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Final Project Report

Development of Landslide Hazard Maps for

St. Lucia and Grenada

The Caribbean Development Bank (CDB) and

The Caribbean Disaster Emergency Response Agency (CDERA)

February, 2006

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Development of Landslide Hazard Maps for St. Lucia and Grenada

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Table of Contents Executive Summary ........................................................................................................................................ 3 1. Introduction ............................................................................................................................................ 6

1.1. Project Understanding ................................................................................................................. 6 1.2. Project Inception Mission............................................................................................................. 6 1.3. Project Study Areas and Expected Map Outputs......................................................................... 7

2. Method of Investigation .......................................................................................................................... 8 2.1. Landslide Inventory Map.............................................................................................................. 9 2.2. Base Map Preparation and Analysis.......................................................................................... 10

2.2.1. Digital Elevation .................................................................................................................... 10 2.2.2. Characterization of Slope...................................................................................................... 11 2.2.3. Characterization of Aspect .................................................................................................... 11 2.2.4. Characterization of Geology.................................................................................................. 12 2.2.5. Characterization of Soils ....................................................................................................... 14

2.3. Base Map Classification and Factor Map Development ............................................................ 17 2.3.1. Geologic and Physiographic Attributes of Landslide Distribution in Saint Lucia.................... 17 2.3.2. Geologic and Physiographic Attributes of Landslide Distribution in Grenada ....................... 24

2.4. Hazard Model & Susceptibility Map Development ..................................................................... 32 3. Hazard Map Description....................................................................................................................... 34

3.1. Landslide Hazard Map Description: Saint Lucia ........................................................................ 34 3.2. Landslide Susceptibility Map Description: Grenada................................................................... 36 3.3. Map Utilization ........................................................................................................................... 38

3.3.1. General Statement: Map Use and Limitations....................................................................... 38 3.4. Specific Recommendations for Utilization of landslide Hazard Maps ........................................ 39

3.4.1. Development Planning Considerations ................................................................................. 39 3.4.2. General Site Planning and Design Considerations ............................................................... 40 3.4.3. Landslide Specific Considerations ........................................................................................ 40

4. Hazard Mapping Workshop.................................................................................................................. 42 4.1. Workshop Goals ........................................................................................................................ 42

4.1.1. Landslide Hazard Presentation ............................................................................................. 42 4.2. Workshop Observations and Issues .......................................................................................... 42

4.2.1. Workshop Observations and Issues: Grenada...................................................................... 43 4.2.2. Workshop Observations and Issues: Saint Lucia.................................................................. 43

4.3. Workshop Conclusions and Recommendations ........................................................................ 44 4.3.1. Recommendations for Landslide Hazard Mapping: Grenada ............................................... 44 4.3.2. Recommendations for Landslide Hazard Mapping: Saint Lucia............................................ 44

5. Appendices .......................................................................................................................................... 46 5.1. Project Team Members on Inception Mission............................................................................ 46 5.2. Inception Mission Itinerary ......................................................................................................... 47 5.3. Brief on Inception Meetings ....................................................................................................... 49 5.4. Attendance Sheets for Inception Meetings ................................................................................ 51 5.5. Hazard Workshop Presentations ............................................................................................... 52

6. References........................................................................................................................................... 53



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EXECUTIVE SUMMARY The Caribbean Development Bank (CDB) through the Disaster Mitigation Facility for the Caribbean (DMFC), and the Caribbean Disaster Emergency Response Management Agency (CDERA) through the Caribbean Hazard Mitigation Capacity Building Program (CHAMP), have collaborated on a multi-phased project to support the development of national hazard mitigation plans in Saint Lucia, Grenada and Belize. The hazard mitigation planning process is expected to extend over a two and half year timeframe from November 2003 to March 2006 and comprises five distinct but inter-related phases. The Hazard Mapping and Vulnerability Assessment (HMVA) phase of the national hazard mitigation plan development process is especially important by providing a firm foundation for the development of goals, objectives, intervention strategies and mitigation actions for each of the three pilot states. The development of landslide hazard maps for Saint Lucia and Grenada is one component of a series of ongoing hazard mapping consultancies intended to inform vulnerability assessments in each of the three pilot states. The vulnerability assessments, in turn, will inform the national governments and Plan Development Committees (PDCs) of the existing vulnerability in order to develop and prioritize hazard risk reduction strategies and actions. The landslide mapping project was initiated by combined project inception meetings and field reconnaissance in Saint Lucia and Grenada. The consultant project team met with the respective HMVA Subcommittees to discuss the technical approach to developing the landslide susceptibility maps. In Saint Lucia, it was decided to expand the initial boundaries of the Pilot Study Area in the Castries watershed to include additional lands to the north where landslides were known to have occurred. Both HMVA Subcommittees requested that the consultant project team include in the final report recommendations on best management practices to reduce the frequency and severity of landslide events. For Grenada, the major study area covers the entire island but does not include the adjacent island of Carriacou, which also falls under Grenada’s jurisdiction. In consultation with CDB and the Government of Grenada, the Pilot Study Area was shifted to the area surrounding the community of Florida which was found to be more prone to landslides and landslips than the initially proposed study area in St. George and St. David parishes. The field reconnaissance spanned over five days in early September of 2005 and was conducted by an engineering geologist, environmental planner and geographer, in addition to local government representatives with intimate knowledge of the islands who helped locate recent and historical landslide events. Once landslides were located, the field reconnaissance team took a Geographical Positioning System (GPS) reading and then evaluated the physiographic, geologic and human influences that may have played a role in causing the landslide. The field reconnaissance review of previous technical studies and evaluation of the spatial distribution of landslide events led the consultant project team to identify five factors that were most important in causing landslides. They include:

• Slope – the steepness of the hillslope, expressed as a percentage

• Slope Aspect – the orientation of the hillslope to the prevailing winds

• Elevation – used as a surrogate for the influence of rainfall intensity

• Geology – the underlying bedrock units from geologic surveys

• Soils – soil mapping units from soil surveys



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A geographic information System (ArcGIS), a computer-driven geographic mapping program, was used to overlay and combine the physical, geologic and soils data necessary to create the landslide susceptibility maps. A hazard mapping methodology was developed that systematically combines factors in a GIS model to provide insight into landslide susceptibility. The mapping methodology can be divided into four steps: Step One – Landslide Inventory Map. Landslide occurrences were mapped and observations noted included landslide type, location along hillslope, slope angle, depth of landslide whether shallow or deep seated, and the nature of the bedrock including rock type and degree of weathering. This inventory will be provided in both hard copy and digital formats to provide a landslide chronology that can be updated over time. Step Two – Base Map Preparation. A series of base maps related to the five factors noted above were compiled from the Common Digital Database (CDD), prepared under a previous CDERA/CDB consultancy. The base maps were converted to particular formats for each island to facilitate analysis at the proper map resolution. Step Three – Base Map Classification and Factor Map Development. The landslide inventories allowed the project team to determine the frequency of landslide events that occurred within different geologic and soil mapping units, in addition to selected categories of elevation, slope angle and slope aspect. The project team was able to query the database to determine the number of landslides within each category, calculate the percentage of landslides within that category, determine the total area of a category and finally, the percentage of area for that category compared to the total area of the study area. These calculations allow an estimation of relative landslide susceptibility based on a ratio comparing the percentage of land in a specific category to the percentage of landslides mapped in that same category. This ratio provided the project team with a map for each factor influencing landslide susceptibility and quantitative means to rank the relative importance of each factor. Step Four – Hazard Model and Susceptibility Map Development. A susceptibility mapping model was prepared for each island that reflected the varying influences of slope, elevation, aspect, geology and soils. The two models used a simple mathematical overlay process that adds the susceptibility ranking for the corresponding cells of each factor map together. The output of the model was reclassified into five susceptibility categories: Very Low, Low, Moderate, High, and Severe. These five categories provide an indicator of landslide susceptibility in the Pilot Study Area on St. Lucia and throughout Grenada. The landslide susceptibility maps for Saint Lucia and Grenada will be distributed to the respective governments in several different formats. In addition to a digital format, hard copy maps for the Saint Lucia Pilot Study Area will be provided at a ratio of 1:10,000. For Grenada, digital and hard copy formats will be provided at 1:25,000 for the island-wide map and at 1:10,000 for the Pilot Study Area surrounding the village of Florida. There are several limitations that must be considered in utilizing the landslide susceptibility maps for Saint Lucia and Grenada. The five categories represent the potential for landslide events not their actual occurrence. Study limitations included the time available for the field work and field evaluations constrained by road access to landslide sites. For major road and infrastructure projects, site-specific geologic investigations are recommended. That being said, the landslide susceptibility maps provide an important tool for development review and physical planning functions. The maps are an important input for any vulnerability assessment in that they provide an understanding of landslide hazard and contribute to the development of national hazard mitigation plans. Section 3 of the report provides a series of recommendations on utilizing the landslide susceptibility maps and best management practices for minimizing the potential for man-caused landslide events. They include



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development planning considerations, general site planning considerations and specific recommendations for reducing landslides alongside roadside cut and fill slopes. The governments of St. Lucia and Grenada might want to consider a tiered review process where developers would be required to submit additional information through the Environmental Impact Assessment (EIA) process on major development projects areas while a more simplified checklist approach could be used for single family residential construction.



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1. INTRODUCTION

1.1. PROJECT UNDERSTANDING

The Caribbean Development Bank (CDB) through the Disaster Mitigation Facility for the Caribbean (DMFC), and the Caribbean Disaster Emergency Response Management Agency (CDERA) through the Caribbean Hazard Mitigation Capacity Building Program (CHAMP), have collaborated on a multi-phased project to support the development of national hazard mitigation plans in Saint Lucia, Grenada and Belize. The hazard mitigation planning process is expected to extend over a two and half year timeframe from November 2003 to March 2006 and comprises of five distinct but inter-related phases. The Hazard Mapping and Vulnerability Assessment (HMVA) phase of the national hazard mitigation plan development process is especially important for providing a firm foundation for the development of goals, objectives, intervention strategies and mitigation actions for each of the three pilot states. The development of landslide hazard maps for Saint Lucia and Grenada is one component of a series of ongoing hazard mapping consultancies intended to inform vulnerability assessments in each of the three pilot states. The vulnerability assessments, in turn, will inform the national governments and Plan Development Committees (PDCs) of the existing vulnerability in order to develop and prioritize hazard risk reduction strategies and actions. The landslide hazard maps will be prepared utilizing the Common Digital Database (CDD), developed under a previous consultancy, which contains baseline data, including natural and anthropogenic features, necessary for the HMVA. The CDD is designed to be compatible with and is expected to be integrated into existing and/or planned national geographic information system (GIS) databases.

1.2. PROJECT INCEPTION MISSION

The project inception meetings were coupled with field reconnaissance in Saint Lucia and Grenada. The Project Director, Jeffrey Euwema, coordinated meetings with the respective HMVA Subcommittees while the Hazard Mitigation Specialist, Pieter de Jong and the Engineering Geologist, Stanley Wharton, conducted field reconnaissance with the support of knowledgeable government representatives. The amount of time available for the inception mission was constrained by budget considerations. Inception meetings were held on September 6, 2004 in St. Lucia and September 8, 2004 in Grenada. The inception meetings were followed by two field days in the Castries watershed pilot area (September 6 and 7) and three field days for Grenada (September 8 – 10). A government vehicle and driver were made available to the field reconnaissance team and the existing road network was used to find recent and historical landslide events. During the inception mission, various types of geospatial data were also reviewed and collected from a variety of different sources and formats. Data Collection efforts focused on augmenting the Common Digital Database. For each BMC initial data collection efforts found data to be held in a variety of locations. Emphasis was placed on collecting descriptive documentation of the data itself.



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1.3. PROJECT STUDY AREAS AND EXPECTED MAP OUTPUTS

Saint Lucia The Pilot Area boundaries were discussed during the HMVA briefing. During this discussion, HMVA Subcommittee members indicated they had concerns about the extent of the pilot area. Base maps were presented via an overhead projector to invoke discussion about the delineation of the pilot area. HMVA members indicated that there were landslide concerns immediately outside of the study area. The outcome was to expand the pilot area to include settlements to the north of the Castries River watershed that formed the boundaries of the initial pilot area. The new study area boundary was delineated, verified and agreed upon by both Mr. David Alphonse and Mrs. Glenda Charles of the Saint Lucia Physical Planning Section. Both Mr. Alphonse and Mrs. Charles are members of the HMVA Subcommittee. It was agreed the output scale for map for St. Lucia would be: � High-resolution map at a 1:10,000 scale for the expanded study area within the Castries watershed. Grenada There are two study areas for the landslide mapping project in Grenada. The major study area covers the entire island of Grenada but does not include the adjacent island of Carriacou, which also falls under the jurisdiction of the Government of Grenada. The Terms of Reference for this landslide mapping project also included a Pilot Study Area for portions of St. George and St. David parishes. This Pilot Study Area boundary was chosen, in part, because of the availability of high resolution topographic data. The consultant project team recommended that this Pilot Study Area be deleted because the area is not prone to landslides. In consultation with CDB and the Government of Grenada, the Pilot Study Area was shifted to the area surrounding the community of Florida which is very susceptibility to landslides and landslips. It was agreed the output scale for map for Grenada would be: Grenada � Island-wide map at a 1:25,000 scale1; and, � High-resolution maps at a 1:10,000 scale of Florida.

