A Multi- Factor Approach to Landslide and Flood Susceptibility Mapping

ByJacob Opadeyi Phd, Leah Fouchong BSc, Gabrielle Thongs MSc, and Roxann Smith BSc

Department of Geomatics Engineering and Land ManagementThe University of the West Indies

St. AugustineTrinidad

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,

Wrightson Road, Western Main Road in Cocorite, South Quay, Maraval River

Flood Events in Trinidad

Eastern Main Road

Trinidad Guardian 1 Nov 2010

Landslide Events in Trinidad

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.

• 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.

Instability Factors (Con’t)

Instability Factors

Classifications Susceptibility Level Rank

Lithological Formation

FormationsAlluvium, Navet, Palmiste Very High 5Moruga, Lengua, Brasso, Manzanilla, Maracas, Galera High 4

Toco, Maraval, Rio Seco, Springvale, Nariva, Chaudiere, Cipero, Morne L'Enfer,Mud Volcanoes Cones & Flows,Gros Morne, Karamat, Mayaro, San Fernando

Moderate 3

Cedros, Diorite, Laventille,Guayamara, Tompire, Chancellor, Guayamara, Talparo, Lopinot, Cushe, Cunapo, Concord, Gautier, Naparima, Lizard Springs,Chaudiere

Low 2

Sans Souci, Arima, Water, Pointe-a-Pierre, Cruse, Erin Very Low 1

Classification and Landslide Factor Ranking

Classification and Landslide Factor RankingInstability Factors

Classifications Susceptibility Level

Rank

Tectonic Features

Proximity (m)0-100 Very High 5100-200 High 4200-300 Moderate 3300-400 Moderate 3400-500 Low 2500-600 Low 2> 600 Very Low 1

Slope Angle Angles (degrees)48 - 87 Very high 527- 48 High 415 - 27 Moderate 35 - 15 Low 20 - 5 Very Low 1

Tectonic Features

Classification and Landslide Factor RankingInstability Factors

Classification Susceptibility Level

Rank

Road Network Proximity (m)

0-100m Very high 5

101-200m Very high 4

201-300m High 3

301-400m High 3

401-500m Moderate 2

501-600m Moderate 2

>600m Low 1

Classification and Landslide Factor RankingInstability Factors

Weight Classifications Susceptibility Level

Rank

Drainage Network

0.06 Proximity (m)

0-100m Very high 5

101-200m Very high 4

201-300m High 3

301-400m High 3

401-500m Moderate 2

501-600m Moderate 2

601-700m Moderate 2

>700m Low 1

Classification and Landslide Factor RankingInstability Factors

Classifications Susceptibility Level

Rank

Land Use ClassesBarren land Very High 5Urban or built-up land High 4

Rangeland Moderate 3Agricultural land Low 2Forest land Very Low 1Wetland Very Low 1Water Very Low 1

Classification and Landslide Factor Ranking

Factors Classification Scheme Rank Rating

Rainfall (mm)

178- 226 Very High 5

126 - 178 High 4

78 - 126 Low 3

40 - 78 Moderate 2

1 – 40 Low 1

Cartographic Model - Landslide

Ranking Weighting

Landslide Factor Weighting: Scenario A

Instability FactorsScenario A %

Tectonic Features 1 0.14Lithological Formation 1 0.14Slope Angle 1 0.14Land Use 1 0.14Drainage Network 1 0.14Road Network 1 0.14Rainfall 1 0.14Total 7 1

Landslide Factor Weighting: Scenario B

Instability FactorsScenario B

%

Tectonic Features 2 0.18

Lithological Formation 2 0.18

Slope Angle 2 0.18

Land Use 2 0.18

Drainage Network 1 0.09

Road Network 1 0.09

Rainfall 1 0.09Total 11 1

Landslide Factor Weighting: Scenario C

Instability FactorsScenario C

%

Tectonic Features 1 0.09

Lithological Formation 3 0.27

Slope Angle 3 0.27

Land Use 1 0.09

Drainage Network 1 0.09

Road Network 1 0.09

Rainfall 1 0.09Total 11 1

Landslide Factor Weighting: Scenario D

Instability FactorsScenario D

%

Tectonic Features 1 0.08

Lithological Formation 3 0.23

Slope Angle 3 0.23

Land Use 3 0.23

Drainage Network 1 0.08

Road Network 1 0.08

Rainfall 1 0.08Total 13 1

Landslide Factor Weighting: Scenario E

Instability FactorsScenario E

%

Tectonic Features 2 0.20

Lithological Formation 2 0.20

Slope Angle 2 0.20

Land Use 1 0.10

Drainage Network 1 0.10

Road Network 1 0.10

Rainfall 1 0.10Total 10 1

Factor Modelling - Flooding

Flood Factors• Rainfall (mm) • Slope (%) • Land Use • Drainage density • Road (% each watershed) • Elevation (m)

