PHYSIOGRAPHY 



29 



TABLE 1 



Areas of Environments at Enewetak Atoll 

 (in kilometers) 



torn visible in aerial photographs is limited to depths of 15 

 to 20 m and is usually located only on the rim of the 

 lagoon. The only structures which are visible from the sur- 

 face in the central lagoon are coral pinnacles which reach 

 within less than 15 to 20 m of the surface. 



There arc two major channels between the lagoon and 

 ocean (Figs. 2 and 3). The first is the "deep" channel, 

 between Medren and Japtan, which is nearly 60 m deep in 

 places but is relatively narrow. It averages only about 1.4 

 km in width between Japtan and Medren, but the deepest 

 portion (below 40 m depth) is only about 600 m wide. 



During tidal changes, swift currents flow in and out of this 

 channel It is exposed to the easterly swell from the ocean 

 and allows such swell to enter the lagoon in its vicinity. 

 The swell, combined with wind-produced chop due to the 

 open fetch of the channel and currents flowing out of the 

 lagoon (counter to the wind direction), often produces 

 extremely rough conditions in the channel. 



The deep channel splits into two branches just west of 

 Jedrol Island leaving an area of shallow reef in between 

 with minimum water depths of about 6 m (Fig. 2). This 

 wedge-shaped reef gradually deepens both to the west and 

 north until it essentially merges with the lagoon bottom. 

 Near its easternmost extremity, a ferro-cement barge — the 

 "Concrete No. 9," locally called the "cement ship" — ran 

 aground, resulting in a distinctive marker of this site. The 

 bottom slopes away at about a 45° angle into the 

 branches of the deep channel which begin to flatten out at 

 about 40 m depth. 



The second major passage, the "wide" channel, is 

 located at the south end of the atoll between Enewetak 

 and Ikuren. It is no more than 15 to 18 m deep but 

 stretches 10 km between the islands (Fig. 3). Since it is 

 considerably shallower than the deep lagoon bottom, it 

 resembles a sill. The currents in its vicinity are essentially 

 unidirectional, out of the lagoon (Atkinson et al., 1981), 

 but their speed is determined by the tide. Although the 

 wide passage does not directly face the ocean swells, the 

 swells are refracted somewhat around the southern end of 

 Enewetak Island and enter the lagoon through this open- 

 ing. This, combined with waves from the lagoon and the 



Fig. 2 Aerial view of the deep, narrow channel entrtuice to the lagoon between Medren and 

 Japtan Islands on the eastern, windward side of the atoll. [Photo by P. L. CoUn.] 



