There are a number of good ways to measure the amount of water in a stream
or a canal. What method of measurement you should use will depend on several
factors:
The accuracy of the result needed;
The quantity of water present in the stream or canal you will measure;
The equipment you have available to use.
Let us compare various methods. Table 3 will help you to compare various methods and to select
the one best suited to your needs. Each of these methods is fully explained
and illustrated in the following sections.
Does not
vary greatly,
114 l/s or smaller,
or does vary greatly from small to large
High
For recording flow over a period of time
Wood, sheet metal or corrugated roof sheeting;
tools for working with wood or metal; shovel, pick, line,
level,
measuring stick
Weir,
rectangular
Does not vary greatly and is
greater than 114 l/ s
NOTE: * very simple; ** more difficult; *** most
difficult.
3.1 Quick rough estimate
This is a very simple method to measure approximate water flow
in very small streams. You do not need any special equipment for
this estimate.
Drop a leaf in the water flow of the stream you want to measure.
Walk in the direction the leaf is floating at a normal pace
for about 30 metres or 35 paces.
See how far the leaf floats during the time you are walking and
estimate the water flow as shown in the examples.
Examples
If you find that your water requirements are no greater than those
seen in the examples, you do not need to make any more water flow
measurements.
If you find that your water requirements are greater
than those seen in the examples, you should use one of the more accurate
methods to measure the water flow so you will be sure that you have
sufficient water available.
3.2 Bucket method
This is a simple method for measuring a very small flow of less
than 5 l/s with very high accuracy.
Begin to build a small dam of earth across
the stream to stop the flow. You can use wood poles, bamboo or tree
branches to hold the earth in place while you build the dam.
When the dam is about half built, put in a pipe about 5-7 cm in
diameter and about 1-1.5 m long. This pipe can be made of bamboo.
Finish building the dam across the stream so that all the water
flow passes through the pipe.
Find at least two buckets or other, similar containers which you
can use to catch the water flowing through the pipe. You will also
need a bottle or other, smaller 1-litre container.
Using the 1-litre container, count the number of litres needed
to fill the buckets with water, in order to find how much each bucket
will hold.
Example
Using one bucket after the other, catch all the water flowing through
the pipe for 1 minute (60 seconds). Count how many buckets you can
fill during that time. Calculate the total water flow (in l/s).
Example
3.3 Float method
This is a method for measuring small to large water flow with medium
accuracy. This method is best used in streams with calm water and
during periods of good weather for if there is too much wind and
the surface of the water is rough the float may not travel at the
normal speed.
Prepare a float
A good float may be a piece of wood or a smooth tree branch about
30 cm long and 5 cm wide or a small well- capped bottle 10 cm tall,
containing enough matter (such as water, soil or pebbles) so that,
when it floats in the stream, the top of the bottle is just above
the surface.
Where to measure
Find and mark a length AA to BB along the stream, which is straight
for a distance of at least 10 metres. Try to find a place where
the water is calm and free from water plants so the float will flow
easily and smoothly.
Find the average water velocity
Ask a friend to place the float in the middle of the stream, a
few metres upstream from line AA, and to release it gently into
the current. Stand at line BB and using a watch, measure exactly
the time (in seconds) it takes the float to travel the distance
from AA to BB.
Repeat this measurement three times. Place the float in the water
and note how long it takes to travel the distance from AA to BB
three different times.
Note: if one of the three measurements is greatly
different from the other two, take a fourth measurement and use
this one.
Example
Now you can calculate the average time it has taken
the float to travel from AA to BB. Add the three measurements and
divide the sum by 3.
Example
Find the surface water velocity (in m/s)
by dividing the distance from AA to BB (in this example, 10 m) by
the average time (in seconds) and multiply this result by 0.85 (a
correction factor) to estimate the average water velocity of
the stream.
Example
Find the average width
Measure the width (in m) of the stream in a number of places. Take
the measurement that occurs most frequently as the average width.
Example
Find the average depth
Measure the water depth (in m) of the stream at several points
along its width. Take half of the deepest measurement as an approximation
of the average depth.
