Estimation of Water Flow Rate Q Using Average Cross-section

Dec 16
2009
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Equation of interest: Area x Average speed x 80% Friction Factor = Q,  
the estimated average stream flow rate.

 

By measuring the rate of travel for a floating object traveling down the main flow of a stream and then multiplying by the average cross-sectional area. One can determine the average volume flow rate or Q directly. Please note that this method is only an estimation, and will have inaccuracies due to anomalies in the channel and issues surrounding the float chosen, etc. The main difficulty in carrying out this measurement has to do with the care and problems in accurate measurement of the streams cross-sectional profile between the points B-B’.

 

Procedure: 

  1. Pick a fairly regular part of the stream with about the same cross section and curvature for a 100 foot distance.
  2. Measure a 50 to 100 foot section or race course of your stream bed. The length between point A and B. will be used to measure the velocity of the float.
  3. Select a float that will be somewhat neutrally buoyant, such as an orange. Plus it’s biodegradable :-)
  4. The goal is to have it float just at or under the surface down through the race course between point A and B.
  5. Use a stopwatch to time, several runs, tossing your float in upstream from section A while starting the watch as the float crosses section A and stopping the watch just as the float crosses section B.  Repeat this sequence 5 or 10 times and average the measured times. The average is obtained by adding the times up and dividing by the number of times that you measured the elapsed time. Throw out any times that are grossly apart from each other.
  6. Now measure the cross sectional area of the creek by measuring the distance from the surface to the bottom of the creek (Use a level reference line see diagram in this post.) Each distance must be taken using the same horizontal interval, say 1 foot. Now add up the depth measurements and divide by the number of measurements. This is your average cross sectional depth. Multiply by the interval width and you have average area.
  7. Multiply average stream velocity x average cross section area x friction correction factor of 0.8. Due to friction, bottom irregularities, etc. this is the least accurate measurement. It is likely only about 15-20% accurate at best. Concrete channels are best and rough streambeds the worst cases for using this method. Still, it will be better at stream flow estimation than a rough estimate or wild guess.

Diagram: Stream Flow Measurement Using a Float, Stopwatch and average cross sectional area estimate.

Stream Flow Estimation By Direct Measurement of Speed x Cross section Area

Stream Flow Estimation By Direct Measurement of Speed x Cross section Area

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
For more on this method visit this US EPA Water Flow Rates document.
Written by Jess in: Hydropower planning, Small & Micro Hydro | Tags: , ,

Measuring the flow rate Q from an open pipe

Oct 02
2009
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How to Measure your Water Flow Rate Q from an Open Pipe:

There are times when it’s necessary to estimate the flow rate from a stream constrained to flow in a water jet flowing from an open pipe. This flow measurement method doesn’t require us to have precision fluid flow measurement instrumentation, other than a straight edge and plumb bob. Flow meters or weirs would likely be more accurate, but sometimes all you have is a filled pipe with a jet of water streaming out. If that’s the case, then try this method.

Pipe Flow Direct Q Measurement Diagram A

Pipe Flow Q Direct Measure Diagram A

There are two basic pipe flow measurement methods used to make this estimation. The first pipe flow measurement method involves pipes that are completely filled; that is they have no air in  the water pipe above the fluid exiting the pipe, the other method used is a partially filled pipe. The completely filled pipe flow case will be fairly easy to measure utilizing the table enclosed below. The other method we will cover in another SmallHydro.com blog post.

Meanwhile let’s examine how to measure the pipe flow rate Q from a water jet squirting from a full pipe:

What is needed to measure the completely filled pipe flow case is a simple straight edge ruler and a plumb bob marked off at an appropriate pre defined distance, in this case we use 13 inches. To utilize this method one simply measures out an appropriate extra distance parallel to the initial pipe exit flow direction up to an intercept point with the plumb bob line that is 13 inches below that point. See  flow measurement diagrams A, B & C.

You will move the ruler parallel to the pipeline until the plumb bob just touches the outer edge of the falling stream of water. By measuring this intercept distance X. for the given 13 inch plumb bob Y. value, you then simply use the table below with your pipe diameter and X value to look up the flow in gallons per minute or GPM.  You can convert these values to your desired standard units of measurement as well.

This water flow estimation technique is used in agriculture to measure irrigation flow from an open pipe. This method can also be used for smaller flows,  if a stream can be constrained temporarily to completely fill the inside of a pipe and flow out the end above the ground or stream tail water enough to not interfere with the plumb bob.

Keep in mind that if the pipe is small enough, direct measurement can be made by filling the bucket and timing the amount of time it takes to fill it, we’ll discuss this in another Small Hydro power stream flow measurement post and link to it later.

So, Let’s take a look at the measurement technique, see pipe flow measurement procedures, and diagrams A., B. and C. :

Tools needed to measure your hydropower system pipe flow rate Q:

  1. Straight edge ruler marked in inches
  2. Plumb bob or thinweight and string at least 2 feet long
  3. A pipe completely filled with the water flow jet about 18-24 inchs up in the air above any interference.

