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 Table of Contents

Title: Effects of dredging in Water Bay, St. Thomas
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00096169/00001
 Material Information
Title: Effects of dredging in Water Bay, St. Thomas
Alternate Title: Water pollution report - Caribbean Research Institute ; 2
Physical Description: 11 p. : chart ; 28 cm.
Language: English
Creator: Caribbean Research Institute
Publisher: Government of the Virgin Islands, Dept. of Health, Division of Environmental Health
Place of Publication: St. Thomas, USVI
Publication Date: 1969
Copyright Date: 1969
Subject: Marine biology -- Caribbean Sea   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Spatial Coverage: United States Virgin Islands
General Note: "3rd printing, February, 1971".
Statement of Responsibility: Caribbean Research Institute.
 Record Information
Bibliographic ID: UF00096169
Volume ID: VID00001
Source Institution: University of the Virgin Islands
Holding Location: University of the Virgin Islands
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 27261827


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Full Text




Contrted By





Rbert P. vamEepoel

November, 1969

(3rd Printing, Febrtary, 1971)


I. INTRODUCTION. . . . . . . .1





VI. SUMMARY ...... .. . . . 9

VII. APPENDIX . . .. . . 11


Thu ,.Pot Pancea the dm nd camebdm.M .bhkb wasuvh frow Pon-
himimry inswuptiwem, of due effects of dr.fting hWater say. St. 1bmma.
TI, coaae..tin fes, ameporumpser wee oam~pkshed with the meoetae. df
Piz .iet-day. of etfoot by -nsbe e othe staff of thW Caribbeean eumoeb

Tb. dwe sod raeomasdafo in geow sh and~mui 50.1lwe..
contributed by Dr. David BJMandsh Dr. Jobs, Adenw; snhtboe tohm iiws
hiImIW sod enolog by Robert Brody. noe report smee poepsed bee tei
doeal .seovsia by tb. .1gb... who sawlated sekch al. thes tbisd Amid ch e-
sewaionse. Moth df ik. mooledl as derived frome recomteeiom on 0 MO
15 October. 19G6; eneessh tar. Ieotro uips 26AM 80 S~epember. aid edo
about 26 Masrch, in the swinog before doedgieg hmpa. A@rlsl -hI Ipb
-er tal.. ee8 Avgesth 1969.

- 1-


Water Bay is situated on the north shore of St. Thomas, about two
miles from the eastern-most tip. It opens to the east to the Leeward Pas-
sage, an Inter-island channel from the Atlantic Ocean to Pillsbury Sound;
and it is bounded on the north by Coki peninsula, on the west by Pineapple
Beach Club site, and on the south by the island land mass proper, ending
at Footer Point which separates Water and Smith Bays. Part of the south-
er bshore is the slope of Mt. Ptesart, which rises to 200 feet elevation.
Between Footer Point and Mt. Pleasant is a lowland are which formerly
was a salt pond, and which until recently had been used as a garbage dump.
This area was overfilled with the dredging spoils. The sub-littoral area of
the hey Is about three million quare feet, with a seaward opening o a north-
south axis of about 1, 500 feet and a westerly extension of about 2,000 feet.

Dredging last spring was allowed under Department of Interior Sub-
merged iLands Permit No. 10 and U. S. Army Corps of Engineera Permit
SASJ Permits (69-66). The Department of Interior Permit expires 31
December 1972. These permits allow the dredging and removal of 600,000
cubic yards of material; and Special Condition No. 5 of Department of Interior
Permit No. 10 requires that the dredging be confined to water depths greater
than 25 feet. This condition was "to Insure preservation of the nearshore
submarine r-eource." The permitted pays to the Office of Territories. U.S.
Department of Interior. $0.10 for each cubcle yard removed, subject to annual
recoonsidrautn of the fee.

