Vulnerability Report & Adaptation Strategies
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INVENTORY AND ANALYSIS: SEA LEVEL RISE AND CLIMATE CHANGE
Introduction
Sea level is rising according to one hundred years of records from the Portland, ME tide gauge.
Along with this change, storms are becoming more frequent and intense, and damages are
increasing. The important question for the Town of Bowdoinham is: “How should the Town
respond and adapt?” This chapter inventories the best available data on historical and recent
trends in sea level change, and offers the best available current predictions for the future. This
Chapter establishes the rational basis on which the Town’s policy response to sea level rise is
based.
This chapter does not delve into the underlying causes of the observed changes in sea level. The
underlying causes of sea level rise are being debated nationally and internationally, and are the
subject of extensive scholarly investigation worldwide. This chapter simply acknowledges the
sea level changes as documented over the past century at the Portland tide gauge. Whether the
cause is man-made or natural or a combination of both is irrelevant. Whatever the cause may be,
it is important for the Town to react to changing physical conditions.
1. Trends in Sea Level Rise
As shown in this section, there is a clear historical pattern of sea level rise which began about
11,000 years ago and which is still occurring today. For the past couple thousand years there has
been a pattern of only minor, gradual increases in sea level, although the most recent data
appears to be showing an increasing rate of sea level rise.
Scientists believe that there are two dominant components to what is happening when global sea
level is observed to increase. The first is thermal expansion, as the ocean temperature warms.
The second is volumetric increase when the volume of water in the ocean increases, caused by
melting glaciers and ice sheets located on land.
When scientists look at data on a specific piece of coast, like at the shoreline of Bowdoinham,
Maine, there are some local reasons for changes in sea level, other than those at the global level.
Some movement of the land up or down is left over from the end of the last ice age. As the crust
of the earth in this area was covered with thousands of feet of ice, it sank in response, just like
when you lie down on a mattress. When the ice age ended, the land experienced “isostatic
rebound,” as the crust bounced back up. Some isostatic rebound is still happening today after
thousands of years, but the effect now is very slight. However, in the past, this phenomenon had
a tremendous effect on Maine. In other parts of the United States, there was no ice age, and
sinking of the land or “subsidence” is a problem. This is pronounced in the Chesapeake Bay area
and the Louisiana coast, where higher rates of sea level rise are happening right now, compared
to Bowdoinham, Maine. It should also be noted that seasonal wind patterns can change tide
levels during different periods within each year. In our area, tides will run lower during periods
of sustained northeast winds during the winter which blow water offshore.
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Thirteen thousand years ago at the end of the last ice age, the land in Maine was so crushed by
ice that sea level was 230 feet (70 meters) higher than it is today. At eleven thousand years ago
after the ice had receded, the land rebounded so that sea level was about 200 feet (60 meters)
below today’s levels. Continued melting of ice filled the oceans, and in the last five thousand
years, levels in Maine have been very stable. It is important to note that this is the period when
our modern beaches and wetlands that we know today were formed. See Figure 1.
Figure 1: Elevation of Sea Level in Maine – 13,000 Years Ago to Present
Studies of marshes in Wells show that in the last five thousand years (the area shaded in blue in
figure 1) the rate of change in sea level has leveled off from over 3/64 inch (1mm) per year to
only 1/64 inch (0.2 mm) per year about a thousand years ago. This data was derived by
radiocarbon dating of marsh borings. See Figure 2.
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Figure 2: Sea Level Change in Wells, ME – 5,000 Years Ago to Present
The peaceful period of gradual sea level rise that has been experienced for the past several
thousand years appears to be over. The Portland tide gauge shows that over the last hundred
years, since 1912, sea level has been rising at a rate of 1.9 mm per year. That would be 7½
inches (190 mm) during this period. This mirrors global ocean changes, as measured from
orbiting satellites, of about 5/64 inch (1.8 mm) per year. See Figure 3.
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Figure 3 – Portland Tide Gauge – Mean Sea Level – 1912 to 2011
Similar results are found up and down the coast, as documented at nearby tide gauges. See
Figure 4.
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Figure 4 – Recent Rates of Sea Level Rise – Portsmouth to Eastport
Not only has the pace of sea level rise picked up over the last hundred years, the rate is
increasing, and is up substantially since 1993. For the last 20 years or so, the rate of sea level rise
has increased to 11/64 inch (4.3 mm) per year, or 17 inches (430 mm) per century. See Figure 5.
