Introduction
Headland breakwaters are structures, often made of rock,
that can occur as a single breakwater or a series of breakwater that create a series of pocket beaches (Figure 1).
These types of structures are used worldwide for the protection of the coast and to stabilize beach nourishment.
Much of the research and applications have centered on
open ocean applications and the Great Lakes (Suh and
Dalrymple, 1987). The use of headland breakwaters in
estuarine systems has been limited except for the United
States’ mid-Atlantic coast. The Chesapeake Bay, in
particular, has numerous examples of this application
(Hardaway and Gunn, 2010; Hardaway and Gunn,
2011). This is due in part, to a long history of installations
where beach habitat can be considered as a trade-off for
encroachment onto state-owned bottom.
The Chesapeake Bay estuarine system like many
others in the United States and around the world consists
of a variety of shorelines that vary from low, upland
banks and marshes to beaches and dunes to high bluffs.
Erosion of these shorelines becomes significant when
fetch exposure, the distance over open water that wind
can blow and generate surface waves, exceeds a few kilometers. Erosion becomes severe when shorelines are
exposed to fetches exceeding 16 km or more. Critical erosion has been defined as erosion that immediately
threatens upland improvements and infrastructure no
matter what the fetch exposure. The use of headland
breakwaters coupled with beach fill to create stable
pocket or embayed beaches for shoreline management
has become somewhat commonplace. Over the last
25 years, numerous research and project installations
have paved the way for widespread usage (Hardaway
and Gunn, 1991; Hardaway et al., 1995; Hardaway and
Byrne, 1999; Douglass and Pickel, 1999; Hardaway and
Gunn, 1999a; Hardaway and Gunn, 1999b; Hardaway
and Gunn, 2002).
Properly designed and installed headland breakwaters
with beach fill for pocket beaches and wetlands plantings
provide shore protection and create a “full” coastal profile with three components, the beach, backshore, and
dune, which enhances habitat, creates a tertiary buffer
for upland runoff and groundwater, and provides access
for recreation. Establishing vegetation zones within the
headland breakwater system is a critical design element
since wetlands grasses also create an erosion-resistant
turf during storm events. Along the existing beaches on
the open bay and broad rivers of the Chesapeake
Bay estuarine system, dune grasses can only survive
above a stable berm. Intertidal grasses must reside in
sheltered regions. This coastal profile also accommodates environmental permitting requirements of habitat
enhancement.
Design considerations
Perhaps the most important parameters in headland breakwater design are the width and elevation of the beach in
the gaps of the breakwater system (i.e., minimum bay
beach size). The beach must be high and wide enough to
offer protection, usually for the base of a graded upland
bank, under design storm conditions. Design storms are
at least the 25-year event, and the breakwater itself should
withstand the 100-year or greater storm.
In headland breakwater design, the beach morphology
emulates nature so the beach profile should be assessed
first when designing any headland breakwater system.
The design of the beach begins with establishing the minimum design beach width (Bm) and profile in the context
of a stable pocket beach that will be held between headland breakwaters (Figure 2). This will determine the
amount of beach nourishment required. With Bm
established, the breakwater length (Lb), the breakwater
gap (Gb), and the bay indentation distance (Mb) are determined and depend on the wave environment. Hardaway
et al. (1991) developed relationships between these design
parameters. In particular, the statistical relationship
between breakwater gap (Gb) and the bay indentation
distance (Mb) should be 1.65:1. Hardaway and Gunn
(2002, 2010) further assessed this relationship and found
it to be a usable first step in the design process.
Numerous studies, as documented in Chasten et al.
(1993), show that as a breakwater is lengthened relative
to its distance offshore, a tombolo is more likely to
develop. A tombolo is an essential element in headland
breakwater systems. The tombolo must be developed with
the addition of beach nourishment in Chesapeake Bay
projects since natural sand supply generally is limited.
It appears that as breakwater length approaches the design
wave length by twofold, it can better hold a tombolo,
particularly when the breakwater acts as a headland in
a multiple breakwater unit system. The level of tombolo
attachment may vary from attachment above high water
to a low water connection.
Suh and Dalrymple (1987) demonstrated that when
the gap between two diffraction points (i.e., the ends of
adjacent breakwaters) becomes approximately twice the
incident wave length or more, the shoreline behind each
breakwater responds independently as if there is no interaction among breakwaters. This mechanism might provide the response of the tangential section of the spiral
bay or pocket beach as it orients itself into the dominant
direction of wave approach. Wave length is an important
parameter in wave diffraction and wave refraction both
of which are important mechanisms in breakwater wave
attenuation and pocket beach configurations.
Bodge (1998) offers the 1/3 rule for the relationship of
breakwater gap (Gb) to bay indentation (Mb) or the maximum offset of the embayed beach from a line connecting
adjacent breakwaters. Bodge (2003) provides formulae to
assist in developing this ratio and notes that it is
a combination of the static equilibrium bay (SEB) model
and his research to define mean low water around an
embayed coast.
The Coastal Engineering Manual (CEM, 2000) defines
the minimum beach (Bm) width as Ymin, the minimum
HEADLAND BREAKWATERS
351
Headland breakwaters are structures, often made of rock,
that can occur as a single breakwater or a series of breakwater that create a series of pocket beaches (Figure 1).
