Content uploaded by Bryan J Legare
Author content
All content in this area was uploaded by Bryan J Legare on Nov 14, 2018
Content may be subject to copyright.
1
Mapping storm tide pathways
in the Little Beach Area
of Chatham, Massachusetts
Prepared by
Mark Borrelli, PhD
Steve T. Mague
Bryan Legare
August 2018
2
Introduction
The northeast storms of January and March 2018 caused extensive flooding from the Atlantic
Ocean in the Little Beach area of Chatham. After multiple storms the flood waters did not recede
leaving low-lying areas with standing water. The Center for Coastal Studies (CCS) was asked to
map the location of storm tide pathways for the Town using a technique developed by CCS for
the Coastal Resiliency Grant program administrated by the State’s Office of Coastal Zone
Management. Unlike traditional “bathtub” or inundation modeling, the storm tide pathways
method combines: 1) contemporary and historical elevation data associated with community
storm and astronomical tidal profiles; 2) contemporary topographic information from the most
recent Lidar data; and 3) field verification and mapping of threshold elevations that, when
exceeded by storm tide levels, activate pathways that convey destructive flood waters inland. A
key benefit of this method is elimination of the need for assumptions related to: winds; waves;
storm frequency, intensity, or probability; rate(s) of sea level rise, etc., which contribute to the
uncertainties associated with coastal inundation models. Further, maps of storm tide pathways
provide communities with the information necessary to respond to approaching storms and to
plan for future events.
Overview of Methodology
The term storm tide as defined by the National Atmospheric and Oceanic Administration
(NOAA) refers to, ‘…the total observed seawater level during a storm, which is the combination
of storm surge and astronomical [predicted] tide.’ A storm tide pathway represents a mapped
location (latitude, longitude, and elevation) where water will begin to flow inland during a flood
event. Both location and elevation of each pathway are then incorporated into a database that is
linked to maps stored in a GIS database.
The highest resolution, and most recent elevation data available for the Little Beach area was
used in our preliminary analysis. Lidar (a remote sensing method of collecting detailed elevation
data), collected by the United States Geological Survey (USGS) on April 4th 2014 to conduct
Post-Hurricane Sandy studies, was downloaded from the Massachusetts Coastal Zone
Management (MCZM) online mapping tool, MORIS
(maps.massgis.state.ma.us/map_ol/moris.php). These data were brought into a state-of-the-art
3
data visualization software package (Fledermaus™) and used as the basis for initial mapping of
storm tide pathways. After further desktop analysis, the horizontal and vertical position of each
potential pathway was verified in the field using survey-grade, GPS equipment. While in the
field, the topographic setting was further evaluated to ensure that potential pathways, not visible
on the Lidar were not overlooked. This process is an iterative sensitivity analysis to identify
pathways associated with the maximum water elevation recorded during previous storms and
high water events, such as spring tides, king tides, and other periodic or episodic ‘nuisance
flooding’.
After the pathways have been field verified, the extent of inundation potential of each pathway is
mapped in ½-foot increments starting from the elevation that water first begins to flow over the
pathway and up to the maximum study elevation (11.0 ft NAVD88). These elevations typically
begin at the highest predicted tide of the year and proceed to a maximum elevation represented
by the storm of record plus three feet. The analysis is extended to elevations 3 feet beyond the
storm of record to identify pathways that have never flooded but will be susceptible to future
flooding with projected increases in sea level rise. These types of insights can be invaluable to
town managers and planners, residents and other stakeholders.
Tidal Datums
The effects of storm tides on coastal communities are dependent on many factors including:
shoreline orientation (e.g., east facing v. south facing); tide range (e.g., the elevation of mean
high water (MHW) in Boston Harbor is 4.31 ft NAVD88 while that of Stage Harbor is 1.52 ft
NAVD88); topography of adjacent upland; 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
elevation); nature of coastal landforms (e.g., rock shorelines of the north shore v. the dynamic
sandy shorelines of Cape Cod); and vertical relationship between historical community
development and adjacent water levels.
