Flagler_2020_Geotechnical_Investigation.pdf

Prepared for:

Flagler County, FL Board of County Commissioners

Prepared by:

Olsen Associates, Inc.

2618 Herschel Street Jacksonville, FL 32204

(904) 387-6114 (Fax) 384-7368 www.olsen-associates.com

February 2020

Flagler County, FL

Beach and Dune Restoration Project

GEOTECHNICAL INVESTIGATION

FOR OFFSHORE SEDIMENT

BORROW AREA

i Geotechnical Investigation for Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Beach and Dune Restoration Project

Geotechnical Investigation for Offshore Sediment Borrow Area

Prepared for: Flagler County Board of County Commissioners

Prepared by: Olsen Associates, Inc., Jacksonville, Florida

February 2020

EXECUTIVE SUMMARY

This report describes the data collection and analyses conducted to develop an offshore sand borrow site proposed for excavation to restore two reaches of eroded beach and dune along approximately 4.1 miles (6.6 km) of shoreline in Flagler County, FL. The restored beach and dune will protect the upland infrastructure (including Florida Highway A1A), enhance and protect the environmental habitat, and restore and maintain the recreational access and use of the beach. These two project reaches are located immediately adjacent to and are contiguous with the Flagler County Federal Hurricane and Storm Damage Reduction Project (Federal Project).

The Flagler County Beach/Dune Restoration Project (the Project) will restore previously unconstructed beach shoreline reaches where devastating berm and dune losses associated with Hurricane Matthew in 2016 and Hurricane Irma in 2017 resulted in severe beach erosion and infrastructure damage. The project site has not been previously nourished and requires an estimated 1.3 million cubic yards (Mcy) of sand, measured in-place on the beach, for initial construction, and an estimated 500,000 cubic yards (cy) is anticipated for a future renourishment.

Accounting for 15% loss in project losses, a borrow area containing 2.1 Mcy of beach-compatible sand is required for initial construction and one future renourishment.

The proposed borrow area, designated as the Flagler County Borrow Area (FCBA) is located approximately 10 NM offshore of the City of Flagler Beach shoreline (Figure EX.1) and lies within Federal waters. The project borrow area lies within a large sand deposit that was previously delineated and investigated by USACE as part of the Flagler County Hurricane and Storm Damage Reduction Project Feasibility Study and Environmental Assessment (Feasibility Study) -- Borrow Area 3A (USACE, 2015). FCBA occupies an area of roughly 345 acres (hectares) of seabed.

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Flagler County Beach/Dune Restoration Project olsen associates, inc.

The borrow area lies within a large offshore sand deposit and contains beach-compatible sand, as verified through the analysis of representative Vibracores. It contains approximately 2.3 Mcy of sand above -62.5 ft, NAVD88 which is accessible by dredge. The sand volume is expected to accommodate the Project design requirements (2.1 Mcy).

Figure EX.1: Location of the Flagler County Local Project beach and dune fill and the offshore borrow area. The location of the adjacent Federal Project beach and dune fill and offshore borrow area is also shown.

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Flagler County Beach/Dune Restoration Project olsen associates, inc.

The proposed borrow sediment is characterized by the retrieval of Vibracore samples, which are located throughout the borrow area. Sixteen (16) Vibracores were collected in January 2019 by the USACE Jacksonville District in the vicinity of FCBA; fifteen (15) of which are used to describe the sediment composition within the borrow area. One (1) additional Vibracore from 2011 was recovered within the borrow area during the sand source exploration associated with the Feasibility Study (USACE, 2015). No other relevant geotechnical information in this borrow area is known to exist.

Analysis of the available geotechnical information confirmed the existence of a sand resource within FCBA that has been found to be suitable for use as beach fill along the Flagler County beach in terms of grain color, size, and size distribution. This information was used to delineate the horizontal and vertical location and dimensions of the borrow area to be used for construction of beach fill. The borrow area is also subdivided into zones. These zones have been delineated according to sediment characteristics and considering dredge access and operation within the areas.

The color composite Munsell Values within the defined borrow area is about 5.8. The color composite Munsell Hue and Chroma for the borrow area were determined to be 10Y and 1, respectively. Sediment composites exhibit a median grain size of about 0.23 mm and an average mean grain size of about 0.27 mm. All of the sampled borrow area material falls well within the range of material sizes that occur on the project beach and consists mostly of fine to medium-grained sand-sized quartz. Overall, the proposed borrow area sediments are slightly larger in size compared to the native beach, resulting in an overfill ratio of 1.0, based on the Dean and James- Krumbein methods. Comparison between the borrow area zones and native sediment are summarized in Table EX.1.

The borrow area has been designed to allow access by dredge to the maximum amount of available beach-compatible sand. It has also been designed to minimize the creation of unnatural depressions in the seafloor following sand removal. A uniform dredge cut elevation at -62.5 ft, NAVD was chosen as a maximum dredge depth to avoid non-compatible beach sand, and it will result in a most level seafloor after excavation. The three FCBA dredge zones, A, B and C, contain 1.2 Mcy, 475,000 cy and 665,000 cy above -62.5 ft NAVD88, respectively. These zones have been established with consideration of the shape and configuration of sand thickness. The zones have also been configured to maximize the linear distance along each cut lane to accommodate hopper dredge access and excavation. The delineation of the borrow area zones will direct the contractor upon excavation, ensuring the sediment is homogenized in terms of size distribution and color before delivery to the beach.

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Flagler County Beach/Dune Restoration Project olsen associates, inc.

Table EX.1: Summary comparison of the native beach sediment to the proposed borrow area composite sediment with overfill ratios.

