HomeMy WebLinkAboutStorm Surge and Inundation Pathways - 2016- 2 -
Empowering Coastal Communities to Prepare for and Respond
to Sea Level Rise and Storm-related Inundation:
A Pilot Project for Nantucket Island
Massachusetts Office of Coastal Zone Management’s
Coastal Resiliency Grant program | FY 2015 RFR ENV 15 CZM 03
Prepared by
Mark Borrelli, PhD
Steve T. Mague
Theresa L. Smith
Bryan Legare
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PROJECT BACKGROUND AND OVERVIEW
The need for coastal managers to be better prepared for low frequency storm-related
inundation from such events as Sandy and Katrina, as well as periodic nuisance flooding
associated with increasing astronomical tides was the impetus for this project. Recognizing that
many coastal municipalities often do not have the resources or expertise to manage and maintain
a complicated Geographic Information System (GIS), this project uses existing topographical
information and actual water levels from historical storm events to identify pathways through
which coastal floodwaters may be carried. With results linked to local tide levels, in conjunction
with National Weather Service (NWS) storm surge forecasts, one goal of this study is to provide
information that local emergency managers can use to take action in preparation for, and
response to, coastal storm threats.
The second objective of this project is to provide managers with information that will
help identify longer term local planning needs in response to changing and uncertain
environmental conditions. As coastal populations, both seasonal and year-round, continue to
increase, much attention has been focused on the subjects of climate change and sea level rise.
With regard to the latter, many scientists have concluded that sea levels are not only rising, but at
an increasing rate. As shown in Figure 1, projections vary from a low of 0.15 meters (0.5 feet) to
a high of 2 meters (>6 feet) by the end of this century. Such a broad range creates significant
issues for coastal managers faced with identifying potential hazards to, and vulnerabilities of,
property and infrastructure; prioritizing protection actions; and the need to undertake actions in
the face of the unavoidable uncertainties inherent with century-scale sea level rise projection
scenarios. Traditionally (and necessarily) short-term actions and effective responses,
implementable at the local level, are not easily defined within the context of sea level rise
discussions. Significantly, the results of this study are by design independent of the various sea
level rise scenarios and can be used, therefore, to develop action plans within the planning
horizon of most communities.
In addition to the issue of defining a suitable planning horizon, the ability of coastal
managers to effectively and efficiently recognize potential vulnerabilities and to educate
residents and community leaders about the threats associated with storm tides (the combination
of storm surge and the astronomical tide level) and flooding has been severely limited by the lack
of regional-scale, accurate elevation data. For example, Flood Insurance Rate Maps (FIRMS),
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produced by the Federal Emergency Management Agency (FEMA), have long been a standard
planning resource for coastal communities. These maps, however, were intended to facilitate the
determination of flood insurance rates and lack the planimetric detail necessary for focused
planning efforts. Until recently the accuracy of relatively low cost topographic data has been
appropriate only for general planning at regional scales and not for identifying storm-tide and
flooding impacts over timeframes that meet the needs and budgets of most municipalities.
Numerical modeling of storm surge, sea level rise, waves, or sediment transport (coastal erosion)
can be effective for regional efforts to understand coastal evolution, but can also be cost
prohibitive. Furthermore, these models are often too coarsely-scaled to inform local decisions
facing coastal managers and municipalities.
Figure 1. Relative sea level rise scenario estimates (in feet NAVD88) for Boston, MA. Taken from, Sea Level Rise:
Understanding and Applying Trends and Future Scenarios for Analysis and Planning. Massachusetts Office of
Coastal Zone Management, December 2013. Available at: http://www.mass.gov/eea/docs/czm/stormsmart/slr-
guidance-2013.pdf.)
Based on the long range projections of sea level rise and the catastrophic damages
associated with large coastal storms, much attention has been placed on long term strategies to
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reverse current climate trends and to slow the rate of, or reverse, sea level rise. Strategies to
reduce Green House Gas (GHG) emissions and promote green energy to deal with rising
temperatures, glacial ice melt, and thermal expansion of seawater over the next hundreds of years
are being discussed and debated at the international, national, and state levels. Clearly the
planning and costs to confront these issues are long term and capital intensive. Lost in these
discussions are viable hazard planning strategies that can be adopted and implemented at the
local level within the shorter planning horizons and financial means of local municipalities.
Reflective of the limited financial and technical resources of coastal communities and
their unique geography, local responses and strategies to sea level rise and climate change will be
more successful when considered in the context of short-term planning horizons, limited
financial and technical resources, and frequently changing leadership. Specifically, the short
term planning should identify actions or responses that are:
1) Achievable within an appropriate time frame (e.g., 30 years)
2) Implementable with current technology
3) Financially feasible
4) Politically viable (i.e., not extreme – e.g., wholesale retreat)
5) Adaptable to future scenarios
6) Focused on both infrastructure and natural resources
While sea level rise projections are clearly critical for longer term planning considerations,
particularly for large scale efforts, actual storm tide elevations can provide an effective means of
characterizing coastal hazard vulnerability for local planning actions. Using historical data to
identify potential storm tide heights, coastal flooding extents, and areas of potential vulnerability
provides important, high certainty planning information to local communities with several
benefits. First, using historical storm tides to identify coastal hazard vulnerabilities removes sea
level rise and the disparity of projections (Figure 1) from the discussion of the most appropriate
sea level rise elevation to use to develop short term planning responses. Sea level rise
notwithstanding, storm tides of these magnitudes have been experienced in the past and are very
likely to be experienced again in the future. Second, storms of record provide an accurate, actual
(i.e., indisputable) reference elevation that towns can plan for when history repeats itself. Finally,
as discussed below, using emerging data gathering technologies to identify storm tide impacts,
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will yield valuable information that can be used by coastal communities to plan and implement
ground level strategy in response to sea level rise.
