Tides and Currents in Fanning Atoll Lagoonl
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Tides and Currents in Fanning Atoll Lagoonl B. S. GALLAGHER,2 K. M. SHIMADA,2 F. 1. GONZALEZ, JR.,2 and E. D. STROUP2 As PART OF the Fanning Island Expedition simple fashion. In each case, a straight section 1970, selected physical studies were conducted was established by stringing a cross-channel line, in the atoll lagoon. The major effort was the along which bottom topography was measured. measurement of volume, salt, and heat transports (At North Pass, there is, in addition to a main through the three main atoll openings over a channel, an expanse of shoals which were un 24-hour period. In addition, lagoon and ocean covered at low water. This area was ignored tides were recorded, and a cursory survey was resulting in an estimated maximum local volume made of circulation in a small, reef-enclosed transport error of 13 percent.) A single station pond within the lagoon. was chosen at a point along the section that appeared to be in the main flow (see Figs. 1 and 2). At this location, current velocity, tem TRANSPORT STUDY perature, and salinity were sampled each hour, Fanning Island is a roughly oval atoll whose within 0.5 m of the surface. Water level was lagoon, although almost entirely enclosed, does read hourly from a tide staff mounted in a exchange water with the surrounding ocean. sheltered spot near shore. Figures 1 to 3 show Tidal flow in and out of the lagoon occurs at the measurements. The various transports four locations and amounts to roughly 5 percent through these passes were computed by assuming of the lagoon volume over a semidiurnal cycle. the measured parameters to be constant over the channel cross sections. About 90 percent of this exchange takes place at Flow through English Harbor channel was English Harbor, through a channel with a sampled in far greater detail because it was maximum depth of approximately 8.5 m and a expected to be overwhelmingly important in the least width of 290 m. On the east side of the total-exchange picture. It has been reported atoll, opposite English Harbor, is a shoal open (U.S. Hydrographic Office, 1952) that currents ing (Rapa Pass) which handles about 2 percent of the total exchange. A similar channel to the in the channel exceed 5 knots. Taking measure ments from a skiff would be very difficult at best north (North Pass) accounts for roughly 5 in such a flow. Consequently, transports were percent. Air photos show one additional, mean monitored along a semicircular section enclosing dering route which mayor may not lead from the lagoon end of the channel and through which the lagoon to the sea. This possible fourth a more moderate flow could be expected. Figure opening, located about 5 km south of Rapa Pass, 4 shows the section and a smoothed fathometer was ignored in our study. trace along it. Anchored buoys were emplaced Exchange between ocean and lagoon was at stations 1 to 6. The section was traversed monitored for 24 hours, starting at local noon continuously by a skiff which was made fast to on January 7, 1970. Current velocity, tempera each buoy in turn. Seventeen cycles of measure ture, and salinity were recorded at each of the ments were completed during the 24-hour study. three passes. At each buoy, current velocity, temperature, and Both North and Rapa passes were treated in salinity were measured at the depths shown in Figure 4. These data were integrated over the 1 Hawaii Institute of Geophysics Contribution No. cross section to obtain transport figures. 3 Aver- 359. This research was funded by the National Science Foundation under grant GB-15581. 3 Details of all calculations, lists of instruments 2 Department of Oceanography and Hawaii Institute used, and a description of the buoy-anchoring method of Geophysics, University of Hawaii, Honolulu, are given in Hawaii Institute of Geophysics Report Hawaii 96822. 70-23. 191 192 PACIFIC SCIENCE, Vol. 25, April 1971 oNORTH PASS Channel cross section 160 r-----,-r----,......,.-----r----r-----r--------, 80 ~ 120 / 60 ~ / \ E E c: 80 / \ 40~ ~.- / \ .J:: / \ -CI 40 20 'ijj 'I \ .J:: 'I \ c c 0 0 .." ...Q) \ // , ..... :::>- Current speed '-... _ ./ \ I u 6 40 \ / '-'"" 1200 80 ... 60 o c: ~.- 40 c: ...c ... 20 Q) E :::> o o >:; I o I 20 1200 FIG. 1. Channel cross section showing station position (solid dot and vertical line), tidal height and current speed, and volume transport for North Pass during the 24-hour transport study. Dense patches of Turbinal'ia that nearly reach the surface at low tide are located between 20 to 40 m from the east shore. Cross-sectional depths are relative to a tidal height of 13 cm. Absolute sea level is arbitrary. Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 193 RAPA PASS Or-----,-------,-----r->---r---.,...-----.-----r- ..c. Q. Q) 0.8 o v 60 80 Q) <II "- E ,..- v 40 I\ I\ ""tl.~ I \ '" .:c Q) I \ " \ Q) 20 I ", 40·2> Q. / , Q) <II I \ ..c. I , cQ) 0 / , 200 ~~ ""tl ~ ~ / Current speed \ ~ 0 " ~- ..... _-; " u 20 v" , o 1200 1600 1200 7 Jan,1970 v ~ 40 ME -; c 20 0' Q. C 0 o~ ~ ~ ~ 0 Q)E 20 L-. l......- l......-::--__l......- "-- "--__----' ~ 1200 1600 2000 0000 0400 0800 1200 ~ 7 January 1970 8 January FIG. 2. Channel cross section with station position (solid dot and vertical line), tidal height and current speed, and volume transport for Rapa Pass during the 24-hour transport study. Cross-sectional depths are relative to a tidal height of 38 em. Absolute sea level is arbitrary, ages of all the temperature and salinity measure each period of inward flow, a jet developed and ments at buoys 2, 3, and 4 for each measurement extended into the lagoon from the channel itself. cycle are presented in Figure 5, along with the The jet appeared at buoys 2, 3, and 4, where volume transport through the entire channel. speeds exceeding 3 knots could be found. On A recording current meter was later placed either side of the base of the jet, eddying near the surface at buoy 3, the site of strongest motion, which could be observed visually, ap flow. The resulting record is shown in Figure 6. peared in the measurements of volume transport at stations 1, 5, and 6. During the ebb, however, RESULTS OF THE TRANSPORT STUDY there was outward flow at all locations across Volttme our section, funneling into the channel. Figure Currents through the channel at English Har 7 illustrates these patterns in the volume trans bor displayed an interesting pattern. During ports at each buoy. The same jetlike flow also 194 PACIFIC SCIENCE, Vol. 25, April 1971 100 35.5 80 j 0354 60 . ~ ... o o .~35.3 40.s ~ I c c o 1 20 .-... ~ 35.2 Q) o E ..... :::) 35.1 LVolume transport 1200 1600 1200 7 January - RAPA PASS ou 30 .-----=----.------,.------,-----.-----.------........, u Q) - \ ~ ~ 28 ...... ..!'_J"emperature ~/ M 35.3 :> .... ,,,,,,,,,, --, ~-------" E ~ ~, o - I , I - ~ 26 'v....1 ''../ 40 "t: o-e ~ o >.35.2 / \/ \ Q. c ." c ~ 24 ....Volume :, 20 c ~ ......., .~ 35.1 transport o .-e c./) Q) :::) E 35.0 20 02 ~ 1200 1600 2000 0000 0400 0800 1200 7 January 1970 8 January FIG; 3. Salinity, temperature, and volume transport for North and Rapa passes during the 24-hour transport study. developed on the seaward side of the channel tion.4 The lagoon's response to the ocean tide during ebb and could often be seen from shore. would be an intriguing subject for further study. Another noteworthy feature of the current was The first semidiurnal cycle of transport studied its rapid reversal, with relatively short periods coincided with a typical tidal excursion. During of weak flow between strong ebb and strong this period (1440 to 0120) 0.025 lan3 of water flood. This indicates that the lagoon has a non was computed to have entered the lagoon linear response to tidal excitation. The presence through the three passes. The computed out- of nontidal overtones is seen in the current and lagoon tide records displayed together in Figure 4 There probably is also some error in the relative 6. As one would expect, the current shows phases of the curves. in Figure 6; the current should not lag the lagoon-tide derivative. We cannot find a strong relation to the tide's rate of change, but timing mistake in either record and can only point both processes clearly reflect nonlinear distor- out this discrepancy. Tides and Currents In Fanning Lagoon-GALLAGHER ET AL. 195 OA 1 2 3 4 5 6 B 2 lagoon 4 2 3__ "~5 N~1''''-'- 6; ..s4- 'A ' B ...c 0.6 Q.) 0 ~ 8 ocean 10 o 2652 0 200 400 600 ~s Distance (m) FIG. 4. Smoothed channel cross section (left) and buoy locations (right) for English Harbor channel. Depths (in meters) at which measurements were most often taken during the 24-hour transport study are indicated for each buoy. flow was 0.026 km3. The agreement between 107 kg) also matched inflow (86 X 107 kg) these numbers is gratifying, because the tidal within our limits of accuracy (see Fig. 8). Salin records show no net change in lagoon water ity followed no discernible pattern (see Fig. 5). level over this particular period. Moreover, At North and Rapa passes, inflow exceeded the measured exchange, 6 percent of the outflow by 2.0 X 107 kg of salt (Figs. 9 and mean lagoon volume, agrees with the value 10). Salinity in these passes decreased during estimated using tidal range and mean depth in flow into the lagoon and increased during out the lagoon.