Can living shorelines survive the rising seas?

Take a sneak preview into our new issue, which publishes this Friday and turns the Spotlight on conservation in marine habitats. The feature includes a Practitioner’s Perspective on designing climate‐resilient living shorelines, which Molly Mitchell and Donna Marie Bilkovic discuss here. Look out for an additional post bringing together all the papers in the Spotlight soon.

Living shorelines are a form of shoreline protection that mimics natural marshes and beaches.  They stabilize the shoreline and prevent erosion, but also provide habitat for birds, fish, crabs, snails and other animals.  Similar to their natural counterparts, they can help reduce wave energy from big storms.  This can protect coastal homes and prevent some flood damages. Due to their many benefits, living shorelines are a key component of coastal resilience plans in many areas. Unfortunately, similar to many coastal environments, living shorelines are at risk of drowning due to sea level rise.

Natural marshes have two paths to adapt to rising waters.  They can follow the tides inland, moving naturally up the shoreline.  This process works as long as there aren’t any barriers (e.g. walls) or steep banks behind them. The other process involves building up elevation by accumulating soil/sand on the shoreline surface.  Although these paths are both available to living shorelines, they are often designed in a way that limits the shorelines’ ability to take advantage of the natural processes necessary to persist under sea level rise.

Over the years of researching living shorelines, we have collected some design best practices that should help keep living shorelines functional into the future.

The most important consideration is siting of a living shoreline. Consider the two examples below:

In the first picture, the land behind the living shoreline is low in elevation and there are no barriers preventing the marsh from migrating across the landscape as the tide rises.  In the second photo, there is a steep bank behind the living shoreline.  This means that, as the tide rises and the front edge of the living shoreline drowns, the back edge will not have much space to move into. The marsh will get narrower and narrower, and eventually disappear.

It’s important to keep in mind that this doesn’t mean that the living shoreline in the second photo shouldn’t have been built.  Until it drowns, it is still providing erosion protection and habitat for a variety of animals.  It just means that this living shoreline has a more limited lifespan, and won’t improve long-term coastal resilience.

Living shorelines 3In areas where there are barriers to migration, the next important consideration is design features that will help the living shoreline marsh accumulate sediment and build its surface elevation.  We recommend dense marsh grass coverage as one of the best ways to help the marsh raise its elevation.  The roots produced by the dense grass take up space in the sediment and help raise the surface.  Dense vegetation also encourages sediment to fall out of incoming tidal waters.

Placing sills or oyster reefs on the front edge of the living shoreline marsh (as shown in the first two photographs) can help retain sediments. These structures help reduce wave energy, allowing more sediment to fall out of the tidal waters.  They also prevent erosion of the marsh edge and are necessary in higher energy areas.

In some marshes, animals may also help bind sediments, which will help with overall marsh stabilization.  In our Mid-Atlantic marshes, ribbed mussels are a desirable species for sediment stabilization. However, we have found that living shorelines sometimes have low populations of these mussels.  Encouraging more mussels to populate the living shorelines would provide additional resilience and we are actively working on strategies to accomplish this.

Living shorelines 4This photo shows a living shoreline with an unusual number of mussels and a sill of the marsh edge. We would expect this living shoreline to do a better job of accumulating and holding sediment.  Characteristics that might make the living shoreline more resilient to sea level rise.

We believe that living shorelines are not just a temporary solution to stabilizing shorelines.  With the proper design and some coordination between coastal engineers and ecologists, they can be self-sustaining.  Thoughtful design of our living shorelines will promote resilient coastlines and reduce sea level rise impacts.

Read the full Practitioner’s Perspective, Embracing dynamic design for climate‐resilient living shorelines for free in Journal of Applied Ecology.

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