Validation of High-resolution ADCP Measurements of TKE Dissipation Influenced by Waves

This study evaluates the capability of the new Origin 600 (Sonardyne) high-resolution five-beam Acoustic Doppler Current Profiler (ADCP) to measure turbulence in shallow coastal waters influenced by surface waves. Evaluation was done through comparison between bedframe-mounted, upwards-facing ADCP data and a free-falling MicroStructure Sampler (MSS) shear probe, in turbulent flow conditions characteristic of the Tamar River channel in Plymouth Sound, UK. While commonly used to measure water column velocity, the low resolution of conventional ADCPs severely reduces their ability to estimate finescale turbulence. The new Sonardyne Origin 600 ADCP allows for much higher along-beam resolution of up to 1.2 cm, granting an unprecedented outlook into water column turbulent characteristics. ADCP-based Turbulent Kinetic Energy (TKE) dissipation estimates were computed through a second order structure function for each ADCP beam. Sets of 5, 11 and 17 radial separations and spatial velocity averaging combinations of 10, 25 and 50 cells per bin were compared to reference MSS-measured dissipation data, for six measurement segments ranging from 10 to 18 minutes. Surface wave field conditions above the ADCP unit were evaluated through power spectrum density performed on wave heights computed from ADCP echo intensity, estimating wave induced TKE dissipation contamination. Although further research is required to fully evaluate the accuracy of their TKE dissipation estimations, this study found that a high-resolution ADCP is a versatile tool to continuously evaluate finescale turbulence in shallow and energetic water columns, especially when compared to a lower-resolution MSS. High-resolution ADCP data can also be binned after capture, allowing for a tailored approach when measuring differing water column conditions.