These types of structures are used worldwide for the protection of the coast and to stabilize beach nourishment.
Much of the research and applications have centered on
open ocean applications and the Great Lakes (Suh and
Dalrymple, 1987). The use of headland breakwaters in
estuarine systems has been limited except for the United
States’ mid-Atlantic coast. The Chesapeake Bay, in
particular, has numerous examples of this application
(Hardaway and Gunn, 2010; Hardaway and Gunn,
2011). This is due in part, to a long history of installations
where beach habitat can be considered as a trade-off for
encroachment onto state-owned bottom.
The Chesapeake Bay estuarine system like many
others in the United States and around the world consists
of a variety of shorelines that vary from low, upland
banks and marshes to beaches and dunes to high bluffs.
Erosion of these shorelines becomes significant when
fetch exposure, the distance over open water that wind
can blow and generate surface waves, exceeds a few kilometers. Erosion becomes severe when shorelines are
exposed to fetches exceeding 16 km or more. Critical erosion has been defined as erosion that immediately
threatens upland improvements and infrastructure no
matter what the fetch exposure. The use of headland
breakwaters coupled with beach fill to create stable
pocket or embayed beaches for shoreline management
has become somewhat commonplace. Over the last
25 years, numerous research and project installations
have paved the way for widespread usage (Hardaway
and Gunn, 1991; Hardaway et al., 1995; Hardaway and
Byrne, 1999; Douglass and Pickel, 1999; Hardaway and
Gunn, 1999a; Hardaway and Gunn, 1999b; Hardaway
and Gunn, 2002).
Properly designed and installed headland breakwaters
with beach fill for pocket beaches and wetlands plantings
provide shore protection and create a “full” coastal profile with three components, the beach, backshore, and
dune, which enhances habitat, creates a tertiary buffer
for upland runoff and groundwater, and provides access
for recreation. Establishing vegetation zones within the
headland breakwater system is a critical design element
since wetlands grasses also create an erosion-resistant
turf during storm events. Along the existing beaches on
the open bay and broad rivers of the Chesapeake
Bay estuarine system, dune grasses can only survive
above a stable berm. Intertidal grasses must reside in
sheltered regions. This coastal profile also accommodates environmental permitting requirements of habitat
enhancement.
Design considerations
Perhaps the most important parameters in headland breakwater design are the width and elevation of the beach in
the gaps of the breakwater system (i.e., minimum bay
beach size). The beach must be high and wide enough to
offer protection, usually for the base of a graded upland
bank, under design storm conditions. Design storms are
at least the 25-year event, and the breakwater itself should
withstand the 100-year or greater storm.
In headland breakwater design, the beach morphology
emulates nature so the beach profile should be assessed
first when designing any headland breakwater system.
The design of the beach begins with establishing the minimum design beach width (Bm) and profile in the context
of a stable pocket beach that will be held between headland breakwaters (Figure 2). This will determine the
amount of beach nourishment required. With Bm
established, the breakwater length (Lb), the breakwater
gap (Gb), and the bay indentation distance (Mb) are determined and depend on the wave environment. Hardaway
et al. (1991) developed relationships between these design
parameters. In particular, the statistical relationship
between breakwater gap (Gb) and the bay indentation
distance (Mb) should be 1.65:1. Hardaway and Gunn
(2002, 2010) further assessed this relationship and found
it to be a usable first step in the design process.
Numerous studies, as documented in Chasten et al.
(1993), show that as a breakwater is lengthened relative
to its distance offshore, a tombolo is more likely to
develop. A tombolo is an essential element in headland
breakwater systems. The tombolo must be developed with
the addition of beach nourishment in Chesapeake Bay
projects since natural sand supply generally is limited.
It appears that as breakwater length approaches the design
wave length by twofold, it can better hold a tombolo,
particularly when the breakwater acts as a headland in
a multiple breakwater unit system. The level of tombolo
attachment may vary from attachment above high water
to a low water connection.
Suh and Dalrymple (1987) demonstrated that when
the gap between two diffraction points (i.e., the ends of
adjacent breakwaters) becomes approximately twice the
incident wave length or more, the shoreline behind each
breakwater responds independently as if there is no interaction among breakwaters. This mechanism might provide the response of the tangential section of the spiral
bay or pocket beach as it orients itself into the dominant
direction of wave approach. Wave length is an important
parameter in wave diffraction and wave refraction both
of which are important mechanisms in breakwater wave
attenuation and pocket beach configurations.
Bodge (1998) offers the 1/3 rule for the relationship of
breakwater gap (Gb) to bay indentation (Mb) or the maximum offset of the embayed beach from a line connecting
adjacent breakwaters. Bodge (2003) provides formulae to
assist in developing this ratio and notes that it is
a combination of the static equilibrium bay (SEB) model
and his research to define mean low water around an
embayed coast.
The Coastal Engineering Manual (CEM, 2000) defines
the minimum beach (Bm) width as Ymin, the minimum
HEADLAND BREAKWATERS
351