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R. Frank (Roswell Frank) Busby.

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Woods Hole Oceanographic Institution

Data Library 6c Archive

McLean Laboratory

Atlas Collection



y



TR-189



TECHNICAL REPORT



OCEAN BOTTOM RECONNAISSANCE OFF THE
EAST COAST OF ANDROS ISLAND, BAHAMAS



Prepared by:

ROSWELL F. BJJSBY ,

CHESTER V. BRIGH

and

ANDRES PRUNA



Oceanographic Development Division




MAY 1966



I




U. S. NAVAL OCEANOGRAPHIC OFFICE

WASHINGTON, D. C. 20390

Price $1.10




RFGLiytD

1988 JAN 2 /.;•! li : 2 1

ONR, BOSTON



FILE



ABSTRACT



A visual and photographic survey of the shore and ocean bottom
to 600 feet (100 fathoms) depth was performed off selected sites
on the east coast of Andros Island, Bahamas. Climatology and
oceanographic conditions are also presented. Survey methods
employed both SCUBA divers and a manned submersible (Perry
Cubmarine PC3-B) to provide direct visual observation and
measurement of bottom features.

The Andros shoreline was observed to vary considerably in mor-
phology and composition: barrierreef development ranges from
well-developed to virtually non-existent from north to south along
Andros Island. A marginal escarpment commencing offshore
between 14 and 16 fathoms was observed to descend almost
vertically to 100 fathoms. Various morphologic features of the
escarpment suggest a lowering of sea level and subsequent sub-
aerial and intertidal erosive processes which have been active
in the recent geologic past.



ERRATA - TR l89



m^4>i



Page
9

12



1st Paragraph,
last sentence

Figure 5, caption



25


2nd paragraph,
i+th sentence


55


Plate 25(A)




Plate 25(D)



Read:



Read:



Read:



Read:



Read:



easterly" for
westerly"

Fresh Creek" for
Deep Creek"

Visibility" for
Visability"

CAVES" for
CORES"

ESCARPMENT" for
EXCARPI4ENT"




i o
: m

: □



V'/ond-s^lole OceanographiclDStiMion
_ATLAS ^^GAZE«?ff?^LECT!ON




J



^sm



FOREWORD



This report presents the results of visual and photographic
underwater surveys from the shoreline to a depth of 600 feet
along selected routes off the eastern coast of Andros Island,
Bahamas, Additional oceanographic and meteorological data are
included which were obtained during previous surveys. This is
the U. S, Naval Oceanographic Office's first survey utilizing
SCUBA equipped divers and submersibles and represents a further
expansion of our data collection and survey capabilities to
provide a more lucid presentation of ocean environmental
charact eris ti cs ,

The survey was performed to provide information applicable
to the installation of submarine cables in support of the U. S.
Navy's Atlantic Undersea Test and Evaluation Center.




ODALE D. WATERS, JR.

Rear Admiral, U. S, Navy

Commander

U. S. Naval Oceanographic Office



Woods Hole Oceanographic institution

ATLAS -GAZETTEER COLLECTION



TABLE OF CONTENTS



J



Page

FOREWORD iv

LIST OF FIGURES v

LIST OF TABLES vi

LIST OF PLATES vii

INTRODUCTION 1

Field Methods 1

Geologic Setting 4

Previous Studies 5

CLIMATOLOGY AND OCEANOGRAPHY 6

Climatology 6

Tides 6

Waves 9

FRESH CREEK (SITE 1) 11

SALVADOR POINT (SITE 2) 14

BIG WOOD CAY (SITE 3) 16

GOLDING CAY (SITE 4) 18

DEEP CREEK (SITE 6) 19

HIGH POINT CAY (SITE 7) 22

UPPER MARGINAL (RIM) ESCARPMENT 24

SUMMARY AND CONCLUSIONS 26

REFERENCES 29

FIGURES

1. Location of Survey Sites 2



FIGURES (Cont'd)

