Physically-based dynamic modelling of the effect of land use changes on shallow landslide initiation in the Western Ghats of Kerala, India
Summary of PhD thesis to fulfil the requirements for the degree of Doctor on the authority of the Rector Magnificus of the University of Utrecht Prof. Dr. J.C. Stoof publicly defended on Monday 13 December 2010 at 12.45 in the Academiegebouw, Utrecht , Netherlands by Sekhar Lukose Kuriakose.
The highland region forming the western slopes of the Western Ghats of Kerala state, India, is increasingly affected by shallow landslides and consequent debris flows. Many researchers suspected that the spatio-temporal probabilities of shallow landslide initiation in this area are dependent on the land use which in turn determines the mechanical and hydrological effects of vegetation on slope stability. A physically-based spatially distributed shallow landslide initiation model named STARWARS (Storage and Redistribution of Water on Agricultural and Revegetated Slopes) + PROBSTAB (Probability of Stability) model was identified as the most suitable for this research. This model was calibrated and validated (Figure 1) for the Aruvikkal catchment (Figure 2). Climatic and hydrological data necessary for this was acquired from 13 automated open stand pipe piezometers, one automated discharge station (stage gauge) and one automated weather station starting from May 2007 onwards. Time series analysis revealed that the discharge of the catchment will respond within 1 hr after a rainfall event while perched water level may need about 6 hrs.
Figure 1: Observed stream discharge (Obs) and rainfall compared to the stream discharge predicted by STARWARS (Calibration period: Feb to Sep 2008; Validation period: Oct to Dec 2007)
Figure 2: The relative location of Aruvikkal catchment and the distribution of monitoring instruments in the catchment
Soil properties necessary and the soil depth were measured at representative locations and spatially interpolated using geostatistical techniques. Depending on the dominant plant species in each land use unit and the soil depth the corresponding root reinforcement applicable was derived from measured root tensile strength, pullout strength (Figure 3), root diameter and root density data using the perpendicular root model. Nine species of plants were tested of which Teak (Tectona grandis) trees offered the highest amounts of net root reinforcement.
Figure 3: Root pullout test in progress in an experimental pit in the study area
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In order to evaluate the effects of long term and short term land use changes on slope stability in the region, historic (1913 to 2008) land use maps, soil depth maps and root reinforcement maps were derived. In addition the land use maps and corresponding root reinforcement maps of two future scenarios (2016 and 2058) were also derived. These data sets were used to simulate the corresponding slope stability and probability of failure conditions based on the 1985 rainfall time series which included the most extreme daily rainfall that has caused at least one shallow landslide in the study area in the past 57 years. |
In light of the empirical observations and the modelling results it can be concluded that a continuous rain storm of 4 to 26 hrs, totaling about 106 to 290 mm can cause a steep rise in the perched water table up to critical levels in regolith filled bedrock depressions and the persistence of this level for ~10 hrs may lead to shallow landslides in the study area. It was evident that land use changes which occurred in the early part of 20th century have reduced the root cohesion and altered the soil depth significantly by terracing. The land use changes from the pre-plantation (1913) scenario to present (2008) have resulted in an average increase in the potential area of failure by 43% and the spatio-temporal probability of failure by 49% (Figure 4).
Figure 4: Predicted factor of safety (FOS) and probability of failure (PF) maps of Aruvikkal catchment, corresponding to the land use scenarios of 1913, 1973, 2001, 2008, 2016, 2058 and the no-root cohesion condition (NRC), considering a 100 year return interval daily rainfall timeseries
Thus, the research showed that in the study area the transition probability of land use change (and consequently the changes in soil depth and root cohesion) outweighs the rainfall quantity in determining the spatio-temporal probability of shallow landslide occurrence. This is in contradiction with the commonly held belief that the temporal probability of shallow landslides can be quantified with only the return probability of landslide-causing extreme rainfall events. Hence, transition probabilities of land use should be assessed and incorporated in regional scale landslide hazard assessments that utilize heuristic and stochastic techniques, especially in such anthropogenically modified terrains.
The outputs of this research was published as 6 peer reviewed ISI articles, an article to Encyclopedia of Agrophysics (Springer Publication), an invited contribution to the FAO Status-of-Knowledge Collaborative Report on Forests and Landslides, one book chapter (Springer Publication), a solicited oral presentation in the European Geosciences Union (2008), 7 conference publications and 4 pre-prints for ISI journals which are in the different stages of peer review. Three M.Sc students, namely Mr. Sanjaya Devkota (ITC, M.Sc AES 2008), Ms. Tuba Zhara (ITC-IIRS, M.Sc AES 2010) and Mrs. Bindhu C. Thomas (ITC, M.Sc GEM 2010) contributed to this research through their respective M.Sc thesis.
The research was supervised by Prof. Dr. V.G Jetten (ITC), Prof. Dr. Steven de Jong (UU), Dr. C.J van Westen (ITC), Dr. L.P.H van Beek (UU) and Mr. G. Sankar (CESS) with support from the Faculty of Geoinformation Science and Earth Observation (ITC), University of Twente, Centre for Earth Science Studies (CESS; www.cessind.org), Thiruvananthapuram, India and Department of Physical Geography, University of Utrecht (UU).
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Curriculum Vitae |
Sekhar Lukose Kuriakose was born on 17 March 1979 in India. After finishing his BSc Geography at the University of Kerala, he continued his MSc Applied Geography at the University of Madras in India. From 2004 till 2006 he joined the MSc in Geo-information Science and Earth Observation under the IIRS-ITC Joint Education Programme. In 2006 he started his PhD in Physical Geography at the University of Utrecht and the International Institute for Geo-information Science and Earth Observation (ITC) which he finished in 2010.
He worked as a GIS consultant at the Prambikulam Wildlife Sanctuary, Dept. of Forests and Wildlife, Govt. of Kerala, India from 2001 till 2003. From 2002 till 2004 he was working as a GIS consultant at Enter Technologies Pvt. Ltd, Technopark, Thiruvananthapuram, India while he was working as Management Plan and GIS specialist at the Gulf of Mannar Marine National Park, Dept. of Forests and Wildlife, Govt. of Tamil Nadu, India before starting his MSc in 2004.
His research interests include Monitoring and modelling of earth surface processes, with special interest on shallow landslides; Mechanical effects of vegetation on earth surface processes; Scientific philosophy, research methods and history of science; Sensitivity and uncertainty analysis; and GIS and EOS for hazard, vulnerability and risk assessment.