Native Beach

Composite

0.13 0.18 0.57 2.11 0.23 1.04 19.2 0.23 10YR 7 2 1.00 1.00

Borrow Area

FCBA‐A

Composite

0.16 0.23 0.45 1.88 0.27 0.94 20.1 1.58 10Y 6 1

Borrow Area

FCBA‐B

Composite

0.16 0.22 0.41 1.92 0.26 0.91 21.1 1.70 10Y 6 1

Borrow Area

FCBA‐C

Composite

0.16 0.23 0.45 1.87 0.27 0.88 20.6 1.67 10Y 6 1

Folk and Ward (1957) Method Method of Moments

Passing

#230 (%)

Carbonate

Content

Mean

(mm)

Mean

(phi) D84 (mm)

D50 (mm)

(Median) D16 (mm)

Overfill

Dean

(2000)

Overfill

James

(1974)

Sorting

(phi)

Munsell Color

Value ChromaHue v

Flagler County Beach/Dune Restoration Project olsen associates, inc.

TABLE OF CONTENTS

EXECUTIVE SUMMARY ........................................................................................................... i

INTRODUCTION

1.1 Project Description

1.2 Native Beach Sediment

BORROW AREA

2.1 Geologic Setting

2.2 Past Investigations

2.3 Location and Configuration

2.4 Available Sediment Data

2.5 Excavation

BORROW AREA SEDIMENT

3.1 Sediment Composition and Grain Size

3.2 Sediment Color (Borrow Area)

3.3 Compatibility

CONCLUSIONS

REFERENCES

APPENDICES

Appendix A: Native Beach Gradiation and Frequency Distribution Curves

Appendix B: Core Logs, Photos, Driller Fieldnotes, Gradiation Curves, and

Granulometric Reports

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Flagler County, FL

Prepared for: Flagler County Board of County Commissioners

Prepared by: Olsen Associates, Inc., Jacksonville, Florida

INTRODUCTION

This report describes the data collection and analyses conducted to identify and develop an offshore sand borrow site proposed for excavation to restore two reaches of eroded beach and dune along approximately 4.1 miles of shoreline in Flagler County, FL. The Flagler County Beach/Dune Restoration Project (the Project) will protect the upland infrastructure including Florida Highway A1A, enhance and protect the environmental habitat, and restore and maintain the recreational access and use of the beach, an important economic resource for the City of Flagler Beach and Flagler County. These two project reaches are located immediately adjacent to and are contiguous with the Flagler County Federal Hurricane and Storm Damage Reduction Project (Federal Project). The project will restore areas where devastating beach and dune losses associated with Hurricane Matthew in 2016 and Hurricane Irma in 2017 resulted in severe beach erosion and infrastructure damage. As will be discussed, the borrow area identified for the Project contains enough beach-compatible sand to support an initial restoration and at least one maintenance renourishment over the 15-year permit life.

The Project sediment borrow area lies within a large offshore sand deposit identified and investigated by the USACE Jacksonville District in 2011 as part of the Flagler County Hurricane and Storm Damage Reduction Project Feasibility Study and Environmental Assessment (Feasibility Study) (USACE, 2015). The USACE designated the deposit as Borrow Area 3A and designed a portion of it for use in the Federal nourishment project. This study utilizes the available geotechnical information within the remaining portion of 3A to identify a sufficient volume of beach-compatible sediment to support the Local Project. In total, the USACE collected four (4) Vibracores in 2011 and forty-eight (48) Vibracores in 2019 in Borrow Area 3A. The majority of the Vibracores are located within the Federal borrow area and are not available for use by the Local Project. The available data for the Local Project borrow area design include one (1) Vibracore collected in 2011 and sixteen (16) Vibracores collected in 2019 by the USACE. These data originate from sampling efforts conducted by the USACE to support the Federal Feasibility Study (2011) and the design and regulatory review of the Federal Project

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

(2019). Geotechnical laboratory work for the sand search investigation was conducted by Meskel & Associates Engineering in January 2019, a contractor to the USACE Jacksonville District.

This study also utilizes a hydrographic multi-beam survey of the seabed conducted by the USACE Jacksonville District in May 2019 and a cultural resources study of the borrow site performed in July 2019 by Panamerican Consultants, Inc., a contractor to the USACE Jacksonville District.

1.1 Project Description

The Flagler Beach/Dune Restoration Project is an engineered beach nourishment project along approximately 4.1 miles (6.6 km) of Atlantic Ocean shoreline in Flagler County, FL, consisting of a northern and a southern reach. The project reaches are located in the City of Flagler Beach between FDEP R-monuments R-64.5 and R-80 (North reach), and R-94 and R- 101 (South reach) (Figure 1.1). These two project reaches are located immediately adjacent to and are contiguous with the Federal Project (R-80 to R-94). The details of the Federal Project are described in FDEP permit number 0378136-001-JC.

The Project will include the placement of up to 1.3 Mcy of sand for the initial nourishment with an expected renourishment interval of 11 years, which is consistent with the renourishment interval for the Flagler County Federal project reach (FDEP P/N 0378136-001- JC). Future renourishment volumes will depend upon project performance and are expected to require between 400,000 and 500,000 cy of sand. Therefore, it is anticipated that a total of about

1.8 Mcy will required for the Project over the life of the 15-year permit.

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Figure 1.1: Location of the Flagler County Beach/Dune Restoration Project reaches and proposed offshore Flagler County Borrow Area (FCBA).

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

1.2 Native Beach Sediment

The material located on the Project beaches is comprised of mainly shell (calcite) and quartz sands. The shell is derived from outcroppings originating near the coastline during the Anastasia formation known as coquina, and it is responsible for providing the unique orange color seen in Flagler County (Figure 1.2). The following quantitative analysis was performed on physical samples collected along the project site to determine composite sediment characteristics of the native beach.

Figure 1.2: Sediment on Flagler beach comprised of orange shell fragments and quartz.

