In contrast to riverbank stabilization projects, efforts to stabilize slip areas need not give so much attention to long-term elasticity of shrubs or high-cost tending schedules, provided that we select a suitable combination of plants to support stand development. The important point is to find a proper mix of pioneer and succession shrubs.
Considering that plants need time to grow but that slopes must be stabilized quickly, we have to use auxiliary measures to bridge the gap, especially timber–a rapidly regrowing material of high sustainability.
Based on some examples taken from my 19 years of practice at the special operation for soil protection, torrent control, and avalanche defense in South Tyrol, Austria, this article describes how slope failures can be stabilized by soil bioengineering methods, which plants are most suitable for this purpose, and the limits of some of the methods used.
Käsebach/Deutschnofen Landslide
In 1985, a violent thunderstorm ravaging Deutschnofen in the Eggen Valley led to several landslides along the Käsebach rivulet as a result of the toes of slopes being swept away by high water and bed-load transport. Check dams were constructed at the damaged section and the cut slopes stabilized at the toe with blockstone embankments in the same year.
The broken-up soil material (at 1,250 m above sea level) mainly consisted of scree quartz-porphyry characterized by a high rate of coarse clay. Blocking strata at the flatter section above used for grazing made for an extremely water-logged slope, and I had to wear waders for the first site inspection. The most urgent work was to build a ring drainage system, which ran above and lateral to the slope and drained into the Käsebach. It was a standard drainage system, using an 80-mm drainage pipe and a 500- to 600-mm gravel bed.
To stabilize the loose water-logged material, I proposed hedge brush layers, which consist of willow branches and rooted deciduous trees. For this method, it is important to use highly transpiring broad-leaved species such as whortle willow (Salix myrsinifolia), mezereon willow (Salix daphnoides), common willow (Salix alba), gray alder (Alnus incana), ash (Fraxinus excelsior), harewood (Acer pseudoplatanus), bird cherry (Prunus padus), and mountain ash (Sorbus aucuparia).
Terraces of 1—1.5 m in depth are run across the slope at a slight inward angle and covered, as much as possible in parallel, with willow branches and rooted deciduous trees, the inside strengthened with round timber. The plants are then topped with the earth obtained from the terrace above. Cross-laid willow branches (a method that is customary in some regions) will not take root as easily because cavities will remain under the branches, and willows grow roots only when fully covered with earth. The shrubs should stick out from the soil by not more than 100-150 mm to avoid drying and to give better support to the vertically growing new shoots. The spaces between layers are planted by hydroseeding.
In April 1986, immediately following installation of the drainage system, the hedge brush layers were put in place. They quickly took root and grew to 1 m within six months. The mezereon willows and gray alders in particular grew rapidly. The plants had such an immediate draining effect–at this time no more water was flowing at the outlet of the drainage pipes–that I was taken by surprise.
In June 2000, when I took students of the Vienna University of Agricultural Sciences to the stabilized slope within the scope of a soil bioengineering excursion, the shrubs were already grown to 4-6 m. Of the willows, only the mezereon willow is still highly visible. The others had withered away, having served their purpose. Gray alder and harewood dominate the stand, while spruce is appearing in the undergrowth. No tending has been performed so far, nor will it be necessary in the future.
Suldenbach/Prad-Stilfs Landslide
The raging thunderstorms of July 1987, which caused the Frana Val Pola landslide in the Veltlin Valley, blockage of the Adda River, and the deaths of several people, also ravaged the Ortler area and damaged roads, houses, and torrent-control systems at Sulden and Trafoi.
Two of the failures were stabilized as follows. As an emergency measure, consolidation dams were built at the Suldenbach rivulet upstream of Prad/Vinschgau using large blocks (laid in concrete), and the slope toe was similarly stabilized with boulders. Similar to many other landslides, this one had produced a cone of loose material, which we stabilized in the spring of 1989 using the hedge brush layer method described earlier. Here, however, the willows and rooted deciduous trees planted were more drought-resistant species such as the purple willow (Salix purpurea), hoary willow (Salix eleagnos), ash, and bird cherry. The adjoining steep slope sections (lateral glacier moraines rich in silicate) and spaces between hedge brush layers were planted by hydroseeding. Once again, the shrubs successfully took root, although not as well as in the Eggen Valley because of the drier climate in the Vinschgau: they grew to just 2-3 m after four years.
At the upper edge, the soil had been settling and cracking, causing the center section, which was stabilized by hedge brush layers, to bulge so that a supporting structure had to be installed.
Trafoierbach/Trafoi Landslide
The disastrous thunderstorms of July 1987 also caused a landslide at the Trafoierbach Stream, which–at a height of 150 m and a width of 100 m–was considerably larger than the two failures described above. The failure occurred at 1,400 m above sea level, and it had a gradient of 40-45° at the lower part and 55-65° above.
The loose debris facing down-valley was stabilized with hedge brush layers in the spring of 1992; the breaking and heavily eroding inner section above was supported by log crib walls in 1993. The very steep slope could not be slanted at the top edge because the spruce and larch growth above is just as steep (55-65°). Material must thus be expected to break away continuously, so that log crib walls could be constructed only at the less inclined lower half. The log crib walls are intended to retain the falling material and thus gradually reduce the inclination of the upper slope section. The walls were planted with altitude-resistant rooted species such as goat willow (Salix caprea), harewood, and mountain ash. Purple willow and whortle willow were added to the hedge brush layers. As shown in the photos, the layers grew well. The purple willow in particular grew to 2 m in just eight years (when the students walked through the layers, they were almost invisible from below). Plants in the log crib walls are constantly covered and damaged by falling material so that their growth is slow and stunted. The crib walls themselves retain the material as expected.
Looking at these two soil bioengineering methods, we clearly see that shrubs certainly have a draining and stabilizing effect, especially when they consist of pioneer plants such as willows that rapidly produce a root system of high tensile strength. Since willows have a limited life span on slopes (as compared to those growing at flowing water), woody successor plants are required to sustain and foster the plant population. But where extremely adverse forces prevail or where quick stabilization is required, plants on their own are limited in their effect. They require assistance from timber, which supports plants in many ways and passes on the stabilizing action when it rots.
