United States
Environmental Protection
Agency
Mater
Office of Uater
Regulations and Standards
Washington, O.C. 20460
October 1981
TAMPA BAY RECONNAISSANCE
SURVEY
An Appraisal of the
Benthic Environment of Three Alternative
Dredged Material Disposal Sites
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TABLE OF CONTENTS
Page
Abstract ii
List of Tables iii
List of Figures iv
I Introduction i
II kethods 6
III Results 10
A. Site 1 10
B. Site 2 19
C. Site 3 29
D. Site Comparison 35
Appendix I 36
Appendix II ^3
i
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ABSTRACT
The Tampa Bay Reconaissance Project was conducted by the Environmental
Protection Agency from October 4 through October 8, 1981, in response to
concern over the location of an ocean disposal site for dredged material
from Bayboro Harbor, St. Petersburg, Florida. Using a combination of
side-scan sonar and diver-obtained ground truth, the benthic environments
of three alternative dredged material disposal sites were mapped, observed
and photographed. The three sites are located between 13 and 24 miles west
of Tampa Bay in the Gulf of Mexico (Figure 1).
Side-scan sonar recordings and fathometer readings from Alternative
Sites 1 and 2 indicated little topographic relief. The highest elevation in
either site was less than 3 meters higher than the lowest point. Fathometer
readings also showed little change in relief on Alternative Site 3. Divei
observations at 25 locations revealed hard-bottom habitats at all stations
within the boundaries of Site 1, hard bottom communities on the easterr
stations of Site 2, and similar conmunities at some stations within Site 3,
Bottom sediments collected at the sites were sand with shell hash. Site :
sediments revealed some silt cover.
ii
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LIST OF TABLES
Page
Table
1
Site and Station Coordinates
4
Table
2
Summary
of Data Collection
5
Table
3
Summary
of Divers Observations
of
Site
1
J3
Table
4
Summary
of Divers Observations
of
Site
2
26
Table
5
Summary
of Divers Observations
of
Site
3
33
iii
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LIST OF FIGURES
Page
Figure 1 Location of Site Surveyed 2
Figure 2 Xerox of Side-Scan Sonar Recording 7
Figure 3 Sample Diver Observation Sheet 9
Figure 4 Record of Path of Side-Scan Sonar at Site 1 11
Figure 5 Site 1 Dive Stations 12
Figure 6 Photograph Station 1A 15
Figure 7 Photograph Station IB 15
Figure 8 Photograph Station 1C 16
Figure 9 Photograph Station ID 16
Figure 10 Photograph Station IE 17
Figure 11 Photograph Station 1H 17
Figure 12 Photograph Station IF 18
Figure 13 Record of path of Side-Scan Sonar at Site 2 20
Figure 14 Site 2 Dive Stations 21
Figure 15 Photograph Station 2F 22
Figure 16 Photograph Station 2G 22
Figure 17 Photograph Station 2C 23
Figure 18 Photograph Station 2D 23
Figure 19 Photograph Station 2H 24
Figure 20 Photograph Station 2J 24
Figure 21 Photograph Statoin 2K 25
Figure 22 Site 3 Dive Stations 29
Figure 23 Photograph Station 3D 30
Figure 24 Photograph Station 3F 30
Figure 25 Photograph Station 3G 31
Figure 26 Photograph Station 31 31
Figure 27 Photograph Station 31 32
Figure 28 Photograph Station 3H 32
iv
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I. Introductioi
During the first week of October 1981, Environmental Protection Agency
(EPA) divers and scientists performed a reconnaissance survey of the benthic
environment of three alternative sites for the disposal of dredged material
from Bayboro Harbor, St. Petersburg, Florida. Using a combination of side-
scan sonar and diver-obtained ground truth, the three alternative dredged
material disposal sites were mapped, observed, and photographed.
This survey was carried out in response to local concern over the
existing interim dump site approximately 14 miles off the mouth of Tampa
Bay. Numerous citizen groups had urged that an alternative site be located
and utilized, contending that detrimental effects of dredged material on
hard-bottom communities within and. nearby the existing dredged material dis-
posal site had already occurred.
The seaside communities of Holmes Beach, Anna Maria, and Longboat Key,
along with Manatee County, helped finance an ecological study of the effects
of dredge material at the interim disposal site. The resulting Mote Marine
Laboratory technical report, entitled An Ecological Study of Effects of
Offshore Dredge Material Disposal With Special References to Hard-botton
Habitats in the Eastern Gulf of Mexico, concluded that the disposal ol
Bayboro Harbor dredge material was having a severe degrading effect upon the
benthic environment. It recommended discontinuation of disposal of dredged
material at that site and a search for a more appropriate disposal site.
