Riprap Revetments and Living Shorelines Explained

Not every eroding shoreline needs a vertical wall. Two other approaches are common: riprap revetments, which armor a sloped bank with heavy stone, and living shorelines, which use vegetation, sand and natural or nature-based structures to stabilize sheltered shores. Each works differently from a bulkhead, and each has conditions where it makes sense and conditions where it does not.

This page compares the three approaches. For the background on why shorelines erode, see waterfront erosion explained.

How vertical walls interact with waves

USACE's coastal design manual notes that revetments, bulkheads and seawalls mainly protect only the upland behind them and can all reflect wave energy that erodes the bottom or beach in front. The problem is greatest for vertical structures, which are nearly perfect wave reflectors, and progressively less for curved, stepped and rough sloped structures.

NOAA's 2015 living shoreline guidance makes the same point about bulkheads specifically: the vertical face reflects wave energy, causing erosion along the toe. It adds that shoreline hardening can contribute to loss of shallow intertidal habitat, fringing marsh and submerged aquatic vegetation. For how toe scour threatens walls themselves, see signs of bulkhead failure.

Riprap revetments

How a revetment is built

USACE describes a revetment as an armor layer, one or more filter layers and toe protection. The armor can be randomly placed stone or interlocking concrete units. The filter keeps underlying soil from washing out while allowing drainage. The toe resists undermining at the bottom of the slope.

The Virginia Institute of Marine Science explains that rock revetments have sloped, rough faces that decrease wave reflection and therefore bottom scour, and that the upland bank can be graded to improve protection. VIMS describes revetments as an effective last line of defense in high-energy settings.

Limits of riprap

• Space. A sloped revetment occupies more horizontal footprint than a vertical wall.
• Wave size. USACE's manual states that graded riprap is not recommended where wave heights exceed 5 feet; larger waves call for heavier, more uniform armor stone or units.
• Habitat and runoff. VIMS notes that in most cases no vegetation grows riverward of a revetment and that there is often no buffer to filter runoff.
• Overtopping. Revetments can be overtopped in storms.
• Access. USACE notes that randomly placed stone can hinder access to the water.

Stone is also used at the base of existing bulkheads as toe protection. VIMS observes that many deteriorating Chesapeake Bay bulkheads have been reinforced or replaced with revetments.

Living shorelines

What counts as a living shoreline

NOAA defines a living shoreline as a range of stabilization techniques along estuarine coasts, bays, sheltered coastlines and tributaries, with a footprint made up mostly of native material. It uses vegetation or other natural elements, alone or with harder structures such as oyster reefs or rock sills for stability, and keeps the natural land-water interface continuous.

NOAA places shoreline treatments on a green-to-gray continuum, from vegetation only, through hybrid designs with sills or breakwaters, to fully built structures. Vertical bulkheads of wood, steel, vinyl or concrete, and stone revetments or breakwaters without living components, are on the gray end and are not considered living shorelines.

What the evidence shows

NOAA's guidance summarizes several findings from the research literature:

• Salt marshes can dissipate wave energy; NOAA cites a study finding Spartina marshes reduced wave energy by 50 percent within the first 2.5 meters, while noting most studies examined low- or medium-energy waves.
• Marsh and sill designs can trap sediment and become more stable as plants and reefs grow.
• On sheltered North Carolina coasts, marshes with and without sills outperformed bulkheads during Hurricane Irene in 2011, while 75 percent of regional bulkheads surveyed were damaged, according to a study NOAA cites.

NOAA also cautions that no technique, hard or living, is guaranteed to prevent loss of infrastructure in the most severe storms, and that living shorelines need maintenance such as replanting and debris removal.

Where living shorelines suit

NOAA's guidance focuses on sheltered coasts not exposed to open-ocean waves. Living shorelines need room for a gentle slope or marsh fringe, enough sunlight for vegetation and wave energy that plants and sills can withstand. They are generally not a substitute where a vertical edge and deep water at the shore are required, which USACE identifies as a reason bulkheads are used.

Side-by-side comparison

Regulatory trends

Policy has been moving toward softer approaches. A 2007 National Research Council report on sheltered coasts recommended that the regulatory preference for permitting bulkheads be changed to favor more ecologically beneficial solutions, noting that armoring was often simpler and faster to permit. A 2015 analysis by Gittman and colleagues, summarized by the Ecological Society of America, estimated that about 14 percent of U.S. tidal shoreline had been hardened.

In Virginia, the Code of Virginia now directs the Marine Resources Commission to permit only living shoreline approaches unless the best available science shows they are not suitable, according to VIMS. Other states have their own rules. Check requirements through the bulkhead permit guide and the Permit Navigator.

Choosing among the three

Final suitability depends on local conditions, exposure and professional design. Many projects combine approaches, such as a toe revetment in front of an existing wall or a marsh sill on part of a property. For wall materials, see the Material Comparison Explorer, and for broader strategies, see shoreline erosion control.