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Measuring Flow
The second major step in evaluating your
site’s hydro potential is measuring the
flow of the stream. Stream levels change
through the seasons, so it is important
to measure flow at various times of the
year. If this is not possible, attempt
to determine various annual flows by
discussing the stream with a neighbor,
or finding U.S. Geological Survey flow
data for your stream or a nearby larger
stream. Also keep in mind that fish,
birds, plants, and other living things
rely on your stream for survival. Never
use all of the stream’s water for your
hydro system.
Flow is typically expressed as volume
per second or minute. Common examples are
gallons or liters per
second(orminute),and cubic feet or cubic
meters per second(orminute).Each can be
easily converted to another,as follows:
1 cubic foot = 7.481 gallons
1 cubic meter = 35.31 cubic feet
1
cubic meter = 1,000 liters
Three popular methods are used for
measuring flow—container, float, and
weir. Each will be described in detail
below.
Container Fill Method
The container fill method is the most
common method for determining flow in
micro hydro systems.Find a location
along the stream where all the water can
be caught in a bucket.If such as pot
doesn’t exist,build a temporary dam that
forces all of the water to flow through
a single outlet.Using a bucket or larger
container of a known volume,use a
stopwatch to time how long it takes to
fill the container. Then divide the
container size by the number of seconds.
For example, if your container is a
5-gallon paint bucket and it takes 8
seconds to fill, your flow is 0.625
gallons per second (gps) or 37.5 gallons
per minute (gpm).
Float Method
The float method is useful for large
streams if you can locate a section
about 10 feet (3 m) long where the
stream
is
fairly consistent in width and depth.
Step1.Measure
the average depth of the stream. Select
a board able to span the width of the
stream and mark it at 1-foot(0.3m)
intervals. Lay the board across the
stream, and measure the stream depth a
teach 1-foot interval. To compute the
average depth, add all of your
measurements together and divide by the
number of measurements you made.
Step2.Compute
the area of the cross-section ou just
measured by multiplying the average
depth you just computed by the width of
the stream. For example, a 6-foot-wide
stream with an average depth of 1.5 feet
would yield a cross-sectional area of 9
square feet.
Step 3.Measure
the speed. A good way to measure speed
is to mark off a 10-foot (3 m) length of
the stream that includes the point where
you measured the cross-section.
Remember, you only want to know the
speed of the water where you measured
the cross-section, so the shorter the
length of stream you measure, the
better.
Use a weighted float that can be clearly
seen—an orange or grapefruit works well.
Place it well upstream of your
measurement area, and use a stopwatch to
time how long it takes to travel the
length of your measurement section. The
stream speed probably varies across its
width, so record the times for various
locations and average them.
Weir Method
A weir is perhaps the most accurate way
to measure small- and medium-sized
streams. All the water is directed
through an area that is exactly
rectangular, making it
very easy to measure the height and
width of the water to compute flow.
This kind of weir is a temporary dam
with a rectangular slot, or gate. The
bottom of the gate should be exactly
level, and the width of the gate should
allow all the water to pass through
without spilling over the top of the
dam. A narrower gate will increase the
depth of the water as it passes through,
making it easier to measure.
The depth measurement is not taken at
the gate itself because the water depth
distorts as it moves through the gate.
Instead, insert a stake well upstream of
the weir gate and make the top of the
stake exactly level with the bottom of
the weir gate. Measure the depth of the
water from the top of the stake.
Once the width and depth of the water
are known, a weir table is used to
compute the flow. The weir table shown
here is based on a gate that is 1 inch
(25 mm) wide. Simply multiply the table
amount by the width (in inches) of your
gate. For example, assume your weir gate
is 6 inches wide, and the depth of the
water passing over it is 71/2
inches. On the left side of the table,
find “7” and move across the row until
you find the column for “+1/2”.
The table shows 8.21
cfm flow for a 1-inch gate with 71/2
inches of water flowing through it.
Since your gate is 6 inches wide, simply
multiply the 8.21 by 6 to get 49.26 cfm.
A weir is especially effective for
measuring flow during different times of
the year. Once the weir is in place, it
is easy to quickly measure the depth of
the water and chart the flow at various
times.
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