Accurate Elevation Data, Record Storm Tides and Potential Pathways
Over the past ten years, light detection and ranging (lidar) surveys have emerged as a
cost-effective source of coastal elevation data. Covering broad geographic areas with horizontal
accuracies on the order of 3 meters (~10 feet) and vertical accuracies on the order of 15-30 cm
(0.5-1.0 feet), this relatively high resolution topographic information can be used by coastal
managers as the initial basis for developing inundation scenarios that communicate threats
associated with coastal storms. Although continuing to improve in vertical accuracy, the use of
lidar alone to map areas of storm vulnerability and to develop community response strategies
remains limited. Recognizing these limitations, current guidelines for inundation modeling using
lidar elevation data sets with vertical accuracies of 15 cm (0.5 feet) recommend analyses be
performed at increments of 58.8 cm (~2.0 feet), a resolution clearly too coarse for the
development of local action items. This base level information, however, when supplemented
with area-specific high resolution elevation data, can be used to accurately identify and prioritize
potential coastal hazards at the local level in a cost effective manner.
This project began in the fall of 2014, at that time the best available lidar data suited for
the purposes of this study was from 2010; FEMA collected lidar data in Coastal Massachusetts
and Rhode Island. The horizontal and vertical accuracies of this publicly accessible
contemporary elevation data provide a reliable base map and can be used as the foundation for
local action planning. All of the desktop analysis had been completed using the 2010 lidar.
During the course of this study the United States Geological Survey (USGS) collected lidar data
on Nantucket as part of the “Sandy Project’. This project involved collecting data in and around
areas effected by Hurricane Sandy. These data were of better quality and wider coverage than the
2010 lidar data. After the USGS data were released all of our work was checked against the old
lidar and it was determined that all of the final spatial (elevation) data would come from the
newer, more accurate lidar data. This involved duplicating almost the entire desktop analysis
portion of the study, but was done to provide Nantucket with the best available elevation data in
the final product.
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Using lidar data to guide accurate fieldwork, this study mapped the low-lying locations
through which the elevated water levels associated with coastal storms might flow into
Nantucket, Massachusetts. These locations are referred to herein as “storm tide pathways”). The
term ‘storm tide’ refers to the rise in water level experienced during a storm event resulting from
the combination of storm surge and the astronomical (predicted) tide level. Storm tides are
referenced to datums, either to geodetic datums (e.g., NAVD88 or NGVD29) or to local tidal
datums (e.g., mean lower low water (MLLW)). Storm surge refers to the increase in water level
associated with the presence of a coastal storm. As the difference between the actual level of the
storm tide and the predicted tide height, storm surges are not referenced to a datum. Another
contemporaneous study funded through the same grant program uses the term “inundation
pathways’, this term is interchangeable with storm tide pathways (STP).
Generally, STPs, by virtue of their elevation relative to the elevation of the storm tide,
provide a direct hydraulic connection between coastal waters and low lying inland areas.
Examples of pathways that may serve as direct hydraulic connections include: low spots in built
environment (e.g., roads, walkways, dikes, seawalls, etc.); low lying infrastructure that can serve
as unintended conduits (e.g., storm water system, sanitary sewers, electrical/utility conduits); and
low spots in natural topography (e.g. low lying earthen berms, barrier beaches, and dune systems
susceptible to erosion and breaching). Low-lying infrastructure can also serve as unintended
conduits (e.g., storm water system, sanitary sewers, electrical/utility conduits), however, analysis
of potential conduit hydraulics should be evaluated by a qualified engineer to accurately assess
potential vulnerabilities.
PROJECT METHODOLOGY
Spatial Analysis
Based on the characterization of the Nantucket Harbor tidal profile discussed below, the
analysis begins in the lab by using state-of-the-art software to superimpose various water levels
on the existing elevation (lidar) data in three dimensions to identify potential STPs. To compile a
list of potential STPs this desktop analysis relies on best available synoptic elevation data to
serve as the study are base map for the project GIS.
For this purpose, the latest lidar data were downloaded from the NOAA website
(https://coast.noaa.gov/digitalcoast/). This website has default settings for horizontal and vertical
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reference datums, spheroid and projection parameters, and units (metric vs standard). For the
purposes of this study CCS staff altered the default download parameters for ease of use within
several software packages. Regardless of the spatial parameters, the positional information
within the lidar are not altered and the final data products are referenced to MLLW datum for
Nantucket Harbor, to simplify use at the local level and for quick and direct comparison with
NOAA’s real time tide station located in Nantucket Harbor.
The data are downloaded in a raster format and brought into ESRI’s ArcGIS software
where the raster is divided into smaller tiles. These lidar tiles are then brought into QPS’s
Fledermaus data visualization software. While acquired by CCS as an integral component of its
Seafloor Mapping Program, the Fledermaus software package has proven to be an ideal platform
for the initial desktop identification of STPs with the accuracy of the initial analysis limited
primarily by the uncertainty and resolution of the lidar itself.
The power of Fledermaus lies in its ability to work with very large data files quickly.
Individual files can be multiple GBs in size, yet Fledermaus very rapidly moves through the data
for visual inspection (‘fly-throughs’) and similar functions. A horizontal plane, representing a
specific water elevation can be added to a Fledermaus project or ‘scene’ and that plane can be
changed to simulate the increase or decrease in water level until it first reaches an STP.
Another invaluable feature of the data visualization software is the ability to drape a 2-
dimensional data set, such as a vertical aerial photograph, over a 3D dataset (lidar). This helps
the analyst to better document the STP and also to gain valuable information as to the substrate
conditions and landscape setting in which the potential STP is located. For example, an STP
found on or near a naturally evolving coastal feature such as a beach or dune is characterized
differently than one atop a concrete wall or other relatively static feature. This observation serves
to inform the field team to examine naturally evolving areas closely and to be vigilant for other
potential STPs that may be in close proximity to the identified point but, due to the dynamic
nature of the area, not present in the lidar. This characterization is also important for a final
assessment of the most appropriate way to address an STP in a critical area.
As an accurate and synoptic dataset, the terrestrial lidar collected in 2010 by FEMA was
selected to be the base map for the desktop analysis and STP identification phase. With metadata
reporting horizontal and vertical accuracies of +/- 1.0 m and +/- 0.15 m respectively, this dataset
proved to be a reliable source from which to compile the initial STP base map and facilitated the
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fieldwork phase discussed below. Again, as discussed above, the 2013-2014 USGS lidar was
used to produce the final datasets.
