First Look at the Water Level Bench

Today I had the chance to work in the lab a little bit. I spent a while just sweeping up a lot of grit and dirt.

Since the bench has been completed I put some water in it and took a series of photos. Hopefully those will be useful for Ken to look at as he works on his software.

Ken also has been in contact with Ruby Mehrubeolgu, who has some interesting water/machine vision work going on down in Corpus Christi. A lot of grant solicitations emphasize collaboration between universities, so it is possible we could work together on something if interests, timing, etc, align.

BAE (Ag) Shop

I mentioned the BAE shop in my last post. Here's a link! It details all the machines and capabilities of the lab.

BAE Machine Shop

Keys to the Lab

Today I got my keys to the lab where I'll be working! It's an old lab that's been out of use for quite a while, so there is a lot of cleanup to do. I'm not sure how involved I will be in that, but it might contribute to a slower start to the project than I hoped.


I took a tour of the Ag Shop. It's excellent ~ all kinds of tools and machines in there to create stuff with. Hopefully after next week I'll have access to some of the tools in order to make a (very simple) camera mount. And for items I don't know how to create, it sounds like the shop personnel will be able to make just about anything.

In the meantime, I'm working on a preliminary set of deadlines. Dr. Birgand will look it over and help set it up on a realistic time table.

Gig

As of today I officially landed a part time gig to work as an undergraduate researcher with the NCSU BAE department this summer. (See archive posts for details on BAE.) I'll be working with Dr. Birgand and also with my friend Ken Chapman. We're working on some gadgetry to detect stream stage. More later!

Vacation Stream Gauge

In our travels through VA, WV, PA, OH, & IL, I have been seeing stream gauges all over the place!

Here's one on the Potomac River in Paw Paw, WV:

Hydrometry - Float Operated Gauges

Float gauges provide a mechanical means to measure water height.

Here are the basic components:

Float
Line (Tape, or Wire)
Pulley (Float wheel)
Counterweight

Recorder

The line is attached to the float on one end and a counterweight on the other. It hangs on a pulley. The recorder keeps record of the pulley shaft rotation, which can be translated into water height. Recorders have traditionally been paper on a drum, but more are being converted to digital recorders that keep track of the pulley shaft rotation.

Typically, float gauges are installed in stilling wells, which help to produce stable measuring conditions.

There are several sources of error that must be considered when using the float operated gauges.

Systematic Effects:
1.) Float lag: The float tends to lag behind the true water level as the float rises and falls. This can be minimized by selecting an appropriate diameter float and sufficiently large float wheel.

2.) Intake pipe effects: The intake pipe (horizontal pipe in figure to the right) diameter can also introduce measurement error. A diameter that is too large can reduce the effectiveness of the stilling effect of the well by allowing water to surge in and out of the well. However, a too small diameter can introduce measurement lag because of head loss in the intake pipe.

3.) Chimney Effect: How far the intake pipe extends into the river cross section also impacts measurement due to chimney effect. This can produce a systematic underestimation of the water height.

Other Concerns:
4.) Counterweight: The counterweight location must be configured so the counterweight does not hit the float as it drops. Ideally, the counterweight is never submerged, as the effect of bouyancy on the counterweight will affect measurement.

5.) Tape/Wire: Kinks or twists in the line can cause measurement error. In addition, surges of water can potentially unseat the wire from the pulley.

Float Gauges in Stilling Wells
(Source: USGS)

Hydrometry Introduction

I found this book at the NC State Library:

Hydrometry, 3rd edition: A comprehensive introduction to the measurement of flow in open channels
UNESCO-IHE Lecture Note Series
Wubbo Boiten, Wageningen University, Delft, The Netherlands

Here's the publisher's description.

Based on this book and other sources I may find along the way, I hope to post a few times about the basic measurements taken along streams and rivers.

Initially, water resource measurements can be broken into two categories; quantity and quality. On the quantity side, we want answers to questions such as: do we have enough to drink, is there a risk of flooding in a certain location, are impermeable surfaces such as parking lots creating too much increase in stream flows, etc. The quality side seems obvious because we all want clean drinking water, but there are also less obvious concerns such as sediment transport, nutrient levels, wildlife impacts, and so on.


First let's look at quantity measurement.
When news about a flood hits the airwaves, we're always given stats like flood stage and water height. So water height is necessarily of interest. How do we measure it?
In talking to Dr. Birgand and in reading this book, I find five basic measuring techiques for water height.
1.) Staff Gauge
2.) Float/Counterweight Systems
3.) Pressure Transducers
4.) Bubble Gauges
5.) Ultrasonic sensors

The image on the left is a USGS Stream Gauge Station near Pullen Park in Raleigh, NC. In the background, you can see a white "stick" in the water. That is a staff gauge, which looks a lot like a measuring stick.


In addition to height, we also like to know how much water is flowing in a stream or contained in a reservoir. In streams this is measured as volumetric discharge, in units such as cubic meters per second or gallons per minute. Here are some classic methods for calulating discharge:
1.) Velocity Area method
2.) Slope Area method
3.) Stage Discharge method
4.) Acoustic Methods
5.) Electromagnetic Methods

I've used the Velocity Area method for calculations in class. The remaining four methods I hope to learn more about.

There's plenty to read about quantity measurements, so I think I'll leave the discussion of quality for another time.