b. Derived transport rates (net and gross transport rates). 



c. Effective threshold for longshore sand transport (calm and 

 near -calm wave events). 



The first two items are treated within GENESIS in combination with appropriate 



input file preparation, and the third item is treated in wave data file 



preparation prior to running GENESIS. 



Multiple transport rates 



193. Waves arriving at the shore are typically produced by several 

 independent generating sources. Long-period swell waves were probably 

 generated from distant storms, whereas the shorter period "chop" or sea waves 

 were produced by local winds. Indeed, the WIS hindcast provides information 

 for both sea waves and swell. The modeler may have to deal with even more 

 than two wave sources. For example, for the southern coast of California, 

 three independent wave sources coexist during parts of the year: Northern 

 Hemisphere swell, local sea waves, and the Southern Hemisphere swell which 

 arises from storms as far away as the Antarctic Ocean. The Southern Hemi- 

 sphere swell occurs mainly in the interval from May through October and, in 

 some years, may be the dominant transporting wave component along the coast of 

 the southern California Bight. 



194. The situation of multiple wave sources is handled through the 

 assumption that each wave source gives rise to an independent longshore sand 

 transport rate. GENESIS then calculates a total longshore sand transport rate 

 at each grid point i by linear superposition. Let Q L m be the transport 

 rate at grid point i produced by source m , of which there are M wave 

 sources. The total transport rate at i is 



M 



(36) 



GENESIS uses this quantity to calculate shoreline change. 



195. As discussed in the next chapter, the interface of GENESIS 

 requires specification of the number of wave sources (called "NWAVES" 

 instead of M as above). The file holding wave data must similarly reflect 

 this number by containing wave data in sequence for the M sources at each 



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