Tides and Currents in Fanning Atoll Lagoonl

B. S. GALLAGHER,2 K. M. SHIMADA,2 F. 1. GONZALEZ, JR.,2and E. D. STROUP2

As PART OF the Fanning Island Expedition1970, selected physical studies were conductedin the atoll lagoon. The major effort was themeasurement of volume, salt, and heat transportsthrough the three main atoll openings over a24-hour period. In addition, lagoon and oceantides were recorded, and a cursory survey wasmade of circulation in a small, reef-enclosedpond within the lagoon.

TRANSPORT STUDY

Fanning Island is a roughly oval atoll whoselagoon, although almost entirely enclosed, doesexchange water with the surrounding ocean.Tidal flow in and out of the lagoon occurs atfour locations and amounts to roughly 5 percentof the lagoon volume over a semidiurnal cycle.About 90 percent of this exchange takes place atEnglish Harbor, through a channel with amaximum depth of approximately 8.5 m and aleast width of 290 m. On the east side of theatoll, opposite English Harbor, is a shoal open­ing (Rapa Pass) which handles about 2 percentof the total exchange. A similar channel to thenorth (North Pass) accounts for roughly 5percent. Air photos show one additional, mean­dering route which mayor may not lead fromthe lagoon to the sea. This possible fourthopening, located about 5 km south of Rapa Pass,was ignored in our study.

Exchange between ocean and lagoon wasmonitored for 24 hours, starting at local noonon January 7, 1970. Current velocity, tempera­ture, and salinity were recorded at each of thethree passes.

Both North and Rapa passes were treated in

1 Hawaii Institute of Geophysics Contribution No.359. This research was funded by the National ScienceFoundation under grant GB-15581.

2 Department of Oceanography and Hawaii Instituteof Geophysics, University of Hawaii, Honolulu,Hawaii 96822.

simple fashion. In each case, a straight sectionwas established by stringing a cross-channel line,along which bottom topography was measured.(At North Pass, there is, in addition to a mainchannel, an expanse of shoals which were un­covered at low water. This area was ignored­resulting in an estimated maximum local volumetransport error of 13 percent.) A single stationwas chosen at a point along the section thatappeared to be in the main flow (see Figs. 1and 2). At this location, current velocity, tem­perature, and salinity were sampled each hour,within 0.5 m of the surface. Water level wasread hourly from a tide staff mounted in asheltered spot near shore. Figures 1 to 3 showthe measurements. The various transportsthrough these passes were computed by assumingthe measured parameters to be constant over thechannel cross sections.

Flow through English Harbor channel wassampled in far greater detail because it wasexpected to be overwhelmingly important in thetotal-exchange picture. It has been reported(U.S. Hydrographic Office, 1952) that currentsin the channel exceed 5 knots. Taking measure­ments from a skiff would be very difficult at bestin such a flow. Consequently, transports weremonitored along a semicircular section enclosingthe lagoon end of the channel and through whicha more moderate flow could be expected. Figure4 shows the section and a smoothed fathometertrace along it. Anchored buoys were emplacedat stations 1 to 6. The section was traversedcontinuously by a skiff which was made fast toeach buoy in turn. Seventeen cycles of measure­ments were completed during the 24-hour study.At each buoy, current velocity, temperature, andsalinity were measured at the depths shown inFigure 4. These data were integrated over thecross section to obtain transport figures.3 Aver-

3 Details of all calculations, lists of instrumentsused, and a description of the buoy-anchoring methodare given in Hawaii Institute of Geophysics Report70-23.

191

192 PACIFIC SCIENCE, Vol. 25, April 1971

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Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 193

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FIG. 2. Channel cross section with station position (solid dot and vertical line), tidal height and currentspeed, and volume transport for Rapa Pass during the 24-hour transport study. Cross-sectional depths are relativeto a tidal height of 38 em. Absolute sea level is arbitrary,

ages of all the temperature and salinity measure­ments at buoys 2, 3, and 4 for each measurementcycle are presented in Figure 5, along with thevolume transport through the entire channel.

A recording current meter was later placednear the surface at buoy 3, the site of strongestflow. The resulting record is shown in Figure 6.