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Figure 5 – Portland Tide Gauge – Mean Sea Level – 1993 to 2011
Not only has the pace of sea level rise picked up locally here in Maine, but the rate of rise has
picked up on global sea level, as measured by orbiting satellites. Since 1993, global sea level has
risen at a rate of 8/64 inch (3.2 mm) per year, or 12.5 inches (320 mm) per century. See Figure 6.
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Figure 6 – Global Rates of Sea Level Rise
2. Best Predictions of Future Sea Level Rise
Having an overall understanding of past changes to sea level is essential to understanding the
range of projections of future conditions. There seems to be widespread consensus in the science
community that sea level will continue to rise in the coming century. Nobody can know today
whether or not this will actually prove true, or how much the sea level will actually change by
any given point in time in the future. Still, a rising sea level has planning implications for coastal
communities like Bowdoinham. Some buildings, roads and public facilities will be impacted on a
daily basis or during storms where overall rising sea levels will worsen storm-related impacts.
While the degree of certainty is unknown, it is nonetheless important to gaze into the crystal ball
and consider the range of likely alternatives.
For planning purposes, and under the rules of the Sand Dune Act administered by the Maine
Department of Environmental Protection, 2 feet (0.6 M) of sea level rise is expected by 2100.
The current trends since 1993 shown on our local tide gauge, as well as the satellite
measurements of the global ocean levels, are showing faster increases than we have seen before.
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The amount of sea level rise in the last 100 years since 1912 has been about 7.5 inches (190
mm). The next hundred years will probably be triple that, according to the projections of the
Intergovernmental Panel on Climate Change (IPCC) and reach at least two feet. The IPCC
projections do not include contributions from the melting of glacial, land-based ice sheets.
Figure 7 shows that if you superimpose the Portland tide gauge data from 1993 to the present, as
well as the satellite measurements of the global ocean level, that sea level rise during the last 20
years is tracking the HIGHEST PROJECTION curve of the IPCC.
Figure 7: Recent Data Compared to Past Predictions
There is another factor at work, which should be considered by the Town of Bowdoinham, which
makes the prediction of one foot (0.3 M) of sea level rise by 2050, and 2 feet 0.6 M) of sea level
rise by 2100, conservative numbers. Geologists are measuring that ice sheets on land in
Greenland and the Antarctic are melting, which could add substantial amounts of water to the
world’s oceans. This has not been a factor over the past hundred years. The recent SWIPA report
(Snow, Water, Ice, and Permafrost in the Arctic) by Rignot and Others, from March 2011,
predicts that “if the current Antarctic and Greenland ice sheet melting rates continue for the next
four decades, their cumulative loss could raise sea level by 5.9 inches (150 mm) by 2050. When
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this is added to the predicted sea level contribution of 3.1 inches (79 mm) from glacial ice caps
and 3.5 inches (89 mm) from ocean thermal expansion, total sea level rise could reach 12.6
inches (320 mm) by the year 2050.” (More information is available from the American
Geophysical Union, via the Web: http://www.agu.org).
Figure 8 shows a review of 9 recent peer reviewed studies that indicate between one and six feet
of sea level rise by the year 2100, when the influence of ice sheets is included in sea level rise
scenarios. The center of the red bar on the right side of the figure is the middle of all predictions,
at around 4 feet (1.2 M). Most newer authors are factoring in a contribution from ice sheet
melting processes, which drives their predictions higher.
Figure 8: A Range of Sea Level Rise Predictions Through Year 2100
It seems clear from these expert projections that Bowdoinham can expect and should plan for
some degree of sea level rise in the coming years. How much and in what timeframe are the key
unknowns.
Vulnerability Assessment – Sea Level Rise
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The Maine Geological Survey prepared a vulnerability assessment for the Town of
Bowdoinham, predicting how many miles of roads and railroads, and the number of buildings
that might be flooded by sea level rise alone, in the coming decades. MGS also predicted these
impacts to roads, railroads, and buildings, if a 100 year storm occurred, on top the risen sea level,
in the coming decades.
The analysis started by determining the height of the highest tide of the year that occurs today,
and using that as the starting point for the assessment. Table 1 shows, in the first row, that the
highest tide of the year occurring today in Bowdoinham is 4 feet above sea level (in NAVD 88
units). The amount of surge predicted in the Town’s FEMA flood study, from a storm that has a
1 in a hundred chance of occurring in any given year (the 1% or100-year storm), would be 4.7
feet on top of the Highest Annual Tide, bringing the total water level up to 8.7 feet. With 3.3
feet of sea level rise (1 meter), the highest annual tide of the year would reach 7.3 feet. Table 1
summarizes the various water heights that would occur with different amounts of sea level rise
during the highest annual tide of the year (the Spring Tide) and with a 100-year storm on top of
the Highest Annual Tide.