The characteristics of the native beach sediments were quantified by the USACE, Jacksonville District (USACE, 2015). Sediment samples from the beach were taken along shore-perpendicular transects at five R-monuments within the project limits: R-65, R-67, R-73, R-93, and R-97. Along each transect, eight samples were collected. These were located approximately at the dune, mid-berm, mean-tide level, -3 ft, -5 ft, -10 ft, -15 ft, and -20 ft elevations. Laboratory testing of these samples consisted of visual examination for estimated shell content, Munsell color estimate, mechanical sieve analyses for grain size distribution, and carbonate testing by acid leaching. Appendix A contains the gradation and frequency distribution curves for the native beach samples.

The collected samples were used to describe the average native sediment characteristics within the project area. A composite grain size distribution for each cross-shore transect was developed by applying a weighting scheme to appropriately account for sample location and spacing. By using a cross-shore distance weighted average, the results more adequately reflect the non-uniform spacing of samples. Some limitations of the available data included a lack of

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

specific location and elevation data corresponding to the dune toe and berm samples. Due to the variability of the dune alongshore, and uncertainty in the sample location, the dune sample was not utilized in the calculation of the cross-shore composite. To approximate the cross-shore location of the berm samples, the seaward-most occurrence of the +10 ft NAVD88 contour was used. The horizontal locations of each sample elevation were determined based on the July 2011 beach profile survey. Ultimately, the cross-shore composite for each transect was generated using six sand samples: mid-berm (+10 ft), mean-tide level (-0.73 ft), -3 ft, -5 ft, -10 ft, -15 ft.

The -20 ft sample was not included in the native composite because it is below the depth of closure.

To compute the cross-shore weighted average composite grain size curve for each transect, an “influence distance” for each of the six samples was determined. The influence distance was measured as the horizontal cross-shore distance between the midpoints of adjacent samples. The berm sample influence area was extended 20 feet towards the dune to account for the average berm width (and lack of precise sample location data). The result of the cross-shore weighting is a composite grain size distribution for each transect.

To determine a final grain size distribution that represents the native beach sediment along the project shoreline, the cross-shore transect composites were averaged using a weighting that considered the alongshore distance between transects. Table 1.1 lists the composite grain size analysis for each transect, the complete project area, and the two individual project segments (north and south). Table 1.2 lists summary statistics for the composite curves and includes information on the color and carbonate content.

The Munsell Color Order system classifies color based on three attributes: Hue, Value and Chroma. The native beach color is described with a Munsell Hue that ranges from 7.5YR to 10YR, Munsell Value that ranges from 6 to 8, and Munsell Chroma that ranges from 1-3.

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Table 1.1: Cross-shore weighted average composite profiles for each sampling transect, complete project composite, northern project area, and southern project area in Flagler County.

Table 1.2: Summary statistics for the composite grain size distributions developed to represent the native beach material in Flagler County.

Sieve No. Diameter (mm) Phi (φ) R65 R69 R73 R93 R97

3/4" 19.000 ‐4.25 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

3/8" 9.500 ‐3.25 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

#3.5 5.600 ‐2.50 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

#4 4.750 ‐2.25 100.0 100.0 99.9 100.0 100.0 100.0 100.0 100.0

#5 4.000 ‐2.00 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9

#7 2.800 ‐1.50 99.8 99.7 99.8 99.9 99.9 99.8 99.8 99.9

#10 2.000 ‐1.00 99.4 99.2 99.3 99.6 99.6 99.4 99.3 99.6

#14 1.400 ‐0.50 97.8 97.2 96.7 97.8 98.2 97.3 97.0 98.0

#18 1.000 0.00 94.2 93.4 90.5 93.4 94.8 92.6 91.9 94.2

#25 0.710 0.50 89.6 89.2 83.5 88.5 90.9 87.2 86.1 89.9

#35 0.500 1.00 83.0 85.1 77.5 84.6 87.2 82.1 80.4 86.1

#45 0.355 1.50 74.9 80.6 72.2 81.4 82.7 77.0 74.8 82.2

#60 0.250 2.00 65.0 72.8 65.5 76.7 74.7 69.7 67.2 75.6

#80 0.180 2.50 41.2 51.6 46.6 58.6 56.8 50.0 46.8 57.5

#120 0.125 3.00 6.3 7.9 9.0 17.8 11.4 10.0 8.2 14.1

#170 0.090 3.50 0.6 1.5 1.5 2.6 1.6 1.5 1.3 2.0

#200 0.075 3.75 0.5 1.0 0.9 1.6 1.1 1.0 0.9 1.3

#230 0.063 4.00 0.5 0.9 0.8 1.4 1.0 0.9 0.8 1.1

0.25 0.23 0.26 0.21 0.21 0.24 0.25 0.21

2.00 2.13 1.94 2.23 2.22 2.07 2.00 2.23

2,902 3,857 8,661 2,772 3,799 21,991 15,420 6,571

Mean (mm)

Alongshore Distance (ft)

Mean (phi)

Cross‐shore Weighted Composites, Percent Passing (%) South

Composite

North

Composite

Complete

Composite

Value Chroma

R‐65 0.14 0.20 0.53 2.00 0.25 0.99 17.1 0.52 7 1‐2

R‐69 0.13 0.17 0.46 2.13 0.23 1.02 17.3 0.89 6‐8 1‐3

R‐73 0.13 0.19 0.73 1.94 0.26 1.14 22.9 0.84 6‐7 1‐3 R‐93 0.12 0.16 0.47 2.23 0.21 1.06 22.0 1.35 6‐7 1‐3

R‐97 0.13 0.17 0.39 2.22 0.21 0.97 16.9 0.96 7 1‐3 Native Beach

Composite

0.13 0.18 0.57 2.11 0.23 1.04 19.2 0.91 6‐8 1‐3

Monument D16 (mm) D50 (mm)

(Median) D84 (mm) Mean (phi)

Method of MomentsFolk and Ward (1957) Method

7.5YR‐10YR

Munsell Color

Passing

#230 (%) Sorting (phi)

Carbonate

Content (%) Mean (mm)

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Figure 1.3 plots the cumulative grain size distribution of the native beach composite along with the grain size distribution of the individual samples used to create the representative native beach composite. Figure 1.4 plots the frequency distribution of the native beach composite along with the frequency distributions of the individual samples used to create the representative native beach composite.