Example
Calculate the water flow
To calculate the water flow (in m3) multiply the average
water velocity (in m/s) by the average width (in m) and by the average
depth (in m).
Example
Note: remember that 1 m3
= 1 000 l so multiply by this to convert water flow measurements to
litres per second (l/s).
Example
3.4 Float and cross-section
method
This is a simple method for measuring small to large water flow
with an accuracy somewhat greater than the float method described
in Section 3.3. Like the float method, it is best used in calm water
and during periods of good weather when there is little wind. You
will need to prepare a float as you were shown in
the previous section.
Find a length along the stream that is straight for a distance
of at least 20 metres. Try to find a place where the water is calm
and free from water plants so the float will float easily and smoothly.
Mark it with stakes on both sides of the stream at points AA and
BB and stretch a line between the stakes.
Find the average cross-section
The cross-section of the stream will be different at the beginning
(AA) and the end (BB). You will need to find the average cross-section.
Measure the water depth (in m) five times at equal distances across
the stream at point AA.
It will be easier to record the measurements you take at points
AA and BB if you prepare a small drawing as a record sheet to write
them on.
When you have taken all the measurements
at point AA, add the five depth figures and divide by 5 to find the
average water depth at AA.
Example
The cross-section (in m2 ) at
point AA is the average depth multiplied by the width of the stream.
Example
Now take measurements at point BB as you
did at point AA to find the average depth, stream width and cross-
section at BB.
Example
To calculate the average cross-section
of the stream at points AA and BB add the two cross-section values
you found and divide by 2.
Example
Find the average water velocity
Now you must find the average water velocity using a float as
described in previous section 3.3. Have a friend
put the float in the middle of the stream, a few metres upstream
from line AA, and release it gently into the current. Stand at
line BB and, using a watch, measure exactly the time (in seconds)
it takes the float to travel the distance from AA to BB.
Repeat the measurement at least three times and calculate the
average time by adding all the measurements and dividing by the
number of measurements you have taken. Now divide the distance
from AA to BB by the average time to find the surface velocity
of the water, and multiply this by 0.85 (a correction factor)
to estimate the average water velocity.
Example
Calculate the water flow
To calculate the water flow (in m3/s) multiply the
average water velocity by the average cross-section.
Example
To express this water flow in litres
per second (l/s), multiply the result (in m3/s) by 1000.
Example
Note: you can increase the accuracy
of this method if you increase the distance from A to B to 30
m, 50 m or even 100 m. A greater distance between A and B is
especially recommended if the stream is fast flowing. The faster
the water flow, the greater the distance should be.
Note: you can also increase the accuracy of this
method if you increase the number of time measurements to 5,
7 or even 10.
But remember...
The longer the time measurement, the less the number of
measurements required;
The longer the time measurement, the greater the difference
between each figure will be.
Example
3.5 Dye stain and cross-section method
This is a method for measuring small and large water flow with
medium accuracy. In this method, water-staining dye is used
instead of a float to measure the water flow.
Measure the time (t1, in seconds) it takes for the
front of the dye stain to reach line BB.
Drop a small amount of dye in the middle of the stream a little
above line AA. This will form a dye stain in the water.
Note: potassium permanganate and fluorescein are
suitable dye solutions that may be available from chemical suppliers.
Measure the time (t2, in seconds) it takes for the
end of the dye stain to reach line BB.
Calculate the average
time it takes the front and back of the dye stain to reach line
BB by adding t1 and t2 and dividing the
result by 2.
Example
Calculate the water velocity (in m/s) by dividing
the distance from AA to BB (in m) by the average time (in s).
Note: when you use a dye stain you do not have
to multiply the water velocity by a correction factor as you
do when using a float.
Example
Calculate the average cross-section
of the stream as described in section 3.4.
Example
The water flow equals the water velocity
multiplied by the average cross-section.
Example
Note: you can increase the accuracy of this method if
you increase the distance from AA to BB or increase the number of time
measurements as described in the previons section.
3.6 Weir methods
Weirs are commonly used to measure small and large water flow
with high accuracy. They are especially useful for recording water
flow over a period of time.