Page

2. Sketch Chart of Survey Sites 3

3. Seasonal and Annual Wind Rose at Mangrove Cay,

Andros Island ^

4. Wind Wave Rose Histogram, Tongue of the Ocean 10

5. Fresh Creek: Bathymetry and Survey Area 12

6. Salvador Point: Bathymetry, Survey Track, and

Location of Marker Buoys 15

7. Big Wood Cay: Bathymetry, Survey Track, and

Location of Marker Buoys 17

8. Golding Cay: Bathymetry, Survey Track, and

Location of Marker Buoys 20

9. Deep Creek: Bathymetry, Survey Track, and Location

of Marker Buoys 21

10. High Point Cay: Bathymetry, Survey Track, and

Location of Marker Buoys 23



TABLES



1. Summary of Meteorological Observations at Mangrove

Cay, Andros Island (1959-1960) 7

2. Tidal Differences Relative to Hampton Roads, Virginia

at Various AUTEC Sites 9

3. Comparison of Possible Wave Heights in the Tongue of

the Ocean for Different Fetch Orientations 11



VI



PLATES

Page

1. Fresh Creek: Aerial Photographs of Site; Beach
Face and Lagoon-Reef Configuration. Lagoon Bottom

at 6 Feet Depth 31

2. Fresh Creek: Barrier Reef and Outer Platform 32

3. Fresh Creek: Outer Platform and Rim Escarpment 33

4. Salvador Point: Aerial Photograph of Site; Beach Face

and Low Tide Terrace 34

5. Salvador Point: Lagoon Bottom 35

6. Salvador Point: Patch Reef and Barrier Reef 36

7. Big Wood Cay: Aerial Photograph of Site; Beach Face

and Low Tide Terrace 37

8. Big Wood Cay: Low Tide Terrace and Lagoon Bottom

at Buoys 1 to 4 38

9. Big Wood Cay: Lagoon Bottom Buoys 1 to 4 and 5 59

10. Big Wood Cay: Outer Platform 40

11. Big Wood Cay: Outer Platform and Rim Escarpment .... 41

12. Golding Cay: Shoreline and Cay Rock 42

13. Golding Cay: Bottom From 25 to 36 Feet Depth 43

14. Golding Cay: Bottom from 45 to 120 Feet Depth 44

15. Golding Cay: Outer Platform Edge and Rim Escarpment . . 45

16. Deep Creek: Backshore Area, Country Rock, and Beach

Face 46

17. Deep Creek: Lagoon Bottom and Patch Reefs 47



PLATES (Cont'd)

Page

18. Deep Creek: Outer Platform 48

19. High Point Cay: North Shore of Cay Above and Below

Water 49

20. High Point Cay: Bottom at Buoys 3 and 4 50

21. High Point Cay: Bottom at Buoys 7 and 9 51

22. High Point Cay: Bottom at Buoys 10 and 11 52

23. High Point Cay: Outer Platform Edge and Rim Escarpment . 53

24. Western Marginal (Rim) Escarpment from Outer Platform

Edge to 600 Feet Depth 54

25. Drawings Showing Major Marginal Escarpment Features ... 55

26. Representative Photographs of Rim Escarpment from 210

to 312 Feet Depth 56

27. Representative Photographs of Rim Escarpment from 343

to 450 Feet Depth 57

28. Drawings of Major Shallow-Water Features Along the

Courses Surveyed 58



vill



INTRODUCTION



Between 5 through 20 May 1964, the U. S. Naval Oceanographic Office
in conjunction with the U. S. Naval Photographic Center performed an
onshore and underwater cable route survey off the eastern coast of
Andros Island, Bahamas. Surveys commenced ashore and proceeded under-
water offshore along predesignated bearings to a depth of 130 feet.
Survey sites were Salvador Point, Big Wood Cay, Golding Cay, and
Deep Creek. Following this phase of the survey a similar operation
was pursued off Fresh Creek and High Point Cay in June 1965. In
addition, a manned submersible (Perry Cubmarine PC3-B) was employed
to investigate the bottom to a depth of 600 feet off Fresh Creek,
Big Wood Cay, and Golding Cay (Figures 1 and 2).

