Index-Velocity Rating Development
Long term or continuous discharge measurements in large streams/channels have historically been virtually impossible to perform with any acceptable degree of accuracy. With the introduction of Acoustic Doppler Current Profilers and the use of the index-velocity method, high accuracy continuous discharge measurements are now possible. C.C. Lynch & Associates, Inc. personnel have years of experience collecting stream discharge measurements, developing stage-discharge rating curves, and developing index-velocity ratings. Contact us today to discuss how we can assist you on with your stream flow measurement needs.
What is an Index-Velocity Rating?
You may be familiar with a stage-discharge rating which simply correlates water level to a known discharge. In many situations, the stream hydraulics do not allow for this basic correlation and therefore an index-velocity rating is used. An index-velocity rating is accomplished by identifying a correlation between the average water velocity in a specific area within the cross section as measured by an index-velocity meter and the average velocity in the entire cross section as measured by a moving boat ADCP. This correlation establishes an offset that is applied to the measured index velocity so that when the discharge is calculated by the equation Q = Area X Velocity, the calculated discharge measured by the index-velocity meter closely matches the measured discharge of the moving boat ADCP.
What Steps Are Involved in Developing and Index Rating?
- Identify a suitable location which includes reviewing historical and seasonal discharge/water level information.
- Install index-velocity meter.
- Survey channel cross section.
- Collect discharge measurements with moving boat ADCP while simultaneously collecting velocity measurements with index-velocity meter. Discharge measurements should be collected over expected range in water levels and velocities.
- Evaluate data using a regression analysis.
- Repeat discharge measurements and data evaluation to include all ranges of expected water levels and velocities or as frequent as the channel bed may change shape.