What is a weir?
A weir is an obstruction placed across a stream that forces
all the water to flow through a notch in the weir.
There are weirs of many types and designs. In this section we
will discuss two types, the triangular weir and
the rectangular weir.
In both the triangular and the rectangular weir the notch used
has sharp edges so the water flowing over the weir
will touch only a fine line and the notch width is
smaller than the stream width (contracted weir).
When a weir is in place across the stream it raises the upstream
water level. To be efficient, a weir should create a sufficient
vertical drop between the notch bottom and the downstream water
surface. In such a case, the water will fall free,
and air can circulate beneath the water as it overflows.
The crest of a weir is the bottom edge of the
weir notch. In a rectangular weir the crest length is the width
of the notch. In a triangular (or V-notch) weir the crest length
is zero.
The head of the weir is the vertical distance
from the weir crest to the undisturbed upstream water surface.
Advantages and disadvantages of weirs
Advantages:
They allow for easy and accurate flow measurement;
They are easy to build and require only little maintenance;
small, floating debris will easily pass through the notch;
They are durable.
Disadvantages:
They require considerable head-loss for proper operation;
Large pieces of floating debris can become caught in the
notch and change the water flow;
Changes in flow can occur, for example, if debris becomes
caught in the weir, silt builds up behind the weir, etc.
Where to install a weir
A weir should be installed in a channel that, upstream from
the weir, is straight for a minimum distance at least 10 times
greater than the length of the weir crest.
To increase accuracy, place the weir at the lower end of a
long pool sufficiently wide and deep for the water flow to approach
the weir slowly, regularly and without any eddies.
The water velocity immediately upstream from the weir should
not exceed 0.14 m/s.
Place the weir where the upstream water level
(behind the weir) will not cause abnormal water losses by flooding
the banks next to the stream, or water infiltration loss into
the upper soil of the stream banks, which were not under water
before. You must be particularly careful in flat countryside
or where there are channels or ditches next to the stream that
will be below the new water level behind the weir.
How to choose a suitable weir
First estimate the stream flow by using the float and cross-section
method described in Section 34.
Use a triangular weir if the stream flow to be measured:
Does not vary greatly from season to season and is generally
smaller than 114 l/s;
Does vary greatly from small to large flow or large to small
flow.
Use a rectangular weir if the stream flow to be
measured:
Does not vary greatly, and is generally greater than 114 l/s.
How to design a triangular weir
A triangular weir or V-notch weir has a notch that is a right
or 90° angle. Both edges of the notch must be sharp and no more
than 3 mm thick.
To obtain accurate water flow measurements
with a triangular weir, be sure that:
The water head is greater than 5 cm;
The crest height, above the stream bottom upstream from
the weir, is greater than two to three times the head;
The water drop behind the weir is high enough to create
a sufficient vertical drop so the water will fall free.
Note: before you begin to build the weir, plan
carefully in order to meet the above requirements of head, crest
height and water drop. Be particularly careful about the stream
width (if possible more than seven times the maximum water head)
and the depth of the stream where you plan to install the weir.
After the weir is built it will be difficult to change it.
When you estimate water flow using a triangular weir, the error
will tend to increase as the head decreases. Under field conditions,
if you have fulfilled the requirements listed, the error will
generally be limited to 10 percent. In a triangular weir, if you
want to decrease the error further you can increase the notch
depth, within the limits stated above, which
will increase the head.
The following notch depths (in cm) are required for the sizes
of water flow (in l/s) shown:
20 cm, flow less than 15 l/s;
30 cm, flow 15 to 45 l/s;
40 cm, flow 45 to 65 l/s;
50 cm, flow 65 to 110 l/s.
If the water flow is more than the largest water flow figure
shown above (110 I/s) you will have to approximate the required
notch depth. Using Table 4, find the head
(in cm) corresponding to the maximum water flow (in I/s) to
be measured and add about 10 cm to the head value to obtain
the corrected notch depth.