During the course of gathering information for the preparation of ar
Environmental Impact Statement, the Interstate Electronics Corporation,
(IEC), an oceanographic engineering firm under contract to EPA, receivec
numerous suggestions for alternative disposal sites from local divers,
commercial and sports fishermen, and the Corps of Engineers. Eventually,
three distinct areas were identified as potential alternative sites.
Unfortunately, IEC and EPA had sparse information on these locations at
alternative disposal sites.
-1-
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St. Petersburg
j Existing interim <
_ disposal site nJ
Figure 1. Location of sites surveyed
Nautical Miles
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The objectives of this study were to: \.w prov 1UC d pttjpoi.uo4. uv«v*.*r
tion of the alternative sites and adjacent areas; (2) create a photographic
record of the benthic environment at the sites; and (3) qualitatively des-
cribe the biological communities present. No chemical or quantitative bio-
logical analyses were performed during this reconnaissance study.
The alternative sites examined were square, each measuring 1.4 nautical
miles on a side and 1.96 nautical miles square (Figure 1). Site 1
(southwest) is located 16.5 miles from the mouth of Tampa Bay; Site 2
(near-in) is located 13 miles from the mouth of the Bay and one, mile south
of the Safety Fairway; Site 3 (deep water) is located 24 miles from the
entrance to Tampa Bay and due west from the Safety Fairway. The exact
locations can be determined from Table 1. A summary of the data collected
on the three sites is shown on Table 2.
-3-
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Table 1. Site and
Station Coordinates
Sed iment
Diver
Site/
Sample
Survey
Station
Loran C/Lopa B
Latitude/Longitude
1 NW corner
27°29.2IN/83°2.30,W
1 NE corner
27°29.2,N/83°0.70,W
1 SE corner
27°27.8,N/83°0.60'W
1 SW corner
27°27.8'N/83°2.30'W
X
1 A
14141.4/44700.5
X
1 B
14140.6/44696.3
X
1 C
14139.7/44693.2
X
X
1 D
14138.5/44694.0
X
X
1 E
14137.3/44701.0
X
X
1 F
14136.6/44706.3
X
X
1 H
14138.9/44707.3
2 NW corner
27°34.3'N/82"58.18'
2 NE corner
27"32.8'N/82658.18'
2 SE corner
27°32.8'N/82"59.7'W
2 SW corner
27°34.3,N/82°59.7,W
X
2 A
14159.8/44699.4
X
2 C
14154.0/44717.1
X
2 D
14157.0/44721.2
X
2 E
14156.9/44718.0
X
2 F
14158.6/44714.8
X
2 G
14160.6/44707.1
X
2 H
14164.6/44711.3
X
2 J
14160.0/44723.0
X
2 K
14159.6/44724.9
3 NW corner
27°37.5,N/83,,9.6,W
3 NE corner
27°36.1,N/83°9.6'W
3 SE corner
27836.1,N/83°11.2,W
3 SW corner
27°37.5,N/83°11.2,W
X
3 A
14136.0/44829.6
X
3 B
14134.3/44834.4
X
3 C
14135.9/44829.5
X
3 D
14141.0/44825.2
X
3 E
14144.0/44824.8
X
X
3 F
14140.2/44816.8
X
X
3 G
14141.0/44839.5
X
X
3 H
14139.4/44831.0
X
X
3 I .
14140.5/44807.6
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Table 2. Summary of Data Collection
10/04/81 Side-scan sonar recordings taken on Site 2 and fathometer traces
for 16 transects of Site 2 completed.
10/05/81 Diver observations, photographs, are taken at Site 2: Stations:
A, C, D, E, F, G, and H.
Side-scan sonar recordings taken on Site 1 and fathometer traces
for 17 transects of Site 1 completed.
10/06/81 Diver observations, photographs, and sediments samples taken at
Site 2: Stations: J & K, and Site 3: Stations: A, B, and C.
Side-scan sonar recordings attempted at Site 3 but halted when
Loran C was cut off.
10/07/81 Diver observations, photographs and sediment samples taken at
Site 1: Stations: A, B, C, D, E, F, and H.
10/08/81 Diver observations, photographs and sediment samples taken at
Site 3: Stations: D, E, F, G, H, and I.
-5-
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II Methods
The survey began with side-scan sonar recordings of the alternative
sites. The recordings were ground-truthed by dive teams who took photo-
graphs and sediment samples, and noted what biological communities were pre-
sent at each station.
