GIS AS A TOOL IN THE ASSESSMENT OF NATURAL HAZARD
VULNERABILITY, ANTIGUA AND BARBUDA
Eva Hodgkinson-Chin, M.Sc.* and Cassandra Rogers, Ph.D.**
*Department of Surveying and Land Information, The University of the West Indies, St.
Augustine, Trinidad
** Department of Civil Engineering, The University of the West Indies, St. Augustine,
Trinidad
Abstract: Under the USAID/OAS Post-Georges Disaster Mitigation (PGDM)
project, a national natural hazard mitigation plan is being developed for Antigua
and Barbuda. An assessment of the vulnerability of critical facilities to individual
natural hazards is a key information source for the plan. The paper describes the
use of ArcView GIS and MS Access to semi-automate the vulnerability
assessment procedure, in order to facilitate its use by disaster managers and
planners. The vulnerability assessment procedure developed by the National
Atmospheric and Oceanic Administration (NOAA) consists of a three-stage
process involving the identification and prioritization of hazards, hazard mapping
and a critical facilities analysis. The latter involves the creation of an inventory of
critical facilities and the weighted assessment of each facility in terms of damage
history, structural vulnerability and operational vulnerability for each hazard
identified. Zoned maps of relative hazard are then combined with maps of critical
facilities to assess the locational vulnerability of individual facilities. A facility
vulnerability score (FVS) is then calculated for each facility and hazard type. The
project utilized a Microsoft Access 2000 database of the critical facilities and
linked the data required for spatial analysis to ArcView 3.2. An SQL query
calculated the facility score for damage history, structural vulnerability and
operational vulnerability. The query was imported into the GIS and a process of
spatial and tabular manipulations used to assess the locational vulnerability and
calculate the FVS. Facility vulnerability layers were developed for each hazard to
identify priority zones for disaster mitigation.
INTRODUCTION
The Post Georges Disaster Mitigation (PGDM) program is one of three
components of the Hurricane Georges Reconstruction and Recovery in the
Eastern Caribbean program. The PGDM is responsible for implementing the
disaster mitigation capacity building component and seeks to reduce the
vulnerability of population and economic activities to natural hazards.
The vulnerability assessment component of the mitigation plan requires that
critical facilities are identified and data on their disaster history, structural and
operational vulnerability are collected. This data is to be entered into a database
and integrated into a GIS. The key layers in the GIS are hazard maps prepared by
specialists in the areas of wind, drought, storm surge, floods, coastal and stream
erosion and earthquakes. Other features such as agricultural land, major roads,
bridges and telecommunication installations are also included in the GIS.
VULNERABILITY ASSESSMENT PROCEDURE
The vulnerability assessment procedure developed by NOAA consisted of the
following:
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Identification and prioritization of hazards
Creation of individual hazard maps
Creation of an inventory of critical facilities
Assessment of each facility in terms of damage history, structural
vulnerability and operational vulnerability for each hazard identified.
The locational assessment of facilities within hazard zones
The calculation of a hazard facility vulnerability score (FVS) for each
facility.
Each facility is assessed separately for each hazard in terms of its damage history,
structural vulnerability, operational vulnerability and locational vulnerability. The
FVS is defined by the formula:
FVS = (V+L) HPS
Where “V” is the Facility Vulnerability and is the total of the damage history,
structural vulnerability and operational vulnerability scores, “L” is the Locational
Vulnerability and “HPS” is the Hazard Priority Score.
Table 1 indicates the facility vulnerability assessment criteria and associated
rankings. Values are incrementally numbered with “0” representing no history or
no vulnerability.