With such variation in natural and human-altered characteristics, the initial step in the
identification of storm tide pathways for a community is development of a datum-referenced
tidal profile that characterizes average tidal heights, nuisance flooding, and historical storm tides.
4
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), to be useful this
tidal profile must include datum-referenced storm tides of the past, including the elevation of the
maximum storm tide experienced (i.e., the storm of record) for the area, and future sea level rise.
In March of 2017, the Center for Coastal Studies installed a HOBO™ U20 water level logger in
Outermost Harbor anticipating a potential new inlet formation through South Beach. On April 1,
2017, a new inlet formed across from Outermost Harbor approximately 2 miles to the south of
the Fish Pier in Chatham Harbor. The water level logger is referenced vertically to NAVD88 (a
geodetic datum) and water elevations are logged every six minutes. After collecting these data in
NAVD88 they can be converted to local tidal datums by developing monthly averages for these
values (Appendix A). Following standard NOAA-COOP procedures these values were used to
translate one month of datum-referenced tidal observations into 1983 – 2001 National Tidal
Datum Epoch (NTDE) values for comparison with datums published for NOAA Stations
#8447435 (Aunt Lydia’s Cove) and #8447505 (Stage Harbor) (NOAA, 2003). For more
information see Appendix A.
Table 1 represents the tidal profile constructed to complete the storm tide pathway analysis for
the Little Beach area. Rounding to the nearest foot from the table, the maximum storm tide
elevation considered in this analysis was 11.0 ft (13.7 ft MLLW). This elevation represents the
maximum elevation recorded by CCS at Outermost Harbor during the January 4, 2018 nor’easter
of 7.76 ft NAVD88 (10.42 ft MLLW) plus 3 feet, rounded up to the nearest ½ foot increment
(10.76 ft → 11.00 ft). Prior to this event, the storm of record reported by NOAA for Aunt
Lydia’s Cove (Sta # 8447435) was 6.95 ft NAVD88 (9.76 ft MLLW). To evaluate potential
nuisance flooding associated with more frequent non-storm tidal events, the lowest elevation
considered in the STP analysis was 2.80 ft NAVD88 (5.46 ft MLLW), which was rounded down
to the next ½ foot increment from the elevation of mean higher high water (MHHW) calculated
for Outermost Harbor (2.80 ft → 2.5 ft), rounding up would have missed water between 2.8 to
2.99 ft.
5
Table 1. Outermost Harbor Storm Tide Profile
Results
Initial analysis of the Lidar data using visualization software yielded 42 potential storm tide
pathways throughout the Little Beach study area. Each location was inspected by the 3-person
field team and, where necessary, storm tide pathways (STP) were adjusted when it was
determined that the 2014 Lidar did not represent 2018 conditions. Based on this field work, the
final STP dataset developed for this project contains 36 storm-tide pathways (Table 2). These
Table 2. Summary of Storm Tide Pathways. See Appendix B for more information.
Source of
Water
Pathways
Standard
(STP)
Spillway
(STP-S)
Roadway
(STP-R)
Unverified
(STP-U)
Atlantic
Ocean
15
1
9
0
5
Stage
Harbor
21
1
12
4
4
TOTAL
36
2
21
4
9
Outermost Harbor Tidal Profile
(Adjusted to 1983 - 2001 NTDE based on CCS Tide Readings)
NAVD88 (FT) MLLW (FT) Comments
Storm of Record
plus 3 Feet
10.76 13.42
Upper Limit of Storm
Tide Pathway Analysis
Coastal Storm
1/4//2018
7.76 10.42
Storm of Record
Based on CCS Observations
Coastal Storm
1/3/2014
6.95 9.61
Based on Highest Observed Storm
Records for Aunt Lydia's Cove NOAA
Sta. #8447435
Maximum 2018
Predicted High
6.37 9.03
From 2018 NOAA Tide Predictions
MHHW 2.80 5.46
Adjusted to 1983-2001 NTDE
MHW 2.28 4.94
Adjusted to 1983-2001 NTDE
MSL --- ---
Adjusted to 1983-2001 NTDE
MTL 0.11 2.77
Adjusted to 1983-2001 NTDE
MLW -2.06 0.60
Adjusted to 1983-2001 NTDE
MLLW -2.66 0.00
Adjusted to 1983-2001 NTDE
6
pathways were further characterized based on whether the source of the inundation was Stage
Harbor (Nantucket Sound) or the Atlantic Ocean. One STP was found to be in a position where it
could potentially be flooded in either direction (Figure 1).