The Water Bay bottom was subjected to spoiling on two prior occasion;
first In 1961 for the original boat dockage and construction fills, and then again
in 1965 for beach extensions and construction materials for the Pineapple Beach


Water Clarity

The site was visited in late September and early October. On the
first visits, turbidity was not nearly as high as on the latter visits, with
visibility initially along the northern shore up to about 20 feet. The weather
had been exceptionally calm for several weeks, but between visited had re-
turned to a near normal late summer/early fall condition; sea state of 2-3
and wind of 10-15 hmote. At the last visit bottom could not be clearly seen
anywhere la the bay in depths over 5 feet. Turbidity remained high off-shore
east of Cold Point to depths of St feet, and gradually diminished along the
north shore of the peninsula, ver a 100 150 yard stretch westward in
Leeward Pasnage, to normal clarity. Corals have been killed, as a result
of deposition of sand particles, nearly to the eastern edge of Cold bathlag
beach. Large schools of hbitfish were observed sauth of Cold Point, and
are being preyed upon by birds; so apparently the vertebrate fauna is not
being albrtantitlly affected. Water clarity Is markedly improved along
Pineapple Beach Club in water depths less than 10 feet -- the western 15('
of the bay.

Causes of Sub-littoral Mortalities

Two primary modes are/were In operation :

1. Turbidity eliminates or reduces light penetration, and the
coral algae symbloais are killed. Those symblonts supply the proper
metabollsm of coral wastes and allow calcium carbonate fixation by the
ooral animal. When the symbioat algae die, the corals die.

2. Siltation causes clogging of the coral animal tissues and
literally smothers the corals. There are apparently two mechanisms
of silltatio operative in the bay.

a) initially, the dredge work disturbs the normal bottom
over of algae, grasses, and coral, killing some by actual physical dis-
ruption and suspending a large quantity of sediments of all sizes in the
water mass. The large particles settle out within a few hours or days
and fall upon the corals. If a sufficient. mouot of sediment fala upon a
coral, the load will be too great to be removed by the coral's tentacular
structure and death will earue.


b) A second problem occurs as a result of the fine mater-
ial put into supensian by the dredge activities. In Water Bay ths is com-
plicatod by the apparent stratlgraphf of the sediments on the small south
shore beach. Dredge work removed a suffioent amount of sand to permit the
sand on the beach area to "slump" bmick into the offshore waters. The ter-
restrial land under and behind the btach contains very fine clay particles
and movement of this material accompanied (or followed) the change in beach
tospgraphy. The extremely line clay particles are held in suspension In
the water, and those larger ones which settle kill by clogging or simply
covering the low coral growth (e.g., Poritea) with a slurry of clay-silt.
The effects of this situation are obviously of much longer duration than those
of the larger particles, and they are very much in evidence throughout
Water Bay.

Inspection of Dredge Cuta

Figure I lia roan asecton sketch of a line from about the tip of
Cold Poinat southwad Into the dredged area. The bottom areas from Cokl
Point south for about 100 yards are floored with corals to i depth of ap-
proximately 25 feet. Below this coral zone iS a narrow strip of sand with
the genss Cymodocea and noa-caloareous algae ie g., Caluerpa). This
gasee community Is Interrupted by a distinct trench, made by the dredge,
which reaghly follows the eortOir of the point. The trench profile at the
section line cts into the slope below the reef at 30 35 feet depth; it is
about 15 feet wide. The bottom of the trench is 46 feet and the level rises
on the offshore side to 40 feet. The dredge out described above was followed
for about 1/mile underwater. For almost all of the length, it follows ex-
actly a 28 foot depth contour. Presumably the dredge operator bad control
of this as the lico followed extended at the constant depth first west. then
north, than wst, thenaosth.

- 4 -


This section summaries the effeta of the dredging which caused
the preheat high turbidity oonditton io the bay and the eroalon of the small
beach area on the south ahore of the bay.

Erosion of the South Beach

The small beach on the south side of the bay shows evidence of
rapid erosional retreat. The hben has been out baok to form a nearly
vertical cliff several feet above ea level. Substantial portions of the
root systems of several large trees have been exposed. The beach it-
self is coarse, gravelly, and contalso little Band. It is steep and arrow.
Some beachrack la exposed near water level. The beachrock shows no
evidence of long exposure to the sea: it has not been attacked by nartLne
borere nor does it support corla or other attached organeims; algae
cover is extremely thin and spotty.