Table 1 – Facility Vulnerability Assessment Criteria and Scores
Criteria
Description
Score
Damage History None
0
Minor
1
Moderate
2
Repetitive / Significant
3
Structural
No
0
Vulnerability
Yes
1
Operational
No Effect
0
Vulnerability
Minimal
1
Significant
2
Life Threatening
3
Damage history assesses the extent and frequency of damage to a facility by a
hazard. The determination of structural vulnerability is based on such information
as building age, condition, maintenance, materials of construction, retrofitting and
the existence of un-repaired damage. Operational vulnerability is defined as the
extent to which the functions of a facility will be affected by a hazard. It is the
most subjective of the facility assessment criteria as it ‘factors in’ an assumption
of damage history and structural vulnerability. The operational vulnerability
score for facilities that function as shelters reflects this function. However, the
subjectivity of the assessment procedure is balanced by the system design, which
allows for the re-assessment of facilities at any time.
“L” is the locational vulnerability of a facility. It is determined by the hazard
susceptibility of its location. Each hazard theme was zoned in terms of degree of
hazard and scores assigned. Table 2 indicates the hazard zones and their scores.
Table 2 Locational Vulnerability and Scores
Locational
Score
Vulnerability
None
0
Low
1
Moderate
2
High
3
Very High
4
The HPS was generated from a prioritized listing of the hazards to which the
island is prone and which were weighted incrementally. The HPS listing for
Antigua and Barbuda consisted of the six hazards listed in Table 3.
Table 3 Prioritized Hazards by HPS
Hazard
HPS Score
Winds / Hurricanes
6
Drought
5
Storm Surge
4
Floods
3
Coastal and Stream Erosion 2
Earthquakes
1
GIS Design
The design developed seeks to semi-automate the procedure for the assessment of
facility vulnerability. It also facilitates the wider database requirements of the
National Office for Disaster Services (NODS) in Antigua and Barbuda. It
provides through tabular query the data required for vulnerability analysis and
enables the calculation of the FVS within the spatial environment.
Figure 1 indicates the main components of the Vulnerability Assessment GIS. The
design utilizes the approach of developing a database of critical facilities in MS
Access 2000 and linking the data required for spatial analysis to a point feature
theme of critical facilities in ArcView 3.2. The Access database is extensive and
includes detailed data on facilities and their associated buildings. The ArcView
project includes layers of the various hazards, critical facilities and other layers
required for analysis such as topography and population distribution.
VULNERABILITY ASSESSMENT GIS
MS Access
ArcView 3.2
Data on
Facilities
Hazard layers
Critical Facility layer
Other layers e.g..:
roads, population
Figure 1. Main Components of GIS
Figure 2 is a cartographic model of the assessment process indicating the
relationship between the MS Access and ArcView components. It indicates the
creation of a critical facilities table in MS Access by a select query on the main
facilities table and the joining of the query to the critical facilities theme table in
ArcView. The hazard zone value from each hazard layer is then assigned to the
critical facilities layer and the FVS calculated.
MS Access
Facilities Table
MS Access
Select query
ArcView
Critical Facilities
Query Table
Join
ArcView
Facilities Point
Theme Table
ArcView
Critical Facilities
Layer
Critical Facilities
Layer
ArcView
Assign data by
location
Critical Facilities
Theme Table with
assigned values
ArcView
Calculate FVS
Hazard Layer
ArcView
Figure 2. Cartographic Model of Vulnerability Assessment
FVS
MS ACCESS 2000 DATABASE DEVELOPMENT
The development procedures within MS Access 2000 included:
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The review of existing data and identification of entities and attributes
The classification and coding of attribute data
The creation of tables, primary and foreign keys
The definition of relationships
Creation of data entry and retrieval forms
Data entry
Creation of GIS query
Data was reviewed to identify entities, attributes and facilitate the classification
and coding of data. Entities were identified and attributes determined. The
objective was to create normalized, non-redundant table structures. Three entities
were identified Facilities, Buildings and Contacts.
Attributes were classified and coded to facilitate data entry and analysis. Table
linkages were defined utilizing the Tools Relationships window in MS Access
2000. Update and delete referential integrity was enforced between the Facility
and Building tables. Delete referential integrity was enforced between the
Facilities table and the Contact table. Figure 3 indicates the tables and
relationships established.
