Printable Version

Determining a Potential Microhydropower Site's Flow

The quantity of water falling from a potential microhydropower site is called flow. It's measured in gallons per minute, cubic feet per second, or liters per second.

You can use your site's flow calculation along with its head calculation to determine the site's potential power output.

The easiest way to determine your stream's flow is to obtain data from these local offices:

• The U.S. Geological Survey
• The U.S. Army Corps of Engineers
• The U.S. Department of Agriculture
• Your county's engineer
• Local water supply of flood control authorities.

If you can't obtain existing data, you'll need to conduct your own flow measurements. You can measure flow using the bucket or weighted-float method.

Bucket Method

The bucket method involves damming your stream with logs or boards to divert its flow into a bucket or container. The rate at which the container fills is the flow rate.

For example, a 5-gallon bucket that fills in 1 minute means that your stream's water is flowing at 5 gallons per minute.

Weighted-Float Method

Another way to measure flow involves measuring stream depths across the width of the stream and releasing a weighted-float upstream from your measurements. Due to water safety concerns, this method isn't recommended if the stream is fast-flowing and/or over your calves.

You will need:

• An assistant
• A tape measure
• A yardstick or measuring rod
• A weighted-float, such as a plastic bottle filled halfway with water
• A stopwatch
• Some graph paper.

With this equipment you can calculate flow for a cross section of the streambed at its lowest water level.

Instructions:

1. First, select a stretch of stream with the straightest channel, and the most uniform depth and width possible.
2. At the narrowest point, measure the width of the stream.
3. Then, holding the yardstick vertically, walk across the stream and measure the water depth at one-foot increments. To help with the process, stretch a string or rope upon which the increments are marked across the stream width.
4. Plot the depths on graph paper to give yourself a cross-sectional profile of the stream.
5. Determine the area of each section by calculating the areas of the rectangles (area = length × width) and right triangles (area = ½ base × height) in each section.
6. Next, from the same point where you measured the stream's width, mark a point at least 20 feet upstream.
7. Release the weighted-float in the middle of the stream and record the time it takes for the float to travel to your original point downstream. Don't let the float drag along the bottom of the streambed; if it does, use a smaller float.
8. Divide the distance between the two points by the float time in seconds to get flow velocity in feet per second. The more times you repeat this procedure, the more accurate your flow velocity measurement will be.
9. Multiply the average velocity by the cross-sectional area of the stream.
10. Then multiply your result by a factor that accounts for the roughness of the stream channel (0.8 for a sandy streambed, 0.7 for a bed with small to medium sized stones, and 0.6 for a bed with many large stones). The result will give you the flow rate in cubic feet or meters per second.

Stream flows can be quite variable over a year, so the season during which you take flow measurements is important. Unless you're considering building a storage reservoir, you can use the lowest average flow of the year as the basis for your system's design. However, if you're legally restricted on the amount of water you can divert from your stream at certain times of the year, use the average flow during the period of the highest expected electricity demand.