The purpose of the survey was to provide information on the
nature of the bottom for future submarine cable, installations off
Andros Island in support of the U. S. Navy's Atlantic Undersea Test
and Evaluation Center (AUTEC). Additional information regarding
oceanography and climatology was obtained prior to these operations
to fulfill other project requirements.



FIELD METHODS



SCUBA surveys to a depth of 130 feet were conducted from aboard
R/V AQUANAUT out of Nassau, New Providence Island. Predesignated
onshore cable termination sites were located using aerial photo-
graphs and survey monuments from previous site surveys. Survey
tracks were established by positioning a transit over the onshore
site marker and sighting along the proper bearing. At a measured
distance from the shoreline, and guided by shore signals, styrofoam
marker buoys were placed at intervals along the track. Subsequent
to installation of the buoys, divers took still and motion pictures,
samples, and measurements of the bottom at each buoy and between
buoys where a change in bottom characteristics occurred.

Diving operations with PC3-B commenced immediately seaward of
the barrier reef off Fresh Creek, in the center of the boat channel



78«*00'



77»30'




25»
00



240
30'



24»
00



78° 00'



77°30'

FIGURE I. LOCATION OF SITES



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between Big Wood and Gibson Cays, and several hundred yards east
of the center of Golding Cay. In all cases the commencement point
was established with Decca Mark X and the submersible then proceeded
due east along the bottom to a depth of 600 feet.

Still photographs ashore were taken with a Press Type 4" x 5" still
picture camera; underwater stills were taken with a 35 mm "Calypso"
camera. Motion pictures were made underwater with a "Sampson" under-
water camera using artificial and natural light.



GEOLOGIC SETTING



Andros is the largest of the Bahamian islands. It is centered
at approximate latitude 24''29'N., and longitude yy'SZ'W., on the
Great Bahama Bank, and constitutes the western land boundary along
the majority of the Tongue of the Ocean (Figure 1). Barely more
than 30 miles wide, the island is approximately 90 miles long. A
network of tidal channels and marshes, calcareous mud, and mangrove
thickets characterize the geography of the west coast of the island.
Conversely, the east coast is well defined by alternating deposits
of sand and beachrock. A barrier-type reef extends along the eastern
coast of Andros and is separated from the island by a shallow lagoon
which does not exceed 3 fathoms in depth.

A major part of the island is covered by low shrubs and trees
supported by a well-indurated, medium to fine-grained, oolitic
limestone. There is virtually no topsoil covering the limestone
country rock; instead, a few inches of humus collects here and there
in the cavities and depressions honeycombing its surface.

The lagoon averages one mile wide and a maximum 2 to 3 fathoms
deep. The lagoon floor is an extension of the Andros country rock
and is covered by a varying thickness of medium- to coarse-grained
calcareous sand. In some areas (Fresh Creek, Salvador Point, Deep
Creek) the sand is as much as 5 feet thick, while in other areas
(Big Wood Cay; Golding Cay; High Point Cay) only a thin, shifting
veneer is present.



The reef off the eastern coast of Andros extends from the Berry
Islands in the north to Pigeon Cay in the south. It varies in
development, does not present a solid barrier throughout, and is
more pronounced at some locations than at others. According to
Newell et_ al (1957), the reef consists of a narrow ridge of oolite
which rises in places to form small cays, and in certain areas has
been overgrown with algae-coral encrustations which form a broad
underwater ridge several hundred feet wide.

Seaward of the reef an outer platform extends eastward a quarter
of a mile or more. The platform terminates abruptly at a depth
between 14 and 16 fathoms where a sharp break-in-slope occurs which
serves as the boundary delineating deep from shallow water. At the
break-in-slope an escarpment descends almost vertically to depths
between 100 and 120 fathoms. Below this depth, the slope gradient
decreases continuously toward the deep sea floor in the center of
the Tongue of the Ocean. The uniform slope seaward of the outer
platform is interrupted in several places by terraces which parallel
the barrier reef.



