1 1 Laser Doppler Velocimetry 

 and Sediment 

 Concentration in the 

 Bottom Boundary Layer at 

 SUPERTAIMK 1 



Introduction 



Background 



Understanding of near-bed dynamics of wave boundary layers is central to 

 sediment transport predictions in the nearshore region. Wave-induced motion 

 at the bed produces a thin oscillatory boundary layer where shear is high and 

 turbulence generation is vigorous. The bulk of the coarse suspended sediment 

 stays close to the seabed, and the transport rate is determined primarily by the 

 concentration and velocity profiles in the near-bed region. For these reasons, 

 the velocity field, the concentration profile, and sediment flux constitute the 

 essential variables of scientific interest in sediment transport study. Past 

 measurements of the velocity structure and sediments in full-scale laboratory 

 flows do not exist, although smaller-scale flows have been extensively 

 examined in the laboratory. 



This work is driven by the need for improved models of sediment trans- 

 port. Modeling the dynamics of the turbulent bottom boundary layer in an 

 environment of waves amounts to dealing with the turbulence closure problem 

 and modeling the turbulent Reynolds stresses in some relation to the mean 

 quantities. By tuning the closure parameters, observed and predicted velocity 

 profiles can be made to agree (Grant and Madsen 1979; Trowbridge and 

 Madsen 1984a,b; Davies 1988). In contrast, sediment transport modeling in 

 the boundary layer has not been as successful for two major reasons. First, it 



'Written by Yogesh C. Agrawal, Northwest Research Associates, Inc., Paul A. Hwang, Quest 

 Integrated, Inc., and Joan Oltman-Shay, Northwest Research Associates, Inc. 



Chapter 1 1 LDV in the Bottom Boundary Layer 



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