particles will experience shear from those eddies with fluctuation times i^ 

 equal to or greater than Xp. 



Let us now examine the relaxation time t^ of sediment particles of 

 various sizes. For simplicity, we assume Stokes flow and hence the settling 

 speed Wg is proportional to the square of the particle radius. Assuming a 



3 



particle density (p^) of 2 gm/cm , the results are summarized in Table 6.2. 



Table 6.2 

 Settling Speeds and Relaxation Times of Sediment Particles 



2 



2r gCP.-Pf) 



y^ = 



Particle Radius s g^p T^=Wg/g 



(ym) (cm/sec) (sec) 



1 2.18x10 2.22xlo" 



5 6.45x10 ' 6.55xl0"^ 



10 2.18x10 2.22xlo" 



2. 



.18x10" 



■k 



6. 



.45x10' 



■3 



2. 



.18xl0" 



■2 



6. 



.45xl0" 

 2.18 

 54.5 



■1 



50 6.45x10 ' 6.55x10 ** 



100 2.18 2.22xl0"^ 



500 54.5 6.55x10 



Based on the results in Tables 6.1 and 6.2, one can compute a critical 



particle radius r^ at which the relaxation time t^ is equal to the time scale 



of dissipation eddy t^ under expected ranges of coastal turbulence 

 (Table 6.3). 



Table 6.3 

 Critical Particle Radii at Various Dissipation Rates 



2 3 

 e (m /sec ) 0.001 0.01 0.1 1 10 



r^ (um) 367 212 116 67 37 



130 



