Hello,
Does anyone know how a USGS water level and flow gage actually works?
I can imagine various types of floats or sensors for the level, but the FLOW gage is a mystery.
While I can understand how water flow can be measured using a weir of a known cross section and a velocity reading, how can flow be measured in a sand bottomed river?
For example, consider the Chippewa River at Durand Wisconsin, it’s about 200 feet wide, and usually around 2 feet deep. The channel meanders as sandbars shift. The Wisconsin river anywhere in its final 100 miles would be similar.
Thanks
Paul
You can look this up somewhere,…
– Last Updated: Mar-20-06 3:24 PM EST –
... but as I understand it, they normally measure water elevation and flow at a location with well-defined banks, typically between highway-bridge abutments. Periodically, the bottom of the river between the abutments is surveyed so that the cross-sectional area available for flow at any water level can be calculated. Finally, the velocity must be determined, and I imagine this is measured at various elevations and used for future reference (the current speed would be expected to be the same for a given water level every time the river is at that level, barring such anomalies as ice jams, or just the reduced cross-sectional area due to ice cover, and that is why gauge sites provide elevation but do not provide a flow rate when there is ice on the river).
Have you ever seen a bridge with one or two small winches mounted on the side? That used to be pretty common, and I think they used those winches to lower measuring devices into the river. In Iowa, I've seen bridges with a "chair-lift" suspended on cables across the river alongside the bridge. I beleive they use those for surveying the river bottom between the bridge abutments.
I know of one interesting situation regarding current speed vs flow rate. At the location of the USGS Gauge on the Upper Yahara River in my town, the current reverses direction every 40 minutes or so during normal flow rates in the warm season, or, if the flow rate is high, the current alternates between faster than average and slower than average, again on about a 40-minute cycle. This is caused by a seche in Lake Mendota which only "settles down" once in a while when the lake has been calm for an extended time. Sudden high wind, or the abrupt cesation of high wind can also cause the river to flow backward at a pretty extreme speed for a while when the wind causes a "pileup" of water on the near side of the lake, or piled-up water on the opposite side of the lake settles back to normal when the wind quits. The gauge at this location uses some kind of sonar to measure current speed via doppler effect, as I recall. I suppose a similar device could be used at standard guage locations, though so far, I haven't heard of that being the case.
Rating curves
They establish a rating curve for flow vs. gage height for each location and then later use the gage to determine the flow.
You can measure the flow with a flow meter at different depths and locations across a stream. For bigger rivers, at least, it is easier to do with acoustic dopplar these days. That gives you nearly continuous velocities along a profile.
http://www.nws.noaa.gov/os/hod/SHManual/SHMan040_rating.htm
Oh, oh, I know this one!
At the risk of upsetting a hydro tech out there, I’ll take a stab at this one. Calculating flow through a wier is based on the known physics of water flow through a wier, and as you point out this won’t work in streams that have irregular shaped bottoms. Estimating flow in rivers starts with a direct measurement of streamflow. Basically, you start with a cross section of the river you’re interested in. Then you directly measure the velocity of water flowing down the river at many points - say, every 2 feet across the stream, near the streambed, half way to the surface, and at the surface. There are tools to do this with, as haresfur pointed out this step has been modernized in recent years. Via some cool equations you can then estimate the water velocity(ft3/s)throughout the cross sectional area of the river. Now you just multiply the cross sectional area of the river (ft2) by the velocity of the water flowing through the river (ft/s) to get total streamflow (ft3/s).
As you do this a bunch of times you can predict the flow (ft3/s) based on how high the river is. Then you just need to monitor the height of the river (which is easy to do in real time) and you can estimate the streamflow. The devices that monitor stream height in real time talk to the computers back at the office via a phone line and, the conversion from stream height to streamflow is made and the ‘real time’ streamflow estimates are calculated & posted online.
Drop me an email if you really want to get the math-nerd version.
discharge water
A gage house with either a well or a pressure sensing devise is set up on a river. This devise in most cases will only measure the stage of the water. During a normal year the water will be at high (runoff), medium, and low stages. A measurement will be made of discharge ft3/s and this discharge will be connected with the each stage. A graph is drawn using stage vs discharge so that for every stage (Ex 2.37 ft) a corresponding discharge (Ex 267 ft3/s) can be calculated.
Normally there is a device in the gage house that transmits the stage every 1 to 4 hours to a satilite and then gets retransmited to earth. This is how the stage gets into the computer. The computer calculates the discharge. Every so often, usually about every 6 weeks, the gage is measured and corrected or recalibrated. Small corrections are normal.
The discharge measurement is made by dividing the river into small sections, measuring the width, depth, and velocity of each section and adding them together for total disharge - ft3/s.
If you want more info most USGS sites will supply information or I would be glad to answer questions. I hope the above info helps.
Case. Sorry about spelling errors.
GAUGE
for goodness sakes, GAUGE!