2.2.3 Using Sand Fences
Fencing materials include canes and tree branches inserted directly into the sand
and wooden slats, plastic, and jute fabric attached to fence posts. Using sand
fences with different characteristics and configurations can result in considerable
variety in foredune topography and vegetation. Straight fences placed parallel to
the shore appear to provide the most economical method of building protective
dunes (CERC 1984; Miller et al. 2001). Zig-zag fences can create wider dunes
with more undulating crest lines and more gently sloping dune faces that are closer
to the shapes of natural dunes (Snyder and Pinet 1981). Paired fences can create a
broader based foredune with a rounded crest that can look more natural
(Schwendiman 1977). Side spurs perpendicular to straight alongshore alignments
can increase trapping rates in locations of strong longshore winds. Multiple lifts of
fences can create a higher dune with much greater volume than single lifts (CERC
1984; Mendelssohn et al. 1991; Miller et al. 2001).
Sand accumulation efficiency and morphological changes depend on fence
porosity, height, inclination, the scale and shape of openings, wind speed and
direction, the number of fence rows, separation distance between fence rows, and
placement relative to the existing topography. Fencing with a porosity of about
50 %, with space between open and closed areas of less than 50 mm, can fill to
capacity in about a year where appreciable sand is moving, but vegetation planting
may still be required to stabilize the surface (CERC 1984) and establish a more
natural trajectory.
A fence placed at the seaward limit of natural vegetation or at an existing
foredune line may be far enough landward to survive a wave attack during small
storms and have a wide source of wind-blown sand seaward (CERC 1984). Where
diagnostic vegetation is lacking, dunes built with fences appear likely to survive
storms at least a 1 year apart if they are built landward of the upper-most storm
wrack line. The decision about where to place fences on a beach widened by fill or
storm wave overwash could be made taking into account whether it is desirable to
create a dune with space devoted to low wet habitats (slacks) between landward
and seaward ridges or a higher, drier, and more continuous dune with multiple
ridges (Nordstrom 2008). In these cases of a widened beach, the landward fence
could be at least 100 m from the active beach, as suggested by Dahl and Woodard
(1977) and Miller et al. (2001).
Sand fences or vegetation plantings are often used to stabilize newly-formed
bare areas, forming gaps in the existing dunes. Depending on how, when or
whether gaps are sealed, a foredune could reflect the differences in topography and
cyclic reversals in vegetation succession found on a more dynamic natural landform or the more uniform sequences found on a stable landform. Constructing new
fences is a more traditional and conservative approach, but one that should be
evaluated more carefully considering the lower restoration potential of the
resulting linear dune.
22
K. F. Nordstrom and N. L. Jackson
Fencing materials include canes and tree branches inserted directly into the sand
and wooden slats, plastic, and jute fabric attached to fence posts. Using sand
fences with different characteristics and configurations can result in considerable
variety in foredune topography and vegetation. Straight fences placed parallel to
the shore appear to provide the most economical method of building protective
dunes (CERC 1984; Miller et al. 2001). Zig-zag fences can create wider dunes
with more undulating crest lines and more gently sloping dune faces that are closer
to the shapes of natural dunes (Snyder and Pinet 1981). Paired fences can create a
broader based foredune with a rounded crest that can look more natural
(Schwendiman 1977). Side spurs perpendicular to straight alongshore alignments
can increase trapping rates in locations of strong longshore winds. Multiple lifts of
fences can create a higher dune with much greater volume than single lifts (CERC
1984; Mendelssohn et al. 1991; Miller et al. 2001).
Sand accumulation efficiency and morphological changes depend on fence
porosity, height, inclination, the scale and shape of openings, wind speed and
direction, the number of fence rows, separation distance between fence rows, and
placement relative to the existing topography. Fencing with a porosity of about
50 %, with space between open and closed areas of less than 50 mm, can fill to
capacity in about a year where appreciable sand is moving, but vegetation planting
may still be required to stabilize the surface (CERC 1984) and establish a more
natural trajectory.
A fence placed at the seaward limit of natural vegetation or at an existing
foredune line may be far enough landward to survive a wave attack during small
storms and have a wide source of wind-blown sand seaward (CERC 1984). Where
diagnostic vegetation is lacking, dunes built with fences appear likely to survive
storms at least a 1 year apart if they are built landward of the upper-most storm
wrack line. The decision about where to place fences on a beach widened by fill or
storm wave overwash could be made taking into account whether it is desirable to
create a dune with space devoted to low wet habitats (slacks) between landward
and seaward ridges or a higher, drier, and more continuous dune with multiple
ridges (Nordstrom 2008). In these cases of a widened beach, the landward fence
could be at least 100 m from the active beach, as suggested by Dahl and Woodard
(1977) and Miller et al. (2001).
Sand fences or vegetation plantings are often used to stabilize newly-formed
bare areas, forming gaps in the existing dunes. Depending on how, when or
whether gaps are sealed, a foredune could reflect the differences in topography and
cyclic reversals in vegetation succession found on a more dynamic natural landform or the more uniform sequences found on a stable landform. Constructing new
fences is a more traditional and conservative approach, but one that should be
evaluated more carefully considering the lower restoration potential of the
resulting linear dune.
22
K. F. Nordstrom and N. L. Jackson