On a recent boat trip in the Intracoastal Waterway from Palm Coast to Fort Matanzas, it was interesting to notice the variety of shorelines present in the region. They ranged from fully-vegetated salt marsh to eroding banks to various styles of seawalls.
In looking at a series of aerial maps of the region, we can see that the width of the Intracoastal in northern Flagler County doubled from 1942 to 2008. This increase in width is probably mostly due to erosion caused by boat wakes. It is not surprising, therefore, that property owners have installed vertical concrete, wooden or other structures to prevent their property from eroding further.
While these “hardscapes” can serve to protect one piece of property, they can result in increased erosion on adjacent or opposite properties, because wave energy becomes reflected off the hard walls. Seawalls also can become undermined from below or behind, causing them to fail and collapse.
An alternate strategy for preventing coastal erosion is being promoted in Florida by the U.S. Fish and Wildlife Service, and many local governments and state agencies are supporting that effort. “Living Shorelines” have proven effective in many locations, including the Chesapeake Bay and the Indian River Lagoon.
Living Shorelines use natural shoreline ecosystems to absorb wave energy without causing erosion. Living Shorelines extend from the upper bank of the property to below the water level. They typically include a variety of plants, including salt marsh grasses and/or mangroves as well as structural elements such as oyster shell, or even riprap. The overall concept of Living Shorelines is to provide habitat which will grow and change as water levels change (unlike seawalls which are a fixed height). Having structural elements in the shallow water, and plants which are rooted below water level but grow above the water helps absorb wave energy.
Invasive plants such as Brazilian pepper (Schinus terebinthifolius) have proven to be poor stabilizers of shorelines, as seen in the Indian River Lagoon during the 2004 hurricane season. Native marsh grasses, on the other hand, are effective at resisting erosion and dissipating wave energy—especially when planted on shallow slopes. In these cases, they can reduce wave energy by 50% in the first 8 feet of vegetation, and by 100% over a distance of 100 feet.
Other native coastal plants such as sea oxeye daisy, saw palmetto and cabbage palms are fairly resistant to shoreline erosion. Sea grapes tend not to hold up to erosion very well. Mangroves can also provide a good buffer to wave energy—the wider the mangrove fringe, the better the buffering effect.
In many locations, property owners do not own enough land to create the desired shallow slope for a living shoreline. In these cases, riprap structures made from large coquina rock or limestone rock can be used. Larger diameter rocks (3-6’) are less likely to be shifted by wave action than smaller rocks. Coquina and limestone rocks will settle and will also erode; however they are also likely to become vegetated, which will increase their aesthetic appeal and may help stabilize the shoreline further.
If the rocks that are used are too small, they will tend to shift at the toe of the slope, causing the entire rock wall to collapse. Seawalls and articulated block structures tend to be vertical (or steeply sloped) and are prone to scouring. Waves can cause erosion of soil from behind articulated blocks, and from behind seawalls, causing the collapse of these structures.
Living Shorelines help to preserve the natural, historic and aesthetic values of coastal properties. From an engineering standpoint, they provide acceptable to very good performance in hurricanes (based on the Indian River Lagoon sites during the 2004 hurricane season, when category two and category three hurricanes passed directly over the region.) They are typically less expensive to install than rock shorelines or seawalls. They may, however, be more cumbersome to get permitted as they require approval and/or permits from a variety of agencies.
Many successful Living Shoreline projects have been completed in Florida. In the Pelican Island National Wildlife Refuge, oyster shell was used to create a submerged breakwater, allowing areas to be filled and planted with marsh and mangrove vegetation. These planted areas then naturally expanded, restoring 0.84 acres which had previously eroded from the island in the Indian River Lagoon.
At the Cargill Fertilizer plant in Tampa, an eroding shoreline was re-sloped and planted with marsh grasses in 1990; by 2003 it had become a healthy mangrove/salt marsh ecosystem.
At John Lloyd Park, across from Port Everglades in South Florida, limestone boulders were used to create a protected area which naturally vegetated with mangroves over a 20- year period. This mangrove habitat is directly across from the shipping docks.
The St. Johns River Water Management District is currently conducting salt marsh restoration projects within Gamble Rogers and North Peninsula state parks.
For additional information:
Living Shorelines, May 28, 2009 U.S. Fish and Wildlife workshop (http://www.fws.gov/northflorida/Coastal/2009_LSL_Wkshp/2009_LSL_Wkshp_Agenda.htm)