2.2. Maps that Depict Site-Specific Scenarios for
Wetland Accretion as Sea Level Rises along the
Mid-Atlantic Coast

Authors: James G. Titus, U.S. Environmental Protection Agency
Russ Jones, Stratus Consulting Inc.

Richard Streeter, Stratus Consulting Inc.

This section should be cited as:

Titus, J.G., R. Jones, and R. Streeter. 2008. Maps that Depict Site-Specific Scenarios for
Wetland Accretion as Sea Level Rises along the Mid-Atlantic Coast. Section 2.2 in:
Background Documents Supporting Climate Change Science Program Synthesis and
Assessment Product 4.1, J.G. Titus and E.M. Strange (eds.). EPA 430R07004. U.S. EPA,
Washington, DC.


-------
[ 176 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

Abstract

This paper develops maps and a data set
depicting a set of site-specific assumptions for
wetland vertical accretion developed by a panel
of wetland scientists. The panel had drawn
polygons on USGS 1:250,000 scale topographic
maps. For each polygon, for each of three sea
level rise scenarios, the panel indicated whether
tidal wetlands within the polygon would be lost,
keep pace, or be marginal. This paper describes
how we converted the hard-copy polygons into a
GIS database and created a set of maps to
concisely depict the panel's findings.

2.2.1. Background

In Section 2.1, Reed et al.1 explain the basis for
an expert panel assessment of the ability of
coastal wetlands to keep pace with rising sea
level along the mid-Atlantic Coast from the
south shore of Long Island to the Virginia/North
Carolina border. That assessment was a part of
EPA's effort to assess the possible vulnerability
of tidal wetlands to rising sea level, which also
depends on coastal topography2 and coastal
development.

This paper describes our efforts to create a GIS
data layer and maps to depict the panel's
assessment. The panel produced a set of marked-
up hard copy USGS 1:250,000 scale maps and a

:Reed, D.J., D.A. Bishara, D.R. Cahoon, J. Donnelly, M.
Kearney, Alex Kolker, L.L. Leonard, R. Orson, and J.C.
Stevenson. 2008. Site-Specific Scenarios for Wetlands
Accretion as Sea Level Rises in the Mid-Atlantic Region.
Supporting Document for CCSP 4.1, Question 3. New
Orleans, LA: Department of Earth and Enviromnental
Sciences University of New Orleans.

2In Chapter 1, Titus and Wang develop a data set and maps
expressing coastal elevations relative to spring high water,
which is approximately the upper boundary of tidal
wetlands. See Titus and Wang, 2008, Maps of Lands Close
to Sea Level along the Middle Atlantic Coast of the United
States: An Elevation Data Set to Use While Waiting for
LIDAR, in Background Documents Supporting Climate
Change Science Program Synthesis and Assessment Product
4.1: Coastal Elevations and Sensitivity to Sea Level Rise,
EPA 430R07004, Washington. DC: U.S. EPA.

set of spreadsheets. We used the hard copy maps
to define our polygon boundaries and the
spreadsheets to provide descriptions about those
polygons (i.e., attributes).4

The panel drew polygons on the hard copy maps
to approximately identify the areas associated
with five primary geomorphic settings, with
several subsettings. The USGS 1:250,000 scale
topographic maps show roughly where wetlands
exist; but they do not delineate the actual
wetlands. Therefore, we construed each polygon
as representing the panel's intent to identify an
area within which all tidal wetlands could be
associated with one of the following geomorphic
settings or subsettings:

1.	Tidal Fresh Forests

2.	Tidal Fresh Marsh

3.	Estuarine/Brackish Channelized Marshes

a.	Meander

b.	Fringing

c.	Island

4.	Back Barrier Lagoon Marsh

a.	Back barrier/Other

b.	Active flood tide delta

c.	Lagoonal fill

5.	Saline Marsh Fringe

Each polygon on the maps had an index number.
The associated spreadsheets provided:

•	Polygon index number

•	Region (as described in the panel report)

•	Two columns for geomorphic setting and
sub setting,

•	Three columns for the panel's prognosis
for wetland accretion under three
alternative sea level rise scenarios

•	Place name (optional)

•	Special explanation (if appropriate).

