Final Sub-Bottom Seismic Survey Report:

Offshore Sand Borrow Site Assessment

St. Johns County, Florida

Submitted to

By

January 22, 2009

Gary A. Zarillo, Ph.D., PG and John Bishop, M.S. Scientific Environmental Applications, Inc. (S.E.A.) 5575 Willoughby Dr. Melbourne, Florida 32934 Ph/fax: 321.254.2708 Email seapp1@aol.com

Jeffrey R. Tabar, P.E. PBS&J, Coastal & Waterways Division 5300 West Cypress Street, Suite 200 Tampa, Florida 33607

Executive Summary

A sub-bottom seismic survey was conducted over several shoal features in shallow waters

offshore of St. Johns County to determine the potential volume of beach quality sand that may be

present. The shoals were selected using the existing data sets described in the earlier reconnaissance

report to St. Johns County. These data sets were assembled from the U.S. Army Corps of Engineers,

the Beaches and Shore Program of the Florida Department of Environmental Protection and the

Florida Geological Survey. From these sources and sources published in the scientific literature, a

geologic model was developed that can be used as an exploration tool for identifying deposits of

clean sand on the inner continental shelf in the form of linear sand shoals and larger sand banks.

The model indicates that the cleanest sand free of silts and clays are most likely to be present below

the surface of the crest of the shoals and thin to a minimum thickness over the topographically

lower flanks of the shoal. On this basis, approximately 125 miles of high seismic profile data were

collected at frequencies between 500 Hz and 5 kHz. Analysis of the sub-bottom seismic profiles

consisted of digitizing the topographic surface and the base of the acoustic backscatter that indicate

sand deposits. The interpretation was aided by a series of core boring logs in draft format that were

provided by the Jacksonville District of the Corps of Engineers. The results of the interpretations

were assembled into a GIS software platform for mapping and volumetric analysis. Based on the

final analysis the beach quality sand is likely to be contained within 5 to 10 ft below the topographic

surface of the shoals. Consistent with the geologic model: the thickest deposits are below the crest

and thin to near zero at the base of the shoals at depths of about 55 feet and lower. The total volume

of sand present in 5 shoals is calculated at approximately 157 million cubic yards. It is cautioned,

however, that the actual volume of beach quality sand may be substantially lower pending further

investigation based on additional geologic and acoustic data required to develop specific borrow

sites. Recommendations are made for selection of specific areas best suited for recovering beach

quality sand.

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Table of Contents

Executive Summary ..................................................................................................... i

1.0 Introduction ............................................................................................................1

2.0 Project Area Description .......................................................................................1

3.0 Geologic Setting......................................................................................................3

4.0 Geologic Model for Exploring Borrow Areas Offshore of St. Johns County ..6

5.0 Methods ................................................................................................................ 10

6.0 Survey Results ..................................................................................................... 14

6.1 Overview...................................................................................................................... 14

6.2 Shoal 1 ......................................................................................................................... 15

6.3 Shoal 2 ......................................................................................................................... 18

6.4 Shoal 3 ......................................................................................................................... 21

6.5 Shoal 4 ......................................................................................................................... 25

6.6 Shoal 5 ......................................................................................................................... 29

7.0 Potential for Beach Compatibility of Offshore Sand Resources .................... 33

8.0 Conclusions and Recommendations.................................................................. 39

9.0 References ............................................................................................................ 44

1.0 Introduction

Previous work by the U.S. Army Corps of Engineers, the Florida Department of

Environmental Protection (FDEP), the Florida Geologic Survey (FGS), and the U.S. Minerals

Management Service (MMS) on existing data and geological features along the northeast coast

of Florida demonstrated that significant sand resources may exist in the coastal waters offshore

of St. Johns County. These resource shoreface connected ebb shoals, linear sand shoals are at or

beyond the 3nm limit of Florida State waters. To further define the potential volume of beach

quality sand resources, a sub-bottom acoustic survey was conducted consisting of 125 miles of

track lines. These data were then calibrated using core borings taken in the area by the U.S.

Army Corps of Engineers between 1976 and 2006. This report summarizes the results of the sub-

bottom survey and provides an estimate of the total potential volume of beach quality sand as

well as potential sand volume contained in the individual shoals.

2.0 Project Area Description

The project area is within State and nearshore Federal waters offshore of St. Johns

County, Florida where several prominent individual sand ridges and large sand banks occur.

Figure 1 shows the sand ridges and sand banks with significant relief; those included in this

study are labeled. These sand bodies and many others along the inner shelf of east Florida were

first described by Meisberger and Field (1975) in a reconnaissance survey to identify sand and

gravel resources on the inner continental shelf. The largest individual feature in the area was

termed Shoal A6 (Meisberger and Field, 1975). Others were designated and are shown as shoals

and sand banks in Figure 1. The ebb shoal deposits related to the evolution of St. Augustine Inlet

were also noted in the 1975 analysis as a possible source of sand.

The most prominent feature in the project area is the A6 shoal situated approximately 6

miles directly offshore of the north boundary of the project limit (Figure 1). This shoal can be

considered to have three components as shown in Figure 1. The topographic base of the A6 shoal

is in about -65 feet water depth, whereas the maximum elevation at the crest of A6 is about -40

ft. MSL. The crest of the A7 shoal is within 4.5 miles of the center of the onshore project area.

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An additional feature included in the survey is termed Shoal 1 and Shoal 2, which are segments

of a narrow linear shoal extending directly south of A7. The features selected for survey in this

study were included due to their proximity to the beach renourishment area and their likely

potential to yield clean sand based on previous work.

Figure 1. Locations of sand banks and sand ridges along the inner continental shelf of Northeast Florida near the project area.