RESULTS OF THE TRANSPORT STUDY

Volttme

Currents through the channel at English Har­bor displayed an interesting pattern. During

each period of inward flow, a jet developed andextended into the lagoon from the channel itself.The jet appeared at buoys 2, 3, and 4, wherespeeds exceeding 3 knots could be found. Oneither side of the base of the jet, eddyingmotion, which could be observed visually, ap­peared in the measurements of volume transportat stations 1, 5, and 6. During the ebb, however,there was outward flow at all locations acrossour section, funneling into the channel. Figure7 illustrates these patterns in the volume trans­ports at each buoy. The same jetlike flow also

194 PACIFIC SCIENCE, Vol. 25, April 1971

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Salinity, temperature, and volume transport for North and Rapa passes during the 24-hour transport

developed on the seaward side of the channelduring ebb and could often be seen from shore.Another noteworthy feature of the current wasits rapid reversal, with relatively short periodsof weak flow between strong ebb and strongflood. This indicates that the lagoon has a non­linear response to tidal excitation. The presenceof nontidal overtones is seen in the current andlagoon tide records displayed together in Figure6. As one would expect, the current showsstrong relation to the tide's rate of change, butboth processes clearly reflect nonlinear distor-

tion.4 The lagoon's response to the ocean tidewould be an intriguing subject for further study.

The first semidiurnal cycle of transport studiedcoincided with a typical tidal excursion. Duringthis period (1440 to 0120) 0.025 lan3 of waterwas computed to have entered the lagoonthrough the three passes. The computed out-

4 There probably is also some error in the relativephases of the curves. in Figure 6; the current shouldnot lag the lagoon-tide derivative. We cannot find atiming mistake in either record and can only pointout this discrepancy.

Tides and Currents In Fanning Lagoon-GALLAGHER ET AL. 195

o 2652~s

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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 areindicated for each buoy.

flow was 0.026 km3 . The agreement betweenthese numbers is gratifying, because the tidalrecords show no net change in lagoon waterlevel over this particular period. Moreover,the measured exchange, 6 percent of themean lagoon volume, agrees with the valueestimated using tidal range and mean depth inthe lagoon. In spite of these consistency checks,however, the transport figures calculated fromour measurements should not be regarded asexact. Inaccuracies in current speeds, directions,and areas of channel cross sections lead to avolume transport uncertainty of +-15 percent.

There was a net inflow of water throughNorth and Rapa passes during the first, semi­diurnal tidal cycle monitored. The amount was0.56 X 10-3 km3, or 0.1 percent of the lagoonvolume. Thus, there is an overall flow across thelagoon which must exit at English Harbor; thisflow is probably driven by wind and wave trans­port over the windward and northern reefs.

Salt

During the first transport cycle, the computedtotal salt transport into the lagoon (90 X 107

kg) matched (within our limits of accuracy)the total salt transport out of the lagoon (91 X107 kg).

At English Harbor channel, outflow (90 X

107 kg) also matched inflow (86 X 107 kg)within our limits of accuracy (see Fig. 8). Salin­ity followed no discernible pattern (see Fig. 5).

At North and Rapa passes, inflow exceededoutflow by 2.0 X 107 kg of salt (Figs. 9 and10). Salinity in these passes decreased duringflow into the lagoon and increased during out­flow. Because these passes are shallow, solarheating may cause the levels of evaporation inthe lagoon to exceed open-ocean levels, at leastlocally near the mouths of the passes. Thehigher salinity outflow could be partly caused bythe mixing of incoming seawater with moresaline lagoon water. However, this cannot beproven. If the ocean water merely flowed in andout of the lagoon, with no mixing, estimatesshow that local evaporation in the lagoon wassufficient to account for the observed salinityincrease.

Heat

In the first transport cycle, during which netvolume transport was zero, net heat transport(see Figs. 8 to 10) was also zero to withinthe accuracy of our measurements. Total inflow(68 X 1013 calories) was closely matched byoutflow (66 X 1013 calories). There was a netinflow of heat at the small passes (1.6 X 1013

calories), but this reflected the particular phas-

196 PACIFIC SCIENCE, Vol. 25, April 1971

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FIG. 6. Aanderaa current meter record at buoy 3, English Harbor channel, and lagoon tide. Absolute sealevel is arbitrary.

Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 197

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ing of the diurnal temperature curves with thesemidiurnal current variations, and may havehad no particular significance. It does seemstrange, however, that the temperature variationswere diurnal and did not show the effects of thetidal flows which appeared in the salinity curves.The disagreement in the periodicities of the

temperature and salinity occurred at both passes.It seems likely that this illustrates the rapid re­sponse of temperature in this shallow-water en­vironment to the large diurnal variation in theflux of radiant energy. The diurnal variationin evaporation was proportionately very muchsmaller, so that no diurnal salinity fluctuation is

198 PACIFIC SCIENCE, Vol. 25, April 1971

100 ENGLISH HARBOR 100

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obvious in the record. At English Harbor, therewas no significant temperature variation. Thisagrees with other observations which indicatethat the water which entered and left throughthe channel was essentially open-ocean water.Changes incurred by mixing with lagoon waterwere too small to be detected.

Mixing

Replacement of water in the lagoon appearsto be a slow process. A major reason for this isthe fact that the lagoon was divided into numer­ous, small ponds by interconnecting line reefs,many of which were almost uncovered at lowwater. Flow was thus restricted to a shoal sur­face layer and to meandering passes through thereefs. An exception was the area receiving flowthrough the channel at English Harbor. Lagoon­ward from the channel, the bottom was scattered

with large coral formations, but these were iso­lated and did not seriously baffle the flow. Thewater here was relatively deep and quite clear.The clear water contrasted markedly with theextremely turbid water found throughout therest of the lagoon, and the boundary betweenclear and turbid water appeared to be verticaland often only a few meters wide. This bound­ary was sometimes observed moving in responseto tidal inflow and outflow through the channel.These facts, together with the temperature andsalinity records from the transport study, indi­cate that very little mixing occurred betweenlagoon water and the water that participates inthe predominant tidal exchange. Mixing islikely to be relatively more important very lo­cally near North and Rapa passes where no topo­graphic or turbidity boundaries were apparentand the water was shoal. Such mixing, because

Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 199

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of the line reefs and because the volume ex­change through these passes was relatively verysmall, would not greatly reduce lagoon-wideresidence times.

One possible lower limit on residence timecan be calculated by using the observed volumeof net inflow through the two small passes. Ifthis inflowing water is assumed to mix com­pletely throughout the lagoon before exiting atEnglish Harbor, then we have

lagoon volumeresidence time =

rate of net exchange

.41 km3

---------- = 11 months..56 X 10-3 km3/11 hr

Tides

The main tidal records were kept near theCable Station on the northwest side of theisland; readings were taken in the lagoon andin the ocean. The ocean gauge was fastenedto an outhouse piling a few meters seaward fromthe water's edge, while the lagoon record wasobtained at the end of a small pier. The mea­surements were used to make daily predictionsfor biologists on the expedition who were in­volved in nearshore collecting. The tide curves,shown in Figure 11, are useful for examiningranges and phases. However, the records are notrelated to any common reference level. More­over,. the cur~es contain several interruptionsoccasIOned by mstrument malfunctions, and no

200 PACIFIC SCIENCE, Vol. 25, April 1971

2000 RAPA PASS 2000

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attempt was made to maintain a constant zero­level between fragments of a given record.

The observed tidal range in the lagoon wastypically 40 em. This provided an immediateestimate of the volume of tidal flow. Half thetidal range was 5 percent of the mean lagoondepth estimated by the geologists, which impliesthat 5 percent of the lagoon volume took partin tidal exchange. Direct measurement of thevolume transport through the passes confirmedthis value. The range in the lagoon was roughlyhalf that in the ocean outside.

The lagoon tide lags that of the surroundingocean by a typical period of 1 hour, 40 minutesas shown in Figure 12. The figure also showstides measured in North and Rapa passes dur­ing the transport survey and the tide outside theCable Station. Although the measurements inthe passes were taken roughly halfway betweenlagoon and sea, the curves appear to be aboutthe same in phase and in range as the ocean tidemeasured at the Cable Station.

There is a temptation for practical reasons tosee whether the ocean tide at Fanning Island is

Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 201

related in some simple way to Honolulu tides.The Honolulu tide is shown plotted against theFanning Island tides in Figure 11. No relation­ship that could be used to give rough Fanningpredictions based on Honolulu records is ap­parent. Semidiurnal phase lags, based on veryscanty records, varied from 16 minutes to 1hour, 42 minutes for high water and from-48 minutes to 2 hours for low water. Theamplitudes were also quite different.

