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Introduction
The shallow marine shelf surrounding the Commonwealth of Puerto Rico has an area of about
3,500 km2 within the 200 m isobath, or nearly two-thirds that of the land area
of the island. Deposits of sand and gravel on the insular shelf are of major importance to
Puerto Rico because of their potential for low-cost development and because suitable
onshore sources have been depleted. On an island where more than 85 percent of the
population lives within 7 km of the sea and is dependent on the tourism that its beaches
attract, it is necessary that scientific data related to oceanographic processes acting in
the coastal zone and on the insular shelf be available to help formulate public policy
regarding residential and commercial development along the coast, beach replenishment, and
future utilization of marine resources.
Hurricane Hugo struck Puerto Rico on September 18, 1989, with maximum sustained winds of
225 km/hr and a minimum sea-level pressure of 934 mb causing extensive damage to the
coastal infrastructure (Rodríguez and others, 1994). Studies
were conducted to assess the effects of the hurricane (hereafter referred to as Hugo) on
three critical coastal resources: beaches, offshore sand deposits, and coral reefs (Schwab and Rodríguez, 1992). Schwab and others
(1996b) presented the findings of high-resolution seafloor mapping investigations
related to the Hugo study conducted on the northern insular shelf, offshore of the town of
Luquillo (Fig. 1).
The Luquillo study area was directly in the path that Hugo followed (Fig.
2). A combination of storm-induced waves and sea-level surge caused minor to moderate
erosion of the beaches in the Luquillo area (Rodríguez and Webb,
1990; Rodríguez and others, 1994). Considering the intensity of Hurricane Hugo, the
impact on coastal erosion rates was surprisingly limited. However, historical
shoreline-change data shows that some shoreline segments in the vicinity of Luquillo were
eroded during the past 50 years (Thieler, 1993; Schwab
and others, 1996a). In an attempt to determine the ultimate fate of sediment removed
from the coastal zone, detailed seafloor mapping was conducted using high-resolution
sidescan sonar, and seismic-reflection techniques followed by bottom sediment sampling and
sedimentologic analysis. Prior to these investigations, it was generally unknown where the
sand eroded from the coast by major storms had gone, or if the material was recoverable.
The purpose of this report is to present the findings of Schwab and
others (1996a, 1996b) in digital form. Storing data digitally has numerous advantages
over analog methods. Once the data is in digital form creation of maps at varying scales
or using subsets of data is accomplished quickly and easily. In addition, transferring of
data between users becomes simpler. The exchange of digital data allows a wider range of
options in terms of analysis and interpretation of those data. In areas of long-term
studies or monitoring studies, data collected at different times can be merged and/or
directly compared. The full resolution sidescan imagery presented in this report are
contained in the maps directory of the Openfile 98-204 CD-ROM in ARC/INFO format.
Study Area
Reconnaissance seafloor mapping studies based on widely spaced seismic-reflection data and
bottom samples have shown that the sediment cover on the narrow insular shelf around
Puerto Rico is patchy and diverse with little lateral continuity (e.g.,
Pilkey and others, 1987). This variability in sediment distribution is thought to be a
reflection of the wide range of physical and biological factors affecting sedimentation on
this low-latitude insular shelf (Schneidermann and others, 1976).
The steep and narrow nature of the northern insular shelf and its exposure to high-energy
wave conditions are thought to promote rapid cross-shelf sediment transport (Bush, 1977, 1991; Pilkey and others, 1978; Grove and others, 1982; Pilkey and others, 1984).
The shelfbreak in the Luquillo study area occurs at a water depth of about 70 m and
borders the landward slope of the Puerto Rico trench. The shelf off Luquillo ranges in
width between 4 and 10 km and has a slope that is steep (0.3° to 0.7°) in comparison to
typical continental shelves. The bathymetry of the study area is relatively irregular due
to outcropping Pleistocene eolianite deposits (Kaye, 1959). Two
well-developed submarine canyon systems extend far onto the shelf in the study area, one
off the mouth of the river Río Mameyes and another 4 km to the east (Fig. 1). The most important factors affecting
sedimentation in the Luquillo study area are thought to be shelf bathymetry, wave energy,
and coral reefs and other rock outcrops on the sea floor.
