Flow measurements tell us the amount of water that is flowing past a specific point in a stream at a given time. Irrigation withdrawals, precipitation, season, stream shape and local groundwater conditions are some ways flow vary. Flow is typically recorded in cubic feet per second (cfs) or cubic meters/second (cms). Flow measurements are performed for natural (rivers, streams) and engineered (outfalls, ditches, canals) surface water bodies.
Stream flow measurements for our purposes of assessment are to determine pollutant loads, characterize water quality conditions and the effect of hydrologic conditions on aquatic life.
Flow is often measured while wading, however when unsafe, it may be done from bridges, especially in deep or high-flow areas. While the tools used to measure water flow vary, the general principle remains the same. Familiarity with the different procedures associated with the use of a wading rod (e.g. Marsh-McBirney or MagnaRod) or flow probe (e.g. Global Water Flow Probe) will lend itself to other methods. Go here for PDF quick instructions.
To determine flow in a stream you must know the stream’s average velocity (feet per second) and the cross sectional area (square feet). These are multiplied, which gives you flow.
(velocity*width*depth) or (ft/sec * ft * ft = ft3/sec or CFS).
Because velocity and depth vary across the stream, you will take a series of measurements across the stream.
The number of measurements to make depends on the size of the stream.
- Typically, your measurements should be at least 0.3 feet apart.
- The subsection of stream between each measurement should have 10% or less of the total discharge.
Velocity and depth vary across the stream, so we will be taking a series of measurements depending on the size of the stream (see "conducting a measurement"). Measurements will be at least 0.3 feet apart.
Average velocity in a wadeable stream is located at 0.6 times the total depth at any site, measured from the top (see figure below).
- Wading rods will automatically put your velocity meter at this depth.
- The Globe velocity meter automatically calculates an average velocity.
Calculate the area associated with each velocity measurement by using the mid-point method (see figure).
To calculate total flow, just add up all the flows for the smaller cross sectional areas (box).
Total Flow (combination of each box) = (W1*SD1*SV1) + (W2*SD2*SV2) + (W3*SD3*SV3) +...
Where W = width of the box; SD = Stream depth; SV = Stream velocity; 1,2,3 are each location/box along the transect.
We provide a spreadsheet that will do this for you, but it’s always good to check your calculations to make sure they make sense.
Choosing a Site:
Establish a stream cross section for flow measurement to occur. Desirable characteristics for the site location include:
- A straight section of stream, away from stream bends;
- Stream flow approximately parallel to stream banks;
- A constant stream gradient;
- No obstacles protruding from water surface (i.e. stones, plants, bridge piers).
When establishing the cross section, look for an area of laminar, smooth flow with minimal obstructions. Obstructions, including large rocks, can be moved out of the way of the cross section, but only before flow measurements begin, never during the measurement. If wading, depth should not exceed 3 feet.
Conducting a measurement:
- Stretch a tape measure across the stream and attach it on both ends. Make sure it
is perpendicular to the flow and it is tightly stretched.
- Measurements are taken along the tape measure the interval given below:
- If stream width < 10 feet, collect data every 0.5 feet; and take first reading at 0.25 feet (half of interval) from edge.
- If stream width > 10 feet, collect 20 evenly divided measurements across the entire stream; take the first reading at half of the determined interval from edge
Collecting flow data works best with a team of two people. One person handles the flow meter while the other records the stream width, depths, and velocities.
- Make sure the flow meter is reading flow in feet per second (ft/s) and the allotted time for reading flow is set at twenty seconds.
- The sensor must be facing upstream and both members of the team must take care to stand to the side of the flow meter, to avoid impacting the measurement.
- The operator will measure the actual water depth using the wading rod and call it out to the recorder who will record it on the field sheet.
- The wading rod is then adjusted to the correct depth – putting the flow meter at 60% of the water depth, measured from the surface.
- Begin measuring velocity once the depth is set. After 20 seconds, the operator will call out the average velocity to the recorder and then continue to the next site.
Calculating the flow:
Depth and velocity values need to be converted into a flow measurement by using the Excel template found below, or using the following calculation.
Remember, flow is (velocity*width*depth) or (ft/sec * ft * ft = ft3/sec or CFS)
Total Flow is a combination each measurement across the stream cross-section.
(Combination of each box) = (W1*SD1*SV1) + (W2*SD2*SV2) + (W3*SD3*SV3) +...
Depth and velocity values can be converted into a flow measurement using this Excel template.
Find Utah DWQ Stream Flow SOP here.
Simple Instructions for using the MagnaRod