SHORE PROTECTION
C. Scott Hardaway, Jr.
Virginia Institute of Marine Science, College of William &
Mary, Gloucester Point, VA, USA
Synonyms
Shore erosion control
Definitions
Shore protection along estuarine coasts generally occurs
when shorelines are actively eroding. Shoreline erosion,
a natural process, is primarily a function of a rising sea
level and the impinging local wave climate.
Introduction
Many of the processes that govern erosion (and accretion)
on the open ocean coasts also apply to estuarine coasts, but
compared to the typically long linear nature of open
coasts, estuarine coasts are more sheltered and exhibit
a more irregular configuration. Estuarine coasts often display very distinct geomorphic compartments containing
a complex mix of resources that may vary from compartment to compartment. The relatively lower wave energy
conditions found along estuarine coasts create unique
environments that foster habitats and ecological communities, such as marshes and mudflats, not typically found
on open coasts (NRC, 2007).
Extent
Just about every estuarine shoreline can experience erosion, but typically when fetch exposures exceed about
1.6 km, land loss by wave action becomes more common
(Hardaway and Byrne, 1999). Documented land loss from
shoreline erosion is found in numerous estuaries around
the USA including Chesapeake Bay, Long Island Sound,
Delaware Bay, Puget Sound, and Mobile and Galveston
Bays. Land loss in Chesapeake Bay over the 100-year
period from 1850 to 1950 is estimated at over 190 km
2
(Singewald and Slaughter, 1949; U.S. Army Corps of
Engineers, 1973; Byrne and Anderson, 1978). The
response by land owners is to protect their property.
Types
Techniques used to address erosion along estuarine coasts
may be placed into broad categories, such as those proposed by Nordstrom (1992), Rogers and Skrabel (2001),
and Rogers (2005). Most guidelines and reports on shore
protection employ the same basic concepts to discuss
approaches such as structural or “hard” methods versus
nonstructural or “soft” approaches (USACE, 1981, 1984;
New York Sea Grant, 1984; Virginia Marine Resources
Commission, 1989; Ward et al., 1989; Pile Buck, 1990;
Maryland Department of Natural Resources, 1992;
Eurosion, 2004).
Shore protection methods along estuarine coasts are
wide ranging but can be grouped into two basic types,
defensive and offensive. These are described below.
Defensive
Perhaps the most common method of estuarine shoreline
protection is to harden the shore with bulkheads, seawalls,
or revetments. The primary goal of hardening the shore is
to protect the coast from wave attack by creating a barrier
to the erosive forces, waves, and currents. According to
the NRC (2007), traditional hardening methods often utilize local materials such as stone, wood, and concrete
and are built using techniques familiar to local marine
contractors and waterfront property owners. While wood
bulkheads are common in many areas, stone walls
(riprap revetments), constructed of local rock, have
become more widely used in areas where rock is available.
Bulkheads, typically made of wood, are actually
retaining walls that consist of the elements shown in
Figure 1. The wood is usually treated for longevity in the
estuarine environment. The vertical sheet piles are usually
tongue and groove and are either driven or “jetted” into
place. Horizontal members called wales run along the face
of the sheet piling in one or two levels. Tie rods or tie
backs are positioned every 1.8–2.4 m and anchored landward to hold the pilings or wales as shown. The tie rods are
often placed in conjunction with outside pilings for stability. For a typical bulkhead design, the sheet piles should be
in the substrate at least as much as the length above. Also,
the structure should allow for groundwater and wave
overtopping to be released through the wall via weep holes
or other drainage feature.
Some walls are made of concrete and rely on
a foundation or footing for stability. They may be referred
to as seawalls although this is generally afforded nomenclature for open ocean coasts. The footing is built first,
and then the vertical wall is built upon it. These must be
properly connected with re-bar or other connecting member. Having them both poured in place at the same time is
best. The wall is a gravity structure meant to withstand
wave action and hold the land side in place. As with bulkheads some type of drainage system to allow for groundwater and wave overtopping may be required.
Sloped stone walls are commonly called rock revetments, although revetments can be made of other materials
such as concrete units, concrete mats, or gabions. Stone
revetments armor the front face of the eroding shoreline.
They are commonly constructed with one or more layers
of graded riprap (Figure 2). The eroding bank is often
graded to provide a subgrade for the placement of filter
fabric, bedding stone, and then armor. Two layers are preferred, but one made on large stone can be used. Armor
stone must be of sufficient size to withstand the design
storm wave condition.