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3.0 Geologic Setting

The geologic evolution and geologic setting of the project area as it relates to sand source

potential was reviewed in the reconnaissance report (Zarillo, 2008a). Briefly, the most important

aspect of the geologic setting is the genesis of layers of unconsolidated sands and silty sands

overlying formations rich in carbonate. Early investigations by the U.S. Army Coastal

Engineering Research Center (Meisberger and Field 1975, 1976) showed that sand resources on

the inner continental shelf are arranged in discrete linear sand shoals or larger sand banks. A

scattering of core borings and surficial grab samples allowed sandy textures to be classified as

fine quartz rich sands that are poorly graded (well sorted) having a low carbonate content to

medium to coarse sands of low quartz and high carbonate content consisting of a mix of shell

fragments and other carbonate content. The possible genesis of shoals in the nearshore and

littoral environments by a combination of tidal inlet migration and sea level rise during the

Holocene transgression was also reviewed in the reconnaissance report (McBride and Moslow,

1991).

Additional information on the geologic setting of the sand bodies of the Northeast Florida

inner continental shelf can be found in a series of studies sponsored by the U.S. Mineral

Management Service (MMS) and conducted by the Florida Geologic Survey (FGS) beginning in

the early 1990’s (Nocita et al., 1991) and extending through 2004 (Phelps et. al.,2004).

The FDEP sponsored a regional investigation entitled the Florida Northeast Coast

Reconnaissance Offshore Sand Search (ROSS) that began in 2001(URS, 2007). The results of

the Northeast regional study were archived in the ROSS database (URS, 2007) along with much

of the historical and more recent data sets pertaining to sand resources of the inner continental

shelf of Florida. The most recent sand source study in the area, an effort by the Jacksonville

Engineering District of the U.S. Army Corps of Engineers, is summarized in a draft report yet to

be officially released. The draft report contains a summary of core borings taken in 1976, 1998,

and 2003. These data were particularly helpful in understanding and calibrating the sub-bottom

acoustic records taken for this study. Figure 2 shows the distribution of these cores relative to the

A6 and A7 Shoals and other sand bodies in the area.

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The most recent MMS investigation in the project area was completed by Scientific

Environmental Applications, Inc. (S.E.A) and the Louis Berger Group (LBG) (Zarillo et al.,

2008). This study was focused on the potential for environmental and biological impacts that

might be associated with excavations from the vicinity of the A6 shoal. Results of this study

indicate that multiple borrow cuts across the A6 shoal are unlikely to cause impacts at the

shoreline. However, a relevant finding of the (Zarillo et al., 2008) study is that the surficial sand

layers of the A6 and

Figure 2. Location of core borings collected by the Jacksonville Army Corps Engineering District between 1976 and 2006. Pattern areas indicate the location of major shoals in the area. Lines J-J’ and H-H’ indicate lithologic cross sections developed by the Jacksonville District, which are discussed later in this report.

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similar shoals in the area can be reworked and transported by long period waves generated by

storms whether locally or from distal storms in the North Atlantic Ocean. For instance, Figure 3

shows the net topographic changes predicted over the crest of the A6 shoal for a 12-month period

during which several hurricanes pass over the Florida Coastal Ocean or generated long period

energetic waves in a distal fetch area which eventually dispersed to the Northeast Florida

Continental Shelf. The cumulative impact of storms over long periods is not expected to alter the

overall morphology of a shoal, the upper 2 to 3 feet of sediment is expected to be relatively

coarse and free of silt and clay

Figure 3. Predicted topographic change over the surface of the A6 shoal after 2 years of simulation (from Zarillo et al., 2008).

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4.0 Geologic Model for Exploring Borrow Areas Offshore of St. Johns County

Based on previous work on shelf sand ridge deposits and studies specific to the Northeast

Florida Shelf, an efficient search for beach quality sand can be based on a Geologic Model.

Shoals of the Northeastern Florida shelf described by Meisberger and Field (1975) and more

recent studies by MMS and the FDEP are consistent in scale and orientation to the sand ridges

discussed in notable papers by (Duane et al., 1972; Swift et al., 1972; Stubblefield et al., 1984;

McBride and Moslow, 1991; Snedden et al., 1994, 1999). Most of these investigations were

focused on the Eastern U.S. Atlantic continental shelf and emphasized barrier-island retreat and

inlet-related models for the origin of sand ridges attributed to shoreline retreat mechanisms,

including shoreface-attached ridges and ebb shoal retreat paths. The paper by McBride and

Moslow provided a conceptual model of the possible relationships among sea level rise, the

related shoreface transgressive process, and the associated evolution of tidal inlet shoals. Figure

4 from McBride and Moslow (1991) shows the possible evolutionary steps in the development of

linear sand shoal attachment to the shoreface and later isolation on the inner continental shelf. In

this conceptual model, inlets breach barrier island systems undergoing transgression with rising

sea level. As the inlets migrate alongshore, the ebb shoal systems extend both alongshore due to

inlet migration and cross-shore due to shoreface transgression.

For sand resource evaluation it is important to understand the structure and

sedimentologic indices of sand ridges. Since these features are found in both modern

environments as linear sand ridges and in ancient environments as porous sandstone petroleum

reservoir rocks, models of sand ridge deposits are used to identify the shallow marine

environment from well logs. Figure 4 shows the idealized ridge and inter-ridge sediment types

common to inner shelf environments. This idealized model of a coarsening upward sequence

rests on a pre-Holocene surface and begins with mixtures of organic-rich sediments or silts and

clays typical of the restricted back barrier environment followed by the typical inter-ridge

sediments that can be mixtures of sand and fine grain sediments. The upper two units are

characteristic of post-transgressive sands that have been re-worked from inlet shoals and

shoreface sands into discrete linear sand bodies composed of medium to coarse sand. The cross-

bedding and lack of biogenic structures in the top unit of the idealized model represents the

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continued reworking of the modern sand ridges by inner-shelf physical processes including

waves and storm or tide-generated currents. This is consistent with modeling results obtained in

the MMS study by Zarillo et al. (2008).