Water flow to measure (l/s)
Corresponding head (cm)
Required notch depth (cm)
180
44.5+10
55
260
51.5+10
62
390
60.5+10
71
How to design a rectangular weir
The type of rectangular weir discussed in this section has
a rectangular notch with a crest length that is less than the
width of the stream. All three edges of the notch must be sharp
and no more than 3 mm thick.
To obtain accurate water flow measurements
with a rectangular weir, be sure that:
The water head is greater than 5 cm;
The crest length is at least
15 cm and should, preferably, be greater than three times
the maximum water head to be measured;
The crest height above the stream bottom upstream
from the weir is greater than two to three times the head;
The distance from the sides of the notch to the sides
of the stream channel should be greater than two times
the maximum water head to be measured;
The water drop behind the weir is high enough
so the water will fall free.
Note: before you begin to build the weir, plan
carefully in order to meet the above requirements of head, crest
height and water drop. Be particularly careful about the stream
width (if possible more than seven times the
maximum water head) and the depth of the stream where you
plan to install the weir. After the weir is built it will be
difficult to change it.
When you estimate water flow using a rectangular weir, the error
will tend to increase as the head decreases. Under field conditions,
if you have fulfilled the requirements listed, the error will
generally be limited to 10 percent. In a rectangular weir, if
you want to further decrease the error you can reduce the crest
length, within the limits stated above which
will increase the head. Table 5 may help you
to do this.
The following notch depths and crest lengths (in cm) are required
for the water flow range values shown:
30 x 60 cm, flow 80 to 120 l/s;
40 x 90 cm, flow 120 to 300 l/s;
55 x 120 cm, flow 300 to 600 l/s;
75 x 180 cm, flow 600 to 1500 l/s.
To define notch dimensions for a rectangular weir other than
those shown above, you can use the white upper
part of Table 5. Locate the maximum water flow (in l/s) to
be measured, keeping the crest length (in cm) as
small as possible. Read horizontally the corresponding head (in
cm) and add 10 to 15 cm to find the notch depth you
should use.
Example
How to build and install a weir
How you build and install a weir will depend on the speed of
the water flow and the size of the stream.
In a flowing stream
If there is a slow water flow or the stream is small, you may
choose to build the weir on the bank, where it is dry and easier
to work, and install the weir in the flowing stream after it is
finished.
With a very small stream, a weir that has been built on the bank
can be installed by pounding it into place or digging it into
the sides and bottom of the stream while the water is flowing.
Note: if there is a fast water flow or the stream
is large, you may choose to build the weir in place in the stream.
The larger the stream, the larger the weir will have to be; it
may turn out to be too large and heavy to build it on the bank
and place it in the stream after it is finished.
By diverting the water
When the stream is large and you must build the weir in place,
you will have to divert the water from the stream channel around
the place where you will put the weir while you are building it.
To divert the water, dig a ditch from a point in the stream bank
upstream from the place you will put the weir to a point in the
stream bank below the weir.
Build a barrage just below the upstream end of the diversion
ditch. When the water backs up behind the barrage it will flow
through the ditch, around the site and back to the stream.
To prevent the water diverted downstream from flowing back into
the site you may have to build another barrage below the weir.
When the water has been diverted and the site is dry, you can
begin to build the weir in place. After the weir is finished,
remove the barrages and let the water back in the stream channel.
It will soon reach its constant level and begin to flow through
the notch.
Position of the weir in a stream
A weir must be placed or built in the stream
in a vertical positionand on a line across the streamperpendicular to the water flow.
Mark the position you have chosen for the weir by stretching a
line across the stream from bank to bank at a right (90°) angle
to the water flow.
Drive a row of strong wood stakes into the stream bed along the
line. Use a level to make sure the stakes are vertical. This row
of stakes will help you to position the weir properly, whether
it has been built on the bank before installation or built in
place in the stream.
When you are installing a weir that has either been built on
the bank, or is being built in place in the stream where the water
has not been diverted, position the weir on the upstream side
of the vertical stakes so that the flow of the water holds the
weir in the correct position against the stakes.
After the weir has been well installed in the banks and bottom
of the stream, you can remove the vertical stakes if the weir
needs no additional bracing. If the water flow is strong and additional
bracing is needed, remove only the stakes behind the notch.
