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    Flow Rate Research Methodology


The information provided in this section of the Pathfinder Science Stream Monitoring Project is based on the Missouri Volunteer Water Quality Monitoring Program, a partnership project between the Missouri Department of Conservation (MDC) and Natural Resources (DNR), the Conservation Federation of Missouri (CFM) and the Citizens of Missouri.

Instructions for Calculation of Stream Discharge

Select a stream section that is relatively straight, free of large objects such as logs or boulders, with a noticeable current, and with a depth as uniform as possible. Stretch the tape measure across the stream. The "0" point should be anchored at the wetted edge of the stream. The end of the tape measure should be anchored at the opposite end so that it is taut and even with the other wetted edge.

Step 1. Determine the stream cross sectional area. The first step in determining cross sectional area is to measure and calculate the average stream depth. In the table below, record depth measurements at one foot intervals along the tape measure you have stretched across the stream. The depth must be measured in tenths of a foot (ie 1.7 feet equals one foot and seven tenths). DO NOT MEASURE DEPTH IN INCHES.

Interval Number Depth in Feet Interval Number Depth in Feet Interval Number Depth in Feet
1   11   21  
2   12   22  
3   13   23  
4   14   24  
5   15   25  
6   16   26  
7   17   27  
8   18   28  
9   19   29  
10   20   30  
Sum   Sum   Sum  


The average depth is calculated by dividing the sum of depth measurements by the number of intervals at which measurements were taken.

Sum of Depths (Feet)÷Number of Intervals=Average Depth (Feet)


The final step in calculating the cross sectional area is to multiply the average depth (in feet) by the stream width (in feet) at the point where the tape measure is stretched across the stream.

Average Depth (Feet)xStream Width (Feet)= Cross Sectional Area (Feet)2

Step 2. Determine the average velocity for the stream.

For a stream less than ten feet in width, select three points in the stream approximately equal distances apart for velocity measurements. For streams greater than ten feet in width, no fewer than four velocity measurements should be taken at approximately equal distances across the stream. For example, if the stream were eight feet wide, then velocity measurements would be taken at approximately two foot intervals across the stream in order to derive three measurements. If the stream were sixteen feet across, then the velocity measurements would be taken at approximately three foot intervals across the stream in order to derive four measurements. This method of measuring the stream velocity will insure that velocity measurements are recorded for the slow and fast portions of the stream.

Once you have determined the number of velocity float trials you need to complete, measure the water's surface velocity in the following manner: Select two points located equal distance upstream and downstream from the tape measure you have stretched across the stream. Determine the distance between these two points and record this value (in feet) in the distance box below. Count the number of seconds it takes a neutrally buoyant object, such as an orange or an apple, to float this distance. Record this time (in seconds) in the table below for each float trial you complete.

Velocity Float Trials
Trial NumberTime (seconds)
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
Sum 

Distance Floated (Feet)             

The next step in calculating the surface velocity is to determine the average float time. Average float time is equal to the sum of float times (in seconds) divided by the number of float trials.

Sum of Float Times
(seconds)
÷Number of TrialsAverage Float Time
(seconds)

The final step is to divide the distance floated (from the box above) by the average float time.
Distance Floated
(Feet)
÷ Average Float Time
(seconds)
Average Surface
Velocity (Feet per Second)

Water in a stream does not all travel at the same speed. Water near the bottom travels slower than water at the surface because of friction (or drag) on the stream bottom. When calculating stream discharge the water's velocity for the entire depth (surface to bottom) needs to be determined. Therefore, you must multiply the average surface velocity (from above) by a corretion factor to make it represent the water velocity of the entire stream depth.

Choose the correction factor that best describes the bottom of your stream in the area where of the stream that you measured the surface velocity.

Correction value based on Stream Bottom Type:

    Rough, loose rocks or coarse grave - use a correction value of 0.8
    Smooth, mud, sand or hard pan rock - use a correction value of 0.9

Multiply the correction factor by the average surface velocity to calculate the corrected average stream velocity.
Correction ValuexAverage Surface Velocity
(Feet per Second)
=Corrected Average
Stream Velocity
(Feet per Second)


3. Calculate the Stream Discharge.

Multiply the cross sectional area (feet)2 from Step 1 by the corrected average stream velocity (feet/second) from Step 2.

Cross Sectional
Area
x Corrected Average
Stream Velocity
(Feet per Second)
= Stream Discharge
(Feet)3per Second or
Cubic Feet per Second
(CFS)

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