UTILIZING GEOLOGICAL AND GEOTECHNICAL PARAMETERS TO

CONSTRAIN OPTIMAL SITING OF MID-ATLANTIC BIGHT OFFSHORE

WIND PROJECTS

by

Alia Ponte

A thesis submitted to the Faculty of the University of Delaware in partial fulfillment of the requirements for the degree of Master of Science in Geology

Spring 2016

© 2016 Alia Ponte All Rights Reserved

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UTILIZING GEOLOGICAL AND GEOTECHNICAL PARAMETERS TO

CONSTRAIN OPTIMAL SITING OF MID-ATLANTIC BIGHT OFFSHORE

WIND PROJECTS

by

Alia Ponte

Approved: __________________________________________________________ John Madsen, Ph.D. Professor in charge of thesis on behalf of the Advisory Committee Approved: __________________________________________________________ Neil Sturchio, Ph.D. Chair of the Department of Geological Sciences Approved: __________________________________________________________ Mohsen Badiey, Ph.D. Acting Dean of the College of Earth, Ocean, and Environment Approved: __________________________________________________________ Ann L. Ardis, Ph.D. Senior Vice Provost for Graduate and Professional Education

iii

ACKNOWLEDGMENTS

I would like to thank my advisor, Dr. John Madsen for his constant guidance

and support throughout my time at the University of Delaware. Your knowledge and

encouragement has been invaluable to my success and I am forever grateful. Thank

you as well to my committee members Dr. Jeremy Firestone, Dr. Susan McGeary, and

Mr. Bill Wall. I would also like to thank Coty Cribb for both his work on the side-

scan data and his constant support and friendship inside and outside of the classroom.

To Uji and Michelle, I cannot say thank you enough for your constant reinforcement

and willingness to listen to my struggles whenever I was at my breaking point. Lastly,

to my family, thank you for your unwavering support and encouragement.

iv

TABLE OF CONTENTS

LIST OF TABLES ....................................................................................................... vii LIST OF FIGURES ..................................................................................................... viii ABSTRACT .................................................................................................................. xi Chapter

1 INTRODUCTION .............................................................................................. 1

1.1 Overview and Background ........................................................................ 1 1.2 Focus Area: Mid-Atlantic Bight and Maryland WEA .............................. 2 1.3 Geologic Framework ................................................................................. 5

1.3.1 Existing Studies in the Area .......................................................... 8

1.4 Objectives and Hypotheses ........................................................................ 9

2 METHODOLOGY ........................................................................................... 10

2.1 Data Acquisition ...................................................................................... 10 2.2 Chirp Sub-bottom Profiles ....................................................................... 13

2.2.1 Data Processing ........................................................................... 13

2.2.1.1 Data Import ................................................................... 13 2.2.1.2 Bottom Tracking ........................................................... 14

2.2.2 Digitizing Features of Interest ..................................................... 18

2.2.2.1 Paleochannels ............................................................... 18 2.2.2.2 Surficial Sand Sheet ..................................................... 21 2.2.2.3 Depth of Major Reflection Events ................................ 23

2.3 Multibeam Bathymetry ............................................................................ 24 2.4 Side-Scan Sonar ....................................................................................... 25

2.4.1 Data Processing ........................................................................... 25

v

2.4.2 Sediment Classification ............................................................... 27

3 SHALLOW STRATIGRAPHY OF THE MARYLAND WEA ...................... 30

3.1 Introduction ............................................................................................. 30 3.2 Eustatic Sea-Level Change ...................................................................... 31 3.3 Stratigraphic Units ................................................................................... 32

3.3.1 Unit 1: Holocene Sand Sheet ....................................................... 34 3.3.2 Unit 2: Transgressive Coastal Lithosomes .................................. 35 3.3.3 Unit 3: Fluvial Incisions during Late Pleistocene to Early

Holocene Glacial Intervals .......................................................... 36 3.3.4 Unit 4: MIS 5 Interglacial Deposits ............................................. 38 3.3.5 Unit 5: Middle Pleistocene .......................................................... 40

3.4 Paleochannel Systems ............................................................................. 41 3.5 Conclusions ............................................................................................. 44

4 SUITABILITY OF THE WEA AND ADJACENT REGIONS ....................... 45

4.1 Introduction to Marine Spatial Planning ................................................. 45 4.2 Suitability Model ..................................................................................... 46

4.2.1 Step 1: Identify Parameters ......................................................... 46 4.2.2 Step 2: Defining Scale and Suitability ......................................... 47 4.2.3 Step 3: Create a Work-path ......................................................... 47

4.3 Discussion ................................................................................................ 53

5 IMPLICATIONS FOR FOUNDATION SELECTION AND DEVELOPMENT ............................................................................................. 56

5.1 Introduction ............................................................................................. 56 5.2 Geotechnical Considerations ................................................................... 56 5.3 Foundation Types .................................................................................... 57

5.3.1 Monopile ...................................................................................... 59 5.3.2 Jacket/Lattice Structures .............................................................. 60 5.3.3 Gravity Base ................................................................................ 61 5.3.4 Suction Bucket (Caisson) ............................................................ 61

5.4 Discussion ................................................................................................ 63 5.5 Foundation Conclusions .......................................................................... 66

6 CONCLUSIONS .............................................................................................. 69

vi

6.1 Foundation Recommendation .................................................................. 69 6.2 Future Work ............................................................................................. 70

REFERENCES ............................................................................................................. 71 Appendix

SUITABILITY RECLASSIFICATION MAPS ............................................... 78

A.1 Introduction ............................................................................................. 78

vii

LIST OF TABLES

Table 3.1: Toscano et al. & Williams stratigraphic units of coastal MD/DE. Summarized from Toscano et al. (1989) and Williams (1999) ............... 31

Table 4.1: Suitability scale with corresponding numeric values. ................................. 47

Table 4.2: Summary of ranking and reclassification of each vector dataset. ............... 51

Table 4.3: Overall suitability scale for the WEA ......................................................... 54

viii

LIST OF FIGURES

Figure 1.1: Mid-Atlantic Bight BOEM designated WEAs and New York and North Carolina areas of interest for potential offshore wind projects. ................ 3

Figure 1.2: U.S. Mid-Atlantic offshore wind speeds for 90m hub height above sea level (NREL, 2014) ................................................................................... 4

Figure 1.3: Coastal bays & drainage systems surrounding the Maryland WEA (data from USGS national map database) .......................................................... 7

Figure 2.1: Track line map showing N/S and E/W survey lines throughout the area. . 12

Figure 2.2: Example of a profile with A. 150 scalar and B. 250 scalar. ...................... 14

Figure 2.3: Example of a sub-bottom profile A. prior to bottom tracking and B. after bottom tracking. .............................................................................. 15

Figure 2.4: Example of a profile A. before and B. after the application of AGC. ....... 16

Figure 2.5: Example of a profile where waves have created interference A. before and B. after the swell filter has been applied. ......................................... 17

Figure 2.6: Example of a profile with a feature of interest A. before digitizing and B. after digitizing with a blue polyline. ................................................... 18

Figure 2.7: A. Profile with a potential paleochannel system and B. same profile with interpreted spatial extent of channel. ............................................... 19

Figure 2.8: A. Point data imported from SonarWiz prior to B. creation of polygons (paleochannels) based on endpoints/horizontal extent from profiles. ..... 20

Figure 2.9: Example of a A. profile with a dune present and B. same profile with Quick Thickness tool (dotted vertical red line) recording the thickness of the dune. .............................................................................................. 22

Figure 2.10: A. Exported surficial sediment data and B. Nearest Neighbor method. .. 22

ix

Figure 2.11: Example of a A. major reflection event/boundary distinguished by different reflectivity intensities and B. same profile with Quick Thickness tool (dotted vertical red line) marking the depth of the reflection. ................................................................................................. 24

Figure 2.12: A. Bathymetric map showing data gaps and B. corrected bathymetric map with filled data gaps. ........................................................................ 25

Figure 2.13: Full-scale side-scan base map showing reflection intensity (Cribb, 2015). ....................................................................................................... 27

Figure 2.14: A. User defined sediment classification output from ArcGIS and B. Automatic bottom classification output from ENVI (Cribb, 2015). ....... 29

Figure 3.1: Changes in sea level along the Mid-Atlantic coast with associated MIS stages (Krantz et al., 2009). ..................................................................... 32

Figure 3.2: Representative cross-section of the stratigraphic units within the MD WEA as identified in this study. .............................................................. 33

Figure 3.3: Profile with surficial sand sheet ridge/swale topography as well as ravinement surface T1 marked in red. ..................................................... 35

Figure 3.4: Portion of a profile A. with potential Unit 2 lithosomes and B. with Unit 2 digitized. Note that profile also exhibits presence of Unit 3 but, was cut out to emphasize Unit 2. .................................................................... 36

Figure 3.5: A. Profile exhibiting Unit 3 channel with highly organized B. chaotic infill. ........................................................................................................ 38

Figure 3.6: Profile exhibiting major reflection corresponding to ravinement surface T2 in red. ................................................................................................. 39

Figure 3.7: Possible Pleistocene-Pliocene boundary interacting with bottom-multiple. ................................................................................................... 41

Figure 3.8: Map of WEA paleochannels and those mapped in surrounding studies. ... 43

Figure 4.1: Flowchart demonstrating step by step work-path taken to create the suitability map ......................................................................................... 52

Figure 4.2: Final suitability map showing optimal areas for development. ................. 55

Figure 5.1: Four types of offshore wind foundations (IPCC, 2012). ........................... 58

x

Figure 5.2: The twisted jacket foundation combining both jacket and monopile design features (de Villiers, 2012). ......................................................... 68

Figure A1: Reclassified bathymetry data. .................................................................... 78

Figure A2: Reclassified slope data. .............................................................................. 79

Figure A3: Reclassified paleochannel data. ................................................................. 79

Figure A4: Reclassified surficial sediment type data. .................................................. 80

Figure A5: Reclassified mobile sediment (Unit 1) data. .............................................. 80

Figure A6: Reclassified anomaly data. ......................................................................... 81

xi

ABSTRACT

As the offshore wind energy sector expands due to government mandates, a

thorough understanding of the geologic setting of potential project sites becomes an

essential component in the design process. Geophysical and geotechnical parameters

yield vital information on the sediments and/or rocks that are present. The variable

distribution of sediments, with concomitant variations in geotechnical properties, has

significant implications for the selection (e.g., monopile, suction caisson, gravity base,

jacket), design, location, installation, and subsequent scouring in the vicinity of wind

turbine foundations. Identifying suitable sites based on sediment types allow for

optimized engineering design solutions. Because foundations represent approximately

25% of total offshore wind project expenditures, reducing foundation costs with

geologic suitability in mind could significantly decrease required initial investments,

thereby expediting project and industry advancement.