Building a weir of wood
You can build a weir of close-fitting wood boards or planks
held together by upright pieces of wood on both sides.
The thickness of the wood you should use will depend on the
width of the stream and the force of the water flow. For a very
small stream you can use light wood, but for a large and fast
stream you will need to use heavy wood or timber.
Measure the width of the stream and
the distance from the tops of the bank to the stream bed to find
out what size of weir you must build. A weir must be built high
enough and wide enough so that it can be driven well into the
stream banks and bottom to give the necessary support and to prevent
water leakage around the sides and under the weir.
Build the weir in such a way that there
is enough space in the centre between the uprights for the size
of notch you will need.
Cover the joints between the wood boards or planks with strips
of wood to prevent water leakage.
After the weir has been built, you are ready to cut the notch
in the upper edge.
Triangular weir
How to construct a 90° triangular notch in wood:
Find the centre point on the top edge of the weir;
On each side of the centre point, measure and mark a distance
equal to the depth of the notch (say 30 cm) you will use;
Connect the end of this line with the two marks on the top
edge of the weir. You have constructed a right angle (90°)
triangular notch;
Using a wood saw, carefully cut out the notch;
Check the notch you have cut with a square to see that it
has a 90° opening and that all other measurements are exact;
If necessary, strengthen the boards or planks you have cut.
Do this with wood bracing on the downstream side
of the weir;
File both sides of the notch to an angle with a sharp edge
of no more than 3 mm on the upstream side of
the weir.
Rectangular weir
How to construct a rectangular notch in wood:
Find the centre point on the top edge of the weir;
On each side of the centre point, measure and mark a distance
equal to half of the crest length, say 30 cm (crest length 60
cm), you will use;
At each of these two marks, draw a right angle line downwards
equal in length to the depth of the notch;
Connect the ends of these two lines. You have constructed
a rectangular notch;
Using a wood saw, carefully cut out the notch;
Check the notch you have cut with a square and a mason's level
to see that the crest edge is at a 90° angle to the sides and
that all the other measurements are exact;
If necessary, strengthen the boards or planks you have cut.
Do this with wood bracing on the downstream side
of the weir;
File all sides of the notch to an angle with a sharp edge
of no more than 3 mm on the upstream side of the
weir.
When the weir is in position in the stream
and has been well built into the banks and stream bottom, make sure
it is watertight. Pack the joints between the boards
with moss, clay or greasy cotton waste. Fill all the holes along
the bottom and sides of the weir by packing them with clay, sod
or turf.
Building a weir with other materials
You can also build a weir with sheet metal or corrugated roof
sheeting.
Sheet metal
The thickness and strength of the sheet metal you will have
to use will depend on the speed of the water flow and the size
of the stream.
When you cut a notch in sheet metal be careful that the edges
are straight and sharp. You might ask the local blacksmith to
help.
Note: if a weir made of sheet metal needs additional
bracing, leave the vertical stakes you have put in the stream
to mark the position of the weir, but be sure to remove those
stakes in front of the notch.
Corrugated roof sheeting
Corrugated sheeting is usually easy to find in large sheets
and much less expensive than sheet metal.
Corrugated sheets have the disadvantage of bending along the
corrugations. If the corrugations are placed across the stream
and the weir is well built into the banks and stream bottom,
the sheet will be quite strong.
A notch cut in a corrugated sheet will
be irregular and will give a less accurate result. To avoid this,
fit a section of wood into the centre of the sheet and cut the
notch in this as described earlier for a regular
wood weir.
Note: if a weir made of
corrugated sheeting needs additional bracing, leave the vertical
stakes you have put in the stream to mark the position of the
weir, but be sure to remove those stakes in front of the notch.
For accurate measurement it is essential that your weir:
While you are installing a weir or building a weir in place,
check this regularly.
Using a weir to determine water flow
A weir is used to determine water flow by measuring the
head, or the difference between the level of the crest
of the weir and the water level upstream from the weir.
