© 1998 Streamline Publications

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. S&S Seeds 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.
S&S Seeds consultants can customize Living Channel Liner to meet your precise needs and bioregion.

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