To illustrate how geological and geotechnical data can be used to inform site

selection for foundations, geophysical data were analyzed and interpreted (chirp sub-

bottom profiling, side-scan sonar, and multibeam bathymetry) from the Maryland

Wind Energy Area (WEA). Side-scan sonar data from the WEA show three distinct

acoustic intensities; each is correlated to a general bottom sediment grain size

classification (muds, muddy and/or shelly sand, and sand with some gravel). Chirp

sub-bottom profiles reveal the continuity and thicknesses of various depositional

layers including paleochannel systems. Paleochannels consist of heterogeneous infill;

creating undesirable condit���� ��� �������� �� ����� ���� �������� ����

xii

provides a suitability model for how the interpretation of geophysical and geotechnical

data can be used to provide constraints on, and reduce uncertainties associated with,

foundation location and type selection.

Results from this study revealed 5 distinct subsurface units. The oldest (Unit

5) originated from Middle Pleistocene during Marine Isotope Stages (MIS) 5 & 6.

The youngest (Unit 1) consists of the modern surficial sand sheet sediments which

have been eroded and reworked during recent Holocene transgression. Several

distinct paleochannel systems incise the study area. Though data beyond the

boundaries of the study area are scarce a southeasterly channel direction along with

results from previous studies suggest these systems originated from Maryland coastal

bays. An integrated marine spatial planning approach identified the southernmost

portions of the study area as the most unsuitable for wind energy development.

Conversely, the same analysis determined that the central-eastern section of the WEA

is most suitable. Correlating these data with parameters governing foundation

selection revealed that piled-type foundations (either lattice or monopile) are most

appropriate for the study area, although suction bucket caisson foundations cannot be

definitely ruled out as a possible design solution.

1

Chapter 1

INTRODUCTION

1.1 Overview and Background

Climate change concerns resulting from increasing fossil-fuel generated carbon

dioxide emissions, coupled with highly volatile energy prices has led policy-makers to

support the use and further development of ������� ��������� � �� � �����

production (United States Energy Information Administration (EIA), 2015). In 2008,

the United States Department of Energy (DOE) developed a modified scenario where

wind energy would supply 20% of domestic electricity needs by 2030, with offshore

resources accounting for 18% of the total wind capacity (DOE, 2008). Subsequently,

onshore wind energy in the United States (US) has rapidly expanded with electricity

generation in 2013 reaching a high of 167,663 gigawatt-hours (GWh), accounting for

4.1% of total net production (National Renewable Energy Laboratory (NREL), 2014).

However, progress within the US offshore wind industry has been limited. As

of 2014, in the US there were 560 operating onshore wind facilities and 0 offshore

(DOE, 2015). While there are numerous areas that have been designated for potential

development (i.e., Bureau of Ocean Energy Management (BOEM) Wind Energy

Areas (WEAs)), several factors associated with offshore wind including the relatively

high levelized cost of energy (LCOE), instability of federal and state policies, complex

and long regulatory timelines, the necessity of developing local supply chains and the

logistics associated with construction, operation and maintenance have served as

barriers (DOE, 2015).

2

Foundations for offshore wind turbines represent approximately 25% of total

project expenditures (DOE, 2011). Foundations support the wind turbine tower

(typically constructed of steel and/or concrete) and the principal turbine subsystems,

including the rotor and blades, hub, drive train and nacelle (Manwell, 2009).

Minimizing foundation costs would aid industry advancement by lowering overall

project costs, required initial capital expenditures (CAPEX) and the LCOE.

Unlike onshore, offshore foundations are subject to a dynamic ocean

environment (Westgate and DeJong, 2005). As such, these foundations must be able to

withstand extreme horizontal and overturning moment loading resultant from waves,

wind, currents and potential debris and ice drift (Westgate and DeJong, 2005). Turbine

size (and thus loading), water depth and soil/sediment type and distribution govern the

optimal type of foundation used for a given project (Dean, 2010). Proper geological,

geophysical and geotechnical investigations aid in the siting, type selection and design

of foundations and can equate to savings worth millions of dollars (Feld, 2006). The

need for the most effective foundation selection is becoming increasingly more

important as the industry trends towards deeper, larger and thus even more costly

foundations (Westgate and DeJong, 2005; DOE, 2015).

1.2 Focus Area: Mid-Atlantic Bight and Maryland WEA

The US Mid-Atlantic Continental Shelf is the site of various proposed offshore

wind projects (Figure 1.1). A portion of the shelf referred to as the Mid-Atlantic Bight

(MAB) extending from Long Island to North Carolina, currently contains five BOEM

designated WEAs. The MAB is an area with consistently high wind speeds capable of

supporting project development (NREL, 2014) (Figure 1.2). It is a geologically

complex area (e.g., Meade, 1969; Milliman et al., 1972; Field, 1980; Knebel, 1981). In

3

addition to water depths, MAB surficial and subsurface sediments will play a critical

role in the type and design of foundation selected, and placement of turbines within

the designated WEAs. This thesis research project uses the Maryland WEA, and its

geological setting, as a model for how within the MAB region, the use of existing

geophysical and geotechnical data provides constraints on the siting of potential wind

projects and the selection of the optimal foundation type for these projects.

Figure 1.1: Mid-Atlantic Bight BOEM designated WEAs and New York and North Carolina areas of interest for potential offshore wind projects.

4

Figure 1.2: U.S. Mid-Atlantic offshore wind speeds for 90m hub height above sea level (NREL, 2014)

In this thesis, an assessment of the geological framework and sediment

distribution and characteristics of the Maryland WEA is used to delineate preferred

areas for development. A thorough analysis of geophysical data provided by high-

resolution sub-bottom profiling and side-scan sonar, and available geotechnical data is

used to place constraints on optimal foundation types. Particular emphasis is placed

on the analysis of data towards the prospect of employing a suction bucket foundation

design. Suction buckets are particularly advantageous because they use substantially

less steel, require a seabed penetration on the order of 10m or less and are more easily

transported, emplaced and subsequently removed (Bakmar et al., 2009). All of the

5

aforementioned factors result in significant cost reduction for suction buckets when

compared to other foundation types (Bakmar et al., 2009).

1.3 Geologic Framework

The MAB, encompassing the Maryland WEA, is characterized by a unique

surficial and shallow subsurface framework that evolved predominantly under

conditions of Quaternary sea-level regression and transgression (e.g., Knebel, 1981).

The MAB is considered a sediment-starved shelf, where the majority of mobile river

sediments remain trapped in estuaries (Meade, 1969). Thus, sediments currently

characterizing the MAB continental shelf surface (termed the surficial sand sheet by

Knebel, 1981) originated during the last sea-level lowstand (Milliman et al., 1972;

Field, 1980). Irregular distribution of the surficial sand sheet is primarily a result of

Holocene patterns of coastal and shelf currents (Milliman et al., 1972; Prusak and

Mazzullo, 1987).

The most prominent subsurface features along the MAB are paleochannels,

infilled channels of former river systems that flowed across the exposed shelf during

time periods of lower sea level (Twichell et al., 1977; Swift et al., 1980; Knebel and

Circé, 1988; Chen et al., 1995; Murphy, 1996; Boss et al., 2002; Nordfjord et al.,

2009; Childers, 2014). During the most recent sea-level lowstand approximately

20,000 years before present, the MAB was exposed as a land surface to nearly the

present-day continental shelf break, allowing drainage systems to erode seaward and

incise river valleys (Cronin et al., 1981; Toscano and York, 1992; Duncan et al.,

2000). These paleochannels are characterized by heterogeneous infill resulting from

deposition of sediments during subsequent sea-level transgression (e.g., Belknap and

Kraft, 1981). This heterogeneous infill is characterized by varying geotechnical

6

properties like highly elastic clays and silts, which deform under intense pressure, and

organic sediments with gas-filled pore spaces, creating undesirable conditions for

foundation placement (Dean, 2010). The Maryland WEA, located adjacent to the

Delaware River watershed, including several inland and coastal bays, is underlain by

multiple paleochannels (Figure 1.3).

7

Figure 1.3: Coastal bays & drainage systems surrounding the Maryland WEA (data from USGS national map database)

8

1.3.1 Existing Studies in the Area

Subsurface imaging is dependent on many soil properties, not exclusively

sediment type, and thus understanding the results of geophysical surveys requires a

thorough knowledge of a region�s geologic history. To correlate the Maryland WEA

data with the surrounding MAB region, a thorough investigation into existing

geological, geophysical and geotechnical data took place. Significant emphasis was

placed on finding available geotechnical studies including, but not limited to:

boreholes, cores and core logs, vibracores, CPT tests and grab samples. Geotechnical

studies provide information including specific parameters of soil which govern the

type of foundation that may be employed (Dean, 2010). These parameters include:

shear strength, plasticity, cohesion, moisture content and porosity. Geotechnical

parameters are critical in establishing an accurate representation of MAB surficial and

sub-bottom sediment conditions.

One of the biggest hurdles faced in this desktop study was the lack of publicly

available ground-truthing data. Geologic studies on the Maryland coast and the entire

MAB have been primarily constrained to the inner continental shelf. One study

conducted by Toscano et al. in 1989 extends along the Delmarva Peninsula and

eastward, ending just south of the Maryland WEA. Close in proximity, this study

provides valuable insight into the seismic facies and sedimentary characteristics of the

Maryland inner and outer continental shelf. Similarly, many other geophysical studies

have been conducted on the inner continental shelf of Delaware, Virginia, New York,

and New Jersey (Chen et al., 1995; Foyle, 1997; Hobbs, 1997; Mallinson et al., 2005;

Nordfjord et al., 2009; Thieler et al., 2014; Metz, 2015). There have also been a few

select geophysical and geotechnical efforts done on the outer continental shelf of New

Jersey and New York (Knebel and Wood, 1979; Nordfjord et al., 2006; Nordfjord et

9

al., 2009). Through thorough analysis and correlation, these studies offer valuable

information about MAB geology that provides a basis for the interpretation of the

Maryland High Resolution Geophysical Resource (HRG) Survey.

1.4 Objectives and Hypotheses

The overarching goal of this thesis research project is to define the geologic

setting of the Maryland WEA. Initial future offshore wind development on the

continental shelf of Maryland will be confined to this area, which has been designated

by the BOEM. The geological setting can provide unique and vital information that is

critical for making final location and design decisions within the Maryland WEA.

Varying geotechnical characteristics should be considered with other first-order

factors including wind resources, water depths, wave and current conditions, access to

onshore grid infrastructure and ecological and human impacts, in determining optimal

sites for MAB offshore wind projects.

The primary objectives of this thesis project are to:

1. Locate potential areas, based on the geological setting and associated

geotechnical properties of the sediments, within the Maryland WEA that are

optimal for the siting of an offshore wind project

2. Place constraints on the suitability of the various types of offshore wind

turbine foundations, including monopile, gravity base, jacket/lattice, with a

special emphasis on suction bucket foundation designs for potential use in the

MAB in general, and the Maryland WEA, in particular.

