Role of Vegetation in Slope Stability

• By John Bornsworth
• October 20, 2015

Vegetation (trees, shrubs, grasses, flower and ground covers) helps stabilize slopes in numerous ways. Native plants provide wildlife habitat, are adapted to our native soils, can adapt better to climate disruptions, and interact with each other in ways hybrid/exotic plant communities do not. If you have indicators of potentially unstable slopes, call us. Peninsula Environmental Group’s arborists and ecologists are trained in marine and freshwater shoreline processes and their plant communities.

Vegetation management of urban, forested, coastal, marine shorelines, freshwater shorelines, and other riparian zones should conserve and maintain plant cover to reduce surface soil erosion and landslide styled movements of soil. The relative effectiveness of vegetation in any site will be a function of quality of vegetation, topography, slope, hydrology, geology and soils.
Surface Soil Erosion (particle by particle erosion) Ways in which vegetation and specifically herbaceous plants prevent soil erosion include (adapted from Gray 1982)

1. Interception – foliage and plant residues absorb rainfall energy and prevent soil compaction from raindrops.
2. Restraint – root system physically binds or restrains soil particles while above ground residues filter sediment out of rain and stormwater runoff.
3. Retardation – above-ground residues increase surface roughness and slow velocity of runoff.
4. Infiltration – roots and plant residues help maintain soil porosity and permeability
5. Transpiration – depletion of soil moisture by plants delays onset of saturation and runoff.

Erosion occurs when rainfall dislodges soil particles and carries them off a slope, forming rills and gullies that can trigger landslides. Raindrops hitting the soil surface can also seal the soil particles and make a crust that prevents infiltration and increases surface flow speeds, creating runoff. Of all these processes, retardation (slowing) of mobile surface waters and infiltration (increased soil porosity) both substantially reduce impacts to particle soil erosion. Soil structure also plays a substantial role. Interception (absorption) of rainfall energy and transpiration (soil moisture wicking) reduce heavily in the winter when plants drop their leaves. Evergreen plants, as opposed to deciduous plants, do retain interception qualities throughout the winter, but their ability to wick moisture from the ground is still heavily reduced.

The primary factors of vegetation that affect mass-movements in slopes, particularly shallow sliding in slopes include (adapted from Gray 1982):

1. Root reinforcement – roots mechanically reinforce a soil transfer of shear stresses in the soil to tensile resistance in the roots.
2. Soil moisture modification – Transpiration and interception by the foliage limit buildup of soil moisture stress.
3. Buttressing and arching – anchored and embedded stems can act as buttress piles or arch abutments in a slope, counteracting shear stresses.
4. Wind-throwing – destabilizing influence from turning moments exerted on a slope as a result of strong winds blowing downslope through trees.

Root reinforcement, soil moisture modification (reduction), and buttressing and arching normally increase slope stability. Surcharge, and wind-throwing are having a net destabilizing effect. My experience is that the net destabilizing effects of trees (surcharge and wind-throwing) are minimal compared to the benefits healthy plants and healthy root systems provide to surface soils.

To learn more about Peninsula Environmental’s slope rehabilitation services click here.

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*D.H. Gray and A.T. Lesier, Biotechnical Slope Protection and Erosion Control: Van Nostrund Reinhold, New York, 1982. Print.

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