The frequency plot describes the relative percent composition of various sediment grain sizes in the native beach. From a qualitative review of Figure 1.4, it appears that that a majority of the sediments sampled on the native beach have a mean grain size that ranges from about 0.1 to 0.3 mm. The composite mean grain size is 0.23 mm. Some of the discrete samples contain sediments which are greater than 0.3 mm, some being as large as 2.0 mm. However, the occurrence of these larger sediment sizes is relatively small compared to the overall range of sediments in the native beach and is attributable to the presence of shell fragments in the sample(s).

To further understand how the various sediment sizes are distributed across the beach, Figure 1.5 presents the average of frequency distributions of sediments sampled at the noted vertical locations across the beach profiles. It is evident from this presentation that the occurrence of the larger gain size materials (shell and shell hash) is limited to samples collected on the berm and at mid-tide elevations.

Flagler County Beach/Dune Restoration Project olsen associates, inc.

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Flagler County Beach/Dune Restoration Project olsen associates, inc.

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Flagler County Beach/Dune Restoration Project olsen associates, inc.

FREQUENCY (%)

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Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

BORROW AREA

2.1 Geologic Setting

Florida occupies a portion of the geological unit known as the Floridian Plateau (USACE, 2015). The Floridian Plateau is a partly submerged platform that represents the seaward extension of the coastal plain of Georgia and Florida. It is nearly 500 miles long and up to 400 miles wide (Shrober and Obreza, 2008; USACE, 2015). The submerged portions of the plateau define the area of the continental shelf that extends into the ocean to a depth of approximately 300 feet (USACE, 2015). The plateau has existed for millions of years alternating between dry land, due to periods of relative drops in sea level, and shallow sea, during periods of inundation.

Its core consists of metamorphic rocks buried beneath a thick layer of sedimentary rock composed mostly of limestone (USACE, 2015). A wide variety of mineral deposits are left behind during each dry land exposure which have formed the present-day sandy beaches, offshore bars, and barrier islands in Flagler County (Randazzo and Jones, 1997; USACE, 2015).

This study explores one of the offshore sediment deposits identified by the USACE as a sand source for nourishment of the Flagler County shoreline.

2.2 Past Investigations

In 2004, the USACE sponsored a reconnaissance level investigation of potential sand sources offshore of Flagler County. This effort identified four potential sand source areas offshore of Flagler County and near sections of the County shoreline in need of beach restoration and maintenance.

In support of the USACE efforts, a sand search operation was undertaken by Halcrow, Inc. in August 2010. The purpose of this study was to determine if a volume of beach-compatible sand sufficient for the proposed Federal project existed offshore of Flagler County (Halcrow, 2010). The report reviewed five potential areas as a sand source, including the four areas identified by the 2004 reconnaissance study (Areas 2-5) and one additional area (Area 1) (Figure 2.1). Area 1 includes 21 square miles and is located 2 miles offshore of Flagler. Area 2 comprises an area of 16 square miles and is significantly further from Flagler County’s shoreline (5 miles) than Area 1 and partially offshore of Volusia County. Area 3 covers an area of 13 square miles and is located further offshore, 10 miles from the project area. Area 4 contains two square miles of seabed, located on the highly energetic ebb shoal of the Matanzas Inlet and Area 5 is located in the Intracoastal Waterway between Matanzas Inlet and the southern Flagler County line.

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Figure 2.1: Location of potential sand borrow areas to be used for beach and dune restoration in Flagler

County (Halcrow, 2010).

Area 1 was considered the most suitable potential borrow site due to its advantageous proximity and likelihood of having similar, compatible sand on a large seabed feature. This area was the primary focus of the Halcrow investigation. Area 2 was potentially suitable for nourishment with confirmation from surface sampling data, but it was deemed less desirable due to its location relative to the project area. Similarly, Area 3 was deemed potentially suitable for nourishment from surface sampling, but was not further investigated by Halcrow due to its displacement from the project site1. Halcrow (2010) recommended Areas 4 and 5 be abandoned from further study. Material in Area 4 was not considered suitable for detailed geotechnical investigation since extracting the required project volume from this site would likely have adverse implications on adjacent coastline. The relatively small size of the area, as well as likely encountering problems with acquiring approval to dredge in the vicinity of the inlet were additional reasons that this area was eliminated from further consideration. Area 5 was not likely to yield a sufficient quantity of material and did not meet other project objectives.

1 Area 3 of Halcrow, 2010, contains the currently proposed borrow areas for the Federal and Local projects.

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Ultimately, Halcrow (2010) found that Area 1 did not contain a sufficient volume of potentially beach-compatible sand to accommodate the need of the Federal project. The report concluded that instead of providing sand for the initial restoration project, material from Area 1 could potentially be used for future maintenance nourishment events. However, the material may be deemed impractical to extract given the limitation from FDEP to leave a 2-ft buffer of sand above incompatible layers. Area 2 and Area 3 were further investigated by the USACE in 2015 and are shown in Figure 2.2. Area 2 (divided into 2A, 2B and 2C, Figure 2.3) was estimated to contain 5.5 Mcy of beach-compatible sand with a composite mean grain size of 0.26 mm (1.98 phi). Area 3 was reduced to a selected portion (Area 3A, Figure 2.3) and was estimated to contain 20 Mcy of beach-compatible sand with a composite mean grain size of 0.29 mm (1.79 phi). Overfill Factors for 2A, 2B, 2C and 3A were also calculated and equal 1.21, 1.10, 14.62, and 1.06, respectively. Area 2C was thus deemed unsuitable for Flagler County beaches (USACE, 2015). The remaining Areas 2A, 2B and 3A were deemed compatible for Flagler County beaches and sediment characteristics from the study are summarized in Table 2.1.