10

Chapter 2

METHODOLOGY

2.1 Data Acquisition

All data processed and analyzed within this thesis research project were

collected during a High-Resolution Geophysical Resource (HRG) Survey by Coastal

Planning and Engineering, Inc. for the Maryland Energy Administration from July 4 to

August 31, 2013. The survey, covering the Maryland Wind Energy Area (WEA) and a

surrounding buffer zone, includes: multibeam bathymetry, side-scan sonar,

magnetometer, shallow-penetration chirp sub-bottom profiler and medium penetration

multi-channel sparker seismic-reflection data. This thesis project focuses primarily on

the analysis of the chirp sub-bottom profiler, side-scan sonar and multibeam

bathymetry data.

As summarized in the 2014 Coastal Planning & Engineering, Inc. report, the

chirp sub-bottom profiles were collected using an EdgeTech 3200 sub-bottom profiler

with a 512i towfish. The sub-bottom data were merged with positioning information

from ultra-short baseline (USBL) and C-Nav differential global navigation satellite

system (DGNSS) data via Hypack® hydrographic survey and processing software.

The sub-bottom profiler was operated using a 5 millisecond (ms) pulse length, at a

60% power level, with a frequency sweep of 1.0 to 10.0 kilohertz (kHz) and a ping

rate of 7 hertz (Hz). At an average vessel speed of 4.0 knots and a sampling interval of

7 Hz, the distance between individual chirp pings is approximately 30 centimeters

(cm). Assuming an average frequency of 5.5 kHz, a seismic velocity of 1,500 meters

11

per second (m/s) and applying the 1/4th wavelength criterion (e.g., Telford et al.,

1990), the vertical resolution of boundaries between acoustically differing sub-surface

layers using the chirp system is estimated to be on the order of 10 cm. The horizontal

resolution of the chirp system using the first Fresnel zone as an approximation (e.g.,

Telford et al., 1990), assuming a frequency of 5.5 kHz and a velocity of 1,500 m/s,

would be 1.65 meters (m) at a depth of 5 m, and 2.33 m at a depth of 10 m below the

towfish.

Side-scan sonar data were collected using an EdgeTech 4200-HFL side-scan

sonar system running Discover acquisition software with a 300/600 kHz dual

frequency towfish. The digital side-scan data were also merged with positioning data

from the USBL and C-Nav DGNSS systems via Hypack. The side-scan data were

collected in high definition mode with a 100 m range scale (200 m swath). Multibeam

bathymetry data were collected using the Reson SeaBat 7125 dual head system. A

total of 1024 beams per sweep and 5190 square kilometers (km2) of multibeam data

were collected. A Sea-Bird Electronics Inc. sound velocity probe was used to record

real-time sound velocity at the multibeam transducer head. Similar to the chirp sub-

bottom and side-scan surveys, navigation data were collected using the Hypack

positioning system.

The chirp sub-bottom and side-scan sonar data were collected along 157

principal survey north-south trending tracklines spaced at 150 m intervals and 28 east-

west tie lines spaced 900 m apart (Figure 2.1) (Coastal Planning & Engineering, Inc.,

2014). The bathymetry data were also collected along the principal survey tracklines,

and, to ensure full coverage of the bottom, along 75 m spaced supplemental tracklines

in shallow areas (Coastal Planning & Engineering, Inc., 2014).

12

Figure 2.1: Track line map showing N/S and E/W survey lines throughout the area.

13

2.2 Chirp Sub-bottom Profiles

2.2.1 Data Processing

Chirp sub-bottom profiles were processed and analyzed using Chesapeake

Technologies, Inc. SonarWiz 5 software. Processing consisted of a step-by-step

sequence as summarized below.

2.2.1.1 Data Import

During collection, profiles were digitally recorded in EdgeTech jsf format.

Profiles were imported into a new SonarWiz project with a geographic projection of

UTM 1983 18N and a manual jsf scalar value of either 150, or 250 when unusually

low amplitude returns were observed, on initial profile plots. The manual scalar is a

multiplying factor applied to the sonar returns that scales them within a 16-bit integer

range for optimal display (Chesapeake Technology, Inc., 2014). The larger the scalar,

the greater the multiplication of return amplitudes (Chesapeake Technology, Inc.,

2014). The 150 and 250 values used were determined by experimentation, varying the

scalar between 1 and 500 and observing the strength (neither too faint (i.e., scalar too

low) nor too dark (i.e., scalar too high) of the sonar returns shown on initial plots of

the data (Figure 2.2).

14

Figure 2.2: Example of a profile with A. 150 scalar and B. 250 scalar.

2.2.1.2 Bottom Tracking

After profiles were imported, they were processed using the bottom-tracking

function in the Digitizing View of SonarWiz. ��� ���� ��� �� ��� ��� �������

this function compares the amplitudes of successive data points, identifies when an

initial large increase in amplitude occurs (e.g., a high amplitude signal is generated by

the reflection of acoustic energy at the water-seafloor boundary) and defines this

position as the seafloor, or bottom. To ensure that the seafloor position is correctly

defined, the function allows the user to specify blanking, duration and threshold

factors. Blanking is the distance (depth) below the towfish where the function will

initiate the search for the increase in amplitude due to bottom reflections, duration is

15

the number of consecutive pings to examine to determine the along-track continuity of

the high-amplitude bottom reflections, and threshold is a comparison of the amplitude

of the bottom reflections to a median value, which aids in the identification of the

probable seafloor location (Figure 2.3).

Figure 2.3: Example of a sub-bottom profile A. prior to bottom tracking and B. after bottom tracking.

After bottom tracking to enhance sub-bottom reflection events, automatic gain

control (AGC) was applied to data occurring beneath the defined seafloor. AGC is a

method to adjust (or gain) the amplitudes of time-varying signals relative to an

16

average output level such that even if the amplitudes were continuously decreasing

(e.g., due to spherical divergence and absorption), the output level would be

maintained (Telford et al., 1990). AGC is performed by determining average signal

amplitudes within relatively short sample time periods, and adjusting applied gains

(intensity) within the sample periods such that the output signal level is relatively

constant (Telford et al., 1990). In the application of AGC in SonarWiz 5, all

amplitudes of signals were applied a resolution of 30 and an intensity of 25 with

amplitudes above the seafloor set to zero, eliminating any �noise� within the water

column (Figure 2.4).

Figure 2.4: Example of a profile A. before and B. after the application of AGC.

17

An additional function that was often used was a swell filter. This function

eliminates the effect of vertical towfish motion on the chirp profiles due to surface

waves causing the tow vessel to heave. The effect of heave on the towfish causes the

bottom, and subsequent sub-bottom, reflections to have an along-track sinusoidal

pattern that mimics wave motion. This function eliminates these unwanted signals,

which disrupt the coherence of the profile, by smoothing it in continuous intervals.

The interval is specified by the user based on the approximate period of wave motion.

The values used for this project ranged from 3.0 � 4.0 seconds (s) (Figure 2.5).

Figure 2.5: Example of a profile where waves have created interference A. before and B. after the swell filter has been applied.

18

2.2.2 Digitizing Features of Interest

After processing by bottom tracking, applying AGC and, if needed, swell

filtering, the chirp profiles were then examined, major reflection events associated

with features of interest were identified and these major events were digitized. Within

the Digitizing View window the Insert Polyline tool was used to digitize the boundary

around or in-between major features. This tool allows the user to create digital

polylines by clicking along or around the identified feature to create a series of

inflection points that will form the line (Figure 2.6). The major features of interest in

this study were: 1) the locations of paleochannels, 2) the extent (horizontal and

vertical) of the surficial sand sheet and 3) the boundaries of older (deeper) geologic

features.

Figure 2.6: Example of a profile with a feature of interest A. before digitizing and B. after digitizing with a blue polyline.

2.2.2.1 Paleochannels

The identification of potential paleochannel systems involved examining the

chirp profiles in Digitizing View and searching for reflections in the sub-bottom that

19

appeared to disrupt, or cut-across, generally horizontal reflections (Figure 2.7). The

disrupting reflections dip from shallow to deeper depths, and are interpreted as the

edges of paleochannels. Paleochannels are the most prominent subsurface feature

identified in geologic studies across the region (Twichell et al., 1977; Swift et al.,

1980; Knebel and Circé, 1988; Chen et al., 1995; Murphy, 1996; Boss et al., 2002;

Nordfjord et al., 2009; Childers, 2014). The conditions that characterize these systems

consist of broad channels that have disrupted previous depositional sequences by way

of erosion. Subsequently they are in-filled and buried by heterogeneous sediments,

distinct from the surrounding stratigraphic layers (Belknap and Kraft, 1981).

Figure 2.7: A. Profile with a potential paleochannel system and B. same profile with interpreted spatial extent of channel.

20

Once a potential paleochannel was identified, its endpoints were marked with a

contact point, and the latitudes and longitudes were recorded. This allowed the

identification of the full horizontal extent (width) of the paleochannel at the given

location. This process was repeated for the entire WEA. The points were exported as

shapefiles and ����������� �� ���� �� ������ ArcGIS 10.2 software (Figure 2.8).

Within ArcGIS, traces of the paleochannels were then created as polygon shapefiles

(Figure 2.8).

Figure 2.8: A. Point data imported from SonarWiz prior to B. creation of polygons (paleochannels) based on endpoints/horizontal extent from profiles.

21

2.2.2.2 Surficial Sand Sheet

Identification of the surficial sand sheet involved analyzing each of the chirp

sub-bottom profiles in Digitizing View. The Quick Thickness function was used to

extract data from profiles that exhibited surficial dune and shoal-like features. When a

dune was identified, the Quick Thickness tool was used to determine both the vertical

and horizontal extent. This was done by clicking on the profile at the base depth of

the dune and subsequently clicking again on the top of the dune directly above it. This

then created a feature that calculates and stores the thickness based on the distance

between the two points (Figure 2.9). To measure the horizontal extent, the Quick

Thickness function was used every 100 m until the entirety of the dune/shoal was

mapped (Figure 2.9). Areas that exhibited a mobile sand layer less than or equal to 0.5

m were not mapped using this function.

All Quick Thickness feature data were exported as an ASCII file. Once the

entire study area was analyzed, the files were combined and imported into ������

ArcGIS 10.2 as non-gridded xyz point data in a projection of UTM 1983 18N. Since

the surficial sediment layer had an irregular distribution, many regions had little to no

mobile sediment and as such, portions of the study area were left without data points

(Figure 2.10). To fill in the entirety of the WEA, the Nearest Neighbor interpolation

method was used (Figure 2.10).