Figure 4. Model of a coarsening upward sequence capped by clean sand deposits of a linear sand ridge (from Tillman 1985).

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Figure 5. Lithology of ICONS core CERC 176 from the crest of the A6 shoal and core CERC 174 from the crest of the A7 shoal.

The few existing published core boring data from the area near the A6 and A7 shoals are

consistent with the geologic model described above. These data were discussed in the

Reconnaissance Report (Zarillo, 2008a) and are exemplified by the core logs in Figure 5. The

sediment in core CERC 174 from the crest of the A7 shoal contains about 9 feet of clean sand

over a base of compacted gray clay. A similar sequence is present in Core 176 from the crest of

A7. Lithology found in the core borings collected by the Jacksonville Army Corps Engineering

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District is also consistent with thee geologic model of sand ridge stratigraphy. Figure 6 from the

unpublished report by the Jacksonville District shows clean sand reaching a maximum thickness

of about 11 feet under the crest of the A6 Shoal, whereas the layer of clean sand is much thinner

in the flank area of the shoal before silts and clays are reached (cross-section J-J’, Figure 2). The

relationship between shoal topography and lithology is also well illustrated in Figure 7, which

shows a series of core borings from the A6 and A7 shoals along cross-section H-H’ shown in

Figure 2. In both figures the sand layers are likely to yield beach quality sand.

Figure 6. Lithology found in 4 core borings distributed across the A6 shoal along line J-J’ shown in Figure 2 (from U.S. Army Corps of Engineering Draft Report, 2007).

A review of existing data in Jacksonville District report (U.S. Army Corps of

Engineering Draft Report, 2007) and the Reconnaissance Report (Zarillo, 2008a) indicates that

beach quality sand is present under the crest areas of the A6 and A7 shoals. The data set included

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the Inner Continental Shelf Sediment and Structure (ICONS) Study conducted in the 1960’s and

more recent sub-bottom data collected by the FDEP ROSS database, and data collected by the

Florida Geologic Survey. Full interpretation of previously collected data was limited due to the

lack of high resolution sub-bottom acoustic reflection records that allows volumetric estimation

of beach sand and definition of specific borrow cuts areas. In the following sections the sub-

bottom survey methods, results, and volume interpretation are presented. The final interpretation

of the potential volume of beach sand is based on a combination of the sub-bottom acoustic

records and core borings that were located on, or near the seismic track lines.

Figure 7. Lithologic profile along line H-H’ (Figure 2) showing variable thickness of clean sand within the shoals in Federal waters offshore of St. Johns County, FL (from The U.S. Army Corps of Engineering Draft report, 2007).

5.0 Methods

The sub-bottom survey using continuous seismic reflection methods was conducted over

a period of 5 days beginning on September 15, 2008. The final two days of surveying were

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conducted on October 1st and 2nd due to weather restrictions. Figure 8 shows pattern of survey

track lines covering approximately 125 miles. The components of the A6 and A7 shoals were

numbered for convenience. Two additional linear shoals features immediately to the south of A7

designated Shoals 1 and Shoal 2 were also included since core borings collected by the Corps

indicated the presence of clean sand layers. These shoals are partially in Florida State waters

within 2 to 5 miles of the beach fill project area (Figure 8).

The east-west oriented lines were mostly spaced at a 2,000-foot interval. In Shoals 1 and

2 the lines were spaced at 1,000 feet since the total area to be covered was much more limited

compared to the other larger shoals. Due to limited time only five north-south or diagonal lines

were completed one for each shoal areas as shown in Figure 8.

Figure 8. Pattern of survey lines used to collect acoustic sub-bottom profiles in each shoal area.

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The sub-bottom profiles were collected using the EdgeTech 3200-XS Sub-bottom

Acoustic Profiling System. The acoustic transducer is packaged in the EdgeTech SB-512i Tow

Vehicle. The Edge Tech SB-512i applies CHIRP (Compressed High Intensity Radar Pulse)

technology and has a total frequency range of 500 Hz-12 kHz. After test runs at the beginning of

the survey, it was determined that an optimal frequency range of 500 Hz -5 kHz provided the

best detail and penetration in the Northeast Florida Survey area. Figure 9 shows the retrieval of

the Edge Towfish at the close of the survey work day. As the survey proceeded some

adjustments were made to optimize the acoustic backscatter from the sub-bottom sediments. This

usually consisted of decreasing the upper frequency range to 2.7 kHz.

Figure 9. EdgeTech SB-0512i Tow Vehicle.

Precision navigation HYPACK© software was used to acquire location coordinates from

a Trimble Differential Global Positioning System (GPS). The sub-bottom data were recorded in

the Society of Exploration Geophysicists (SEG-Y) format in which acoustic reflector strength,

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coordinates, and 2-way travel time to and from the transducer are recorded. Acoustic profile lines

that resulted from the sub-bottom survey were analyzed using Chesapeake Technology, Inc.

sonar analysis software.

The survey data were filtered in order to remove some of the electronic and acoustic

noise. The filtering process was kept to a minimum to achieve the best resolution possible. The

interpretation of the sub-bottom digital records using the Chesapeake Technology, Inc. software

consisted of loading the digital profiles to see the entire array of tracklines in a GIS-like over

view. From this point in the individual profiles lines were selected for further analysis one at a

time. The strategy was to use the existing core boring provided by the Jacksonville District to

make an interpretation of acoustic backscatter that represents sands free of silt and clay. An

example is shown in Figure 10 where a survey line across the A6 shoal was compared with the

lithology of a core boring located near the line. The figure demonstrates that the sand layer

identified by the core lithology provided a stronger backscatter readily differentiated from that of

the silt and clay rich sediment below. Below the unit of sand, stronger and distinctive acoustic

backscatter was probably related to a buried rock surface and possibly occasional inclusions of

rock fragments and shell layers (Figure 10).

