58 



ATKINSON 



Salinity % 



20' 



34.0 



15' 



O 



o 



O 



3 



a 

 E 

 .* 



10< 



35.0 



— I — 



crt26 



4°N /' O^^' 



/ ff/ // 10°N /Vo / 



Ot 27.0 



600 M 

 -800 I PE 



Fig. 1 Temperature-salinity relationships of Western North Pacific Central Water (WNPC) and 

 Pacific Equatorial Water (PE) in the region of the northern Marshall Islands. The solid lines show 

 the temperature-salinity relationship at different latitudes. The insert shows depth of transition 

 zone between the water masses. [Drawn from Barnes et al.. 1949.] 



Breaking waves on the fore-reef and the back-reef 

 determine sand transport in the following ways Cross-reef 

 currents carry sand from the fore-reef and the reef flat to 

 the lagoonward rim of the reef, building and eroding 

 islands. Ephemeral sand spits develop on the margins of 

 the islands; this sand is sorted and distributed by long- 

 shore transport from lagoon waves and back-reef currents. 

 Two general patterns of sand grain size have been deter- 

 mined for Bikini (Emery et al., 1954): (1) grain size 

 increases across the reef flat from ocean to lagoon, then 

 inside the lagoon, grain size decreases until a depth of 

 approximately 15 m is reached; and (2) grain size 

 decreases from the middle of the seaward beaches toward 

 the ends of the islands and decreases from each end of the 

 island to the middle of the lagoon beach. 



Two processes are apparently responsible for the distri- 

 bution of sand: (1) high energy cross-reef currents carry a 

 large suspended load, depositing sand as they slow down; 

 and (2) the continual breaking of seaward and lagoonward 

 waves on the islands transports sand along the shore. The 



high energy currents are formed from oceanic swells 

 breaking on the fore-reef, and the long-shore currents are 

 formed by lagoon wind waves breaking on the beach. In 

 the future, sand transport by currents at Enewetak could 

 be studied as a function of wind speed and direction, surf 

 height, and swell direction. 



The tides at Enewetak Island are usually in good agree- 

 ment with the U. S. Navy Tide Tables. However, lagoon 

 and ocean tide records show differences in amplitude, tim- 

 ing, and tide curve shape. When the reef is awash at 

 Enjebi, wave setup produces ocean tides with a mean 

 water level 0.3 to 0.5 m above the mean lagoon level; at 

 Enewetak Island, the differences are small but significant 

 (Buddemeier, 1981) (Fig. 2). Buddemeier also analyzed 

 long-term differences between the Japtan gauge and a 

 lagoon gauge at Biken (see Fig. 2 for location). His analy- 

 sis showed that while the tide records were similar in 

 amplitude and frequency composition, the Biken highs are 

 broadened and the lows narrowed by about 1 hour. Based 

 on an average difference in tide elevation betwfeen Japtan 



