Flow volume calculations based on three-dimensional current and net orientation data

Abstract

We use three-dimensional current and spatial orientation data from an acoustic Doppler current profiler (ADCP) to analyse volume flow estimates for a series of oblique net tows. Net volumes are calculated for both upcast and downcast tows of variable duration between the surface and 2200 m depth. Standard deviations for volume calculations are less than 3% of the total volume in all nets. The effects on volume bias of ignoring pitch, vertical flow speed and flow angle relative to the net face are discussed. Results indicate that exclusion of the vertical component of flow in the volume calculations produces an underestimate of the net volume for downcast tows and an overestimate of the net volume for upcast tows. For our towing configurations, the magnitude of the volume bias ranges from 5 to 45% of the total volume. We find that the volume bias for a given net can be accurately predicted (within 6%) from the estimated average vertical velocity derived from the depth range and duration of a low, provided that both accurate horizontal flow (flowmeter) and net pitch data are available. Flowmeter data alone are not sufficient for estimating bias in volume flow calculations.