Taking the Measure of Rivers

Taking the Measure of Rivers

a Carolina Paddler Article

Text and Photography by Alton Chewning

–Carolina Paddler has published two previous articles on river gauges and how water height and discharge are measured.  “The Gist of the USGS” gave background on the government service that provides most of the river level information paddlers use.  “Gage, Stage and Discharge” went into depth on how USGS gages work.  In this article we’ll follow two USGS hydrologic technicians as they take measurements on rivers.  This article builds on the other articles, so it’s best understood by reading the previous ones. We have repeated some information here to help with understanding what the technicians are doing.

Eric Sadorf reading Haw River at Bynum gage.

We meet Eric Sadorf at the Bynum gage on a cold morning in late November. Not much rain has fallen in the last few months and the Haw River is very low.  Sadorf gets out of his truck wearing a flannel shirt partly unbuttoned and a t-shirt.  It’s 22 degrees and his clothes seem skimpy for working on the water.  However, Eric has been working with the USGS since 1995 and taking stream flow measurements since 2007.  He knows what he’s doing and is used to working outdoors.

We drive past the gate to the gage location, passing what Eric thinks is two archaeologists shifting soil through boxes, looking for artifacts.  People are interested in all the Haw has to offer, even on a frigid day like this.

We stop a little upriver from the gage station.  Eric says the bank near the gage is steep and this bank has a more gradual slope making it easier to get on the water.

He begins pulling out gear: waders, two “toy” boats and a hi-viz jacket – a “float coat.”  So this is why he was underdressed.  The float coat is both an insulated jacket and a PFD.

Two little boats rest on the frosted grass.  Each boat is equipped with a battery pack and a device called an ADCP.  Eric will use the smaller red one.  The other is a back-up.  This will be his primary means of taking flow measurements today.

Here is an explanation of ADCP’s from the “Gage, Stage, Discharge” article:

Lara Chapman: “We use Acoustic Doppler Current Profilers, ADCP for short, to measure discharge. A technician like me would sit in a cable car and bring the car out over the river. We have a rope attached to a small boat which holds our ADCP transducer. We take the boat back and forth across the river to measure the discharge in various places.”

The car and cableway Lara describes still rest near the Bynum gage, but are seldom used today.  The Bynum cableway is the last remaining one in the Raleigh district. More frequently, technicians like Eric use a kayak to tow the little ADCP boats back and forth across the river. 

Eric hauls the kayak and gear to the bank and gets in the water.  Nearby on the bank is an older structure.  I ask what it is, and Eric says it’s a pumping station for the town of Bynum, no longer in use.

Eric checked measurements on several rivers the previous two days. Each day he saw otters.  He hopes to see more today. One of the perks of the job.

The cable car sits on its tower, awaiting a few final rides across the Haw.  Sometime in the next few years it will be decommissioned and taken down.

The stilling well will be gone, too, like the cableway and the towers.  They have served their purpose and technology changes.

Below, on the bank, Eric is doing a couple of preliminary measurements.  He checks the water temperature with a separate device.  Today it is 43 degrees. The ADCP can also measure temperature but sometimes the reading is unreliable, so it must be confirmed against another device.  This confirmation of one method against another is a staple of the USGS technicians.  They want to be sure of their data.

Eric also must check the speed of any shifting materials on the riverbed.  The Haw rarely has much movement on the bottom, particularly at low flows like today.  The USGS gage reading earlier this morning was 3ft., 176 cfs.  Eric says on other rivers, like the Neuse in Goldsboro or Kinston, the bottom is sandier and can contribute to the water velocity reading they get.  This must be factored into the computation of the water’s discharge rate.  With these preliminaries completed, Eric can begin his primary measurements, criss-crossing the river slowly, the ADCP trailing him like an obedient dog.

Eric must make four passes back and forth.  If things go well this will take about 20 minutes.  If the readings seem off, he will go upstream a little and try the process again.  Some days he will do three rivers, some days four.  Today the Haw is his only river.  In the afternoon he will service equipment at the B. Everett Jordan dam.