22

Figure 2.9: Example of a A. profile with a dune present and B. same profile with Quick Thickness tool (dotted vertical red line) recording the thickness of the dune.

Figure 2.10: A. Exported surficial sediment data and B. Nearest Neighbor method.

23

2.2.2.3 Depth of Major Reflection Events

Boundaries between potential stratigraphic layers can be identified from

contrasting returns. Different sediment properties such as type, density, shape and

porosity combine to reflect different values. Areas that exhibit contrasting changes in

the intensity of the return contain differing properties, which may be indicative of a

change in sediment type (Figure 2.11). However, because a change in reflection

intensity is dependent on multiple factors, a thorough knowledge of the geologic

history and other studies is required to produce a valid conclusion of the conditions.

After a major reflection was identified, the Quick Thickness tool was used to

calculate its depth below the surface. This was done by clicking on the profile at the

depth and location the event occurred, and then clicking again on the top of the water

column. This created a feature that calculates and stores the depth based on the

vertical distance between the two points (Figure 2.11).

24

Figure 2.11: Example of a A. major reflection event/boundary distinguished by different reflectivity intensities and B. same profile with Quick Thickness tool (dotted vertical red line) marking the depth of the reflection.

2.3 Multibeam Bathymetry

Bathymetry data were processed using ������ ���� �� � ��������. The files

were imported as non-gridded xyz point data with the projection UTM 1983 18N.

Using the Point to Raster tool, the data were converted and merged into a raster file

with a cell size of 3.4 m (Figure 2.12). Upon importing and converting the data, it was

discovered that some data gaps existed. In order to fill in the gaps, data from the 2007

National Ocean Service (NOS) Hydrographic Survey were retrieved from the online

National Geophysical Data Center (NGDC) NOAA database. The data were imported

and converted into a raster using the Point to Raster tool. Using the Mosaic to New

Raster tool, the two datasets were then merged with the WEA data prioritized,

permitting the NOS survey to only fill in the gaps (Figure 2.12).

25

Figure 2.12: A. Bathymetric map showing data gaps and B. corrected bathymetric map with filled data gaps.

2.4 Side-Scan Sonar Profiles

2.4.1 Data Processing

Side-scan data were processed and interpreted by Coty Cribb (2015) as part of

his University of Delaware Undergraduate Senior Thesis Project. In this senior thesis;

the side-scan data were processed using Chesapeake Technologies, Inc. SonarWiz 5

software. A full description of his work can be found in his thesis however, a brief

summation of his methodology is included below.

26

Side-scan sonar profiles were imported into a new SonarWiz project with a

geographic projection of UTM 1983 18N. After import, they were bottom tracked

using both automatic and manual functions, the latter when the automatic function was

unable to correctly determine the position of the seafloor.

Based on the intensity of side-scan sonar returns, bottom features of interest

such as changes in sediment type and morphology were digitized by manually tracing

their extent and generating polylines or polygons that outlined their locations.

Obstructions and anomalies such as shipwrecks were identified by inserting a contact

point, recording their location and a description of the feature.

To create a full-scale side-scan sonar map of the entire WEA, an electronic

gain normalization (EGN) was applied to each separate side-scan mosaic (collection of

profiles in separate projects). This technique raises the contrast between low and high

amplitude returns, thus making it easier to differentiate between varying reflection

intensities. ���� ������ �� ��� ����� ��� �������� ����� �� �� at locations

where there was overlap of side-scan sonar returns from different tracklines, the value

used was the mean of the intensities of the overlapping values. The mosaics were then

exported as ERDAS IMAGINE image files and merged with their neighbouring

mosaics in ArcGIS 10.2 to create a full-scale reflection intensity base map of the WEA

(Figure 2.13).

27

Figure 2.13: Full-scale side-scan base map showing reflection intensity (Cribb, 2015).

2.4.2 Sediment Classification

Cribb (2015) used two methods of bottom sediment classification: user-defined

and automatic classification. As summarized from his thesis, once the WEA basemap

was imported into ArcGIS, a user-defined sediment classification map was formed by

creating new shapefiles and drawing polygons around distinct changes in reflection

intensity. Polygons were then assigned one of three colors based on like reflection

28

intensities. Black represented fined grained sediment, gray for medium (sand-like)

sediment and white for coarse grained gravels (Figure 2.14).

Cribb (2015) imported the ERDAS IMAGINE WEA basemap into ENVI to

generate an automated sediment classification. A supervised classification method

was chosen whereby the user trains the program by selecting regions with backscatter

returns associated with three of the classes representing a different sediment type and

��� ����������� ��� � � areas. The values that correspond to each class were

selected using a polygon creator tool, whereby polygons were drawn around each

reflection intensity. After multiple examples of each sediment type were chosen, a

minimum distance algorithm was calculated based on the pixel values inside the

polygons. Using ENVI, Cribb (2015) processed the base map and categorized local

pixel clusters based on how well they fit into each sediment class grouping (Figure

2.14).

29

Figure 2.14: A. User defined sediment classification output from ArcGIS and B. Automatic bottom classification output from ENVI (Cribb, 2015).

30

Chapter 3

SHALLOW STRATIGRAPHY OF THE MARYLAND WEA

3.1 Introduction

In this chapter, an interpretation of the seismic and lithologic data in the

vicinity of the Maryland WEA shallow subsurface is presented. If available, a critical

factor in geological/geophysical studies is the incorporation of cores and/or

geotechnical data. Cores provide ground-truthing information necessary to confirm

results inferred from seismic datasets. Geotechnical data provide physical parameters

of the sediments/rocks encountered, and are of vital importance to design engineers

planning to place structures in the area of study. Unfortunately, core and geotechnical

data within the region were scarce, and noticeably absent from the Maryland WEA at

the time that this thesis project was carried out.

To develop a model of the sedimentary environment without the presence of

cores and geotechnical data, the findings in this project were compared to the results

of studies conducted in surrounding areas. Significant emphasis was placed on studies

by Toscano et al. (1989) and Williams (1999). These investigations were not only

geographically the closest but, also the most comprehensive and relevant geologically

(Table 1).

31

Table 3.1: Toscano et al. & Williams stratigraphic units of coastal MD/DE. Summarized from Toscano et al. (1989) and Williams (1999)

Unit Age Toscano et al. Identifier

Williams Identifier

1 Holocene Q5 A 2 Holocene Q4 B

3 Late Pleistocene / Early Holocene

Q3 C

4 Middle / Late Pleistocene

Q2 D

5 Early Pleistocene /

Pliocene Q1 E

3.2 Eustatic Sea-Level Change

During the late Tertiary through the Quaternary, sea level has vertically

fluctuated by approximately 150 m along the U.S. Mid-Atlantic coast (Figure 3.1)

(Krantz et al., 2009). To date, several studies have correlated cycles of sea-level

transgression and regression with continental shelf stratigraphy (Shideler, 1972;

Toscano et al., 1989; Foyle, 1997; Williams, 1999; Duncan, 2000; Nordfjord et al.,

2005, 2006, 2009; Metz, 2015). Slow sea-level change induces strong reworking and

erosion of sub-bottom sediment (e.g., Belknap and Kraft, 1981). Rapid sea-level

change is associated with minimal shoreface erosion and preservation of depositional

sequences (e.g., Belknap and Kraft, 1981). Sea-level lowstands allow for the

migration of river channels across the exposed continental shelf (e.g., Toscano et al.,

1989; Williams, 1999; Duncan, 2000; Nordfjord et al., 2009). These channels are then

infilled from subsequent sea-level transgression (Belknap and Kraft, 1981).

32

Figure 3.1: Changes in sea level along the Mid-Atlantic coast with associated MIS stages (Krantz et al., 2009).

3.3 Stratigraphic Units

In this thesis project, through a detailed analysis of the chirp sub-bottom

seismic profiles, five major stratigraphic features were identified (Figure 3.2). The

oldest units are interpreted to be representative of early Quaternary regression and

conversely, the youngest of modern Holocene transgression. The most prominent

features among the sub-bottom profiles are incised channel features and their infill.

The channel infill is complex, ranging from highly organized to chaotic (Figure 3.5).

Based on the theory of superposition, it is interpreted that layers incised by, or below,

the channels are older in age, while the channels, their infill and sequences above them

are younger.

33

Figure 3.2: Representative cross-section of the stratigraphic units within the MD WEA as identified in this study.

34

3.3.1 Unit 1: Holocene Sand Sheet

The uppermost stratigraphic unit mapped in this project has an irregular

distribution throughout the WEA ranging from 6 m thick to a thin veneer of less than

0.5 m (Figure 3.3). The unit is interpreted to represent modern transgressive Holocene

sands with migrating ridge and swale topography, most likely due to scour produced

by storm events (Knebel, 1981). Regions containing the thickest deposits of Unit 1

occur in the southern half of the study area, whereas the northeastern area shows

minimal thicknesses (Figure 2.10). This uppermost unit can be correlated with

������� �� ��� (1989) �� � ��� � ���� (1999) �� � �� �������� (2000) Surficial

���� ������ ��������� �� ��� (2009) Holocene Sand Ridges and Metz�� ��� �! Unit 5

Holocene Sands.

There is a strong reflection in the chirp data that occurs at the base of the

surficial sand unit (Figure 3.3). This reflection, herein referred to as T1, is interpreted

to be associated with a ravinement surface representing a transgessive unconformity

separating reworked modern shelf sediment of Marine Isotope Stage (MIS) 1 and 2

(i.e., the surficial sand unit), and older depositional sequences ranging from MIS 3 " 5

(Figure 3.1) (Krantz et al., 2009). This ravinement is referred to as A1 and T by

Toscano et al. (1989) and Nordfjord et al. (2009), respectively. Ravinement surfaces

are erosional unconformities formed during transgression. Rising sea level erodes the

previously deposited sediment creating a distinct boundary between the underlying

(older) and newly deposited sediment (Catuneanu et al., 2011).

35

Figure 3.3: Profile with surficial sand sheet ridge/swale topography as well as ravinement surface T1 marked in red.

3.3.2 Unit 2: Transgressive Coastal Lithosomes

Where identified, this unit is always below Unit 1 (when present) and truncated

by the transgressive ravinement surface (as shown by reflection T1) formed during the

LGM. The thickness of Unit 2 varies from 1 to 3 m. This unit is sparsely observed in

the Maryland WEA and primarily occurs in regions with paleochannels (Figure 3.4).

The strong character (i.e., high amplitude) of the reflection at the base of this unit

suggests a dramatic change from the underlying heterogeneous channel infill (of Unit

3) and Unit 2 to distinctly different muddy sediments of perhaps estuarine and back-

barrier origin.

This unit is interpreted as possible remnants of transgressive coastal lithosomes

formed during a Holocene transgression. Rapid sea-level rise from 20 � 10 thousand

37

al., 1989; Chen et al., 1995; Murphy, 1996; Boss et al., 2002; Nordfjord et al., 2005;

Childers, 2014; Metz, 2015).

