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A Descriptive Study of the Physical Oceanography of Kaneohe Bay, Oahu, Hawaii

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Title:A Descriptive Study of the Physical Oceanography of Kaneohe Bay, Oahu, Hawaii
Authors:Bathen, Karl H.
LC Subject Headings:Oceanography--Hawaii--Kaneohe Bay.
Kaneohe Bay (Hawaii).
Date Issued:Jun 1968
Publisher:Hawai'i Institute of Marine Biology (formerly Hawai'i Marine Laboratory)
Citation:Bathen KH. 1968. A descriptive study of the physical oceanography of Kaneohe Bay, Oahu, Hawaii. Honolulu (HI): Hawai’i Institute of Marine Biology, University of Hawai’i. Report No.: 14.
Series:HIMB Technical Reports
No. 14
Abstract:The results of a thirteen-month hydrographic survey of Kaneohe
Bay are presented in this primarily descriptive study. The subjects
covered in the study are the hypsographic conditions, tides, circulation
patterns, volume transports, sewage distribution, heat budget,
precipitation, runoff and the distribution of water properties in the
Bay. The interrelations between these subjects are also examined.
A primary conclusion of the study is that the bathymetry of the
Bay has a controlling effect on the circulation. This is due to the
large reef areas that restrict the flow, especially from the southeast
basin. The flow in turn governs the volume of water transported into
and out of the Bay, the areas of heat gains and losses, the amount of
stratification, and the sewage distribution in the Bay.
The amplitude and phase of the tides in the Bay were determined.
The wind was found to exert an influence on the differences between
the Honolulu and Kaneohe Bay tides. Stronger trade winds increase
the amount of time a tide in the Bay preceeds the Honolulu tide and
also increase the tidal height in the Bay relative to Honolulu.
The tidal records for the Bay show an unexpectedly high number
of free oscillations. The theoretical free oscillations for the Bay
were therefore calculated and the results are compared with the observed
oscillations. The observations indicate a predominance of
single and binodal free oscillations; however, the influence of these
oscillations on the current velocities in the Bay is small.
The circulation patterns present in the Bay during an incoming and
an outgoing tide were determined. Both patterns show considerable
variation in current velocity and direction depending upon the location
in the Bay. The overall current patterns, however, are very consistent.
The northwest half of the Bay has more active circulation than the
southeast half, because there are fewer flow restrictions in the northwest half of the Bay. Circulation patterns below 4 m in the deeper inshore portions of the Bay are very consistent I and frequently circulate
in opposite directions to the surface circulation. The current
patterns during the changing tides reverse at some locations as in
the southeast channel. The amount of this now reversal is dependent
upon the tidal cycle and the wind. A considerable amount of water is exchanged across the entrance
reefs and in the two channels during each tidal change. The net now
through the inshore portion of the Bay, however, is limited since a
majority of the water exchanged is the surface water that is close to
the Bay entrance. The result is the rushing rate in the inshore
portion of the Bay is low. The Bay accumulates water from precipitation, runoff, and the addition of sewage. This accumulated volume
leaves the basin without tidal exchange transports, appearing as a
daily net volume outflow from the Pay. Since the precipitation and
evaporation are approximately equal, this net outflow from the Bay is
essentially a result of the addition of runoff and sewage to the Bay.

The circulation throughout the Bay determines the distribution of
temperature, salinity, oxygen, and phosphate. The distribution of
these properties was determined each month. The results, presented
in maps for the surface, 5 m and 10 m indicate the Bay is more stratified in the summer than during the winter. An analysis of diurnal
measurements shows the range of daily water property variations due to
the combined effects' of heating, circulation, and tides, is approximately
l/5th of the seasonal variations.
The yearly mean water temperature in the Bay was higher than the
open ocean, implying that heat must have been advected from the Bay
during the year. A computation using the temperature and volume of the
mean monthly net and exchange transports showed such a heat loss.
The Bay must, therefore, have an annual heat gain at the surface. Such
an annual heat gain was demonstrated by computing the heat exchanged
at the surface of the Bay. The results of an alternate method for computation
of the heat exchanged at the surface of the Bay shows a
slight loss of heat throughout the year. Heat losses from the surface
of the Bay are prevalent on the reefs while heat gains are generally
found in the deep inshore areas and areas adjacent to the stream mouths.
Runoff contributes to the stratification in the Bay and also influences
the seasonal distribution, of phosphate on the surface of
the southeast basin. The mixing during winter storms prevents stratification
in the basin. As a result, the phosphate, temperature, and salinity values in the southeast basin during the winter are more
uniform with depth. In the summer the stratification in the basin
causes the sewage and runoff to remain on the surface, resulting in
greater phosphate, temperature and salinity variation in both the
surface and deep water of the basin.
A time series analysis, used to correlate the preceipitation
measured at Mokuoloe Island with the stream runoff into the valley,
revealed a very gradual increase in stream runoff into the southeast
basin over the past thirty-one years. This increase is attributed to
an increase in urban construction in the drainage area of the streams
emptying into the basin.
Pages/Duration:355 pages
Appears in Collections: HIMB Technical Reports

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