Learn How We're Mapping Sea-Level Rise in Southwest Washington

By Jamie Robertson, Conservation Geographer
and Molly Bogeberg, Marine Conservation Coordinator

When scientists and agencies produce sea level rise projections due to a changing climate, maps can show where water will flood inland as seas rise. These maps create better understanding of how higher waters will impact coastlines, communities and the environment. Sea level rise maps can be very helpful for coastal planners as they consider the safest places to develop and where to conserve habitats to utilize their protective services.

How are sea level rise projections determined?

Earlier this year, The National Oceanographic and Atmospheric Administration (NOAA) published localized sea level rise projections for points around Washington state as well as other locations around the United States, the Caribbean and Pacific Islands. To arrive at these projections, NOAA conducted an in-depth review of recently published literature on global mean sea level rise projections to the year 2100 for six greenhouse-gas emission scenarios from low to extreme. Different greenhouse-gas emission scenarios are thought to have different effects on the rate of ice melt. Scientists are also working to incorporate the upward or downward movement of the Earth (called vertical land movement) and other coastal conditions to give communities a clearer picture of how higher sea levels will affect local shorelines.

 Salt marsh on Willapa Bay. Photo by Keith Lazelle.

What is sea level rise mapping?

Sea level rise mapping simulates where land areas that currently do not get covered by tides may become underwater as sea levels rise.  For the tidal regions of Grays Harbor County and Pacific County in southwest Washington, The Nature Conservancy mapped areas that were underwater when tides reached the "Mean Higher High Water" (MHHW) line in 2010 and which may be underwater by 2030, 2050, and 2100.  The MHHW line can sometimes be seen on seashores where those tides deposit a line of debris at the top of the shore, which does not get washed away during typical tides. The animated map below of Raymond, Wash., shows increasing sea levels using a high greenhouse-gas emission scenario from 2010 to 2100. By 2100, much of the city becomes inundated.

How is sea level rise mapped?

To map sea level rise, we started with an elevation map created by the Federal Emergency Management Agency (FEMA) through a method called Light Detection and Ranging or LiDAR. This detailed map is produced from data collected with lasers emitted from a plane flying back and forth over the landscape.

Elk River in Grays Harbor County. Photo by Joel Rogers.

Next we used a “bathtub” method to raise the waters over the detailed elevation data. As the name implies, this method assumes that the rising water fills the bathtub and spreads outward over previously dry land until it reaches the predicted level of rise. Where high enough, natural hills and cliffs and human-made features like sea walls, dikes, levees and tidal gates act as the bathtub walls. In our maps, we show both the area covered by sea-level rise and the low-lying areas at risk of flooding if rise continues above the "bathtub" walls.

How is The Nature Conservancy helping?

The Nature Conservancy is working with the state Department of Ecology, Sea Grant and others through the Washington Coastal Resilience Project (WCRP)  to update Washington’s coastal planning guidance on sea-level rise. Maps, like the one above, can allow communities to visualize flooding potential and hopefully educate development and restoration decisions to maintain resilience along a changing coastline.