PREVIOUS STUDIES



Prior to this study, considerable effort was expended on AUTEC
site surveys in connection with base constnaction and dredging.
Other studies, not connected with the project, provided oceano-
graphic information of general and specific interest.

NAVOCEANO performed bathymetric surveys of the lagoon and
reef areas off all the sites, as well as in the Tongue of the
Ocean, which provided coverage of the areas studied herein.
Private organizations performed surveys of the land sites for
construction purposes and provided detailed topographic maps.
In some instances, the contractors made soil borings in the
lagoon to determine the unconsolidated sand thickness.

Newell et al_ (1957) charted the entire Andros reef and their
results provTded an excellent and detailed guideline for this
study.



Tidal measurements initiated by NAVOCEANO (IMR 0-21-64) in 1963
have been reported and are continuing at Salvador Point and Fresh
Creek .

DeLeonibus (1960) calculated theoretical wave heights under varying
conditions of wind and fetch in the Tongue of the Ocean.

Meteorological data were provided by the Bahamas Meteorological
Service which maintains a permanent observation station at Mangrove
Cay, Andres Island, and other islands throughout the Bahamas.



CLIMATOLOGY AND OCEANOGRAPHY



Monthly air temperatures, humidity, rainfall, and wind speed over
a four year period at Mangrove Cay is presented in Table 1. Seasonal
and annual wind roses as measured from November through October 1963
are shown in Figure 3. No wind directions were available from 1957
to 1960.

Winds approaching hurricane strength are most probable in the
months of August, September, and October when 79 percent of the
hurricanes occur. An average of two hurricanes invades the Bahamas
annually.



TIDES



Tidal corrections to be applied to tide predictions at Hampton
Roads, Virginia, from Fresh Creek, Salvador Point, and Pigeon Cay
are presented in Table 2. Tides in the Bahamas are semi-diurnal
with a diurnal inequality, and show a mean range of approximately
3 feet. Tidally induced currents in the lagoon are generally low
speed (<0.2 kts). In the Andros Bights (e.g., between Big Wood
and Gibson Cays) the current can reach 1 knot.







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SPRING




AUTUMN




WINTER



LEGEND




CALM < I



I — 6 KNOTS PERCENTAGE FREQUENCY SCALE

7—16 KNOTS 10 10 20 30 40 50 60 70%

17— 28 KNOTS LuiilimJ 1 1 1 1 1 1 1

29-40 KNOTS



FIGURE 3. SEASONAL AND ANNUAL WIND ROSE AT MANGROVE CAY,
ANDROS ISLAND, NOV. 1962 THROUGH OCT. 1964.



Table 2
Tidal Differences Relative to Hampton Roads Predictions

High Water Low Water

Time Height Time Height
hr. rain. ft. hr. min. ft.



Fresh Creek -1


12


+0.6


-1


36


+0.7


Salvador Point -1


41


+0.9


-1


42


+0.7


Pigeon Cay -1


05


+0.9


-1


05


+0.7



WAVES



The barrier reef provides an effective breakwater for waves generated
in and outside of the Tongue of the Ocean. Consequently, much of the
shoreline on Andros is protected. A wave height of one foot inside
the windward lagoon is rare. Along the eastern shore of Golding Cay
and High Point Cay, where there is a sheer rock wall over 15 feet
high, waves may, however, exceed the one foot height. There is
no barrier reef to protect either island. Consequently, the sea-
ward side of the island receives the full force of all westerly
waves .

In the Tongue of the Ocean, theoretical wave heights of 11.3 feet
can be generated at a wind speed of 26 knots. DeLeonibus (1960)
presented Figure 4 which represents observed seasonal wave heights,
direction, and frequency in the Tongue of the Ocean. From the same
report ( Ibid.) Table 3 summarizes the most probable wave heights
from all directions for wind speeds ranging from 10 to 26 knots
under unlimited duration at the downwind edge of the fetch.