Figure 1. Storm tide pathways for the Little Beach Area. Thirty-six (36) pathways are color-coded to reflect
elevation at which storm tide water will begin to flow at those locations. All elevations are relative to NAVD88 feet
for their respective sources of water, Atlantic Ocean (circles) and Stage Harbor (square). One STP, a blue circle
overlain onto a blue square, (noted by white arrow) could be flooded from both bodies of water. Aerial photograph
taken in June 2017 (provided by the Town of Chatham).
There are several types of STPs included in this dataset: standard Storm Tide Pathways (STP)
discussed above; ‘spillways’ (STP-S); ‘roadways’ (STP-R); and unverified (STP-U) (Table 2).
These sub-types were developed to reflect different on-the-ground morphologies and techniques
needed to identify and/or describe potential inundation at these locations.
The ‘standard’ STP can be described as a relatively narrow low-lying area where flowing water
is directed inland by the natural topography or human-altered landscape (Figure 2A). As opposed
to the discrete point-like nature of the standard STPs, the term ‘spillway’ is used as a way to
reflect the low relief of the area. The spillway STPs are situated in very flat areas and are
representative of long broad weir-like formations (Figure 2B). Actions planned to mitigate
7
spillway STPs generally must be implemented along a wide area and designed in conjunction
with detailed topographic surveys in order to minimize associated flooding during future events.
While difficult to visualize, spillway STPs are often of greatest concern because of the associated
broad, flat areas of inundation with undefined pathways for controlling flood waters. Spillway
STPs were the most prevalent (21 out of 36) in the Little Beach area.
Figure 2. A. Example of a standard type of storm tide pathway (STP). In this instance a human-altered STP. B. An
example of a ‘spillway’ STP near the Outermost Harbor Marina.
A roadway STP (STP-R) designation is used for those inundation pathways that generally impact
roadways with little to no flooding in other areas of concern. Four (4) STP-Rs were mapped in
this study on Morris Island Road, the only access into or out of the area and a potential concern
for emergency vehicles.
8
Finally, 9 STP-Us were identified in low-lying areas that would likely experience flooding,
however, the precise location of the pathway could not be obtained. Unverified STPs (STP-U)
are defined to be STPs that were identified during the Lidar analysis but for various reasons
could not be located or occupied by the field team. For example, the elevation data used for this
study is a ‘bare earth’ Lidar data set. Since these data are processed to remove vegetation, (trees,
bushes, beach grass, salt marsh, etc.) and structures (houses, buildings, etc.) occasionally STPs
that appear accessible during the analysis are found to be inaccessible in the field. In addition,
while not data-related, where STPs were clearly located on private property (Figure 3) they were
not occupied if property owners could not be asked for permission.
Figure 3. Example of an STP-U. This was an unverified STP as it is located on private property. The red dot is the
approximate location of the unverified pathway determined during the Lidar analysis.
Based on the results of the field survey, a total of 15 storm tide pathways were identified with the
potential to convey storm tide waters to Little Beach from the Atlantic Ocean. Thirteen of the
pathways occurred at elevations ranging between 4.3 – 6.0 ft NAVD88 and should be of primary
interest to the town. Significantly, spillway STPs make up 9 out of the 15 pathways identified.
While this is perhaps intuitive given the flat topography of the area, 8 out of 9 of these pathways
are below 6.0 ft NAVD88 and, as noted above, may present more of a challenge to address due
to the length of affected areas.