Suez Pond

A large part of Fanning Island Lagoon issubdivided into small ponds by interconnectingline reefs that reach nearly to the surface. Atypical example, called Suez Pond in this paper,was chosen for intensive geological and chem­ical study; we attempted to provide some sup­porting information about water circulation.(See Roy and Smith in this issue of Pacific Sci­ence for a further description of the pond it­self. )

Water movements in the pond proved to beextremely complex and usually too weak to mea­sure quantitatively with standard current meters.Winds, tides, and thermohaline forces all ap­peared to be important in the circulation. Fur­thermore, processes on the bordering reefs andin adjacent ponds seem to have had an influenceas well. Our time and instrumentation were al­most totally inadequate for the task of de­scribing the circulation, and we are able to makeonly qualitative comments.

During much of the year Fanning Island liesin the southeasterly tradewinds, which blowacross the lagoon at mean speeds of 10 knotsfrom 135°. There are diurnal speed variationswith afternoon winds being 10 to 20 percentstronger than morning winds (New ZealandMeteorological Service, 1956). Short, choppywaves up to 0.3 m high were generated withinthe lagoon itself. With exceptions to be notedlater, surface flow across the pond and adjacentreefs was in the direction of the wind. Withinthe pond, the directly wind-driven layer wasless than 1 m deep with speeds on the order of0.1 knot. We found no simple pattern of returnflow at depth; currents at 3 m were variablein direction with speeds too low to measure(< .05 knot). Sand from the tops of the adja­cent reefs was found deposited almost exclu-

sively along the windward boundary of thepond, indicating the directional consistency ofthe wind-generated waves and currents over thereefs, and supporting the finding that currentsat depth are weak.

Tidal effects were superposed and sometimesmasked by the wind-driven flow. Currents weremeasured at several locations on the shoal reefsbounding the pond, both during rising and dur­ing falling tides. At Suez, in a channel roughly1 m deep and 5 m wide dredged through theleeward reef, an ebb flow moved against thewind at 0.1 to 0.2 knots. Tidal reversals werenot detected at any other location. Tidal cur­rents almost surely crossed the reefs at otherplaces, and future measurements should includea study under conditions of no wind. To thenorth and west (and not communicating withthe pond) is a meandering pass which can befollowed through the reefs from the Cable Sta­tion to English Harbor. It is possible that thisserves as a channel for overall southward flowin the area; the ebbing tide and the necessaryreturn of wind-driven surface transport may beminimal in Suez Pond itself.

We attempted to trace subsurface flow withdye, but found this to be of limited value be­cause of the high turbidity in the pond. On oneoccasion, however, we were able to detect sub­surface movement against the wind. Dye intro­duced in a vertical streak near the windwardedge of the pond broke into two patches. Asurface patch moved with the 9-knot wind at amaximum estimated speed of 0.4 knot, andbecame too diffuse to see after about 40 minutes.A second patch, extending downward fromabout 0.5 m, moved up against the windwardreef at an estimated speed of 0.06 knot andcould not be seen coming to the surface. Thedye may have moved downward, or it may haveascended very slowly and become too diffuse todetect in the surface flow. Dye injected as apoint source in the same location at 3 m depthwas never detected throughout 2 hours of ob­servation.

Distributions of properties showed a compli­cated pattern of inhomogeneities. Salinity wassampled at depth at five locations during a 3­hour period. At four locations salinity increasedby .01 to .03%0 from the surface to the bottom,and one station showed a decrease of .01%0'

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204

c~20I-

PACIFIC SCIENCE, Vol. 25, April 1971

1200

FIG. 12. Tidal heights at North and Rapa passes, and in the lagoon and the ocean near the Cable Station,during the 24-hour transport study. Absolute sea level is arb:trary.