The three sea level rise scenarios were current
rate, current rate + 2 mm/yr, and current rate + 7
mm/yr. For each of these three scenarios the
spreadsheet provided a prognosis for wetland
accretion for each polygon. In most cases, the
prognosis was one of three possibilities: keeping

4In a GIS polygon layer, an attribute table associates
information with each polygon.


-------
[ SECTION 2.2 177 ]

pace, marginal, and loss (see Section 2.1, Reed et
al. for description). In a few cases, however, the
panel's original assessment was "marginal/loss"
for a particular sea level rise scenario.5

2.2.2. Conversion of the Panel's
Output to a GIS Dataset

Our final data set provides two layers:

•	"Raw" consists of the polygons created by
the panel (and the associated attributes),
which identify the geomorphic setting.

•	"Wetlands" is a coastal wetlands data set,
with attributes that identify the geomorphic
settings and wetland accretion potential as
defined by the panel.

The Raw Data

Our objective was to convert the hand renderings
into a digital data set suitable for use in a GIS.
The polygons provided by the panel included
tidal wetlands, nontidal wetlands, dry land, and
open water; but the information developed by the
panel applies only to the tidal wetlands within
the polygon. We also inspected the results of our
digitizing to identify and remedy those cases
where a literal digital conversion of what the
panel drew was inconsistent with the panel's
intent. For example, the polygon boundaries did
not include all of the tidal wetlands in some

areas, because the USGS 1:250,000 scale
topographic maps do not show all wetlands or
indicate the head-of-tide (above which wetlands
are nontidal).

The first step toward creating a data set was to
create a tracing of the polygons according to a
procedure developed by Russ Jones. The key
aspects were to faithfully trace the panel
polygons and the registration marks from the
USGS maps. Dana Bishara of the University of
New Orleans overlayed Mylar sheets on top of
the 1:250,000 USGS maps and manually traced
the polygons and registration marks, and sent
them to Jones.

The second step was to digitize the polygons.
Jones provided the Mylars to Digital Data
Services, Inc. (Lakewood, Colorado), who
scanned them to a digital format in color at 300
dots per inch in Tagged Image File Format (tif).
Richard Streeter digitized the polygons into a
GIS using raster-to-vector conversion software.6
See Figure 2.2.1.

The third step was to overlay the polygons with a
wetlands data set. Jones and Streeter created
quad-specific maps in a GIS by overlaying the
polygons on top of the EPA coastal wetlands
data set (Chapter 1, Titus and Wang, see note 2).
Figure 2.2.2 shows the initial "raw" product
from this overlay, for the Salisbury (Maryland)
quadrangle.

5These cases were all either along the South Shore of Long
Island or in the Virginia Beach/Chesapeake area.

6ESRI, 2005, ArcScan software, v. 9.1, Redlands, CA:
Environmental Systems Research Institute.


-------
[ 178 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

Figure 2.2.1. Polygons Created by Wetland Accretion Panel Assessment: Salisbury Quadrangle. The
wetland accretion panel drew polygons on 1:250,000 USGS quads. Panel staff then traced the
polygons onto Mylar. The black lines define subregions; the other colored lines define polygons
representing wetlands of a given geomorphic setting.


-------
Figure 2.2.2. Overlay of the Polygons from Wetland Accretion Panel with a Wetlands Data Set:
Salisbury Quadrangle. Each of the shaded polygons has an index number or letter; wetlands outside
the shaded polygons were unassigned and had to be corrected. The light red lines that are not the
boundary of a shaded polygon delineate the subregional boundaries. Note that that the shaded
polygons do not include ail of the tidal wetlands along Chincoteague and Indian River bays, nor the
upper portions of the Choptank, Nanticoke, and Pocomoke rivers.