The upper and lower boundaries of acoustic backscatter signals most likely correspond to

the vertical extent of sand layer digitized on each profile line along with a series of horizontal

coordinates of the sand layers along each line. This provided a digital record for all seismic

profile lines consisting of x,y,z data in the form of easting and northing coordinates in the Florida

State Plane system along with the thickness (or isopach) of the sand layer.

These files were then imported into ArcGISTM for further analysis and interpolation of

the sand isopach (thickness) and potential volume of beach quality sand available within each

shoal. The resulting analysis provides a visualization of the sand isopach for each shoal

represented in both 2 and 3-dimensional formats. The spatial analysis capabilities of ArcGISTM

were also used to extract the total volume as defined by the sand isopach analysis. In the

following sections the results of the analysis are presented for each shoal feature along with

examples of the sub-bottom records and a comparison with lithologic data from the core borings.

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Figure 10. Examples of the interpretation of sub-bottom seismic records with the aid of core lithology. The example is from Shoal A6.

6.0 Survey Results

6.1 Overview

The results of the sub-bottom survey are presented in a series of 2- and 3-dimensional

representations of the sand isopach found within each shoal feature. The topography of each

feature is described along with the total potential volume of sand contained within. However, it

is emphasized that the recoverable beach quality sand volume for each of the shoals may be

much lower than the total volume potential. Further analysis for each area would be required to

define specific borrow cuts. Recommendations on how to proceed in developing borrow sites

with respect to the required geologic and sedimentologic data are provided in the final

conclusions of this report.

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6.2 Shoal 1

Shoal 1 as identified in this project from existing bathymetric data and cores collected by

the Army Corps of Engineers is part of a more or less continuous, narrow shoal centered about 3

nautical miles directly offshore of St. Augustine Inlet as shown in Figure 8. Figure 11 represents

the topography of the shoal in shaded relief compared to the surrounding general bottom

topography shown in contours. At the highest point of the crest the shoal reaches an elevation of

approximately -43 feet MSL. The shoal is approximately 4-miles long and about 0.5 to 0.75

miles wide to the base of the shoal at about -55 feet MSL.

Figure 11. Topography of Shoal 1. Maximum elevation at the crest is approximately -43 feet MSL.

Figure 12 shows the sand thickness or isopach of clean sand below the surface of the

shoal. The sand isopach varies from near zero to a maximum of about 8 feet below the surface of

the shoal. Figure 13 shows the computer generated sand isopach from seismic records in a 3-D

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perspective view from the south east. This view emphasizes the fact the sand distribution is not

uniform in thickness but is irregular having discrete peaks of sand thickness and areas where the

sand overburden is thin. This indicates planning borrow cuts in this shoal feature, and ultimately

in all of the shoals, will have to be carefully designed to eliminate silt and clay-rich sediment

below the clean sand.

Figure 12. Isopach of sand below the surface of Shoal 1 interpreted from the sub-bottom seismic record in combination with core lithology.

Based on the seismic survey of Shoal 1 the volume of sand contained in the isopach

between the topographic surface and the sand layer as interpreted from the sub-bottom records

may yield as much as 6.1 million cubic yards. Figure 14 shows a perspective view of the

topographic surface and a surface defined by the base of the sand layer in the isopach of Shoal 1.

The sand volume is then contained between these two surfaces that are separated by a vertical

distance of between 2 and 8 feet.

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Figure 13. Perspective view of the Shoal 1 isopach from the southwest showing irregular distribution of sand distribution. Vertical exaggeration is 175x.

Figure 14. Perspective view of the Shoal 1 topographic surface in the shaded blue over the surface defined by the base of the sand layer interpreted from seismic records shown in the blue-green shades. The view is from the southwest and the vertical exaggeration is 175x.

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6.3 Shoal 2

Shoal 2 as defined in this survey begins about 3.5 miles offshore of St. Augustine Inlet

and extends approximately 4.4 miles to the south. The length of the shoal is about 4.4 miles at

the -55 foot topographic contour (Figure 15). The minimal elevation of the shoal reaches –45 ft.

MSL.

Figure 15. Topography of Shoal 2. Minimum depth at the crest is approximately -45 feet MSL.

The isopach of clean sand below the topographic surface is shown in Figure 16. The

topography surrounding the shoal is shown for reference. Sand thickness varies between near

zero and about 10 feet. A perspective view of the isopach is shown in Figure 17, which

emphasizes the variability of sand deposits over the silt and clay-rich sediments below.

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Figure 16. Isopach of sand below the surface of Shoal 2. Contours are in feet showing the topographic features surrounding the shoal.

Figure 17. Perspective view from the southwest of the Shoal 2 isopach emphasizing the variability of sand layer thickness above silts and clays.

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Figure 18 shows the interpretation of the north-south oriented seismic line along the crest

of Shoal 2. One of the core borings collect by the Jacksonville District was located within 100

feet of the line and is shown in the figure. The core indicates about 3.5 feet of sand below the

topographic surface, whereas the sub-bottom profile indicates about 5 feet of sand in the vicinity

of the core boring. The difference can be attributed to local variability and distance of the core

boring from the seismic profile line.

Figure 18. Interpretation of north-south seismic profile line across Shoal 2. Lithology of a core boring near the line is shown.

The total volume of sand potentially available in Shoal 2 is approximately 11.5 million

cubic yards. The perspective view of the Shoal 2 topographic surface and the base of the sand

layers are shown in Figure 19. The isopach illustrated in Figures 16 and 17 is bounded by these

two surfaces.

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Figure 19. Perspective view of the Shoal 2 topographic surface in the shaded blue over the surface defined as the base of the sand layer from seismic records. The view is from the southeast and the vertical exaggeration is 175x.

6.4 Shoal 3

Shoal 3 corresponds to A7 as defined by Meisberger and Field (1975, 1976).