Variations of .04%0 over distances of a few hun­dred meters occurred at all depths. One of thestations was repeated after 24 hours, and salin­ity had increased almost 0.3%0 at all depths. Wefound no water of this higher salinity in thepond on the previous day, so the rapid replace­ment of the 8-m water column at this stationdoes indicate vertical movement. The salinitydata indicate thermohaline circulation which wasvery irregular in space and in time. Probablypatches of high-salinity water originated overthe wide, shoal, boundary reefs; were advectedinto the pond; and, perhaps with nighttime cool­ing, gave rise to vertical circulations which canhave small horizontal dimensions compared tothe pond. Below about 3 m, the pattern of suchcirculation would be further complicated by thevery irregular topography.

The distribution of extinction coefficients inthe water column is shown in Figure 12, Royand Smith, this issue. Several factors appeared.A patch of high turbidity, covering about one­third of the pond's area, was apparent at thesurface and extended down to about 2 m.Within the patch, suspended load decreasedwith depth, indicating that the patch was quiterecently formed. The horizontal distributionstrongly implies that this highly turbid waterentered the pond through Suez Canal duringthe falling tide and flowed out over the surface.Shallow tidal flow against the wind also ap-

peared to bring turbid water from the leewardreef into the pond to the east of Suez. At depthsbetween 2 m and 4 m additional horizontalpatches of suspended load could be seen. Theywere less intense and had no apparent connec­tion with those at the surface; they may havebeen remnants of surface patches in the processof settling. (Particulate matter of this size hasa settling rate on the order of 1 m/day.) Below4 m the turbidity became almost uniform hori­zontally, with an intensity that appeared to beclose to an average value for the overlyingwater.

The salinity and suspended load data suggestsome possible general conclusions about mixingin the pond. The uniformity of turbidity atdepth, along with the settling rate of the parti­cles, suggests that the pond had a mixing timeof less than about 5 days. Salinity indicates thatthe mixing time certainly exceeded a few hours,and that at least part of the mixing was asso­ciated with diurnal, thermohaline processes. Onemight suspect that deeper portions of the pondare filled by relatively dense water that is re­newed only infrequently by unusual events.However, there were no indications of stagna­tion; oxygen concentrations were close to satura­tion at all depths. Thus, there must be a fairlyregular downward transport of buoyancy, indi­cating an external source of mixing energy. Themost probable agent is the wind-driven flow,

Tides and Currents in Fanning Lagoon-GALLAGHER ET AL. 205

but we have insufficient measurements for pro­posing a circulation model which could explainthe mixing and the observed distributions ofproperties.

Should members of a future expedition wishto obtain a comprehensive picture of physicalprocesses in the pond, they would face a diffi­cult task. We suggest from our experiences thatthey employ very sensitive current meters (re­sponsive to 1 em/sec or less) to see whetherany average, overall patterns of movement canbe discerned. Great effort would be required toensure no motion of the meters themselves;they cannot simply be lowered from a single­anchored skiff. Hourly vertical profiles shouldbe obtained at as many stations as possible (atleast four on a line roughly parallel with thewind and through the center of the pond) . Weadvocate this direct approach because the patchyand transient nature of property distributionswould limit the value of these profiles for in­ferring motion unless they were based on verydense sampling. If a general circulation patternwere found, then temperature, salinity, and tur­bidity measurements could be planned to com­plement and take advantage of it. Such a planshould be designed to maximize the density ofreadings in both time and space; variations that

occur rapidly and over short distances are to beexpected. On-the-spot instrument readout wouldbe extremely valuable.

ACKNOWLEDGMENTS

We thank C. J. Berg, Jr., R. E. De Wreede,E. B. Guinther, Dr. E. A. Kay, G. S. Key, andJ. P. Villagomez for their help during the 24­hour transport study; Dr. D. C. Gordon formaking all salinity determinations; Dr. K. J.Roy for his help in obtaining fathometer recordsof the bottom topography between buoys atEnglish Harbor channel; and E. G. Gilley forkeeping the boats operational.

LITERATURE CITED

NEW ZEALAND METEOROLOGICAL SERVICE.1956. Line Islands. Wellington, Meteorologi­cal Notes II-B.

RoY, K. J., and S. V. SMITH. 1971. Sedimenta­tion and coral reef development in turbidwater: Fanning Lagoon. Pacific Science, vol.25, no. 2, pp. 234-248, 15 figs.

U.S. HYDROGRAPHIC OFFICE. 1952. Sailing di­rections for the Pacific islands. III. The south­central groups. Washington, HydrographicOffice Publication 80.