1 Please note that base map data used for production of maps was at a scale of 1:25,000; however, final map was plotted at 1:30,000 scale in

ArcGIS.



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2. METHOD OF INVESTIGATION The hazard mapping methodology utilized in this study systematically combines several factors in a GIS model to provide insight into landslide susceptibility. The mapping methodology developed in this study can be divided into four (4) steps: Step 1: Landslide Inventory Map Step 2: Base Map Preparation and Analysis Step 3: Base Map Classification and Factor Map Development Step 4: Hazard Model & Susceptibility Map Development Step 1: Landslide Inventory Map Field reconnaissance allowed project team members to map recent and historical landslides. Locations were captured using a GPS. GPS coordinates were synchronized to the appropriate coordinate mapping system and each landslide “mapped” digitally. Mapping was field checked by noting landslide sites on hard copies of either 1:10,000 or 1:25000 topographic maps in order to corroborate actual location of landslide events. Landslide occurrences were mapped and observations of the landslide type, location along hillslope, slope angle, depth of landslide whether shallow or deep seated, and the nature of the bedrock including rock type and degree of weathering were taken into account. Step 2: Base Map Preparation and Analysis A series of base maps were prepared which included landslide inventory map, elevation model, slope angle, slope aspect as well as geology and soil maps. To determine what geographic parameters contribute to landslide susceptibility, the project team converted data to raster format. For existing data in raster format, a 25 meter raster grid was utilized for Grenada and a 10 meter raster grid was utilized for St. Lucia. General vector polygon data such as soils and geology were converted to raster grids that matched the resolution for each respective island. Step 3: Base Map Classification and Factor Map Development Within the ArcGIS 9.1 environment a series of analyses were run to relate the landslides to each data set. The landslide inventory map was then overlaid on each base map and a series of spatial queries were utilized to classify individual base maps into landslide susceptibility factor maps. A series of GIS spatial queries determined the number of landslides within each geologic or physiographic category of each data set. GIS queries and overlay techniques were utilized to calculate the percentage of landslides and total area of each geologic and physiographic category and finally, the percentage of area of abovementioned categories compared to the total area of the study area. These spatial queries allowed an estimation of relative landslide susceptibility based on a comparison ratio (percentage of land in a geologic/physiographic category related to percentage of landslides mapped in the same category). This ratio provided the project team with a quantitative measure to rate the primary factors according to their relative susceptibility. Comparison ratios were then used to develop susceptibility factors for each data set. Distinct categories within each data set (i.e. elevation interval, soil type, geology unit, etc.) were classified according to susceptibility levels based on the outcomes of the spatial analysis, review of descriptive documents and field observation. This resulted in the development of reclassified factor maps for elevation, slope angle, slope aspect, soils and geology.



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Step 4: Hazard Model & Susceptibility Map Development A susceptibility mapping model was prepared for each island that reflected the varying influences of slope, elevation, aspect, geology and soils. The resulting factor maps were reclassified, converted to raster format and modeled spatially. A susceptibility mapping equation model for was developed for each island that used a simple mathematical overlay process that adds the susceptibility ranking for the corresponding cells of each factor map together. The output of the model was reclassified into five susceptibility categories: Very Low, Low, Moderate, High, and Severe. These five categories provide an indicator of landslide susceptibility in the Pilot Study Area on St. Lucia and throughout Grenada.

2.1. LANDSLIDE INVENTORY MAP

The inventory for landslides in both St. Lucia and Grenada were developed using field reconnaissance and mapping. Field reconnaissance conducted on both islands was limited because the Project Team was restricted in identifying and evaluating landslide events by the existing road network. Landslide occurrences were mapped and observations of the landslide type, location along slope, slope angle, depth of landslide whether shallow or deep seated, and the nature of the bedrock including rock type and degree of weathering were taken into account. Landslide occurrence was recorded along with the type of bedrock noted. The degree of human activity on the hill slopes and the density of housing including waste water disposal system and drainage were also noted. Data was collected using a general registration system. GPS coordinates were synchronized to the appropriate coordinate mapping system and each landslide “mapped” digitally. Mapping was field checked by noting landslide sites on hard copies of either 1:10,000 or 1:25000 topographic maps in order to corroborate actual location of landslide events. Digital data was downloaded and interfaced with a GIS. Base maps were utilized for field orientation, however, GPS allowed project team members to collect the data without having to be mindful of their exact orientation with either base map or the satellite imagery. Over forty (40) points were recorded for St. Lucia and two hundred and forty five (245) for Grenada. These numbers were reduced from 40 to 14 for SLU and 245 to 170 in GRN during the evaluation since initially recorded ‘data’ points reflect not only a specific landslide location but also other specific observations related to the mapped area. With two people in the field, field data was captured rather quickly by simultaneously registering fairly exact landslide locations and key data related to factors contributing to their generation. Each landslide was assigned an identification number, the landslide type was estimated and general physiographic characteristics of the area were noted. Recorded data was further distilled to reflect actual landslide information and these were further classified into the following landslide categories for input into the GIS:

• Rock Slide

• Rock Fall

• Debris Slide (deep seated or shallow seated)

• Debris Flow

• Creep This approach to landslide mapping proved beneficial in providing access to landforms and rock types susceptible to landslide activity. The limited timeframe for field assessment, however, did not allow for a detailed analysis of each landslide site. The length and width of each landslide was not measured nor was the total area of disturbance estimated.



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As aforementioned, field data capture was confined to primary and secondary roads. Although many landslides were mapped in lower elevations (i.e. along roads), field observation pointed to a greater incidence of landslides in higher elevations. For instance, during the mapping of landslides in Grenada many landslides and landslide prone areas were identified which were located off the road in hilly and sometimes inaccessible parts of the island. Many of these landslides occurred during Hurricane Ivan in 2004. Therefore, there were biases in the field data that resulted in a poor correlation between elevation and the frequency of landslides (i.e. data was counter intuitive to field analysis). While field mapping in Saint Lucia was confined to lower elevations, the consultant project team observed that landslide frequency was also closely related to increases in elevation. Field observations however, indicated that slope class was the most important factor in determining landslide occurrences in the Saint Lucia Pilot Area. The data captured, although limited in the study area, showed a direct relation to increases in elevation and increases in the percentage of slope. The following section provides a detailed description of the methodology and a resolution of the biases encountered in the field data.

2.2. BASE MAP PREPARATION AND ANALYSIS

Data collection efforts focused on augmenting the Common Digital Database. For each BMC, various types of geospatial data were collected from a variety of different sources and formats. In order to determine what geographic parameters contribute to landslide susceptibility, the project team converted data to raster format. A twenty-five (25) meter raster grid was utilized for Grenada and a ten (10) meter raster grid was utilized for St. Lucia. General vector polygon data such as soils and geology were converted to raster grids that matched the resolution for each respective island.

2.2.1. Digital Elevation Saint Lucia A new DEM was created for the expanded pilot area for St. Lucia. This involved utilizing high resolution, 2 ft. contour data that were derived originally from 1:2,500 map sheets of St. Lucia. Graphic data and attribute fields were verified and “zero” values were deleted for the DEM creation. Surface analysis functionality of the ArcGIS 3D Analyst extension was to create a Triangulated Irregular Network (TIN), using the aforementioned input layers. The output matches the grid resolution of the initial pilot area (DEM of 10 meters). Grenada Land surface elevation categories were simplified into six (6) categories for assessment and were based on a 25-m resolution DEM that was generated from 1: 25,000 quadrangles with a contour interval 25 ft below 250 ft and 50 ft above 250 ft. Output was verified against original topographic information and existing DEM and TIN files that were developed for the original pilot area.



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Table 2.1 Elevation Categories

Saint Lucia Grenada � 0-50 meters � 50-100 meters � 100-150 meters � 150-200 meters � 200-250 meters � 250-300 meters � 350-400 meters � 450m-

� 0-100 meters � 100-200 meters � 200 -300 meters � 300- meters

2.2.2. Characterization of Slope The DEM was processed and used for the gradient or slope analysis for both islands. The gradient was developed using the slope function in ArcGIS 9.1. The slope function computes a gradient at each grid cell based on the maximum change in elevation from eight surrounding cells. The slope grid was created using a 5 cell (50 m) radius to define the neighboring cells used to compute the mean. The slope gradients categories utilized are listed below:

Table 2.2 Slope Gradients Categories

Saint Lucia Grenada � 0-10% � 10-20% � 20-30% � 30-40% � 40-50% � 50-60% � 60%-

� 0-15%; � 15-30%; � 30-45% � 45% -

2.2.3. Characterization of Aspect The DEM will be processed and used to create an aspect (direction of inclination of ground surface) grid using the aspect function in ArcGIS 9.1. The aspect function determines the aspect of each DEM cell by identifying the down-slope direction of the maximum rate of change in elevation between the DEM cell and its neighboring cells. Aspect values, which range from 0° to 360°, were divided into three broad ranges according to whether the slope faces to the lee of, to the prevailing wind or is neutral to prevailing winds. In general, the northeast and east facing slopes are subject to the prevailing winds and as a result, get more precipitation on an annual basis than other slope aspects. The slope aspect categories utilized are listed below:

� Prevailing Wind Aspect (45 degrees to 135 degrees) � Neutral to Prevailing Winds (315 degrees to 45 degrees and 135 degrees to 225 degrees) � Aspect in Lee of Prevailing Winds (225 degrees to 315 degrees)



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2.2.4. Characterization of Geology Saint Lucia The geology map was developed in 1984 and was included as part of the OAS Development Atlas. During the inception briefing, HMVA Subcommittee members requested that the project team map the geology in “unclassified areas” within the pilot area since this area represented a fairly large part of the geology of Castries Although this mapping exercise went beyond the initial scope of the project, the project team agreed to map “unclassified areas” within the Pilot Area by extending generalized geology into unclassified areas. The areas that were mapped include undeveloped or partially developed lands that are susceptible to landslide hazards (i.e. sloping lands). Additional refinement to areas mapped as “undifferentiated volcanics” was made by defining specific bedrock types within this generic bedrock classification. The geology for densely urbanized areas, which are primarily flat, was not mapped. New geology boundaries were verified by a rapid field assessment of areas and represent only estimates using rock outcrop data. No boring data was utilized in refining the 1984 geology map but careful observation of rock exposures especially in the “unclassified” area enabled identification of the previously unmapped rock types and extension of related bedrock types on the Geology Map. The table below depicts all of the thirty-three (33) geology types found in Saint Lucia.

Table 2.3 Geology Types, Saint Lucia

No. Geology Unit No. Geology Unit 1 Alluv, Beach & Terrace S 17 Horneblende Andesite 2 Belfond Pumice Flow Tuff 18 Andesite Porphyritic 3 Belfond Pumice Fall 19 Andesite Breccia 4 Belfond Dome Lava 20 Columnar Andesite 5 Terrace Blanche Dome Lav 21 Andesite Ash Altered And 6 Piton Agglomerate 22 Mudflow 7 Piton Dome Lava 23 Andesite Agglomerate, Mu 8 St. Phillips Agglomerate 24 Rhyolite 9 St. Phillips Lava Flow 25 Andesite

10 Pale Andesite Dome Lava 26 Basalt,Andesite,Aggl,Tuf 11 Andesite Pumice Flows, T 27 Basalt,Agglomerate 12 Andesite Agglomerate Cal 28 Limestone granular 13 Dark Andesite Cones 29 Thin bedded Tuff 14 Prophyritic Basalt 30 Agglomerate Tuffs, Tuffs 15 Aphyric Basalt 31 Agglomerate Tuffs, Tuffs 16 Alteresd Andesite Porphy 32 Unclassified

33 Crater



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In Castries, three main geological types are exposed on the surface: � (25) Andesite � (26) Basalt, andesite (occasionally bedded) agglomerate, tuff, and � (27) Basalt agglomerate Some terrace deposits of alluvial rocks also exist in the northern parts of Castries. Observation of the relative degree of weathering for each rock type was noted and use of an island wide landslide evaluation by DeGraff in 1985 was beneficial for comparisons of landslide susceptibility in particular rock types across the island. Generally, mapping of rock types in the Castries area was easily corroborated with the Geology Map and contributed to the degree of accuracy in the analysis. Andesitic types dominate basaltic types for landslide prone areas. Rapid assessment of rock type included observations of rock fabric, color, degree of weathering and weathering patterns, dip of beds, grain size and mineral composition. Grenada The geology map was developed in 1981 as part of a reconnaissance study of the geothermal resources conducted by Geothermica Italana. The geology base maps (1:25,000) were digitized by the Ministry of Agriculture in 1994. The geology map was classified into nineteen (19) bedrock geology classes:

Table 2.4 Geology Types, Grenada Code Description Code Description

As Alluvial and Superficial Deposits(Recent)

St

Tufton Hall Formation(Late Eocene-Early Oligocene)

B Beach T Town L Lake Va Lake Antoine Volcanics(Pleistocene) Ld Lava Domes Vb Grand Anse Bay Volcanics(Miocene-Pliocene)

M Mangrove

Vc Mount St. Catherine Volcanics(Pliocene-Pleistocene)

P Airport Ve South East Mountain Volcanics(Miocene) Ps Point Saline Beds Vg Mount Granby Volcanics(Miocene-Pleistocene)

S Swamp

Vi Indifferentiated Volcanics, mainly reworked(Pleistocene)

Sg Great River Beds Vl Levera Hill Volcanics(Miocene)

Vm Mount Craven Volcanics(Early Miocene-Pleistocene)

Supporting documentation was not present in the Ministry of Agriculture, Physical Planning, or in the Documentation Center. A large part of the Geology Map is described as “Undifferentiated Volcanics”. Refined geologic information for specific landslide prone areas is provided in the landslide inventory with additional information on the nature of the geology found within the undifferentiated volcanics mapping unit.



