Having calculated the water quantities in the watershed it will be interesting to calculate the flow of water in the stream which drains Lake Wintergreen. This can be conveniently done by the float method where the stream flows over a concrete bottom and between concrete sides under the Wilbur Cross Parkway. Although this method isn’t the most accurate, it doesn’t require construction in the stream and should be sufficient for the purposes of this course and provide an understanding of what stream flow means.
Two points are chosen at least 30 feet apart and are marked on both banks. The cross sectional area of the stream is calculated at each end. (If there were more variation in the shape of the stream, more cross sections should be taken to get a good average.) A float is then placed in the water at the upstream mark and is timed until it reaches the downstream mark. The result in square feet of section times the feet of distance traveled divided by the time in seconds equals the cubic feet per second, times 7.48 gallons per cubic foot equals gallons per second.
The inaccuracy in this method lies in the friction between the water and the concrete on the sides and bottom which slows down the water. Concrete has a lower friction effect than most natural stream bottoms, but this method will still overestimate stream flow.
This simple measurement could be repeated several times to see how the stream flow varied. The stream flow could also be multiplied by time until it reached a year to see how it compared with the calculated excess from the Lake Wintergreen watershed. It should be remembered that the stream flows for about 3000 feet from the lake to the parkway and during this distance it receives water by the same processes which provide water to the lake. Seasonal variation in streamflow should also be kept in mind. Most streams have highest flows in late winter and spring, and lowest flows in late summer, early fall. In this case the lake should act as a capacitor would in an electric circuit, moderating and evening out the stream flow.
Once the flow is determined it will be interesting to determine the power that would be available from the stream to turn a turbine to produce electricity. The formula for gross horsepower is the minimum water flow (in cubic feet/sec.) times the gross head in feet divided by 8.8.
If the minimum flow measurement isn’t available, whatever measurement is available should be used as long as a lower limit to year round power production is understood. Head is the measurement of the distance the stream falls. Just south of the Nature Center entrance the stream falls very quickly so this would be a good place to think about a water power site. Head is traditionally measured with a transit, making a series of level lines and height measurements. It might be easier and more interesting with the students to use the hose method. Here a siphon is started from the higher water area using a long garden hose. The lower end is raised until the water stops flowing. At this point the height of the hose is about the same at both ends. The distance of the downstream end of the hose above the stream is the head. This may have to be done several times if the head is high.
The gross horsepower can then be calculated. Factors which will reduce this amount of power in an actual water power installation, besides lower stream flow, are friction losses in the system used to carry the water from the higher area to where the power unit would be located at the low point. Other factors which reduce final power are turbine efficiency less than 100% and transmission losses from production point to the power use site. Several sources of information about small generators are listed on the resource sheet.