The level of the water actually passing over the crest of the
weir will not be as high as the water level upstream because,
as water flows closer to the weir, the level begins to drop
before it flows over the crest.
To measure the head, or the constant upstream water level
equivalent at the weir, you will have to transfer a point equal
to the crest height at the weir to another point upstream where
the water level will be constant.
Find the upstream point by measuring a distance above the weir
that is a least 10 times the depth of the weir notch.
Preparing an upstream point to measure the
head when you have diverted the water flow
If you have diverted the water from the channel to build the
weir it will be easier to prepare this upstream point.
Drive a stake into the stream bottom
near the bank at the upstream point you have selected. Use a mason's
level and straight board and transfer the height of the weir crest
to the stake. Continue to drive the stake down until the top is
at the same height as the weir crest.
Now let the water flow back into the channel. Be sure to close
the diversion ditch so that when the level rises behind the weir
no water will be lost in channels or ditches or by infiltrating
or flooding (see p. 63). When the constant
upstream level has been reached, the top of the stake will be
under water.
Check to see that the weir is built properly and all requirements
have been met.
Find the head by placing a measuring stick, with the zero mark
at the bottom, on top of the stake and reading the depth figure
at the surface of the water.
Preparing an upstream point to measure the head when you have
not diverted the water flow
If you have not diverted
the water from the channel to build the weir, you will have to
prepare this upstream point while the water is in the channel.
Drive a stake into the stream bottom near the bank at the upstream
point you have selected.
The stake should be tall enough to remain above the surface
when the level of the water reaches its maximum height.
Hold a measuring stick, with the zero mark at the bottom, in
the weir notch. The length of the measuring stick should be
a bit longer than the notch is deep. Using a
mason's level and straight board, transfer the height of
the top of the measuring stick to the stake and mark it.
Remove the measuring stick from the notch, place it beside
the stake and tie the top of it to the stake, even with this
mark.
Find the head by reading the depth figure on the measuring
stick at the surface of the water.
We have seen previously that triangular weirs are generally
used for measuring small water flows while rectangular weirs
are used for measuring large water flows. For this reason, measure
the head in a triangular weir with a measuring stick graduated
in half -centimetres and the head in a rectangular weir with
a measuring stick graduated in centimetres.
Note: when you are measuring the head at the upstream
point, be careful not to disturb the water surface (by standing
in the water, for example), which may make the head reading
inaccurate.
Maintaining a weir
To insure accurate water flow estimates using a weir you must
maintain it regularly:
Clean the weir and remove floating debris caught in the
notch;
Remove any silt that builds up on the upstream side of the
weir;
Control the erosion of the stream bottom on the downstream
side of the weir;
Check the alignment of the weir, both vertical
(from the surface of the water) and perpendicular to the flow of the
water;
Check that the weir is watertight;
Check that the zero mark on the upstream measuring stick
is equal to the weir crest.
How to calculate water flows using a weir
Triangular weir
When you use a triangular weir, measure
the head value (to the nearest half -centimetre) on the upstream
measuring point. When you have found the head value, use Table
4 and find the water flow (in l/s).
Examples
Note: remember that triangular weirs are best suited
to measuring water flows of 114 l/s or smaller. When using Table 4,
all values higher than F = 114.08 l/s and H =37 cm will become less
and less accurate as H and F increase above these values.
TABLE 4 Water flow estimates using a triangular or V-notch weir
NOTE: The grey section is less accurate.
Rectangular weir
When you use a rectangular weir, measure
the head value (to the nearest centimetre) on the upstream measuring
point. When you have found the head value, use Table
5 and, in the column corresponding to the weir crest length,
find the water flow (in l/s).
Example
If you find in-between or odd-number
head values, you will have to approximate to find the correct
water flow value. Only even-number head values are shown in Table
5.
Example
If the crest length of your weir is greater than 30 cm
and is not listed in Table 5 (e.g., 40, 50, 70 and 80 cm), you
can calculate the water flow value by using the 10-cm
column, at the right of the table, and by following the
steps as in the next example below.
Find the head value in the right-hand column of the table.