The lowest elevation storm tide pathways are those conveying storm tide water from Stage
Harbor. These elevations range from 3.68 to 9.79 ft NAVD88; however, the tidal restriction near
9
326 Morris Island Road minimizes the likelihood that water from Stage Harbor would be able to
flood these pathways. Of the 21 pathways that flood from Stage Harbor, 13 are tidally restricted.
Further, all of the STPs that are below 8.50 ft NAVD88 pass through the same tidal restriction. A
total of 15 out of 21 pathways that flood from Stage Harbor cross Morris Island Road at varying
depths. Based on the results of the field survey, this tidal restriction will be vital to continue to
aid in the control of flooding from Stage Harbor into the future.
Conclusions
The land in and around Outermost Harbor Marina is a flat, low-lying area made more vulnerable
to inundation with formation of the new inlet on April 1st, 2017. Many storm-tide pathways are
located in this area. As evidenced by the January and March 2018 storms, they are among the
lowest pathways that can convey storm tide water to much of the Little Beach area.
Although 36 pathways were identified in this study, it appears that focusing on the general area
proximate to Outermost Harbor Marina would reduce the more frequent flooding of the Little
Beach area associated with the more frequent, less powerful coastal storm tide events (<6 ft
NAVD88). As shown in Figure 4, the area of focus for these mitigation efforts is located
generally to the southwest of the Marina continuing to the dune field to the northeast. It should
be noted that the line provided in Figure 4 signifies the area of interest alongshore and does not
represent the location of a proposed structure.
Figure 4. The orange line represents an area that could be addressed to reduce or eliminate storm tide flooding for a
significant area of Little Beach lying below elevation 5 ft NAVD88 that experiences inundation from the Atlantic
Ocean. All elevations are in feet and refer to NAVD88.
10
Due to the relatively flat topography throughout the Little Beach area, solutions for many
individual pathways may be unfeasible. For this reason, the results of this study would suggest
that a micro-regional approach that focuses on potential solutions to multiple pathways of similar
elevation over broad areas may be the most productive approach to consider. As with any design
solution, a qualified engineer should be consulted to design and evaluate various alternatives.
A similar approach should be implemented for a detailed evaluation of the tidal restriction
crossing the storm tide pathway from Stage Harbor that passes under Morris Island Road. This
low-lying area, if flooded from Stage Harbor, could also inundate areas that are typically only
flooded from the Atlantic Ocean. Given the complex nature of the hydrologic and hydraulic
connections in this area, solutions to storm tide related flooding must necessarily consider
inundation possibilities from both Stage Harbor and the Atlantic Ocean.
Recommendations
Recognizing that addressing storm tide pathways with elevations exceeding 8.0 ft NAVD88
present significant design challenges, we recommend that in the short term, the following actions
be explored to minimize associated flooding:
1. Address the low-lying storm tide pathways in areas proximate to Outermost Harbor Marina
2. Address the storm tide pathway low-lying areas at the end of Starfish Lane.
3. Evaluate and manage the existing tidal restriction under Morris Island to maximize its
potential to control storm tide related flooding.
4. Conduct a detailed tide study of Stage Harbor and Outermost Harbor tides to understand the
relationship between tidal flow in, and between, Nantucket Sound and the Atlantic Ocean
and the potential effects on storm tide pathways of the Little Beach area.
11
APPENDIX A: Calculating Local Tidal Datums
An NTDE represents a specific 19-year period adopted by the National Ocean Service (NOS) as
the official period over which tide observations are taken and reduced to obtain mean elevations
of tidal datums (e.g., mean lower low water, etc.) at various tidal stations along the East, West,
and Gulf Coasts (Gill & Schultz,. 2001). A nineteen-year period is used to compute tidal datums
because it is the closest full year to the 18.6-year nodal cycle, the period required for the
regression of the moon’s nodes to complete a circuit of 360 degrees of longitude (Gill & Schultz,
2001; NOAA, 2003). The NTDE is used as the fixed period of time for the determination of tidal
datums because it includes all significant tidal periods, is long enough to average out the local
meteorological and seasonal temperature effects on sea level, and by specifying the NTDE, a
uniform approach is applied to the tidal datums for all stations.