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The main geological types exposed in the mapped area are represented by the following: � Undifferentiated Volcanics, mainly reworked of Pleistocene age � South East Mountain Volcanics of Miocene age � Great River Beds � Mt. Granby Volcanics, Miocene to Pleistocene age � Mt. St. Catherine Volcanics Pliocene to Pleistocene age � Mt. Craven Volcanics, Early Miocene to Pleistocene age Rapid field assessments were conducted over the island to assess the rock types within the different mapped types in order to differentiate the key geological units involved in landslide activity. Rock characteristics such as rock fabric, color, degree of weathering and weathering patterns, dip of beds, grain size and mineral composition were identified and observations made on slope steepness and other physical characteristics. A key observation, however, was that the rock type regarded as “Undifferentiated Volcanics” was identified as generally volcaniclastic sediments that reflected a range of physical characteristics. Several observations made by geologists who have conducted previous investigations in Grenada have noted that the island experienced several different volcanic episodes from the Miocene to the Pleistocene. These observations were identified in the field as the rock types displayed varying episodes of volcanic and sedimentary deposition. Weaver (1989) noted that at least five different volcanic centers of activity can be identified across the island; each dominated by either basalt or andesite eruptions at different stages of the island’s geological history. These volcanic centers of activity, namely North Domes, South East, Mt. Maitland, Mt. Granby-Fedon’s Camp and Mt. St. Catherine, are associated with basalt and andesitic lava flows and are also associated with volcaniclastics, tuff or scoria deposits. Earle (1923) also observed the predominance of relict massive un-decomposed lava scattered on slopes at locations such as Black Bay and Woodford. Field mapping was used to corroborate with the literature for an in-depth assessment of observations of rock slides. In particular, several areas were found to have large volcanic lava boulders scattered on the upper sections of slopes. The presence of these large boulders was observed to be associated to the presence of relict landslides and their presence was further used in adjusting the susceptibility rating for rock slides in several areas.

2.2.5. Characterization of Soils Saint Lucia The soils map was developed in 1966 by the Soil Research Unit of the University of the West Indies, St. Augustine, Trinidad. It is important to note that the soil mapping was primarily based on topography, drainage, parent material and not according to pedology which emphasizes how the soils originated. The approach taken by the surveyors reflected the need to produce a survey that would be of most use for the agricultural community, not for its potential use in geotechnical investigations. During the inception briefing, HMVA Subcommittee members requested that the project team map soils in “unclassified areas” within the Pilot Area. Although this mapping exercise went beyond the initial scope of the consultancy, the project team agreed to map “unclassified areas” within the Pilot Area. Again, soil classes were extended in unclassified areas and were based only on visual interpretation in the field. No boring data was utilized in the extension of the soil mapping units into the unclassified areas. Mapping was based on the field assessment and a review of supporting documentation, particularly topography. The table below depicts all of the fifty-eight (58) soil classes found in St. Lucia.



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Table 2.5 Soil Classes, Saint Lucia

No Soil Type No. Soil Type 1 Annus Clay 30 Jean Batiste Silty Clay 2 Anse Clay 31 Latille Clay Loam 3 Assor Clay 32 Mabouya Silty Clay 4 Balembouche Gritty Clay 33 Made Land 5 Bare Rock 34 Mahaut Silty Clay Loam 6 Beach Sands 35 Mangrove Swamp 7 Becune Loam 36 Marquis Clay 8 Belfond Clay Loam 37 Michel Gritty Clay 9 Bocage Stony Clay 38 Moreau Clay 10 Calfourc Silty Loam 39 Panache Silty Clay Loam 11 Canelles Clay 40 Parasol Clay 12 Casteau Gravelly Boulder 41 Piaye Silty Clay 13 Cliff 42 Quarry 14 Cochon Silty Clay Loam 43 Quilesse Silty Clay 15 Deglos Silty Clay 44 Rabot Clay 16 Delomel Clay 45 Raveneau Clay 17 Dry Pond 46 Regnier Stony Clay 18 Dugard Clay 47 Richefond Fine Sandy Cla 19 Esperance Clay 48 Rozette Gritty Clay 20 Excessively Steep Slopes 49 Salina 21 Falaise Stony Loam 50 Soucis Silty Clay Loam 22 Franciou Stony Clay 51 Sulfur Springs 23 Garrand Clay Loam 52 Swamp 24 Gomier Stony Clay Loam 53 Troumasse Loam 25 Hardy Clay 54 Urban Area 26 Haut Clay Loam 55 Vanard Peat 27 Ivrogne Stony Clay 56 Venus Loam 28 Jalousie Clay 57 Warwick Clay 29 Jambette Stony Silty Cla 58 Zenon Gravelly Bouldery

Nine (9) soil mapping units occur in the Castries Pilot Area; they include:

� Hardy Clay (45) � Canelles Clay (24) � Bocage Stony Clay (20) � Assor Clay (25) � Jambette Sony Silty Clay Loam (15) � Anse Clay (42) � Franciou Stony Clay (32) � Delomel Clay (43) � Gommier Stony Loam (31) Grenada The soil map was developed in 1959 by the Soil Research Unit of the University of the West Indies, St. Augustine, Trinidad. The soil base maps were digitized in 1994 by the Ministry of Agriculture. The authors of the soil survey note that of the five factors recognized in soil formation (parent material, climate, topography, vegetation and time), climate and topography are the most important in understanding soil



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formation in Grenada. Climate is the most important single factor, specifically differences in total annual rainfall and in the length of the dry season. The parent rocks are very similar in mineralogy and may be discounted as a major factor leading to variation in soil types; however, parent material was found to influence the frequency of occurrence of landslide events. The geological bedrock is also relatively young, therefore time, has not yet been an import soil-forming factor. The digitized soil map was classified into forty-two (42) soil unit types.

Table 2.6 Soil Types, Grenada Code Soil Type Code Soil Type 1 Woodlands Clay Loam 30 Capitol clay loam 2 Plains Clay Loam 31 Capitol clay loam ( stony bouldery phase ) 3 Plains Loamy Sand 32 Capitol clay loam (very steep shallow phase) 4 Plains Sandy Loam 35 Capitol clay loam ( drier areas ) 5 Bonair Bouldery Sandy Loam 36 Capitol clay loam ( stony bouldery phase in drier areas) 6 La Tante Clay Loam 37 Capitol clay loam (very steep and shallow phase in drier

areas) 7 Hope clay 40 Belmont clay loam 8 Pearls clay 41 Belmont clay loam (stony and bouldery phase) 9 Simon clay loam 42 Belmont clay loam (very steep and shallow phase) 11 Hillsborough loamy sand 43 Palmiste clay loam 12 Lauriston clay 44 Palmiste clay loam (very steep and shallow phase) 20 Sabizan clay loam 50 Tophill Stony Clay 21 Woburn clay loam 51 Belair Stony Clay 22 Woburn clay loam (stony

/bouldery phase) 52 Betish Clay Loam

23 Hartman clay U Urban area 24 Limlair clay M Made land 25 Concord clay loam RW River wash 26 Concord clay loam (stony

bouldery phase) BS Beach sand

27 Parnassus clay SS Salt swamp 28 Perseverence clay SL Salina 29 Perseverence clay (stony

bouldery phase ) MA Mangrove swamp

Although soil boundaries were initially delineated for agricultural and not geotechnical purposes, the soil survey does contain important information for evaluating how soils and the underlying weathered bedrock may influence the occurrence of landslide events.



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2.3. BASE MAP CLASSIFICATION AND FACTOR MAP DEVELOPMENT

The landslide inventory map was then overlaid on each base map and a series of spatial queries were utilized to classify individual base maps into landslide susceptibility factor maps. A series of GIS spatial queries determined the number of landslides within each geologic or physiographic category of each data set. GIS queries and overlay techniques were utilized to calculate the percentage of landslides and total area of each geologic and physiographic category and finally, the percentage of area of abovementioned categories compared to the total area of the study area. These spatial queries allowed an estimation of relative landslide susceptibility based on a comparison ratio (percentage of land in a geologic/physiographic category related to percentage of landslides mapped in the same category).

2.3.1. Geologic and Physiographic Attributes of Landslide Distribution in Saint Lucia In St. Lucia, the project team utilized a 10 meter DEM to determine elevation, slope and slope aspect. General vector polygon data such as soils and geology were converted to 10 meter raster grids for purposes of analysis. The discussion below describes the results of this analysis and trends that connected geographic and geologic features to landslide occurrence.

2.3.1.1. Analysis of Elevation Landslide frequency was greatest in land surface elevations from mean sea level and 50 -100 meters above mean seal level. Forty-six (46) percent of the mapped landslides fell within the latter category. Thirty (30) percent of the mapped landslides occurred in elevations between 0-50 meters. Fifteen (15) percent fell into elevation above 200-250 meters (Table 2.7) while the lowest, approximately eight (8) percent, occurred between 150- 200m.

Table 2.7 Elevations and Associated Landslides, St. Lucia

ELEVATION 0-50m 50-100m 150-200m 200-250m

Landslide 4 6 1 2

% of landslide 30.8 46.2 7.7 15.4

Area 3602568.497 1557295.585 766541.410 309870.012

% of area 48.3 20.9 10.3 4.2 % LANDSLIDE / % AREA 0.6 2.2 0.7 3.7

The results of this analysis are counter-intuitive in that one would expect landslide frequency to increase with elevation because this criterion is often considered a surrogate for rainfall. In the volcanic islands of the Lesser Antilles, annual rainfall is strongly and positively correlated with elevation. The results observed in Table 2.1 can be readily explained because of the bias involved with the field reconnaissance which was constrained to the existing road network and access, mainly restricted to low to mid-elevations. The Island of Saint Lucia lies between the sub-tropical high pressure belt of the Atlantic Ocean and the Equatorial low pressure belt and has a tropical climate. The main features of Saint Lucia’s climate are uniformly high temperatures all year round mitigated by the north-east trade winds which freshen during the dry season. In winter, the sub-tropical anticyclone of the North Atlantic comes south as the equatorial trough retreats to the equator. Warm dry air covers the island as a rule and the limited rainfall during the dry season usually results from cold fronts descending from the North American continent. In summer, the



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equatorial trough moves north again and Saint Lucia is affected by the inter-tropical front. Intense rainfall during the wet season is caused by storms generated along the inter-tropical front, in addition to convectional thunderstorms usually occurring in the afternoons (St. Lucia Soil and Land-use Survey, 1966). Rainfall varies from an annual average of about 50 inches at Moule a Chique in the extreme south and at Cap Point in the extreme north of island, to over 160 inches near Mt. Gimie in the mountainous south central portion of the island. Over the greater part of the Saint Lucia Mountains, the range is from 70 to 100 inches per annum. The rainfall distribution increases with altitude and the rainfall map clearly shows a concentric distribution.

Table 2.8 Elevation and Susceptibility Factors, St. Lucia

Elevation Susceptibility

Factor

0-50m 1

50-100m 2

100-150m 3

150-200m- 4

200-250m 5

250-300m 6

350m- 7

The small size of the Saint Lucia Pilot Area allowed the project team to provide a higher resolution of elevation classes and we have chosen to include seven (7) classes.

2.3.1.2. Analysis of Slope Hillslope angle is an essential component of any landslide susceptibility analysis. As hillslope angle increases, shear stress in soil and unconsolidated material generally increases. In the Saint Lucia Pilot Area, most landslides occurred on slopes between 0-30 degrees. The highest percentage of landslides (54%) was concentrated on slopes between 20-30 degrees. The second highest frequency and percentage (31%) occurred in slope percentages between 10-20 degrees. The frequency of mapped landslides dramatically decreased in the 30-40 degree category.

Table 2.9 Hillslope and Associated Landslides, St. Lucia

SLOPE 10-20 degree 20-30 degree 30-40 degree

Landslide 4 7 2

% of landslide 30.8 53.8 15.4

Area 2831715.8 1851422.0 410447.0

% of area 37.9 24.8 5.5

% LANDSLIDE / % AREA 0.8 2.2 2.8

When the frequency of landslide events is adjusted for the area covered by the three slope classes presented in Table 2.9, the predicted increase in landslide events is readily apparent as slope increases. For the Saint Lucia Pilot Area, slope class was the most important factor in determining landslide



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occurrences. Only thirteen (13) landslide events in the Pilot Area were identified and evaluated; however, the project team assumes that many other historical landslide events have occurred but were masked by subsequent vegetative cover or development. Table 2.10 presents the susceptibility factors assigned to seven (7) slope classes. The small size of the Pilot Area allowed the project team to provide a higher resolution for mapping the influence of slope on landslide susceptibility and the following seven (7) slope classes were created.