A. Side-scan Sonar
The sonar recordings were taken with a Klein Model 421 that was towed
approximately six meters above the ocean floor at a speed of five knots.
The sonar was set to record 150 meters to each side so that a transect
covered 300 meters across (Figure 2). A Loran C X-Y plotter was used to
chart the sonar's position as the recordings were taken. Site location was
determined by use of Loran C coordinates obtained from NOAA Chart 11412,
26th Ed., July 1981.
B. Diver Observation
Diver observations (by SCUBA) were made at 25 stations within and
adjacent to the three proposed alternative sites. At each station two
divers were dispatched with a reference rod, a sediment sampling tube, and
two Nikonis cameras. One camera contained Tri-X 400, black and white film,
and the other camera contained high-speed Ectachrome film. To ensure that
photographs would be correctly matched to stations, a frame was shot of a
reference board marking site and station before each dive.
The initial dive plan called for each dive team to tie off a line from
the anchor line and swim a 20-meter radiu6 circle. The divers stopped at
six points along their route and took shots representative of the benthic
environment. tn addition, divers took shots of any interesting features or
biological activity they came across. Divers were instructed to include the
reference rod in all shots.
-6-
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* ~ -1 si t *' ? - i 'i *'1. 11 i j ; i
:K".*>';?.;i!'fh * 5: ^ -7»sf. f -j
*- »>h
¦7 ; r :??r.
» ¦ X ¦ ; / J .. : Vijsgi^jw ^ Ij-'i
#; -¦ to m
*J ' * r. * ' s I ^ * ./ ?-y }»-* j* ~tff r i
, r - i% »ik' i v
it
i- *'. ;-- - ^
- art .*£. i-' ^ - '
^r«r-v^^g
Direction of vessel
2. Xerox of Kline Model 4^1 side-scan sonaar recording^
gfyV ri
wi" "' }
f- hi.'" ?-k
-------
This procedure proved too time consuming, so the plan was modi fed so
that divers drifted along the bottom for approximately 10 minutes per sta-
tion. Pictures were taken at time intervals, and whenever something of
interest appeared. The distance covered during the dive depended on current
and wind.
After each dive, the divers were immediately debriefed and their sight-
ings recorded on a diver observation sheet (Figure 3). Table 1 gives exact
Loran C coordinates for all stations examined.
C. Sediment Samples
Sediment samples were taken using a plastic hand corer with rubber
stoppers. Where possible, divers would insert the core sampler as deep as
possible into the sediment, insert the top plug and then by sliding a hand
underneath, insert the bottom plug. Samples were taken only to confirm
visual observations and for comparison with other stations.
D. Bathymetry
Fathometer traces were taken concurrently with side-scan sonar record-
ings. The traces were correlated with Loran C coordinates and used with the
sonar recordings to determine diver stations at each site.
-8-
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SITE# STATION#
DIVER OBSERVATION SHEET
Diver names
Depth (feet)
Visibility (feet)
Description of physical characteristics:
Bottom composition: hard bottom General relief: flat
sand/shell ( or %) ledges
clay outcrops
silt patch reefs
other other
Ripples perpendicular to (heading)
Bottom sample taken: . #
Description of animal and plant communities: ¦ hard corals
soft corals
sponges
stars
urchins
lobster
crab
Other comments:
Camera# Roll# Frames
Figure 3. Sample Diver Observation Sheet
Camera# Roll# Frames
-9-
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Ill Results
A. Site l-Southwest
The side-scan recording of Site 1 revealed numerous changes in the
degree of reflectivity. This is caused by changes in the bottom sediment
composition varying from sand, to shell hash, to clay, to limestone. The
harder the benthic strata, the stronger the reflection. The recordings
indicate that a major portion of the benthic environment is hard-bottom or
shell hash. The recordings also reveal that there were no large ledges or
crevices (2 meters or greater) in or around the site. No wrecks or
artificial reefs were discovered. Side-scan sonar coverage exceeded the
boundaries of the site by about 0.3 nautical miles (Figure 4).
Depth at Site 1 varied from 18 to 20 meters, as revealed by fathometer
tracings. The traces showed several locations where small trenches, 1 to
1.5 meters deep, might exist, but divers were unable to verify them.
Seven stations were examined on Site 1 (Figure 5). Table 3 summarizes
the depth, visibility, bottom composition, and animal and plant communities
observed. At six of the seven stations, 1 A, 1 C, 1 D, 1 E, 1 F, and 1 H,
the divers discovered hard-bottom flat terrain with outcropings ranging up
to 2 feet in height. Highly diverse and productive hard-bottom communities
were present at all stations.