Topographically Shoal 3 reaches a minimum depth at the crest of about -40 MSL above the

surrounding sea floor, which is between 55 and 60 below MSL (Figure 20). This shoal along

with Shoal 4 (A6) is one of the larger shoal features on the inner continental shelf of northeast

Florida covering a total area of about 6 square miles. The isopach shown in Figure 21 is based on

sand deposits that range between near zero on the lower flanks of the feature to about 8 feet

below the higher elevations of the crest.

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Figure 20. Topography of Shoal 3. Minimum depth at the crest is approximately -39 feet MSL. Contours are in feet MSL.

Figure 22 shows the sand isopach of Shoal 3 in a perspective view from the southwest.

Similar to the other shoals included in this survey the thickness of sand deposits is variable and

correlated with topography. Figure 23 is an example of a sub-bottom acoustic profile from Shoal

3 and compares the lithology of one of the core borings obtained from the Jacksonville District

with the seismic interpretation. Core VBSJS06-13 shown in Figure 23 reached a depth of only

about 4 feet before terminating, possibly because a resistant clay surface prevented further

penetration. In this area of Shoal 3, the acoustic backscatter from the sand layer is strong and

easily identified as being about 5 feet thick.

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Figure 21. Isopach of sand below the surface of Shoal 3. Contours the topographic features surrounding the shoal are shown in feet. Maximum sand thickness below the topographic surface according to seismic data is about 8 feet.

Figure 22. Perspective view from the southwest of the Shoal 3 isopach.

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Figure 23. Interpretation of north-south seismic profile line across Shoal 3. Lithology of core boring VBSJS06-13 located within 20 feet of the profile line is shown.

From the isopach calculation the total volume of sand contained in Shoal 3 could reach

approximately 31 million cubic yards. The location of the sand between the topographic surface

and bottom of the sand layer is illustrated in Figure 24. Again, it is cautioned that the recoverable

beach quality sand from Shoal 3 is likely to be much lower than the potential volume pending a

more comprehensive analysis from additional data from core borings and additional seismic data

collected at higher spatial resolution.

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Figure 24. Perspective view of the Shoal 3 topographic surface in the shaded blue over the surface defined as the base of the sand layer from seismic records. The view is from the southeast and the vertical exaggeration is 175x.

6.5 Shoal 4

Shoal 4 as identified in Figure 8 is the largest shoal included in this study covering a

total area of about 20 square miles. This shoal can be best described as a sand bank, rather than a

discrete linear shoal, since is covers a broad area and includes multiple lobes each having a

distinctive crest (Figure 25). In addition, both Shoal 4 and Shoal 5 discussed in the next section

are part of the larger A6 Shoal system first described by Meisberger and Field (1975) in an early

report by the U.S. Army Corps of Engineers Coastal Engineering Center as part of the Inner

Continental Shelf Sediment and Structure Study (ICONS) to identify sand and gravel resources.

The isopach calculated from the sub-bottom profiles indicates clean sand deposits of sand up to 9

feet thick below the topographic surface (Figure 26). Along the flanks of the shoal at depths of

50 to 55 feet below MSL, sand deposits are likely to thin to near zero.

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Figure 25. Topography of Shoal 4. Minimum depth at the crest is approximately -35 feet MSL. Contours are in feet MSL.

Figure 27 shows the sand Isopach for Shoal 4 in perspective view from the southwest

illustrating the variability in sand thickness over this large shoal. Figure 28 is an example of the

interpretation of the sub-bottom acoustic data from this shoal. A good correlation was found

between the lithology found in the core boring data provided by the Jacksonville Engineering

District and the sub-bottom records. Additional seismic and sedimentologic data would be

required at greater spatial resolution to define specific borrow areas in Shoal 4. In many areas of

this shoal and the other shoals included in this study, the overburden of clean sand above silts

and clays averages between 4 and 5 feet, but can vary from the average by a foot or more over

relatively short distances. Thus, borrow excavations would have to be designed to account for

spatial variability to avoid including silts and clays.

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Figure 26. Isopach of sand below the surface of Shoal 4 Contours showing the topographic features surrounding the shoal are in feet. Maximum sand thickness below the topographic surface according to seismic data is about 9 feet.

Figure 27. Perspective view from the southwest of the Shoal 4 isopach.

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Figure 28. Interpretation of east-west seismic profile line across Shoal 4. Lithology of core boring VBSJS06-38 located approximately 400 feet from the profile line is shown.

The volume of sand contained in Shoal 4 is approximately 96 million cubic yards

according to calculations from the isopach analysis. This is more than the combined volume of

the other four shoals included in this study. Again the recoverable volume from borrow cuts that

may be designed for this shoal will be restricted by the need to avoid the inclusion of silts and

clays and possibly by other environmental restrictions. Finally, Figure 29 illustrates the form of

a sand isopach contained between the topographic surface and base of the sand deposits in Shoal

4.

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Figure 29. Perspective view of the Shoal 4 topographic surface in the shaded blue over the surface defined as the base of the sand layer from seismic records. The view is from the southwest and the vertical exaggeration is 175x.

6.6 Shoal 5

The final shoal included in this study is another component of the A6 system and is

situated just to the west of the north end of Shoal 4. Most of this shoal is contained within

Florida State waters as shown in Figure 8. Shoal 5 is approximately 3 square miles in area and

rises to a maximum elevation (minimum depth) of about -40 feet MSL (Figure 30). The analysis

of the sub-bottom records indicates that sand deposits are up to 8 feet thick (Figures 31). The

distribution of sand deposits as interpreted from the sub-bottom seismic data is more uniform in

thickness compared to the other shoals as illustrated by the perspective view of the isopach

shown in Figure 32.

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Figure 30. Topography of Shoal 5. Minimum depth at the crest is approximately -45 feet MSL. Contours are in feet MSL.

31

Figure 31. Isopach of sand below the surface of Shoal 5. Contours showing the topographic features surrounding the shoal are in feet. Maximum sand thickness below the topographic surface is about 8 feet.