In 2019, the USACE Jacksonville District conducted a more detailed investigation of the geotechnical conditions within Borrow Areas 2A and 2B as part of the design and permitting of the Flagler County Hurricane and Storm Damage Reduction Project (Figure 2.3). These two borrow areas were the closet to the planned sand placement area and were considered to be the most economical sand source for beach and dune construction. The purpose of this effort was to more accurately describe the sediment conditions. Additional Vibracores were collected and sediments therein were evaluated in a laboratory for sediment characteristics and quality.

Preliminary coordination with the Florida Department of Environmental Protection (FDEP) concluded that the accessible material contained within Borrow Areas 2A and 2B would not be suitable for beach placement. The material was found to have an overall fines content that would not meet FDEP Sand Rule criteria with respect to fines content and color (F.A.C 62B-41.007j).

The percentage of fines (amount passing the No. 230 sieve) within the material was too high and the sand color was too dark, compared to native beach conditions, to meet Sand Rule criteria and be considered suitable for placement along the Flagler County beaches. As a result, the USACE elected to eliminate Borrow Areas 2A and 2B from further consideration for use along the Flagler County shoreline.

Once Borrow Areas 2A and 2B were no longer available for consideration, the only remaining option with potentially suitable sediment characteristics and sand volume for the Federal project was Borrow Area 3A (Figure 3A). Borrow Area 3A is located about 6 miles farther offshore than 2A and 2B but contains significantly more sand volume and numerous high ridges that were found to contain suitable materials in the 2015 study. As such, the USACE Jacksonville District redirected design and permitting efforts to Borrow Area 3A. Details regarding the development of Borrow Area 3A as a sand source for both the Federal and non- Federal projects are described in the following sections.

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Figure 2.2: Location of potential sand borrow Areas 1, 2 and 3 to be used for beach and dune restoration in Flagler County (USACE, 2015).

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Figure 2.3: Location of potential sand borrow Areas 2A, 2B, 2C and 3A to be used for beach and dune restoration in Flagler County (USACE, 2015).

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

Table 2.1: Summary of Sediment Characteristics (USACE, 2015).

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

2.3 Location

The project will be constructed using sand from Flagler County Borrow Area (FCBA), which is located approximately 10 NM offshore of the City of Flagler Beach shoreline in Federal waters (Figure 2.2). The borrow area lies within Borrow Area 3A, previously delineated by the aforementioned USACE Feasibility Study. The Project borrow area lies in about 60 feet of water and occupies roughly 345 acres (139 hectares) of seabed.

Figure 2.2: Location of the proposed project borrow area for the Flagler County, FL Beach/Dune

Restoration Project.

Offshore Sediment Borrow Area:

Flagler County Beach/Dune Restoration Project olsen associates, inc.

2.4 Available Sediment Data

Borrow Area 3A was first investigated by the USACE Jacksonville District in 2011 as part of the Feasibility Study (USACE, 2015). A total of four (4) Vibracores were taken inside BA-3A. The cores indicated the presence of a significant volume of beach-compatible sediment.

In January 2019, the USACE collected forty-eight (48) Vibracores in the western corner of BA- 3A to support the development of sand borrow areas for the Federal Project and the Flagler County Local Project. These Vibracores were collected using the USACE coring vessel “Snell”.

The Vibracores were 3-inches in diameter and collected by vibrating a steel sample tube into the seabed. The Vibracores are vertical samples of up to 20 feet of sediment within the data collection area. After collection, the cores are then taken to a geotechnical lab, logged and processed to determine the nature of sediments within the core including general sediment characterization, grain size composition, color and shell content. Top of core boring elevations for the logs have been adjusted to match the elevations represented by the May 2019 hydrographic multi-beam survey. All available Vibracores are presented in Figure 2.3.

Figure 2.3 details the May 2019 bathymetric conditions from the survey of Borrow Area

3A and all the available Vibracores within its boundaries. The seafloor elevation ranges between -54 and -65 ft throughout BA-3A. This range is also typical of seafloor adjacent to BA-3A along and extending at least 1,000 ft from all four sides. Borrow Area 3A contains linear ridges of sediment oriented from SW to NE reach elevations of -54 ft. Two ridges are observed in the Federal Project borrow area, and one is observed in FCBA. As well as sitting higher in the water column, ridges are desirable to dredge since they likely contain coarse, beach-compatible sediment. Multiple depressions in the seafloor are also observed both in between ridges and to the northernmost and easternmost sides of BA-3A. Depressions are desirable to avoid while dredging due to deeper seafloor conditions and the likelihood that they contain fine material that is not suited for beach fill.

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Figure 2.3: Plan view of the Borrow Area 3A and Vibracores collected within. Bathymetry represents the results of a USACE multi-beam survey collected in May 2019 (ft, NAVD).

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Following review of the available cores and preliminary core logs, USACE Jacksonville District staff selected the northwest portion of Borrow Area 3A as the sand source for the Federal Project. The Federal borrow area includes 32 of the 48 cores collected in 2019 for the 50-year Federal project. As such, Flagler County was relegated to developing a borrow area for the non- Federal project using the remaining 16 cores and the representative area (Cores 33 through 48).

