150 



ADCP had two specific problems: electrical cross-coupling between the con- 

 ductors in the cable between the transducer head and the electronics, and a 

 significant velocity bias error at short pulse separation. These errors led to 

 the breakdown of the velocity resolution scheme {Experiment Procedures — 

 Pre-processing and filtering) in highly dynamic conditions or when the hori- 

 zontal velocity was 50 cm/sec or above. For single-ping data files, the data 

 are partly recoverable through ambiguity resolution methods that can be ap- 

 plied in post-processing. Multiple-ping data are unrecoverable. 



Instrument 2. The main problem with the 600-kHz data was the lack of 

 adequate instantaneous dynamic range. Many of the wave runs contain data 

 that far exceed the 42-db range in the 5-cm layer close to the bottom. The 

 output saturated the system and the data are not fully recoverable for the most 

 dynamic events. 



Summary 



Review of experiments and results 



The objectives of the ADCP measurements at the SUPERTANK project 

 were fulfilled, but the scientific content of the data is not as good as was 

 anticipated, and the data quality is mixed. First, the statistics of the velocity 

 data from the offshore region were obtained for each run (Appendix H). 

 Detailed time series, however, are garbled for the larger orbital velocities, due 

 to ambiguity errors. Second, much was learned about the design of acoustic 

 backscatter and velocity systems for use in the surf zone. The lessons, how- 

 ever, were not small perturbations on what was already known, but a radical 

 departure from pre-conceived ideas about dynamic range. Third, the 

 possibility of using BB-ADCPs to measure turbulent stress and kinetic energy 

 was evaluated. The conclusion, however, was that a measurement scheme 

 that has previously worked well for deep-ocean conditions was inadequate in 

 the surf zone. Even if all hardware and software problems are fixed, the 

 temporal resolution must be increased by one or two orders of magnitude 

 before the system is robust enough to allow routine measurements of any flow 

 other than the mean current. For backscatter measurements, the BB-ADCP 

 beams should ideally be multiplexed, the dynamic range must be increased, 

 and the filters opened up to properly resolve the bottom boundary layer. 



Recommendations 



The authors recommend that no further work be done with the present data 

 set. Although acoustic methods have a future as a tool to uncover the mech- 

 anism for energy transfer and transport in the surf zone, the hardware used in 

 SUPERTANK must be radically improved before these goals can be reached. 



Chapter 8 ADCP Measurements at SUPERTANK 