Figure 3. Critical Facilities Database – Tables and Relationships
Forms were used to facilitate data entry and retrieval. Figure 4 shows the
Vulnerability Form. All the forms use drop down lists, where applicable to
facilitate data entry and error catching. As Figure 4 indicates, users select
descriptive text but numeric values are entered into the database. Check boxes
were not utilized, as ArcView 3.2 tables do not recognize the data type and spatial
analysis cannot be conducted on data entered in this manner.
Figure 4. Vulnerability of Facilities Form
GIS Query
A “GIS” query was built in the Access database to provide the link to ArcView
3.2. It consisted of calculated fields that summed the damage history score,
structural vulnerability score and operational vulnerability score of each facility
for each hazard type. This generated the “V” score in the FVS. The query also
selected those facilities that were defined as critical using the shelter and type of
facility (code) fields. This allowed for flexibility in the definition of critical
facilities.
ARCVIEW 3.2 DEVELOPMENT
The database development conducted within ArcView 3.2 included connecting
the GIS query to the facilities theme table and the spatial processing of the hazard
and critical facilities layers. The ArcView 3.2 environment was used to determine
the locational vulnerability of facilities and calculate the final FVS.
A facility feature theme was created and the facility identifier from the Access
facility table was added to the theme table. The Access GIS query table was
added to the project and joined to the theme table on the common data item of
GIS_ID.
Figure 5 details the spatial analysis conducted. The Geo-processing wizard in
ArcView was used to assign the hazard zone values from the hazard maps to the
features of the critical facilities table on the basis of location. This procedure
added the scores as a field to the facilities table. A new field was then added to the
table and calculated to the FVS formula.
Assign data from hazard theme to
facilities theme on basis of
location
Add new numeric item to
facilities table
Calculate new item to
(V+L)HPS
Total Facility Vulnerability
Score (FVS)
Figure 5. Spatial Manipulations in ArcView 3.2
Connecting to the GIS Query
The SQL Connect function of ArcView 3.2 connects the ArcView 3.2 project to
the MS Access 2000 database. The project is connected to all fields in the GIS
query. Once the ArcView 3.2 project is saved with the connection established the
project will seek to connect to the database every time it is opened.
The SQL Connect function makes the query available as a table in the ArcView
3.2 project window. This table is joined to the critical facilities theme table on the
common data item of GIS_ID. The resulting critical facilities theme table includes
“V” values for each hazard.
Assessing Locational Vulnerability
The geo-processing extension of ArcView 3.2 was used to assign the hazard zone
values to the critical facilities theme table and to determine the locational
vulnerability. Scores are assigned on the basis of location as indicated in Table 2.
After processing, the critical facilities table has assigned to it the locational
vulnerability scores from each hazard zone. The locational vulnerability score
field of each hazard theme must be named differently to ensure that the fields are
retained when the facilities table is subjected to further processing. To achieve
this a new field is added to the table and the field values are calculated to the
locational scores field. A naming convention of “hazardtype+L” was used e.g:
WindL
Calculating the FVS
The FVS for each hazard is calculated within a new numeric field which is added
to the facilities table. The field is calculated to the FVS formula: (V+L)HPS using
the Field | Calculate command. The “V” value originates from the GIS query, the
“L” value is assigned by geo-processing and the HPS is determined by the hazard
prioritization weighting. Figure 6 indicates the Field Calculation window in
ArcView 3.2 and the entry of the WindFVS formula. Note that the HPS for
Hurricane / Wind was 6 and must be manually entered into the formula.
Figure 6. Calculating WindFVS
RESULTS
The vulnerability assessment produces critical facility vulnerability scores. The
FVS is a relative weighting of each facility in terms of its vulnerability to a
particular hazard. Hurricane winds were identified as the most significant hazard
of Antigua and Barbuda. Table 4 indicates the categories used to zone this hazard
and their lower and upper bounds.
