Shore Protection and Retreat: Shoreline Armoring

6.1.1.1 Shoreline Armoring

Shoreline armoring involves the use of structures to keep the shoreline in a fixed position or to prevent flooding when water levels are higher than the land. Although that term is often synonymous with “shoreline hardening”, some structures are comprised of relatively soft material, such as earth and sand.

Keeping the shoreline in a fixed position

Seawalls are impermeable barriers designed to withstand the strongest storm waves and to prevent overtopping during a storm. During calm periods, their seaward side may either be landward of a beach or in the water. Seawalls are often used along important transportation routes such as highways or railroads (Figure 6.1a).

Bulkheads are vertical walls designed to prevent the land from slumping toward the water (Figure 6.1b). They must resist waves and currents to accomplish their design intent, but unlike seawalls, they are not designed to withstand severe storms. They are usually found along estuarine shores where waves have less energy, particularly in marinas and other places where boats are docked, and residential areas where homeowners prefer a tidy shoreline. Bulkheads hold soils in place, but they do not normally extend high enough to keep out foreseeable floods. Like seawalls, their seaward sides may be inland of a beach (or marsh) or in the water.

Retaining structures include several types of structures that serve as a compromise between a seawall and a bulkhead. They are often placed at the rear of beaches and are unseen. Sometimes they are sheet piles driven downward into the sand; sometimes they are long, cylindrical, sand-filled “geo-tubes” (Figure 6.2). Retaining structures are often concealed as the buried core of an artificial sand dune. Like seawalls, they are intended to be a final line of defense against waves after a beach erodes during a storm; but they can not survive wave attack for long.

Revetments are walls whose sea side follows a slope. Like the beach they replace, their slope makes them more effective at dissipating the energy of storm waves than bulkheads and seawalls. As a result, revetments are less likely than bulkheads and seawalls to cause the beach immediately seaward to erode (USACE, 1995), which makes them less likely to fail during a storm (Basco, 2003; USACE, 1995). Some revetments are smooth walls (Figure 6.3b), while others have a very rough appearance (Figure 6.3a).

Figure 6.1 Seawalls and bulkheads (a) Galveston Seawall in Texas (May 2003) and (b) bulkheads with intervening beach along Magothy River in Anne Arundel County, Maryland (August 2005) [Photo source: ©James G. Titus, used with permission].

Figure 6.2 Geotube (a) before and
(b) after being buried by beach sand at Bolivar Peninsula, Texas (May 2003) [Photo source: ©James G. Titus,
used with permission].

Figure 6.3 Two types of stone revetments (a) near Surfside, Texas (May 2003) and
(b) at Jamestown, Virginia (September 2004) [Photo source: ©James G. Titus, used with permission].

Protecting Against Flooding or Permanent Inundation

Dikes are high, impermeable earthen walls designed to keep the area behind them dry. They can be set back from the shoreline if the area to be protected is a distance inland and usually require an interior drainage system. Land below mean low water requires a pumping system to remove rainwater and any water that seeps through the ground below the dike. Land whose elevation is between low and high tide can be drained at low tide, except during storms (Figure 6.4a).

Dunes are accumulations of windblown sand and other materials which function as a temporary barrier against wave runup and overwash (Figure 6.4b, see also Section 6.1.1.2).

Tide gates are barriers across small creeks or drainage ditches. By opening during low tides and closing during high tides, they enable a low-lying area above mean low water to drain without the use of pumps (Figure 6.5).

Storm surge barriers are similar to tide gates, except that they close only during storms rather than during high tides, and they are usually much larger, closing off an entire river or inlet. The barrier in Providence, Rhode Island (Figure 6.6) has gates that are lowered during a storm; the Thames River Barrier in London, by contrast, has a submerged barrier, which allows tall ships to pass. As sea level rises and storm surges become higher (see Chapter 9), these barriers must be closed more frequently. The gates in Providence, Rhode Island (Figure 6.6), for example, are currently closed an average of 19 days per year (NOAA Coastal Services Center, 2008).

Figure 6.4 (a) A dike in Miami-Dade County, Florida (June 2005), and (b) a newly-created dune in Surf City, New Jersey (June 2007) [Photo source: ©James G. Titus, used with permission].

Figure 6.5 The tide gate at the mouth of Army Creek on the Delaware side of the Delaware River. The tide gate drains flood and rain water out of the creek to prevent flooding. The five circular mechanisms on the gate open and close to control water flow [Photo source: courtesy NOAA Photo Library].

Figure 6.6 Storm surge barriers. (a) Fox Point Hurricane
Barrier and Providence River Bridge, Providence, Rhode Island (August 2008) and (b) Moses Lake Floodgate, Texas City, Texas (March 2006)
[Photo sources: (a) Marcbela; (b) ©James G. Titus, used with permission].

Roadmap: 6.1 Techniques for Shore Protection and Retreat

6.1.1.2 Elevating Land Surfaces Go Forward

For previous reports focused on the implications of rising sea level, go to More Sea Level Rise Reports.

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Shore Protection and Retreat: Shoreline Armoring

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6.1.1.1 Shoreline Armoring

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