Historical vertical aerial photographs of the Puerto Rico shoreline were examined using
the methodology outlined in Danforth and Thieler (1992a, 1992b)
for the purpose of delineating shoreline change from the period of 1901 to 1987 (Thieler, 1993). Although the temporal component of the data varies in
the Luquillo study area (Thieler and Danforth, 1993), a general
trend of minor erosion on the northwest side of the headlands with accretion on the
southeast sides has occurred on Punta Miquillo, Punta Picúa, and possible Punta
Embarcadero (Fig. 3). The shoreline of the embayment
between Punta Percha and Punta Embarcadero is eroding at rates as high as 2 m/yr while the
shoreline along the embayment between the town of Luquillo and Cabeza Chiquita is, in
general, eroding at rates of 1 to 2 m/yr on the west side and accreting at similar rates
on the east side (Fig. 3). The relative importance of the longshore versus offshore
components of sediment transport in these shoreline change rates is unknown.
Although Puerto Rico lies within the plate boundary zone between the North American and
Caribbean plates, an area of high seismicity and tectonic activity (Sykes
and others, 1982; Mann and Burke, 1984), evidence of active
faulting was not evident on data collected during this study.
Methods
The application of high-resolution seismic reflection and sidescan sonar techniques to
seafloor mapping is essential to adequately determine the complex spatial variability of
the bottom morphology and sediment texture. Therefore, marine geologic surveys were
conducted in April/May 1991 aboard the research vessel JEAN A using a sidescan sonar
system, seismic-reflection profiling systems, and a surface grab sampler. These acoustic
data are archived at the U.S. Geological Survey, Woods Hole Field Center of the Coastal
and Marine Geology Program, Woods Hole, Mass. Additional bottom samples were collected in
June 1992 using the research vessel BORIKEN.
The sidescan sonar survey over the Luquillo insular shelf was conducted using a Klein 100
kHz sidescan sonar system; total swath width per trackline was 200 m, and tracklines were
spaced to provide continuous sidescan sonar coverage of the sea floor. The sidescan data
were logged digitally at a sampling rate that resulted in a 0.1 m pixel size in the across
track direction following the methodology outlined in Danforth (1997).
A median filtering routine (Malinverno and others, 1990) was
applied to the sidescan data to remove speckle noise, resulting in a 0.4 m pixel size. The
data were then further processed and mosaicked using procedures described by Danforth (1997). This mosaic was then used as a basemap for the
subsequent sampling phase of the investigation. Additional digital processing and
mosaicking was carried on in 2 stages. The first stage used the routines developed by Chavez (1986) as modified by Paskevich (1992)
for application to high-frequency sidescan sonar imagery. This mosaic (Gowen
and others, 1993) covered a 13 km2 area of the Luquillo insular shelf (Fig. 1) and represents the digital values used for more
extensive analyses of the geophysical and sedimentological data described later in this
paper. The second stage of digital mosaicking included the complete sidescan dataset which
is presented in this report. The initial preparation of the sidescan sonar tracklines is
the same as described by Paskevich (1992) with the digital mosaicking accomplished using
the PCI Remote Sensing software package as described by Paskevich
(1996). For the complete Luquillo mosaic, the sidescan sonar data were mapped with a
resolution of 2 m/pixel. Due to the large area covered by the sidescan survey and
limitations on computer disk space and memory, the digital mosaic was broken down into 3
overlapping sections. Once all sections were completed, they were then merged into a
single image.
Concurrent with the acquisition of the sidescan sonar imagery, approximately 650 km of 3.5
kHz and Huntec Sea Otter seismic-reflection profiles were collected. These data were
recorded using an analog EPC recorder. Bathymetry and sediment thickness of Holocene-aged
sediment were determined from these seismic-reflection data.
Bottom sediment samples were obtained using a Shipek grab sampler for the 1991 field work
and a Van Veen sampler in 1992. Grain size analysis of the samples was conducted using a
combination of wet sieve and Coulter counter techniques following the methodology of Poppe and others (1985). Analyses of calcium carbonate content were
performed at Puerto Rico's Department of Natural Resources sedimentological laboratories
using methods described by Carver (1971).