Divers found hard-bottom patch reefs at six, (1 A, IB, 1 C, ID, IE,
and 1 H) of the seven stations (Figures 6, 7, 8, 9, 10 and 11). For the
purposes of this report, a hard-bottom patch reef is defined as a benthic
environment containing a rich and diverse assemblage of invertebrates, fish,
and macroalgae. The divers on Station 1 F did not observe the diversity of
plant and animal communities as was seen at the other stations in Site 1
(Figure 12); the reason for this is unclear.
-10-
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Figure 4. Record of the path covered by the side-scan sonar at Site 1.
-11-
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tV
A
V
Figure 5. Site 1 (south west) dive stations.
A
N
Nautical Miles
.25 .5 .75
-12-
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Table 3. Summary of Diver Observations at
Site 1 (south west)
Depth
(feet)
Visibility
(feet)
Station
Bottom
Composition
General
Relief
Animal
and Plant
Communities
65
30-40
hard bottom,
some
sand/shell
flat with
outcrops
hard corals
soft corals
sponges
urchins
filamentous
algae
iacks
grunts
Halimeda
Caulerpa
65
40
sand/she11
flat with
outcrops
hard corals
soft corals
sponges
stars
urchins
mo Husks
grouper
jacks
reef typeo
Grecilleria
c
66
40
hard bottom
with
sand/shell
flat
hard corals
soft corals
sponges
urchins
crabs
reef fish
D
68
30
hard bottom
with (depres-
sions)
flat
hard corals
soft corals
sponges
stars
urchins
crab
reef fish
64
40-50
hard bottom
with (shell
hash over rock)
flat
hard corals
soft corals
sponges
groupers
reef fish
-13-
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Table 3. Summary of Diver Observations
Site 1 (south west) (Cont'd)
Depth
visibility
Bottom
General
Animal
(feet)
(feet)
Composition
Relief
and Plant
Station
Communities
F
68
30
hard bottom
flat with
hard corals
outcrops
soft corals
sponges
urchins
crabs
reef fish
algae
G (scratched due to time limitations)
H 68 30 hard bottom flat hard corals
soft corals
sponges
urchins
crabs
reef fish
algae
-14-
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Figure 6. Benthic environment at Station 1A, the northeast
corner of Site 1. Divers reported one rock
outcrop per 1,000 feet, and 6 inch depressions,
8 to 10 feet in diameter (2 per 1,000 feet).
Scale: White reference rod is black banded at
one foot intervals.
Figure 7. Station IB. Divers reported 40% of site
alternating sand and shell in windrow fashion
with small outcrops of reef habitat (1-1.5 feet
wide and 1-1.5 feet high). Entire station
contained sponges, macroalgae, and fish.
-15-
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Figure 8. Station 1C, southeast corner. Divers discovered
a hard-bottom habitat* with outcrops and
crevices (6-9 inches).
Figure 9. Station ID, southeast. Divers found a hard
bottom area with relief to 1 foot. Hard-bottom
was covered by a 1 to 1 1/2 inch sand layer.
-16-
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Figure 10. Station IE, south central area. Sixty percent
exposed rock with the remaining 40% consisting
of rock covered by shell hash.
Figure 11. Station 1H. Southwest corner. Hard bottom over
entire station.
-17-
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Figure 12. Station IF, southwest corner. Rocky bottom
covered in part by shell hash.
-18-
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B. Site 2-Near-in ¦:
As in Site 1, the side-scan recordings of Site 2 appeared as a mosaic
of highly reflectiye and less refective areas. This is again caused by
changes in sedimentscomposition. No areas of strong relief were seen in the
recordings, indicating an absence of large ledges or crevices, 2 meters or
greater, in or aroqpd the site. Further, no wrecks or artificial reefs were
revealed. Side-8C4jt sonar coverage exceed the boundaries of Site 2 by about
0.3 nautical miles.- (Figure 13).
The fathometer traces, run simultaneously with the sonar scan, revealed
that the depth of Sjte 2 varied from 11.5 to 15 meters. There is a gradual
downslope across Site 2 from east to west. The traces revealed a site of
generally low relief with minor ridges or plateaus.
