The effect of forest fire on mass movement in Lebanese mountainous areas Rouba Ziade ´ A , Chadi Abdallah B,C and Nicolas Baghdadi A A IRSTEA, UMR TETIS, 500 rue J.F.Breton, BP 5095 Montpellier, F-34916 Montpellier, France. B National Council for Scientific Research and Remote Sensing Center, PO Box 11-8281, Beirut, Lebanon. C Corresponding author: chadi@cnrs.edu.lb Abstract. Mass movements are major hazards that threaten natural and human environments. In Lebanon, the occurrence of mass movements increased by almost 60% between 1956 and 2008. Forest fire has emerged as an additional hazard: it destroyed over 25% of Lebanon’s forests in a period less than 40 years. This paper investigates the potential effect of forest fire on the occurrence of mass movements in the Damour and Nahr Ibrahim watersheds of Lebanon. Mass movement and forest fire inventory maps were produced through remote sensing using aerial and satellite images. Forest fire was included as an additional factor in mass movement induction, and its effect was quantified from Landsat images through the normalised burn ratio (NBR) index. A field study was conducted to substantiate the mass movement inventory and NBR maps. Following the standardisation of the effect factors into layers using geographic information systems, the weight factor of each layer for inducing mass movements was evaluated using the modified InfoVal method, and a mass movement susceptibility map was generated. Exceeded only by changes in land cover, the NBR produced the highest weights, making forest fire burn severity the second highest factor influencing mass movement occurrence in the study areas. Additional keywords: GIS, InfoVAL, normalised burn ratio, remote sensing. Received 15 May 2013, accepted 12 March 2014, published online 1 August 2014 Introduction Mass movements such as landslides, earth flows, block falls and erosion are among the most invasive hazards in the Medi- terranean basin, causing surface instability and promoting land degradation (Poesen and Hooke 1997; Atkinson and Massari 1998; Duman et al. 2005; Bou Kheir et al. 2008; Goswami et al. 2011). In Lebanon, mass movement results in human deaths and causes damage to the natural and built environments esti- mated at US$10–$15 million annually (Abdallah 2012). The occurrence of mass movement is influenced by many pre- conditioning factors conveyed through terrain parameters, inc- luding geology, slope gradient, soil, mean annual rainfall and land cover (Zinck et al. 2001; Abdallah et al. 2005; Kamp et al. 2008 ; Hadji et al. 2013). Mass movement is generally triggered by events or factors such as abundant rainfall, earthquakes and land use changes (Rodrı ´guez et al. 1999; Huang et al. 2012; Mugagga et al. 2012). Forest fire is considered a significant, if not the primary, foundation of geomorphological and hydrological modification in fire-prone areas over both short and long time scales. Apart from its visible effect on reducing vegetation and litter, forest fire can be an essential rock-weathering agent (Journaux and Coutard 1974; Dorn 2003; Larsen et al. 2006), and is able to alter soil properties through the removal of organic material and deterioration of both soil structure and porosity (DeBano 2000; Andreu et al. 2001; Certini 2005; Shakesby and Doerr 2006; Moody et al. 2009; Stoof et al. 2010; Shakesby 2011). However, not all fires have the same effect on ecosystems. This mainly depends on the fire severity, fire intensity and burn severity, which in turn dictate the ecosystem response through runoff, erosion or even regeneration (Bento-Gonc ¸alves et al. 2012). Perhaps the most discussed effect of forest fire is enhanced water repellency, termed the hydrophobic characteristic of soil particles (DeBano 2000; Arcenegui et al. 2008; Rodrı ´guez- Alleres et al. 2012; Beatty and Smith 2013). Water repellency diminishes the attraction of soils to water, causing the soil to oppose wetting for periods ranging from a few seconds to hours, days, weeks or months (Doerr et al. 2000). Enhanced water repellency has considerable geomorphological and hydrological consequences, including reduced soil infiltration ability, increased overland flow and enhanced soil erosion. In general, water repellency in fire-affected Mediterranean-type climatic regions, such as in our Lebanese study area, has been reported in pine forests (Arcenegui et al. 2008). In contrast, the most commonly reported weathering effect of fire is spalling (Zimmerman et al. 1994), in which flakes of a substance are broken off a larger solid body, leading to surface failure. Vertical fracturing affecting an entire rock boulder has been reported, as have irregular linear and curvilinear fractures (Ballais and Bosc 1994). Fire-blackened boulder surfaces are CSIRO PUBLISHING International Journal of Wildland Fire http://dx.doi.org/10.1071/WF13077 Journal compilation Ó IAWF 2014 www.publish.csiro.au/journals/ijwf