RESTRICTED 



8 



3. OCEAJ^JOGRAPIIY OF THE LAGOON 



3.1 Currents 



Three methods were used to measure currents: (a) current 

 meters, "by which the velocity was determined at various depths 

 from surface to bottom and the direction to a depth of about 100 

 feet, the limit of visibility; (b) cirrrent poles, v;hich deter- 

 mined the average drift of the upper fifteen feet of water over 

 periods of from eight hours to a day and a half: (c) dye marker, 

 which was used primarily in the channels and over the reefs, 

 where other methods were impracticable. 



Figure 3 shows the general drift of the surface v/ater of 

 the lagoon as determined by current pole observations. Data 

 obtained by all three methods are presented in Figures 4 to 7, 



The circulation of the lagoon as determined by the current 

 measurements is as follows: 



(a) Over the eastern and northern reefs, continuous inflow 

 results from the fact that outside currents and wave action 

 maintain a gradient in water level between the outer reefs and 

 the lagoon amounting to about 1.5 feet. 



(b) Continuous outflow occ\irs through the v/estern part of 

 Enyu Channel. The volume of this flow is a little more than 

 half the inflow over the reefs. 



(c) Elsewhere on the periphery of the lagoon the direction 

 of flow changes with the tide. The ebb is stronger than the flood 

 throxigh the southwestern passes, 



(d) The dominant features of water movement inside the la- 

 goon are a wind-driven surface current flowing in a generally 

 WSWT direction and a return current along the bottom. 



^. (e) The surface current extends to a depth of 40 feet or 

 more. Its velocity varies with the wind as shown in Figiire 8. 

 Throughout the entire lagoon the cxirrent is influenced to some 

 extent by the tide, decreasing on the flood and increasing on 

 the ebb and with a more pronoimced southerly component on the 

 ebb. Near the southwestern passes the flood tide is strong 

 enough to reverse the surface cxorrent. 



(f ) Part of the surface current leaves the lagoon through 

 the passes and channels with each ebb tide; however, the outflow 

 accounts for only about 30^ of the total transport into the 

 western end of the lagoon. The remainder sinks and returns as 

 an ENE bottom ciirrent, carrying with it some outside water that 

 has come into the lagoon on the flood tide. The bottom ciirrent 



