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a. Provide background velocity data for the deep end of the wave chan- 

 nel, the "offshore" region. 



b. Increase understanding of how acoustic systems must be designed and 

 operated to extract information about the concentration of suspended 

 sediment, resuspension events, and near-bottom transport. Several 

 special equipment tests were made during the experiment as part of 

 this objective. 



c. Evaluate the performance characteristics of a newly developed 

 2.4-Mhz Broadband Acoustic Doppler Current Profiler (BB-ADCP). 

 This included the capability of the instrument to measure turbulent 

 quantities (e.g., <«' 2 >, <w' 2 >, and <u'w'> , where u' and w' are 

 the horizontal and vertical turbulent velocities and triangular brackets 

 denote time averages) as well as an assessment of its feasibility as a 

 tool to resolve and quantify highly dynamic sediment transport pro- 

 cesses such as resuspension. 



Scope 



It is emphasized that the instrumentation and data sampling strategies were 

 of an experimental nature. As could be expected, some of these elements 

 were not as successful as desired, and, in hindsight, the project served as a 

 rude awakening to the highly dynamic flow regime found in the nearshore 

 region. First, the variation in the acoustic signal strength (5 to 6 orders of 

 magnitude) greatly exceeded expectations. Second, the BB-ADCP was found 

 to have its own set of problems caused by the large wave orbital accelerations. 



As a result, the wealth of information gathered during the project has 

 primarily been used as an instructive lesson in instrument design for dynamic 

 flow regimes. The analysis of data has had direct impact on instrument design 

 and implementation, both for current meters and acoustic backscatter profilers. 



The impact of the data on instrument design is not addressed in this report. 

 Instead, emphasis is placed on objective (a), the average conditions in the 

 offshore region of the wave channel. Data were collected for the first 

 2 weeks of the SUPERTANK project and cover both erosive and accretionary 

 conditions. The Experiment Apparatus section describes the equipment being 

 used, its physical configuration, and its location in the channel. The Experi- 

 ment Procedures section discusses the range normalization of the backscatter 

 data and findings that have implications for the interpretation of the velocity 

 data. The Data Analysis section contains a description of the data analysis. 

 The final section contains the summary. As an additional reference, Appen- 

 dix H includes: a complete listing of all the data files, one sample set of 

 2.4-Mhz data from each run, and a personal-computer- (PC-) based conver- 

 sion program for reading and separating the binary data files. All data files 

 are stored on two magnetic tapes and can be retrieved on a PC-based Colora- 



Chapter 8 ADCP Measurements at SUPERTANK 