Field Work
At the completion of the desktop analysis, all potential STPs were compiled into a
database with x, y, z coordinates, uploaded into the Center’s GPS, and an extensive fieldwork
assessment program conducted to verify the presence or absence of each STP. The stored
coordinate file and GPS were used to navigate to each potential STP and when confirmed by
visual inspection of the three-person team, an accurate horizontal and vertical location obtained.
With the collection of GPS data, lidar data loaded into a laptop to assist with fieldwork were
inspected for each point in real-time. This served three purposes: first to compare the real-world
position of the STP with that found during the desktop lidar analysis; the second to verify the
positional accuracy of the STP itself; and lastly to serve as a qualitative check on the positional
accuracy of the lidar data.
Significantly, using the GPS instrument to navigate to the location of a potential STP also
afforded the field crew the opportunity to investigate alternative or additional STPs based on
visual inspection of the area. Many coastal sites have very low relief (relatively flat) and
verifying whether an STP existed, its exact location, and the direction of water flow required
professional judgment facilitated with experience in the principles and practices of land
surveying as well as a thorough knowledge of coastal processes.
A Trimble® R8 GNSS receiver utilizing Real-Time-Kinematic GPS (RTK-GPS) was
used for all positioning and tide correction field work. The Center subscribes to a proprietary
Virtual Reference Station (VRS) network (KeyNetGPS) that provides virtual base stations via
cellphone from Southern Maine to Virginia. This allows the Center to collect RTK-GPS without
the need to establish a terrestrial base station or to post-process the GPS data, reducing
mobilization and demobilization costs and streamlining the field effort.
The Center undertook a rigorous analysis of this system to quantify the accuracy of this
network (Mague and Borrelli, in prep). Over 25 National Geodetic Survey (NGS) and
Massachusetts Department of Transportation (DOT) survey control points were occupied with
published coordinate values relating to the Massachusetts Coordinate System, Mainland and
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Island Zones (horizontal: NAD83; vertical NAVD88). Control points were distributed over a
wide geographic area of Cape Cod and Nantucket.
Multiple observation sessions, or occupations, were conducted at each control point with
occupations of 1 second, 90 seconds, and 15 minutes. To minimize potential initialization error,
the unit was shut down at the end of each session and re-initialized prior to the beginning of the
next session. The results of each session (i.e., 1 second, 90 second, and 15 minute occupations)
were averaged to obtain final x, y, and z values to further evaluate the accuracy of short-term
occupation. Survey results from each station for each respective time period were then compared
with published NGS and DOT values and the differences (error) used to assess and quantify
uncertainty. Significantly, there was little difference between the error obtained for the 1 second,
90 second, and 15 minute occupations. The overall uncertainty analysis for these data yielded an
average error of 0.008 m in the horizontal (H) and 0.006 m in the vertical (V). An RMSE of
0.0280 m (H) and 0.0247 m (V) and a National Standard for Spatial Data Accuracy (95%) of
0.0484 m (H) and 0.0483 m (V).
This fieldwork program is a critical step for several reasons. First, lidar collected via low
flying aerial surveys and the post-processing involved can introduce uncertainties that exaggerate
or diminish features in three dimensional data that could obscure or conflate the presence and
scale of a storm-tide pathway. This has been shown to be particularly evident in cases of ‘bare
earth’ models where elevations tend to be “pulled up” in areas adjacent to where buildings are
removed and “pulled down” in areas of bridges or where roads cross streams or creeks (Figure
2).
Second, the use of an RTK-GPS instrument provides the best possible accuracy for
acquiring and verifying 3-dimensional positional data, such as lidar. Thus the GPS data can
corroborate, or refute the presence of STPs identified in the desktop analysis. Further, due to the
dynamic nature of coastal environments, fieldwork and careful visual inspection are critical steps
to ensure that potential STPs not identified by the desktop analysis are documented while in the
field.
Finally, due to the ephemeral characteristics of the areas proximate to the shoreline, even
the most current lidar is rapidly out of date in certain areas. Consequently, GPS fieldwork is
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critical to identify preliminary STPs that appeared in the lidar but no longer exist due to changes
in landform.
Figure 2. Example of ‘pull-up’. The image is an aerial photograph draped over the 2010 lidar for Nantucket. The
area above the red dotted line should have been removed by the ‘Bare Earth’ algorithm but a remnant still remains.
The 20-40 cm remnant is a small fraction of the building height but still represents 67 – 133 years of sea level rise
(at the current rate of 3 mm/yr).
With the completion of the fieldwork, the team returned to the lab to remove points
determined not be STPs and to add new STPs identified and documented as part of the
fieldwork. In addition, the database was expanded to incorporate labels for all STPs including
position, elevation, substrate condition, and other pertinent descriptive information. This
comprehensive database, associated with individual STP shapefiles, was then incorporated into
the project GIS for further analysis. Importantly, the database was annotated to note those areas
where the lidar was found to correlate poorly with current conditions or real-world position as
determined by the GPS observations and professional judgment was necessary to accurately
represent the STP.
With the compilation of the comprehensive STP database, the file is brought into ESRI’s
ArcGIS to visualize STP locations and provide a working tool for local managers to: 1)
proactively address STPs prior to storm events; 2) prepare for approaching storms; and 3) to plan
for longer-term improvements to mitigate other STPs.
Recognizing that accurate field delineation of the extent of inundation for each STP is
beyond the scope of the project (i.e., GPS mapping of a flood zone for each STP), the USGS
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lidar data were used in 2 interactive ways to provide additional utility by visualizing STP
inundation levels. The first depiction is referred to as the Pathway Activation Level (PAL). This
data layer is developed to show the approximate extent of inundation for the elevation at which
water begins to flow over an STP. It is delineated as the contour representing the STP elevation
as extracted from the lidar. For example, based on the GPS fieldwork, an STP with a PAL of 8.6’
MLLW implies that the moment the water reaches this elevation water will begin to flow inland
via the STP. Using the data visualization software, a water elevation of 8.6’ MLLW is then used
to trace the area that is hypothetically inundated (assuming storm tide water levels are
maintained long enough for the entire area to become flooded). If a storm tide recedes after
reaching the PAL then this depiction can be viewed perhaps as a “best” case scenario for impacts
associated with a specific storm tide. If water levels were to continue to rise above the PAL,
(i.e., higher than 8.6’ MLLW), however, obviously more area would be inundated leading to the
need for a second way to visualize STPs.