Find both the water flow value shown in the column where
the crest length is smaller than the actual crest length of
the weir you are using and the water flow value in the 10-cm
column.
Calculate the number of additional 10-cm lengths the actual
weir crest has, compared with the smaller crest length you
have found in the table.
Now multiply the water flow value found in the 10-cm column
by this number and add the result to the water flow value
corresponding to the smaller crest length.
The result is the corrected water flow value for the actual
crest lengt.
Example
If you are measuring a water
flow that is 130 l/s or less,you can use Table
6. Do this by finding the head (in cm) on the left scale of
the table and follow this value horizontally across until you
reach the curve that represents the correct crest length. Bring
this point vertically down to the bottom scale and read there
the water flow (in l/s).
Example
TABLE 5
NOTE: The accuracy of the water flow values decreases
when head values are greater than one third of the crest length.
Water values in this table are divided into three sections:
white, darker and lighter grey. The values in the white section
are the most accurate. In the other two sections, the accuracy
decreases as the head increases toward a value equal to the
crest length.
1With full end contractions and sharp edges. 2Approximate water flow for each additional 10 cm
of weir crest (for crest lengths of 30 cm or longer and for
values in the unshaded upper part of the table only).
TABLE 6
1With full end contractions and sharp edges
3.7 Water flow through
a straight pipe
This is a method to estimate water flow through a relatively
short, straight pipe, from a higher level to a lower level,
and can be used, for example, when you fill or empty a pond.
To use this method you will have to find the head (in cm).
If water flowing from a higher level to a lower level flows
out of the pipe above the water line of the lower level,
you can find the head by measuring the vertical distance (C.L)
between the surface of the water above and the centre line
of the pipe below.
If water flowing from a higher level
to a lower level flows out of the pipe below the water
line of the lower level, you can find the head by measuring
the vertical distance between the surface of the water above
and the surface of the water below.
To
find the head, first prepare a constant point to
measure from. You can do this by using a
mason's level and a straight board, or a line level and
a string tied between two stakes.
Place a straight board on the top of the bank. Make sure
it is horizontal by using a mason's level. If the board is
not horizontal, prop it up with stones until it is. Find the
head by measuring downwards on both sides of the bank and
taking the difference between the two measurements.
Example
Another way to find the head is to drive a stake into the
bank on each side. Put the stakes in the water a little out
from the edge of the bank. Using a
line level, tie a string between the two stakes in a horizontal
position. Find the head by measuring downwards on both sides
of the bank and taking the difference between the two measurements.
Example
When you have found the head value, find the water flow using
Table 7 for pipes with an inside
diameter smaller than 9 cm, or Table
8 for pipes with an inside diameter larger than
9 cm. Do this by finding the head value (in cm) on
the vertical scale of the table and follow horizontally across
until you reach the curve that marks the size of pipe you
are using. Now look down to the bottom scale where you can
read off the water flow (in l/s).
Examples
TABLE 7
TABLE 8
3.8 Water flow through a
siphon
This is a method to estimate water flow through a relatively
short, curved tube called a siphon, from an upper level to a lower
level, and can be used, for example, when you fill or empty a
pond. As with the pipe method just described, to use this method
you will need to calculate the head (in cm).
How to make a siphon
A siphon can be made from a length of rubber or plastic tube
that is long enough and pliable enough to reach over the bank
from the upper water level to the lower water level.
How a siphon functions
A siphon will function only when there is a difference in
the two water levels and the end of the tube at the lower
level is below the end of the tube immersed in the water at
the upper level.
Measure the head,
the difference between the surface of the water on the upper
level and the surface of the water on the lower level, by using
a mason's level and
a straight board, or a line level and a string tied between
two stakes, as shown in the previous section.
Example
When you have found the head value, find the water flow using
Table 9 for siphons with an inside diameter
smaller than 9 cm, or Table 10 for siphons
with an inside diameter larger than 9 cm. Do this by finding
the head value (in cm) on the vertical scale of the table
and follow horizontally across until you reach the curve that
marks the correct size of siphon. Now look down to the bottom
scale where you can read off the water flow (in l/s).