Table 2.10 Hillslope and Susceptibility Factors, Saint Lucia

Slope Susceptibility Factor

0-10% 1

10-20% 2

20-30% 3

30-40% 4

40-50% 5

50-60% 6

60%- 7

2.3.1.3. Analysis of Aspect For the Saint Lucia Pilot Area, sixty-one (61) percent of mapped landslides fall into a neutral category indicating that slope aspect did not play an important role in slope failure. Interestingly, the remaining landslides split into expected categories. Twenty-three (23) percent of mapped landslides were on windward orientated slopes, while fifteen (15) percent of mapped landslides were in lee of prevailing winds.

Table 2.11 Aspect and Associated Landslides, St. Lucia

ASPECT 225-315 (leeward) Neutral 45-135 (windward)

NUMBER VALUE_-1 VALUE_0 VALUE_1

Landslide 2 8 3

% of landslide 15.4 61.5 23.1

Area 1431273.22 4996430.85 945518.03

% of area 19.4 67.8 12.8

% LANDSLIDE / % AREA 0.8 0.9 1.8

Given that the entire Pilot Study Area in Saint Lucia was located on the leeward side of the Island and partially masked from prevailing winds by higher elevations to the east, the observed results presented in Table 2.11 are understandable. Although aspect was not considered a driving factor in determining landslide susceptibility, the influence of aspect can be observed in the fact that landslides were approximately three times more likely to occur in hillslopes facing the prevailing winds than in hillslopes in the lee of prevailing winds. The largest landslide in Castries at Black Mallet faces the prevailing winds. Hence, slope aspect was carried over and included in the mapping model developed for Saint Lucia. Based on the relative occurrence of landslide events in the slope aspect categories, susceptibility factors of were broken into three broad ranges according to whether the slope faces are, in lee of (-1), normal (0), and to the prevailing wind (1).



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2.3.1.4. Analysis of Geology Due to the limited amount of data and geologic units in the Saint Lucia Study Area, historical data was utilized. In particular, an inventory of landslides from DeGraff (1984) was utilized to determine the frequency of landslides in particular bedrock types and to gain some insight as to the influence of bedrock types on the frequency of landslide events. The results indicate DeGraff’s inventory of landslides occurred to some level in almost all of the bedrock types (twenty-five (25) of the thirty-three (33) geology map units island-wide). One hundred and ninety-nine (199) or twenty-eight (28) percent of all of the mapped landslides fell into Andesite Ash Altered Andesite (No. 21). Andesite Agglomerate Cal (No. 12) had the second highest count with one hundred and four (104) or fifteen (15) percent of the mapped landslides. Other geologic map units with a high incidence of landslide activity included Belfond Pumice Flow Tuff (No. 2), Andesite Agglomerate Cal (No 12), Andesite Porphyritic (No.18), Andesite Agglomerate, Mu (No. 23), Basalt, Andesite, Aggl.,Tuffs (No.26), and Agglomerate Tuffs, Tuffs (No. 30). The table below documents the frequency of landslide events by geologic bedrock type island-wide.

Table 2.12 Frequency of Landslides by Geology Type, Saint Lucia

GEOLOGY Landslide

% of landslide Area

% of area

%LANDSLIDE / %AREA

2 Belfond Pumice Flow Tuff 44 6.2 39616809.397 6.6 0.94

3 Belfond Pumice Fall 13 1.8 6286756.863 1.0 1.75

5 Terrace Blanche Dome Lav 1 0.1 1340132.470 0.2 0.63

6 Piton Agglomerate 21 2.9 11939161.889 2.0 1.49

7 Piton Dome Lava 18 2.5 4070405.822 0.7 3.74

8 St. Phillips Agglomerate 8 1.1 2665668.417 0.4 2.54

10 Pale Andesite Dome Lava 9 1.3 4108026.507 0.7 1.85

11 Andesite Pumice Flows, T 6 0.8 2814050.548 0.5 1.80

12 Andesite Agglomerate Cal 104 14.6 46038571.597 7.6 1.91

13 Dark Andesite Cones 13 1.8 8819875.624 1.5 1.25

14 Prophyritic Basalt 3 0.4 5708750.077 0.9 0.44

15 Aphyric Basalt 1 0.1 7136625.215 1.2 0.12

17 Horneblende Andesite 11 1.5 3004260.857 0.5 3.10

18 Andesite Porphyritic 42 5.9 48602769.093 8.1 0.73

20 Columnar Andesite 2 0.3 1692420.907 0.3 1.00

21 Andesite Ash Altered And 199 27.9 114429369.120 19.0 1.47

22 Mudflow 5 0.7 9268404.773 1.5 0.46

23 Andesite Agglomerate, Mu 40 5.6 59812988.916 9.9 0.57

25 Andesite 17 2.4 23649420.972 3.9 0.61

26 Basalt,Andesite,Aggl,Tuf 41 5.8 75985174.963 12.6 0.46

27 Basalt,Agglomerate 12 1.7 10319819.441 1.7 0.98

28 Limestone granular 1 0.1 197088.866 0.0 4.29

30 Agglomerate Tuffs, Tuffs 38 5.3 15213119.901 2.5 2.11

31 Agglomerate Tuffs, Tuffs 25 3.5 11691696.829 1.9 1.81

32 Unclassified 39 5.5 49549686.159 8.2 0.67

The results of the data collected in Castries during our brief field reconnaissance indicate that only two (2) of the three (3) different geologic units in the Pilot Area contained mapped landslides. Of these, the greater number of landslides fell into the Andesite class. Eight (8) or sixty-one (61) percent of the landslides fell into



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the Andesite geologic map unit, while five (5) or thirty-eight (38) percent fell into the Basalt Agglomerate bedrock type.

Table 2.13 Geology and Associated Landslides, Saint Lucia

GEOLOGY Andesite (No 25)

Basalt,Agglomerate (No. 27)

Landslide 8 5

% of landslide 61.5 38.5

Area 13085870.33 3762374.80

% of area 61.8 17.8

% LANDSLIDE / % AREA 1.0 2.2

Analysis of the data used in the DeGraff study was significant in revealing the bedrock units that reflect the higher susceptibility to landslide activity across the island. The data is significant also in that the andesite beds represent the most highly prone bedrock to landslide activity due to the deep weathering potential of the bedrock. The single largest landslide in Castries located at Black Mallet is located in weathered andesite beds which contain ash and pyroclastics material. This rock type corroborates with the data in DeGraff’s study in that andesite beds with ash material show a high susceptibility to landslides across the island especially since these rock types easily weather under tropical conditions. The field data showed a range in debris slide and creep landslide types with the most common being shallow seated surficial landslides in both the andesites and the basalt where a well defined residual soil weathering zone is developed. Typically these areas are densely populated and affected by poor waste water drainage on the slopes. Black Mallet is located on a fairly steep north east facing hillside south of the airport. The Black Mallet landslide occurred where the two main rock types are juxtaposed and the bedrock in the landslide area represents weathered andesite agglomerate and ash overlying more competent and less weathered andesite and basalt. This landslide which was studied by several consultants in the past was compared to other landslides in the area during field reconnaissance to observe the susceptibility of similar rock types and it was found that Black Mallet probably represents a separate case since similar deeply weathered bedrock had not been observed elsewhere. Based on the relative occurrence of landslide events in various bedrock types noted in the deGraff study, the following susceptibility factors were incorporated in the Saint Lucia landslide mapping model (Table 2.14).

Table 2.14 Geology and Susceptibility Factors, Saint Lucia

Geology Type Susceptibility Factors

Andesite 3.5

Basalt, Andesite, Aggl, Tuf 1

Basalt Agglomerate 2

Urban Area 0

2.3.1.5. Analysis of Soils Once again the limited amount of data and soil classification units in the Saint Lucia Pilot Area precipitated the need to utilize historical data. In particular, DeGraff (1984) was utilized to gain some insight into the



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influence of soil class units on the frequency of landslides that occurred throughout the island. The results indicate historical landslides are present in the majority of the soil classification categories and fell into thirty-eight (38) of fifty-eight (58) soil classification units island-wide. Based on this analysis a small subset was broken out to look at the frequency of landslides island-wide for the soil types found within the Pilot Area (Table 2.15).

Table 2.15 DeGraff Island-wide Data for Pilot Area Soil Types, Saint Lucia

Soil Type Landslide % of landslide Area % of area %landslide / %area

HARDY CLAY 54 7.6 44913188.4 7.5 1.0

CANELLES CLAY 38 5.3 44149047.1 7.3 0.7 BOCAGE STONY CLAY 8 1.1 12188445.9 2.0 0.6

ASSOR CLAY 6 0.8 7974972.2 1.3 0.6 JAMBETTE STONY SILTY CLAY LOAM 4 0.6 3720890.0 0.6 0.9

ANSE CLAY 8 1.1 27145784.7 4.5 0.2 FRANCIOU STONY CLAY 41 5.8 35654184.0 5.9 1.0

DELOMEL CLAY 4 0.6 15037885.6 2.5 0.2

URBAN AREA 1 0.1 6387555.3 1.1 0.1 GOMMIER STONY LOAM 3 0.4 6167112.5 1.0 0.4

The same type of analysis applied to other factors was then conducted using the landslides mapped during field work and related to soil types found in the study area (Table 2.16).

Table 2.16 Pilot Area Soil Types with Associated Landslide Activity, Saint Lucia

SOIL Bocage Stony Clay Franciou Stony Clay Gomier Stony Clay Loam

Landslide 3 3 7

% of landslide 23.1 23.1 53.8

Area 499929.14 855747.00 1178565.21

% of area 6.7 11.5 15.8 % LANDSLIDE / % AREA 3.4 2.0 3.4

Fifty-three (53) of the mapped landslides fell into Gomier Stony Clay Loam soil classification, while forty-six (46) percent of the landslides were found in Bocage Stony Clay (23.1%) and Franciou Stony Clay (23.1%) soil classification units. These results from the Pilot Study Area proved inconclusive and had to be augmented by soil survey information describing the characteristics of the nine (9) soil types found in the Pilot Study Area. � Hardy Clay (45). Imperfectly drained shallow soils over andesitic or basaltic agglomerate. Soils of the

peripheral hills and glacis slopes. � Canelles Clay (24). Soil type developed from very recent ash covering well-weathered latosolic soil.

Composite steepland soils of the interior hills and mountains.



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� Bocage Stony Clay (20). The soil survey identifies this soil, among six soil types island-wide, as being landslide prone. It is a colluvial soil, formed where sediments have accumulated. This soil type is found in the central core of hills and mountains where colluvial materials have accumulated at the toe of steeply-sloping hillsides. Derived from andesitic material with a relatively deep soil profile.

� Assor Clay (25). Composite steepland soil type of the interior hills and mountains. Derived from very

recent ash covering well-weathered latosolic soil. This soil type is an imperfectly-drained variant of Canelles Clay.

� Jambette Stony Silty Clay Loam (15). A skeletal soil found on very steep rocky land. The soil type

has a thin mantle of dark brown stony light-textured soil over andesitic or basaltic agglomerates and especially over basaltic dykes and lava flows.

� Anse Clay (42). Imperfectly-drained soils over andesite agglomerate. Seepland soil type of the interior

hills and mountains. � Franciou Stony Clay (32). Imperfectly-drained shallow soils over andesitic or basaltic agglomerate.

Soil type of the peripheral hills and glacis slopes. � Delomel Clay (43). Poorly-drained soils over andesitic or basaltic agglomerate. The soil characteristics

include expanding lattice clay of the montmorillonitic type which can cause problems with foundations following cycles of dry and extremely wet periods.

� Gommier Stony Loam (31). This soil type is very similar to the Jambette soil series but the parent

material is basalt rather than andesitic agglomerates. The susceptibility ratings presented in Table 2.17 reflect the frequency of occurrence of landslides documented in the DeGraff study with the exception of Bocage Stony Clay soil type which was weighted with one additional rating point to reflect the findings of the 1966 Soil Survey.

Table 2.17 Soil Type and Susceptibility Factors, Saint Lucia

Soil Type Susceptibility Factor

HARDY CLAY 4

CANELLES CLAY 3

BOCAGE STONY CLAY 4

ASSOR CLAY 3 JAMBETTE STONY SILTY CLAY LOAM 4

ANSE CLAY 2

FRANCIOU STONY CLAY 4

DELOMEL CLAY 2

URBAN AREA 0

GOMMIER STONY LOAM 3



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2.3.2. Geologic and Physiographic Attributes of Landslide Distribution in Grenada In Grenada, the project team utilized a 25 meter DEM to determine elevation, slope and slope angle. General vector polygon data such as soils and geology were converted to 25 meter raster grids for purposes of analysis. The discussion below describes the results of this analysis and trends that connect geographic features to the frequency of landslide occurrence.

2.3.2.1. Analysis of Elevation Landslide frequency was greatest in land surface elevations from mean sea level and 100 meters above mean seal level. Sixty-one (61) percent of mapped landslides fell within this category. Forty-four (44) percent of the mapped landslides occurred in elevations between 100m to 200 m and dramatically decreased above that range to eight (8) percent between elevations between 200 - 300m. No mapped landslides fell into elevation of 300m and above (Table 2.18).