Offshore from the Beach

The ollowioag five ones are f ted:

1. Itard-packed, current-swept sand pavement.

2. Firm sand with small oecillation ripples.

S. Outcrops of a well-atratifled, coheslve clay.

4. Rtmnaacs of a small reef or coral bank (mostly dead).

5. Loose, soupy bottom with a 2" 12" layer of reworked
clay overlyieg dead coral and clean and.

The clay (3) outcrops at water depths of from 4 6 feet. The zo0n
of reworked clay (5) extends out tin the bay from a depth of about 8 feet. Its
outer boudary was not found although it presumably occupies an area of
several acres near the south beach.

Timing of the Beach Erselon

The present beach ahows every evide-ne of having been eroded very
recently (probably within the past few months). The cliff cut in t hebrm
and the fresh beachrock are moot eloquent in this regard, but all the observed


feature support this conclusion. Furthermore. the beach is still actively
eroding. The rook Jetty constructed Just east of the beach appear to have
had little or o effect (it may even have tcreased the rate of beach erosion).

Clay Outcropping Offshore from the Beach

This could not have been deposited in the bay as wee see t today. It
to probably part of a larger clay deposit that underlies the present berr and
beach. At some it.se i the past, the berm must have been farther out in the
bay so that it formed a lagoon or salt pond, which served a. a protected
basin for clay deposition.

The present exposure of clay is a direct consequence of the erosion
of the beach, which In turn was most probably touched off by dredging of
sand close to shore.

EffeAts of Erosion of the Clay

Some of the clay that hae been exposed to wave action in the shallow
waters off the beach has moved downslope to form zone 5, referred to above.
The rest of it has been (and continues to be ) thrown into e suspension and
moved elsewhere In the bay. The latter has produced a level of turbidity
which has reduced visibility to a few inhes near the nutcrops of clay and
has had a noticeable effect am underwater visibility elsewhere, particularly
atomi the southern margin of the bay. The reworked cly of zone 5 is also
contributing to this turbidity and loes of visibility.

Contrast with SiltiUg Caused by Dredging

Thin deposits of fine sand and ailt stemmlng from the dredging opers-
tione are apparent in the northern half of the bay and have been responsible
for partial killing of coral and other reef orglanims along the north shore of
the bay. Presumably, this silting took place during and shortly after the
dredging and was geographically controlled by water current patterns at the
time. This sand depoeition also was found along the north shore of Cold
peninsula, into varying magnitude.

The sequence of events leading to the exposure and erosion of the clay
on the south side of the bay Is entirely differeDt and has quite different impli-
cations. The clay is now exposed in extremely shallow water and may be ex-
pected to provide I continuous supply of suspended material. Thus, elevated
turbidity affecting the marine ecology and aesthetic value of the bay may be
expected to continue until the clay supple is exhausted or until corrective
measures are taken.


General Sueey

Aonroor pagnapta along both south and north shores are nearly all
secrotic; labyrinthform corals fp Montastrea) are somewhat Iss
severely damaged, but obviously not well.

The margins of several poaches of Thalassia along the western
section of the south shore are abruptly curtailed and depth increases radi-
cally at these margins. These Thalaseia patches are in water of 6 8 feet
depth; the depth of the margins i 12 15 feet. The turbidity la these areas
is high; calcareous fines are notable to depth of 8" 12" before caleareous
sands Mealldn eto.) are felt. Dredging was done in this area prior to
the work done this year. and it appears the ires from the new dredging Ieve
settled into the old cut.

The corals in water 10 25 feet deep are adapted to a lower'lighb
regime and show less marked deleterious effects. There is evidence that
1 2 inches of sand have accumulated around the base of the octocorala.
killing some few coral polype bet not the entire colony. This is also suggest-
ed by the lack of small hard coral growth on the surface. many dead mall
(e.g., 2" 3" diameter) hard corals beneath the sand and a general lack of
encrustIng colounal organisms (spoeges, tualcates, mileporine (fire) corals).

South Shore

There a interimittent coral kill on Acrpera plmata (Elkhoarn),
Por ores t (Finger Coral) and their associated coeleaterate communities.
In the area immediately below Mt. Pleasant peak and west, Acrenra asows
partially dead colonies with end size particles atop horizontal coral limbs.
Immediately north of the small beach area west of Footer Point and on to the
west for approximately 300 yards there is nearly complete extirpation of
Acona and almost equal kill of Porhes. From the beach to Footer Point
there i again the intermittent Idll.