For this reason and to increase the utility of the STP data and make visualizations more
user-friendly inundation levels were grouped into ranges. Referred to as Inundation Ranges
(IRs), these layers were representing ranges of water levels for the entire study area rather than
creating PALs for every STP and all potential flood elevations. The IR visualizations are based
on a series of iterations of potential inundation scenarios, including nuisance flooding.
Reviewing various scenarios, that the lower end of the IR range reflects the highest Spring tide
of the year. Due to low tide range (approx. 3.0 feet) and the relatively low relief of Nantucket’s
coastal areas, range elevations reflect 0.5’ foot intervals. The IR visualizations can be used to
approximate conditions up to a maximum elevation of the Storm of Record plus three feet. In
addition to providing an upper limit to project elevations, it was felt that extending half foot
ranges to his elevation the provides a useful representation of future sea level rise scenarios with
practical implications for local managers. The development of Inundation Ranges for the entire
Island of Nantucket was not included in the project description but it was felt that this would be
an invaluable product for the coastal managers and was included in the final deliverables.
NANTUCKET HARBOR TIDAL PROFILE
As discussed above there is a clear need for reliable sea level rise projections to inform
long-term planning efforts and public policy decisions. For shorter term community planning
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decisions and real-time decisions required by Emergency Managers, Public Works Departments,
Harbormasters and Coastal Resource Managers, however, historical data and measurements
obtained from actual coastal storms and related storm tides can provide an important source of
baseline information.
In addition to major inundation that often accompanies coastal storms, as relative sea
level continues to rise, many coastal communities are also beginning to experience minor
flooding associated with the higher tides of the month (e.g., spring tides). Often referred to as
nuisance flooding since it is rarely associated with dramatic building and property damage, this
type of inundation is becoming more frequent with chronic impacts that include overwhelmed
drainage systems, frequent road closures, and the general deterioration of infrastructure not
designed to withstand saltwater immersion (NOAA, 2014).
As noted in the Methods section, the first step in assessing IPs involves the development
of a tide profile for the community of interest that characterizes elevations of local astronomical
tides, historical storm tides, and the possibility of nuisance flooding. In addition to the more
common tidal datums of mean high water springs (MHWS), mean higher high water (MHHW),
mean high water (MHW), and mean sea level (MSL) this tidal profile should include datum
referenced storm tides of the past, including the elevation of the maximum storm tide
experienced (i.e., the storm of record), and capture potential future storm tides (and sea level
rise) by adding four feet to the storm of record.
As mentioned above the term storm tide refers to the rise in water level experienced
during a storm event resulting from the combination of storm surge and the astronomical
(predicted) tide level. Storm tides are referenced to datums, either to geodetic datums (e.g.,
NAVD88 or NGVD29) or to local tidal datums (e.g., mean lower low water (MLLW)). Storm
surge refers to the increase in water level associated with the presence of a coastal storm. As the
difference between the actual level of the storm tide and the predicted tide height, storm surges
are not referenced to a datum.
In addition to the magnitude of a storm surge, the time at which the maximum surge
occurs relative to the stage of the astronomical tide is a critical component of the maximum
storm tide elevation experienced during any particular storm. The significance of when the
maximum storm surge occurs relative to the stage of the astronomical tide is illustrated by the
following example.
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Based on fifty years of readings, the storm of record for the Nantucket Tide Gauge
(#8449130) occurred on October 30, 1991 (Halloween Gale of ‘91) at approximately 5:00 p.m.
with a maximum storm tide elevation of 7.87’ MLLW (5.78’ NAVD88). Occurring
approximately an hour before the time of the predicted or astronomical high tide, the
contributing storm surge was approximately 4.2 feet. Although the maximum storm surge for
this storm was 4.6 feet because it occurred at 3:00 p.m. while the tide was still rising, the
associated water elevation was “only” 7.18’ MLLW or approximately 0.7’ lower than the
eventual storm of record.
By comparison, the maximum storm tide elevation recorded on the same gauge during
the blizzard of January 27, 2015 (Coastal Storm Juno) was 10.25’ MLLW (8.16’ NAVD88).
Occurring shortly after the predicted high tide, this elevation resulted from the combination of an
astronomical tide height of 6.88’ MLLW (4.79’ NAVD88) and a storm surge of 3.37 feet.
Although the maximum storm surge for this event was observed to be approximately 4 feet,
because it occurred close to the time of the predicted low water, the corresponding storm tide
elevation was only 1.0’ MLLW (-1.1’ NAVD88). Had the maximum storm surge occurred
approximately 6 hours earlier at the time of the astronomical high tide, the resulting storm tide
elevation would have been approximately 7.7’ MLLW (5.6’ NAVD88), approaching the
elevation of the storm of record. Recognizing the significance of not only the magnitude of the
predicted storm but the time it will occur relative to the stage of the tide, the Southern New
England Weather Forecast Office of the National Weather Service (SNEWFO-NWS) has been
working on an experimental website (accessible at http://www.weather.gov/box/coastal) that
estimates storm surge and total water level for various locations as coastal storms approach New
England.
The effects of storm tides on coastal communities are dependent on many factors. These
include:
The shoreline orientation of the community (e.g., east facing v. south facing shores);
the elevations of astronomical tides (e.g., the elevation of mean high water in Boston
Harbor is 4.31 feet NAVD88 v. the elevation of mean high water for Woods Hole is 0.56’
NAVD88);
general characteristics of astronomical tides (e.g., the average range (MHW-MLW) of
Boston tides is 9.49 feet while that of Woods Hole tides is only 1.79 feet);
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topography (e.g., the elevation of the land relative to the community tidal profile);
nearshore bathymetry (e.g., the deeper the water relative to shore, the greater the potential
wave energy); topographic relief (i.e., a measure of the flatness or steepness of the land
with flatter areas more sensitive to small changes in water levels);
the nature of coastal landforms (e.g., the rocky shorelines of the North shore v. the
dynamic sandy shorelines of Cape Cod); and
the vertical relationship between historical community development and adjacent water
levels (e.g., development in Boston began in the early 17th century with the water levels
at that time influencing the elevation of not only pile supported structures but large scale
landmaking efforts).
