The bottom orbital currents due to these waves could be computed from 

 linear wave theory and taking into account of the local water depths. 

 Figure 7.17 shows the maximum bottom orbital currents at stations 8, 10, and 

 17. The currents are much stronger at stations 8 and 17 in the immediate 

 vicinity of the north shore, while rather weak at station 10 in the middle of 

 the Sound. 



Bottom shear stresses at these stations, averaged over the wave cycles, 



are shown in Figure 7.18. Average bottom stresses at stations 8 and 17 are on 



2 2 



the order of 3 dyne/cm , but less than 1 dyne/cm at station 10. Bottom shear 



stresses due to the tidal currents at the same time period are shown in 



Figure 7.19. At the same station, the tide-induced bottom stresses are 



generally an order of magnitude smaller. The bottom shear stresses due to the 



combined effects of tidal currents and wind-driven currents are somewhat 



stronger and are shown in Figure 7.20. For the mid-Sound station, the tide- 



and wind-induced bottom stress is comparable to the wave-induced stress and 



both may contribute to the entrainment of sediments. For the north-shore 



stations, however, the wave-induced stress is still an order-of -magnitude 



stronger. Entrainment of sediments at these stations would primarily result 



from the wave effect. 



Wave Climate and Bottom Stresses During 9/01/80 to 9/10/80 



The wave climate and wave-induced bottom stresses during 1 September to 

 10 September 1980 are also computed. As shown in Figure 7.21, the wind 

 velocity shifted from mild southeasterlies to strong northeasterlies and then 

 to strong easterlies, before it diminished eventually. The significant wave 

 heights at stations 8, 10 and 17 during this period are shown in Figure 7.22. 

 High waves up to 3 feet occurred during 3 September to 5 September. The 

 wave-induced stresses, as shown in Figure 7.23, increased substantially from 

 the middle of 3 September. Due to the increasing fetch associated with the 

 easterly winds, the bottom stresses become increasingly stronger from 

 station 8 to station 10 and station 17. 



Computations on wave heights and wave periods presented above were 

 obtained by means of a spectral wave model. The results were found to be 

 generally higher than those obtained from the SMB model (CERC, 1973), which is 



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