Streamgaging

Methods used

Acoustic Dopler Velocity Meter (ADVM) method

The stream channel cross section is divided into vertical subsections, and an ADVM is used to track the movement of particles suspended in the water. The ADVM uses sound pulses to measure the velocity of particles at a specific point in the stream subsection column. Subsection velocities are multiplied by corresponding subsection areas and the values summed to compute total discharge for the cross section. 

Acoustic Doppler Current Profiler (ADCP) method

ADCPs are larger hydroacoustic instruments set in floating pontoons and moved across the measuring section of the river. They use sound pulses to measure velocity of moving particles in a column of water directly below the ADCP using the principles of the Doppler Effect. By measuring the frequency shift of sound reflecting off particles in the water, the ADCP creates a highly accurate, continuous profile of both water speed and direction across the entire channel.

Index-Velocity method

Measuring streamflow velocity for use in computing continuous-record discharge is often done during challenging conditions and weather environments. Until the advent of ADVMs, it was difficult and more labor intensive 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, within the predetermined range of the total cross section is used in conjunction with mean velocity from discharge measurements of total streamflow to develop an equation that related the partial velocity measurement to the mean velocity in the cross section. This multi-step method is described as the index-velocity method. This method is often used to compute continuous records of streamflow in reaches with difficult and non-standard conditions like variable backwater or storage. The index-velocity method uses two ratings (or relationships) to compute discharge, 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 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. 

Image Velocimetry method

Image Velocimetry is an experimental non-contact method for measuring streamflow. Cameras record the movement of features on the water surface, and Space-Time Image Velocimetry (STIV) techniques, which utilize image analysis to capture and analyze the movement of surface features over time; used to calculate water velocity from the imagery. River discharge is then computed by combining the measured velocity with the river's cross-sectional area obtained from surveys or water-level-based channel measurements. At the New Jersey Water Science Center, videos are collected using fixed cameras or uncrewed aircraft systems and paired with field‑surveyed or established stage‑area data to produce accurate flow measurements. You can find more details about image velocimetry here: Discharge Estimation Using Video Recordings from Small Unoccupied Aircraft Systems | Journal of Hydrology