Trying to halt a natural process like erosion might seem as foolhardy as King Canute commanding the tide from rising, but new treatments can help insure that wind and rain won’t run off with your landscape design.

Any project planned for California, especially those in the arid and semiarid regions, must take the effects of erosion into consideration. Wind will always be a factor?EUR??,,????'??+even a velocity of just 8 to 9 miles per hour 6 inches above dry, bare ground can turn soil into dust and blow it away. The washing away of soil by water is an even greater problem. The principal factors affecting erosion are rates of rainfall (or irrigation) and runoff, soil type, the type of surface protection, and slope configuration.

Erosion is accelerated in urbanized watersheds due to man’s changes to the environment. These changes upset the system’s delicate balance and sometimes set into motion processes that are difficult to reverse.

Construction and developments can reduce a watershed’s absorption capacity, leading to a greater volume of runoff. Removal of protective vegetation exposes soil to the erosiveness of wind and water. Grading and other disturbances of the soil reduce the physical stability of the soil mass and make it more susceptible to the various forces that cause soil movement. Urbanization often results in flow concentrations that accelerate erosion and reduce infiltration. Even landscaping can lead to erosion: irrigation adds water to the system, which increases runoff.

Erosion Control Methods

Fortunately, a wide variety of erosion control approaches and methods are available today to help mitigate these impacts. From an environmental standpoint, it’s always preferable to use the restoration of vegetation as a primary approach. However, when the changes have resulted in high-velocity concentrated flow, some sort of structural method may have to be used. A variety of structural methods utilize both natural and manufactured products, and they can often be combined with vegetation to provide a more visually attractive and environmentally compatible result.

Short term practices– such as mulching, applying a soil binder, or the use of matting, netting or blankets–can be used to prevent erosion until vegetation or other long-term treatment is established. The range of products is wide and can be customized for the site conditions. The temporary treatment should be porous for water infiltration, conserve soil moisture by reducing evaporation, minimize soil temperature changes, and be nontoxic.

Long-term measures include permanent vegetative stabilization that should be long-lived and require little maintenance. A diverse plant assemblage that reflects the environmental characteristics of the site with perform best whether maintained or not; the diversity enhances the success of the vegetation by reflecting natural processes, providing overall stability, and displacing alien weeds. A complex root system encourages the formation of a water-stable soil structure. A varied canopy and ground cover complex provides a multi-layered protection of the ground from wind and raindrop impact. The selection of plant types should be based on specific site conditions and level of maintenance foreseen.

If non-irrigated native vegetation is selected, then timing is important. A planting window and soil moisture requirements at the time of planting should be tightly specified. A technique that works well for native regrowth is to scarify, chip, and save the top 4 to 6 inches of topsoil and vegetation at the start of site grading. The resulting topsoil-seed mulch can later be respread on the finished grade surfaces.

A positive form of site alteration is to place 6-inch silt fences on large, steep cut slopes. Plant development is often poor on these sites because the absence of soil and vegetation allows runoff and erosion to minimize regrowth of vegetation. The silt fences will slow runoff and trap sediment, allowing plants to develop behind them.

Slope length and gradient are major factors affecting erosion. Although flattened slopes expose more surface area to be eroded, their erosiveness is lessened and both the construction of erosion protection measures and the long-term effectiveness of revegetation is enhanced.

Although smooth fill slopes have become common grading practice, they are not beneficial in terms of erosion control and the establishment of vegetation. The surface should be roughened to slow down the runoff velocity and to increase infiltration. The texture of the roughened surface should trend parallel to the slope contours.

The key factor in reducing erosion is control of drainage, and several measures can be taken to reduce the possibility of uncontrolled water flowing down the face of the slope. Slope terracing can reduce the runoff distance and enhance vegetation growth, and should be combined with drainage swales to collect runoff water and guide it to downdrain flumes or slope pipes. Although initially expensive, installation of permanent water control devices assures better erosion protection and reduces maintenance costs.

Leaking or poorly maintained sprinkler systems can cause severe erosion gullies to occur, especially on fill slopes. Care should be taken to maintain the irrigation equipment and not overwater the vegetation. One option that works well on low slopes is to place all sprinkler heads at the base of the slope and irrigate upslope only.

Where concentrated flow velocities are too high for non-structural methods, a more substantial treatment is needed. Some methods available for protection of slopes subject to high-velocity flow are riprap, gabion mattresses, or interlocking concrete blocks, all underlain by a filter fabric. Riprap protection consists of a blanket of large rocks with sufficient size and weight to withstand anticipated water velocities. Gabion mattresses are relatively thin (6- to 9-inch) wire mattresses filled with rock. Interlocking concrete blocks are relatively thin precast concrete blocks that interlock into a continuous mat. As with gabions and riprap, this system is both flexible and permeable.

Wherever possible, grade control structures such as check dams or weirs should be used to slow concentrated flow velocities. These fix the grade at a certain elevation and allow depositation of sediment upstream of the structure. The newly flattened upstream slope serves to slow the flow velocity, spread the water out, and increase recharge into adjacent soils–thus enhancing vegetative growth and reducing the degree of slope protection required.

Carol Forrest is a senior project engineer with Woodward-Clyde Consultants, a nationwide firm of engineering, environmental, and earth sciences consultants (619/224-2911). She is also president of the International Erosion Control Association and will participate in the Erosion Control Workshop at The L.A.T.E. Show ‘87.