Of the nine stations examined (Figure 14) in Site 2, only two, 2 F and
2 C, revealed diverge and productive hard-bottom communities (Figures 15 and
16). Six of the regaining seven stations, 2 A, 2 C, 2 D, 2 H, 2 J, and 2 K,
featured expanses of sand and a paucity of macroinvertebrate (Figures 17,
18, 19, 20 and 21). Based solely upon diver observations, there appears to
be a finger of exposed hard bottom extending from the eastern boundary of
Site 2 almost a mile into the site (Figure 14). Table 4 summarizes the
depth, visibility, bottom composition, and animal and plant communities
observed at the Site 2 stations.
-19-
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V.V5.V
Figure 13.Record of the path covered by the side-scan sonar at Site 2.
-20-
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2A
FigureW. Site 2 (near-in) dive stations.
N
Nautical Miles
U i .25 i -5 ¦: -75 1
I \ < \
-21-
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Figure 15. Station 2F, at center of Site 2. Terrain was
flat with numerous outcrops.
Figure 16. Station 2G, at eastern center of Site 2. Divers
found hard-bottom communities with outcrops and
small ledged up to 2 feet throughout the station.
-22-
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Figure 17. Station 2C, southwestern corner. Divers found a
featureless expanse of sandy bottom .
Figure 18. Station 2D, western side. Station barren except
for several sponges and macroalgae.
-23-
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% §f"
Figure 19. Station 2H, at northeast corner of Site 2.
Bottom environment almost barren.
Figure 20. Station 2J, at northwest corner of Site 2.
Sandy barren bottom.
-24-
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Figure 21. Station 2K, at northwest corner of Site 2.
Sandy bottom with some shell hash.
-25-
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Table 4. Summary of Diver Observations at
Site 2 (near-in)
Depth visibility Bottom General Animal
(feet) (feet) Composition Relief and Plant
Station Communities
A
52
15
sand/shell
flat
sponges
stars
crabs
sand dollars
live organisms
in shells
B
(scratched due
to time
limitations)
C
52
15-20
sand/shell
flat
sponges
stars
crabs
D
51
30
sand/she11
flat
sponges.
benthic macro
algae
F.
56
10
sand/shell
with hard
bottom ribbon
flat
soft corals
sponges
crabs
bottom fish
55
20
hard bottom
with sandy
areas
flat with hard corals
outcrops soft corals
sponges
stars
urchins
crabs
52-55
30
hard bottom
with some
sand
outcrops
and
sraal I
ledges
hard corals
soft corals
sponges
stars
urchins
crabs
banded shrimp
snapper
sea cucumbers
45
20-30
sand/shell flat stars
crabs
sand dollars
fish schools
-26-
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Table 4. Summary of Diver Observations at
Site 2 (near-in) (Cont'd)
Station
Depth
(feet)
Visibility
(feet)
Bottom
Composition
General
Relief
Animal
and Plant
Communities
J
48
10
sand/shell
flat
stars
crabs
K
55 ?
20
sand/shall
flat
stars
-27-
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C. Site 3 - Deepwater
Complete side-scan sonar recordings and fathometer traces were
unobtainable due to the shutting down of the 14000 and 40000 Loran C signals
on October 6, 1981. The Loran C network was decomissioned by the Coast
Guard for maintenance. This forced a change in the method of selecting dive
stations. Instead of picking stations off the sonar recordings, stations
were picked from corner to corner transects (Figure 22).
Fathometer tracings were taken on the transects and showed Site 3 to
vary in depth from 23 to 25 meters. The bottom slopes gradually from north-
east to southwest.
Ten stations were examined at Site 3 (Figure 22). At half of the
stations (3 A, 3 D, 3 F, 3 G, and 3 I) hard-bottom communities were
discovered. (Figures 23, 24, 25 and 26). Divers found a thin layer of silt
followed by a one-to-three inch layer of sand and shell hash over
hard-bottom at Stations 3 A, 3 C, 3 F, 3 G, and 3 I. Bottom fish, sea
robins, lizard fish, grunt, and jacks were found at all stations except 3 6.
Stations 3 A, 3 B, 3 E, and 3 H revealed disolate flat terrain (Figures 27,
28 and 29). Based on diver's observations and the photographic record, the
communitites discovered at Site 3 were not as diverse as those found at the
other two sites. Table 5 summarizes the diver's observations of the
stations Site 3.
-23-
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Figure 22. Site 3 (deep water) dive stations.
N
Nautical Miles
0 .25 .5 .75 1
1 l - 1 J
-29-
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Figure 23. Station 3D at center of Site 3. Divers found one
hard coral (1 foot high) per 1,000 feet, and
multi-bodied sponges.
a. # A
gr ¦
¦M
Figure 24. Station 3F, at southeastern corner of Site 3.
