Small Hydropower & Micro Hydropower

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: 

1. Pick a fairly regular part of the stream with about the same cross section and curvature for a 100 foot distance.
2. 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.
3. Select a float that will be somewhat neutrally buoyant, such as an orange. Plus it’s biodegradable
4. The goal is to have it float just at or under the surface down through the race course between point A and B.
5. 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.
6. 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.
7. 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

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