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Out of Sight... For Now
© 1997, 1998 Streamline Publications
To be effective, erosion
control geotextiles must be applied so they will remain in contact with the soil
they are meant to protect. When the contact is lost, soil beneath the geotextile
will be lost to erosion. What will be preserved is the appearance of an
embankment that is held in place. The reality can be far different and far more
costly.
The Bubba Factor
Michael V. Harding, a certified
Professional Soil Erosion and Sediment Control Specialist, and past president of
the International Erosion Control Association (IECA), identified many of the
ways erosion control geotextiles—commonly referred to as "blankets"—can
lose effectiveness. 
Bubba's success installing geotextiles was limited.
His "Bubba" factor is an
evaluation of how these materials hold up during shipment, storage, rehandling
and application at the job site. He makes the point in his paper, Comparing Best
Management Practices: The Erosion Control Benefit Matrix (ECBM) (IECA,
Proceedings of Conference XXV, 1994), that even the weight and packaging of
these materials can affect their durability. In his analysis of
hydraulically-applied soil sealants and geotextiles, he makes the point that to
be effective both must remain undisturbed, and that Bubba's boots, particularly
in the case of these geotextiles, can become tangled in loose netting and the
fastening devices needed for securing it to the slope. The resulting damage during
installation can destroy the effectiveness of the material's ability to hold
soil. Plant fiber-based erosion control
blankets are subject to decomposition and have a limited shelflife before their
inherent durability suffers. On-site use of blankets degraded in this way can
produce an ineffectual installation.
Keep it Simple
Mr. Harding holds that "...ease of
installation is one variable where dissimilar erosion control approaches can
be compared against each other." He further supports his contention
explaining that as the complexity of the installation process and number of
steps needed increases, that the likelihood of a correct installation decreases.With the comparability of the separate
methods and materials hinging on ease of installation—simplicity—
hydro-seeding presents itself as the hands-down choice over geotextiles for
slopes compatible with its properties. Ease of installation is one of the major
benefits of hydroseeding. Unlike geotextiles, the hydraulically-applied slurry
conforms, particle-by-particle, to the irregularities of grade and surface.
Geotextiles are subject to "tenting" over rocks and stumps. Shrinkage,
or poor installation techniques may lift the fabric up from low spots. The
labor-intense effort necessary for installing these fabrics may result in damage
from the installation (remember Bubba?) before the work is complete.
When Geotextiles Are Best
Certainly, there are reasonable
applications of these geotextile erosion control blankets. The times they are
superior to hydroseeding are: 1) when the growing season is short and plants
cannot stabilize the slope quickly, 2) at high altitudes, or 3) where major
storms are a frequent occurrence. The problem we see is that geotextiles are
often specified for use in places where hydroseed-ing is a far better choice.
There are other factors that speak for selection of hydroseeding techniques over
blankets, the most significant being economy and ease of maintenance.
Maintenance Mr. Harding concludes that "...the
cost of routine maintenance is seldom considered in erosion control planning and
is usually the chief cause for failure of an erosion control system."
Geotextile installations will often
mask slope failures from all but the most intense scrutiny until erosion is too
far along to effectively treat the slope with spot methods. At this point,
maintenance of the slope may well mean a large, unpleasant, unbudgeted surprise.
In the case of wide-area under-blanket rill and gully formations, the migrating
soil could draw the attention of government pollution control authorities or the
wrath of downstream neighbors. When a hydroseeded area has a crust
failure, whether from weather, human or animal activity, the damage is visible
and, with early mitigation, can be cheaply repaired before its effects widen and
worsen. In most of California there are few
annual rain events of a size that will cause significant loss to a
properly-hydroseeded slope. Corrective maintenance for such losses is easy and
inexpensive.
Cost Even the least costly geotextile
installation can run, perhaps, 36 cents a square foot, and the geotextiles' cost
doesn't include seed and fertilizer. A far better way to spend your erosion
control budget is to use hydroseeding—at 3 to 5 cents a square foot,
including seed, biodegradable mulch and a soil binder—then maintain
integrity of the crust with later applications if needed. When considering economy in erosion
control it is important to examine cost versus effectiveness. According to Paul
Albright, owner of Albright Seed Company, "The facts are plain from
rain tower test data that erosion control blankets, even when carefully
installed by experts, are not twice as effective as hydroseeding under
common weather conditions—and they're ten times more expensive. So the
effect is that if you come back in and hydroseed after each major rain event,
you'd still have better erosion control at lower cost.
"When reality and simple
mathematics are applied to the problem hydro-seeding is most often the answer,"
Albright said. "Cost-effective erosion control will depend on low-cost
hydro-seeding initially and low-cost follow-up maintenance. "A slope cloaked in a blanket of
erosion control geotextile may look better than a similar hydroseeded grade, but
it may just be hiding erosion under the rug."
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