Discharge Measurements

The size of a waterway and its flow rate affect its water quality. For example, discharges containing contaminants will have less effect on large swiftly flowing rivers than on small slow streams. This is one reason for measuring flow - to work out the load of contaminants and sediment the waterway is carrying. Because discharge can have a significant effect on water quality, it is important that it is recorded at the time of sampling and, if possible, during the previous few days. It is particularly valuable to know if flows are at low, moderate or high level and if the level is rising or falling. This is because the concentrations of nutrients, turbidity and contaminants tend to be higher when the stream level is rising than when it is falling. There are three ways to measure discharge. A simple method is to see how fast a floating object travels downstream over a chosen distance. This is called the float method. Secondly, flow data can be obtained from US Geological Service or the State Emergency Management Office, if your site is near a gauging station. Thirdly, the velocity head rod (VHR) method can be used. If your group has a flow-meter you should use the method recommended by USGS.

The velocity head rod (VHR) is a fast and inexpensive method of measuring velocity in a stream. The rod can be made using a 3-6 ft long, two-inch wide, very thin, piece of wood. Mark a scale in ½ inch increments on the rod, starting with zero at the bottom of the rod and stopping at 18 inches. The easiest VHR is an aluminum construction straight-edge from your local hardware store or online. It must be at least two-inches wide and 24-36 inches long. Note: A 26-gauge copper sheet or similar should be fastened to the bottom to provide more stability and reduce sinking into the sediment. Below are the steps to determine stream flow using the VHR:

1. Place rod in the water with sharp edge upstream. Measure stream depth on scale.
2. Place rod sideways in the water. This will create turbulence and the water will "jump" or rise above its normal depth; velocity is proportional to this jump. In most cases you cannot accurately measure a rise of < 0.25 inch (1/4 inch).
3. Measure depth of turbulent water next to rod. Subtract stream depth from the turbulent depth reading to obtain the "jump height," or velocity head in inches.
4. Find the stream velocity in feet per second from the table provided on the survey data sheet.
5. Determine the stream velocity at intervals across the stream and average them to obtain the average stream velocity in feet per second. The formula is: V = 8 x √R; where R is the rise in feet and V is velocity.

Cross sectional shape varies with position in the stream, and discharge. The deepest part of channel occurs where the stream velocity is the highest. Both width and depth increase downstream because discharge increases downstream. As discharge increases the cross sectional shape will change, with the stream becoming deeper and wider. The measurement of the cross section is necessary to determine the total discharge, which is the volume and velocity of the water. Measuring the width and depth of the waterway, and multiplying these measurements together determine the cross section. The depth will vary across the stream and so the width and depth should be measured in small intervals and aggregated to determine the total area. ​The measurement of discharge can be an average or a sum total. If you choose an average it should be based on a minimum of five measurements across the stream. If a sum total approach is preferred follow the USGS recommendations as closely as possible.

Resources

• Velocity Head Rod

  The construction and use of the velocity head rod for measuring stream velocity and flow.

• Aluminum Straight Edge

  Fastenal shopping link for thermally bonded 36-inch aluminum straight edge.

• VHR Base

  Example of a velocity head rod base to provide more stability.

• USGS Sum Total Approach

  USGS methods for measuring stream flow.