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Watercourse Erosion Control
The Key is
Under the Mat
California's promised lousy
weather has, indeed, arrived. Questions of erosion control are on many lips as
homes and lives are lost to the most dramatic erosive events of all—landslides.
Soaked soil gives way and reminds us of the temporary nature of man's tenure on
earth. We are helpless to do more than get out of the way until the rain stops.
The sun brings a new opportunity, a
chance to, perhaps, stave off repetition of the destruction the next time "unseasonable
precipitation" comes. That means an active program of erosion control, to
mitigate, and if possible, to prevent recurrence. The hills dry out and we get
to work installing geotextiles, planting hill-holding grasses, reevaluating
drainage patterns. And here is where we notice one of the most difficult erosion
problems—watercourse bank stabilization. Can't avoid the water here, even
if ditches, swales and creeks happen to be dry most of the year. Selecting
plants that will stand the direct buffeting assault of moving water—at
times, very rapidly moving volumes of water—is essential to any reasonable
effort at soil stabilization. Long periods of dry weather further complicate
plant selection.
Hard Problem—Soft Solutions This challenge is the domain of the
Biotechnical Engineer who is charged, in the case of watercourse erosion
prevention, with devising "green" solutions—soft engineering
solutions. Vegetation is clearly the cheapest and
most effective way to stabilize slopes and keep erosion under control. Erosion
occurs when rain drops strike unprotected soil and break off particles of soil
that may then be carried away by the accumulated flow. Vegetation protects the
soil from raindrop impact. Erosion is further reduced by the
irregular configuration of the streambed when vegetation lines it. Water passing
over is slowed and turbulence reduced at the soil-water interface by the
influence of flexible grasses. The reduced velocity is less likely to undermine
the banks; water has more time to infiltrate the soil; and much of the soil that
does become entrained settles out closer to its source when water moves with
less speed. The complex of roots stabilizes the
soil while the plants, themselves, absorb quantities of water that are later
lost to evapotranspiration through leaves. It is normal, though, for a certain
amount of siltation to be carried along with waterflow—ocean beaches will
soon disappear without replenishment of sand.
Getting Grass Going Growing a sustainable stand of the
right vegetation will depend on each situation. In a dry swale that sees water
only occasionally as a result of weather, the plants chosen must establish
before the next damaging rain event and be able to withstand the assault of
rapid water flows. Such areas may have lost topsoil and
may not be able to support revegetation quickly enough. In these cases a
suitable nurse crop will have a better chance of establishing if the exposed
subsoil is amended with a growth promoter such as Albright's TurboStart.
A Solution in Solution The tougher problem is revegetating a
watercourse where water is present and moving—the temptation to use
concrete and "hard" engineering solutions arises. But remedies are
available, that will warm a biotechnical engineer's heart. Albright Seed Company
has pioneered Living Channel Liner—a complex of native riparian grasses
pregrown in a variety of erosion control mats.By growing riparian grasses in a
controlled environment into an easily-portable mat, later establishment along a
flowing stream is virtually unimpeded by the time available. The mature grass
and mat are installed on the streambank with a fully-formed, vigorous root
system ready to invade the soil and build a protective matrix. The mat, which is
pinned in place strengthens the hold of the plants and the presence of the
plants helps prevent the mat being torn loose by water pressure because it
reduces suction at the boundary. This additive effect grows beyond the rated
resistance to waterflow of either component. This arrangement has the advantage of
maximum non-erosive flow rates through the watercourse. Design flow rates
consider the grasses, even tall grasses, will be compressed as flow increases to
about a 4-inch thickness—a durable carpet of protection. Living
Channel Liner eliminates the risk and frustration of growing riparian
grasses from seed while weather and water fight each step.Albright Seed Company
consultants can customize Living Channel Liner to meet your
precise needs and bioregion.
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