Scenario
Existing
0.3 m (1 foot) SLR
0.6 m (2 feet) SLR
1.0 m (3.3 feet) SLR
1.8 m (6.0 feet) SLR
Highest Annual Tide
(ft. NAVD 88)*
4.0
5.0
6.0
7.3
10.0
1% storm
(ft. NAVD 88)**
8.7
9.7
10.7
12.0
14.7
* data from the nearest NOS tidal prediction station; all data in ft, NAVD88
** data from effective FIS, 11/19/1997
Table 1: Heights of the Highest Annual Tide, and the 100-Year Storm,
with Different Amounts of Sea Level Rise
Then GIS analysis (Geographic Information Systems, or computer mapping) was conducted to
estimate the impacts to roads, railroads and buildings, associated with these higher water levels.
These are general estimates based solely on the current building stock and current road and rail
network, and are suitable for planners to understand the relative changes in impacts at varying
states of sea level rise and storm surge.
There are 85.6 miles of roads and 10.3 of rails in the Town of Bowdoinham. Table 2 shows the
mileage of roads and rails that would be flooded, as well as the percentage of total road and rail
miles flooded, with different levels of sea level rise on top of the Highest Annual Tide:
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Scenario (HAT)
Impacted Roads
Miles % impacted*
Impacted Rails
Miles
% impacted*
Existing Conditions
0.3 m (1 foot) SLR
0.6 m (2 feet) SLR
1.0 m (3.3 feet) SLR
0.0
0.0
0.0
0.2
0.0%
0.0%
0.0%
0.2%
0.0
0.0
0.0
0.0
0%
0%
0%
0%
1.8 m (6.0 feet) SLR
1.8
2.1%
0.3
3%
*impacted means the road or rail is covered by water
Table 2: Road and Rail Infrastructure Affected by Sea Level Rise Alone
The next step in the analysis was to model how much flooding might occur, when a 100-year
storm came on top of sea level rise. Table 3 shows the mileage of roads and rails that would be
flooded, as well as the percentage of total road and rail miles flooded, with a 100 year storm on
top of different levels of sea level rise:
Scenario (1% storm)
Miles of Impacted Roads and Rails
%
%
Road Miles
Rail Miles
impacted*
impacted*
Existing Conditions
1.0
1.2%
0.1
1%
0.3 m (1 foot) SLR
1.8
2.1%
0.3
3%
0.6 m (2 feet) SLR
2.2
2.6%
0.7
7%
1.0 m (3.3 feet) SLR
3.4
4.0%
1.5
15%
1.8 m (6.0 feet) SLR
5.1
6.0%
2.9
28%
*impacted means the road or rail is covered by water
Table 3: Road and Rail Infrastructure Affected by a 1% (100-Year) Storm on
Top of Sea Level Rise
Figures 9 and 10 below illustrate the locations of the predicted road and rail inundations. These
are screen shots from a Google Earth project file which was created by MGS for this
comprehensive plan chapter, in order to calculate the results in Tables 1 through 4. The Google
Earth files are on file at the Bowdoinham Town Office. Figure 9 shows the inundation scenario
of 3.3 feet (1 meter) of sea level rise, on top of the highest annual tide, without any storm surge
conditions. In this scenario, the total level of tidal waters would reach 7.3 feet. With this
additional meter of high tide, the water crossings at Brown’s Point and Center’s Point Roads are
overtopped (see red circled areas in figure).
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Figure 9: Road Segments Impacted by 3.3 feet (1 meter) of Sea Level Rise on Top of the
Highest Annual Tide
Figure 10 shows the inundation scenario of a 100 year storm on top of 2 feet of sea level rise, or
a total water level of 10.7 feet. In this scenario, the depth and length of the inundation at the
water crossings on Brown’s Point and Center’s Point Roads increase, and the rails and roads at
the Town landing are also compromised (see red circled areas in figure).