Benthic grasses In this southnide of Water Bay are sparse. In those
grass areas near the beach described above, the epiphytic algae in Ttbalsoia
re lightly coated with a fine clay silt. On the Thalassij patches Immedlately
north of live Acrerap this silt Is less dense but still noticeable.

Beethle algae in the area follow the same pattern. Tie noncalcareoua
speies (Cau ter Di i GigiL) are depleted or absent. In the area
north of the beaheb visibility is reduced to 2 3 feet and virtually anl algae are

The apparent cause of this kill sla twofold. The Acroora in the
inshore (less than 10 feet) area to the wvet shows spotty coral kill with
relatively large diameter algal sand particles still in place upon the
Acroor branches. The entire area in deeper water and also the area
from the rock Jetty at the beach to 300 yard west inshore, shows the ef-
fects of a ewry fle clay-silt which is causing noral death by all of the

North Shore

1. The corals in the shallow surge zone are dominated by
Arp plhnate and ontororals (seafans, etc.). In this area there
has been coral kill at about a level of 2S 50% by the mechanasm of
large particle fall-out.

2. The area on the deep water side of the cut is covered with
fine particles to a depth of 3 8 inches and vegetation is greatly reduced,
or more frequently absent, burled by the silt.


It is clear *uat the bottom alteration this year in Water Bay has re-
mitted In a major ecologlocal disaster for the tub-littoral floor and sessile
fauna. Relatively rapid extirpation of many of the corala, gresaes, and
algae resulted from disruption, covering, or alltation: the highly turbid
water condition continues to destroy more.

This Is a secondary effect of the dredging procedure. When the
bottom cover of grasses and algae are removed the fine particles are no
longer trapped by them and their associated opi flra, and epifauna. Turbidity
is thus Increased. This works in a cycle, since turbidity reduces bottom
light levels and marine grasses cannot carry on sufficient photosynthesis
to rapidly recolonoiz the area. Thus a new community of non-photosyn-
thetics may repisce it. and an anaerobic canditioa might exist. The po-
tential for this is already present in Water Bay where black sediments
underlie the existing layers of light silt. At best, the photosyethetic com-
munity will take a long time to reestablish itself; thus successional process
will take several years as the initial colonizers will have to stabilize the
sediment with root system development and be replaced in time by other
species. The marme grasses repre ent the "climax community" of clear
inshore marine waters, and their success Is the result of several modiflea-
tims of the substratum by several preceding plant communities.

The kill of the coral animals is significant in terms of the long range
health of the reptef since big corals must grow from small corals. Those octo-
corals which have had their lower portions killed will not die immediately.
but a primary means of the growth of fire urals is to utilize the dead lower
surface areas of soft coral branches as an initial area of colonization. The
fire coral then grows up the stem of the octocoral, Idlling the octoooral as
it grows. Approximately 505 of the octocorelB show the pattern of lower
stalk death as a result of rapid sedimentation. If the sand now present is
moved away, expostig the central aidl, a prime habitat for fire coral colon-
ization is opened, and probably most octocorals so damaged will die In the
future by thia mcchanism.

The dredging was done without proper consideration of the nearabore
dynamics. Removal of sand too close to the south shore of the bay caused
dowslope transport of the sand forming the beach, and subsurface clay de-
posits w.re exposed to the water currents. It goes without saying that ased-
ment cores taken In the beach area before dredgig would have shown the
presence of clay elome to the surface, end the present turbidity problem could
have been anticipated or avoided. Now it would seem prudent to survey the
area much more extensively in order tc fully assess the problem, and to learn
more about the basic phenomenon of cloy exposure so that mistakes will not be
made toIn future operations. It would also seem appropriate to investigate as

:ma0 as pma.ibi. Y W d ya aaWm .Of ~Moedyf the pnile.t eobI.eM. This
o-.ld -t--b'~ly le1..1 -1v.ri the bly (o-.a 3 sodS b-)s 0~
-toLag it by spoosteialld drektsg.






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