Other hydrologic technicians like to mount the small boat in front of their kayaks. Eric says he’s tried it in front and behind.  The readings were consistent, but it was harder to control the direction with the boat in front since it doesn’t have a keel.

Eric works out of the Raleigh office (as does Lara), which covers a large area, Winston-Salem to the coast and as far south as Calabash.  Nine hydrologic technicians work this area, shifting territories every year to keep it interesting and to take advantage of what each does best.  The office has a staff of thirty handling various projects.  Another district office in Asheville covers western NC.  The Charlotte office has a small reach, but with many gages clustered around Mecklenburg County and a few in adjoining NC and SC areas.

Eric has made his passes, but he is not satisfied with the readings.  Low water is difficult.  He says he’ll take more measurements just upriver, close to where he put in.

What does a person like Eric do for fun?  Well, many things.  For one, he and his wife like to go down east to some lazy river, jump in a canoe or kayaks and paddle up for a couple of miles, then float back down.  No little red boat in tow.

Lara Chapman measuring Walnut Creek at Buck Jones gage.

Lara Chapman is one of those delightful people who is serious about her work, enjoys doing it and is able to share the excitement with other people.  Not that her work is easy.  She’s outside in the great outdoors but sometimes the great outdoors is freezing or boiling, parched or wet.  And muddy.

Today, Lara is checking a USGS gage at Buck Jones Road in Raleigh. The waterway is Walnut Creek, one of the two flashy creeks in Wake County that cause headaches and heartaches if torrential rains pour.  Today is quiet.  It’s been dry for weeks until the past weekend.  Walnut Creek flashed but not to a dangerous level.  Not this time.

Lara gives a tour of the gage box.  Once you’ve seen the stilling well and the cableway at the Bynum gage, it’s difficult to comprehend this little box does what those impressive structures do.  There is a solar panel on top charging the battery within.  An antenna connects to USGS and to Raleigh’s Alert II system.  Lara points to a white enclosure.

Inside the white box are the inner workings of the “bubbler,” the device that actually measures the height or stage of the water in Walnut Creek.  Again, we’ll refer to our previous article, “Gage, Stages, Discharge”.

“The bubbler works on pressure measurements. Pressurized air is forced down a line to an outlet at the lowest gage height of the river, a point arbitrarily chosen. This lowest point represents zero on measurements and thus no gage readings are below zero. All measurements are positive numbers. The pressurized air at this predetermined low point is “bubbled out.” With one foot of water above it, a certain amount of air pressure is required to force out a bubble of air. With two feet of water above, more pressure is required and so on. This required pressure is correlated to the height of the water column. Lara notes, “It’s a pressure transducer detecting the pressure from the water. We know the density and weight of water so the correlation can be made in feet.” This is the number paddlers read in feet when we check the river level online. We’re seeing what river level ‘bubbled up.’”

Lara looks at the bubbler read out, observing, “Right now it is at 1.01 ft.  I will go down and verify that at our outside reference, the staff plate.” Time to get dirty.

 

After confirming the bubbler’s 1.1-foot reading matches the staff gage, Lara heads up the far bank to check the crest stage gage there.  The heavy rainfall of the previous weekend peaked somewhere around 8 feet.  The CSG should confirm this recording.  Again, from the previous article, here’s an explanation of the CSG:

Crest Stage Gage

It’s a metal pipe that extends several feet into the bank. Inside is a wooden rod with markings in feet and inches. A technician puts some ground cork in a cup at the bottom of the rod. During a major rain event both the river water and the water in the pipe rise, floating the cork pieces. When the water stops and falls, the cork sticks to the rod at the water’s highest point. This gives a peak-stage measurement of a rain event. Such as “Flood waters on the Haw reached over 17 feet today.”

Lara shared, “We tie a point on the CSG into NAVD88, just like the rest of our gage equipment. Then we can take a ruler and measure from our survey point to our cork line to get a peak stage reading.  So, imagine the CSG at Bynum was found to be at 15.632 ft. If we were to measure a cork line at 1.45 feet, we would have a recorded peak of (15.632 ft. + 1.45 ft.) = 17.082 ft.”

