It is important to know not only average stream flow, but also minimums and maximums to be expected. You'll need to estimate minimum flows to ensure that you will always have enough power, and you'll need maximum flows in order to design your system so that it will not create a endanger the turbine installation during peak flooding. If you are familiar with your area, you may be able to recall past conditions. If not, you may have to gather this sort of historical information.
There are three methods of measuring flow rate:
 1. Temporary Dam, Stopwatch and Bucket: The dam and bucket system is suited to a small flow such as a stream or spring. You build a box-like containment to catch all the water, and provide an outlet pipe to spill the entire flow into a large container. Time the period needed to fill the bucket, and you can easily calculate the flow in litres per second.
2. Float and Stopwatch: The float method is useful when you can lay off 10 metres or more of a stream where the width and depth are fairly constant. The following drawings show you how to determine flow with this system.
Mark board at 300mm intervals and place across stream; measure water depth at each mark and record. Then draw across section to determine square metres by either averaging measurements or by subdividing into rectangle and two triangles and calculating the area. Use a weighted float that can be clearly seen, and multiply the metres travelled by float in one minute by square metres of cross section. Multiply this figure by 0.83 for water flow in cubic metres/min. (The 0.83, or 5/6, is difference in flow rate between the water at the stream's surface and the lower depths). To further convert this figure into litres/second, multiply by a factor of 16.67.
Note - the factor of 0.83 is suitable for a smooth wooden trough only, a reduced factor should be used for other materials. For example:
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Concrete channel 0.80 |
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Earth head race 0.75 |
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Rough river bed 0.60 |
3. Temporary Dam and Weir Gate:
The weir board is located at a convenient place, where it can be bedded in and calm, slow moving water can be accumulated behind it. The weir board may be 15 to 75 mm thick, depending on its length. The bottom edge should be embedded in the bed of the stream, and the top edge should be above the surface of the water flowing through the weir. The ends should be bedded in the bank or suitably sealed to prevent water leaking past. The water notch, or opening is cut into the weir board, deep enough to pass all the water and long enough to give a depth over the weir so that the following table can be conveniently used. Widths of the notch of 250, 500, 750 mm or 1 metre will enable the table to be used more easily. Preferably, the width of the notch should be less than half the width of the stream, to keep the water velocity sufficiently low behind the weir. The notch or opening, should have the sill or bottom and the side edges sharpened by cutting away the downstream side. The sill or bottom edge should be set true and level.
The stake is driven into the bed of the stream at one side in slow moving water, and about 1 to 2 metres upstream from the weir board. It should be carefully set so that the top of the stake is level with the sill, or bottom edge of the notch or opening. When the water has reached its greatest depth, a measurement 'h' is taken of the depth of the water over the stake.
The readings of 'h' are used with the table below as follows:
Suppose the notch is 500 mm wide, and the water at the stake 80 mm deep. The water flow reading of 20.1 litres per second is found in the column of figures for the 500 mm notch, level with the 80 in the 'h' column.
Instead of using the stake, it is sometimes easier, but perhaps not so accurate, to mark a scale for the depth 'h' on the weir board. close to the bank and as far away from the notch as possible. It is wise to put a scale at each side and take the average of the two readings in case there is any difference.
[ Introduction ] [ Gross Head Sketch ] [ Measuring Gross Head ] [ Water Flow Tables ]
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