Slope Failure Restoration using Spyder Hoe Assisted Planting on Steep Slope

WALKER'S LANDING ROAD PROJECT

Location: Riondel, British Columbia

Client: Ministry of Transportation and Highways

Objective: To stabilize surficial materials, address surface and seepage water on the slide face and control surface erosion using biotechnical slope stabilization techniques. To establish vegetative cover to reduce rill and gully erosion.

Walker’s Landing Road is a public road located on the east shore of Kootenay Lake in Southeast British Columbia, Canada.

Diverted drainage from a gravel pit located above the road from the failure site caused the original slide. The initial slide occurred on February 2002 due to heavy rainfall events that resulted in increased storm water runoff. The increased velocity of this storm water runoff created a deep vertical scar on the downstream end of the culvert crossing Walker’s Landing Road. The highway maintenance contractor removed the culvert above the slope failure. The vertical scar was then filled with gravel, cobble, and small boulder material while compacted from the bottom up using an excavator. In December 2002 a second failure occurred depositing approximately 90 m3 of sand, coarse gravel and cobbles on the beach below. The original failure covers an area of approximately 18 m in width by 70 m slope distance. The slope angle ranges from 35 to 40 degrees. In March of 2003 Terra Erosion Control Ltd. in association with William H. Wells Consulting was retained to carry out a field assessment and to prescribe a biotechnical slope stabilization treatment.

In May 2003 Terra Erosion Control Ltd. implemented the prescribed treatments. An excavator was used to pull back oversteepened material at the head scarp to make the slope angle consistent with the remainder of the slope. Vegetated lifts, brush layers, fascines, live staking and live pole drains were installed with the combined assistance of a spyder hoe and manual laborers. The species component of the structures was 80% black cottonwood (Populus balsamifera ssp. trichocarpa) and 20% willow (Salix scouleriana). In order to enhance the moisture retaining capacity during the summer drought and provide additional nutrients to the site, a special blended growing medium was applied. These techniques will help stabilize the site by addressing surface erosion, directing surface and seepage water, and establishing a woody shrub component. A mix of grasses and legumes was broadcast, and native seedlings were planted to provide deep- rooted nitrogen fixation to the soils. The site was then monitored in the fall of 2003. Survival and growth were considered excellent even after an extremely hot and dry summer.

The site was visited again in July of 2004 and October 2005.  During the last site visit the survival was estimated to be approximately 90%.  Grasses and legumes were becoming established and native herbaceous species such as Epilobium ang. and Rubus par. were invading the site.   It was noted that the erosion control matting mesh used on a portion of the site was not biodegraded after a period of 3 years.  The average growth on the brush layers was 3.5 m, 2.2 m for the live pole drain, 3.3 m for the drain fascines and 1.5 m for the Alnus tenuifolia seedlings. 

Additional monitoring took place in the fall of 2014. See the photos at the bottom of the page.