Ship navigation was conducted using a shore-base Miniranger Falcon IV transponder
navigation system for the 1991 investigation and a Global Positioning System (GPS)
satellite navigation system in 1992. Using these navigation data, the seismic-reflection
profiles and 1991 bottom sample locations are accurate to within ± 5 m. The sidescan
towfish, however, was not navigated independently of the ship, thus, an additional maximum
error from 15 m in shallow water to approximately 100 m in deeper areas exists in the
sidescan imagery. Navigation on the 1992 sampling cruise aboard the RV BORIKEN had an
accuracy of ± 100 m.
Mapping Results
Numerous maps have been completed in a 105 km2 area of the Luquillo insular
shelf. These maps show sidescan sonar imagery with ship tracklines (Fig.
4), bathymetry (Fig. 5a,b), bottom sample locations (Fig. 6a,b), Holocene sediment thickness (Fig.
7a,b), and a Pleistocene structural horizon (Fig. 8a,b). On
these sidescan images, areas of high acoustic backscatter intensity are white, and areas
of low backscatter are black.
The seafloor in the study area is dominated by a series of eolianite ridges which are
clearly expressed on the sidescan imagery as areas of high acoustic backscatter intensity
with a high degree of backscatter variance (Fig. 9). These ridges
trend roughly east-west across the study area. Although samples of these rock ridges were
not collected for verification, they are thought to be eolianite formed during Pleistocene
time (Kaye, 1959). Similar eolianite dunes are common along the
northern coast of Puerto Rico and crop out to the east on islands northeast of Cabo San
Juan. Using seismic-reflection profiles, the Pleistocene surface, which is correlative
with the surface of the eolianite ridges, can be traced as a strong reflector under the
Holocene sediments (Fig. 10) which, in turn, fill in depressions
on the surface.
The surficial sediment can be subdivided into four facies (Fig. 11)
based on relative degree of acoustic backscatter shown on the sidescan sonar images (Fig. 9) and sediment texture (Fig. 12).
These are: 1) low-backscatter sand - relatively fine-grained sand found in the nearshore
region in areas of relatively low backscatter; 2) high-backscatter sand - relatively
coarse-grained reefal-derived sands and gravelly sands found in areas of relatively high
acoustic backscatter; 3) low-backscatter silt - silt to sandy silt found in areas of low
backscatter on the outer shelf; and 4) intermediate-backscatter sandy silt - sandy silt to
silty sand found in an area of relatively moderate-high backscatter along the shelfbreak.
Textural data from samples are presented in Table 1.
The sand-size fraction of the sediment samples were analyzed for their skeletal carbonate
(coralline algae, Halimeda, Echinodermata, coral fragments, Porifera, Molluska, Gorgonia,
Annelida, Bryozoa, and Foraminifera), non-skeletal carbonate (ooid, aggregate, and
peloid), and terrigenous (quartz, feldspar, and rock fragment) components. The average
compositions of the sand is presented on Figure 13, where they
are subdivided into samples collected from the nearby beaches, those from the
low-backscatter sand facies collected from the inner shelf, those from the low-backscatter
sand facies in close proximity to the eolianite (low backscatter "reefal"
sands), those from the high-backscatter sand facies, and those from the low-backscatter
silt facies.
The increase of the coral fragment component in samples from the beachface and from areas
in close proximity to the eolianite ridges in comparison to that of the low-backscatter
inner-shelf sand is probably due to the proximity of the sediment to coral reefs that
fringe the coast and to corals that are growing on the eolianite (Fig.
11). However, the compositional (Fig. 13) and textural (Fig. 12) similarities of the low-backscatter sand from the beach to
the eolianite suggests that they are the same deposit.
The average composition of the high-backscatter sand acoustic facies (Fig.
13) is also similar to the composition of low-backscatter sand facies with an expected
increase in the Halimeda component, which grows on the eolianite, and an unexplained minor
reduction in the Echinodermata and Porifera components. However, the high-backscatter sand
facies is coarser grained than the low-backscatter sand facies (Fig.
11). Chemical analyses of the low- and high-backscatter sand facies show that the
samples are greater than 85 percent calcium carbonate with all but two samples greater
than 93 percent (Fig. 14). This indicates that there is little
input of terrigenous sediment into this area from the local rivers and that most of the
sandy sediment is locally (biologically) derived from carbonate-producing organisms which
grow on the local hardgrounds. Terrigenous deposition in the study area appears to be
concentrated in the low-backscatter silt found on the outer insular shelf and is expressed
by the relative increase of the terrigenous component of the sand-fraction (Fig. 13) and calcium carbonate contents less than 71 percent (Fig. 14).