Flood Factor Ranking

Factors Classification Scheme Rank Rating

Rainfall (mm)

1 - 40 Low 140 - 78 Moderate 278 - 126 Low 3126- 178 High 4178- 226 Very High 5

Elevation (m)

477.0 – 931.4 Very low 1

286.4 – 477.0 Low 2

147.1 – 286.4 Moderate 3

55.5 – 147.1 High 4

0 – 55.5 Very High 5

Flood Factor RankingFactors Classification

Scheme Rank Rating

Slope (%)

569.1– 2748.7Very low 1

257.7– 569.1 Low 2

85.9 – 257.7Moderate 3

21.5 – 85.9 High 4

0 - 21.5Very High 5

Drainage density (% each watershed)

0 – 0.004674Very low 1

0.004674 – 0.006769Low 2

0.006769 – 0.0083Moderate 3

0.0083 – 0.009508High 4

0.009508 – 0.020628Very High 5

Factors Classification Scheme Rank Rating

Road (% each watershed)

0.000194 – 0.004626Very low 1

0.004626 – 0.009335Low 2

0.009335 – 0.018198Moderate 3

0.018198 – 0.039249High 4

0.039249 – 0.071101 Very High 5

Land Use (related to water absorption and drainage capacities)

Wetland, Forest< Water Very Low 1

Rangeland (Non-wetland) Low 2

Agricultural land Moderate 3

Barren land High 4

Urban or built-up land Very High 5

Flood Factor Ranking

Cartographic Model - FloodRanking

Weighting

Flood Factor Weighting: Scenario A Flood Factors Scenario A %

Rainfall (mm) 1 0.17Slope (%) 1 0.17Land Use 1 0.17Drainage density 1 0.17

Road (% each watershed) 1 0.17

Elevation (m) 1 0.17Total 6 1

Flood Factor Weighting: Scenario B

Flood Factors Scenario B %

Rainfall (mm) 2 0.22

Slope (%) 2 0.22

Land Use 2 0.22

Drainage density 1 0.11

Road (% )1 0.11

Elevation (m) 1 0.11

Total 9 1

Flood FactorsScenario C %

Rainfall (mm) 3 0.33

Slope (%) 2 0.22

Land Use 2 0.22

Drainage density 0.5 0.06

Road (%)0.5 0.06

Elevation (m) 1 0.11

Total 9 1

Flood Factor Weighting: Scenario C

Flood FactorsScenario D %

Rainfall (mm) 2 0.22Slope (%) 3 0.33Land Use 1 0.11Drainage density 1 0.11

Road (% )1 0.11

Elevation (m) 1 0.11Total 9 1

Flood Factor Weighting: Scenario D

Flood FactorsScenario E %

Rainfall (mm) 2 0.22

Slope (%) 2 0.22

Land Use 3 0.33

Drainage density 0.5 0.06

Road (% )0.5 0.06

Elevation (m) 1 0.11

Total 9 1.00

Flood Factor Weighting: Scenario E

Users / Uses of Susceptibility Maps

• The occurrence map can be used to identify areas where historical landslide or flood activities and areas susceptible to future landslide or flood.

• This will highlight potential regions of physical instability and inundation.

• As such , the landslide and flood susceptibility map will prove to be a practical tool in hazard mitigation planning, urban planning, natural resource management and disaster response management.

Suggested UsersSuggested users of this information include but are not limited to them :

• Landuse Planners• Property Developers• Geologists and Geographers• Disaster Management Officers• Local Government Agencies/ Regional Co-operations• Landslide technical specialists• Environmental planners• Researchers • General Public

Suggested Uses

Uses of landslide and flood susceptibility maps:

• Landuse Planning - Provides planners with a practical and cost-effective way to zone areas susceptible to landsliding.

• Development Control - Can be used as a tool to help identify land areas best suited for development (to determine landuse capability) by examining the potential risk of landsliding

• Building Regulation

• Negotiating Insurance Premiums

Thank You