Figure 32. Perspective view from the southwest of the Shoal 5 isopach.

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Shoal 5 included only one of the core borings obtained from the Jacksonville District’s

draft report on the area offshore of St. Johns County. However, a comparison between the

Lithology of this core and the interpretation of the sub-bottom profile nearest to the core shows a

close correspondence. Figure 33 shows the Lithology of Core VBSJS06-49 with an east-west

oriented sub-bottom profile across the north end of the shoal. The acoustic signature of the sand

layer above a layer of clay is distinctive from the acoustic backscatter from the clay layer below.

Figure 33. Interpretation of east-west seismic profile line across Shoal 5. Lithology of core boring VBSJS06-39 line is shown for comparison to the seismic interpretation.

The estimated total volume of clean sand in Shoal 5 is approximately 12.6 million cubic

yards contained between the topographic surface and the base of the sand layer defined from the

sub-bottom records (Figure 34). Figure 34 again illustrates the more limited variation in the sand

isopach compared to the other shoals considered in this survey.

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Figure 34. Perspective view of the Shoal 5 topographic surface in the shaded blue over the surface defined as the base of the sand layer from seismic records. The view is from the southeast and the vertical exaggeration is 175x.

7.0 Potential for Beach Compatibility of Offshore Sand Resources

Based on the results of the seismic sub-bottom survey and the limited number of core

borings available from the Jacksonville District of the Corps of Engineers sand resources

contained in Shoal 4 are most likely to yield beach compatible sand. Figure 35 shows the

bathymetric features of Shoals 3, 4, and 5 along with the position of core borings obtained by the

Corps. The mean sediment grain size of a composite sample taken from approximately the upper

5 feet of sand within each core boring is also shown. A total of only 74 core borings are

available from potential sand source areas offshore of St. Johns County, an area of more than 50

square miles. Among these cores only sixty have detailed grain size data available from core

samples. Only sixteen cores are available from the Shoal 4 and thus trends in sediment texture

within this shoal cannot be established with certainty. Figure 35 shows that cores taken from the

higher elevations of this Shoal contained medium to fine sand having a mean grain size range of

about 0.25 to 0.70 mm.

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Figure 35. Mean grain size in mm of the upper 5 to 7 feet of sand recovered from core borings of Shoals 3, 4, and 5 (Sedimentologic data from the U.S. Army Corps of Engineering Draft Report, 2007).

Sand found in cores taken from depths greater than 50 feet are more likely to have a

mean grain size restricted to fine sand or silty fine sand. The coarser distribution of samples

from the shoal crest areas is due to higher shell content. Figure 36 demonstrates the strong

correlation between mean grain size of samples from the core borings and visual estimates of the

shell content provided by the Jacksonville Engineering District. The concentration of coarse

carbonate material at higher elevations of the shoals is likely due to re-working by waves and

35

strong currents during storms. Evidence for this process was presented in the Reconnaissance

Report (Zarillo, 2008a).

Figure 36. Correlation between mean sediment size and carbonate shell material contained in core borings from the Shoals 3, 4, and 5 (Sedimentologic data from The U.S. Army Corps of Engineering Draft Report, 2007).

The geotechnical analysis of native beach samples collected in the onshore project area

(Zarillo, 2008b) also shows that textures of the beach are dependent on the content of carbonate

mostly in the form of shell fragments (Figure 37). Sediment textures on the existing beach are

characterized by somewhat coarser and broader grain size distribution and higher carbonate

content compared to sediment from the crest of Shoal 4 and the other shoals of the inner

continental shelf. This shell material is probably contributed from erosion of the carbonate rich

Anastasia Formation situated just below the modern barrier island sediments.

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Figure 37. Correlation between mean sediment size and the percent carbonate content of beach sands in the project area.

The role of the Anastasia in providing a portion of the modern beach and barrier was

discussed in the Reconnaissance Report (Zarillo, 2008a). Sediment distribution across the beach

includes coarser sands at higher elevations to finer textures at lower elevations of the subtidal

shoreface as shown in Figure 38, which depicts mean sediment size on the beach and shoreface

at Range 98. The coarser textures of the upper beach are due to higher concentrations of

carbonate in the form of shell fragments. Figure 39 shows the composite mean and median grain

size from a series of sand samples taken across eight beach profiles in the beach fill project area.

The sharp increase in the composite mean at the south end of the project area is most likely due

to the proximity of St. Augustine Inlet. Strong tidal currents moving through the inlet

conveyance channels may scour into the carbonate rich Anastasia Formation exporting this

material to the littoral environment where wave action transports it onto the beach adjacent to the

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inlet. Some of this material can also be distributed to the north by wave-generated littoral

currents.

Figure 38. Range of sediment size across the beach and shoreface in the onshore beach fill project area represented by the Range 98 profile.

38

Figure 39. Mean and median grain size of beach profile composite samples. The increase in grain size at the south end is attributed to the proximity of St. Augustine Inlet where strong tidal current may scour into the coarse material of the late Pleistocene Anastasia Formation.

39

The coarse nature of the existing beach sands is also a likely contributor to the relative

steep profile in some areas of the existing beach. A beach reconstruction using sand from

offshore borrow sources with less carbonate will likely lead to a wider beach having a more

gentle gradient. Coarse carbonate beach sediments consisting of shells fragments having

irregular and flatter shapes may have hydraulic equivalence to finer quartz rich sands. Further the

composite samples from the beach have a mean grain size range only slightly lower than the

range of sediment sizes found in cores on the crest of the Shoal 4 (Figure 35).

