is difficult to specify an appropriate boundary condition for sediment 

 concentration or sediment flux at the bed. Second, data on near-bed velocity 

 and sediment characteristics under full-scale random waves are inadequate and 

 imprecise. The SUPERTANK Laboratory Data Collection Project afforded an 

 opportunity to remedy this limitation. 



Objectives 



Objectives of this program were to measure the wave-driven velocity and 

 sediment concentration profiles over the sand bed, and to derive the sediment 

 flux rates from these measurements. 



Scope 



In this report, the results of co-located velocity and sediment concentration 

 measurements are presented. By using a newly developed fiber-optic laser 

 Doppler velocimeter (LDV), both the velocity field and the sand grain 

 concentration profile in the near-bed region (from 1 to 16 cm above the bed) 

 have been derived. Simultaneously, velocity data outside the bottom boundary 

 layer were acquired by a fixed acoustic current meter BASS (Benthic Acoustic 

 Stress Sensor) at 1.4 m above the bed. A pressure sensor was used to sense 

 water surface displacement. At a later stage, an optical forward scatterometer 

 (OFS) operated by Mr. Jeff Mather of Sea Tech Inc. was placed close to the 

 LDV system to measure the sediment concentration independently. The 

 experimental procedure and representative results from the data sets are pre- 

 sented in the following sections. 



Experiment Apparatus 



The suite of instruments, the LDV, pressure sensor, and BASS, along with 

 the OFS in the last 2 weeks of the experiment, were attached to a traversing 

 mechanism (the profiler, Figure 11-la), secured on a bracket mounted on the 

 inside wall of the wave channel (Figure 11-lb). The profiler outer structure 

 was used to mount the BASS, a pressure sensor, and one OFS, so that these 

 instruments were at fixed locations. The telescopic inside profiling member 

 carried the LDV. The measurement station was located 1.5 m shoreward of 

 Station 19, which is approximately 18.5 m downstream of the wavemaker. 

 Water depth at this station was nominally 3 m. 



212 



Chapter 1 1 LDV in the Bottom Boundary Layer 