Table 2.18 Elevations and Associated Landslides, Grenada

ELEVATION 0-100m 100-200m 200-300m 300m -

Landslide 90 44 13 --

% of landslide 61.2 29.9 8.8 --

Area 3602568.5 1557295.6 1177149.2 --

% of area 48.3 20.9 15.8 --

% LANDSLIDE / % AREA 1.3 1.4 0.6 --

The results of this analysis are counter-intuitive in that one would expect landslide frequency to increase with elevation because this criterion is often considered a surrogate for rainfall. In the volcanic islands of the Lesser Antilles, annual rainfall is strongly and positively correlated with elevation. The results observed in Table 2.11 can be readily explained because of the bias involved with the field reconnaissance which was constrained to the existing road network, mainly restricted to low to mid-elevations. The climate of Grenada is a humid tropical marine type, with little seasonal or diurnal variation and fairly constant trade winds from the east to northeast, strengthening during the dry season. In winter, the sub-tropical anticyclone of the North Atlantic comes south as the equatorial trough retreats to the equator. Warm dry air covers the island as a rule and the limited rainfall during the dry season usually results from cold fronts descending from the North American continent. In summer, the equatorial trough moves north again and Grenada is affected by the inter-tropical front. Intense rainfall during the wet season is caused by storms generated along the inter-tropical front, in addition to convectional thunderstorms usually occurring in the afternoons. The dry season falls usually between January to May with a wet season from June to December. Rainfall varies from an annual average of about 40 inches at Point Saline in the southwest to over 145 inches at Grand Etang in the mountainous center of the island. Outside of these extremes, the range for rainfall throughout the island is generally between 60 to 100 inches per annum, with the leeward slopes being considerably wetter than the windward slopes (eastern). The average annual rainfall is highly correlated with elevation, showing an almost concentric distribution around the mountainous center of the island.



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Although the bias in the field reconnaissance resulted in a poor correlation between elevation and the frequency of occurrence of landslides, the project team did use elevation as a surrogate for rainfall intensity and broke out elevation into the following four susceptibility classes (Table 2.19).

Table 2.19 Elevation and Susceptibility Factors, Grenada

Elevation Susceptibility

Factor

0-100m 1

100-200m 2

200-300m 3

300m- 4

2.3.2.2. Analysis of Slope Hillslope angle is an essential component of analyzing landslide susceptibility. As hillslope angle increases, shear stress in soil and unconsolidated material generally increases. In Grenada, most landslides occurred on slopes that had slope percentages between 0-30%, with the highest percentage (52%) occurring on slopes between 0-15%. The second highest frequency and percentage (35%) occurred in slope percentages between 15-30%. The frequency of mapped landslides dramatically decreased in the categories of 30-45% as well as the 45% and greater category.

Table 2.20 Hillslope and Associated Landslides, Grenada

SLOPE, as percentage 0-15 15-30 30-45 45-60

Landslide 77 52 15 3

% of landslide 52.4 35.4 10.2 2.0

Area 168667504 116568128 38816248 10235625

% of area 50.3 34.8 11.6 3.1

%LANDSLIDE / %AREA 1.0 1.0 0.9 0.7

Again, the bias in the field reconnaissance due to the project team being constrained by the existing road network explains the counter-intuitive results presented in Table 2.13. Grenada, unlike Saint Lucia did not have a previous landslide mapping consultancy which we could expand our data points for analysis purposes. Likewise, the project team attempted to use remote imagery to expand the number of landslide data points at higher elevations and steeper slope angles; however, the imagery could not be used accurately in this analysis. Using professional judgment, the project team used a simple unweighted susceptibility ranking of four slope classes as noted in Table 2.21 below.



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Table 2.21 Hillslope and Susceptibility Factors, Grenada

Slope Susceptibility Factor

0-15% 1

15-30% 2

30-45% 3

45-60% 4

2.3.2.3. Analysis of Geology All of the mapped landslide points fell into 13 geologic map units. The results of the GIS analysis indicate that landslides were present in most bedrock types throughout the island. The highest percentage (40%) was found in the bedrock class of Undifferentiated Volcanics, mainly reworked (Pleistocene). The occurrence of landslides in the broad grouping of Undifferentiated Volcanics was problematic as it did not allow a direct correlation to specific bedrock type. The second highest category of mapped landslides (approx. 15.6 percent of landslides) was found in the South East Mountain Volcanics (Miocene) map unit, while 11.6 percent of mapped landslides were found to fall within the Great River Beds formation particularly in areas with a high degree of hillslope. Although detailed remapping of this area was not within the scope of the study, the project team mapped landslides and attempted to correlate specific bedrock units with landslide occurrence across the entire island. Bedrock type was recorded for each mapped landslide even though the map classification noted that the bedrock was Undifferentiated Volcanics. Careful examination of bedrock in these areas revealed that the rock type was mainly volcaniclastics but these existed in different bedding relationships as exposed on surface from bedded volcaniclastics to interbedded volcaniclastics with lava flows. These volcaniclastic types were predominant in the NW, W, SW and S parts of the areas. Towards the East, the rock type changed from volcaniclastics to ash beds which form part of the South East Mountain Volcanics. Landslide types mapped in these Undifferentiated Volcanics displayed a limited range in landslide types, displaying predominantly rock falls. Volcaniclastic rocks display a high competence and weathering is in the form of block weathering. Road cut slopes are generally steeper. One of the most memorable landslide events occurred near Concord when a large boulder fell unto a bus and killed several persons many years ago. Other landslide types that were mapped, such as creep, which exist in the Undifferentiated Volcanic classified area, may be related to outcropping of incompetent sediments of other rock types such as the Tufton Hall Formation. Landslides mapped in the South East Mountain Volcanics showed a preponderance of residual soil and debris slides activity owing to the weathering of the bedrock ash beds. These residual soils are dominantly reddish colored and display creep characteristics in some areas. Overall however, field data show that the highest occurrence of landsliding occurred in the Undifferentiated Volcanics followed by the South East Mountain Volcanics.



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Table 2.22 Geology and Associated Landslides, Grenada

Geology Unit Landslide % of landslide Area % of area

% LANDSLIDE / %AREA

AS 4 2.7 15497092.0 4.9 0.6

PS 7 4.8 20603949.1 6.6 0.7

SG 17 11.6 19330278.8 6.2 1.9

ST 7 4.8 5664260.5 1.8 2.6

T 1 0.7 2217132.8 0.7 1.0

VA 2 1.4 10996049.6 3.5 0.4

VB 7 4.8 5813298.0 1.9 2.6

VC 13 8.8 24410340.0 7.8 1.1

VE 23 15.6 50893905.0 16.2 1.0

VG 5 3.4 19488230.0 6.2 0.5

VI 59 40.1 120937500.0 38.5 1.0

VL 1 0.7 3450390.0 1.1 0.6

VM 1 0.7 6500747.0 2.1 0.3

Ten (10) landslides represented a mismatch between ground reality and information depicted on maps. These discrepancies occurred when mapped landslides were found in Alluvial and Superficial deposits. On further analysis, the project team found that alluvial deposits, which are usually found in low elevation and valley floors, did not match topographic contour data in these areas. An analysis of field data allowed the project team to relate these landslide locations to adjacent geology groups for analysis. The result was that only four (4) or 2.7 percent of landslides were found to be anomalous in that the mapped location did not match geologic characteristics captured during field mapping. All of the landslides found in alluvial areas were in areas with a high degree of hillslope, indicating that landslides can be attributed to mapping errors in base geology data. A description of the geology types with an occurrence of landslides is provided below: � Alluvial and Superficial Deposits(Recent) (AS) – these sedimentary deposits represent material

which lies on the bedrock and are the result of erosion and deposition � Point Saline Beds (PS) – these bedrock units are comprised of volcaniclastic deposits and are located

in the southern part of the island . � Great River Beds (SG) – The Great River Beds are exposed in the east of the island and are

composed mainly of basalts, andesites, and hornblende andesites in areas of high relief and pyroclastic rock material, agglomerates and ashes in the lower coastal areas. The location and occurrence of these differing bedrock types can be attributed to the sequence in volcanic eruptions.

� Tufton Hall Formation(Late Eocene-Early Oligocene) (ST) – this bedrock type represents the oldest

rock formation on the island and is comprised of clays and shale sedimentary deposits. � Town (T) – rocks exposed in this area represent the Undifferentiated Volcanics in the form of

volcaniclastics located in the city of St. Georges



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� Lake Antoine Volcanics(Pleistocene) (VA) – these rocks are mainly pyroclastics and basaltic volcanic rocks, and reworked volcaniclastics derived from Pleistocene volcanism. Volcanic flows appear interbedded with the pyroclastic rocks and tuff is common.

� Grand Anse Bay Volcanics(Miocene-Pliocene) (VB) – these rocks are composed of interbedded

volcanic flows with pyroclastics � Mount St. Catherine Volcanics(Pliocene-Pleistocene) (VC) – these volcanics dominate the north

central part of the island and represent an andesitic volcanic dome. Bedrock materials are composed of pyroclastic and volcanic flows

� South East Mountain Volcanics(Miocene) (VE) – these volcanics are located in the south central part

of the island and are comprised of volcaniclastics, volcanic flows, tuffs, sometimes bedded, and pyroclastics beds.

� Mount Granby Volcanics(Miocene-Pleistocene) (VG) – the these volcanics were produced by the Mt.

Granby-Fedon Camp volcanic centre and are comprised mainly of volcaniclastics and both andesitic and basaltic lava flows

� Undifferentiated Volcanics, mainly reworked(Pleistocene) (VI) – these rocks are mainly composed

of reworked volcaniclastic beds that derived from weathering and transport of volcanic material from the volcanic centres on the island. Bedded andesitic and basaltic lava flows are at some localities bedded with the volcaniclastics. At some localities, distinct comglomeratic beds with fairly large rounded rock boulders comprise the main bedrock units. Lava flows are very distinct and weather mainly by fracturing. The Undifferentiated Volcanics display fairly wide variability in rock types.

� Levera Hill Volcanics(Miocene) (VL) – These volcanics are dominated mainly by ashes and tuffs from

more recent volcanic activity in geologic time. The area is dominated by an andesitic dome which represents one of the later stages in the volcanic history of the area.

� Mount Craven Volcanics(Early Miocene-Pleistocene) VM – these volcanics are located to the north

of the island and represent the north dome area. The bedrock is composed mainly of andesitic lava flows, domes and volcaniclastic material.



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The break out data suggests that among the various rock types exposed on the island, the Undifferentitated Volcanics (59) showed the highest incidence of landslide events, followed by the South East Volcanics (23) and then by the Great River Beds (17).The only other noteworthy rock formation was the St. Catherine Volcanics (13). These rock types are represented by a range in rock types such as volcaniclastics, lava flows, pyroclastics and tuffs.

Table 2.23 Geology and Susceptibility Factors, Grenada

Code Geology Unit Description # Events %/Landslide /%Area Susceptibility rating Comments

ST Tufton Hall Formation 7 2.6 2

VB Grand Anse Bay Volcanics 7 2.6 2 stabilized, ancient landslide events

SG Great River Beds 17 1.9 4

VC Mt. Saint Catherine Volcanics 13 1.1 4

VI Undifferentiated Volcanics 59 1 4

VE South Est Mountain Volcanics 23 1 4

T Town 1 1 1

PS Point Saline Beds 7 0.7 2

VL Levera Hill Volcanics 1 0.6 1

VG Mt. Granby Volcanics 5 0.5 1

VA Lake Antoinie Volcanics 2 0.4 1

VM Mt. Craven Volcanics 1 0.3 1

AS Alluvial & Superficial Deposits 4 0.6 1

-- All other Geology Types 0 0 0

Each shows a high susceptibility rating of four (4) and represents the rock units that are most prone to various types of landslide failure. Initially, some of the rock types had shown fairly high susceptibility rating based on landslide occurrence. Some of these ratings were further analyzed based on a re-assessment of the mapped data and there was a further adjustment of the susceptibility rating for the Point Saline Beds and Grande Anse Bay Volcanics in particular. The main reason for the adjustment was based on re-assessments of the actual landslide type and its potential in these rock types and the percentage landslide per area. Several of the landslide data points mapped as old rock slides represented the product of block erosion of the pre-existing lava flows exposed high upslope. The scattered lava blocks may or may not be representative of old rock slides. The geology units which showed a high number of events and represented by a high susceptibility rating displayed a variety of failure types. There was a higher incidence of rock falls and a larger coverage area was defined among other types in the Undifferentiated Volcanics than in any other unit. The geology susceptibility factors were therefore determined by the incidence of landslides and the adjustments were duly made for the susceptibility rating.

2.3.2.4. Analysis of Soils By comparing the number of landslides in a given soil type with the percentage of land area containing that soil, insight into the frequency of landslides may be inferred. These insights were augmented by soil survey information describing the soil characteristics that might influence landslide susceptibility. Most of the landslides in Grenada, twenty-three (23) percent, were found in Capitol Clay loam (Soil Unit 30). Capital



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Clay loam accounts for approximately 28.1 percent of the land area in Grenada. Landslides were also numerous in Woburn clay loam (SU 21), Perseverance clay, stony bouldery phase (SU 29) and Belmont clay loam (SU 40). Together these soil units account for approximately thirty-eight (38) percent of the land area.

