Equation of interest: Area x Average speed x 80% Friction Factor = Q,
the estimated average stream flow rate.
By measuring the rate of travel for a floating object traveling down the main flow of a stream and then multiplying by the average cross-sectional area. One can determine the average volume flow rate or Q directly. Please note that this method is only an estimation, and will have inaccuracies due to anomalies in the channel and issues surrounding the float chosen, etc. The main difficulty in carrying out this measurement has to do with the care and problems in accurate measurement of the streams cross-sectional profile between the points B-B’.
Procedure:
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Pick a fairly regular part of the stream with about the same cross section and curvature for a 100 foot distance.
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Measure a 50 to 100 foot section or race course of your stream bed. The length between point A and B. will be used to measure the velocity of the float.
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Select a float that will be somewhat neutrally buoyant, such as an orange. Plus it’s biodegradable
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The goal is to have it float just at or under the surface down through the race course between point A and B.
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Use a stopwatch to time, several runs, tossing your float in upstream from section A while starting the watch as the float crosses section A and stopping the watch just as the float crosses section B. Repeat this sequence 5 or 10 times and average the measured times. The average is obtained by adding the times up and dividing by the number of times that you measured the elapsed time. Throw out any times that are grossly apart from each other.
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Now measure the cross sectional area of the creek by measuring the distance from the surface to the bottom of the creek (Use a level reference line see diagram in this post.) Each distance must be taken using the same horizontal interval, say 1 foot. Now add up the depth measurements and divide by the number of measurements. This is your average cross sectional depth. Multiply by the interval width and you have average area.
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Multiply average stream velocity x average cross section area x friction correction factor of 0.8. Due to friction, bottom irregularities, etc. this is the least accurate measurement. It is likely only about 15-20% accurate at best. Concrete channels are best and rough streambeds the worst cases for using this method. Still, it will be better at stream flow estimation than a rough estimate or wild guess.
Diagram: Stream Flow Measurement Using a Float, Stopwatch and average cross sectional area estimate.
Stream Flow Estimation By Direct Measurement of Speed x Cross section Area
For more on this method visit this US EPA Water Flow Rates document.
February 10th, 2010 at 1:41 am
I am much influenced by lts logic and desings. keep us informing..Thanx
March 8th, 2010 at 6:18 am
CAN YOU PROVIDED ME WITH THE PRODUCTION KILWATT FOR THE PELTON WHEEL?
July 21st, 2010 at 4:36 am
How do you get 80% friction factor? I have read numerous articles on this and have seen this number range from 60% to 90% – all without much explanation. Can you make this more clear?
I’m measuring an open-top concrete channel (average cross-sectional dimension of flow 0.9m x 0.5m, height x width) and though it has some sediment at the bottom I tend to doubt your FF will work. The tight dimensions make me think the FF should be increased, but the fact that it is concrete (relatively smooth) tells me the opposite. Right now my best estimate is 60%. What do you think? My result is putting the flow at 66 Liters/second which I think is high.
September 20th, 2010 at 5:46 pm
Kyle,
This method employs an empirical approach to measurement. That is the development of the method requires correction factore to calibrate the result so it will even be close. Each factor is chosen based on the surface type and the roughness. Rough surfaces cause turbulence and eddies to form. These will impede flow via increased friction near to the surface.
With that said under some circumstances (high flow rates & tight turns) the flow will detach and all sorts of interesting complicated turbulent stuff happens. For now just know the factors are measurements that correct lab models to fit reality a bit better.
If you want a model that gives a bit more accurate estimate of open channel flow, look at the pipeflow site mentioned on another post on here on “smallhydro.com”… they also have channel flow estimation tools in addition to pipeflow.
December 8th, 2010 at 11:20 pm
i wants to design a small hydro power plant (100kw) what are the procedure to measure the water discharge ,in liter /sec.
April 26th, 2011 at 4:47 pm
Great idea ! however i believe it is an expensive exercise. If you are looking at supplying your quarters only, why not consider building a “Tower-Tank” (depending on hight to give max fall) and harness the water from the stream (no enviro configuration in the fauna) into the tank using pumps driven by “tower tank hydro” .. You’ll not only save but also preserve the habitat.
I say this as i have a “tower -tank hydro” at the hight of 10mtrs, running a modified 8Kva alternator, giving power to the whole house with white goods.