Index Velocity
The water velocity measured by a hydroacoustic current meter in a portion of a river can be used as a surrogate or "index" for the mean-channel velocity. Mean-channel velocities computed using index velocity methods can be used to generate records of river streamflow.
The USGS uses fixed hydroacoustic current meters in various deployment configurations (side-looking, upward and downward-looking) for many different applications. Perhaps the most common application is at streamflow-gaging stations to compute streamflow using index velocity methods. The water velocity measured by a hydroacoustic current meter in a portion of a river can be used as a surrogate or "index" for the mean-channel velocity. Mean-channel velocities computed using index velocity methods can be used to generate records of river streamflow. Upward and/or downward-looking fixed deployments have been used to measure tidal current patterns in bays, estuaries, and harbors as well as wind-driven and density currents in lakes.
Real-time Index Velocity Examples
It can be beneficial to view real-time data from operational index velocity stations. The following links provide examples of USGS streamflow-gaging stations with real-time, online data produced from index velocity instrument and methods:
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Chicago River at Columbus Dr at Chicago, Illinois (05536123)
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Columbia River at the Dalles, Oregon (14105700) - long-term acoustic velocity meter (AVM) index velocity site
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Indiana Harbor Canal at East Chicago, Indiana (04092750) - Lake Michigan tributary with rapidly varying flow and frequent flow reversals
Index Velocity Resources
Instrument Page: Descriptions of the types of hydroacoustic current meters commonly used for index velocity applications.
Guidance: Published guidance reports, technical memorandums, and recommended procedures.
Software and Firmware: Information regarding software and firmware for commonly used acoustic Doppler velocity meters and acoustic profilers (side-looking, upward and downward-looking).
Deployments: A summary of mounts and deployments for side-looking and upward-looking applications.
Tech Tips: Quick reference sheets and other tips for deploying index velocity meters and computing discharge using the index velocity method.