Preliminary numerical studies have shown that rhizosphere is likely toplay a key role in controlling the initiation of shallow landslides at Rest and Be Thankful. As a result, plants with root system architecture that enhances lateral subsurface flow shold be privileged when designing vegetation based remedial measures. This project aims at i) investigating quantitatively the role of the rhizosphere in controlling slope hydrology and, hence, slope instability and ii) propose and demonstrate a remedial measure for the Rest and Be Thankful slopes based on the use of vegetation to ‘engineer’ the rhizosphere. Rainfall is a major triggering factor of shallow landslides. When heavy and/or prolonged rainfall events occur, water infiltrates into the slope causing pore-water pressures build-up. This reduces soil shear strength eventually triggering slope instability.
Rainfall-induced shallow landslides may evolve into debris flows. These are characterised by high velocities and long travel distance and are a main cause of property and infrastructure damage, injury, and death. An example of fast-moving rainfall-induced landslides is given by Rest and Be Thankful (A83) in Scotland, which has been subject to frequent landslide activity in recent years with two road closures per annum on average recorded over the last five years.
It is widely acknowledged that the presence of vegetation can influence landslide occurrence and, for this reason, vegetation is often regarded as potential remedial measure. Plants can significantly enhance slope stability by reinforcing the soil due to root anchoring effects and by promoting soil water extraction via transpiration.
There is indeed a third effect that has thus far been little explored. The root system can promote subsurface lateral flow by creating networks of preferential flow. The rhizosphere, the upper portion of soil profile affected by root secretions and soil microorganisms, can therefore act as a lateral drainage, i.e. diverting rainwater from infiltrating downward. However, there are no studies assessing quantitatively the actual impact of the hydraulic ‘diversion’ promoted by the rhizosphere on landslide occurrence.