Exploring how hydrophobic soil layers govern debris flow formation after wildfires


This project will explore, through laboratory testing, how the formation of hydrophobic soil layers initiates deadly debris flows after wildfires.

Project background

Debris flows after wildfires arise due to changes in the hydrologic function of the underlying soil. In the extreme heat, water storage in the organic litter layer is lost, deep infiltration decreases and water concentrates in the topsoil, weakening it and triggering instability. The key driver in this mechanism is that a hydrophobic soil layer forms at some depth below the soil surface: heating the soil to 175–280°C for durations of 5–20 minutes volatises the surface organic layer, which then infiltrates the soil. Volatised organic matter cools and condenses at shallow soil depths (1-5 cm), rendering the soil hydrophobic. The hydrophobic layer can degrade with a prolonged exposure to water. However, intense rainfall, striking the soil slope before the water repellency can degrade, leads to water runoff and, in some instances, a deadly debris flow. This effect is compounded by atrophy of the below-ground biomass due to soil heating, with the consequent loss of its stabilising biomechanical properties.

In this project, we will examine experimentally the combined mechanisms of the formation of the hydrophobic soil layer and the triggering of slope instability and debris flows, building upon our world-leading understanding of hydrophobic soil mechanics, wildfire behaviour, and debris flow phenomena. The results from the project will assist land management agencies, for example the United States Department of Agriculture Forest Service, to plan for, mitigate, and evacuate people from the paths of debris flows.

Research questions

  • What information exists in the literature to explore the occurrence of this multi-hazard, and what triggers and drivers exacerbate it?
  • What concentration of organic matter is needed for a hydrophobic layer to form?
  • How do the thermal gradient and thermal exposure time affect the degree of water repellency?
  • How deep must a hydrophobic layer be to trigger slope instability?
  • What rate of precipitation versus infiltration brings the slope to failure?


Year 1: Perform a systematic literature review to identify, amongst other aspects: critical soil types; heating rates (wildfire case studies); affected and critical topographies; experimental testing approaches; the contribution of water retention to slope stability

Year 2: Design and complete soil heating experiments. Examine the degree of water repellency and relate it to the material soil water retention behaviour and slope stability

Year 3: Design and complete slope stability and debris flow initiation experiments, incorporating hydrophobic layer formation from earlier experiments. Prepare articles for dissemination.


A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills.


A minimum of an Upper Second or equivalent Master's Degree in an Engineering or Geosciences discipline with a focus on soils behaviour, fire dynamics, or debris flow analyses.


DeBano, L. F. 2000. Water repellency in soils: a historical overview. Journal of Hydrology, 231--232, 4-32

Doerr, S. H., Shakesby, R. A. & MacDonald, L. H. 2009. Soil water repellency: a key factor in post-fire erosion?. Restoration strategies after forest fires

Lourenço, S. D. N., Wang, G.-H. & Kamai, T. 2015. Flume tests in wettable and hydrophobic sands: insights into the initiation of wildfire-related debris flows. Proceedings of the 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering, 1085-1088


Apply by Thu Jan 04 2024 at 12:00


Further Information: 

The University of Edinburgh is committed to equality of opportunity for all its staff and students, and promotes a culture of inclusivity. Please see details here: https://www.ed.ac.uk/equality-diversity

Closing Date: 

Thursday, January 4, 2024
Debris flow formation: a) vegetated slope; b) wildfire creates a thermal gradient within the soil and volatises organic matter; c) formation of a hydrophobic soil layer; d) rainfall saturates upper soil layers; e) soil slope stability is exceeded and a debris flow initiates

Principal Supervisor: 

Assistant Supervisor: 


Minimum entry qualification - an Honours degree at 2:1 or above (or International equivalent) in a relevant science or engineering discipline, possibly supported by an MSc Degree. Further information on English language requirements for EU/Overseas applicants.


Tuition fees + stipend are available for Home/EU and International students

Informal Enquiries: