Evaluation of a triple-axis coherent Doppler velocity profiler for measuring near-bed turbulent flow: a field study

Collocated detailed measurements of near-bed turbulent and intrawave flow are important for studying sediment transport processes and seabed evolution. Existing commercially available triple-axis profiling instruments do not provide collocated velocity measurements. To improve the capability to make such measurements a triple-axis coherent Doppler velocity profiler (CDVP) has been developed and tested in the marine environment. The instrument was designed to measure orthogonal velocity profiles within a narrow column of water at 16 Hz within 1 m of the bed with a vertical spatial resolution of 0.05 m. This paper describes the first deployment of the instrument, in a tidal inlet in Portugal during a multidisciplinary study, when CDVP flow velocity measurements were compared with data from other instrumentation. A pragmatic approach was adopted to assess the capability of the triple-axis CDVP, using both an evaluation of internal consistency and an assessment against two commercially available acoustic Doppler velocimeters (ADVs). Measurements of the mean and fluctuating velocity profiles were collected with the triple-axis CDVP, and these have been shown to be internally consistent and to be in good agreement with measurements obtained with the ADVs.