Table 4 – Wind Hazard Categories
Hazard Description Lower Bound Upper Bound
Level
Wind Speed Wind Speed
in meters/sec
in meters/sec
0
None
0
17
1
Low
17
43
2
Moderate
43
50
3
High
50
59
4
Very High
59
100
Source: Wagenseil, R. 2001. Wind and Storm Surge Technical Report. http://www.oas.org/pgdm.
Figure 7 represents the vulnerability analysis result for the island of Antigua with
regards to hurricane winds. It indicates that the long term susceptibility of
Antigua to winds and the FVS of critical facilities. Most of Antigua and in
particular the eastern and central districts are shown to be moderately susceptible.
These areas would generally experience category 2 storm winds with moderate
damage. The western section of the island is of low susceptibility and would most
often experience tropical storm or category 1 strength winds. The south facing
slopes of the southern range are within a highly susceptible zone and would
experience category 3 and 4 winds with extensive damage.
The highly susceptible zones consist largely of woodland and grazing lands. The
exception is along the southeast coast where the southern section of Dockyard and
Shirley Height’s are highly vulnerable. Most of St. John’s is vulnerable to low
winds.
Critical Facilities and Wind FVS
For the purpose of the PGDM Vulnerability Assessment project in Antigua and
Barbuda critical facilities were defined as the following:
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Any facilities that functioned as a shelter
Hospitals and clinics
Government administrative buildings
Airports, Sea ports and Bridges
Power, Water and Telecommunication Installations
Oil and Gas Companies
Protective Services
Hotels and Guest Houses
Historical Sites
The distribution of wind FVS for all critical facilities is shown on Figure 7. The
map indicates that the facilities with the higher FVS are fairly evenly distributed
throughout the north, west and southern sections of the island. Figure 8 shows the
location of facilities that have a high FVS (greater than 33). They are clustered
around St. John’s and scattered evenly throughout the southern half of the island.
Figure 7- Hurricane / Wind Hazard Zones and FVS
The analysis revealed that several key critical facilities had extremely high wind
FVS. The facilities and their FVS are listed below:
Holberton Hospital - 54
Crabbs and Cassada Power Stations - 48
Crabbs Desalination Plant – 48
Friars Hill Power Station - 42
V.C. Bird International Airport – 42
The facilities included the main hospital, main power and desalination plants and
the main airport. In addition, several shelters had a high FVS of 36 and are listed
in Figure 9. The results indicated that further examination was required of these
facilities. For example in the case of the shelters either structure retrofitting or
relocation may be required.
Figure 8. Critical Facilities with High Wind FVS
Figure 9. Shelters in Antigua with Highest Wind FVS
CONCLUSION
The system is designed to allow for the re-assessment and updating of facilities.
This review may also include hazard mapping. Improvements in models and the
availability of data may make available refined mapping and improve the quality
of the assessment process. Map layers can be added or deleted from the project at
any time.
The vulnerability assessment as developed represents an attempt to determine
relative vulnerability so as to inform mitigation priorities. The “FVS” is a relative
score given to each facility to represent its vulnerability to a hazard. The “FVS”
should be used as an indication of the need for further examination of facilities.
The relative contribution of the “V” and “L” factors to the FVS score should be
analyzed to provide a guide to mitigation measures. Facility scores help to focus
mitigation requirements and can assist in the prioritization of activities by
identifying the most vulnerable facilities. Hazard zoning indicates safety zones
and zones which require special measures for safe development. The results can
be used to guide future development and mitigate the effects of hazards on present
development.
REFERENCES
Hodgkinson-Chin, E. 2001. Hazard Vulnerability Assessment for Antigua and
Barbuda, Post Georges Disaster Mitigation Project, USAID / OAS, pp. 46;
www.oas.org/pgdm.
Rogers, C.T. 2001. Training in Natural Hazard and Vulnerability Assessment –
Final Report, Post Georges Disaster Mitigation Project, USAID / OAS, pp. 62.