578
SHORE PROTECTION
C. Scott Hardaway, Jr.
Virginia Institute of Marine Science, College of William &
Mary, Gloucester Point, VA, USA
Synonyms
Shore erosion control
Definitions
Shore protection along estuarine coasts generally occurs
when shorelines are actively eroding. Shoreline erosion,
a natural process, is primarily a function of a rising sea
level and the impinging local wave climate.
Introduction
Many of the processes that govern erosion (and accretion)
on the open ocean coasts also apply to estuarine coasts, but
compared to the typically long linear nature of open
coasts, estuarine coasts are more sheltered and exhibit
a more irregular configuration. Estuarine coasts often display very distinct geomorphic compartments containing
a complex mix of resources that may vary from compartment to compartment. The relatively lower wave energy
conditions found along estuarine coasts create unique
environments that foster habitats and ecological communities, such as marshes and mudflats, not typically found
on open coasts (NRC, 2007).
Extent
Just about every estuarine shoreline can experience erosion, but typically when fetch exposures exceed about
1.6 km, land loss by wave action becomes more common
(Hardaway and Byrne, 1999). Documented land loss from
shoreline erosion is found in numerous estuaries around
the USA including Chesapeake Bay, Long Island Sound,
Delaware Bay, Puget Sound, and Mobile and Galveston
Bays. Land loss in Chesapeake Bay over the 100-year
period from 1850 to 1950 is estimated at over 190 km
2
(Singewald and Slaughter, 1949; U.S. Army Corps of
Engineers, 1973; Byrne and Anderson, 1978). The
response by land owners is to protect their property.
Types
Techniques used to address erosion along estuarine coasts
may be placed into broad categories, such as those proposed by Nordstrom (1992), Rogers and Skrabel (2001),
and Rogers (2005). Most guidelines and reports on shore
protection employ the same basic concepts to discuss
approaches such as structural or “hard” methods versus
nonstructural or “soft” approaches (USACE, 1981, 1984;
New York Sea Grant, 1984; Virginia Marine Resources
Commission, 1989; Ward et al., 1989; Pile Buck, 1990;
Maryland Department of Natural Resources, 1992;
Eurosion, 2004).
Shore protection methods along estuarine coasts are
wide ranging but can be grouped into two basic types,
defensive and offensive. These are described below.
Defensive
Perhaps the most common method of estuarine shoreline
protection is to harden the shore with bulkheads, seawalls,
or revetments. The primary goal of hardening the shore is
to protect the coast from wave attack by creating a barrier
to the erosive forces, waves, and currents. According to
the NRC (2007), traditional hardening methods often utilize local materials such as stone, wood, and concrete
and are built using techniques familiar to local marine
contractors and waterfront property owners. While wood
bulkheads are common in many areas, stone walls
(riprap revetments), constructed of local rock, have
become more widely used in areas where rock is available.
Bulkheads, typically made of wood, are actually
retaining walls that consist of the elements shown in
Figure 1. The wood is usually treated for longevity in the
estuarine environment. The vertical sheet piles are usually
tongue and groove and are either driven or “jetted” into
place. Horizontal members called wales run along the face
of the sheet piling in one or two levels. Tie rods or tie
backs are positioned every 1.8–2.4 m and anchored landward to hold the pilings or wales as shown. The tie rods are
often placed in conjunction with outside pilings for stability. For a typical bulkhead design, the sheet piles should be
in the substrate at least as much as the length above. Also,
the structure should allow for groundwater and wave
overtopping to be released through the wall via weep holes
or other drainage feature.
Some walls are made of concrete and rely on
a foundation or footing for stability. They may be referred
to as seawalls although this is generally afforded nomenclature for open ocean coasts. The footing is built first,
and then the vertical wall is built upon it. These must be
properly connected with re-bar or other connecting member. Having them both poured in place at the same time is
best. The wall is a gravity structure meant to withstand
wave action and hold the land side in place. As with bulkheads some type of drainage system to allow for groundwater and wave overtopping may be required.
Sloped stone walls are commonly called rock revetments, although revetments can be made of other materials
such as concrete units, concrete mats, or gabions. Stone
revetments armor the front face of the eroding shoreline.
They are commonly constructed with one or more layers
of graded riprap (Figure 2). The eroding bank is often
graded to provide a subgrade for the placement of filter
fabric, bedding stone, and then armor. Two layers are preferred, but one made on large stone can be used. Armor
stone must be of sufficient size to withstand the design
storm wave condition.
578
SHORE PROTECTION