Is measuring flow such an easy thing to do?

This is the title of one of the presentations that were given March 22nd and 23rd, 2011 by our group to illustrate some of the uncertainties associated with measuring flow using rating curves.  The presentation can be found directly here. 

Several animations were given during the talk, which we think illustrate quite nicely the fact that in some situations, the hypothesis of a unique relationship may not hold.  This is something already well known, but we thought that seeing some real data helps.

The first animation shows data obtained in a low land stream in North Carolina when 'back water conditions' are regularly obtained (slope 0.01%; dates in 1998).  The curves drawn are sometimes called 'looped stage-discharge curves' and show a hysteresis of the stage-discharge relationship.

The second animation was obtained in a more upland stream in Nozay in France (44; dates in 2004).  The hypothesis of a unique relationship between stage and discharge seems to be a lot better verified.

In both cases, however, there are some uncertainties that may have some consequences, particularly on the cumulative flow volumes, as can be seen on the presentation.

The reason for that is that the rating curves that may be drawn from manual gauging and that are used to calculate instantaneous flow depend on:

1. the presence or not of a hysteresis in the stage-discharge relation
2. the number of points from which the best fit rating curve is established
3. the range of stage and discharge covered to establish the rating curve

Below are two animations showing, for 20 gauging points, how different the rating curves can be depending on when they were obtained.  The first animation with the red dots show dramatic differences between consecutive rating curves and correspond to the lowland stream in North Carolina where back water conditions were known to happen.

The second with the green dots show differences in rating curves for the upland stream in France where the unique rating curve was thought to hold quite well.  It becomes obvious that the curves do not vary nearly as much as they do for the lowland stream, although there are some variations despite the fact that the curve was fit extremely well through the points for each (r²>0.98).

The consequences on the cumulative flow volumes can be seen on the presentation.

Lab Assistance, Fresh Projects

We have been very busy in the lab lately, despite the lack of blog posts! We've welcomed Tyler as a new lab assistant for this semester.

A few random updates...

- As you can see on the GaugeCam website, we've installed a new set of fiducials on the water level bench background. We're referring to these as bowtie fiducials, for obvious reasons!

- We recently completed the construction of a seepage tank. The purpose of this tank is to evaluate different methods of measuring upwelling groundwater flow rates. A parallel project is the development of new ways to continuously measure these flow rates.

- As part of several projects, we've been using an interesting apparatus called a Marriote jar (or siphon). Check it out on Wikipedia!

- Several of us in the lab are being trained on spectrophotometry equipment. It's an exciting experience, because absorbance and fluorescence are very useful in determining the chemistry of solutions! But it's also challenging, because photochemistry relies on numerous assumptions and caveats. Many of the measurements are easy to make, but expertise is required to interpret the results.

How does agricultural drainage work?

In our Theory of Drainage course, we have been developing an understanding of how agricultural drainage works, both qualitatively and quantitatively. This is exciting, because the some of the same concepts can be applied to wetland hydrology, stream to groundwater table interactions and much more!

We resurrected an old Hele-Shaw laboratory model that was used to investigate some of the equations we applied this semester. With Dr. Skaggs' help, we ran an experiment during class to test the equations. For the most part, resulting data showed a strong agreement between the equations and the lab observations, which were recorded by the graduate students pictured here. We did find some areas where the data began to stray from the predicted values, which was also a valuable exercise.

 

For the study originally published (Skaggs, 1973) from this model, measurements were observed manually using optical sights. Today, it would be a great way to apply vision-based liquid level measurement!

SKAGGS, R (1973). "WATER TABLE MOVEMENT DURING SUBIRRIGATION". Transactions of the ASAE(0001-2351), 16 (5), p. 988.

Water Level Camera: Lab Setup

The brown backdrop adjacent to the water level bench may appear like an unremarkable addition, but we're excited to have it installed. Why? Because we're one step closer to testing our new GaugeCam (GRIMe) software!

We've been working with GRIMe for a while now, but we've just added automatic calibration functionality. All we have to do is add fiducials to this backdrop and we're ready to start testing again.

Wondering what the water level bench is all about? We've been blogging about it for over a year at the GaugeCam blog. Click through and check it out!

Thanks to undergraduate researchers Zach and Austin for assisting with the water level bench backdrop.

Constructing Weir Boxes

Hydrologists and Ecological Engineers often utilize weirs as a means to control water flow in open channels. Weirs are especially useful in research, when we want to measure the flow rate of water in a stream, canal or other open channel. As noted in this Wikipedia article, permanent construction (or removal) of weirs has ecological and environmental implications.

In the picture below, Dr. Birgand is working on a weir box which will soon be used in a field study of biogeochemical processes in streams.

This particular control structure contains a V-notch weir. It's easy to see where that name came from when you view the image below.

Here we are off to transport the weir boxes to their installation site.  Imagine the flow (volume per time) of water that will be passing through these boxes!  All in a day's work for Hydrological and Ecological engineers.

Total Station Survey

James and Marc collect survey data near Weaver Labs. 

Ecological engineers use many of the same tools as environmental, civil and construction engineers. In this case they are using a total station, which is a powerful tool that efficiently records accurate survey points.

Job Security for Ecological Engineers

So the other day, Raleigh got a lot of rain.  Creeks rise out of their banks and people get flooded, it happens. However, events such as this, where you see sediment-laden water coming out of a storm pipe should not happen.  Look at the upstream part of the creek to the left of the picture (click on the photo for full size). It is relatively clear even after a large rain event. Compare it to the muddy discharge coming out of the storm pipe. Hmm, anybody want to guess the number one pollutant in North Carolina's streams and waterways?

We figured another shot of the creek would help bring home the point.  By the way, a stream restoration was just completed on this creek a couple of months ago.  

We found that runoff from a construction site about a 1/2 mile upstream was causing all of this sediment loading.   The bright side to all of this, I guess Ecological Engineers have job security for a while!