Table 2.24 Soils and Associated Landslides, Grenada

Soil Type Landslide % of landslide Area % of area

% LANDSLIDE / % AREA

Perseverence Clay (28) 2 1.4 11253382.2 3.6 0.4

Woburn Clay Loam (21) 22 15.1 74674560.1 23.8 0.6

Perseverence Clay Steep and Bouldery (29) 20 13.7 6261226.6 2.0 6.9

Plains Clay Loam (2) 3 2.1 1563166.3 0.5 4.1

Woburn Clay Loam, Stony and Bouldery (22) 7 4.8 7939374.7 2.5 1.9

Belmont Clay Loam (40) 17 11.6 38693555.4 12.3 0.9

Palmiste (43) 11 7.5 4441920.9 1.4 5.3

Plains Sandy Loam (4) 2 1.4 3798073.5 1.2 1.1

Belmont Clay Loam, Steep and Shallow (42) 5 3.4 24626199.1 7.8 0.4

Hartman Clay (23) 3 2.1 6593499.9 2.1 1.0

Palmiste Clay Loam, Steep and Shallow (44) 3 2.1 531756.2 0.2 12.1

Belmont Clay Loam, Steep and Shallow (41) 1 0.7 1471734.8 0.5 1.5

Capitol Clay Loam, Drier (35) 7 4.8 8658235.7 2.8 1.7

Capitol Clay Loam, (30) 34 23.3 88402063.6 28.1 0.8

Capitol Clay Loam, Steep and Shallow (32) 4 2.7 14868883.9 4.7 0.6

Hope Clay (7) 3 2.1 648924.1 0.2 10.0

Capitol Clay Loam, Stony and Bouldery (31) 2 1.4 987769.2 0.3 4.4

When the frequency of landslide occurrence was adjusted to take into account the percentage of land area for each soil type, the resultant ratios fell in four distinct clusters which made the determination of susceptibility ratings relatively easy. The initial ratings were subsequently modified from a review of soil survey information related to the susceptibility of particular soil types to various types of landslide events. Of the forty-two (42) soil types described in the soil survey, seventeen (17) had at least one landslide event that was evaluated during the field reconnaissance. Of the soil types that had recent or historic landslides noted, four were identified in the soil survey as being landslide prone. Those four soil types and a brief description are as follows: � Palmiste Clay Loam (SU 43). Brown soils formed over tuffaceous shales, moderately drained, mainly

in areas with substantial annual rainfall. Noted in Appendix B as being susceptible to landslips; also needing erosion and drainage controls.

� Plamiste Clay Loam, Stony and Bouldery Phase (SU 44). Brown soils formed over tuffaceous

shales, moderately drained, mainly in areas with substantial annual rainfall. As above, susceptible to landslips; presence of large boulders indicates historic landslide events in this unit.

� Belmont Clay Loam, Steep and Shallow Phase (SU 42). “Brown Earth” soils formed over ash or

agglomerate, well drained, mainly in areas of substantial annual rainfall. Appendix B of the Soil Survey indicates danger of landslides and severe erosion potential.



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� Capitol Clay Loam, Steep and Shallow Phase (SU 32). Reddish soils formed mainly over deeply

weathered basic igneous rocks (“Red Earths”), moderately to well drained. Soil Survey indicates danger of landslides and severe erosion potential. The Belmont and Capitol soil types (42 and 32) present the greatest risk of landslide events. Other phases of the Capitol Clay Loam are less prone to landslides.

The susceptibility ratings presented in Table 2.25 reflect the relative occurrence of landslide events mapped during the field reconnaissance plus increasing the ratings of the three soil types identified in the Soil Survey as being landslide prone (the steep and shallow phase of Palmiste Clay Loam already had the highest rating based on the frequency of observed landslides).

Table 2.25 Soils and Susceptibility Factors, Grenada

Soil Type Name # %/Landslide /%Area

Susceptibility Rating Comments

44 Palmiste Clay Loam (Steep and Shallow) 3 12.1 4

Soil Survey noted as prone to landslips (already had 4 rating)

7 Hope Clay 3 10 4

43 Palmiste Clay Loam 11 5.3 4 Soil Survey bump (+1)

29 Perseverence Clay (stony bouldery phase) 20 6.9 3

31 Capitol Clay Loam (stony bouldery phase) 2 4.4 3

2 Plains Clay Loam 3 4.1 3

32 Capitol Clay Loam (Steep and Shallow)1 4 0.6 3 Soil Survey bump (+1)

42 Belmont Clay Loam (Steep and Shallow)1 5 0.4 3 Soil Survey bump (+1)

22 Wodburn Clay Loam 7 1.9 2

35 Capitol Clay Loam (drier) 7 1.7 2

41 Belmont Clay Loam (stony bouldery phase) 1 1.5 2

4 Plains Sandy Loam 2 1 2

23 Hartman Clay 3 1 2

40 Belmont Clay Loam (typical) 17 0.9 2

30 Capitol Clay Loam 34 0.8 2

21 Wodburn Clay Loam 22 0.6 2

28 Perseverence Clay 2 0.4 2

-- All other Soil Types 0 0 1



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2.4. HAZARD MODEL & SUSCEPTIBILITY MAP DEVELOPMENT

A susceptibility mapping model was prepared for each island that reflected the varying influences of slope, elevation, aspect, geology and soils. The two models used a simple mathematical overlay process that adds the susceptibility ranking for the corresponding cells of each factor map together. The output of the model was reclassified into five susceptibility categories: Very Low, Low, Moderate, High, and Severe. These five categories provide an indicator of landslide susceptibility in the Pilot Study Area on St. Lucia and throughout Grenada. The map equations utilized in the development of susceptibility maps for St. Lucia and Grenada are defined mathematically: St. Lucia

SM = E • (HS*2) • Sl • Gl ( St. Lucia: equation 1) where, E is elevation classified, HS is the slope factor multiplied by two, S is a soil susceptibility index and G is a geology susceptibility index Grenada

SM = E • HS • A • Sl • Gl ( Grenada: equation 2)

where, E is elevation classified, HS is the slope factor, S is a soil susceptibility index and G is a geology susceptibility index The E factor is based on the Digital Elevation Model (DEM) as initial input data. The E factor also takes into account rainfall as it is expected to increase with elevation. The HS (slope) and A (aspect) factors are also based on the Digital Elevation Model (DEM) as initial input data. HS is typically calculated to derive slope angle in degrees and/or percentages from DEM. The A factor is derived by identifying the down-slope direction of the maximum rate of change in elevation between the DEM cell and its neighboring cells. The A factors were broken into three broad ranges according to whether the slope faces are, in lee of, normal, and to the prevailing wind.



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Each factor map was utilized as an equation factor or input parameter in the hazard model. ArcGIS 9.1, with the Spatial Analyst Extension was then used to calculate each equation factor in a simple mathematical overlay process that adds the corresponding cells of each grid together. A conceptual design of the hazard model for St. Lucia is depicted in Figure 2.1 below.

Figure 2.1 Model Data Processing: Equation Factor Grids as Model Input, St. Lucia

The SL (soil) and GL (geologic) factors take into account soil and geologic susceptibility to landslides. Soil and geology units were provided susceptibility ratings, which ranged from low, moderate, high and severe values (1-4). These ratings were based on the analysis of data collected in field, observations during field assessments and literature. The soil and geology polygon shapefiles were then converted to a raster grid, with numeric values assigned to each cell. Once the equation factor grids were created and designated as model input parameters, the model was run and the resulting grid was transformed and reclassified into five susceptibility categories: Very Low, Low, Moderate, High, and Severe. The reclassified values serve as an index to be used as an initial indicator of potential landslide susceptibility in St. Lucia and Grenada.

DEM Elevation

Soil Data Conversion

Geology

GEO_SOIL Grid

DEM

Slope

Aspect

GEO_DEM Grid

GEO_DEM Grid

GEO_SLOPE Grid

GEO_ASPECT Grid

GIS Arithmetic Overlay

LANDSLIDE ISLAND

LANDSLIDE PILOT AREA

Data Conversion



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3. HAZARD MAP DESCRIPTION

3.1. LANDSLIDE HAZARD MAP DESCRIPTION: SAINT LUCIA

Based on the evaluation of the data within Castries proper and use of island wide data from DeGraff’s previous landslide mapping work, much of Castries show a moderate to high susceptibility to landslides with only the south central area showing a low susceptibility. Even though a few landslides were mapped in the area during field reconnaissance, each landslide fell within the low to moderate to high susceptibility areas. These landslides are mainly debris slides, some associated with creep, and found mainly in weathered andesites. Of particular importance is the high landslide susceptibility zone represented at Black Mallet/Maynard Hill where the largest single landslide exists in the Castries area. Similar east facing slopes at higher elevation show a high susceptibility to landslides and should be identified as areas of concern (i.e. attention should focus on identifying mitigation to prevent or educe the impact of this hazard). It is important to note that the landslide susceptibility map was developed based on combination of physical factors (i.e. slope, soil, geology and elevation) and did not incorporate the influence of human factors. Although information on density of housing or human settlements was not readily available at the time of this consultancy; this information should not be underestimated in determining susceptibility to landslides. Higher density housing areas such as at Entrepot, Bagatelle Ravine Chabot and Pave where landslides were experienced but fell into low to moderate susceptibility categories, should be checked for further study. Human factors in combination with the identified physical factors can increase the susceptibility rating for several areas where there is a high density of housing on sloping lands. Typically other factors such as poor waste water practices and sewerage disposal, lack of a drainage infrastructure, deforestation and cultivation of slopes in the Castries area may further contribute to a higher susceptibility rating for the abovementioned and other communities throughout Saint Lucia.



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Figure 3.1 Landslide Hazard Map of Castries St. Lucia, West Indies



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3.2. LANDSLIDE SUSCEPTIBILITY MAP DESCRIPTION: GRENADA

The mapping factors, described above, proved to be significant in defining landslide susceptible areas as many of the key areas are represented within the susceptibility zones demarcated as moderate to high in most cases. Landslide types found on the island are mainly debris slides, debris flows, rock falls and creep. All landslides types were found to be located mainly in varied elevation zones of the island and within particular rock and soil types. The island of Grenada can be subdivided into at least three broad susceptibility zones:

� North Central Zone dominated by Mt. St. Catherine Volcanic in the central area, � West Central Zone, and � South Central Zone dominated by the South East Mountain Volcanics in the central area.

The North Central Zone shows a range in landslide susceptibility from low to moderate to severe. The Zone is dominated by rockfalls and ancient rockslides to the west and northwest along the coast and by debris flows in the higher elevations particularly on the higher slopes of Mt. St. Catherine. Rock falls, in particular, are caused by the dimensions by which roads are cut into particular steep slopes. Roads cut into steep sloping lands pose a significant threat to the traveling public. Rock slides, which were identified to be mainly ancient rock slides, are distributed along the higher elevation slopes away from the coastline. The highest elevation slopes at Mt. St. Catherine show the highest susceptibility area to debris flow landslide especially after heavy and prolonged rainfall as experienced during a storm or hurricane. Hurricane Ivan caused the generation of numerous debris flow landslides at Mt. St. Catherine. The West Central Zone shows some similarities to the North Central Zone in the susceptibility rating of areas to landslides in that the distribution of rockfalls along the coast are associated with road cuts. As elevation increases there is an increase in susceptibility to landslides in other areas throughout this zone. Creep is also a dominant feature in localized areas such as Florida, Nesbitt and Mabouya. Other factors such as rock type, soil type and slope become important in these areas as well as in higher elevation areas. Higher elevations in this West Central Zone show a high susceptibility to debris flows which are caused by heavy and prolonged rains as observed after Hurricane Ivan. The South Central Zone reflects a similar range in susceptibilities to landslides and covers a larger area than the other Zones. The lower elevation areas show a low susceptibility to landslides in which the dominant landslide type is ancient rock slides. Slopes are fairly stable to this type of landslide activity and do not pose a threat to reactivation. Debris slides are predominant and occur within the higher elevation from the coast within very deep weathered soil. These areas are characterized by reddish weathered volcanic soils and reflect some of the deepest weathered soil in the study. Areas in upper elevations show a higher susceptibility to landslides and debris flows. Also there seems to be a strong correlation between elevation and heavy and prolonged rainfall events.



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Figure 3.2 Landslide Hazard Map of Grenada, West Indies2

2 Please note that base map data used for production of maps was at a scale of 1:25,000; however, final map was plotted at 1:30,000 scale in

ArcGIS. Please note that a more detailed landslide hazard map was plotted at a scale of 1:10,000 for Florida, Grenada but was not included in the final report.



