was low, but on the other hand, the number of velocity realizations per unit 

 time represented the concentration of sand grains in the data. It is this latter 

 result which makes it possible to derive estimates of sand grain concentration 

 and flux in the SUPERTANK bottom boundary layer data. The number of 

 valid velocity data accumulated over a fixed time is proportional to the 

 sediment concentration and is denoted as n. The same data could also be used 

 for flux estimation in the SUPERTANK project because uniform size sand 

 was used. The LDV system can be modified to record the signal amplitude as 

 a measure of grain size, for deployment in environments where the sediment 

 composition is poorly sorted. 



Pressure sensor. A pressure sensor was mounted on the fixed outside tube 

 of the telescopic profiler for wave-height monitoring. The sensor consisted of 

 a Sensym model ST2015G2 absolute pressure sensor. Amplifier electronics 

 were modified to give a resolution of 0.2 cm of water. Bench tests showed 

 that the short-term stability of the electronics was better than the resolution. 

 Pressure data were sampled, digitized, and stored along with velocity data to 

 ensure synchronization. 



BASS current meter. The BASS current meter measures the difference in 

 acoustic travel time between sound waves traveling with and against the flow. 

 The instrument is fully described by Williams (1985), and only a brief dis- 

 cussion is presented here. The BASS sensor consists of four pairs of acoustic 

 transducers. Each pair is aligned so that the members of a pair receive and 

 transmit signals from and to each other only. The eight transducers are ar- 

 ranged in two sets of four. Each set is placed on a horizontal ring with sen- 

 sors pointing at 45 deg relative to the horizontal plane. The spacing between 

 the two rings is 10 cm so that the acoustic path is approximately 15 cm. In 

 operation, one transducer in a pair fires a short pulse which is received by the 

 other member of the pair a measured time later. Shortly thereafter, the for- 

 mer receiving transducer fires the pulse and the former transmitter detects it, 

 again a measured time later. The difference in the two measured times is the 

 time delay due to fluid velocity. Measurements from the four transducer pairs 

 form an over-determined estimate of the three velocity components. As con- 

 figured, the BASS obtains velocity with a spatial resolution of 15 cm, which 

 implies that it is suitable for velocity measurements at sufficiently great dis- 

 tances from the seabed or boundaries. In the present experiment, the BASS 

 was used to measure current velocity outside the boundary layer. The sample 

 rate was set at 1 Hz. The velocity measurement was stored as a 12-bit word 

 for each of the four axes. 



OFS. A sediment concentration sensor system was added to the LDV- 

 BASS-pressure package in the last 2 weeks of SUPERTANK. The system 

 measured the light scattered by suspended material over a range of forward 

 scattering angles. In this instrument, a laser diode-optics arrangement 

 transmits a divergent beam of light along the sensor housing. Light scattered 

 over a broad range of angles is sensed by a photo diode, molded in the inte- 

 gral package. Because forward scattering is far more intense than backscatter, 

 this device is capable of sensing lower concentration limits. The present 



Chapter 1 1 LDV in the Bottom Boundary Layer 



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