Tybee Island combines human ingenuity with natural resilience in combatting sea level rise

By Sarah Buckleitner

We don’t usually associate sunshine and blue skies with the sort of flooding that can shut down roads and creep into homes. But as sea level rise creeps further inland, coastal communities face a growing number of “sunny day” floods.

While many coastal communities hunker behind seawalls and elaborate levee systems to protect them from the elements, Tybee Island is working with UGA’s Institute for Resilient Infrastructure Systems and Thomas & Hutton, the city’s engineering firm, to create a plan that combines natural resilience with human ingenuity through the use of green infrastructure.

The proposed plan is to attack the problem by building features on the island and surrounding shoreline and marsh that will slow the flow of water toward the island and improve the community’s ability to handle getting its feet wet.

“We have multiple features, and they’re broken into two groups:  shoreline or marsh features and features on the island,” explained Dr. Félix Santiago-Collazo, assistant professor in the College of Engineering. Santiago-Collazo oversees coordinating activities from UGA’s engineering side of the project.

“The biggest feature is what we’ve been calling a ‘marsh barrier,’ because it’s meant to help expand the marsh so that it can better protect the bayside of the island. The marsh grass itself protects the island from surges because it slows the water’s movement. And if you take energy out of the water from that storm surge, then it won’t penetrate as far inland.”

On the island, the design team has focused on a culvert enhancement project, which is proposed to be installed on Sixth Street. This design centers on the installation of a “box culvert”— two large rectangular openings that are roughly 8 by 10 feet wide—a vast improvement over the restricting, two 30-inch diameter pipes that currently carries water.

“This was one of our main priorities to tackle because it’s a quick fix and you get a lot from it. For example, we found that if we instead install a box culvert—then the flow improves greatly, which alleviates flooding. Another side benefit of the box culvert is that it provides a bigger space for wildlife to migrate, which was also a component of all this,” said Santiago-Collazo. ‘

If implemented, these two projects would work in conjunction to help keep the town dry. The marsh barrier will provide a slope where the marsh can migrate to higher ground in response to rising seas, while the culvert will help establish more natural flows in the marsh. Simultaneously, the researchers also recommend raising the height of Venetian Drive, so that it acts as a subtle barrier between the houses beyond and the ocean.

“If we install the marsh barrier, then we’ll have a place where the marsh can migrate. So you have that for the marsh, and then you’ll have a higher barrier, which will be street, which will serve to help stop flooding. And then you have your houses on the back end. The project will have the double purpose of providing room for the marsh to migrate while protecting the island, not to mention increased opportunities for recreation and tourism in the greenspaces that will be created, which is really cool,” said Santiago-Collazo.

Healthy marshes provide a number of benefits to communities, including storm surge and flood protection, purification of toxins, and carbon sequestration, as well as places to recreate and spot the wildlife that call them home.

Dr. Clark Alexander, IRIS affiliate and Director of the UGA Skidaway Institute of Oceanography, has focused specifically on the marsh—assessing its health, and identifying where seawalls and other hard armoring may make it impossible for it to migrate.

“What I’m focusing on in this project is the marsh and its health, including its accretion—or growth rate in relation to sea level rise. This is important to know in understanding whether the marsh is going to be drowned by rising water levels, or whether it is going to be able to migrate onto the upland–and if so, where,” Alexander explained.

The researchers are tackling this problem in a two-pronged attack: measuring how quickly the marsh is growing, and identifying where the problem areas may lie in the future.

“We’ve been mapping where the armoring is within the study area so that we’re aware of where the marsh can migrate–which is essentially where the armoring isn’t. In addition to that, we’ve been looking at marsh accumulation rates, trying to understand whether the marsh is keeping up with sea level rise,” Alexander said.

“As in other Georgia marshes, we find that accretion rates are on the order of 1-2 millimeters a year based on measurements at the Fort Pulaski tide gauge, which is not far from Tybee Island. This is far below what the marshes need to survive in the face of a sea level rise rate of 3.4 millimeters a year.  Given this difference, marshes here will need help keeping up.”

Ensuring that the marsh is in good health and has a place to migrate is essential to protecting the back side of the island.