[ SECTION 2.2 179 ]

The fourth step was quality control of the
polygons created by the panel. Figure 2.2.2
shows some of the issues that we addressed in
this step. In many cases, tidal wetlands7 lie
outside of the geomorphic regions defined by the
polygons, and assignments of geomorphic
regions did not match local conditions (e.g.,
active flood tide deltas were not adjacent to
inlets). In some cases, the tidal wetlands
extended farther inland than the polygons. See,
for example, the tidal wetlands that are not
included in a shaded polygon to the west (inland)
of polygon #3 along Delaware Bay; the extensive

7Titus and Wang (see note 2) generated a wetlands data set
from a combination of National Wetlands Inventory
wetlands and state wetlands data sets.

tidal wetlands along Rehoboth Bay (i.e., the bay
between polygons #6 and #7), and the tidal
wetlands along the upper Pocomoke River (i.e.,
the river that runs through polygon #C). In other
cases, wetlands extend farther into the coastal
lagoons than the polygons drawn by the panel
indicated (e.g., the wetlands along polygons #9
and #10). In some cases, the original polygons
omitted wetland areas, particularly in the upper
reaches of estuaries; so we had no information
on geomorphic setting or wetland accretion
potential for wetlands in those areas (see Figures
2.2.2 and 2.2.3).

In general, the panel's polygon boundaries
needed correction for several reasons:


-------
[ 180 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

(a)	Maps using a coarse 1:250,000 scale
routinely show "scale mismatch" when overlaid
with data created at a finer resolution.

(b)	The panel's polygon boundaries often
omitted large areas of wetlands, because the
USGS 1:250,000 maps do not show all wetlands.

(c)	In some cases, the polygon boundaries did
not track the landforms originally intended (e.g.,
the polygon around an inlet on the 1:250,000
scale map covering open water and missing the
wetlands). This occurred primarily because the
polygons that the panel had drawn were in many
cases drawn to be "indicative" rather than
precise; e.g., on the 1:250,000 map, the polygons
boundaries as drawn sometimes differed from
the actual boundary by approximately 1 cm.

(d)	The panel did not have a watershed map, and
in some cases the boundaries that they drew
unintentionally crossed watershed boundaries or
split a boundary. Many of these errors were
apparent with the wetlands overlay.

We brought these cases to the attention of Reed
and Bishara, who used our overlay to hand-edit
the polygon boundaries to more closely follow
the landforms and thus reflect the original intent
of the panel. Streeter digitized the changes into
the GIS. We then examined the maps a final time
and made a small number of additional
corrections. For example, in Figure 2.2.3, some
tidal wetlands were not part of any "polygon" in
the original panel output. The hand-edits
assigned all of those wetlands to the same
categories as the adjacent estuarine wetlands.
Along the Christina River, this left us with
estuarine wetlands upstream from freshwater
wetlands; so we readjusted polygon 5 to include
the upper portion of the tidal river. The net effect
of these changes was to ensure that all tidal
wetlands would be included in one of the shaded
polygons, and associated with the correct
landform and assigned region.

Wetlands Data Set

Our fifth step was to convert the raw data into a
wetlands data set. This step involved both data
processing and some cartography. Our data
processing step involved importing the
spreadsheets of attributes provided by Reed into
the GIS and joining to the polygon layer via the
index number that was common to both files.
Finally, we transferred the attributes in the panel
polygons to the EPA coastal wetlands data
generated by Titus and Wang via a simple
overlay function within the GIS. The final output
of this fifth step is a polygon wetland data set
with attributed defining geomorphic setting,
accretion potential, and subregion. Figure 2.2.4
is an example of the resulting map.

2.2.3 Creating Maps from the Data

The cartographic step involved devising a
reasonable way to portray the results of the panel
assessment. The three main issues we considered
were readability of small polygons, map colors,
and the map legend.

Readability of Small Polygons

The purpose of the map is to show where
wetlands are likely (or unlikely) to keep pace
with sea level rise. We decided early on to use
wetlands data rather than regional boundaries,
because the area and location of wetlands is an
important consideration. In places where the
wetlands are a narrow fringe or widely dispersed
islands, they are likely to be too small to be seen
on a statewide map drawn to scale—not to
mention a map of the entire mid-Atlantic. We
looked at test maps drawn to scale, and the
freshwater tidal wetlands along the Potomac and
Delaware rivers were particularly hard to see.

