3. ESTIMATES OF WATER FLOW

3.0 Introduction

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.

Note: to find the time it will take to fill or empty a pond using a straight pipe or a siphon, see Sections 3.7 and 3.8.

TABLE 3

Section

Method

Water flow

Accuracy

Remarks

Equipment

3.1 * Quick and rough Small
An approximation
For a quick estimate
None
3.2** Bucket Very small

Very high
Most accurate of all methods
Dam, pipe, buckets,
1 -l bottle, watch
3.3** Float Small to large

Medium

Best for streams with calm water
Float, stakes, line, measuring stick, watch
3.4** Float and cross section

Low to medium

Float, stakes, line, measuring stick, record sheet, watch
3.5** Dye, stain and cross section
Dye, stakes, line, measuring stick, record sheet, watch
3.6*** Weir,
triangular 		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.

 


Where to measure

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 position and on a line across the stream perpendicular 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;
  • At the centre point, draw a right angle line downwards, equal in length to the depth of the notch;
  • 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.

Check to see that the weir is built properly and all requirements have been met, either for the triangular weir or for the rectangular weir.

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).








Example


TABLE 9



TABLE 10