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Index -Velocity and Hydroacoustics

Many streams do not have a direct streamflow-to-gage height (water-surface elevation referenced to a datum) relation. These sites include streams influenced by control structures, general backwater effects, and tidal effects. Historically, computation of streamflow at these sites has been difficult. Deploying stationary acoustic doppler instruments allows the USGS to “index” the mean channel veloc

Index-Velocity Method

Flood Measurement in New Jersey
USGS hydrographer Bob Atkinson preparing an Acoustic Doppler Velocity Meter (ADVM) to make flood measurements at USGS 01399100 Middle Brook at Burnt Mills NJ. 

With ADVMs, measuring and computing continuous records of streamflow can be conducted in challenging environments. Until the advent of ADVMs, it was difficult or impractical to accurately compute streamflow records on tidally affected rivers or rivers affected by backwater. ADVMs can be attached to a fixed structure, such as a pile or bridge pier, and used to measure velocity for a portion of a channel cross section. The measured velocity at a fixed point in the cross section can be used to compute continuous streamflow records. When a site is subject to periods of vertically stratified  directional flow, an ADVM can be mounted at the bottom of the streambed in a vertically oriented or “up-looking” position, and velocities can be measured at multiple points or “bins” throughout the water column.

Measured velocity and stage or water-surface elevation data are used with the index-velocity method to compute continuous records of streamflow. In the index-velocity method, two ratings (or relationships) are developed and maintained—a stage-area rating and an index-velocity rating. The stage-area rating is developed by surveying a stable cross section in the stream near the permanently mounted ADVM. The channel area for a given stage then can be determined from the surveyed cross section. An index-velocity rating is developed by using the relation between the measured mean cross-sectional velocity at the surveyed cross section and the simultaneous index velocity measured with the permanently mounted ADVM. Continuous records of stage and index velocity are converted to a channel area and mean velocity using the respective ratings. The channel area then can be multiplied by the mean velocity over time to compute a continuous record of streamflow.