8.0 Conclusions and Recommendations

The survey of sub-bottom conditions in combination with the earlier reconnaissance

study of the inner continental shelf offshore of St. Johns County indicates that potential beach

quality sand resources are associated with several shoals situated in nearby Federal waters and in

some cases within state waters. Among the five shoals nearest to the area of the beach fill

project, all are likely to contain sand resources that may be accessible if a decision is made to

proceed with the design and permitting of specific borrow areas. Reconnaissance level core

borings and textural analysis of core samples collected and analyzed by the Jacksonville

Engineering District (2007) indicate that the textural properties of sand resources are likely to be

largely in the fine sand range with respect to mean grain size of quartz-rich sediment along with

occasional textures in the medium to coarse sand range when the carbonate content is above

20%.

When the sub-bottom isopach analysis presented in this study is compared and combined

with the lithologic data provided by the Corps of Engineers, it becomes apparent that beach

quality sand resources are most likely confined to a near surface layer averaging about 4 to 5 feet

in thickness. In some locations the thickness of clean sand can expand to 10 feet or more, but this

may occur in limited areas such as the north end of Shoal 4 where one core obtained by the

Jacksonville District contained about 11 feet of clean quartz-rich fine sand. Sub-bottom profiles

in this area indicate a sand isopach of 9 feet or more.

In order to define specific areas among the five shoals for development, the spatial

resolution of the seismic records and the stratigraphic and sedimentologic data from cores would

40

have to be increased. This is particularly true since the vertical extent of clean sand is limited to

an average of about 5 feet, but can spatially and abruptly vary even over short distances. Thus,

borrow cuts would have to be designed to account for spatial variability of clean sand layers to

avoid including silts and clays. Among the shoals that were investigated Shoal 4, the main

component of the A6 shoal system, contains about 60% of the likely sand resources in the area

having a total estimated volume of about 96 million cubic yards of sand (Table 1). Shoal 3,

containing a potential of about 31 million cubic yards, has the second largest potential for beach

quality sand. These are followed in rank by Shoals 5, 2, and 1 respectively as listed in Table 1.

Only a limited number of cores are available to establish the textural properties of sand

resources.

To emphasize the fact that the recoverable sand volume from the shoals will be less than

the potential, Table 1 lists the potential volume of sand resources from each shoal according to a

buffer of 1 foot and 2 feet above the base of the sand layers as determined from the sub-bottom

records. This is a hypothetical calculation, but is consistent with a likely permit requirement for

any borrow cut design that may establish a buffer zone above the sand resource to minimize the

risk of including sediments of high silt and clay content in beach fill material. Table 1 also lists

the ratio between the planimetric area of each of the five shoals and the volume of sand

contained in each shoal. This serves as an “efficiency factor” quantifying the amount of sand

contained in each shoal per unit of areal extent. As can be seen in Table 1 the ratio, having a

dimension in feet, closely approximates the average isopach for each shoal. A ratio calculated

using the surface area of the shoal rather than planimetric areas would be an exact calculation of

the average sand thickness or isopach. Each shoal can be characterized by the total potential of

sand available and the average isopach or efficiency with which clean sand is present in the

shoal.

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Table 1. Summary of sand volume and area of each shoal

Shoal Volume ft3 Volume yd3 Planimetric Area ft2 Ratio* Avg. Isopach ft.

Shoal 1 173,478,928 6,125,139 35,667,181 4.86 4.88

Shoal 2 311,233,902 11,527,170 58,820,704 5.29 5.39

Shoal 3 829,771,917 30,732,262 178,470,209 4.65 4.66

Shoal 4 2,596,039,276 96,149,507 584,230,300 4.44 4.49

Shoal 5 341,228,365 12,638,075 85,184,899 4.01 4.01

Total 4,251,752,388 157,472,153 942,373,292 4.51 4.69

1ft Buffer Volume ft3 Volume yd3 Planimetric Area ft2 Ratio

Shoal 1 137,811,747 5,104,134 35,667,181 3.86

Shoal 2 252,416,356 9,348,745 58,811,231 4.29

Shoal 3 651,327,044 24,123,200 178,371,999 3.65

Shoal 4 2,012,222,053 74,526,668 583,222,292 3.45

Shoal 5 256,070,302 9,484,076 85,094,888 3.01

Total 3,309,847,502 122,586,822 941,167,590 3.52

2ft Buffer Volume ft3 Volume yd3 Planimetric Area ft2 Ratio

Shoal 1 102,144,566 3,783,128 35,667,181 2.86

Shoal 2 193,617,755 7,171,021 58,782,812 3.29

Shoal 3 473,136,124 17,523,543 177,959,591 2.66

Shoal 4 1,429,855,083 52,957,543 581,314,426 2.46

Shoal 5 171,124,234 6,337,928 84,648,875 2.02

Total 2,369,877,763 87,773,163 938,372,886 2.53

* ratio between sand volume and planimetric

42

It is recommended that the selection of specific borrow sites for development be based on

a combination of factors including permitting issues, dredging costs determined by distance from

the beach fill project site, quality of sand, and quantities that may be needed for the initial fill

project and estimated renourishment volumes over the life of the project. For a long term project,

the best candidates for development would be Shoal 3 and Shoal 4 due to the large volume of

sand likely to be present in each. Shoals 1 and 2 have a larger average sand isopach, but their

resources would become limited over time if a buffer above the base of the sand layer is

required. However, these shoals may be useful for small or initial beach fill projects that can be

permitted within Florida State Waters. Development of Shoals 1, 3, 4, and a portion of Shoal 5

would require a permit from the U.S. Minerals Management Service. A significant body of

physical and biologic data has already been collected by the MMS that could support

development of these shoals (Zarillo et al., 2008).

Most of the existing database of core borings and textural properties are from Shoal 4.

Thus, based on the sub-bottom survey and the available in-situ data Shoal 4 is the best candidate

to yield a large recoverable volume of beach quality sand that has similar textural properties to

the existing beach north of St. Augustine Inlet. Figure 40 shows the recommended area over the

crest of Shoal 4 to conduct a higher resolution sub-bottom survey and collect additional core

borings to design a specific borrow area.

