Discharge Calculation of Natural Channel Flows with AFM Data

The AFM (acoustic flow meter) system outputs an index value of the flow regime at the installation site. This index value is referred to as the line velocity, which is an integrated value of flow velocities along the acoustic path. To calculate channel discharges based on the measured line velocities, a site-specific relationship between them must be specified. The process of establishing the relationship is known as the system calibration of the AFM installation. The FVD (flow velocity distribution) models are used to derive the site-specific equations for calculating channel discharges with the AFM outputted line velocities and channel stage values as well as the cross-section geometry data. To facilitate field applications, the discharge calculating equation for a specific AFM installation can be further expressed in simplified form as a function of observed values of line velocities and channel stages. Applications of this procedure at selected AFM(AFFRA) installation sites of the Water Survey of Canada (WSC) indicated that the resulting channel discharge values were consistent with those generated with the conventional current meter measurements. In particular, this procedure worked well for flows under ice cover. Results of case studies for an AFM site on the Red River in two successive winters are presented.