The channel systems are likely Late Pleistocene to Early Holocene in age with

incision occurring during glacial time intervals (e.g., MIS Stages 4 and 2) coinciding

with larger areas of the continental shelf exposed to fluvial processes (Figure 3.1)

(Krantz et al., 2009). Approximately 18 � 20 ka during maximum Wisconsinan

glaciation, sea level was lowered to ~130 m mean sea level (MSL) and these channel

systems incised as far as the present day shelf break (Figure 3.1) (Krantz et al., 2009).

The paleochannels mapped as Unit 3 in the Maryland WEA extend beyond the length

of the study area. It is likely that they continue towards the shelf break and/or merge

with other relict drainage systems.

Thalwegs for the paleochannels mapped in the Maryland WEA have depths to

10 m below the seafloor (bsf). The infill, as shown by the reflections within Unit 3, is

complex and ranges from highly organized to chaotic (Figure 3.5). The basal portions

of the channels consist of mainly fluvial sediments of fine to coarse sands formed at

the height of glacial maximum time intervals (e.g., MIS 4 and 2) (Figure 3.1) (Krantz

et al., 2009). Subsequent Holocene transgression during MIS 3 and 1, and base level

increase resulted in aggradation of sediments representative of changing environments

from fluvial, to lagoonal, to estuarine (as mapped as Unit 2 in this study) (Krantz et

al., 2009). The chaotic nature of the reflections associated with most of the infill made

it difficult to characterize the exact environments of deposition however; there are

several examples of reflections that can be interpreted as showing prograding and

lateral infill, which are indicative of meandering channels (Nordfjord et al., 2005)

(Figure 3.5). Depositional units similar to Unit 3 of this study have been described by

38

Toscano et al.�� (1989) Unit Q3 and W�������� �� ��� �� ��� �� ��� �dentified

as consisting of ����� ������ ��� � ���� ����� �� ������ ������� ����� ����� ��

���������� �!������ �� �� ������ ��"��� � (Toscano et al., 1989). Williams� (1999)

Unit C consisted of coarse sands and gravels. Analogous units to Unit 3 of this study

��!� ���� ���� ��������� �� #������� (2000) �������� ��� � $����%����� (2005)

Channels Unit and &� '�� (2015) Unit 3.

Figure 3.5: A. Profile exhibiting Unit 3 channel with highly organized B. chaotic infill.

3.3.4 Unit 4: MIS 5 Interglacial Deposits

Unit 4 lies beneath Unit 1 (when present) and appears throughout the entire

WEA. It ranges from 5 to 8 m thick and contains widespread subparallel reflections

that most likely represent bedding that occurs within the unit (Figure 3.6). Prominent

incised channeling of this unit by Unit 3 means it must be older than Unit 3 and the

LGM. The base of Unit 4 is defined by the most prominent reflection (herein referred

39

to as T2) encountered in the profiles, approximately -15 to -37 MSL dipping gently

towards the southeast (Figure 3.7). Following Toscano et al. (1989) and Williams

(1999), reflection T2 is interpreted as the basal ravinement surface separating Unit 4

and a deeper Unit 5 (Figure 3.6). Toscano et al. (1989) and Williams (1999)

interpreted this boundary as a transgressive erosional unconformity formed during

MIS 5, with a maximum interglacial time period approximately 130 � 120 ka (Figure

3.1) (Krantz et al., 2009). Unit 4 sediments overlying this unconformity are

interpreted as muds with finely interbedded sands and/or silts resultant of an evolving

marine to estuarine environment; similar to those found by both Toscano et al. (1989)

and Williams (1999) who described the unit as comprised of silty clays with thin, fine

sand laminae.

Figure 3.6: Profile exhibiting major reflection corresponding to ravinement surface T2 in red.

40

3.3.5 Unit 5: Middle Pleistocene

Unit 5 is the lowermost unit identified in the chirp sub-bottom profiles studied

in this thesis project. It is separated from the overlying Unit 4 by reflector T2.

Spatially, Unit 5 appears throughout the study area. Unfortunately, due to the

relatively high frequency and thus lower depth of penetration of the chirp system, the

full vertical extent (thickness) of this unit could not be mapped. The sediments

contained within this unit are interpreted to have originated sometime during MIS 6 or

earlier. A similar unit (Q1) mapped by Toscano et al. (1989) was found to contain

shelly sands. They interpreted this unit to have originated during the middle to late

Pleistocene (Toscano et al., 1989). Williams (1999) also noted the appearance of a

comparable unit in several profiles however, absence of available core data to the

depths needed limited their interpretation. Although there is a lack of data to directly

confirm, Unit 5 is interpreted to be part of the Omar Formation. According to Ramsey

(2010), the Omar Formation is of Middle Pleistocene age and ranges in thickness from

3 to 24 m. It is comprised of quartzose, homogeneous, fine to very fine sand with

scattered medium to coarse laminae commonly overlain by dark-greenish-gray, silty

clay to clayey silt with scattered shell beds and bioherms (Ramsey, 2010).

In some of the chirp sub-bottom profiles, a distinct reflection event ~10 to 15

m bsf was identified. However; oftentimes it occurred at a depth that coincided with

the presence of a multiple reflection between the towfish and the sea-bottom (Figure

3.7). Due to interference from this bottom-multiple, there is insufficient data to

accurately map the position and thus confidently interpret the origin of this reflection.

Based on geological studies in adjacent regions, it is possible that this reflection

represents the boundary between Pliocene- and Pleistocene-age sediments. The

Pleistocene-Pliocene unconformity is identified by Shideler et al. (1972) as reflection

41

R2 and Toscano et al. (1989) as reflection M1 in their data. Sediments below these

reflections likely are of the Beaverdam Formation (Field, 1980; Toscano et al., 1989;

Metz, 2015).

Figure 3.7: Possible Pleistocene-Pliocene boundary interacting with bottom-multiple.

3.4 Paleochannel Systems

As mapped by Unit 3, at least two major fluvial systems (Paleochannels 1 and

2) with channel widths ranging from 600 to 1,000 m, and four smaller channels

(Paleochannels 3, 4, 5 and 6 with widths of 100 to 300 m) can be identified within the

study area (Figure 3.8). All of these channel systems have similar thalweg depths and

trend towards the south to south-east following the general drainage pattern mapped

by Toscano et al. (1989). Observed cross-cutting relationships between the channels

(e.g., Paleochannel 3 cuts-across, and is thus younger, than Paleochannel 2) suggest

that Paleochannels 1 and 2 are older, and would thus have been formed by incision

during earlier glacial time periods (e.g., MIS 4 or 6?) as compared to Paleochannels 3

and 6 which may have been incised during glacial time interval MIS 2. Absence of

42

ground-truthing data proved to be problematic in terms of determining accurate age

estimates and the direct correlation between a given paleochannel and its

corresponding MIS stage, and thus time of incision.

Interpretation of the extent of paleochannels beyond the boundaries of the

study area was difficult. Landward of the WEA, it is likely that the channels

originated from Assawoman Bay and/or Isle of Wight Bay (Krantz et al., 2009).

Studies mapping relict channel systems just north of the study area show much larger

(both wider and deeper) channel incisions than those mapped in this project (Williams,

1999; Childers, 2014; Metz, 2015). Those paleochannels, which extend to the

Delaware Estuary, are larger in dimension likely due to the greater size of the

Delaware Estuary drainage network, distinct from the smaller, more localized bay

drainage system in Maryland (Figure 1.3). In contrast with the data collected in this

project, Williams (1999), Childers (2014) and Metz (2015) observed a more eastward

trending channel system. This is indicative of a possible drainage divide along the

border of Maryland and Delaware, first hypothesized by Williams (1999) and Krantz

et al. (2009). North of the Delaware-Maryland state line, tributaries extended north-

east towards the ancient Delaware River, whereas south of the state line, rivers ran

south-east, eventually joining the Chesapeake (Krantz et al., 2009).

43

Figure 3.8: Map of WEA paleochannels and those mapped in surrounding studies.

44

3.5 Conclusions

The goal of this geophysical analysis was to interpret the shallow sub-bottom

stratigraphy of the Maryland WEA and further correlate it with existing studies in

adjacent regions. The chirp sub-bottom profiles confirm a complex geologic evolution

primarily driven by changes in eustatic sea level over the last 2.5 million years. Five

unique stratigraphic units have been identified, similar to those previously studied in

the MAB. Unit 1 Holocene sands have an irregular distribution throughout the WEA

and MAB. Unit 2 coastal lithosomes have a limited occurrence and appear to be

constrained to areas associated with antecedent channel topography. Unit 3 represents

fluvial incisions during Late Pleistocene to Early Holocene glacial time intervals

followed by infilling during subsequent transgressions. The infill is characterized by

reflections that indicate deposition of heterogeneous sediments representative of rapid

sea-level rise. High amplitude reflections are interpreted to represent unconformities

within the study area including the boundaries between Units 4 and 5. Within the

gently dipping Unit 4, reflections are interpreted to show subparallel bedding and

laminae. Unit 5, the lowermost unit that can be distinguished in the chirp sub-bottom

profiles is interpreted to have originated during the Middle to Late Pleistocene, MIS 6.

A reflection that occurs in close association with a multiple event related with the

towfish and sea-bottom potentially represents the boundary between Pliocene- and

Pleistocene-age sediments.

45

Chapter 4

SUITABILITY OF THE WEA AND ADJACENT REGIONS

4.1 Introduction to Marine Spatial Planning

In Europe, the siting and development process for offshore wind projects has

been fully established. Programs like the Marine Resource Assessment System

(MaRS) utilize hundreds of spatial datasets combined with Geographic Information

Systems (GIS) to create maps for potential offshore energy development in the UK

(Moore, 2009). These maps indicate areas of opportunity and constraint based on a

variety of external factors including wind resource, geology, marine mammals,

shipping lanes, obstructions, etc (Moore, 2009). The usefulness of this integrated

�������� ��� � ���� �� � ����������� ��� ��� �� ������� ������� ���

projects (e.g., van Heteren, 2005; Haasnoot et al., 2014; Golightly and Birchall, 2015).

In the U.S. however, offshore wind projects have generally been managed by the

developer(s) based solely on first-order factors such as wind resource, sand borrow

areas, artificial reefs, fishing and shipping, with site-specific environmental

assessments during the latter stages of development. Prior to leasing, during the

Planning and Analysis stage of the Federal Renewable Energy Program, BOEM

conducts an Environmental Assessment (EA) however, a geological assessment is not

included (BOEM, 2016). This narrowly-focused approach can lead to the discovery of

unforeseen obstacles such as unsuspected boulders and/or marine mammal habits and

migration routes well into the planning process. Adapting to new constraints late in

46

development can drastically increase total project costs and as such, it is critical that in

the US marine spatial planning strategies similar to those in Europe be employed.