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3.3. MAP UTILIZATION

3.3.1. General Statement: Map Use and Limitations This landslide hazard maps are intended primarily for land use and development planning purposes. The hazard maps identify areas that are associated with a very low, low, moderate and high, and severe hazard susceptibility levels. In general this map may be used to: � Guide future development away from high hazard zones; � Identify areas currently under development for which priority should be given to minimize risk; � Identify areas where more detailed mapping and investigation will be required prior to granting of

development approval, especially where existing communities or future development are vulnerable (moderate, high and very high hazard zones)

� Identify areas where development is highly unlikely to be affected by landslides (low hazard). It is important to note that the zones do not imply legal restriction or regulation by zoning ordinances or laws as laid down by government authorities. The map is, however, an important tool that may be used to inform decisions that may help reduce losses through the incorporation of structural (including retrofitting), economic, socioeconomic and environmental preventative and/or mitigation measures. One limitation of these maps are that they may underestimate the true potential for landslides because the landslide database utilized in this study represents only a partial record of the actual number of landslides that have occurred. Another limitation is that only physiographic features were utilized in the hazard mapping model and areas that have been significantly modified as a result of recent development may not be represented as having high susceptibility to landslides. It should be noted and understood that natural changes as well as human-induced changes also affect the susceptibility to this hazard. Therefore, these maps are intended to be used as a general guide to landslide occurrence for the purposes outline above, and not as a predictor of hazard at specific development sites. These maps do not take the place of an on-site survey or professional judgment by a geologist of geotechnical engineer. The lack of or resolution of some input data may have affected map accuracy. The accuracy of the landslide hazard maps can be improved upon as more detailed information of landslide occurrence and more accurate topographic data becomes available.



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3.4. SPECIFIC RECOMMENDATIONS FOR UTILIZATION OF LANDSLIDE HAZARD MAPS

The following section contains a number of considerations regarding the use of the resultant landslide hazard maps in the natural hazard planning process.

3.4.1. Development Planning Considerations Major projects require comprehensive site inventory and analysis to identify hazard areas and determine site development suitability.

For major development projects that would normally undergo an Environmental Impact Assessment (EIA), potential land developers should be required to submit a complete site inventory and analysis including mapping (on topographic maps or other appropriate base map) and/or reports that fully identify all hazard conditions and the implications for development. � Site Inventories: The developer should be required to provide the Physical Planning Unit with

information on soils, geology and landslide susceptibility at the same mapping scale as the development site plan.

� Site Analysis: The analysis information should be focused on the suitability of development for all areas of the site. Common methods of analysis would result in a map that identifies areas of the site with either constraints or opportunities for development that reflect the soils, geology and landslide susceptibility. Corresponding language in land use or development ordinances can define what constitutes these different levels of limitations or developers can be required to submit written justification for their analysis based on site conditions.

� Development Suitability: It is important to clearly define what is acceptable for a site analysis in the different landslide susceptibility categories (i.e., severe, high, moderate, low, and very low). No action on the part of the developer may be required in the low and very low categories. In the severe and high risk categories, the government is justified in requesting additional information and analysis on the part of the developer. This is also important in the moderately constrained areas where some level of site modifications could mitigate site constraints.

In any event, this analysis should be prepared, reviewed and approved before specific development plans are proposed to avoid any unnecessary expense by potential developers and to serve as the basis for all subsequent engineering design and construction activities. Recommendations for minor development projects, single family residences or small lot subdivisions.

Hard copies of the landslide susceptibility maps should be used routinely by the Physical Planning Unit staff when reviewing development proposals for minor projects. If a proposed project is located in a high or severe landslide susceptibility category, the plan reviewers should determine if the site plan can be modified to eliminate or minimize the potential for future landslide events. Modifications might include shifting the location of proposed structures or realigning access to minimize grade and, in general, to be more sensitive to site topography. In any event, the Physical Planning Unit should provide the applicant with a simple checklist with best management practices that could be employed during construction to minimize the potential for future landslide events. Cumulative impacts should also be considered to reduce development impacts on down-gradient property owners. One example to address cumulative impacts is avoiding the concentration of storm water run-off and discharge of grey water by lateral spreading techniques.



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Require conceptual and/or preliminary plans to confirm that proposed development activities will be consistent with site constraints.

Prior to the development of detailed planning, design and engineering documents for a proposed development, a conceptual development plan – identifying boundaries for use areas, main points of vehicular and pedestrian access and circulation, and routing for basic infrastructure such as sewer and water service - should be submitted, reviewed and approved. As part of this process, it is not uncommon for options to be identified and evaluated that respond to the suitability analysis previously prepared. Physical characteristics of the parcels should be analyzed to determine whether a scheme can be developed that minimizes the impact of hazards such as clustering or orienting buildings to take advantage of and avoid complications due to topography and slope. Of particular importance in this stage of the planning process is the determination of site access points. For subdivisions or lot development, the primary use of the property is associated with the buildings and access is usually a secondary consideration. Although access should not dominate development criteria, it is important to be given attention. Access may dictate where buildings can be located, but access should not dominate or overwhelm the primary land development. Also, roadways often serve, intentionally or otherwise, as drainage conveyances for surface flows. Roads can impound small flows, divert existing flows to other areas and concentrate surface flows to new locations. If the drainage aspects of access are ignored, it is likely that they will cause unanticipated erosion and sediment transport down-gradient.

3.4.2. General Site Planning and Design Considerations Require appropriate construction methods based on site conditions.

Many properties can be used for a variety of uses or can accommodate a variety of building designs and styles. However, in areas where properties have numerous development constraints, the options for building size, design and construction techniques may be similarly limited. A logical outgrowth of the review of site development suitability and conceptual plans is the identification of conditions that will influence construction techniques. A primary consideration is the determination of building foundation (i.e., the base upon which a building will be placed) construction methods, which is closely tied to soil and geologic conditions. This important aspect of site development should reflect the nature of the conditions and hazards that are present on any particular property to provide a safe, stable platform for the rest of the building. A good foundation design will often provide protection for the rest of the building, as well as the occupants, from slope failure or creep, uneven settlement of the foundation and, in certain soil types, the expansion/contraction of clay soils under varying hydrologic conditions.

3.4.3. Landslide Specific Considerations As a general rule, BMCs and Participating States should avoid intensive development of any facilities in areas having natural slopes steeper than 50 percent slope and encourage future development in areas where natural slopes are flatter than 20 percent slope, as stable cut and fill slopes can be economically constructed in such areas, reducing the risk of slope failures. Construction of residences and roads on slopes greater than 50 percent are highly prone to landslides and should be discouraged during the subdivision and/or site plan review process (e.g., development review



41

agencies should not accept private roads for public maintenance in areas of slopes greater than 50 percent).

Proper engineering should be used for the design for all major cut and fill slopes, whether the proposed development is publicly or privately developed. Properly designed, constructed, and maintained slopes and drainage facilities can overcome limitations and constraints in many cases. However, the risk of cut and fill slope instability becomes progressively higher and the cost of constructing stable slopes becomes

significantly higher when the natural slopes are steeper than about 35% (19.3° or 2.9H:1V)3. In addition, the following specific guidelines and considerations have been developed from geotechnical investigations of landslide failures primarily associated with roadside cut and fills: � Many of the cut slopes in soil and weathered rock on the island have been made at an inclination of

about 100% (45° or 1H:1V). A number of these cut slopes have failed or are prone to failure, which indicates that cut slopes in these materials need to be less steep. The only exception to this guideline would be for bedrock units that have been found to be inherently stable under extreme cut slopes.

� Experience also suggests that slopes cut into similar weathered rock units tend to be stable at an inclination of about 1½H:1V (e.g., most of the fill slopes that have failed or are showing signs of failure have inclinations steeper than 1½H:1V).

� Fill slopes constructed at inclinations steeper than 2H:1V commonly have slope failures of the outer several feet of soil because normal construction equipment cannot compact soil sufficiently on these relatively steep fill slopes.

� Fill slopes constructed at inclinations steeper than 1½H:1V can have deep-seated failures across the entire fill slope.

If there is uncertainty over any of these concerns it is better to expect the worst and require site specific geotechnical investigations for major development projects. The following recommended guidelines address best management recommendations for residential or commercial developments: � Avoid development within and directly below steep-sided drainages that extend up hillsides. Such

areas are highly susceptible to rock and mud debris flows.

� Require adequate sewage disposal systems for new construction so that hillsides will not become saturated and destabilized; use plastic piping to direct effluent to the sides of existing homes, rather than down slope of the home.

� Install subsurface storm drains for new developments and channel runoff safely away from existing or proposed developments on steep slopes.

� Avoid intensive development in areas of historical landslide deposits or where colluvial soil mapping units have been identified. These areas have severe risks for landslides and structural foundation failures are common.

These guidelines are presented as recommendations only; the respective development review agencies would need to determine what thresholds are appropriate for intervention depending upon the scale of the proposed development project and the level of landslide risk.

3 The three different measures of slope are respectively defined as percent slope, degree of slope, and slope ratio or horizontal distance over vertical rise in distance.



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4. HAZARD MAPPING WORKSHOP Two (2) workshops were held on each BMC. Each workshop was intended to be interactive and structured to provide a background and understanding of how the hazard maps were developed and can be applied in the vulnerability assessments. The workshop was structured in the following manner: On the first day the mapping consultants, including the CIPA Project Team, provided an overview of the hazard mapping methodology, procedures and outcomes, while the second day, although not attended by consultant team, focused on the finalization of hazard maps (i.e. final map layout, format, color, use and limitations).

4.1. WORKSHOP GOALS

The workshop goals were: � To provide participants with a firm understanding on how the landslide hazard maps were developed;

and � To promote dialogue between the primary users of hazard information for utilization in the vulnerability

assessments.

4.1.1. Landslide Hazard Presentation The CIPA Consultant Project Team prepared a PowerPoint presentation for each BMC. The Presentation focused on the hazard mapping methodology, procedures and outcomes. Presentations were facilitated by Mr. Stanley Wharton and Jeffrey Euwema. The presentation format was informal and thus allowed for interaction between the HMVA Subcommittee and the Consultant Project Team (See Appendix 5.5). The presentation included five (5) major sections:

1. Purpose 2. Project Objectives 3. Technical Approach

a. Step 1: Data Collection b. Step 2: Field Reconnaissance and Mapping c. Step 3: Base Map Analysis and Classification d. Step 4: Factor Map Development e. Step 5: Hazard Model and Susceptibility Map

4. Project Outcomes 5. Suggested Map Uses

4.2. WORKSHOP OBSERVATIONS AND ISSUES

The following section describes observations and issues raised during the Landslide Hazard Map presentation in Grenada on January 10, 2006 and Saint Lucia on January 12, 2006.



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4.2.1. Workshop Observations and Issues: Grenada � It was recommended that the discussion of map preparation be expanded in the final report. The

HMVA Subcommittee asked for elaboration on the various types of landslides observed in particular areas of the country.

� The Subcommittee recommended that the consultant project team discuss the rapid nature of field

assessment. A detailed discussion was included by Mr. Stanley Wharton. The consultant project team also explained that a description was already included in the Final Report.

� The Subcommittee requested that the GIS data layer of landslide locations/types that was derived from

field work be provided to the HMVA in both digital and non-digital format. � A discussion was focused on natural vs. man-induced landslides. It was indicated that the purpose of

the map was to identify areas that are naturally prone to landslides and not provide an analysis of how developmental impacts such as roads, clearing, and septic tanks, have contributed to landslide activity. The result of this discussion was to focus on how the map should be utilized for hazard mitigation purposes i.e. development review and physical planning.

4.2.2. Workshop Observations and Issues: Saint Lucia � Mr. Alphonse pointed out that the colors depicting susceptibility levels were not clear, especially

between high and severe categories. He asked if the color of these two susceptibility categories could be changed on the final map. The consultant project team agreed to revise final maps.

� HMVA Subcommittee members recommended that the consultant project team discuss the rapid

nature of field assessment. A detailed discussion was included by Mr. Stanley Wharton. The consultant project team also explained that a description was already included in the Final Report.

� Mr. Wharton also explained how a former study, especially work conducted by Degraff, was

incorporated in the field work and final map development. � HMVA Subcommittee members asked if an environmental sensitivity analysis was utilized for the

development of the landslide hazard map. Mr. Euwema explained that a statistical method was utilized for the development of the landslide susceptibility hazard map

� The Subcommittee asked if the consultant project team considered human-induced factors (i.e.

infrastructure, drainage, settlement density, etc.) in their analysis for the development of the landslide hazard map. Mr. Euwema explained that the consultant project team only utilized physical factors (i.e slope, soil, geology, etc.) in their analysis and did not incorporate the influence of human factors. It was explained that during of inception mission, human settlement or land use information, in digital format, was not readily available.

� It was also pointed out that the Landslide hazard maps did not indicate that high density housing areas

such as at Entrepot, Bagatelle, Ravine Chabot, and Pave are prone to landslides. It was explained that the consultant team used readily available data and that only physical factors were considered in the hazard model. It was agreed that the Consultant project team would conduct a simple overlay analysis to verify its hazard model by overlaying several factor maps on village locations.



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4.3. WORKSHOP CONCLUSIONS AND RECOMMENDATIONS

The following section contains a number of recommendations for future landslide mapping in each BMC.

4.3.1. Recommendations for Landslide Hazard Mapping: Grenada � Develop a computerized Landslide Hazard Register for Grenada. This is needed to map and monitor

landslide incidence both annually and seasonally and for hazard assessments and mitigation purposes. � Conduct a detailed engineering geology site analysis for areas affected by landslide activity. This is

vital for Florida, Nesbitt and Mabouya. These areas require attention to define the risk associated with construction in similar rock/soil types. Several buildings have already been condemned and destroyed by these creep landslides in Grenada. A deep seated site investigation using geophysical techniques could offer an opportunity to define the extent of the affected areas, causative factors and design an appropriate development and mitigation strategy. Since land availability is becoming scarce in lower elevation areas, the evaluation of these affected sites should be considered a priority.

