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close this book A training manual in conducting a workshop in the design, construction, operation, maintenance and repair of hydrams
close this folder Session 3: Water measurement techniques (3 hours)
View the document Handout 3A: Using a Weir
View the document Handout 3B: Using a Weir - diagram
View the document Handout 3C: Weir table
View the document Handout 3C: Weir table - metric
View the document Handout 3D: The float method of measurement

Handout 3D: The float method of measurement

The float method of measurement is a simple procedure for obtaining a rough estimate of the flow of the stream. It will give a ball park figure for looking at the stream's potential. It should not be used for final determination of the hydram system to be used unless the flow rate needed for the ram is such a small percentage of the stream's total flow that what's taken from the stream, for all practical purposes, amounts to a minimal portion of the stream.

The float method is based upon two aspects of the stream: it's cross-sectional area and the velocity of the stream. The cross-sectional area should be determined at some accessible spot in the stream, preferably in the middle of a straight run. Measure the width (w) of the stream. Then, using a stick, measure the depth at equal intervals across the width of the stream (see figure below). Record the depth at each interval and calculate the average depth (d). Now multiply the width (w) by the average depth (d) to get the cross-sectional area (A).

Figure A

Example: The width of a stream, at the point of making depth measurements, is 4 feet. The average depth is 1.1 feet. Therefore, the cross-sectional area (A) is:

square feet

The stream velocity can be determined by choosing a straight stretch of water at least 30 feet long with the sides approximately parallel and the bed unobstructed by rocks, branches or other obstacles. Mark off points along the stream. On a windless day, place something that floats in midstream, upstream of the first marker. A capped bottle partially filled with water works well because it lies with a portion of the bottle submerged and doesn't just ride the surface of the water. Carefully time the number of seconds it takes the float to pass from the first marker to the second. Repeat this process several times and average the results.

Example: The average time for a float to travel between two markers placed 30 feet apart is 30 seconds. The velocity (V) of the float is therefore:

V = 30 feet

30 seconds

V = 1 foot/second

V = 60 feet/minute

The flow rate of the stream can now be calculated by multiplying the cross-sectional area (A) by the stream velocity (V). The usable flow (F) can then be determined by multiplying the stream flow rate by a fraction representing the portion of the stream flow that you can or want to use.

Example: If you will be using 25% of the stream flow, the usable flow (F) is:

F = A x ΒΌ" x .25

F = 4.4 square feet x 60 feet/minute x .25

F = 66 cubic feet per minute

This flow in cubic feet per minute can then be converted to the appropriate units by multiplying by the correct conversion factor:

cubic feet/min x 7.48 = gallons/min

cubic feet/min x 28.3 = liters/min

SOURCE: Micro-Hydro Power, National Center for Appropriate Technology (1979).