 

Procedure to measure potential hydropower system flow Q from a full flowing pipe:

  1. Tie plumb bob so it dangles 13 inches below the bottom of staight edge ruler
  2. Place straight edge ruler along top edge of pipe parallel to exit flow
  3. Shift ruler until plumb bob  just touches outer surface of the flow at 13 inches below the attachment to ruler.
  4. Measure X distance betwee pipe tip and bob string attachmet point.
  5. Measure inside diameter of pipe
  6. Use ‘X’ and Diameter with edges of table to read off flow rate in GallonsPer Minute (GPM)

 

Note: Keep ruler parallel to pipe at all times.

Pipe Flow Q Direct Measure Diagrams Full & Partial flow D-E

Pipe Flow Q Direct Measure Diagrams Full & Partial flow D-E

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Some more views of the same measurement, it works the same regardless of pipe angle. Of course bigger errors can happen at steeper angles.

Pipe-Flow-Q-Direct-Measure-Diagrams-B-C

Pipe Flow Q Direct Measure Diagrams B & C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The following table can be used to look up the corresponding flow rate in gallons per minute or GPM when given your measured X. value.  See the reference equation given below for how this was computed. Also see the OSU irrigation and hydrology site for more information on these irrigation and other water flow measurement methods.  Click the water flow table picture below to get a closer view of the table or to print it  out.

 

Equation for computing Full Pipe Flow Rate in GPM

Equation for Full Pipe Flow Rate in GPM

Note  Disclaimer

- This is a pipe flow estimation table calculated for 13 inches of  ‘Y’ fall. For other ‘Y’ distances use the formula given.  Accuracy is not guaranteed, so check your work to make sure the results are adequate for your application.

 

 

Pipe Flow Direct Measure Flow - Q Chart

Pipe Flow Direct Measure Flow - Q Chart

Written by Jess in: Hydro civil works, Hydropower planning, Small & Micro Hydro | Tags: , , , ,

Open Pipe Flow Chart – Full Flow Calculation Table

Oct 01
2009
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By popular demand, this a larger copy of the flow table mentioned in the Open Pipe Flow Measurement Method post.

Written by Jess in: Hydropower planning, Small & Micro Hydro | Tags: , , ,

Stream Flow Measurement – Conductivity meter – suggestions.

Aug 27
2009
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Q: Can you recommend a Conductivity Meter for Measuring Stream Flow?

— Original Question —
Hydro Turbine - Renewable Energy from Water

Hydro Turbine - Renewable Energy from Water

Subject: Conductivity meter
From: ‘David’
Date: Mon, July 20, 2009 1:39 am
To: <smallhydroblog@smallhydro.com>

Dear Dorado Vista,
Q: Can you recommend a portable conductivity meter that can assist in the measuring of flow in streams using the salt gulp method.  Is there a limit to the maximum flow that can be assessed using this method.  I’m interested in flows that range between 50 l/s to 1000 l/s.

Thanks,
David

— A: SmallHydro’s Conductivity Measurement Answer —


From: jess.blog@smallhydro.com
Sent: 21 July 2009 17:34
To: David
Cc: smallhydroblog@smallhydro.com
Subject: RE: Conductivity meter – suggestions.

David,

Take a look at SpecMeters.com for EC or dissolved salt meters.

That particular page has several EC meters that would work. For DoradoVista ranch work we use the “Field Scout” it is sturdy, easy to use and accurate.  It will measure both Soils & Water too. There are some more Water specific units and even GPS based ones there too. You are likely just going to need the cheapest water unit you can find.

The dissolved salt pulse method likely has no physical limit, but the practical limit will be defined by your instruments sensitivity and the background of dissolved solids (Electrical Conductivity EC variation), the volume of salt should be just enough to see the distinctive EC increase/decrease pulse.

You will need to use fine ground salt to make sure it dissolves quickly into your brine solution. You also should be careful not to use too much salt as it will affect the biosphere if it is left to concentrate in stagnant pools.  Too much salt can take a much longer time to clear out between readings too. Maybe you could try pre dissolving? Most streams carry some salts in them and as such you will need to measure the background Ec and variation as a baseline reference.

Another thing to watch for with this method will be turbulent eddies or swirls in the water streamlines as this will affect the accuracy of your readings. That means you should take several smaller pulses at several different measurement cross-sectional stream-flow positions. This averaging of multiple measurements will make sure your results are free of too much position dependant time measurement error.

Watch for boulders and channel anomalies that will cause these eddies. You will typically want the opposite of what Gold panners want, that is you want swift deep water. Therefore swift deep water outside bends and away from eddies caused by mid channel boulders.  Longer measurement distance will cut down on these errors as long as you can clearly measure the distinct EC reading changes.

Would you be Ok with this thread in a post or question later?

Sincerely,
Jess
DoradoVista, Inc.

—

Jess,

Thanks for replying, I will have a look at the link you have sent.
Also more than happy for you to post this thread.

Thanks again,
David

Written by Jess in: Hydropower planning, Small & Micro Hydro | Tags: , ,
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