Measuring Streamflow Q. Using a Rectangular Slotted Weir

Dec 21
2009
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One method to accurately measure flow Q for small and medium-size streams is through the use of a slotted Weir.

Methods for Measuring Your Stream Flow…

Stream Flow x Fall = Hydro Power

Stream Flow x Head Pressure = Power

Stream levels will change through the seasons, so it is important to measure FLOW at various times of the year. We will need these varied flow measures to create an FDC or flow Duration curve, more on the FDC in a later post. If this seasonal variable flow measure is not possible, attempt to determine various annual flows by discussing the stream with a neighbor, or finding US 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. Especially during low water seasons, avoid using all the water for your hydro system. FLOW is typically expressed as volume per second or minute. This is also called a “FLOW rate” since it is a dynamic volume per time interval. Common examples of volume units are gallons or liters per second (or minute), and cubic feet or cubic meters per second (or minute):

A rectangular slotted Weir consists of a temporary dam structure with a rectangular slot are opening gate.

This slotted Weir gate has the following characteristics;

  1. All stream flow to be measured, Q. is constrained to go through the slotted gate.
  2. The bottom of the rectangular slotted Weir gate is leveled horizontally.
  3. A reference stake or pole is driven into the stream bed below the water line. So that it is exactly level with bottom of the Weir gate.
  4. The stake must be placed upstream at least four times the distance of the maximum Weir gate water depth.
  5. Water must be allowed to exit the Weir gate freely, such that there is an air gap beneath it as it flows over the Weir. A “sharp” 90 degree edge lip helps here.
  6. Water upstream of the Weir must move freely and not have major disturbances.
  7. Water will contract or shrink in width x depth, as it increases speed, when it approaches and flows through the opening.

     

Given both the width and depth of the water flowing over the Weir; it is a simple procedure to look up the value for the water flow using a Weir table.


Measure Stream Flow Q using a Rectangular Weir (contracted) Measure Stream Flow Q using a Rectangular Weir (contracted)

The following table is based on a reference Weir gate 1 inch wide.

An example of use is as follows:

Assume your Weir gate is 1 foot wide or 12 inches, you measure the water passing over it at 6 1/4 inches.

Using the table, you look up 6+1/4 and read 6.2 5 CFM per inch of width.

Multiply 6.25 CFM/in x 12 in = 75 CFM. That’s a pretty decent flow, if you have enough head you may be in business.

FYI – Metric Formula for a rectangular notched Weir is: Q = 2/3 x Cd x , 2g^1/2 x (L – 0.2h) x h^3/2, Where Cd is the coefficient of discharge.

Take Cd = 0.6 (normal case) then Q = 1.8 x (L – 0.2h) x h^3/2 in liters/sec

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Inches
  +0/8 +1/8 +1/4 +3/8 +1/2 +5/8 +3/4 +7/8
0 0.00 0.01 0.05 0.09 0.14 0.19 0.26 0.32
1 0.40 0.47 0.55 0.64 0.73 0.82 0.92 1.02
2 1.13 1.23 1.35 1.46 1.58 1.70 1.82 1.95
3 2.07 2.21 2.34 2.48 2.61 2.76 2.90 3.05
4 3.20 3.35 3.50 3.66 3.81 3.97 4.14 4.30
5 4.47 4.64 4.81 4.98 5.15 5.33 5.51 5.69
6 5.87 6.06 6.25 6.44 6.62 6.82 7.01 7.21
7 7.40 7.60 7.80 8.01 8.21 8.42 8.63 8.83
8 9.05 9.26 9.47 9.69 9.91 10.13 10.35 10.57
9 10.80 11.02 11.25 11.48 11.71 11.94 12.17 12.41
10 12.64 12.88 13.12 13.36 13.6 13.85 14.09 14.34
11 14.59 14.84 15.09 15.34 15.59 15.85 16.11 16.36
12 16.62 16.88 17.15 17.41 17.67 17.94 18.21 18.47
13 18.74 19.01 19.29 19.56 19.84 20.11 20.39 20.67
14 20.95 21.23 21.51 21.80 22.08 22.37 22.65 22.94
15 23.23 23.52 23.82 24.11 24.40 24.70 25.00 25.30
16 25.60 25.90 26.20 26.50 26.80 27.11 27.42 27.72
17 28.03 28.34 28.65 28.97 29.28 29.59 29.91 30.22
18 30.54 30.86 31.18 31.50 31.82 32.15 32.47 32.80
19 33.12 33.45 33.78 34.11 34.44 34.77 35.10 35.44
20 35.77 36.11 36.45 36.78 37.12 37.46 37.80 38.15

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 FLOW at various points in time. Design Flow Even though your Flow may be very high after exceptionally rainy periods, it probably won’t be cost effective to design your turbine system to handle all that water for just a few days of the year. Instead, it makes sense to build a system that uses Flow you can count on for much of the year. This is called Design Flow, and it is the maximum Flow your hydro system is designed to accommodate. Design Flow, along with Net Head, determines everything about your hydro system, from pipeline size to power output.  For more on measuring stream flow you may want to visit Canyon Hydro – Measuring Flow, or British Hydro – Flow Measuring.

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

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: , ,

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