The Blackwater NWR Inundation Model. Rising Sea Level on a Low-lying Coast: Land Use Planning for Wetlands

The Blackwater National Wildlife Refuge (BNWR), on the Eastern Shore of
Chesapeake Bay (figure 1), occupies an area less than 1 meter above sea level.
The Refuge has been featured prominently in studies of the impact of sea level
rise on coastal wetlands. Most notably, the refuge has been sited by the
Intergovernmental Panel on Climate Change (IPCC) as a key example of "wetland
loss" attributable to rising sea level due to global temperature increase.
Comparative studies of aerial photos taken since 1938 show an expanding area of
open water in the central area of the refuge. The expanding area of open water
can be shown to parallel the record of sea level rise over the past 60 years.

The U.S. Fish and Wildlife Service (FWS) manages the refuge to support
migratory waterfowl and to preserve endangered upland species. High marsh
vegetation is critical to FWS waterfowl management strategies. A broad area
once occupied by high marsh has decreased with rising sea level. The FWS needs
a planning tool to help predict current and future areas of high marsh
available for waterfowl.

"Wetland loss" is a relative term. It is dependant on the boundaries chosen for
measurement. Wetland vegetation, zoned by elevation and salinity (figure 3),
respond to rising sea level. Wetlands migrate inland and upslope and may vary
in areas depending on the adjacent land slopes. Refuge managers need a
geospatial tool that allows them to predict future areas that will be converted
to high and intertidal marsh. Shifts in location and area of coverage must be
anticipated. Viability of a current marsh area is also important. When will sea
level rise make short-term management strategies to maintain an area
impractical?

The USGS has developed an inundation model for the BNWR centered on the refuge
and surrounding areas. Such models are simple in concept, but they require a
detailed topographic map upon which to superimpose future sea level positions.
The new system of LIDAR mapping of land and shallow water surfaces has solved
this problem. Our team has developed a detailed LIDAR map of the BNWR area at a
30 centimeter (ca. 1 ft) contour interval (figure 2). The new map allows us to
identify the present marsh vegetation zones and to predict the location and
area of future zones on a decade-by- decade basis over the next century at
increments of sea level rise on the order of 3 cm/decade (ca. 1 inch).

We have developed two scenarios for the model. The first is a steady-state
model that uses the historic rate of sea level rise of 3.1 mm/yr to predict
marsh areas. The second is a "global warming" scenario utilizing a conservative
IPCC model with an exponentially-increasing rate of sea level rise. Under
either scenario, the BNWR is progressively inundated with an expanding core of
open water. Although their positions change in the future, the areas of
intertidal marsh as well as those of the critical high marsh remain fairly
constant until the year 2050. Beyond that time, the low-lying land surface is
overtopped by rising sea level and the area is dominated by open water.

Our model suggests that wetland habitat in the Blackwater area might be
maintained and sustained through a combination of public and private
preservation efforts through easements in combination with judicious Federal
land acquisition into the predicted areas of suitable marsh formation - but for
only the next 50 years. Beyond that time much of this area will become open
water.

[Summary provided by the USGS.]