Structural Erosion Control Solutions
Holding fast, built to last
knows best. This is the premise behind the approach that many contractors have
taken on their sites in recent years. It has been a common trend for contractors
to use bioengineering techniques or soft-armor erosion control solutions,
including erosion control blankets, coir logs, and vegetation alone, to hold
soil in place rather than to opt for hard-armor solutions, such as poured
concrete or riprap.
friendly bioengineering is an approach that can be effective in many, but not
a site with steep slopes, a site at risk for flooding, or sites that must
withstand high-velocity water flows can require help from manmade or synthetic
solutions in order to prevent greater or more widespread damage. Structural
erosion control can stabilize sites that are dramatically challenged by nature
solutions, which can range from retaining walls and gabions to concrete
shoreline protection measures, can provide long-term protection at sites where
soft-armor engineering systems are not enough. Yet many companies find employing
a combination of both of these techniques can promise the best of both worlds
for particularly challenging sites.
Hillside Erosion in Oahu
one thinks of the Hawaiian island of Oahu, erosion typically isn’t the first
image that comes to mind. Yet Hawaii’s department of transportation hired
Prometheus Construction to protect a section of the Pali Highway two years ago
to protect this side of paradise.
site had global instability problems with a history of landslides,” says Cliff
Tillotson, vice president of Prometheus Construction, a company that specializes
in rockfall mitigation and slope stabilization with offices in Santa Barbara,
CA, and Kaneohe, HI.
|Photos: Prometheus Construction
A section of Oahu's Pali Highway that was in need of hillside erosion protection
hydraulic erosion blanket with seed had been used at the site in the past, but
it had failed. Geobrugg Protection System’s TECCO rockfall drape, made of
high-tensile steel wire mesh, was selected to protect the
was chosen, with varying depths of soil nail anchors to tie the hillside back
beyond its slip plane. The anchors were 10 to 30 feet deep,” says Tillotson,
adding that a combination of erosion control techniques was employed on the
project. “For surface erosion, a North American Green C350 coconut-fiber turf
reinforcement mat was installed. The hillside was also hydroseeded with Kikuya
grass before the installation of the blanket and the
workers had to cope with a challenge of timing: the erosion control products’
installation took place during the island’s rainy season.
prevent erosion during construction, before we removed most of the existing
vegetation, we drilled and installed all the anchors that we could reach with
our excavator. Then, we stripped the hillside of its existing vegetation so we
could get the erosion blanket and TECCO to lay flat. Then we did the
hydroseeding and installed the coconut blanket and TECCO,” says Tillotson.
“After this was competed, which only took a few days, we drilled and installed
all the upper anchors.”
slope was approximately 80 to 100 feet high, and bencher drills were mounted on
man-lifts, he explains.
approach tightened the window of vulnerability to erosion from rainstorms here
in Hawaii before we got the permanent system complete with all the anchors in
place,” says Tillotson.
with the rainy weather, measures were taken to ensure proper
also installed temporary irrigation,” he says, “to speed
areas adjacent to the construction site continue to erode significantly, the
TECCO section remains steadfast.
Beach Erosion at Bay in Florida
difficult to watch pieces of your present environment drift away, but that’s
exactly what residents of Hillsboro Beach were forced to witness as they lost
approximately 2 feet of beach annually to erosion. Although a full beach
renourishment took place at the site about nine years ago, the town, located
beside Ft. Lauderdale on Florida’s Atlantic Coast, had to bring in sand
repeatedly to counter the natural erosion caused by storms and winter
decided you just can’t keep replacing sand,” says town commissioner Tom Puleri.
“It’s too costly.”
2008, the town leased Boca Raton, FL–based EcoShore International’s pressure
equalizing module (PEM) system for three years. The system is designed to alter
water pressure within sediment using tubes with drainage outlets to increase the
sediment deposition on a beach. If it didn’t work, the company would remove the
system at no cost.
PEMs are installed vertically into 9- to 10-foot-deep holes.
the contract, there were stipulations on how to determine whether it worked,”
explains Puleri. “They do surveys, approved by the DEP [Department of
Environmental Protection]. If it showed they met the requirement of the
contract, we would pay them 50% of the annual rent.
could not lose more than 25% of the beach, and you had to gain 25%,” he
continues. “As long as you stayed within those two numbers, they considered it
surveys, he says, must be confirmed by the state of Florida. The project can
potentially save the city millions of dollars in erosion control costs in the
future, according to an article in the local newspaper, the Sun-Sentinel,
which reported Broward County beach erosion commissioner Stephen Higgins as
saying the area’s beaches protect $4 billion in public and private
6-foot-long, 2.5-inch-diameter tube contains drainage slots. A small drill
mounted on tracks digs a 9- to 10-foot hole in the sand. The PEMs are then
vertically inserted in these holes in 33 rows.
distances between the tubes’ tops and the existing grade range from 12 to 36
inches, according to a statement by EcoShore. The rows are spaced 50 meters
apart and run perpendicular to the shoreline. The PEMs themselves have been
placed 10 meters apart, according to the company, with “each row beginning from
the mean low water line and extending to the toe of the dune or to structures
that armor the shore.” The installation process at Hillsboro Beach took
approximately three weeks.
there are slightly less than 4 miles of beach at the site, the DEP would allow
the PEM system to be applied only on 1 mile of the site so that it could be
had to wait until turtle season—the laying of the eggs—was over before the
installation,” says Puleri. “That was one of the things we had to contend with.