To provide a framework to aid in the siting of MAB offshore wind projects, I

will adapt a marine spatial planning approach similar to that of the MaRS program.

The output will be a map with a graded scale indicating the best and worst areas

recommended for development. Emphasis is placed on geologic variables, which are

ordinarily thought of as secondary to wind resource, water depths, proximity to

onshore grid infrastructure, and ecological and human impacts. In almost all cases,

geologic variables vary spatially across a study area and should be considered as first-

order elements included at the beginning of the decision making process. Geological

and geotechnical factors are relevant because the three-dimensional variable

distribution of sediments has significant implications for foundation selection (e.g.,

monopile, suction caisson, gravity base), design, location, installation, and subsequent

scouring.

4.2 Suitability Model

4.2.1 Step 1: Identify Parameters

In order to generate a suitability map, water depths, bottom anomalies (e.g.,

shipwrecks and unexploded ordinances (UXO)) and four key geologic factors that

affect the location of potential offshore wind projects within the study area were

identified. The geologic factors are: slope, sediment distribution and type, subsurface

paleochannels and mobile/surficial sediment. Although most geologic �desk����

studies also incorporate available subsurface geotechnical parameters in their models,

47

the lack of available core and geotechnical data for this area prevented including it

within the model.

4.2.2 Step 2: Defining Scale and Suitability

Once the suitability parameters were identified, a scale was devised to rank

them in terms of their impact. To integrate the parameters into a single map, they

must all be quantified (reclassified) on the same scale. Reclassification is necessary

because it allows the user to compare different types of data with different values, all

based on the same ranking scheme/scale. For the purposes of this study, the scale is

comprised of 4 classifications varying from highly suitable to unsuitable. Since GIS

works in discrete numbers, each suitability condition was assigned a corresponding

value on a scale ranging from 1 to 4 (Table 2). Parameters which are highly suitable

receive the lowest value (i.e., 1); less suitable parameters receive increasing values

associated with their decreasing suitability. Thus for the final map output, areas with

the lowest values will be the most suitable and vice versa.

Table 4.1: Suitability scale with corresponding numeric values.

Highly suitable 1 Suitable 2 Slightly Suitable 3 Unsuitable 4

4.2.3 Step 3: Create a Work-path

The next step was creating a work-path. Each dataset is in vector format,

consisting of multiple attributes and various descriptive characteristics. They all need

to be reduced to smaller, simpler data to extract the selected parameter outlined. To

48

extract this information, we must identify the attribute(s) from each dataset that is

most important.

Bathymetry data provided both depth and slope measurements. Shallow water

depths require generally smaller foundations with less material and as such are more

economical. Depending on the geologic evolution of the region, shallower waters may

also correlate with shorter distances from shore. Less distance traversed by vessels

getting to and from port equates to both time and cost savings while shorter shore

connection cables equates to lower equipment costs and less line loses. Depth data

were subdivided into four categories: � 20, 21 < 30, 31 < 40 ��� � �� � (Table 3).

Depth categories � 20 and 21 < 30 m were assigned values of 1 and 2 respectively,

accounting for lower material and transportation costs and that a more diverse

foundation selection exists at these depths (discussed in greater detail in Chapter 5).

Extending to depths beyond these areas increases material and transportation costs

while decreasing the selection of foundation designs.

Slope data were also divided into 4 categories: 0 � 2, 2.1 � 5, 5.1 � 8 and 8.1 �

11° (Table 3). Highly sloping bathymetry is less suitable because foundations are

more economically installed in areas of low relief. Decreasing suitability was

assigned to each category on the assumption that steeply sloping areas require more

dredging and pre-installation preparation than locations which are flat or gently

sloping. Areas with > 8° slope were classified as unsuitable because it is assumed to

be more economical to avoid them, rather than modify them.

Mobile sediment (i.e., Unit 1 sand sheet) data provide thickness values and the

distribution of the mobile sand sheet. Installing foundations in an area of thick,

mobile sediment can leave large portions of the sub-structure exposed after major

49

storm events induced scouring (Whitehouse, 1998). Mobile sediment data were

divided into three categories: 0 � 1, 1.1 � 3 and 3.1 � 6 m (Table 3). They were

assigned values of highly suitable, suitable, and slightly suitable respectively. These

values were based on the assumption that increasing quantities of mobile sand will

require larger and thus more expensive scour protection. No areas were ranked

unsuitable since scour protection is usually constructed of concrete and is relatively

inexpensive in the greater scope of foundation selection and construction.

Paleochannels are infilled with heterogeneous sediment from changing

environments as sea level rises across a continental margin (Knebel and Circé, 1988;

Toscano et al., 1989; Krantz et al., 2009). Foundations, which will be discussed

further in depth in the next chapter, are simpler to design and install in homogeneous

sediment (Westgate and DeJong, 2005; AWS, 2009; Bakmar et al., 2009; Malhotra,

2011; Bhattacharya, 2014). Paleochannels, when present, were classified as slightly

suitable while regions absent of them were ranked highly suitable (Table 3). Although

the presence of these systems is not ideal, they are not a restricting factor. To meet

design criteria and ensure stability, foundations installed in these regions might need

to be piled deeper requiring more material and capital expenditure.

Surficial sediment data and interpreted sub-bottom profiles provide

information on the location of gravels, sands and muds (Table 3). Areas of sand are

advantageous for foundation installation and are more economical and should be seen

as preferred areas for development, thus they are ranked as highly suitable (Dean,

2010). Areas of gravels and muds due to their strength and cohesion properties are

less ideal for foundation installation. Gravels, which pose the largest problem, would

50

have to be collected and then removed from the location and/or broken up using

mechanical equipment.

Areas of bottom anomalies are generally completely off limits for

development. For example, an area where there is a shipwreck cannot be disturbed

due to its historical value (NPS, 2016). The presence of UXO pose hazards that

prohibit foundation installation and must be disposed of according to specific federal

requirements (EPA, 2016). Based on this, any area with an anomaly such as these was

designated as unsuitable (Table 3). In addition, each anomaly was applied a 500 m

radius buffer to ensure safety. This value is considered conservative but, was chosen

because a standard buffer distance could not be found in literature.

Once all six attributes were identified, they were converted into gridded raster

data. After each dataset was converted into raster data, they were reclassified based

on the aforementioned assigned suitability values of 1 to 4. Each individual cell

within the grid has a rank from 1 to 4 based on the ranking system as shown in Table

3. A summary of how each dataset was ranked and reclassified can be seen below in

Table 3 and Appendix A. After every dataset was reclassified, they were added

together using the Raster Calculator to create the final suitability map. Five of the six

attributes were weighted the same, with the anomaly data being weighted of utmost

importance. If an anomaly was present, that area would be classified as unsuitable

regardless of the final value. A flowchart of this model is shown on the next page for

clarification (Figure 4.1).

51

Table 4.2: Summary of ranking and reclassification of each vector dataset.

Attribute Value Suitability

Depth

� �� � 1 21 < 30 m 2 31 < 40 m 3 � �� � 4

Slope

0 � 2° 1 2.1 � 5° 2 5.1 � 8° 3 8.1 � 11° 4

Surficial Sediment Type

Sands 1 Muds/Fine sediment 2

Gravels 3

Mobile Sand Thickness

0 � 1 m 1 1.1 � 3 m 2 3.1 � 6 m 3

Paleochannels No channels 1

Channels present 3

Anomalies No obstructions 1

Obstructions present 4

52

Figure 4.1: Flowchart demonstrating step by step work-path taken to create the suitability map

53

4.3 Discussion

In areas where all six parameters had a classification of one (highly suitable),

the sum of these values is six. Thus the most suitable regions have a minimum value

of six with decreasing suitability as that number rises. The maximum value that could

be attained if the least suitable classification for all six parameters occurred in one cell

is twenty one. However, the highest cell value produced was fifteen, meaning that no

one cell in the entire WEA is unsuitable for all geologic parameters at the same time.

Given the complete restrictions on anomalies, the only locations 100 % off limits to

developers are areas where they are present (Figure 4.2). Regions of highest

suitability are areas where all the datasets totaled to the smallest overall values.

Similarly, regions that are only slightly suitable or unsuitable had the highest values,

corresponding to locations that are not optimal for foundation placement. Since there

are a large variety of parameter combinations that would result in higher overall cell

values, it is difficult to determine the most accurate scale for suitability. For example,

a value of eleven could represent a location where all six parameters were assigned

one or two, and is thus suitable by definition. It could also be characterized by

parameters that have a combination of assigned values ranging from one to four. As

such, it is advised that developers analyze the suitability of each parameter

individually to most accurately understand the region. Individual reclassification

maps for each parameter can be found in Appendix A. For the purposes of this study,

the scale for overall suitability is shown in Table 4.

Based on the results it is clear that the optimal areas for development, based on

equally weighted factors of water depth and the four geologic parameters (slope,

surficial sediment type, mobile sand thickness, and presence of subsurface

paleochannels), are the western section closest to the shore and the central eastern

54

portions of the WEA (Figure 4.2). It appears as though the southeastern portion of the

study area is less suitable due mainly to the greater water depths (Figure 4.2) (Figure

A1). The most notable negative parameters in the western area are paleochannels.

This region of the study area contains several intertwining channel systems that could

require, to meet design parameters, longer foundations installed to deeper depths

beneath the channel systems. The presence and thickness of the mobile sand sheet up

to 6 m in the southern portion of the study area could pose potential long-term

scouring problems (Figure 4.2) (Figure A5). Extensive migration and reworking on

the U.S. Mid Atlantic coast is due primarily to storm related events (Knebel, 1981).

Since survey completion, the study area has subsequently been exposed to 3 hurricane

��� ��������� ������� ����� � � ����� ���� � � � �� ��� ��� �� �� ��� �� ����

sheet have since been reworked and reorganized.

Table 4.3: Overall suitability scale for the WEA

Highly suitable 6 � 8 Suitable 9 � 10 Slightly Suitable 11 � 12 Unsuitable 13 � 15

55

Figure 4.2: Final suitability map showing optimal areas for development.

56

Chapter 5

IMPLICATIONS FOR FOUNDATION SELECTION AND DEVELOPMENT

5.1 Introduction

In 2009, BOEM created an Outer Continental Shelf (OCS) Renewable Energy

Program outlining the process to allow for renewable energy development on the OCS

(BOEM, 2016). This program occurs in four phases: planning and analysis, leasing,

site assessment and construction and operations. After the planning and leasing

stages, prior to wind farm construction and completion, all leaseholders are required to

conduct and submit a Site Assessment Plan (SAP) and Construction and Operation

Plan (COP) (BOEM, 2016).

