something called a "Rating Curve" which is a math equation that is based on the measured discharge and water level.  So, the discharge isn't actually measured in real time, it is caculated from the constantly measured water level. Those dots on the graph are the physical measurements they make.  They have to do it to make sure their equation hasn't changed.  They are really going to have to take a lot more measurements now since the relation between water level and flow aren't exactly the same as before, so their curve will need to be adjusted.
Some google info: https://openecodatalab.github.io/Hydrology-Online/hydrology/7_rating_curve/7_rating_curve.html

Not a hydrologist, but I am Google smart

Here's a link describing how river flow is calculated using the "Manning Equation" https://www.who.int/water_sanitation_health/resourcesquality/wqmchap12.pdf

Significant flood events that move large amounts debris/sediment could affect affect the profile of the river at the measuring station and thus affect the accuracy of flow calculation. If I understand correctly, they would need re-measure the bottom profile of the river to recalibrate the flow calculations.     

Something like the Manning Equation isn't exactly the same thing. The Manning Equation is used for predicting river flow when you don't have all the measurements needed to develop a "Rating Curve". Still useful (since it needs less data), but will have more error than the Rating Curve since you typically only have a few measurements when you are using the Mannings Equation. 

I guess I've fully outed myself as a hydrologist now.

[Roderick]

 

[clarki]

Thanks for the clarification, geomorphin. Being Google smart does create some gaps in my knowledge base

Appreciate your contributions to the discussion. Always interested to learn about the "whys" and the "hows" when it comes to rivers.