And it’s a long process getting the permits. So far, we have had no problems
whatsoever. It seems to be working. We’ll do another survey in six months. We
lost a little beach, but we gained density.”
beach nourishment, involving barges off the coast pumping sand onto the beach,
was used in addition to establishing the PEM system, says Puleri, who is happy
with the way the system has worked so far.
River Flooding in Minnesota
ties and timbers were used to stabilize and prevent erosion in an area along the
Redwood River, one of 13 watersheds that flow into the Minnesota River. But when
this method gave way, it caused a scouring problem within the river itself, and
the area’s soil conservation service turned to Gabions Inc. for a more effective
Niehaus, president of Gabions Inc., a Litchfield, MN–based supplier and
sometime-installer of Terra Aqua Gabions’ products, explains that a bypass had
been installed around the city of Marshall, MN, where the site was located, to
prevent flooding. However, flooding that had taken place in the area prior to
that construction had caused the erosion at the site.
designed a retaining wall. It was joint project with the local soil conservation
agency and homeowners. They engaged us to put it in place,” says Niehaus.
consisted of cleaning up the mess that was there. We cleaned up the scour hole
in the river. It was a one-pass installation.”
approximately 500 feet long, the wall extended to 12 feet high at one end. Terra
Aqua’s gabions are a double-twisted hexagonal woven mesh, made from galvanized
steel with compartmented baskets forming a rectangular box. Natural stone is
placed in the product’s cells.
2004, the company installed a Reno mattress at the site. Stone is evenly
distributed into the equal-sized cells of the double-twist hexagonal woven mesh
that is made of galvanized steel. The rectangular-shaped mattresses, as well as
gabions, have been used internationally for more than a century in a wide
variety of applications, including soil retention, river training, and channel
mattress provided the foundation and scour protection for the wall,” says
Niehaus. “There was a bridge on the state highway next to it. We terminated the
wall when we got to the bridge. There were some storm sewers that also
contributed to the erosion. We also extended the pipe through the gabion
explains Niehaus, the site will revegetate itself as the Reno mattress catches
seed and sediment through the years. Now that the project is complete, he drives
by the site frequently.
held up,” says Niehaus. “It’s doing fine.”
Canal Banks in Tampa
July 2008, parts of the embankment of the C-24 canal located in St. Lucie and
Martin counties in Florida collapsed into the canal as the result of a
hurricane. The manmade canal had held steady since its 1959–1961 construction by
the US Army Corps of Engineers, but hurricanes can test the strongest of support
stabilize the area, Tampa–based R. H. Moore & Associates used Maccaferri
gabion walls for the canal’s banks. Crews also employed turbidity barriers from
Bradenton, FL–based Aer-Flo Inc. to contain sediment during the
floating barriers can prevent sediment and silt from entering water courses. To
be most effective, however, certain measures must be taken, including containing
the silt and sediment in the retention area long enough to permit settling. The
containment area’s current shouldn’t be so intense that the particles are
permitted to migrate past the barrier before settlement can
|Photos: R. H. Moore and Associates
Floating barriers used to prevent sediment and silt from entering water courses in Tampa, FL
barrier’s lower edge is typically kept approximately 1 foot above the bottom of
the waterway. If the lower edge is long enough to drape the bottom, silt will
begin to build up, and if this is permitted to occur over an extended period of
time, the barrier can be drawn below the water’s surface.
double-twisted hexagonal mesh of the Maccaferri gabions’ steel wire is
reinforced by selvedges of heavier wire. These gabions are pre-assembled for
quick installation. The tops of the gabions and the embankment behind them were
four projects with four different contractors were going on simultaneously in
the area, workers also faced a time constraint: the project required a 21-day
affected the choice of contractor.
had easily the quickest response,” says Larry Larson Jr. of R. H. Moore &
Associates, who adds that using local manufacturing can drive down a project’s
costs slightly. “They have great quality and are very well-known in the state of
Protect Stream Bank
flooding is legendary, immortalized in books and song. For a stream restoration
project located in a flood plain in Carrollton, TX, the city hired DCI
Contracting Inc., based in Decatur, TX, to combat a large volume of water
in late February 2008, DCI used Houston–based Modular Gabion Systems’ product
for erosion control at the Hutton Branch Channel site. These gabions were chosen
for their ease of installation compared with twisted-wire gabions that DCI had
used in the past. In addition, “the welded wire gave us a little more of a
uniform look,” says Shannon DiMarco, the project superintendent for
this project DCI chose the ‘roll stock’ welded wire mesh gabion construction
method, instead of the conventional partially assembled gabions,” says George
Ragazzo, general manager of Modular Gabion Systems. “The roll stock method is
one of MGS’s innovations, which allows the contractor to field-form gabions and
gabion mattresses up to 300 feet long continuously and without joints, thus
eliminating all redundant common panels and a great deal of the labor associated
with conventional gabions construction.