Hard-bottom found covered with 2 inches of fine
-30-
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¦
* % ~S
;
' ,'¦%! . JO
V.
B«i*wSS|e
HB
* " s' W.
F .
H
Figure 25. Station 3G, at northwest corner of Site 3.
Divers discovered a hard-bottom covered by a thin
layer of silt and sand/shell hash.
I, I U ms ¦ JU
ggp
Figure 26.. Station 31, at far southeast corner of Site 3.
Station had a layer (1 to 3 inches) of sand/silt
over hard bottom. Depressions about 6 inches
deep with exposed rock.
-31-
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Figure 27. Example of silt cloud caused by diver moving his
hand just above the bottom, Station 3 I.
Figure 28. Station 3H, at northwest corner of Site 3.
Bottom was sandy with a silt layer.
-32-
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Table 5. Summary of Diver Observations at
Site 3 (deep water)
Depth
(feet)
Visibility
(feet)
Bottom
Composition
General
Relief
Station
Animal
and Plant
Communities
88
50
sand/shell
flat
hard corals
soft corals
sponges
crabs
sea robins
small fish
schools
B
95
50
sand/shell
flat
crabs
sand dollars
C
90
60
silt over rock
flat
soft corals
stars
sand dollars
sea robins
red algae
D
90
25
sand over rock
flat
hard corals
with
soft corals
small
lizard fish
outcrops
bottom fish
sand dollars
E
82
30
sand/shell
flat
stars
mollusks
bottom-fish
pinshells
F
85-90
40
sand over rock
flat
hard corals
sponges
stars
crabs
grunts
jacks
sparse algae
90
20
silt over
sand over rock
flat
hard corals
sponges
grunts
j acks
microalgae
mollusks
-33-
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Table 5. Summary of Diver Observations at
Site 3 (deep water) (Cont'd)
Station
Depth Visibility Bottom General
(feet) (feet) Composition Relief
Animal
and ?lant
Communities
85
40-50
silt over-sand flat
stars
crabs
grunts
87
20-25
silt/sand
over rock
flat
hard corals
soft corals
sponges
stars
grunts
1 acks
snapper
-------
D. Site Comparisons
The three sites are similar in their lack of relief and in the gradual
slope deepening from northeast to southwest. In terms of diversity of the
benthic environment, the divers agreed that richest assemblage of organisms
was at all stations in Site 1 and at Stations 2 F and G in Site 2. The
remainder of Site 2, and all of Site 3 appeared to be equal in the paucity
of life.
As a matter of reference, divers were asked to look at the photographs
of hard-bottom patch reefs taken for the Mote Marine Laboratory study on the
existing interim dump site. Divers agreed that only the environments at
Stations 2 F and 2 C resembled the diversity and quantity of life found in
those photographs.
-35-
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APPENDIX I
SIDE SCAN SONAR RECONNAISSANCE
OF
ALTERNATIVE DREDGE SPOIL DISPOSAL SITES
TAMPA,FLORIDA
Prepared for:
U.S. Environmental Protection Agency
Criteria and Standards Division
Ocean Disposal Environmental Impact Statement Task Force
Washington, D.C. 20460
Prepared by:
Interstate Electronics Corporation
Oceanic Engineering Operations
1745 Jefferson Davis Highway
Arlington, Virginia 22202
-------
SIDE SCAN SONAR RECONNAISSANCE
OF
ALTERNATIVE DREDGE SPOIL DISPOSAL SITES
1.0 INTRODUCTION
On October 5 and 6,1981, a side scan sonar reconnaissance was conducted on
two of three alternative dredge spoil disposal sites offshore Tampa, Florida. A survey
of the third site (Alternative Site #3) was precluded when the Loran-C network for
this region was decommissioned for maintenance.
The purpose of the reconnaissance survey was to determine if hard bottom areas
are present within each potential site boundary. The importance of hard bottom areas
lies with the biological communities associated with them. Preferred disposal sites
would be those devoid of significant biological communities.
Hard bottom assessments were accomplished through the acquisition of side
scan sonar data. These data were augmented with bathymetric data collected at the
same time. Based on the side scan records, several locations within the sites were
selected for SCUBA diver operations. Qualitative observations made by EPA divers
served to determine the extent and composition of biological communities, if present,
and to verify features interpreted from the side scan sonar records.