The present NTDE is for the period 1983-2001 and as with all epochs, tidal data is reviewed
annually for possible revision. Regardless of any annual changes, NTDE values for each tidal
station are actively reviewed for revision approximately every 25 years (NOAA, 2003). Since for
comparative and reporting purposes tidal datums are specified with regard to the current 19-year
NTDE, tidal datums computed from month long tidal readings for Outermost Harbor were
translated to the 1983-2001 NTDE using the Modified-Range Ratio Method as described in
NOAA, 2003. As a primary tide station used historically as the control station for Cape Cod tide
information, NOAA tide station #8443970 located in Boston Harbor was used to adjust the
month long Outermost Harbor tide readings to the 1983-2001 NTDE.
All heights are referenced to the North American Vertical Reference Datum of 1988 (NAVD88)
to facilitate comparisons with tidal datum profiles calculated by NOAA for Stage Harbor and
Aunt Lydia’s Cove as shown in Table A-1 . The uncertainty associated with tidal datums
computed from a short series of records (i.e., 1-month versus the 19-year tidal epoch) is
estimated to be 0.13 ft (3.96 cm) (Bodnar, 1981).
1
1
Notwithstanding the generalized accuracy reported for east coast epoch values calculated from one month of tide
readings, the dynamic nature of the most recent southerly inlet indicates that the tidal profile for Outermost Harbor
may not have stabilized. For this reason, this study recommends conducting a detailed study of the tides in this area,
based on longer series of tide readings.
12
Table B-1. Tidal datum profiles for Aunt Lydia’s Cove, Stage Harbor, and Outermost Harbor (NAVD88 feet)
Station: 8447505
Stage Harbor
NTDE: 1983-2001
Accepted: Apr 17, 2003
Station: 8447435
Aunt Lydia's Cove
NTDE: 1983-2001
Accepted: Feb. 9, 2007
CCS Readings
Outermost Harbor
NTDE: 1983-2001
Jan 1 -31, 2018
Datum Description NAVD88 (FT) NAVD88 (FT) NAVD88 (FT)
MHHW
Mean Higher-High Water 1.89 2.69 2.80
MHW
Mean High Water 1.52 2.32 2.28
MTL
Mean Tide Level -0.45 -0.16 0.11
MSL
Mean Sea Level -0.35 -0.05 ---
DTL
Mean Diurnal Tide Level -0.40 -0.06 0.07
MLW
Mean Low Water -2.43 -2.63 -2.06
MLLW
Mean Lower-Low Water -2.69 -2.81 -2.66
GT
Great Diurnal Range -5.80 5.51 5.46
MN Mean Range of Tide 3.95 4.95 4.34
HWI
Greenwich High Water Interval
(hours)
4.5 4.87 4.48
LWI
Greenwich Low Water Interval
(hours)
10.25 11.59 11.58
13
Appendix B-1. Storm Tide Pathways affected by water from the Atlantic Ocean. See GIS data sets for more information.
