UTILIZING GEOLOGICAL AND GEOTECHNICAL PARAMETERS TO ...
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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
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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
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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.
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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
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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
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6.1 Foundation Recommendation .................................................................. 69 6.2 Future Work ............................................................................................. 70
REFERENCES ............................................................................................................. 71 Appendix
SUITABILITY RECLASSIFICATION MAPS ............................................... 78
A.1 Introduction ............................................................................................. 78
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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
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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
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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
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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
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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���� ��� �������� �� ����� ���� �������� ����
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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.
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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).
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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
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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.
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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
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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
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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).
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Figure 1.3: Coastal bays & drainage systems surrounding the Maryland WEA (data from USGS national map database)
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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.
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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
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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).
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Figure 2.1: Track line map showing N/S and E/W survey lines throughout the area.
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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).
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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
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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
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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.
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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.
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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).
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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.