“This research is important because the back sides of islands are much harder to protect. On the front sides of islands, we know how to build dunes and beaches for protection. But the back is just basically a low gradient slide from upland to marsh to intertidal zone to tidal creek. Protecting it is not as straightforward, and it comes with the limitations of people having built right up against the marsh edge, which is a problem,” said Alexander.

To determine just how these components can all work together in the final design, IRIS researchers developed models that simulate the flow of water and test different scenarios that could cause flooding against the green infrastructure features they’ve designed. This includes an entire system intended to improve stormwater management and create an interconnected network of greenspace throughout the island, which will provide recreational opportunities as well as habitat benefits for wildlife, including birds and pollinators.

“My job was to make a combined model that showed how the whole system worked together–including the stormwater infrastructure, improvements that Thomas & Hutton proposed, and the green stormwater infrastructure improvements that our team designed,” explained IRIS graduate student, Matt Chambers, who worked in conjunction with graduate students Daniel Buhr and Haley Selsor from the College of Engineering, as well as landscape architects Alfie Vick, Alison Smith, and Jon Calabria from the College of Environment and Design, Jill Gambill, Coastal Resilience Specialist with the University of Georgia Marine Extension and Georgia Sea Grant, and Craig Landry with the College of Agricultural and Environmental Sciences.

Chambers and his colleagues combined all of the team’s plans by pulling those elements into a numerical model, and then testing them to see whether they could recreate the flooding that citizens had actually observed on the ground.

“To help ground truth this process, they showed us photo evidence of where flooding was happening and we also measured groundwater levels as they changed with tides and rain. And we tried to make it so that the model actually had flooding, or matching groundwater levels, in those places,” Chambers elaborated. “That was quite a difficult process–it took me a while to develop that model, but now we have this testbed where we can try all kinds of different green stormwater infrastructure approaches and their combinations. So we can put in living shorelines, or we can try out rainwater harvesting or we can manipulate different ways of improving the infiltration into the groundwater. We can test sensitivity to rising and fluctuating groundwater levels.”

These models have made clear how green and conventional infrastructure systems can bolster each other and give the research team a clear idea of how much water the green infrastructure can store.

“We’re able to show that our green infrastructure design boosts the function of the gray infrastructure. We were able to see that with the old system, after heavy rainfall events you’d get flooding on the street. But when we put the green infrastructure into our model, that type of flooding was greatly reduced. Green is definitely boosting gray, while also supporting wildlife and habitat, which is great,” Santiago-Collazo said.

The next step is to bring their findings to the community for review so that they can work together to settle on a final design. After that, they’ll pursue permits and funding for construction.

Over the last year, the team has gathered public input through their resident advisory board, geosurvey, where they crowdsourced information on flooding hotspots, and a survey intended to gather information on risk perceptions, experiences with flooding, priorities related to the function of infrastructure (habitat enhancement, cost, flood reduction) and the willingness to pay for these characteristics.

Alan Robertson, project manager for Tybee Island, elaborated on the importance of these outreach efforts. “While the work IRIS has done clearly identifies the importance of including these types of nature-based solutions in Tybee’s resiliency efforts, to date they are difficult to quantify and are new concepts to most people. Public outreach and education are critical in building the political will necessary to make the difficult decisions. Our IRIS partners have developed the nature-based solutions and modeled their effectiveness to help residents visualize the recommendations. Pictures, and in this case animations, do go a very long way in telling a compelling story.”

The team has plans to do more targeted outreach moving forward, where they will engage residents who live at the sites of proposed natural infrastructure projects.

“We’re excited to iterate with the people who stand to benefit from these changes to figure out the best options,” Santiago-Collazo said.

While Tybee Island might be the first to put together such an innovative plan for building resilience in its community, the researchers hope that it won’t be the last.

“Tybee Island is laying the blueprint for how island communities can tackle the challenges of climate change head on, in innovative and unique ways,” said Chambers. “They’re seeing the real impacts of sea level rise, and have decided to do something about it, which makes them a kind of early adopter of these resilience measures. I wouldn’t be surprised if coastal communities around the world end up looking to it as an example.”