Thursday’s high temp was 79, our highest ever recorded for November 7th, and one degree shy of the highest November temperature ever recorded at Celo’s NWS station since records began in 1948.
Jake and Chris, two USGS hydrologists, have been out in Celo this week attempting to calculate the peak flow rate for the South Toe river during Helene. They were sweating under the November sun in their safety-orange outfits, and I buttonholed them on the mostly-restored Seven-Mile bridge to glean as much knowledge as I could.
Celo’s USGS streamgage reports two measurements of the river: height and flow. The stage height is simply measured in feet above its base elevation. The location for the gage was chosen partly because it is in a bedrock section of river, so the riverbed height (2,656.5’ above sea level) doesn’t change much, even after floods. Measuring the height of the river can be as simple as observing where the water line is on a yardstick that begins at the riverbed, but there are higher tech devices that can report live data, and I’ll describe them below. Measuring the river’s discharge – how many cubic feet of water is flowing by each second (cfs) – is much more complicated, and that’s why Jake and Chris spent a couple of full days out here collecting data, and I’ll describe that process too.
Measuring Height
Celo’s USGS South Toe streamgage is actually made up of 3 different styles of gauge. One is the visible yardstick where you can see how high the river is; it only goes to about 10 feet, so it is easily topped during floods, and it has no way of recording or reporting data. The mechanism that reports data online measures the water column by blowing bubbles into the river. As Chris describes, if you blow bubbles through a straw into the bottom of a glass of water, you can feel the difference in resistance between blowing into a glass that has little water vs. one that is full. The bubbler can determine the weight of the column of water by measuring the resistance of each bubble blown. To attain a precise measurement, it has to compensate for the variable atmospheric pressure above the river (the weight of air pressing on the river), so there is a barometer in there as well that continuously calibrates the data from the bubbler.
Neither the yardstick nor the bubbler function when the streamgage goes under water, and the entire apparatus was inundated in both 2004 and 2024. Fortunately deep inside the gage is a backup mechanism, a pressure transducer (PT), which functions like an underwater barometer. Like the bubbler, the PT measures the weight of the water column, and luckily it survived the flood and continued recording data. Since the gauge’s air pressure barometer was flooded, USGS scientists have to find reliable local barometric pressure readings in order to manually calibrate the reading from the PT. In the case of a storm like Helene, the air pressure changes rapidly, so it’s important to correctly line up the time of the river’s crest and retrieve barometric readings with that same time stamp.
Based on the data recorded by the PT, and adjusted for local air pressure, the provisional USGS determination of the river height is 26.06. Last month with the Celo school kids, we measured a crest of 26.25, a difference of just 2 inches. It turns out that the PT only records data every 5 minutes, so it’s very likely that it missed the crest by a minute or two on either side, which could explain the 2” discrepancy.
Measuring Discharge
There is no device inside the gage that can measure discharge, so hydrologists create a formula to extrapolate flow measurements from the river height. This formula creates a ratings curve which has to be calibrated regularly. Helene was so far beyond the normal curve that extra work needs to be done to achieve a half decent flow estimate, and that’s what Chris and Jake have come out here to do. First, they flag debris lines in the surrounding woods, and use surveying tools to create imaginary cross-sections of the river at its peak. As mentioned, the streamgage is located in a section of bedrock, but it also happens to be in the middle of a wide floodplain. The river, normally about sixty feet across at that juncture, was a quarter mile wide when it peaked shortly before 11 am on September 27. It would be very challenging to obtain an accurate cross section there, so Chris and Jake go downstream to where the river flows through a narrows. It’s not exactly a gorge, but hills on either side limit the river’s ability to spread. From their imaginary cross sections of the flood, Chris and Jake can calculate the peak volume at that spot of the river. That’s the first variable in the formula.
The other variable is the speed at which the water is flowing. I’ve previously encountered Jake and other USGS hydrologists measuring the flow with fancy remote control acoustic Doppler current profiler (ADCP) boats. If you do this at lots of different river stages – I often see them out here soon after a high water event, since opportunities to measure the flow at several thousand cfs are few and far between – you can add that data to the formula to extrapolate peak flow at different river heights. A typical flow rate for the river is around 100 cfs. The preferred height for tubing the river is 200 cfs, and boaters start salivating when it tops 500 cfs. During a drought it can drop to 20 cfs or lower. During a heavy rainfall (this happens most years), the flow will rise over 5,000 cfs. The river begins leaving its banks around 11,000 cfs. In 1977, the river height was measured at 17.41’, and calculations used at the time estimated a peak flow rate of 33,000 cfs. In 2004, at a river height of 15.5’, using more modern techniques, the peak flow was calculated at 28,000 cfs. Helene’s floodwaters were 9 feet above the 1977 mark, but since the river spread out so much more, those 9 feet represent an enormous increase in volume. This event was so far beyond their ratings curve, Jake warned me that it’s possible that they won’t come up with a satisfactory estimate. If they do, I’ll be very curious to know how much water was flowing through the river at that historic moment.
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