Analysis of Sidescan Sonar Imagery
The similarity of the composition of the low-backscatter sand and high-backscatter sand
acoustic facies strongly suggests that the relative backscatter intensity displayed on the
sidescan sonar imagery is a function of sediment texture (Fig. 12).
Sidescan sonar imagery is a graphical representation of how sound interacts with the sea
floor. Relative backscatter intensity is a function of a variety of factors including,
seafloor bathymetry, seafloor roughness, the acoustic impedance contrast between the water
and the sea floor, the frequency of the sonar used, and the angle of incidence of the
sound wave front (Urick, 1983). However, there have been few studies
designed to evaluate the relative importance of these factors (e.g.,
Mayer and others, 1993). Geophysical and sedimentologic data were analyzed over part
of the imagery collected over the Luquillo insular shelf (Fig.
1) in order to quantitatively test the relation between relative backscatter intensity
and sediment texture (Gowen and others, 1993). This area was chosen
because of its manageable size (13 km2), because it contained a broad range of
backscatter intensities which were representative of the entire survey area, and because
sediment samples were located in many different backscatter regimes.
Statistical parameters of the initial digital mosaic (Gowen and others,
1993) were collected in 5 x 5 m areas, centered around each sediment sample location.
This 5 m digital "sample size" was chosen due to the navigational accuracy of
the sediment sample locations (± 5m). The statistical parameters of digital number (DN)
values in varied acoustic backscatter regimes were analytically compared with a suite of
textural, compositional, and geochemical properties of the associated sediment samples
(Gowen and others, 1993). The statistical parameters of the sidescan sonar data used in
the analyses were: the mean, standard deviation, and skewness of the DN values in the
sampled area. The sedimentologic variables used in the analyses were: the mean, median,
standard deviation (sorting), skewness, and kurtosis of the sediment grain size
distribution; mathematical combinations of the above; cumulative percentiles of single
phi-units; percent gravel, sand, silt, and clay; percent calcium carbonate content; and
percent coralline algae, Halimeda, Echinodermata, coral fragments, Porifera, Molluska,
Gorgonia, Annelida, Bryozoa, Foraminifera, unknown skeletal fragments, ooids, aggregates,
peloids, quartz, feldspar, rock fragments, and unknown terrigenous fragments. Variables
not used in the analysis included seafloor topography because the data analyzed covered a
relatively flat sediment-covered sea floor; seafloor roughness due to a lack of data,
vehicle altitude (angle of incidence) because the tow-fish was flown approximately 13 m
above the sea floor in this area of the survey with little variation; and subbottom
penetration because it was assumed that the high-frequency signal would not penetrate the
sea floor. Although a rigorous statistical analysis of the data was not conducted, a
comparative graphical analysis of all variables yielded poor correlations with the
exception of the correlation between mean grain size and mean DN value for the 5 x 5 m
sample (Gowen and others, 1993) (Fig. 15). Note that the
exponential relationship suggests that the DN values for finer grained sediments are less
sensitive to changes in mean grain size than are those of coarser grained sediments.
To test this relationship, visual inspection of acoustic facies boundaries were conducted
using scuba diving techniques (Gowen, 1993) and additional sediment
samples were collected in 1992 and analyzed using the same techniques as the 1991 data
(Gowen and others, 1993). Despite potential changes in the sediment distribution over the
preceding year, and the reduced navigational accuracy, these additional sedimentologic
data reinforced the validity of the original correlation (Gowen and
others, 1993): grain size is the dominant factor controlling relative backscatter
intensity with the 100 kHz sidescan sonar system in the study area.
Discussion
Compositional and textural data augmented with the sidescan imagery and sediment isopach
maps (Fig. 7a,b) suggests that fine-grained sandy sediment
derived from the local hardgrounds and any sediment removed from the shoreline resides on
the inner shelf, generally inshore of the eolianite, forming the low-backscatter sand
facies. Seismic-reflection profiles show that a sand deposit, in places up to 20 m thick
(Fig. 7b), has formed landward of the eolianite. The sandy nature of its surface suggests
it is a potential economic resource. In addition, a series of northeast-southwest-trending
"channels" that cut across the eolianite (Fig. 8b),
most likely subaerially formed, have been partially filled by the low-backscatter sand (Fig. 10).
