Modified brush layers (MBLs) provide immediate surface stabilization for landslides and act as a catalyst for natural revegetation. MBLs are brush layers supported on willow wattles, live fascines, short logs, or boards. These biotechnical structures are typically installed in a staggered design, which makes them independent of one another. This arrangement provides small vegetated terraces that can intercept “raveling” slopes, preventing soil and rocks from gathering momentum as they move downslope. Revegetation occurs as the brush-layer cuttings grow and stabilize the soil with their adventitious roots. The terraces also catch native seeds and provide a stable bed for plant establishment.
In drier areas, MBLs are preferable to wattles or fascines alone, as the brush-layer cuttings are more deeply imbedded in the slope and have a better chance for survival. It is important to harvest plant material during the dormancy period, and cuttings should be properly handled for optimal growth. Alder or dead wood may be used for the supporting wattles, but it is best to use locally adapted willow species for the brush layers. MBLs are inexpensive to construct; live materials can often be obtained at no cost, and the only other expenses usually include logs or boards, stakes, rope, and labor time.
Construction of MBLs is relatively simple, and with an efficient work crew a moderate-size landslide can be stabilized within a few days. As with any type of landslide repair, work can be dangerous, and some slopes might be considered too steep or too unstable for this type of stabilization work. Placement of MBLs varies with slope conditions, and on steeper slopes the structures may be spaced at closer intervals. Locations can be marked by flagging or spray paint.
The first step in construction is to excavate a bench, which should be as long as the wattle or other supporting structure. The support should be placed on the outer edge of the bench and securely staked. The terrace is completed by backfilling behind the support with soil. For optimum growth of brush-layer cuttings, the bench should angle slightly downward into the slope. Cuttings should be placed in a crisscross fashion perpendicularly across the wattle, log, or board, with the basal ends into the slope. The branch tips should point up slightly, with 20% extending past the support and exposed to sunlight. Once the cuttings are in place, soil should be backfilled over the lower 80% of the brush-layer cuttings and stomped in between the branches.
This biotechnical erosion control technique has been used successfully to repair and revegetate landslides in British Columbia and the Pacific Northwest. The infamous Stafford Landslide, located on lumber-company land in northern California, was stabilized using this technique. The Stafford slide, which was actually two separate slides that occurred during the same storm event, destroyed several homes (no lives were lost).
Live pole drains (LPDs) were also used on the slide to act as “wicks” in moist areas where slumping or slippage could occur, as they concentrate seepage and allow it to drain onto more stable areas. LPDs are intended to drain away excess moisture from unstable sites by intercepting subsurface flow and surface runoff. The drains are constructed of one or more live fascine bundles, which are similar to willow wattles except that the branch tips are oriented in the same direction. The key to successful LPD construction is to install the drains in areas of seepage, either by excavating a shallow trench or utilizing an existing drainage gully. The bundled cuttings should be placed in the trench and keyed together firmly by wedging the ends together. Construction stakes or live stakes should be used every 1-2 m to firmly secure the LPDs to the slope. The bundles should be backfilled with native soil, with some twigs and branches left above the ground, as they need sunlight to grow.
LPDs are not effective in large, well-defined channels with concentrated flows. They will plug the channel and cause more erosion as the channel adjusts to maintain capacity. Landslides, eroding slopes, and small gullies and drainages can often be successfully treated with live pole drains. Both LPDs and MBLs are good techniques to initiate the process of natural healing.
For more case studies and information on the use of MBLs and LPDs in British Columbia, contact Polster Environmental Services, 5953 Deuchars Dr., Duncan, BC V9L 1L5, Canada; 250/746-8052.
(This Biotech Tip and drawings are adapted from the CD “Bio Draw 1.0: Compendium of Biotechnical Soil Stabilization Solutions,” by John McCullah, Salix Applied Earthcare.)