With such variation in physical characteristics, the initial step in the identification of inundation
pathways for a community is the development of a datum-referenced tidal profile.
The Nantucket Island tide station (#8449130) was first installed by the National Oceanic
and Atmospheric Administration (NOAA), National Ocean Service (NOS) on October 4, 1963.
This station, located on Steamship Wharf, was replaced with the present station on September
18, 1990. As an island, the ability to connect tidal information by traditional survey methods
with former geodetic datums such as the National Geodetic Vertical Datum of 1929 (NGVD29)
presented certain problems. For this reason, until recently, station #8449130 was referenced to
the station and local tidal datums. Benchmarks established at various locations near the
waterfront as early as 1934 served to memorialize the relationship between gauge readings and
these local datums (Table 1).
Primary Benchmark Stamping Designation Year First Established
TBM1 NO 25 1968 844 9130 Tidal 25 1968
TBM NO 22 1934 844 9130 Tidal 22 1934
TBM NO 23 1934 844 9130 Tidal 23 1934
9130 G 1976 844 9130 G 1976
9130 H 1976 844 9130 K 1976
Table 1. Tidal Benchmarks on Nantucket Island
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In June of 2012, NOS established elevations, referenced to the North American Vertical
Datum on 1988 (NAVD88) on an existing National Geodetic Survey control point. Referred to
as Tidal Bench Mark 8449130 K (NGS PID AJ 4032), this benchmark is located across the
Harbor on the grounds of the U.S. Coast Guard Base Brant Point. As a primary benchmark for
Tidal Station #8449130, the center for Coastal Studies (CCS) occupied this point with its RTK
GPS and found close agreement with the reported horizontal (NAD83) and vertical (NAVD88)
values. In addition, CCS recovered and occupied the following benchmarks established for tidal
station #8449130. The results of the survey agreed closely and were used to develop the
following relationship between local MLLW and NAVD88.
ElevationMLLW + 2.09 feet (0.68m) = ElevationNAVD88
ElevationSTATION DATUM - 3.00 feet (-0.91 m) = ElevationMLLW
This relationship between MLLW and NAVD88 is shown graphically on Figure 3. The
tidal datums represent average tidal elevations computed for the 1983 to 2001 National Tidal
Datum Epoch (NTDE). Recognizing that tidal heights vary with location around the Island, on
the strength of the historical tide information, the published tidal datums for the Harbor were
converted from the station datum to NAVD88 and used as the reference throughout the project
area and for direct comparison with the tidal profiles of other areas. Information for NOAA tide
station #8449130 can be found at
http://tidesandcurrents.noaa.gov/stationhome.html?id=8449130#info providing a real-time
source of 6-minute water level observations for Nantucket Harbor and the Sound.
Table 2 depicts the highest tide and storm surge for each year from 1965 to 2015
referenced to NAVD88 and MLLW. The maximum storm surge and the maximum storm tide
rarely occur on the same day. Figure 3 shows a plot of the maximum annual values of Table 2.
While data points are widely dispersed, it is interesting to note that the trendlines for both data
sets indicate an increase in the maximum annual storm tide and storm surge over the past 50
years.
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Table 2 Maximum Annual Storm Tides and Storm Surges recorded at the Nantucket Harbor Tide Station.
(#8449130) since 1965
Maximum Tide Elevation (Feet) *Maximum Storm Surge (Feet)
Year Date NAVD88 MLLW Date Feet
1965 07/01/65 2.33 4.42 2/25/65 1.29
1966 12/29/66 3.01 5.10 1/23/66 2.02
1967 04/29/67 3.51 5.60 5/28/67 3.38
1968 11/22/68 3.41 5.50 11/10/68 2.69
1969 02/10/69 3.31 5.40 2/10/68 2.20
1970 01/08/70 3.11 5.20 12/26/70 2.32
1971 01/01/71 2.80 4.89 3/28/71 1.91
1972 01/02/72 3.51 5.60 2/19/72 2.99
1973 03/23/73 3.54 5.63 3/22/73 3.03
1974 08/18/74 2.90 4.99 12/2/74 1.85
1975 12/22/75 2.63 4.72 11/25/75 2.06
1976 03/17/76 3.12 5.21 1/22/76 1.54
1977 10/14/77 2.81 4.90 1/7/77 2.47
1978 02/07/78 4.04 6.13 2/6/78 2.92
1979 01/25/79 3.70 5.79 1/25/79 2.23
1980 03/22/80 2.81 4.90 3/22/80 2.18
1981 11/16/81 2.91 5.00 12/6/81 1.98
1982 10/09/82 3.17 5.26 4/6/81 2.55
1983 11/25/83 3.20 5.29 2/12/83 2.32
1984 03/29/84 3.05 5.14 3/29/84 3.30
1985 01/05/85 3.03 5.12 3/12/85 1.89
1986 12/31/86 3.22 5.31 11/19/86 2.40
1987 01/02/87 4.25 6.34 11/12/87 2.67
1988 01/26/88 2.59 4.68 4/11/88 1.44
1989 01/07/89 2.76 4.85 2/24/89 1.75
1990 12/04/90 3.08 5.17 10/27/90 2.15
1991 10/30/1991**5.78 7.87 10/30/1991**4.60
1992 12/12/92 4.60 6.69 12/12/92 3.29
1993 12/16/93 3.61 5.70 3/13/93 3.16
1994 03/04/94 2.80 4.89 12/23/94 2.46
1995 12/20/95 3.72 5.81 12/20/95 2.10
1996 01/08/96 3.31 5.40 1/8/96 2.98
1997 01/10/97 3.65 5.74 4/19/97 3.36
1998 01/29/98 3.53 5.62 2/6/98 2.56
1999 12/01/99 2.93 5.02 12/1/99 2.68
2000 12/12/00 2.82 4.91 1/25/00 2.10
2001 03/07/01 4.03 6.12 3/7/01 3.22
2002 11/06/02 3.66 5.75 12/26/02 2.59
2003 01/04/03 3.62 5.71 12/7/03 3.00
2004 11/13/04 2.91 5.00 12/26/04 2.35
2005 01/23/05 4.12 6.21 1/23/05 3.20
2006 01/31/06 3.60 5.69 2/12/06 2.60
2007 11/03/07 4.03 6.12 11/3/07 3.11
2008 01/28/08 3.25 5.34 1/28/08 2.37
2009 06/22/09 3.55 5.64 12/20/09 2.81
2010 01/02/10 3.80 5.89 12/26/10 2.94
2011 09/30/11 2.78 4.87 10/30/11 2.72
2012 10/29/12 3.89 5.98 10/29/12 3.90
2013 02/09/13 4.80 6.89 2/9/13 4.39
2014 01/03/14 4.51 6.60 1/3/14 3.50
2015 01/27/15 5.18 7.27 1/27/15 4.56
* Elevations before 1996 based on hourly readings estimated to be within 0.5' of maximum elevation
