EM 1110-2-1100 (Part V)
31 Jul 2003
(3) Geotextile filled bags.
(a) Geotextile materials or filter fabrics have a long history for foundation mats beneath rubble-mound
structures (See Part VI-5-3); and, they have been used as silt curtains to contain dredged materials in the water
column. They have also been formed into bags and long, sausage-shaped cylinders (called Longard Tubes)
and filled with sand. They have been deployed as revetments for dune protection, as nearshore breakwaters,
and as groins. In the 1980s and 1990s, there has been significant improvement in the quality and durability
of geotextile fabrics, making them suitable for a variety of coastal applications.
(b) The design life of a geotextile filled bag depends on many factors. It is generally less than properly
designed rock structures serving the same function. However, if found to cause negative impacts to adjacent
shorelines, the bags can be cut open and removed with the filled sand remaining on the beach. It is for this
reason that a soft groin field was permitted by the North Carolina Coastal Resource Commission (CRC)
together with a beach nourishment project for South Beach on the western end of Bald Head Island,
North Carolina (Denison 1998). The North Carolina CRC prohibits hardened structures on its ocean coast.
Fourteen 2.75-m diameter and 100-m-long geotextile bags filled with sand were constructed to grade out to -
1.2 to - 1.5-m (mlw) depths at 120-m spacing to form the groin field. The beach was also prefilled with
496,960.7 cu m (650,000 cu yd) of sand to + 1.8 m (NGVD) berm elevation and 25 - 30-m berm width over
3.65 km. Two hurricanes in 1996 removed about 76,455.49 cu m (100,000 cu yd) from the beach, but caused
no damage to the groin tubes (Denison 1998). The long-term survivability of this system has yet to be
determined.
(4) Beach drains.
(a) Beach face dewatering by lowering the groundwater table along the coastline began in Denmark in
the early 1980s, by accident. After installation of a filtered, seawater system for a seaside aquarium, it was
discovered that the sandy beach width increased where the beach parallel, longitudinal pipe intake was buried
beneath the surface (Lenz 1994). Patents were obtained by the Danish Geotechnical Institute (DGI) in many
countries including the United States where the system is called Stabeach by the licensee, Coastal
Stabilization, Inc., Rockaway, NJ.
(b) Lowering the groundwater table is accomplished by draining water from buried, almost horizontal,
filter pipes running parallel to the coastline. The pipes are connected to a collector sump and pumping station
further inland. Gravity drains the groundwater beneath the beach and through the pipes to the sump and then
the water is pumped from the sump. The sand-filtered seawater can be returned to the sea or used for other
purposes.
(c) Lenz (1994) describes laboratory experiments (no reference) and field tests at Stuart, Florida, and
Englewood Beach, Florida. The patent belongs to Hans Vesterby, DGI, Denmark. Vesterby (1994) reviews
the theory and design elements and describes field tests at three sites on the west coast of Denmark in the
North Sea.
(d) Long-term, independent field monitoring is needed to learn more about the functional performance
of dewatering systems. The system by itself does not produce new sand, so that its greatest contribution may
be in increasing the fill life of renourished beaches.
(5) Innovative technology demonstration program.
(a) Many other ideas and devices have been deployed and/or proposed including beach cones (Davis and
Law 1994); ultra-low profile, geotextiles injected with concrete (Janis and Holmberg 1994) and fishnets,
stabilizers and artificial seaweed (Stephen 1994) for erosion mitigation. Most do not satisfactorily address
V-3-90
Shore Protection Projects
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