Figure 2.4 shows a detailed plan view of the Flagler County project borrow area and the

Vibracore located within its boundaries. The sixteen (16) Vibracores available to the Local Project were collected at about 1,000 ft spacing. Fifteen (15) of these describe the sediment within the selected borrow area. Vibracore #37 and its represented area were not included in the borrow area design, as sediments sampled with this core are not considered suitable for beach placement. Additionally, this area lies within a local depression and would not yield significant volume above the cut elevation (see following Section 2.5).

Vibracore logs and photographs, driller fieldnotes, as well as gradation curves and other associated geotechnical data for the 16 cores are provided in Appendix B. An additional Vibracore from 2011 was recovered within the borrow area during the sand source exploration associated with the USACE Feasibility Study (USACE, 2015). This 2011 Vibracore #5 is plotted in Figure 2.4 and the accompanying data is provided at the end of Appendix B. The granulometric data for Vibracore #5 were not used in the present geotechnical analysis due to both its relatively short recovery length (7.7 ft) and geologically consistency with the modern and longer Vibracore #45 (collected in 2019, which is proximal to the 2011 sample).

Typical information obtained from a row of Vibracores is observed in Figure 2.5, along with a representative cross-section of the borrow area elevations from the multi-beam survey.

Samples extracted throughout each Vibracore result in a vertical representation of the existing sediment in the borrow area. The dredge depth elevations and sediment compositions observed are discussed in Section 2.5. In general, if dredged to a depth of -62.5 ft, the typical excavation thickness would be about 4 ft. Also, a uniform layer of suitable sand material lies above an unsuitable lower layer. It is observed that there is no Vibracore data along the ridge of sand most notably observed between Vibracore #’s 39, 40, 44 and 45. The importance of this ridge of sand is further discussed in Section 2.5.

A comprehensive remote sensing survey and analysis of Borrow Area 3A was conducted by Panamerican in July-August 2019. The investigation found that there are no culturally significant resources of concern in Borrow Area 3A, therefore no avoidance measures will be required during excavation of sand from within the area.

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Figure 2.4: Plan view of the proposed Flagler County Borrow Area (FCBA) showing Vibracore locations and Section A, which is presented in Figure 2.5. Bathymetry represents the results of a USACE multi-beam survey collected in May 2019 [ft, NAVD].

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ig u re

.5 en ce p lo t of V ib ra co re d et ai l fr om

S ec ti on

A o f F ig u re

.4 i nd ic at in g th e co m po si ti on o f th e bo rr ow a re a m at er ia l, ba se d on th e re co rd p ro vi de d in th e V ib ra co re lo gs

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2.5 Configuration and Excavation

The Project will be constructed using a trailing suction hopper dredge and traditional hydraulic sand placement with mechanical dune and berm shaping methods. The sand excavated from Flagler County Borrow Area (FCBA, Figure 2.6) is expected to provide sufficient, beach-compatible sand for the initial sand placement event and at least one additional renourishment placement within the 15-year permit life, with a maximum allowable dredge cut depth to uniform elevation -62.5 ft. A 2-foot disturbance buffer below -62.5 to -64.5 ft is also incorporated into the borrow area design. This allows mechanical disturbance below the design depth into material that is also considered compatible with native beach conditions should any of that material be mixed with the borrow area sand during the dredging process. Sand will be delivered to the beach from four offshore mooring points through submerged pipelines. The mooring points and pipelines will be deployed along predetermined pipeline corridors.

Georeferenced corner points A through K in Figure 2.6 represent the FCBA limits. The total area occupies roughly 345 acres (139 hectares) of seabed. The dredge zones A, B, and C are also depicted in the figure. The borrow area design includes a maximum allowable dredge depth to -62.5 ft, plus a 2-foot disturbance buffer to -64.5 ft.

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Figure 2.6: Plan view of the proposed Flagler County Borrow Area (FCBA). Bathymetry represents the results of a USACE multi-beam survey collected in May 2019 (ft, NAVD).

Offshore Sediment Borrow Area:

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Figure 2.7 shows a detailed representation of the May 2019 bathymetry (left) and sediment thickness in feet above -62.5 ft NAVD88 (right) in FCBA. The borrow area has been subdivided into three zones, A, B and C. These zones have been established considering the shape and configuration of sand thickness availability above -62.5 ft and sediment characteristics, principally grain size and color “Value” (see subsequent sections). Dredgable sand thickness varies from about 2 to 9 feet in zone FCBA-A. Within zones FCBA-B&C, the sand thickness varies from about 2 to 7 feet and 2 to 7.5 feet, respectively. The zones have also been configured to maximize the linear distance along one axis (~41° clockwise from North) within each zone to accommodate hopper dredge access and excavation. The location, extent and configuration of these three zones (A, B and C) are also shown in the figure. The zones will be dredged in sequential order starting with FCBA-A. It is expected that all of FCBA-A and a portion of FCBA-B will be used for the initial Project volume (1.5 Mcy) and the remaining portion of FCBA-B and FCBA-C will be used for future renourishment or post-storm repairs to the Flagler County project, as required. It is estimated, based upon the May 2019 hydrographic survey, that FCBA-A contains approximately 1.2 Mcy of sand above -62.5 ft, NAVD88. FCBA- B and FCBA-C contain a total of approximately 475,000 cy and 665,000 cy of sand, respectively. The three areas total 2.3 Mcy, an amount sufficient for initial construction and at least one planned renourishment.