roll stock is supplied in rolls of welded-wire mesh to contract specifications
engineered for the project. It requires less gabion material, less labor, and
less installation time—typically from 40% to 70% less time. The resulting gabion
structure is more structurally sound—no joints, highly flexible, very cost
effective, and aesthetically superior.”
spiral fasteners, commonly used with welded-wire gabion installation, offer a
joint strength of 1,834 pounds per foot and allow stress that develops from
opposing panels to be evenly distributed along the joint, according to Modular
Gabion Systems. The gabions’ self-supporting nature allows for ease of assembly,
and the PVC powder coating that is fuse-bonded to the welded mesh helps to
prevent corrosive liquids from damaging the core wire.
overcame weather challenges at the site to construct massive retaining
used the gabions to build a couple of retaining walls, one 300 feet by 12 feet
tall,” DiMarco says, adding that the company also installed gabions along both
sides of the waterway. “The channel itself is roughly a mile. We’ve installed
7,000 cubic yards so far.”
|Photo: R. H. Moore and Associates
More floating barriers used to prevent sediment and silt from entering water courses in Tampa, FL
used a combination of erosion control techniques at the site during the $4.5
million project, which also includes the addition of three bridges and a
low-water stream crossing. These additional techniques include geotextiles, coir
matting, and a high-performance turf reinforcement mat.
the gabion channels, they installed coconut straw and seed/grass mixes that go
beneath the fabrics,” says DiMarco. “The flow channel of the stream is built
with the gabion basket. It works great. The city of Carrollton is very pleased
with the outcome.”
a slope restoration project at Miller’s Creek in Ann Arbor, MI, required some
revegetation for erosion control, the site also needed a hard-armor solution for
long-term erosion protection.
creek, which snakes beside Huron Parkway, lies within about 50 feet of the
was a restoration of the banks that had already caved in and washed out,” says
Brack Moss, who managed the ongoing project for Posen Construction. “It was a
pretty good area that was already eroded. Some of the trees had fallen into the
company used a Cable Concrete system produced by International Erosion Control
Systems in Ontario, Canada, on the three-quarter-mile
stainless steel cable is integrated into concrete, forming many interlocking
cells that combine to create a mat. The cable’s flexibility permits the mat to
correspond with ground contours. The mats, which can be installed below or above
water levels, arrive at a site as a unit, which can reduce labor
Cable Concrete system is typically paired with a geotextile base cloth. Holes in
this geotextile allow drainage into the subsoil, preventing hydraulic pressure
buildup beneath the concrete mat.
Cabled Concrete was installed in the worst curve, the curve that had the most
erosion,” says Larry Libbrecht of CSI Geoturf based in Highland, MI, which
represented the manufacturer for this site.
mat lined 120 feet of the area and was filled in with topsoil. A variety of
wildflowers, native seed mixes, and prairie grass plugs were added; when they
mature, the vegetation will cover the mat, says Moss. Ninety percent of the
$565,000 project was spent on erosion control.
the road open during construction was a challenge for workers, who found
themselves beside the traffic in places on the site.
|Photos: CSI Geoturf
A slope restoration project at Miller's Creek in Ann Arbor, MI
diverted the water that was in the stream so we could do the work where it was
dry,” says Moss. “Each one we got finished, we’d open it
used on the project was Propex Geosythetics’ Landlok 450 synthetic turf
reinforcement mat, which “allows vegetation while protecting the slope from
erosion,” explains Libbrecht. The mats are designed for both short- and
long-term protection in moderate flow waterways, such as certain creeks. Soil,
seed, and water are held in place using the company’s X3 fiber technology. This
TRM is composed of completely synthetic, ultraviolet-stabilized
also installed a riprap toe at the site that met the water’s edge on one side
and the articulated concrete on the other.
stakes were used at the site, explains Moss, who notes that the process creates
a root system that will prevent erosion as well as providing a natural
aesthetic. Willow was selected because it thrives around watery areas. Sandbar
willow, pussy willow, silky willow, peachtree willow, and black willow were used
at the site. Other vegetation used on site included red-osier dogwood, silky
dogwood, and ninebark.
touch of green will soften the edges of the landscape around the project for
years to come.
severity of erosion—or its potential—that a site faces can greatly affect and
effectively limit a project’s choice of erosion control methods. When a natural
approach is what you want but manmade strength is what you need, consider the
variety of ways you can incorporate erosion control solutions that combine both
methods, providing the kind of strength with a natural appearance clients often
Author's Bio: Based in Morgantown, PA, Tara Beecham writes frequently for Forester publications.