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Figure 10: Road and Rail Segments Impacted by a 1% (100-Year) Storm on Top of 2 Feet
of Sea Level Rise
The last step of the MGS analysis was an estimate of the number of buildings affected by sea
level rise and future storm surges. Table 4 indicates the number of buildings whose center points
would be flooded by the highest annual tide (HAT) with varying amounts of sea level rise, as
well as how many would be flooded if a 100-year storm in the future, came on top of sea level
rise:
Scenario of Sea Level Rise on Top of the
Highest Annual Tide (HAT)
Existing Conditions
0.3 m (1 foot) SLR
0.6 m (2 feet) SLR
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Number of
Buildings
Impacted*
With 1%
By storm on
HAT
top of
only
HAT
0
7
0
12
0
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1.0 m (3.3 feet) SLR
4
23
1.8 m (6.0 feet) SLR
12
46
*impacted means the building point is intersected by water
Table 4: Number of Buildings Affected by Sea Level Rise Alone; by
Sea Level Rise on Top of a 1% (100-Year) Storm
Climate Change and Increased Precipitation
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The last figure in this chapter, Figure 11, shows that water level increases from sea level rise are
not the only component that will affect potential flooding conditions in the Town of
Bowdoinham. It appears in the rainfall records that there is a trend for higher and more intense
rain events that has started in the last twenty to thirty years. Both the annual amount of rain, and
the number of rain storms with greater than one inch of rain in 24 hours have increased:
Figure 11: Record of Increasing Annual Precipitation; and Increasing Number of Rain
Events Per Year, Greater than 1 inch, in a 24-Hour Period, for Portland, Maine
From Climate Change in the Piscataqua/Great Bay Region: Past, Present, and Future
Carbon Solutions New England, Cameron Wake, et al., December 2011.
Other Climate Change Impacts Not Included in This Chapter
It should be noted that other possible climate change impacts were not reviewed in the creating
of this Comprehensive Plan chapter. These include such varied issues as health impacts from
insect-born disease and high heat days in summer, or plant and animal species changes and
agricultural impacts. Only issues of sea level rise and increased water levels were considered.
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GOALS & STRATEGIES
Adapting to Sea Level Rise & Climate Change
Goals
1. To recognize that sea level rise is occurring, and that tidal surges are happening more often,
with higher intensities and water levels.
2. To recognize that overall precipitation is increasing, with more frequent and intense rain and
snow events.
3. To implement strategies so the Town can become more resilient, and bounce back quickly
after damage from a flood occurs.
Strategies
1. Amend the Floodplain Management and Land Use Ordinance to strengthen standards,
beyond the minimum FEMA Flood Insurance Program requirements, for new or replacement
construction in areas that flood.
2. Enforce the Floodplain Management Ordinance requirements that existing buildings be
elevated, after they are significantly damaged in a flood.
3. Elevate roads and bridges, improve road subsurfaces, and increase culvert sizes, when these
items are being replaced, to make them more flood-proof.
4. Include information on the benefits and costs of higher or more resilient designs, when
making capital budget decisions on strategy 3, above.
5. Inventory the Town roads, bridges, drains, and buildings or other properties that are at risk
from flooding, considering sea level rise and increased rainfall, and update the inventory
every 10 years.
6. Participate in Regional or State efforts to obtain better mapping information for making
decisions about adapting to sea level rise and climate change, including aerial photography
and topographic elevation data (such as “LiDAR,” or “Light Detection and Ranging” Data.).
7. Maintain a digital Shoreland Zoning Map, and locate the position of the highest annual tide
level for Bowdoinham, so that the edges of the shoreland zone are accurate on the map, as
sea level rise increases.
8. Participate in the FEMA National Flood Insurance Community Rating System (CRS)
program, so flood insurance policy holders in Bowdoinham can get the best available flood
insurance rates.
9. Consider natural ways to make shorelines more resistant to erosion, such as installing native
plantings and creating dunes, rather than putting in hard structures that may just shift the
location of the erosion problem.
10. Review the Shoreland Zoning Ordinance and consider requiring additional buffer areas
between developed areas and the shoreline.
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11. Encourage public or private purchases of uplands adjacent to tidal marshes, to allow marshes
to migrate inland and slightly uphill, as sea level rises, to prevent these marshes from
drowning in place.
12. Remove man-made blockages or restrictions on tidal saltwater flows caused by roads or other
development, in areas where marshes can be restored to better ecological health and increase
their capacity to store flood waters.
13. Consider tide gates, one-way valves devices on drains, or other engineered controls of tidal
waters only when more passive methods are not workable.
14. Participate in Regional or State studies of sea level rise and climate change impacts, as well
as data collection efforts, as funding is available.
15. Amend the Land Use Ordinance to require the use of porous materials to reduce the area of
impervious surfaces in shoreland areas, to improve drainage.
16. Educate citizens of all ages about sea level rise and climate change risks, and activities to
increase the Town’s resilience.
17. Share information about the Town’s vulnerability to flooding with the Emergency
Management Director, and include it the Emergency Management Plan.
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