2.0 GEOGRAPHICAL SETTING
All three alternative disposal sites are located directly offshore of Tampa Bay
(Figure 1). Alternative Site #1 is located 16 nm west of Anna Maria Island in approximately
20 m of water. Alternative Site #2 lies in 15 m of water to the north of #1 and is
13 nm west of Egmont Key. Alternative Site #3 is farthest from shore being 23 nm
west of Mullet Key. Located in 27 m water depths, it is also the deepest site.
None of these areas have previously received dredge spoil materials.
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3.0 SURVEY PROCEDURE
Prior to beginning side scan sonar operations, two (of four available) Loran
C stations were selected to provide the navigation coverage Loran-C lines of position
for the Whiskey and Yankee stations ware chosen because their angle of intersection
ov
was near normal (90 ) and each station was locally known for consistent local reception.
The accuracy of this network was checked and calibrated using the following
procedure:
Latitude and longitude of the Tampa Bay Lighted Whistle Buoy (Whistler),
which marks the approach to Tampa Bay, was obtained from the NOAA
Coast Pilot 5 and plotted on NOS Chart 11412. The plotted position was
compared to its printed position on Chart 11412.
Loran-C lines of position values (constant time difference lines or TD*s
for Stations Whiskey and Yankee) for the Whistler were scaled from
the chart and compared to measured values obtained with the Loran-C
unit aboard the survey vessel.
Observed differences in TO values were used to establish the network
error. Since the measured values were accepted as valid a,-0.6 microsecond
adjustment was applied to Station Whiskey values when necessary. Station
Yankee proved accurate as charted.
Once the network errors were determined, preplots of the navigation grid for
each alternative site were constructed utilizing a flat bed plotter. This same plotter
permitted real-time tracking and plotting of survey vessel position. Position fixes
were simultaneously marked on all data records every two minutes. As survey vessel
speed averaged 5.5 to 6.5 kts., fixes were obtained every 300-400 m.
Side scan sonar transects were run along TD lines to facilitate the helmsman's
task of steering straight, parallel lines. To maximize the quality of the data records,
the direction in which the transects were run was determined by the daily sea conditions.
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Track lines in Alternative Site #1 were run in a Northwest-Southeast orientation
(Figure 4) while those of Alternative Site #2 were run in a Northeast-Southwest direction
(Figurel3). Line spacings were 250 m at Site #1 dSd 340 m at Site #2 as a consequence
of this procedure. Typically, 16 transects were run over each of the two potential
disposal sites, providing bottom coverage for an approximate four square mile area.
The side scan was operated on a 150 in/channel scale this a swath of 300 m of bottom
was insonified along each transect. Bathymetric data were continuously recorded
during the entire survey.
4.0 INSTRUMENTATION
Navigational control was provided by an EPSCO C-NAV XL Loran-C. This unit
was coupled to an EPSCO C-PLOT 2 flat bed plotter to provide real-time vessel location
plots.
A Klein Associates Model 421 Side Scan Sonar System was utilized to ascertain
potential hard bottom areas within each site. Its components included a 100 kHz
owfish, a tow cable and a two channel wet paper graphic recorder.
Water depth measurements were recorded by a Raytheon Fathometer Model
I 2460 W operating with a 41 kHz hull mounted transducer.
The field survey was conducted aboard the charter vessel MAKAI. This vessel,
I '
111.3 m Hatteras is homeported in Madiera Beach and operated by her owner,
Japtain Troy Coston.
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5.0 INTERPRETATION
Upon completion of the survey for each potential disposal site, the side scan
records were reviewed by Interstate Electronics and EPA personnel to select sites
for subsequent diving operations.
The intent of the diving operations was two-fold: to ground truth the side scan
sonar observations and to provide a qualitative biological assessment of conditions
throughout the sites.
Dive sites were chosen in order to identify selected features appearing on the
side scan records for interpretive purposes and to provide spatially well distributed biological
observations. The number of locations was limited to that which could be comfortably
accomplished in one dive day.
Alternative Site #1
This site can be generally cnaracterized as a relatively flat area in 18-20 m of
water. The bottom gently slopes downward to the south and exhibits no significant
relief. A few small but persistent troughs are evident within the site but are of insignificant
negative relief and slope (-1.2 m over widths of 50-75 m).
The side scan records typically displayed patchy and irregular dark acoustic returns
interspersed with areas of lighter returns. No definitive acoustic shadows were apparent.