A SAP consists of various site characterization surveys including avian, marine

mammal, archeological and geological studies. It is during this phase that geophysical

and geotechnical studies are conducted in order to characterize the sea floor and sub-

bottom sediment. The work completed in this thesis seeks to demonstrate how a

detailed desktop study of previously existing geophysical and geological data can

provide valuable insight on the conditions and suitability of a region, at the beginning

planning stage, prior to leasing, rather than the later site assessment and construction

and operation planning in the development process.

5.2 Geotechnical Considerations

In order to accurately classify sub-bottom sediment type and parameters,

BOEM requires that all leaseholders complete a geotechnical survey during the site

assessment and planning phase of the renewable energy program (BOEM, 2016).

Geologic desk studies like the one conducted in this thesis should be used to make

57

informed decisions on the best locations for the SAP geophysical surveys. Further, a

broad understanding of the geologic framework should be taken into consideration

with the SAP geophysical work when planning where to locate boreholes for

geotechnical analyses. Integration of these datasets and analyses is crucial step

towards selection of the optimum foundation at a given location in terms of both

design and economics (Westgate and DeJong, 2005). Geotechnical investigations

include detailed in-situ and laboratory testing. These tests are used to determine soil

strength parameters (shear strength, pore pressure, overconsolidation ratio, friction

angle and cohesion) associated with various soil conditions (grain size, porosity,

density) (Westgate and DeJong, 2005; Dean, 2010). From these tests, engineers and

geologists can quantify a soil�s ability to withstand scouring, strong vertical,

horizontal and cyclic loading, overturning moments, skirt penetration and settlement

(Westgate and DeJong, 2005).

5.3 Foundation Types

Most offshore wind foundations have been adapted from designs used for rigs

in the oil and gas industry (Byrne and Houlsby, 2006). Unlike oil and gas rigs and

onshore foundations, static loading from the tower, nacelle and rotor are minor in

comparison to the large dynamic horizontal and overturning moments induced by

waves, wind, rotor inertia, ice drift and cyclic fatigue (Byrne and Houlsby, 2006;

Bakmar et al., 2009; Malhotra, 2011; Bhattacharya, 2014). Foundation selection and

design is also based on soil conditions and their load carrying abilities (Westgate and

DeJong, 2005; Bakmar et al., 2009). There are three main types of offshore wind

foundations that currently make up the majority in use: monopiles, jacket/lattice

structures and gravity base (Figure 5.1). There are additional foundation types

58

currently being designed and tested such as the twisted jacket and suction

bucket/caisson (Figure 5.1). Although suction buckets/caissons are primarily still in

the research phase, this emerging foundation technology has the potential to provide

many benefits, both economic and environmental, which are further discussed below.

Due to the promising nature of this design, it will be reviewed for its suitability below.

Figure 5.1: Four types of offshore wind foundations (IPCC, 2012).

59

5.3.1 Monopile

Monopile foundations make up the bulk of offshore wind foundations

(Westgate and DeJong, 2005; AWS, 2009; Bakmar et al., 2009; Malhotra, 2011).

They are optimal in water depths of up to 20 m (Westgate and DeJong, 2005; AWS,

2009). The design consists of a hollow steel tube, less than or equal to 5 m in

diameter with a thickness of approximately 5 to 15 cm. They are constructed onshore

and transported via vessel to location and either driven or hammered into the seafloor

up to depths of 20 to 40 m. The diameter, thickness and base depth vary based on site-

specific parameters. A problem that may be experienced with these foundations is

buckling (Bhattacharya, 2014; Golightly, 2014). Their large length-to-width ratio

creates point loading, which induces more rapid fatigue and degradation compared to

jacketed structures (Bhattacharya, 2014). Installation practices (hydraulic hammering)

for these foundations are the cause of environmental concern due to significant noise

impacts on marine mammals (Bakmar et al., 2009). Decommissioning of these

foundations also has proven to be quite difficult and oftentimes, fragments of the

foundation are left beneath the seafloor at the end of its life cycle (Westgate and

DeJong, 2005).

Monopile foundations can be employed in a variety of sub-bottom

environments; however they are most stable in homogenous, sandy sediment (Kaiser

and Snyder, 2010). Locations with deep waters (> 20 m) and/or an abundance of

coarse grained sediment ranging from cobbles to bedrock are unsuitable because to

meet design requirements the diameter of the monopiles must be increased to the

extent that would be extremely expensive (AWS, 2009). Soft soils such as clays,

muds and organics can result in settling and if possible should be avoided. Scouring

60

of surficial sediment can leave parts of the foundation exposed and as such, extra

measures (scour protection) must be taken into account.

5.3.2 Jacket/Lattice Structures

There are many variations of jacket foundations. They most commonly consist

of 3 to 4 steel legs in a framed lattice structure (AWS, 2009; Malhotra, 2011). Each

leg is connected to the seabed via either a pile or suction bucket. This design uses less

steel than a monopile and its lattice structure provides greater strength against larger

loading, and as such, it is able to withstand the harsher conditions experienced at

greater distances from shore (Malhotra, 2011). Due to their large cross-section, jacket

style foundations are most suitable for mid-deep water conditions approximately 20 to

40 m (Westgate and DeJong, 2005). Although jacket foundations can be installed in a

variety of conditions, their intricate and complex structure results in increased

construction costs and requires large, expensive jack-up vessels (Kaiser and Snyder,

2010). In order to reduce costs and capitalize on design, these structures are typically

not installed in regions suitable for the less expensive monopiles.

Jacket structures, because of their lattice frame, are able to withstand more

heterogeneous soil conditions (Westgate and DeJong, 2005; Kaiser and Snyder, 2010).

Their differing tripod and quadraped designs can transfer loads to the soil axially

versus vertically, making this foundation suitable for almost any conditions (Westgate

and DeJong, 2005; Malhotra, 2011). Since each leg in the lattice structure has a much

smaller surface area, they are less susceptible to scouring and are not commonly

known to need additional protection (Kaiser and Snyder, 2010).

61

5.3.3 Gravity Base

Gravity base foundations, like monopiles, are typically installed in shallow

waters less than or equal to 25 m deep (Westgate and DeJong, 2005; AWS, 2009).

They are comprised of a wide, weighted steel and/or concrete structure that sits atop

the seafloor. In order to maximize stability, gravity base foundations often require

immense amounts of preparation including dredging, filling, leveling and scour

protection (Westgate and DeJong, 2005; AWS, 2009). Advantages of these

foundations include simplistic design and installation procedures, as well as

subsequent decommissioning processes. Along with floating options, gravity base

foundations are relatively non- intrusive because they do not penetrate the subsurface.

Gravity base foundations are highly dependent on the stability of the upper 10

m of the sub-bottom sediments (Bakmar et al., 2009). It is crucial that the sea floor

surface be absent of soft sediments that cannot support a heavy, wide based foundation

(Westgate and DeJong, 2005; Malhotra, 2011). Oftentimes stronger, coarse grained

sediment must be imported to level the sea floor surface, as well as avoiding

placement of the foundation atop muds (Malhotra, 2011). These foundations can still

be employed in deep waters; however the foundation diameter increases greatly,

exerting greater loads and compressional forces to the sediment (Westgate and

DeJong, 2005).

5.3.4 Suction Bucket (Caisson)

Over the last decade, suction bucket foundations have been an emerging

technology in the offshore wind industry. Suction buckets consist of a very large,

hollow steel tube about 10 to 15 m in diameter (Westgate and DeJong, 2005; AWS,

2009; Malhotra, 2011). Although there are no large-scale wind farms currently using

62

this type of foundation, test sites and facilities show a maximum penetration of

approximately 10 m bsf (Malhotra, 2011). Suction buckets are aptly named based on

their installation method. After initial sinking of the bucket into the subsurface due to

its weight, the water trapped between the top of the bucket and the seabed is pumped

out creating a net downward pressure forcing the bucket deeper (Houlsby et al., 2005;

Westgate and DeJong, 2005; AWS, 2009). Once the foundation is securely implanted

in the subsurface, the pumps are removed and the valves are closed. A major

advantage to this installation procedure is the reduction in noise commonly associated

with pile driving, which is an issue with the presence of marine mammals (Koschinski

and Lüdemann, 2013). Suction buckets can also be implemented for monopod, tripod

and quadraped use (Houlsby et al., 2005; Westgate and DeJong, 2005; Byrne and

Houlsby, 2006). Another upside to suction buckets is that even though they are

constructed onshore, their lightweight nature allows them to be floated to the turbine

location without the use of extremely large and costly vessels. Finally,

decommissioning at the end of the lifecycle is easy, as pumps are simply reattached

and water is forced into the bucket cavity, forcing the foundation out of the seabed

(Westgate and DeJong, 2005).

Suction buckets, similar to gravity base foundations do not penetrate to deep

depths and thus rely on sediment strength parameters for only approximately the top

10 m of soil. Due to their unique installation methods, these structures are best used in

areas of homogeneous sediment (Westgate and DeJong, 2005). If the sediment is not

homogenous, varying pore sizes and discontinuities may be created with time resulting

in less suction and stability (Feld, 2006). One limiting design condition for the

monopod suction bucket is the overturning moment (Malhotra, 2011). Due to its

63

current limited use in wind farms, specific design guidelines are lacking and are site

dependent (Malhotra, 2011).

5.4 Discussion

This comprehensive study would be incomplete without consideration of

foundation selection based on the geologic conditions of the Maryland WEA. The

suitability and stability of foundations is dependent upon the surficial sediments and

underlying stratigraphy of the OCS. Constraining three-dimensional sediment

variability is critical towards choosing the most effective foundation design. The

surficial and sub-bottom sediment variability has been inferred and discussed in

Chapters 3 and 4 of this thesis. Five distinct stratigraphic units have been identified,

including a major network of paleochannel systems. Although there is no

geotechnical data available within the study area, sediment strength parameters can be

inferred from and correlated with other studies within the MAB region (e.g., Toscano

et al., 1989; Williams, 1999; Nordfjord et al., 2009).

The shallow (� �� �� ������� ���������� � �� ������� ��� ��������

of highly variable sediment types. Within the general stratigraphic column for the

WEA, data interpretation identifies five distinct units ranging from sands to silty muds

to a heterogeneous mixture of fluvial infill. The diversity of sediment is a direct result

of changing paleoenvironments that are associated with eustatic sea-level change

within the last 2.5 million years (Belknap and Kraft, 1981; Knebel, 1981; Toscano et

al., 1989; Williams, 1999; Krantz et al., 2009; Nordfjord et al., 2009). In the

following paragraphs, foundation suitability considering the surficial sediments and

subsurface stratigraphy of the WEAwill be discussed with respect to the four major

64

foundations types outlined (i.e., monopile, jacket/lattice, gravity base and suction

bucket).