Therefore, in printing these maps, we set the line
widths to be scale-independent, to accentuate
small areas. The net effect is that every tidal
wetland polygon displays on our maps (unless
overlaid with another wetland polygon).


-------
[ SECTION 2.2 181 ]

CD Polygon Boundary: Saline
CD Polygon Boundary: Estuarine

~	Polygon Boundary: Fresh

~	250K Map Boundary
j [ Region Boundary

Open Water
Nontidal Wetlands
¦ Tidal Wetlands

Figure 2.2.3. Overlay of the Polygons from Wetland Accretion Panel and Wetlands Data Set: Wilmington
Quadrangle. The fresh/saline interface in the Delaware River is generally viewed as located near the
Delaware/Pennsylvania border. But freshwater wetlands extend farther downstream, according to the
panel. Polygon 5 represents the freshwater tidal marshes of the Delaware River watershed; the panel
viewed the rest of the wetlands in the Delaware River watershed as estuarine marsh. Although the mouth
of the Christina River into the Delaware River (southwest end of polygon 5) is in the freshwater marsh,
the upstream portions of the river are shown as being estuarine marsh. We treated this as unintentional
and altered the boundaries to show this entire river as freshwater marsh. Note also that that polygons
denoting wetland zonation do not include all of the tidal wetlands on the Delaware side of the Delaware
River and Bay.


-------
[ 182 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

Figure 2.2.4, Wetland data displayed based on attributes provided by the panel for geomorphic
setting. By this point, polygon boundaries had been revised to include most tidal wetlands. Compare
with Figure 2.2.2. A few revisions were still needed, such as along Indian River Bay, where some tidal
wetlands were still outside the polygon boundaries.

Expectation

Loss even at current rates:
Marginal today, loss at +2
rn/yr:

Keeping pace today, marginal
at 2 mm/yr, loss at 7 m/yr

Keep pace +2 mm/yr, loss at
+7 mm/yr

Keeping with +2 mm/yr,
marginal at +7 mm/yr
Keeping pace at +7 mm/yr

Color Reason for Color

Blue

Red

Brown

Yellow
Brown

Light

green

Bold
green

Because it is becoming water anyway
The standard color for a warning

A common color for environmental risk

A compromise between brown and green

Wetlands likely to survive, stay green

Wetlands very likely to survive (remain
green)


-------
[ SECTION 2.2 183 ]

Loss even with today's trend
Marginal today, loss with current +2mnVyr

Marginal today, marginal or
loss with current +2 mm/yr
Keep pace today, marginal with current
+2mnVyr, loss with current +7mnVyr

Keep pace with current +2mnVyr,
loss with current +7mm/yr
Keep pace with current +2 mrrVyr,
marginal/loss with current +7mm/yr
Keep pace with current +2mm'yr.
marginal with current +7mm/yr

Keep pace with current +7mnVyr
Unassigned Tidal Wetlands
Non-Tidal Wetlands
250K Map Boundary
Region Boundary

polygons need revision

Figure 2.2.5. Wetland Accretion potential for polygons in the Salisbury quad. At this point, the polygons
still needed revision around Indian River Bay.

Map Colors

The panel provided one of five accretion
possibilities (keep pace, marginal/keep pace,
marginal, marginal/loss, loss) for each of three
sea level rise scenarios. That specification
seemed to suggest a map for each sea level rise
scenario—which could lead us to an unwieldy
proliferation of maps. Putting all the information
on a single map seemed more desirable.
Fortunately, only 8 of the possible 15
combinations (5 accretion sensitivities by 3 sea
level scenarios) occurred, a manageable number
of colors.8 Ignoring the areas of uncertainty (e.g.,

8During an initial review, the total number of combinations
was reduced to 7, because the only polygon where
wetlands were marginal at +7 mm/yr had been erroneously
denoted as such. We've left that combination within the
legend bar because it is an obvious possibility that may
emerge during subsequent review or in other study areas.

marginal/loss) actually leaves us with only 6
sensitivities, for which we defined the following
colors.

