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A Multi- Factor Approach to Landslide and Flood Susceptibility Mapping. By Jacob Opadeyi Phd , Leah Fouchong BSc, Gabrielle Thongs MSc, and Roxann Smith BSc Department of Geomatics Engineering and Land Management The University of the West Indies St. Augustine Trinidad. Background.
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A Multi- Factor Approach to Landslide and Flood Susceptibility Mapping By Jacob OpadeyiPhd, Leah Fouchong BSc, Gabrielle Thongs MSc, and Roxann Smith BSc Department of Geomatics Engineering and Land Management The University of the West Indies St. Augustine Trinidad
Background • Trinidad and Tobago faces a number of natural disaster issues. • Although the islands are located to the south of the Caribbean, where there is very little threat from hurricanes , the islands are still subject to tropical storms and the associated heavy showers that result in flooding and landslides. • The Northern Range and Central range ,often referred to as the Caroni Basin is considered most vulnerable to the impacts of these heavy showers. • Furthermore the Caroni Basin is under threat from poor landuse practices which include deforestation of the Northern Range, which results in landslides and perennial flooding of the lower regions of the basin.
Background • Greater Port of Spain , the capital is estimated to have a current population of 130,000, with the city itself having some 40-45,000 inhabitants. • Despite this POS is considered one of the main areas for future development regardless of the fact that it is physically constrained and can only handle so much infrastructure and population • The historical growth of the city gives an indication of the urban sprawl, now being experienced in the main urban areas of Trinidad and Tobago. • Physical planning constraints for the city include flooding problems where drainage channel capacity is undermined through the presence of high sedimentation and high amounts of debris. • This is compounded by the areas of informal and formal developments on steep slopes
Background • Flooding has continued perennially throughout Trinidad, particularly flash flooding along the east west corridor at the foothills of the Northern Range. • This flooding often results in landslides on the hillsides. • Floods and Landslides are annual events associated with the rainy season in Trinidad and Tobago. They have caused millions of dollars damage to infrastructure, agriculture and households, floods in particular have even taken lives. • Landslides are a common occurrence in hilly areas on both islands. • Vulnerable communities for floods are generally low-lying areas , in particular: • Port of Spain, • San Juan, • Caroni, • Sangre Grande, • Caparo,
Flood Events in Trinidad • Wrightson Road, • Western Main Road in Cocorite, • South Quay, Maraval River
Landslide Events in Trinidad Eastern Main Road • Trinidad Guardian 1 Nov 2010
Introduction Hazard= Probability of event with a certain magnitude •Triggering mechanism •Type of event •Magnitude: Volume, domino effect, distance, duration, speed etc Vulnerability= Degree of damage. Function of: •magnitude of event, and •type of elements at risk Amount= Quantification of the elements at risk e.g. •Replacement costs of buildings, infrastructure etc. •Loss of function or economic activities •Number of people Source: van Westen, 2005
What is Susceptibility Mapping? • Susceptibility mapping depicts division of land surface into zones of varying degree of stability based on the estimated significance of the causative factors in including the instability (Anabalagan 1992). • Thus susceptibility should be examined as the likelihood that such phenomena occur under the given terrain conditions regardless of the time scale within which a particular event is likely to take place (van Westen et al. 1997, Atkinson & Massari 1998). • The susceptibility term also does not depend on impact factors such as rainfall, earthquake and human activity (Lee & Min 2001).
What is a landslide and flood susceptibility maps? • The landslide and flood susceptibility maps shows areas vulnerable to landsliding based on an estimated significance of causative factors in inducing the instability. • The purpose of the susceptibility map is to convey the relative likelihood of landsliding and flooding using susceptibility categories. • Many susceptibility maps use a color scheme that relates warm colors (red, orange, and yellow) to unstable and marginally unstable areas, and cool colors (blue and green) to stable areas (USGS).
Methodology The selected instability factors were supported by literature, local knowledge and the availability of input data. The factors considered are : • Lithological Formation • Tectonic Features • Slope Angle • Road Network • Drainage Network • Land Cover • Rainfall
Instability Factors • Lithology- rock characteristics are strongly related to the geomorphologic and geologic units of the study area. This is important for landsliding as it is strongly correlated to the nature of the material , steepness of the slope , and its stability . Unconsolidated and less cohesive steep slopes increases the susceptibility to landsliding. • Tectonic Features – landslides are known to be more frequent in areas that are in close proximity to tectonic features such as strikes and dip faults. As the distance away to tectonic features increases, the susceptibility to landslides is likely to decrease. Consequently , areas that are close to tectonic features are more prone than areas far away.
Instability Factors (Con’t) • Slope Angle – contributes to mass movements in terms of acceleration. Steeper slopes are more susceptible than gentle slopes • Road Network- changes in land natural gradient cause by road construction are noted to sometimes influence the occurrence of landslides as a result of ground subsidence activities • Drainage network – areas that are closer to rivers and streams are more susceptible to landslides because of their ability to have higher moisture contents than areas farther away.
Instability Factors (Con’t) • Land Cover – is a well known trigger to landslides. Disturbed vegetation, agriculture on hilly slopes and building construction on unstable slope geological features are examples of landuse activities likely to cause landslides • Rainfall – landslides are more likely to occur in the wet season than in the dry season. They are often associated with heavy rainfall events. Areas with high rainfall are more susceptible to landslides than areas with low rainfall.
Cartographic Model - Landslide Ranking Weighting
Flood Factors • Rainfall (mm) • Slope (%) • Land Use • Drainage density • Road (% each watershed) • Elevation (m)
Cartographic Model - Flood Weighting Ranking
