Table C-1 Appendix C) evaluated at the breaker position and i^s^g i^ 

 the shoaling coefficient evaluated at the wave gage: 



H. - H (4-45) 



Kg or H/H^ can be evaluated from small -amplitude theory, if wave-period 

 information is available from the wave gage statistics. For simplicity, 

 assume shoaling-coefficient ratios as listed in Column 4 of Table 4-12. 

 Such shoaling coefficient ratios are consistent with the shoaling co- 

 efficient of Kg=1.3 (between deepwater and breaker conditions) assumed 

 in deriving P^g (Table 4-9) , and with the fact that waves on the inland 

 sea of the problem would usually be steep, locally generated waves. 



Column 5 of the table is the product fHg (Kg)^/(Kg)g. The sum 

 (1.06 feet) of entries in this column is assumed equivalent to the aver- 

 age of visually observed breaker heights. Substituting this value in 

 Equation 4-44, the estimated gross longshore transport rate is 226,000 

 cubic yards per year. It is instructive to compare this value with the 

 value of 262,000 cubic yards per year obtained from the deepwater example. 

 (See Table 4-11.) The two estimates are not expected to be the same, 

 since the same wave statistics have been used for deep water in the first 

 problem and for a 12-foot depth in the second problem. However, the numer- 

 ical values do not differ greatly. It should be noted that the empirical 

 estimate just obtained is completely independent of the longshore energy 

 flux estimate of the deepwater example. 



In this example, wave gage statistics have been used for illustrative 

 purposes. However, visual observations of breakers, such as those listed 

 in Table 4-4, would be even more appropriate since Equation 4-44 has been 

 "calibrated" for such observations. On the other hand, hindcast statis- 

 tics would be less satisfactory than gage statistics due to the uncertain 

 effect of nearshore topography on the transformation of deepwater statis- 

 tics to breaker conditions. 



4.6 ROLE OF FOREDUNES IN SHORE PROCESSES 



4.61 BACKGROUND 



The cross section of a barrier island shaped solely by marine hydrau- 

 lic forces has three distinct subaerial features: beach, crest of island, 

 and deflation plain. (See Figure 4-41.) The dimensions and shape of the 

 beach change in response to varying wave and tidal conditions (Section 

 4.524), but usually the beach face slopes upward to the island crest - the 

 highest point on the barrier island cross section. From the island crest, 

 the back of the island slopes gently across the deflation plain to the 

 edge of the lagoon separating the barrier island from the mainland. These 

 three features are usually present on duneless barrier island cross sec- 

 tions; however, their dimensions may vary. 



4-III 