USGS gage reading is like this. Very sophisticated technology back by manual verification.  Lara pulls a stick from a tall pipe on the bank.  Looking down the scaled stick she sees a patch of cork stuck in one place.

This patch of cork is where the water rose during the last heavy rain. 

Lara explains, “We just measured the cork line on the CSG which we read at 1.04 feet.  We survey in what we call the CSG pin, which is where this stick lies within the inside of the CSG.  If we have that tied into our gage datum, then we add that elevation with 1.04ft. which we just measured, which will give us roughly 8ft. stage recorded, for outside peak verification.”  So Walnut Creek at Buck Jones Rd. rose to 8 feet.  It’s been much higher.

All of this is done to verify the bubbler is working properly, the technology is working.  This is especially important in flood prone areas. Lara points out, “Raleigh sites along Walnut Creek and Crabtree Creek are tied to the flood early warning system, something that the city of Raleigh does but our data is used for.  There is a well-known flooding issue in the Raleigh area and so we try to provide updates, real time data for the public so they can get notified as soon as possible if their home or apartment is going to flood or if there at an event where they may be trapped by flooding roads.

“For example, at the apartment complex just downstream of this site at Buck Jones Rd, there have been events in the past where they were flooded.  That’s why we have the flood early warning system to warn people when the stage is rising rapidly.”

Unlike the gage at Bynum, where readings are updated every fifteen minutes but only transmitted once an hour, the new technology at Buck Jones allows updates every five minutes.  Furthermore, at several gage locations on Walnut Creek and Crabtree Creek cameras are mounted which provide real time verifications.

Lara has one more chore to do here at Buck Jones Rd.  She needs to measure the discharge, the current in cubic feet per second or “cfs.”  Since this is a small creek, she won’t use a ADCP mounted on a little boat, like Eric did.  Instead, she’ll use a pole mounted device called a Flowtracker.  From the previous article.

For lower flow streams, handheld devices are used, like Flowtracker, a type of ADV(acoustic Doppler velocimeter). Lara said, “This is for wading measurements. I would stand in the stream and measure the flow going by. For lower flows you have to use these because they can go to a very low depth. 90% of my measurements on Walnut Creek are with a Flowtracker.”

Lara: “The only way we can get discharge data is if we go out and measure it. If I make a measurement with an ADCP or a Flowtracker or another piece of equipment, that’s what gives our cfs cubic feet per second, that we publish online.”

First Lara has to stretch a “tag line,” a measuring tape, across the stream so she knows the exact width and to provide marks for the many readings she has to take across the flow.

Lara must collect data for at least 40 seconds at each station and there are 25 stations crossing the creek.

Water depth is recorded at each station.  The pole must be perfectly vertical. It’s exacting work.

Taking the numerous measurements takes time, the whole process requiring 20-30 minutes.

Lara:  “When I hit start it will take 40 seconds of data.  The two different colors, the blue and the red represent separate beams measuring the velocity.”

Okay, what’s the result?  What is the discharge of Walnut Creek at Buck Jones Rd. currently?  The above measurement is just velocity.  Many other factors must be incorporated. Lara can only hazard a guess at this point of the cubic feet per second.

“Since we can’t be out here twenty-four hours a day taking discharge measurements, we developed what are called rating curves. We go through a lot of measurements, through a lot of math, through a lot of calculus, and develop a rating curve so that when a stage reaches a certain height, we know that according to this equation, how much the discharge will be. For example, if a hypothetical river is three feet, that equals … forty cfs. The Haw at three feet would be way more than forty cfs.”

Lara admits, “My guess is the discharge now is probably just over 1 or 1.2 cfs.  Doesn’t seem like a lot.  It’s shallow here.  Across the width of the stream, some parts will be faster.  That’s why we take so many measurements across the width of a stream.”

Let’s keep in mind, in the past this 1.2 cfs brook has suddenly turned into a tidal wave, flooding streets and homes nearby, endangering lives and wrecking dreams.  Thank you, Lara and Eric, and the many others who seek to prevent and predict these flashes of nature’s sudden power.

Paddling Gauges

For those readers who can’t get enough of gauges, look at the homespun method of determining river level used by paddlers in the past and still in use today.