10 



Depth dj is considered the depth of closure and is used in estimating 

 offshore closure limits for use in beach fill design. Hallermeier (1977) 

 defined depth of closure as the minimum water depth at which no measur- 

 able or significant change in bottom depth occurs based on profile surveys. 



To emphasize the importance of differences in wave and sand charac- 

 teristics and wave variability on open sea coasts, Hallermeier (1978, 

 1981b) computed the depths dj and d for 30 sites on the Pacific, Atlantic, 

 and Gulf of Mexico coasts using the wave climate study of Thompson 

 (1977) and data from the Littoral Environmental Observation (LEO) Pro- 

 gram. For the Gulf of Mexico coast (seven sites), the dj and d values 

 were -4.2 m and -9.9 m, respectively. For the Atlantic coast (11 sites), dj 

 and d were -5.7 and -22.1 m, respectively. D^ and d values at the Pacific 

 coast (12 sites) were -6.9 and -42.9 m, respectively. Differences in dj and 

 d values stated above are a result of differences in significant wave 

 height, wave period, and mean sediment grain diameter. 



Boyd (1981) documented that the maximum depth of the initiation of 

 sediment movement (similar to Hallermeier's (1981a) d) at the New South 

 Wales, Australian continental shelf fluctuates with wave conditions (Fig- 

 ure 4). For instance, for wave height of 0.5 m and periods of 7 sec, this 

 depth is -10 m; for wave height of 2 m and periods of 12 sec, this depth is 

 -60 m. 



Kraus and Harikai (1983) defined depth of closure as the minimum 

 depth at which the standard deviation in depth change decreases markedly 

 to a near constant value. 



Birkemeier (1985) compared data from two profiles located in Duck, 

 North Carolina, between August 1980 and December 1982 to Haller- 

 meier's equation by measuring wave conditions that existed between pro- 

 file surveys that exhibited offshore sand movement (Figure 5). 

 Birkemeier (1985) found good agreement with the form of Equation 4, but 

 recomputed the coefficient to better fit the data. He also found reasonable 

 agreement using only H in Equation 5: 



^/ = 1-57//, (5) 



where 



d^ = nearshore limit, or closeout depth relative to mean low water 



//g = peak nearshore storm wave height, which is exceeded only 

 1 2 hr/year 



He stated that this equation is probably site-specific. 



Kraus (1992) conceptualized that the beach profile responds to wave 

 action between two limits, one limit on the landward side where the wave 

 runup ends and the other limit in deeper water where the waves can no 



Chapter 2 Inner Shelf Concepts 



