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:
- Place rod in the water with sharp edge upstream. Measure stream depth on scale.
- 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).
- 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.
- Find the stream velocity in feet per second from the table provided on the survey data sheet.
- 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
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.