A primary design consideration for the borrow area was to minimize adverse impacts to the site. This included dredging the borrow area to a uniform depth and avoiding the creation of large seafloor depressions. Therefore, removing the naturally high areas (ridges) of sediment within FCBA was desirable. In both panels of Figure 2.7, warm colors represent relatively shallow areas, which generally correspond to greater sediment thicknesses above -62.5 ft. A large ridge of sand exists throughout the upper and middle portions of zone FCBA-A, shown as the dark orange feature in the right panel of Figure 2.7. Cross-sections 1 and 2 in Figure 2.7, are plotted in Figure 2.8 with reference to the multi-beam seabed elevation. The associated vertical distribution of sediment (from the available Vibracore data) is also presented in each cross-section. The aforementioned sand ridge is visible in both cross-sections, at about 2,100 ft. As noted in Section 2.3, the ridge is desirable to dredge since it contains the shallowest (-54 ft depths) and the greatest available volume within FCBA. The sand along the ridge is likely among the brightest Munsell Values available based on adjacent Vibracore samples, as discussed in the following chapter. FCBA-A is thus preferential to utilize for initial project construction. It is also evident from Figure 2.8 that a layer of suitable sand lies in the top-most layer of the borrow area, above an undesirable lower layer of clay, silt, and other inorganic materials.

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. 1

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Figure 2.8: Available beach-compatible sand above the maximum excavation design depth at -62.5 ft, NAVD88 for representative Sections 1 and 2. See Figure 2.5 for plan view location.

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BORROW AREA SEDIMENT

In addition to meeting total project volumes, FCBA was designed with consideration for sediment characteristics – principally grain size and color “Value”. The sediment characteristics of the borrow material are compared to that of the native beach to determine compatibility. The methodology used in calculating these parameters for each zone, given the discrete samples in FCBA, is described below. A compatibility analysis between the borrow area sediment and native beach is presented in Section 3.3.

3.1 Sediment Composition and Grain Size

Tables 3.1A, 3.1B and 3.1C list the characteristics of the representative sediment samples collected from the Vibracores located within FCBA-A, FCBA-B and FCBA-C. All of these samples represent sediments above the design elevation of -62.5 ft, NAVD88. The number of samples collected for each core ranges from one to two and the number is generally dependent upon the amount of sand located above the -62.5 ft elevation within each core.

The typical borrow area sediments are represented by a composite, or weighted average of the sediment characteristics for the individual samples. For design purposes, each discrete sample from a Vibracore represents a given volume within the total borrow area. That volume is based on its vertical influence (above the cut limit) and the effective horizontal area of the Vibracore. Thereafter, each discrete sample’s granulometric data was weighted, based on its representative area within the whole, to develop a composite grain size distribution for the project borrow area. The characteristics for the composition distribution include the mean and median grain sizes, sorting values, fines content (passing the #230 sieve), fine gravel content (retained on the #4 sieve), and Munsell color analysis. The mean and sorting values were computed via the Method of Moments methodology. These parameters are calculated based on Vibracore data within the vicinity of each zone. The determination of composite color involved an additional consideration and is discussed separately in the following section.

Figure 3.1 plots the cumulative grain size distribution curve for all discrete sediment samples collected above -62.5 ft, NAVD88, as well as composite grain size distribution curves that are the weighted average of the individual samples for zones A, B, and C. Figure 3.2 plots the representative composite frequency distribution curves.

The borrow area sediments in FCBA-A, FCBA-B and FCBA-C exhibit an average median grain size of about 0.23 mm and an average mean grain size of about 0.27 mm. The sorting value, σ, of the composite was determined to be 0.9 φ for the three areas. The sorting value provides a description to the degree at which sediments in the composite sample are

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similarly sized. Smaller values approaching σ =0.5 indicate very well sorted (or poorly graded) samples in which the sediment grains are similarly sized.

Carbonate content, determined by acid burn testing on select samples and visually on all others, reveals a range of carbonate (shell) content from 9 to 35%, with an average of about 21% for the three zones in FCBA. The amount of material retained on the #4 for FCBA-A, B and C ranges from 0.0 to 2.2%, with an average of 0.63, 0.99, and 0.54%, respectively.

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Table 3.1A: Sediment characteristics of the individual samples comprising FCBA-A.

Core Easting (ft) Northing (ft) Top Elevation

(ft, NAVD)

Lower Design Elev.

(ft, NAVD)

Cut Elev. (ft, NAVD)

Representative Length of Core

(ft)

Representative

Area ft2

Representative Volume

(cy)

Median (mm)

Mean (mm)

Sorting (phi)

% Passing #230

% Retained on #4 sieve

Total % Shell Content:

Color (Value)