Based on side-scan sonar data, the majority of the surficial sediment in the

WEA was classified as medium grained in size (i.e., sands-) (Figure 2.16). Some fine

grained sediments (i.e., muds) occur consistently along the eastern and southern edges

of the WEA. Potential coarse-grained, clusters of gravel are present in the middle of

the WEA (Figure 2.16). Similarly, Toscano et al. (1989), Wells, (1994) and Metz

(2015) also found that sand was the most prominent and consistent surface sediment.

All three studies also reported the limited presence of muds and gravels. Irregular

distribution of the Unit 5 surficial sand sheet is a pattern encountered across the entire

MAB (Knebel, 1981; Toscano et al., 1989; Wells, 1994; Williams, 1999; Nordfjord et

al., 2009; Metz, 2015). Thickness in the study area ranges from 0 to 6 m, with other

studies having identified deposits as thick as 10 m (Metz, 2015). Constant reworking

and movement of the surficial sand layer poses scouring issues for monopile, gravity

base and suction bucket foundations. Studies conducted on abandoned subway cars

(15 � 18 m long and < 4 m wide) in the Redbird Reef study area have measured scour

depths up to 1 m (Raineault, 2013; Metz, 2015).

The size of the foundation anchored in the seafloor is directly related to the

degree of scouring that may occur (Whitehouse, 1998). Foundations with a larger

diameter such as gravity base and suction buckets are associated with increased

scouring. Based on these assessments, it is hypothesized that significant scour

protection will need to be emplaced around all potential foundations except perhaps

jacket style structures. Bathymetry within the Maryland WEA ranges from

approximately 11 to 42 m deep. The significant presence of sand ridges up to 6 m

65

high, coupled with the gently sloping topography has produced slopes ranging from 0

� 11° creating the potential need to complete extensive preparation and leveling in

some areas prior to employment of a gravity base foundation.

Within the upper 10 m of the subsurface stratigraphic column, and in areas

absent of paleochannel systems, there is a rapid transition from Unit 1: Holocene

sands to Unit 4: Interglacial muds to Unit 5: Late Pleistocene shelly sands. Given that

this occurs within the upper 10 m, all foundations except the gravity base would

penetrate through these layers in the subsurface. Suction buckets, requiring ~10 m of

penetration would terminate just beneath the Unit 4 interglacial muds. Since suction

buckets are a newer technology with minimal public data, it is difficult to assess to

what degree of heterogeneity they could withstand and whether or not they would be

an appropriate design for this area.

Due to the high frequencies, and therefore less depth of penetration, of the

chirp sub-bottom system used to collect the data in this study, constraints on Unit 4 are

lacking. The unit is believed to be part of the Omar Formation deposited during the

late Pleistocene. Its full extent is unknown with the limited core and chirp data

available. Toscano et al. (1989) encountered interbedded sands and shelly sands at the

top of this unit. Given this limited information, it is possible that both monopile and

jacket foundations could be installed in this sequence, although further data to confirm

this conjecture is needed.

As imaged by the chirp sub-bottom profiles, paleochannel infill ranges from

highly organized to chaotic. Distinct reflection sequences most likely correlate to

heterogeneous mixtures of sands, muds and gravels. Further, some locations in the

WEA indicate towards the possible presence of coastal lithosomes. Toscano et al.

66

(1989) encountered Quercus stems and Fraxinus, as well as coastal peats in this unit.

Due to the heterogeneity encountered in the paleochannel infill sediments, foundations

that support the entirety of their load within the upper portions of the stratigraphic

column would not be suitable for these areas. Intense loading on this sediment, which

has a mixture of varying cohesion, friction, pore pressure, and other strength

parameters could result in unwanted movement and settling. Gravity base and suction

buckets, which depend entirely upon the upper stratigraphic column, would be most

susceptible to settling in these areas. Variable sediment types could also result in

installation difficulties such as reduced suction/coupling of the foundation skirt of

bucket structures. Monopile and jacket foundations may be installed in these areas

but, would require deeper penetration, and thus larger structures, to be anchored in

more homogenous layers beneath the paleochannel infill sediments.

5.5 Foundation Conclusions

Based on the interpretation of the sub-bottom stratigraphy, there is a high

variability in the shallow sediments (soils) of the Maryland WEA. This suggests that

foundations like gravity base and suction buckets, which support their load within the

uppermost layers, are less suitable for this region. While suction buckets might not be

an ideal fit for this region, future research to more accurately define the optimal

geologic conditions for their installation could suggest otherwise. It is recommended

that continuing research and learning more about the constraints on using the suction

bucket foundation design in muddy sediments (especially muds that contain higher

percentages of silt-sized particles) and within layers of heterogeneous sediments

(sands to muds) be conducted.

67

Given the predominance of surficial sands and muds and variable subsurface

sediments including some gravels, sands and muds, at this time the optimal foundation

types are monopiles or jacket structures with legs anchored with piles. Both of these

foundation types would support their loads within deeper, more homogeneous

sediment. It is likely that monopile foundations, especially if located within the Unit

1: Holocene sands would require scour protection. Ultimately, the most economical

foundation type and design will be determined based on material, fabrication

construction, installation and decommissioning costs.

An alternative option to consider is one of many hybrid foundations such as the

twisted jacket (Figure 5.2). The twisted jacket, also known as the inward battered

guide structure (IBGS) is a foundation developed by Keystone Engineering that

combines features of the traditional monopile, jacket/lattice, and caisson structures

(ISSUU, 2014). This pre-piled jacket typically comprises of more braces, nodes, pins,

and heavy wall sections. In an effort to reduce overall size and scope of materials, the

IBGS has introduced a small guide structure, which is placed upon a caisson. The

guide structure has built-in sleeves that direct the piles into the seabed. This design is

smaller, less complex, and easier to fabricate than the traditional jacket style (LORC,

2011; de Villiers, 2012). The environmental conditions for the IBGS are not unlike

that of traditional jacket foundations. One major difference is its design to transfer its

load axially, rather than laterally. This relieves the soil of the lateral stresses from a

gravitational load, creating a more simple stress under a uniform simple shear. As a

result, the foundation can be used in less than optimal soil conditions (Mechling,

2014).

68

The IBGS uses substantially less steel than that of the four-legged traditional

jacket foundation and one third the amount of components; this makes them easier to

manufacture and construct. Thus, costs of fabrication and installation are reduced,

promoting a more competitive solution (Mechling, 2014). Since these foundations are

smaller and easier to transport, the initial capital investment is considerably lower. It

is also capable of meeting industry needs, allowing for installation in water depths up

to 70 m. The foundation, while new to the wind industry, was initially developed for

the petroleum industry and has a proven durability as it withstood the extreme

conditions of Hurricane Katrina with no damage (ISSUU, 2014). It is suggested that

the IBGS and other hybrid options may provide the most appropriate solution by

combining the advantages of several different foundation technologies.

Figure 5.2: The twisted jacket foundation combining both jacket and monopile design features (de Villiers, 2012).

69

Chapter 6

CONCLUSIONS

6.1 Foundation Recommendation

Overall the stratigraphic sequences identified in this study correlate well with

most other studies in the MAB region. Highly variable shallow subsurface

stratigraphy within the upper 10 m beneath the seafloor caused by multiple sequences

of eustatic sea-level change creates difficult conditions for both gravity base and

suction bucket foundations. The full capabilities of the suction bucket design are not

yet known and because of the advantages they bring in terms of lesser construction

materials, ease of installation and decommissioning, it is suggested that further

research be conducted before they are ruled out.

Complex channel systems, similar to those across the MAB, extend through

the Maryland WEA. Avoiding these regions would minimize the variability in

subsurface sediments encountered, and with monopiles or jacket/lattice structures, the

depth of penetration of these foundations. Scouring poses a significant threat

throughout the entirety of the WEA, and bottom current and wave conditions should

be effectively monitored prior to foundation selection. Should monopiles be chosen,

scour guards will most likely be required. Ultimately, upon confirmation of these

results via a geotechnical analysis, any variation of a pile driven foundation is

appropriate. Construction and installation methods should be taken into account,

given that jacketed structures are more complex during fabrication and typically

require larger, more expensive vehicles during installation. Lastly, it is recommended

70

that exploration of hybrid alternatives such as the twisted jacket could prove to be the

most economical choice.

6.2 Future Work

Availability of geophysical and geotechnical data within the study area and the

whole MAB is scarce. This study is a preliminary examination on the evolution of the

Maryland OCS and the distribution of surficial and subsurface sediments as a result of

this evolution. In order to further constrain the geological setting of the region,

geotechnical surveying must be completed. Specifications for both geophysical and

geotechnical surveys have been outlined by BOEM as part of its Renewable Energy

Program (BOEM, 2016). Deeper penetration seismic survey data can be used to

determine the sub-bottom stratigraphy for foundations that will require a penetration

greater than the chirp profiling system can resolve. Medium penetration multi-channel

sparker seismic-reflection data was collected during the Coastal Planning and

������������ �� ��� ��� ��� ���� ��������� ��� �������������� �� �eyond the scope of

this thesis.

The results from the study described in this thesis can be used as a basis for

locations within the WEA that should be further sampled, analyzed and from

boreholes have geotechnical analyses on the strength properties of the sediments/soils

determined. It is suggested that the methodology of a preliminary geologically-

oriented desktop analysis, such as the work carried out in this thesis, should be

considered for adoption by BOEM to use in its consideration, along with other first-

order characteristics (i.e., wind resource, water depths, proximity to onshore grid

infrastructure, and ecological and human impacts) in the selection of future WEAs

along the eastern coast of the United States.

71

REFERENCES

AWS Truewind, LLC., 2009. Offshore Wind Technology Overview (NYSERDA PON 995, Task Ordere No. 2, Agreement No. 9998). Albany, New York; AWS Truewind, LLC., 21p. (Prepared for the Long Island � New York City Offshore Wind Collaborative).

Bakmar, C.L., Ibsen, L.B., and Liingaard, M., 2009. Buckling of Large Diameter Foundations During Installation in Sand. Design of Offshore Wind Turbine Support Structures: Paper V. Aalborg: Department of Civil Engineering, Aalborg University. (DCE Thesis; No. 18).

Belknap, D.F. and Kraft, J.C., 1981. Preservation Potential of Transgressive Coastal Lithosomes on the US Atlantic Shelf. Marine Geology 42, 429 � 442.

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Appendix

SUITABILITY RECLASSIFICATION MAPS

A.1 Introduction

This appendix includes figures of the six reclassified parameters for the

suitability analysis identified in Chapter 4.

Figure A1: Reclassified bathymetry data.

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Figure A2: Reclassified slope data.

Figure A3: Reclassified paleochannel data.

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Figure A4: Reclassified surficial sediment type data.

Figure A5: Reclassified mobile sediment (Unit 1) data.

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Figure A6: Reclassified anomaly data.