We then defined intermediate colors for two
other, more intermediate specifications: marginal
today, marginal/loss at current +2 mm/yr, loss at
current + 7 mm/yr (orange) and keep pace with
current + 2 mm/yr and marginal/loss at current +
7 mm/yr (yellow). Figure 2.2.5 shows the
resulting map for wetland accretion. The zipped
file with which this data is distributed includes
jpg's for the quads and the regions, as well as an
overview map, following that color scheme. The
reader may notice that the polygon boundaries
and map colors in Figures 2.2.4 and 2.2.5 have
been assigned to most of the tidal wetlands that
had been omitted from the polygons in Figure
2.2.2. However, some of the wetlands around
Rehoboth Bay were still unassigned. Similarly,


-------
[ 184 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

assigning the map colors allowed us to notice a
number of errors. We queried the data to identify
all wetlands that had not been assigned a
geomorphic setting, and looked for other cases
where the geomorphic setting had a clear map
boundary error.9 We corrected the polygons
based on our understanding of the panel's intent
as documented by Reed et al. (see note 1).

Legend

One problematic aspect with maps following the
format of Figure 2.2.5 is that the keys take a lot
of words to repeat the same concepts. A single
color bar would be preferable; but the panel did
not characterize the
wetlands with a single
condition. We
experimented with a pair
of color bars, but people
found that approach too
confusing. The simplest
alternative to a lot of
words appears to be a
table, with a color bar.

(See Figure 2.2.6.)

Maps 2.2.1 and 2.2.2 provide the regional
summary maps that we created based on the
aforementioned considerations. Because the
panel wanted to include subregional maps in the
panel report, we also provided subregional maps.
We do not reproduce those maps here but they
are available with the data product EPA is
distributing.10 The consensus of panel members
was that the accretion map is not valid at large
scales. Therefore, the subregion-specific maps
should not be reproduced without both a warning
and an explanation about why the maps are being
reproduced at this scale.11

Will Wetlands Be Converted to Open Water?

Rate of
Sea Level Rise

















Current rate

Yes

?

?

No

No

No

No

No

Current + 2 mm/yr

Yes

Yes

Yes?

?

No

No

No

No

Current + 7 mm/yr

Yes

Yes

Yes

Yes

Yes

Yes?

?

No

? = Wetlands would be marginal Yes?= Wetland would be marginal or lost

Figure 2.2.6. Legend for wetland accretion map.

9For example, the polygon boundaries did not match—or
the geomorphic setting was different—at a quadrangle
boundary.

11 "Upon release of this report, EPA will make the data set
described in this paper available to all researchers.

11 Given the 1 cm errors in the hand renderings. National
Map Accuracy standards would suggest a 1:5,000,000
scale.


-------
[ SECTION 2.2 185 ]

Geomorphic Setting
( Back barrier lagoon, other
Back barrier lagoon, flood tidal delta
Back barrier lagoon, lagoonal fill
Estuarine marsh
Estuarine, fringe
Estuarine, meander
| Saline fringe
Tidal fresh forest
Tidal fresh marsh

Atlantic
Ocean

Long Island

Map 2.2.1. Geomorphic Setting of Tidal Wetlands: Montauk Point to Virginia Beach


-------
[ 186 MAPS THAT DEPICT SITE-SPECIFIC SCENARIOS FOR WETLAND ACCRETION ]

Atlantic
Ocean

Will Wetlands Be Converted to Open Water?

Rate of
Sea Level Rise







I







¦

Current rate

Yes

?

?

No

No

No

No

No

Current + 2 mm/yr

Yes

Yes

Yes?

?

No

No

No

No

Current + 7 mm/yr

Yes

Yes

Yes

Yes

Yes

Yes?

?

No

? = Wetlands would be marginal Yes? = Wetland would be marginal or lost

Map 2.2.2 Potential for Tidal Wetland Accretion in the Mid-Atlantic: Montauk Point to Virginia Beach.


-------