38_1 (1ft) 670,574 1,899,901 -58.6 -62.1 -62.5 3.5 1,097,496 142,268 0.22 0.24 0.68 1.37 0.17 13.0 6.0 38_2 (3.4ft) 670,574 1,899,901 -62.1 -64.0 -62.5 0.4 1,097,496 16,259 0.27 0.35 1.19 2.24 0.97 27.2 6.0 39_1 (1ft) 671,269 1,900,652 -57.2 -62.0 -62.5 4.8 1,068,120 189,888 0.22 0.26 0.88 1.71 0.15 17.1 5.0 39_2 (4ft) 671,269 1,900,652 -62.0 -64.2 -62.5 0.5 1,068,120 19,780 0.23 0.28 1.03 1.48 1.21 26.8 5.0 40_1 (0.5ft) 671,919 1,901,410 -58.2 -59.6 -62.5 1.4 1,000,620 51,884 0.24 0.32 1.09 1.76 0.89 22.1 6.0 40_2 (1.4ft) 671,919 1,901,410 -59.6 -62.6 -62.5 2.9 1,000,620 107,474 0.20 0.23 0.86 1.83 0.46 17.6 6.0 41_1 (1ft) 672,582 1,902,160 -58.6 -62.9 -62.5 3.9 1,027,485 148,415 0.24 0.32 1.21 1.06 1.83 22.1 6.0 42_1 (1ft) 673,238 1,902,919 -57.1 -60.7 -62.5 3.6 954,990 127,332 0.24 0.30 1.03 1.34 0.49 27.1 6.0 42_2 (3.6ft) 673,238 1,902,919 -60.7 -62.7 -62.5 1.8 954,990 63,666 0.21 0.24 0.88 1.97 0.34 13.8 6.0 43_1 (1ft) 671,359 1,899,252 -57.8 -61.0 -62.5 3.2 363,884 43,127 0.24 0.28 0.86 1.18 0.18 25.7 6.0 43_2 (3.1ft) 671,359 1,899,252 -61.0 -64.0 -62.5 1.5 363,884 20,216 0.23 0.27 0.98 1.11 0.87 16.7 5.0 44_1 (1ft) 672,038 1,900,000 -58.2 -61.3 -62.5 3.1 387,873 44,534 0.21 0.23 0.73 2.32 0.00 23.2 6.0 44_2 (3.1ft) 672,038 1,900,000 -61.3 -64.3 -62.5 1.2 387,873 17,239 0.22 0.24 0.74 1.62 0.10 13.9 6.0 45_1 (0.5ft) 672,731 1,900,741 -58.9 -61.0 -62.5 2.1 365,862 28,456 0.23 0.27 0.90 2.58 0.65 31.6 6.0 45_2 (2.1ft) 672,731 1,900,741 -61.0 -64.0 -62.5 1.5 365,862 20,326 0.22 0.26 1.03 2.33 1.63 14.4 6.0 46_1 (2ft) 673,347 1,901,502 -60.6 -64.6 -62.5 1.9 391,470 27,548 0.22 0.26 0.92 2.25 0.56 19.4 6.0 47_1 (1ft) 674,019 1,902,270 -58.0 -61.6 -62.5 3.6 362,163 48,288 0.32 0.39 1.10 1.06 1.22 25.0 6.0 47_2 (3.6ft) 674,019 1,902,270 -61.6 -63.6 -62.5 0.9 362,163 12,072 0.21 0.26 1.01 1.73 1.29 24.2 5.0

1,128,771 0.23 0.27 0.94 1.58 0.63 20.1 *

* Color composite is estimated in Table 16.2A.

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Table 3.1B: Sediment characteristics of the individual samples comprising FCBA-B.

Core Easting (ft) Northing (ft) Top Elevation

(ft, NAVD)

Lower Design Elev.

(ft, NAVD)

Cut Elev. (ft, NAVD)

Representative Length of Core (ft)

Representative

Area ft2

Representative Volume

(cy)

Median (mm)

Mean (mm)

Sorting (phi)

% Passing #230 % Retained on #4 sieve

Total % Shell Content:

Color (Value)

33_1 (1ft) 669,837 1,900,572 -57.5 -60.1 -62.5 2.6 959,985 91,732 0.23 0.28 0.78 1.77 1.34 16.1 6.0 33_2 (2.6ft) 669,837 1,900,572 -60.1 -64.1 -62.5 2.4 959,985 85,332 0.22 0.25 0.80 1.99 0.46 23.0 5.0 34_1 (1ft) 670,507 1,901,300 -59.4 -63.4 -62.5 3.1 966,249 110,940 0.23 0.29 1.14 1.44 2.22 20.3 6.0 35_1 (0.5ft) 671,166 1,902,094 -61.1 -62.7 -62.5 1.4 991,872 51,430 0.24 0.29 0.97 1.28 0.34 35.0 6.0 36_1 (2ft) 671,834 1,902,839 -58.5 -62.6 -62.5 4.0 997,488 147,776 0.21 0.24 0.86 1.82 0.38 19.0 6.0

487,210 0.22 0.26 0.91 1.70 0.99 21.1 *

* Color composite is estimated in Table 16.2B.

Table 3.1C: Sediment characteristics of the individual samples comprising FCBA-C.

Core Easting (ft) Northing (ft) Top Elevation

(ft, NAVD)

Lower Design Elev.

(ft, NAVD)

Cut Elev. (ft, NAVD)

Representative Length of Core

(ft)

Representative

Area ft2

Representative Volume

(cy)

Median (mm)

Mean (mm)

Sorting (phi)

% Passing #230 % Retained on #4 sieve

Total % Shell Content:

Color (Value)

43_1 (1ft) 671,359 1,899,252 -57.8 -61.0 -62.5 3.2 649,858 77,020 0.24 0.28 0.86 1.18 0.18 25.7 6.0 43_2 (3.1ft) 671,359 1,899,252 -61.0 -64.0 -62.5 1.5 649,858 36,103 0.23 0.27 0.98 1.11 0.87 16.7 5.0 44_1 (1ft) 672,038 1,900,000 -58.2 -61.3 -62.5 3.1 662,751 76,094 0.21 0.23 0.73 2.32 0.00 23.2 6.0 44_2 (3.1ft) 672,038 1,900,000 -61.3 -64.3 -62.5 1.2 662,751 29,456 0.22 0.24 0.74 1.62 0.10 13.9 6.0 45_1 (0.5ft) 672,731 1,900,741 -58.9 -61.0 -62.5 2.1 588,507 45,773 0.23 0.27 0.90 2.58 0.65 31.6 6.0 45_2 (2.1ft) 672,731 1,900,741 -61.0 -64.0 -62.5 1.5 588,507 32,695 0.22 0.26 1.03 2.33 1.63 14.4 6.0 46_1 (2ft) 673,347 1,901,502 -60.6 -64.6 -62.5 1.9 647,922 45,595 0.22 0.26 0.92 2.25 0.56 19.4 6.0 47_1 (1ft) 674,019 1,902,270 -58.0 -61.6 -62.5 3.6 595,338 79,378 0.32 0.39 1.10 1.06 1.22 25.0 6.0 47_2 (3.6ft) 674,019 1,902,270 -61.6 -63.6 -62.5 0.9 595,338 19,845 0.21 0.26 1.01 1.73 1.29 24.2 5.0 48_1 (1ft) 672,132 1,898,585 -60.1 -63.3 -62.5 2.4 925,614…