# The power of moving water.....

How do we measure current? In some books I’ve read, I seem to recall seeing the flow of rivers described in c.f.s.—I understand this to mean cubic feet per second. I could be wrong—it may have been c.f.m—cubic feet per minute. What is the generally accepted form of measurement? And how is this measured? What instrument[s] is/are used?

Our local river has been running high and fast for about the past two weeks. Between the powerful current and the fact that the banks of the river are now well up into the trees and brush, it is extremely difficult to travel upstream right now.

What I’d like to know is this: Is there a simple way to measure the current? Is there maybe a hand-held device that one could hold overboard in the current to measure the power of the river on any given day? I realize that the actual numerical value of c.f.s [or c.f.m] won’t ultimately mean anything, but I’m just curious.

-Rob

No I don’t think so…
“Is there a simple way to measure the current?”

If I remember my engineering courses from the dark ages. You can only measue the flow at that point, the current needs to be integrated accross the whole cross sectioal area of a river channel. As all kayakers know the flow is very different at all points of a cross sectional area. Flows are estimated based on gauge heights, known topography, and measured flow velocities. Nowdays I believe they even use dopler radar to monitor wataer speed at the surface and calculate flows based on cross sectional area. I’m sure some of the whitewater enthusiasts here can give you much more information.

I use the internet

http://waterdata.usgs.gov/ct/nwis/rt

This is the bible, as long as it’s been updated. Common sense and experience with a given watershed is the second factor in the equation.

This is the general site
http://waterdata.usgs.gov/nwis

Flow meter
You can measure velocity with a flow meter. There are different versions of these, but some look like a wind aenometer.

The river channel dimensions have been established at USGS gaging stations so the volume of water can be measured and are the easiest source of information. Sadly, many of these are closing due to lack of funding.

One method for your own purposes

– Last Updated: Mar-11-07 12:13 PM EST –

You say it's extremely difficult to travel upstream right now, and that makes it sound like you went out on the water in some kind of boat. If that's the case, and if you have a GPS unit, you can drift with the current and measure the boat's speed with the GPS. The GPS-measured boat speed will be the same as the speed of *that portion of the current you are coasting in* (the current speed will be different elsewhere).

This won't be the average current speed in many cases, but you may find it interesting. For example, you'd be able to see how the current is much faster in the center of any zone of main flow, and slower near the edges, and slower still alongside the shore in many cases. Just make sure that when you take your speed reading, that the boat really IS drifting the same speed as the current (it can take a while for any of the boat's own drift through the water to come to a stop, and I've seen people fail to recognize this when measuring current speed by this method). When you are going the same speed as the current, debris in the water will neither move ahead nor fall behind the boat's position, and inserting a stationary paddle blade in the water will exert no control over what the boat does, as long as you aren't on any eddylines (except perhaps for some momentary little random "bumps" from turbulence).

I've done this quite a few times when out on the water with other people, and I've found that most people's visual estimate of current speed to be about twice as fast as what GPS measurement shows it to be (but only if they are trying this for the first time).

– Last Updated: Mar-11-07 1:11 PM EST –

On your home waters, it's easy to pick out visual markers (bridge piers are a good place to look or add them) and base your assessment of the current from them.

I say "home waters" because that's what it sounds like you are talking about. Your own experience at different flow levels is your best guide.

Both cubic feet per second and water depth are meanless without context. Your own experience, perhaps coupled with that of a local club, provide the necessary context to make numbers meaningful. And if in doubt, don't mess about.

GPS aid
The GPS method of measuring local flow velocity is a good idea. As an aid, if you threw some dry leaves or whatever onto the water surface you’ll be able to judge if you’re drifting with the current, plus or minus (see below). To measure surface flow velocity in a water channel, I used to videotape the passage of little paper circles from a hole-punch over a known distance, then use the camera frame rate to find the speed.

As a footnote, due to air drag, the flow velocity below the surface layer can be higher than at the surface. That’s why it’s a good idea to use the boat-drift method - the vertical variation in velocity is effectively integrated by the boat hull to give an average velocity over the part of the river that matters - the part that touches your boat. You can then compare that figure to how fast you are able to paddle.

Powerful Current ,River in the Trees
Sounds like a fair description of flood stage to me.

If you don’t already know, read up on strainers and the hazards of rivers in flood.

If you do already know and you’re going out anyway be careful! Yeah and have fun and show us the pictures.

But mostly be careful.

Tommy

Feel the power with your arm -
Use a 1" x 4" piece of wood about 7’ long and a 1/2" piece of plywood 1’ x 1’ square. Attach the 1’ square piece of plywood to the end of the 7’ long piece so that 6’ remains extended from the square. Plywood screws will work fine.

You can use this instrument from the shore. For accurate relativity, you should conduct your tests at different times from the same place. Hold the end of the board and place the 1’ square piece into the current so that it is just fully submerged. Feel the power. Note the river level on the shore. Conduct the test as often as you wish, feeling the current power and noting the river level in your mind.

You can compromise on the technical aspects by using a yayak paddle, instead.

When you are beginning your paddling excursion, you will come to know what to expect by noting where the waterline is on the shore, and that is all that counts.

Thanks…
Thanks to all of you for your input on this. I asked a pretty vague question about an ever-changing medium. You folks came up with some good ideas to think about and to keep in mind.

Tommyc1, thanks for the safety notice. Good call. You’re right—this river is almost at “flood stage”. I am aware of the hazards involved with high water and I respect the strainers and the incredible power of the moving water.

I, and my wife, had a beautiful downstream paddle today. We steered right down the center [the river did most of the work] about 7.5 miles in the afternoon sunshine.

Lots of waterfowl out there today! Do any of you know the Bufflehead? We have about 30-40 Bufflehead ducks in this stretch of the river right now. They are too cool!

-Rob