Living shorelines are being used more often in coastal Alabama to fight erosion and protect habitat, and researchers at the University of South Alabama say some projects are unintentionally harming the shorelines they are meant to protect. Their research is showing that living shorelines must be custom-designed for each location to be successful.
To create a site-specific living shoreline, projects should use principles from each of the fields of ecology, geology, oceanography and engineering to develop effective projects, according to the researchers.
“What restoration practitioners should realize is that just knocking some of the wave energy down is not always good for the shoreline,” said Scott Douglass, a professor in USA’s Department of Civil Engineering. “It can be good at some locations and disastrous at other locations.”
Douglass is working with Bret Webb, also of USA’s Department of Civil Engineering, on the Mississippi-Alabama Sea Grant-funded research project.
“Living shorelines” refers to management practices that use strategic placement of plants, stone, sand fill and other structural organic materials, such as oyster reefs, to provide shoreline stabilization and protection of marsh vegetation and habitat viability. Living shorelines may be used in appropriate areas as alternatives to bulkheads, rip-rap and other hard structures.
The first-year research results in this two-year project have not shown that the living shorelines breakwater reef project just south of Alabama Port on Mobile Bay’s western shore has slowed the rate of erosion.
Another living shoreline project with unexpected effects is one of the most visible projects in Alabama. Along Alabama Highway 193 (the Dauphin Island Parkway, south of Bayfront Park and north of Jemison’s Bait Shop), the project has had little effect on shoreline recession, according to the research findings. Preliminary results indicate the project may have altered the natural shoreline habitat. What used to be mostly sandy beach backed by marshes, typical of most of Mobile Bay’s natural shoreline, has become an eroding, dying marsh more typical of the north shore of the Mississippi Sound.
The preliminary USA wave basin laboratory findings indicate that some reef structures used in living shorelines project cannot reduce the wave energy levels enough for salt marsh grasses like Spartina alterniflora to thrive along the shoreline. To date, there is little to no natural growth or colonization of any wetland species into the flats in the lee of the breakwater reefs.
Testing in the wave lab
Graduate students at USA are testing living shoreline reef/breakwater models in a laboratory wave basin. Preliminary results have quantitatively measured the wave attenuation characteristics of a variety of reef structure types commonly used in Alabama: oyster shells in bags, oyster shells in vertical steel triangular-shaped cages and hollow, concrete pyramids. It is clear that the wave transmission across these structures is very sensitive to tide levels and the height of the structure. Too high, and it ceases to work as intertidal habitat. Too low, and it will not attenuate waves enough to allow for healthy marsh vegetation growth in its lee. And whether it is high or low, if placed in the wrong location, it can cause damage to the shorelines.
One of the exciting new findings from the preliminary laboratory results is a relationship between the structure geometry, wave attenuation characteristic and wave steepness (a parameter not previously considered in most design guidance), according to team leaders. One implication of this discovery is that it may be possible to “tune” a structure’s geometrical design to the site-specific wave climate and more effectively remove the storm wave energy while allowing gentler waves to pass through.
Living shorelines toolkit
As part of the project, the USA team is developing a pre-planning toolkit to help coastal homeowners, non-profit groups, municipalities and government agencies conceptualize and design living shoreline projects that are best suited for their locations.
Several projects in coastal Alabama have achieved their design goals. They have stabilized the shoreline while creating and restoring various intertidal habitats including beaches, wetlands and oyster reefs. This research project will incorporate proven methods into the toolkit.
The research team also is working with the American Society of Civil Engineers to establish a nationwide living shorelines database.