Such records were characteristic of the entire site but were moist obvious along the
eastern edge of the site. Similar records were collected in the northwestern region
but were of lesser contrast and intensity. Records for the central portion of Site #1
were even less distinctive. This kind of record is indicative of changes in bottom composition
(material of different composition provides returns of varying acoustic strength) or
of low amplitude bedforms (small outcrops, low hummocks slightly exposed reef or
rock structure, etc.). At times (particularly in the east and west boundary region)
these variations in the record^ were distinctly linear in nature which suggests low
sand waves, large ripples or "windrows" of sediment of noticeably different composition.
-An-
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Diver observations confirmed these interpretations. Hard bottom with very
little topographic relief was observed throughout the site. Dives made in the east
and southeast sections of the site revealed areas of outcrops 0.3-0.5 m high on a flat
hard bottom. Areas of sand/shell and shell hash, sometimes as long windows and other
times as lenses, punctuated the sea flow. Small 0.15 m depressions were also observed
on one dive in the northeast corner.
Outcrops were less evident in the central portion of the site as were distinct
changes in sediment composition or cover. Observations made in the western sections
were similar to those in the east revealing a number of outcrops over hard bottom.
No variation in bottom composition was commented upon however on dives in this
area.
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Alternative Site #2
Site #2 is considerably less monotonous in topography than is Site #1. Water
StlMloUXT
depths are here, typically ranging in depth between 11.5-15 m.
A generally flat and slightly rolling or undutaling bottom characterizes the central
portion of the site. It is surrounded in varying degrees by areas experiencing more
dynamic elevation changes. These latter areas exhibit variations in elevation on the
order of 3 to 4 m over 250 m distances. Certainly, they are small in magnitude in
absolute terms, but are very distinctive in a relative comparison of the two sites.
This kind of slope would be difficult for a diver to readily discern and indeed no such
observations were recorded. Nonetheless, the crests of these more positive features
appear to take a northwest-southeast orientation and may be sand waves.
The side scan records appear chaotic with distinct and intermingled reflectors
superimposed on surfaces that are themselves alternating in the intensity of the reflected
signal. No significantly abrupt topographic features, either positive or negative in
elevation, are observed. Acoustically this area appears similar to Site #1. The recortfe
are thought to show areas with crisp definite changes in sediment type, distribution
and low exposure of bedforms much more so than Site #1. Commonly, but not invariably,
the dark reflectors appear to be associated with flatter (hard?) topography. This may be
the result of a thinly covered hard bedding plane.
Interestingly, the divers' logs indicate that most locations were flat, and of a
sand/shell matrix. Few observations mentioned changes in sediment character or texture.
As some dives on the eastern boundary did indicate outcrops (0.6 m high), exposed
ledges, rubble, and some windrowing of sediment or shell, it is felt from review of
the records that more areas on the east boundary are of a similar nature, than have
been reported during the diving operations.
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APPENDIX IT
The Principle of Side-Sean Sonar
Conventional echo sounding techniques employed to record a profile of the
sea floor beneath a survey vessel collect data in a very narrow path, or ^swath-width,"
along each survey line. The nature and magnitude of relief between lines is therefore
lacking, and bathymetric maps are prepared based upon interpolation of sea floor
topography in these unsurveyed areas. Hie concept of using acoustic transmissions
in a direction other than vertical led to the development of the side-scan sonar.
Unlike the echo sounder which employs a vertical beam (or cone) of sound, side scan
sonars generate a fan-shaped beam which sweeps from the vertical out to both the
right and left of the vessel track. Echos returning from objects increasingly further
from the vessel track appear on a dual-channel analog record as "returns" increasingly
further from the center of the record (the path directly beneath the vessel). A good
analogy to this type of survey is that of a person with a flashlight in a darkened room
moving the light beam away from his foot. As the beam sweeps across the floor reflections
are returned which reveal objects by both reflected light and by the shadows the
objects cast. In the case of side-scan sonar, sound is substituted for light, and the
objects with positive relief return an echo, while depressions appear as acoustic "shadows,"
areas which are not "insonified," and hence return no sound energy.
Interpretation of Side-Scan Sonar Records
From the above, it is clear that a large object rising well above the sea floor
would appear as a "strong return," that is, as a dark mark on the analog record upon
which the increasing strength of the echo is reflected in the increased darkness of
the printed signal. With examination of the size and shape of the printed signal, and
considering the size of the acoustic shadow "behind" the object, it is possible to estimate
it's height and length.
Care must be taken when absolute dimensions of objects appearing on side-scan
records are required. As in echo sounding, there is an inherent distortion imposed
upon the analog record. This exaggeration is the result of several factors - ship's
speed, scale (swath-width) settings, paper speed through the recorder, and others.
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