Working with nature to create a stronger, more resilient dune system

Thu, 06/24/2021 - 03:17pm

During bouts of severe weather, including tropical storms and hurricanes, coastal dune systems provide crucial protection for coastal communities, but these dunes can be torn apart by high winds and strong surf. However, the very surf that at times causes damage is also delivering natural materials that may prove to be the key to a more resilient dune system.

Researchers at the U.S. Army Engineer Research and Development Center (ERDC), along with partners at the U.S. Army Corps of Engineers (USACE) Mobile District and The University of Southern Mississippi (USM), are developing methods to use this natural material – known as wrack – to build coastal dunes crucial to protecting the coastline during hazardous weather events like hurricanes.

As part of the USACE Regional Sediment Management (RSM) Program, researchers are conducting a field study in which wrack that naturally washes up on the beach is collected and placed on existing dunes to incrementally increase dune stability.

“This research is important because we are trying to assess ways to improve the resiliency of dunes by mimicking the natural growth cycle of the dunes,” Eve Eisemann, a research physical scientist with ERDC’s Coastal and Hydraulics Laboratory, said. “By utilizing this wrack material that is naturally washing up on the beach, we are not only providing a wrack management solution for coastal communities to keep their beaches clean, but we are providing a way to help the dunes grow.”

Eisemann, who is leading the study on the Mississippi Gulf Coast, said the method of “wrack-cycling” could become an effective strategy for improving the efficiency of beach re-nourishment and man-made dunes like those along Mississippi’s coastline.

Unlike previous methods to construct man-made dunes, where dunes are built to an elevation and then planted with vegetation, researchers are evaluating how a man-made dune responds when biological material is incorporated into the dune incrementally, mimicking the natural growth cycle of dunes. 

“By improving dune resilience and dune growth utilizing these natural materials, we are helping reduce the need for future dune restoration construction projects and can maybe even reduce the need for beach nourishment in the future,” Eisemann said.

Previous ERDC efforts demonstrated vegetation plays a significant role in dune stability, creating a more robust dune that can better withstand the effects of wave action and storm surge.

In the study currently being conducted along three 600-foot segments of beach, Eisemann collects wrack by hand and then places it strategically on the test dunes. Over time, the wrack collects sand, aiding the dune’s growth. And, as the wrack naturally breaks down, it provides nutrients to thriving beach vegetation, such as sea oats, further strengthening the dune through its network of roots.

While time consuming, the method has generated positive results, even in the face of significant weather events such as Tropical Storm Cristobal and Hurricane Zeta, which lashed the test site during the 2020 hurricane season.

“We have collected five surveys now and have been able to take a look at how the elevations have changed across the area from last year, and have captured the impact from Tropical Storm Cristobal, Hurricane Zeta and the passing of a number of other hurricanes that did not come quite as close,” Eisemann said. “We were able to observe that the dune lost a lot of volume, of course, during some of those hurricane impacts, but we have also noted visible benefit of the wrack placement along the segment of treated dunes. You can see a little bit of accretion happening, a filling in of some of the gaps that were there before.

“Hopefully, by the end of this, we will be able to see a very distinct performance difference between those dunes that have had wrack placed and those that have not.” 

Helping provide high-quality surveys of the area utilizing the most precise equipment available is the team at USM’s Gulf Coast Geospatial Center, which leverages state-of-the-art survey-grade equipment and a team of experienced staff, faculty and students.

“The precision instrumentation we operate allows us to model vertical changes on very small scales,” William Funderburk, deputy and interim director of USM’s Gulf Coast Geospatial Center, said. “Wrack-cycling adds to that small vertical scale difference whether it is deposited or washed away from whatever significant type of episodic event the coast experiences.”

Instrumentation used for the precise measurements includes a variety of land-based – or terrestrial - and aerial, drone-based LiDAR (light detection and ranging) devices used over a multi-day survey.

That experience of both the geospatial science and understanding of the local environment has made USM’s Gulf Coast Geospatial Center a vital partner in the project.

“The network-integrated survey-style workflows we use allows us to construct the most precise and accurate spatial models that can be built at this time,” Funderburk said. “Our knowledge is local. The research we have been conducting across the coast and across the barrier island chain over the past 20 years provides us an in-depth knowledge of elevation and habitat-type relationships – precise elevation on the scale of centimeters.”

With an increasing percentage of the U.S. population living in coastal areas, protecting these communities is of utmost importance, and strong, resilient dune systems are key barriers against threatening weather systems.

According to the National Oceanic and Atmospheric Administration’s National Coastal Population Report, 40 percent of the U.S. population lives within coastal areas that account for less than 10 percent of total land within the United States. And, even in the face of increasing threats from severe storms and rising sea levels, the report said, “this trend is persisting despite growing concerns about coastal hazards,” adding even more importance to a resilient dune system.

“A strong dune system serves as a natural barrier to storm surge and storm waves during tropical storms in the Gulf Coast region. Being a natural barrier, they can protect really valuable inland infrastructure as well,” Eisemann said. “Additionally, a well-developed dune system can provide important habitat to a number of coastal species like sea turtles, who lay their eggs on the dune toe, as well as various bird and vegetation species. This can help communities save money on clean up and add value to the dunes as a natural resource.”

Photos by Khary Ratliff, U.S. Army Engineer Research and Development Center