** Storm of Record - Note: Maximum water elevation did not occur at time of maximum storm surge.
- 17 -
Figure 3 Plot of Maximum Annual Storm Tides and Storm Surges recorded at the Nantucket Harbor Tide Station.
(#8449130) since 1965. Note upward trendlines.
Based on historical research, Table 3 ranks the top twenty storm tides experienced in
Nantucket Harbor since 1963. Referenced to NAVD88 and MLLW, the increase in elevation
between the top 5 tides is
approximately a 1 foot; between the
top ten tides approximately a foot and
a half; and between the top twenty
tides approximately 2 feet. The relative
closeness of these rankings further
illustrates the importance of the not
only the magnitude of storm surge but
when it occurs relative to the time of
high tide.
Using this information and
historical resources, Table 3 represents
the tidal profile used to screen IPs for
Nantucket Harbor and the surrounding
areas. APPENDIX A includes a list of
references considered in developing
this profile. As shown by the table, the
Nantucket Harbor (Station 8449130)
Highest Recorded Water Levels
Rank Date NAVD88 (Ft.) MLLW (Ft.)
1 10/30/1991 5.78 7.87
2 1/27/2015 5.18 7.27
3 2/9/2013 4.80 6.89
4 12/12/1992 4.60 6.69
5 1/3/2014 4.51 6.60
6 1/2/1987 4.25 6.34
7 1/23/2005 4.12 6.21
8 2/7/1978 4.04 6.13
9 3/7/2001 4.03 6.12
10 11/3/2007 4.03 6.12
11 10/29/2012 3.89 5.98
12 1/2/2010 3.80 5.89
13 12/20/1995 3.72 5.81
14 1/25/1979 3.70 5.79
15 11/6/2002 3.66 5.75
16 1/10/1997 3.65 5.74
17 1/4/2003 3.62 5.71
18 12/16/1993 3.61 5.70
19 1/31/2006 3.60 5.69
20 6/22/2009 3.55 5.64
Table 3. Top 20 historical storm-tides recorded at the Nantucket
Harbor Tide Station. (#8449130) since 1965
- 18 -
maximum storm tide elevation considered in this analysis was the storm tide of record plus 6 feet
(13.78’ NAVD88). A review of the NOAA tide charts for Nantucket Harbor indicated that the
maximum astronomical high water predicted for 2015 was 2.21’ NAVD88. To evaluate potential
nuisance flooding associated with more frequent non-storm tidal events, this elevation was used
as the beginning level for the IP analysis.
Table 4. The Nantucket Tidal Profile.
- 19 -
For reference purposes, Table 4 also includes coastal flood stages established by the Southern
New England Weather Forecast Office of the National Weather Service (SNEWFO-NWS) as
part of its experimental Coastal Flood Threat and Inundation Mapping webpage. Original levels
referenced to local MLLW have been converted to NAVD88 for comparative purposes. The
flood stages and corresponding action alerts can be summarized generally as follows:
Action Stage: NWS or a partner/user needs to take some type of mitigation action in
preparation for possible significant storm related impacts.
Minor flooding: minimal or no property damage is anticipated but there may be some
public threat associated with a possible storm.
Moderate Flooding: some inundation of structures and roads and some evacuation of
people and/or transfer of property to higher elevations are anticipated.
Major Flooding: extensive inundation of structures and roads and significant evacuation
of people and/or transfer of property to higher elevation are anticipated.
Due to the low relief (flatness) of waterfront areas, such as the downtown harbor region, where a
slight vertical rise in water level can affect large horizontal areas, it is clear why the elevations
defining the lower stages are so close.
A word about datums
A datum is a reference point, line, or plane from which linear measurements are made.
Horizontal datums (e.g., the North American Datum of 1983 (NAD83)) provide a common
reference system in the x, y-dimension from which a point’s position on the earth’s surface can
be reported (e.g., latitude and longitude). Similarly, vertical datums provide a common reference
system in the z-direction from which heights (elevation) and depths (soundings) can be
measured. For many marine and coastal applications, the vertical datum is the height of a
specified sea or water surface, mathematically defined by averaging the observed values of a
particular stage or phase of the tide, and is known as a tidal datum (Hicks, 1985).1 It is important
to note that as local phenomena, the heights of tidal datums can vary significantly from one area
1 The definition of a tidal datum, a method definition, generally specifies the mean of a particular tidal phase(s)
calculated from a series of tide readings observed over a specified length of time (Hicks, 1985). Tidal phase or stage
refers to those recurring aspects of the tide (a periodic phenomenon) such as high and low water.