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Figure 40. Recommended area for additional core samples and sub-bottom profiles to define specific areas for excavating beach quality sand.

The exact location and size of an individual borrow cut or multiple borrow excavations

will depend of the volume of material required over the life of the beach restoration project. The

sub-bottom survey lines can be placed within the existing survey pattern to provide a sub-bottom

profile spacing of 500 feet in the east-west direction and approximately 1000-foot spacing for

north-south or diagonal lines. Typically the spacing of core borings to determine the textural

properties of the borrow sands should be about 1000 feet apart. The combination of sub-bottom

seismic profiles and core borings will allow the construction of detailed lithologic cross-sections

depicting the lateral and vertical extent of potential beach sand from which a borrow excavation

can be designed.

44

9.0 References

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the Atlantic Inner Continental Shelf, Florida to Long Island. In: Swift, D.J.P., Duane, D.B.,

Pilkey, O.H. (Eds.), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and

Ross, Stroudsburg, PA.

McBride, R.A., Moslow, T.F., 1991. Origin, evolution, and distribution of shoreface sand ridges,

Atlantic inner shelf, U.S.A. Mar. Geol. 97, 57-85.

Meisberger, E.F and Michael E. Field, 1975. Geomorphology, Shallow Structure, and Sediments

of the Florida Inner Continental Shelf, Cape Canaveral to Georgia Technical Memorandum

No. 54 U. S. Army Corps of Engineers Coastal Research.

Meisberger, E.P., and Field, M.E., 1976. Neogene sediments of Atlantic inner continental shelf

off northeastern Florida: American Association of Petroleum Geologists Bull., v. 60, No. 11,

p. 2019-2037.

Nocita, B.W., Papetti, L.W., Grosz, A.E., and Campbell, K.M., 1991, Sand, gravel and heavy -

mineral resource potential of Holocene sediments offshore of Florida, Cape Canaveral to the

Georgia Border: Phase I: Florida Geological Survey Open File Report 39, 29 p.

Phelps, D. C., Hoenstine, R.W., Balsillie, J.H., Ladner, L.J., Dabous A., Lachance M., Bailey K.,

and Fischler C., 2004. A geological investigation of the offshore area along Florida's

northeast, year 2 annual report to the United States Department of Interior, Minerals

Management Service: 2003-2004: Florida Geological Survey, unpublished report. DVD

Snedden, J.W., Tillman, R.D., Kreisa, R.D., Schweller, W.J., Culver, S.J., Winn, R.D., 1994.

Stratigraphy and genesis of a modern shoreface attached sand ridge, Peahala Ridge, New

Jersey. J. Sediment. Res. 64, 560-581.

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Snedden, J.W., Kreisa, R.D., Tillman, R.W., Culver, S.J., Schweller, W.J., 1999. An expanded

model for modern shelf sand ridge genesis and evolution on the New Jersey Atlantic shelf.

In: Bergman and Snedden, J.W. (Eds.) Isolated shallow marine sand bodies: Sequence

stratigraphic analysis and sedimentologic interpretation. SEPM Spec. Publ. 64, 147- 163.

Stubblefield, W.L., McGrail, D.W., Kersey, D.G., 1984. Recognition of transgressive and post-

transgressive sand ridges on the New Jersey continental shelf: reply. In: Tillman, R.W.,

Seimers, C.T. (Eds.), siliciclastic shelf sediments. SEPM Spec. Publ. No. 34.

Swift, D.J.P., Kofoed, J.W., Saulsbury, F.P., Sears, P., 1972. Holocene evolution of the shelf

surface, central and southern Atlantic shelf of North America. In: Swift, D.J.P., Duane, D.B.,

Pilkey, O.H. (Eds.), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and

Ross, Stroudsburg, PA.

Tillman, R.W., Martinsen, R.S., 1984. The Shannon shelf ridge sandstone complex, Salt Creek

Anticline area, Powder River Basin, Wyoming. In: Tillman, R.W., Seimers, C.T. (Eds.),

Siliciclastic shelf sediments. SEPM Spec. Publ. 34, 85-142.

U. S. Army Engineer District, 1975. Duval County Beaches, Florida general design

memorandum: Jacksonville, Department of the Army, Jacksonville District, Corps

U. S. Army Engineer District, 1998. St. Johns County, Florida shore protection project: General

reevaluation report with final environmental assessment: Jacksonville, Department of the

Army, Jacksonville District, Corps of Engineers, 81 pp.

U. S. Army Engineer District, 1990a. Duval County, Florida shore protection project

reevaluation study: Jacksonville, Department of the Army, Jacksonville District, Corps of

Engineers, 56 pp.

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U. S. Army Engineer District, 1990b. St. Johns County, Florida beach erosion control project:

Special report, St. Augustine Beach nourishment: Jacksonville, Department of the Army,

Jacksonville District, Corps of Engineers, 58 pp.

U. S. Army Engineer District, 2007. St. Johns County, Florida Shore Protection Project

General Investigation: Geotechnical Report for Investigation Offshore Borrow Areas. 40pp.

Zarillo, G.A. 2008a. Final Reconnaissance Report: Offshore Sand Borrow Site Assessment

St. Johns County, Florida. Report to PBS&J. 17pp. + Appendix.

Zarillo, G.A. 2008b. Geotechnical Analysis of Native Beach Samples Collected from

St. Johns County, Florida. Report to PBS&J. 17pp.+ Appendix.

Zarillo, G.A., Reidenauer, J.A., Zarillo, K.A., Reyier, E.A., Shinskey, T., Barkaszi, M.J., and

J.M. Shenker, 2008. U.S. Minerals Management Service OCS Study 2008-060 Draft

Biological Characterization and Numerical Wave Model Analysis within Borrow Sites

Offshore of Florida’s Northeast Coast Report-Volume I Contract No. 1435-01-05-CT-39075

+ Volume II: Appendices. 270p.