Activation Level
Final_ID
Station
Easting
Northing
Elevation
Type
Status
Nav88 m
Nav88 ft
MLLW m
MLLW ft
Range_MLLW
Range_Navd
1
21
420210.1
4612942
2.732
STP-S
Verified
2.73
8.96
3.49
11.46
11.51 ft - 12.00 ft
9.01ft - 9.50ft
2
24
420332.2
4612998
1.703
STP-S
Verified
1.70
5.59
2.46
8.09
8.01 ft - 8.50 ft
5.51ft - 6.00ft
3
30
420408.6
4613027
1.344
STP-S
Verified
1.34
4.41
2.10
6.91
7.01 ft -7.50 ft
4.51ft - 5.00ft
4
29
420406.9
4613036
1.493
STP-S
Verified
1.49
4.90
2.25
7.40
7.01 ft -7.50 ft
4.51ft - 5.00ft
5
28
420387.2
4613099
1.453
STP-S
Verified
1.45
4.77
2.21
7.27
7.01 ft -7.50 ft
4.51ft - 5.00ft
6
25
420410.7
4613099
1.421
STP-S
Verified
1.42
4.66
2.18
7.16
6.51 ft - 7.00 ft
4.01ft - 4.50ft
7
26
420420.3
4613085
1.354
STP-S
Verified
1.35
4.44
2.11
6.94
6.51 ft - 7.00 ft
4.01ft - 4.50ft
8
27
420452.9
4613044
1.332
STP-S
Verified
1.33
4.37
2.09
6.87
7.01 ft -7.50 ft
4.51ft - 5.00ft
9
42
420416.7
4613169
0
STP-U
Unverified
1.39
4.56
2.15
7.06
7.01 ft -7.50 ft
4.51ft - 5.00ft
10
31
420551
4613064
1.581
STP
Verified
1.58
5.19
2.34
7.69
7.01 ft -7.50 ft
4.51ft - 5.00ft
11
32
420630.3
4613091
0
STP-U
Unverified
1.62
5.31
2.38
7.81
7.51 ft - 8.00 ft
5.01ft - 5.50ft
12
34
420677.4
4613104
0
STP-U
Unverified
1.83
6.00
2.59
8.50
8.51 ft - 9.00 ft
6.01ft - 6.50ft
13
40
420819.9
4613240
0
STP-U
Unverified
1.48
4.86
2.24
7.36
10.51 ft - 11.00 ft
8.01ft - 8.50ft
14
41
420899.9
4613383
0
STP-U
Unverified
3.04
9.97
3.80
12.47
12.51ft - 13.00 ft
10.01ft - 10.50ft
15
5
420655.3
4613298
1.311
STP-S
Verified
1.31
4.30
2.07
6.80
6.51 ft - 7.00 ft
4.01ft - 4.50ft
14
Appendix B-2. Storm Tide Pathways affected by water from Stage Harbor. See GIS data sets for more information.
Activation Level
Final_ID
Station
Easting
Northing
Elevation
Type
Status
Nav88 m
Nav88 ft
MLLW m
MLLW ft
Range_MLLW
Range_Navd
1
20
420164.7
4612981
2.708
STP-R
Verified
2.71
8.88
3.47
11.38
11.51 ft - 12.00 ft
9.01ft - 9.50ft
2
19
420169.6
4612999
2.926
STP-S
Verified
2.93
9.60
3.69
12.10
12.01 ft - 12.50 ft
9.51ft - 10.00ft
3
18
420104.8
4613056
2.98
STP-S
Verified
2.98
9.78
3.74
12.28
12.01 ft - 12.50 ft
9.51ft - 10.00ft
4
17
420129.2
4613054
2.594
STP-R
Verified
2.59
8.50
3.35
11.00
11.51 ft - 12.00 ft
9.01ft - 9.50ft
5
16
420122.5
4613074
2.895
STP-S
Verified
2.90
9.50
3.66
12.00
11.51 ft - 12.00 ft
9.01ft - 9.50ft
6
15
420139
4613095
2.942
STP-S
Verified
2.94
9.65
3.70
12.15
11.51 ft - 12.00 ft
9.01ft - 9.50ft
7
14
420144.1
4613099
2.95
STP-S
Verified
2.95
9.68
3.71
12.18
12.01 ft - 12.50 ft
9.51ft - 10.00ft
8
13
420152.7
4613115
2.955
STP-S
Verified
2.95
9.69
3.71
12.19
12.01 ft - 12.50 ft
9.51ft - 10.00ft
9
12
420176
4613131
2.891
STP-S
Verified
2.89
9.48
3.65
11.98
12.01 ft - 12.50 ft
9.51ft - 10.00ft
10
11
420219.4
4613161
2.983
STP-S
Verified
2.98
9.79
3.74
12.29
12.01 ft - 12.50 ft
9.51ft - 10.00ft
11
10
420260.7
4613203
2.936
STP-S
Verified
2.94
9.60
3.70
12.10
11.51 ft - 12.00 ft
9.01ft - 9.50ft
12
9
420375.6
4613247
1.122
STP-R
Verified
1.12
3.68
1.88