� Conduct a detailed geotechnical analysis in areas affected by rockfalls to determine severity, zones of

relative risk, and remediation since these landslides directly affect the traveling public. Remediation measures should focus on the relative severity of zones and will be part of the engineering geological assessment.

� Conduct a detailed investigation of the primary road network for landslide hazards. Develop a more

focused study to define levels of risk especially since the primary road network acts as key communication and emergency route before, during and after a major hurricane or other natural disaster. Traditionally, this type of investigation has not been conducted for the islands but there is need to investigate the levels of risk from major landslides and volume of debris that could block vital access to the outlying villages. Many islands in the Caribbean, including Grenada, have single access road networks which when blocked usually lead to isolation of villages and have a negative impact to the entire economy.

� Conduct a re-mapping of the geology of the island of Grenada. This mapping effort is required to define

the rock types exposed. Arising from the workshop was wider recognition of the poor quality and low resolution of surficial geological data upon which important decisions are to be made especially for the generation of hazard models for different hazard types. In Grenada, large expanse of the volcanic formations have been described as “Undifferentiated” and these require further definition especially for landslide mapping and seismic zonation work. A more detailed mapping of the geology would assist in identifying landslide-prone rock types and aid in landslide hazard modeling and creation of a digital landslide hazard register.

4.3.2. Recommendations for Landslide Hazard Mapping: Saint Lucia � Undertake a landslide hazard mapping analysis of the island of St. Lucia to define areas of relative risk

to landslides perennially and during hurricanes and storms. The last island-wide mapping was conducted in 1985 by Jerome DeGraff and a comprehensive assessment should be performed at least within a 10 year time frame as severe weather events are becoming more prevalent in the Caribbean.



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� Develop a computerized Landslide Hazard Register for Saint Lucia. This is needed to map and monitor landslide incidence both annually and seasonally and for hazard management in the future.

� Conduct a detailed engineering geology site analysis for areas affected by landslide activity. Special

attention should focus on Black Mallet, Entrepot, Pave and Boucage Ravine Chabot in Saint Lucia. These areas require attention to define the risk associated with construction in similar rock/soil types. A deep seated site investigation using geophysical techniques could offer an opportunity to define the extent of the affected areas, causative factors and design strategy. Emphasis should be placed on the impact of human factors such as density of housing and drainage infrastructure or lack thereof on landslide activity. This is important as informal residential development is encroaching on steeper and less stable areas. An engineering geology evaluation of these affected sites should be considered a priority.

� Conduct an investigation of the primary road network for landslide hazards. Develop a more focused

study to define levels of risk especially since these networks act as key communication and emergency access routes before, during and after a major hurricane or other natural disaster. Traditionally, this type of investigation has not been conducted for the islands but there is need to investigate the levels of risk from major landslides and the volume of debris that could block emergency access to outlying villages. Many islands in the Caribbean, including Saint Lucia, have single access road networks which when blocked usually lead to isolation of villages and loss in revenue nationally.

� A re-mapping of the geology of both Castries city area and the island of Saint Lucia is required to

define the rock types exposed. Arising from the workshop was wider recognition of the poor quality and low resolution of the surficial geological data upon which important decisions are to be made especially for the generation of hazard models for different hazard types. The city of Castries is one such area since key areas included in the landslide hazard mapping were previously unmapped. Similarly there are certain parts of central Saint Lucia which are unmapped and may be included in an island wide hazard analysis. A more detailed mapping of the geology would assist in identifying landslide prone rock types and aid in landslide hazard modeling and creation of a digital landslide hazard register.



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5. APPENDICES

5.1. PROJECT TEAM MEMBERS

Jeffrey Euwema, Director Council for Information and Planning Alternatives, Inc. 2305 Calle Laurel, Suite 910 San Juan, Puerto Rico 00913 Tel. 787.982.3811 Email: [email protected] Mr. Pieter de Jong, AICP Hazard Mitigation Specialist 20700 Westerly Road Poolesville, MD 20837 Unites States of America Tel: 301.518.3451 Email: [email protected] Mr. Stanley Wharton Engineering Geologist Pax Vale Santa Cruz, Trinidad Trindad and Tobago Tel. (868) 676-1688 Email: [email protected] Mr. Hiro Takeshita 400 Springbrook Dr. Silver Spring, MD 20904 Tel: 301-622-0240 e-mail: [email protected]



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5.2. INCEPTION MISSION ITINERARY

LANDSLIDE MAPPING INCEPTION MISSION

NATIONAL HAZARD MITIGATION PLAN DEVELOPMENT

Saint Lucia September 5 and 6, 2005

Time Activity Representatives Involved

Day One 8:30 am Field Assessment Team Mobilization 8:30am – 9:00am Map Review and Mobilization 9:00am – 5:00 pm Field Assessment

Field Assessment Team to be mobilized. Two CIPA Project Team members (de Jong and Wharton) will initiate field work on Day One with the support of the designated national focal point (field work).

9:00am Meeting with Hazard Mapping and Vulnerability Assessment Subcommittee 1. Welcome & Introductions:

National Emergency Management Agency (NEMO)

Mr. Julian Du Bois, Deputy National Disaster Coordinator

2. Hazard Mapping in relation to National Hazard Mitigation Plan development

Dr. Cassandra Rogers, Caribbean Development Bank

3. Project Expectations 4. Project Objectives 5. Technical Approach 6. Data Needs and Collection 7. Project Execution & Timeframe

Mr. Euwema, CIPA Project Director briefs the HMVA Subcommittee

11:00am – 5:00pm GIS Breakout GIS Breakout – CIPA Staff will use the Common Digital Database document to collect and review available data layers. CIPA will identify data gaps and collection strategies.

CIPA Project Team, Country GIS Representatives, and other persons with knowledge of data status. National hazard mapping technical contact to lead national effort.

Day Two 8:30am Field Assessment Team Mobilization 8:30am – 9:00am Map Review and Mobilization 9:00am – 5:00pm Field Assessment

Entire CIPA Project Team to be mobilized, with designated national focal point (field work).



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LANDSLIDE MAPPING INCEPTION MISSION

NATIONAL HAZARD MITIGATION PLAN DEVELOPMENT

Grenada September 7-10, 2005

Time Activity Representatives Involved

Day One 8:30 am Field Assessment Team Mobilization 8:30am – 9:00am Map Review and Mobilization 9:00am – 5:00 pm Field Assessment

Field Assessment Team to be mobilized. Two CIPA Project Team members (De Jong, Wharton) will initiate field work on Day One with the support of the designated national focal point (field work).

9:00am Meeting with Hazard Mapping and Vulnerability Assessment Subcommittee 1. Welcome & Introductions –

National Disaster Management Agency (NADMA)

Mr. Sylvan McIntyre, National Disaster Coordinator

2. Hazard Mapping in relation to National Hazard Mitigation Plan development

Mr. Sylvan McIntyre, National Disaster Coordinator


Mr. Euwema, CIPA Project Director briefs the HMVA Subcommittee

11:00am – 5:00pm GIS Breakout GIS Breakout – CIPA Staff will use the Common Digital Database document to collect and review available data layers. CIPA will identify data gaps and collection strategies.

CIPA Project Team, Country GIS Representatives, and other persons with knowledge of data status. National hazard mapping technical contact to lead national effort.

Day Two/Three 8:30am Field Assessment Team Mobilization 8:30am – 9:00am Map Review and Mobilization 9:00am – 5:00pm Field Assessment

Entire CIPA Project Team to be mobilized, with designated national focal point (field work).



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5.3. BRIEF ON INCEPTION MEETINGS

The landslide mapping project for Saint Lucia (SLU) and Grenada (GRN) is part of the effort to develop landslide hazard maps as part of the national hazard plan development process. Brief the HMVA Subcommittee on the technical approach, timeframe and outputs of the landslide mapping consultancy. In SLU, the lead project consultant, Jeffrey Euwema, met with the HMVA Subcommittee to discuss the landslide mapping mission. Dr. Cassandra Rogers of the CDB was also present and facilitated introductions to emergency management staff and participants. Shortly thereafter, Mr. Euwema facilitated a PowerPoint presentation which outlined:


After the presentation, discussion focused on data collection needs and issues. Mr. Euwema discussed a series of preliminary data needs based on a review of the Common Database Document (CDD) for SLU. Mr. Julian Dubois, Deputy Director of the NEMO, designated Mr. David Alphonse of the Ministry of Planning as the POC for geospatial data concerns. A series of base maps were then presented and discussion focused on the definition of the project study area. Mrs. Glenda Charles expressed concerns about an area of landslide risk not being included in the pilot study area. This area, she explained, included a mountain ridge north of the defined study area. Mrs. Charles asked if the study area could be extended to include this area. Dr. Rogers indicated that the area could be extended based on availability of geospatial data. Dr. Rogers tasked both Mr. Alphonse and Mr. Euwema to assess the data needs for including this new area as to extend the pilot project area. Other concerns raised during this meeting focused on the format of the map workshop that is part of this consultancy. Attention focused on the use of GIS tools for hazard mapping as well as the application of the hazard information in natural hazards planning. In GRN, Jeffrey Euwema met with the HMVA Subcommittee to discuss the landslide mapping mission. Mr. Sylvan McIntyre of NERO facilitated introductions to emergency management staff and participants. Shortly thereafter, Mr. Euwema facilitated a PowerPoint presentation which outlined:


After the presentation, several important questions were posed by Mr. David James of the Agency for Reconstruction and Development (ARD). His concerns focused on input data (i.e. scale of base map information), field assessment team familiarity with the island, especially landslide prone areas, and the application of the information in hazard mitigation and development planning decisions. Related to development practices, Mr. James mentioned that many of the roads are being built without appropriate cut/fill slopes making them prone to both rock falls and landslides.



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He asked if the resulting hazard information could be used to for detailed engineering design. Mr. Euwema mentioned that the resulting hazard maps should not be used in lieu of detailed engineering studies, but could be used for broad based planning decisions. Mr. Euwema further explained that the resulting hazard maps were to be used as part of a national hazard mitigation program and those special concerns such as engineering design recommendations could be incorporated into an action plan. In regards to a more focused field assessment, Mr. Sylvan McIntyre indicated that knowledgeable government personnel would assist the project team in its assessment and that critical areas would be identified prior to field reconnaissance. Discussion focused on identifying landslide prone areas. Areas mentioned included the communities of Florida, Brizan, Gouyave, and Chatameille. Mr. Euwema asked if there were any areas of concern with the pilot project area. Many of the participants stated that most of the landslide concerns were north of the pilot area. Mr. Euwema presented a list of data concerns for Grenada. His initial concerns focused on the absence of geology and soils GIS layer in the CCD. Mr. Mason explained that both these layers were available in digital format and indicated that he would make them available. Mr. Euwema inquired if there was supporting documents that described the physiographic characteristics of each of these layers. Mr. Euwema asked about the availability of aerial photography or satellite imagery. Mr. Mason indicated that IKONOS imagery data was available for the entire island. Mr. James also indicated that the ARD was getting ready to request a statement of qualifications from firms interested in performing Lidar mapping of the island. He indicated that the data would not be available for the landslide mapping project. To close discussions related to GIS data, Mr. Sylvan McIntyre asked Mr. Mason to work with Mr. Euwema in order to facilitate the project team’s data collection efforts.



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5.4. ATTENDANCE SHEETS FOR INCEPTION MEETINGS

Saint Lucia

Name Ministry Mr. Julian DuBois NEMO Mrs. Sherma Lawrence Statistics Department Mr. David Alphonse Physical Planning Section Mrs. Glenda Charles Physical Planning, Survey and Mapping Mr. Chamberlain Emmanuel MCWT/PU Mr. Alva Francis SLASPA Mr. Trevor Alfred Cable and Wireless Mrs. Sharon Emmanuel Cable and Wireless Mrs. Lydie Gleyson MCWT/PU Dr. Cassandra Rogers Caribbean Development Bank

Grenada

Name Ministry Mr. Keith Simon Ministry of Finance Mr. Michael Mason Ministry of Agriculture Mr. Sylvan McIntyre NADMA Mrs. Bernadette McGillwary NADMA Mr. David Jones Agency for Reconstruction and Development

(ARD) Mr. Garth Andrew Ministry of Works



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5.5. HAZARD WORKSHOP PRESENTATIONS



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6. REFERENCES DeGraff, Jerome V., Landslide Hazard on St. Lucia, West Indies, U.S. Department of Agriculture forest Service, Fresno, CA, December 1985. Earle, Kenneth W., Geological Survey of Grenada and the Grenadines, October 1923. Island Resources Foundation, Grenada: Country Environmental Profile, St. Thomas, U.S.V.I., 1991. Latin American Energy Organization, Reconnaissance Study of the Geothermal Resources of the Republic of Grenada, December 1981. Organization of American States, Department of Regional Development, Saint Lucia Development Atlas, St. Lucia 1984. Stark, J.; Green, A.J. and Lajoie, P., Soil and Land-Use Surveys No. 20, St. Lucia, Regional Research Centre U.W.I., Trinidad, W.I., October 1966. Vernon, K.C.; Payne, Hugh and Spector, J., Soil and Land-Use Surveys No. 9, Grenada, Regional Research Centre U.W.I., Trinidad, W.I., June 1959.

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