A series of elongate areas of high-backscatter on the sidescan imagery seaward of the
eolianite ridges are composed of the high-backscatter sand facies (Fig. 11). The sediment
composition and sidescan imagery (Fig. 16) show that these
coarse-grained sand and gravelly sand deposits were derived from the eolianite and
transported in a general offshore direction. These high-backscatter "wisps" on
the sidescan sonar images are oriented northeast/southwest, roughly perpendicular to the
shoreline (Fig. 11). Although sediment cores are not available,
continuous internal reflectors on subbottom profiles suggest that these high-backscatter
sands form relatively thin deposits that form elongate furrows in the surrounding
low-backscatter silt facies. Similar features (termed rippled scour depressions) have been
described in a variety of shelf and nearshore settings and are typically found in a
shore-normal orientation (e.g, Cacchione and others, 1984). How
these furrows initially form is unknown, but studies suggest that coarse-grained sediment
plays an important role in furrow initiation, as small furrows appear to form where rows
of mobile coarser grained sediment have been aligned by bottom currents (Flood,
1981). The rippled scour depressions may then develop as a result of enhanced erosion
created by the greater surface roughness, with localized abrasion or scour around
particles on the bed (Allen, 1969). Both Cacchione and others (1984)
and Black and Healy (1988) suggest that bottom stresses due to waves
are probably the major component contributing to the resuspension of bottom materials.
However, Cacchione and others (1984) suggest the concurrent
transporting action of a quasi-steady current (such as a current flowing generally seaward
across the inner shelf during storm-induced downwelling events) is a more likely cause for
the elongate furrows, while Black and Healy (1988) suggest that the
ripples and the furrows are formed as a direct result of bed mobilization by convergent
waves.
Although the oceanographic processes responsible for the offshore net sediment transport
direction on the Luquillo insular shelf are unknown, the northeast/southwest orientation
of the high-backscatter wisps seaward of the eolianite ridges (Fig.
11) show that it is dominant throughout the study area. It is reasonable to infer that
the enhanced bottom stresses are capable of resuspending and transporting coarse-grained
sand from the eolianite ridge seaward onto the low-backscatter silt acoustic facies, while
the inner shelf fine-grained sand is effectively trapped in the nearshore zone by the
eolianite ridges. We speculate that the high-backscatter wisps seaward of the eolianite
ridges, although depositional features, are maintained as depressions due to enhanced
erosion created by the relatively greater surface roughness of the high-backscatter sand
facies in comparison to the surrounding low-backscatter silt facies; enhanced erosion
retards deposition of terrigenous silt within the depressions.
Summary
Applications of image analysis techniques used in this study include seafloor mapping and
resource evaluation. This methodology enables the creation of sediment texture
distribution maps that can be used to locate and evaluate natural resource potential. The
spatial distribution of sedimentary facies can be mapped in much greater detail by
combining sidescan sonar data and conventional sampling. In addition, the strong
correlation found between mean grain size and mean DN value (Fig. 15)
introduces the potential to false-color enhance the sidescan imagery in order to present
the acoustic and grain-size data together in a single image (Gowen, 1993).
These investigative and analytical techniques can be extremely useful to insular
territories and nations that need to effectively utilize their offshore resources. Placing
the sidescan sonar imagery, bathymetry, Holocene sediment thickness, sediment samples, and
Pleistocene structural horizon into ARC/INFO coverages allows the combination of these
differing data types to identify the distribution and compute the volume of this potential
offshore sand resource. In addition, this digital format of the data is easily accessible
for use in a variety of applications.
Acknowledgements
The success of this program was based on the broad range of technical support provided by
the USGS Puerto Rico Marine Geology Program and the Program for the Study of Developed
Shorelines directed by Orrin H. Pilkey, Duke University. Navigation support of offshore
surveys was provided by Barry Irwin and Juan Trias and shipboard support was provided by
Milton Carlo. Helpful reviews of the manuscript were provided by Bill Danforth, Jane Denny
and Dave Twichell. In addition, we appreciate the cooperation of the captain and crew of
the research vessel JEAN A, provided by the Department of Natural Resources, Commonwealth
of Puerto Rico. Funding for this program was provided by the Office of the U.S. Secretary
of the Interior, the Honorable Manuel Lujan.
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