6.18
6.01 ft - 6.50 ft
3.51ft - 4.0ft
13
8
420405.5
4613269
1.352
STP-R
Verified
1.35
4.44
2.11
6.94
6.51 ft - 7.00 ft
4.01ft - 4.50ft
14
7
420456.5
4613281
1.391
STP-S
Verified
1.39
4.56
2.15
7.06
7.01 ft - 7.50 ft
4.51ft - 5.00ft
15
42
420416.7
4613169
0
STP
Unverified
1.39
4.56
2.15
7.06
7.01 ft - 7.50 ft
4.51ft - 5.00ft
16
6
420514.9
4613287
1.389
STP-S
Verified
1.39
4.56
2.15
7.06
7.01 ft - 7.50 ft
4.51ft - 5.00ft
17
5
420653.5
4613304
1.286
STP-S
Verified
1.29
4.22
2.05
6.72
6.51 ft - 7.00 ft
4.01ft - 4.50ft
18
4
420609.6
4613379
0
STP-U
Unverified
1.55
5.09
2.31
7.59
7.51 ft - 8.00 ft
5.01ft - 5.50ft
19
3
420703.3
4613385
0
STP-U
Unverified
2.03
6.66
2.79
9.16
9.01 ft - 9.50 ft
6.51ft - 7.00ft
20
2
420716.5
4613410
0
STP-U
Unverified
2.10
6.89
2.86
9.39
9.01 ft - 9.50 ft
6.51ft - 7.00ft
21
1
420726.9
4613446
0
STP-U
Unverified
2.11
6.92
2.87
9.42
9.01 ft - 9.50 ft
6.51ft - 7.00ft
15
APPENDIX C: GIS Data
Several shapefiles were created to document the spatial extent water would cover under different
inundation scenarios. Data are binned at six-inch intervals, vertically, beginning with 5.0 ft
MLLW (2.5 ft Navd88). The layer “Little_Beach_Range” contains each separate shapefile
polygon for each ½ foot elevation interval. Each polygon was extracted at the upper value of the
six-inch interval and is so labeled. The shapefile titled “Little_Beach_Range.shp” contains all the
individual polygons in the previously described layer in one shapefile. The combination of these
files into one polygon allows for easy display of different ranges simultaneously or sequentially.
A complimentary data set was created titled “Stage_Harbor”. Data sets titled “Little_Beach”
document the extent of water from the Atlantic Ocean direction and data sets titled
“Stage_Harbor” identify water flowing from the Stage Harbor, Nantucekt Sound direction.
The Layer “Little_Beach_Data”, include a point shapefile of the final location of the pathways.
The “Little_Beach_Inundation_Pathways.shp” includes the location and all associated data in
both NAVD88 and MLLW vertical datums, as well as the range into which they correspond. The
layer titled “Little_Beach_Individual” contains polygons that describe the spatial extent
inundation would occur if water rose to that elevation and are labeled to correspond with specific
inundation pathway shapefiles. A complimentary data set was created titled “Stage_Harbor”.
Several shapefiles were created containing data from the desktop analysis and the field portion of
the data. In the “Desktop and Field” layer, “Desktop_Inundation_Pathways.shp” describe all 42
locations identified through the initial desktop examination. The “Inundation_Pathway_all.shp”
file, includes every pathway identified in the desktop analysis and the status of each (rejected or
included) and associated data collected from field surveying. The data set
“Inundation_Pathway.shp” includes the final 36 inundation pathways identified throughout the
study.