In situ acoustic Doppler equipment mounted to USGS streamgaging statio
Acoustic Doppler technology also has been developed for in situ deployments. 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, aquatic plant growth, general backwater effects, and tidal effects. Historically, computation of streamflow at these sites has been difficult. Deploying in situ acoustic Doppler instruments allows the USGS to “index” the mean channel velocity. At these “indexvelocity” sites, a mean channel velocity is computed by dividing the measured streamflow by the cross-sectional area (from a standardized area rating). The computed mean velocity then is compared to the velocity reported by the in situ velocity meter. Multiple measurements can be used to produce an index-velocity rating.