^



JAN-MAR




JULY-SEPT



^



APR-JUNE



TX^



OCT -DEC



LEGEND



ICALM^" '"""■■ '■

WAVE HEIGHT (FEET)




10 20 30



SCALE (Vo)



100



3 5 8 12



WAVE HEIGHT (FEET)
358 12 01358 12 01358 12



I I I I I
JAN-MAR



"TT



APR-JUNE



I I I I I i
JULY-SEPT



1 — I I I I
OCT-DEC




100



1



3 5 8 I2>120 1 3 5 8 I2>I2 1 3 5 8 12
WAVE HEIGHT (FEET)



1 3 5 8 I2>I2



FIGURE 4. OBSERVED SEASONAL WAVE HEIGHTS, DIRECTION AND FREQUENCY
IN THE TONGUE OF THE OCEAN.

10



Table 3

Comparison of Possible Wave Heights in the

Tongue of the Ocean for Different Fetch Orientation





Max. Fetch


Wind S]


peed (knots)


Wind


Length


10 14


18 22 26


Direction


(N.M.)


Significant


Wave Heights (:


NW - SE


120


1.4 3.2


6.1 10.0 11.3


N - S


70


1.4 3.2


6,1 6.2 6.3


NE - SW


20


1.4 2.8


3.0 3.4 4.3


E - W


20


1.4 2.8


3.0 3.4 4.3



Some swell can occur as a result of persistent northeasterly winds
and a considerable amount may be refracted around Clifton Point on
the western end of New Providence Island into the TOTO.



FRESH CREEK (SITE 1)



Site 1 is two miles south of the entrance to Fresh Creek on
Andros Island. No specific bearing was followed during the Site 1
survey. Instead, the entire area delineated in Figure 5 was
reconnoitered and representative bottom photographs were taken.
Survey efforts concentrated on the barrier reef and outer platform
where divers were towed underwater on plane boards normal to the
reef along tracks positioned with Decca.

The backshore area consists of a sand dune 3 to 4 feet high
covered by low shrubs and bushes which terminate sharply at the
beach face (Plate 1). The beach consists of a medium-grained,
calcareous sand sloping gently seaward. The first several hundred
yards into the lagoon a clean, sandy bottom is present; proceeding
toward the reef the sand thickness decreases and overlies the
extension of Andros country rock into the lagoon. The lagoon
bottom supports a thinly populated community of various marine
organisms. In places where the bottom has little or no sand
veneer the organisms have concentrated to form a local topo-
graphic high 3 to 5 feet above the general lagoon depth of



11



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43'



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240
42'



240
43'



SURVEY AREA



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ANDROS
ISLAND



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240

42'



77'»45'

FIGURE 5, DEEP CREEK. BATHYMETRY AND SURVEY AREA. DEPTH IN FATHOMS
AND FEET. SOUNDING DATUM M.S. L. -0.5 FEET. APPROXIMATE
SCALE 1:17,500.



12



10 feet.

In the Bahamas, localized areas of luxuriant marine growth
(corals, algae, sea fans, etc.) are called "heads." This term
will be applied to describe such features here.

The lagoon is 1500 yards wide and transition from lagoon to reef
is abrupt and marked by a sharp decrease in depth and an increase
in bottom organisms.

Ranging between 200 to 400 feet wide, the barrier reef provides
a sharp delineation between lagoon and outer platform (Plate 1).
The reef is dominantly populated by Milleporidae , Acropora Palmata,
Agaricia agaricites and various alcyonarians (Plate 2). At low
tide the reef breaks the surface in several areas and, although
a few channels provide passage between lagoon and outer platforni,
generally presents a solid barrier in the area reconnoitered.
Approximately 100 yards south of the southern boundary of the
area studied, the solidarity of the reef gives way to scattered
heads separated by wide channels 10 to 12 feet deep.


1 3

Online LibraryR. Frank (Roswell Frank) BusbyOcean bottom reconnaissance off the east coast of Andros Island, Bahamas → online text (page 1 of 3)