- 20 -
to another in response to local topographic and hydrographic characteristics such as the geometry
of the landmass, the depth of nearshore waters, and the distance of a location from the open
ocean (Cole, 1997).2
As most coastal residents know, tides are a daily occurrence along the Massachusetts
coast. Produced largely in response to the gravitational attraction between the earth, moon and
sun, the tides of Massachusetts are semi-diurnal - i.e., two high tides and two low tides each tidal
day.3 Although comparable in height, generally one daily tide is slightly higher than the other
high tide, correspondingly, one low tide is lower than the other (Table 5). Tidal heights vary
throughout the month with the phases of the moon with the highest and lowest tides (referred to
as spring tides) occurring at the new and full moons. Neap tides occur approximately halfway
between the times of the new and full moons exhibiting tidal ranges 10 to 30 percent less than
the mean tidal range (NOAA, 2000a.)
Tidal heights also vary over longer periods of time due to the non-coincident orbital paths
of the earth and moon about the sun. This variation in the path of the moon about the sun
introduces significant variation into the amplitude of the annual mean tide range and has a period
of approximately 18.6 years (a Metonic cycle), which forms the basis for the definition of a tidal
epoch (NOAA, 2000a). In addition to the long-term astronomical effects related to the Metonic
cycle, the heights of tides also vary in response to relatively short-term seasonal and
meteorological effects. To account for both meteorological and astronomical effects and to
provide closure on a calendar year, tidal datums are typically computed by taking the average of
the height of a specific tidal phase over an even 19-year period referred to as a National Tidal
Datum Epoch (NTDE) (Marmer, 1951). The present NTDE, published in April 2003, is for the
period 1983-2001 superseding previous NTDEs for the years 1960-1978, 1941-1959, 1924-1942
and 1960-1978 (NOAA, 2000a).
Frequently, tidal datum elevations are correlated to a fixed reference surface known as a
geodetic datum. Two common reference systems adopted as standard geodetic datums for
vertical measurements are the National Geodetic Vertical Datum of 1929 (NGVD29) and the
2 For example, the relative elevation of MHW in Massachusetts Bay is on the order of 2.8 feet higher than that
encountered on Nantucket Sound and 3.75 feet higher than that of Buzzards Bay.
3 A tidal day is the time or rotation of the earth with respect to the moon, and is approximately equal to 24.84 hours
(NOAA, 2000a). Consequently, the times of high and low tides increase by approximately 50 minutes from calendar
day to calendar day.
- 21 -
North American Vertical Datum of 1988 (NAVD88). NGVD29 was derived from mean sea level
observations at twenty-one (21) tide stations in the United States, including Boston Harbor, and
five (5) in Canada. It is important to note that although it is often referred to as Mean Sea Level
of 1929, the relationship between NGVD29 and local mean sea level is not consistent from one
location to another and, therefore, NGVD29 should not be confused with local mean sea level.
Table 5. Common Tidal Datums (Source: NOAA, 2000b).
As an island and presumably because of its distance offshore connecting Nantucket to a
geodetic, mainland vertical datum does not lend itself to traditional surveying methods and
NGVD29 was never formally adopted for the Island. As a result, Island datums, based on local
tidal information and memorialized by benchmarks, were adopted for work requiring a tidal
connection. Probably the most widely used, the Half-tide Level of 1934, was based on tidal
readings of the 1930s and was used for among other things as the vertical plane of reference for
the initial series of Flood Insurance Rate Maps (FIRMs). An example of another tidal datum
encountered on the island is the Mean Low Water datum of 1992 (MLW of 1992), which
presumably was derived based on tidal observations in the early 1990s.
NAVD88, is a fixed vertical reference system, similar in nature to NGVD29, derived
from the height of a primary tidal bench mark located at Father Point, Rimouski, Quebec,
Canada. NAVD88 is slowly replacing NGVD29 as the official vertical datum for most federal
- 22 -
and state agencies in the United States and as a geodetic datum can be used to compare
elevations directly (heights and soundings) at different geographic locations.
Based on field work conducted by CCS using RTK-GPS, relationships between the
various island vertical planes were developed. While not the objective of this study Figure 4
shows the relationship between the various vertical datums encountered during this project and is
included here for informative purposes. Care should be exercised when converting from one
datum to another and the results verified depending on the purpose of the work.
Datum NAVD88 NAVD88 NAVD88 MLLW
Figure 4 Contemporary & Historical Datum Relationships for Nantucket Harbor. Contemporary data based on 1983
– 2001 NTDE and a Mean Range (Mn) = 3.04 feet.
PROJECT RESULTS
The Storm Tide Pathway Final Dataset
The desktop analysis of the lidar data yielded 129 potential STPs throughout the study
area (Figure 5). Each STP was inspected by the 3-person team in the field and the collected
RTK-GPS data downloaded to the laptop where it was compared against the lidar. Frequently,
based on these observations, the potential STP was moved when the team determined the 2010
- 23 -
Figure 5. Top: initial set of STPs identified in desktop analysis of lidar (n = 129). Middle: Final, color-coded Storm
Tide Pathways (n = 76). Bottom: final STPs underlain by initial set of STPs (white).
- 24 -
lidar was not representative of the 2015 real-world terrain. All field adjusted STPs were located
accurately by RTK-GPS for later incorporation into the comprehensive database.
The final dataset contains 76 storm-tide pathways with the areas around Madaket and downtown
Nantucket Harbors containing the greatest densities of STPs (Figure 5). A comparison of the
preliminary and final datasets demonstrates the value of the fieldwork, where 53 (47%) of the
potential STPs identified during the desktop analysis were eliminated. Finally, approximately 7
potential STPs were either removed or not able to be verified (discussed below) along the south
shore and Siasconset beaches where field inspection revealed that points: were found to be
higher than represented on the lidar or the maximum study elevation (storm of record plus 6
feet); had moved since the lidar was acquired in 2010: did not provide an inundation pathway
that threatened natural or human resources (e.g. the pathway lead to a hollow between primary
and secondary dunes); and/or could not accessed.
As an erosion dominated shoreline (see CZM Shoreline Change Project,
http://www.mass.gov/eea/agencies/czm/program-areas/stormsmart-coasts/shoreline-change/,
accessed 6/21/16) this area of the Island is subject to significant wave action that contributes to
frequently shifting landforms and topography and periodic overwashes in the vicinity of the
Great Ponds. As a dynamic environment, the south shore is an example of the type of shoreline
for which the use of lidar to identify potential STPs is less useful since it can be quickly out of
date. Further, relying on field locations of actual STPs can also be misleading as the shoreline
continues to evolve daily.
Recognizing the ephemeral nature of some STPs along an active coast, a more viable
management approach may require a more contemporary reference framework than that
provided by lidar or archived GPS locations of STPs. Further, for these areas other factors such
as wave heights and general topographic information may provide more effective information
upon which to base real-time emergency management decisions.
As discussed earlier, PALs and IRs were developed for each STP, with the excpetion of
those classified as unverified (discussed below). Initially, the Individual Ranges (IR) were
initially developed at 1-foot intervals, however, due to the low relief (i.e., little change in
elevation for an extended distance) of Nantucket coastal areas, IRs were developed at ½ ft range
intervals to provide more effective management information. In addition to the need for tighter
- 25 -
IR range intervals, the low relief along areas of the island, also highlights the vulnerability of
these areas to relatively small increases in sea level.
Types of Storm Pathways included in the Database
There are several types of STPs included in this dataset: the standard Storm Tide Pathway
(STP) as discussed above, the ‘spillway’ (STP-S); the ‘roadway’ (STP-R); and the unverified
(STP-U) (Table 6). The sub-types were developed to reflect different on-the-ground
morphologies and techniques used to characterize potential inundation at these locations.
Table 6. Breakdown of Storm Tide Pathways
Pathways Standard (STP) Spillway (STP-S) Roadway (STP-R) Unverified (STP-U)
76 21 5 23 27
The ‘standard’ STP is described as a relatively narrow low-lying area where flowing
water is conveyed inland by the natural topography (Figure 6). The term “spillway” is a term
developed to reflect the low relief of the area. An STP-S is situated in very flat areas and are
representative of long broad weir-like formations as opposed to the discrete point-like nature of
the standard STPs. Actions planned to mitigate spillway STPs generally require action along a
broad area and detailed topographic surveys in order to minimize associated flooding during
future events. While difficult to visualize, these areas are of interest precisely because the
characteristic that makes them function like a spillway (i.e., a broad flat area of inundation with
no clear, narrow pathway for floodwaters to enter), often makes it difficult to modify in ways
that will control inundation. The downtown area has several spillway STPs, including walls,
natural and man-made berms, and a dike. Not restricted to a discrete point or conveyance, these
types of STPs (i.e., STP-S) most likely will require more comprehensive, area-wide approaches
to mitigate inland inundation than that required for conventional STPs.
- 26 -
Figure 6. South of downtown Nantucket. Top: two STPs with corresponding Pathway Activation Levels.Bottom
Roadway STPs (termed STP-R) were delineated as they are associated with inundation
that only effects roadways. Based on the fieldwork, 23 STP-Rs were documented along central
and eastern sections of Polpis Rd and the western end of Madaket Rd. Since this study does not
- 27 -
calculate the probability of flooding events nor perform an engineering analysis of the open-
channel flow, the STP-R reflects only the location of an inundation pathway and the potential
area affected based on lidar elevations and the GPS survey data since access along these roads
may be impacted. Another way to think of STP-Rs is that the primary hazard associated with
them is the water flowing over the road. If water crosses a road and continues inland to inundate
more areas, the point at which the water crossed the road was characterized as an STP and not an
STP-R. (Therefore, an STP could flood a road, but an STP-R ‘only’ floods a road).
Finally, unverified STPs (STP-Us) were defined to be potential STPs identified during
the desktop analysis that could not be occupied by the field team. As discussed above, the lidar
used for this study is a “bare earth lidar data set (typical for these types of analyses) from which
vegetation, (trees, bushes, beach grass, salt marsh, etc.) and structures (houses, buildings, etc.)
were removed from during processing (hence the ‘bare earth” designation). As a result, certain
low spots found in the lidar analysis could not be accessed or were otherwise inaccessible (e.g.
private property) (Figure 7).
Figure 7. Example of an STP-U. This was characterized as an unverified STP due to its locationon private
residential property.
Many of the 27 STP-Us compiled in this study are located in low areas that will
experience inundation but, due to the above fieldwork limitations, the precise location of the STP
could not be documented through GPS occupation. To provide for a complete database, the
- 28 -
potential location of these STPs was retained in the database and cited accordingly so that they
can be located should occupation be possible.
The data layers developed for this study were also used to prioritize the present and future
projects the town has identified within its 2014 Coastal Management Plan
(http://www.nantucket-ma.gov/281/Coastal-Management-Plan-Work-Group). The 2013-14
USGS lidar was used to map the Individual Ranges (extent of inundation) for the entire island
from the highest spring tide of the year to the Storm of Record + 6ft. This was done to help
prioritize the projects within the CMP regardless of the presence or absence of an STP. Though
ground-truthing the extent of this inundation was beyond the scope of this study, this analysis
and mapping was undertaken to provide the town with the best possible data set going forward
and in fact this island wide mapping, was not in the project proposal. The 22 locations noted
within the CMP for current and potential projects were overlain onto the IRs and the locations
relative to the STPs and other potential hazards were documented (Figure 8). Center staff worked
with Town staff to convey the impact these new data will have on project design and
implementation.
Figure 8. Extent of inundation based on 2013-14 USGS lidar for the entire island. Elevations are in MLLW ft. based
on NOAA’s Nantucket Tide Gauge. Red dots are locations of projects within the town’s 2014 Coastal Management
Plan.
- 29 -
This study is deterministic rather than probabilistic, the focus was on creating a high-resolution
map of where inundation would occur and when, or at what water level, inundation would begin.
The uncertainties associated with quantifying the how and why of coastal flooding, the modelling
of storm surge, sea level rise, waves, etc. are prohibitive when dealing with inundation events at
the local level by coastal managers. These uncertainties and others are largely removed by the
‘where and when’ of mapping storm tide pathways.
- 30 -
Appendix A
A Summary of References reviewed Concerning Major Coastal Storm Events,
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