User s Guide to Storm Hazard Maps and Data

Transcription

1 Storm Hazard Assessment for St. Lucia and San Pedro/Ambergris Caye, Belize User s Guide to Storm Hazard Maps and Data Prepared For: Caribbean Development Bank Advanced technology and analysis solving problems in science, engineering, commerce, and public policy. R&D Division, 1204 E 49 th Street, Savannah, Georgia 31404, USA

2 Development of, Wave, and Storm Surge Hazard Maps for Belize and St. Lucia Users Guide for Storm Hazard Data Sets Introduction This document describes the storm hazard data sets and maps produced for Belize and St Lucia from the perspective of a potential user. It consists of four major sections. The first section discusses how hazard zones are defined and the implications of these definitions. The second section provides an overview of the maps and data sets produced in this study. The third section provides sample applications of the data. The final section comprises user notes for specific data sets. A detailed description of the methodology used to develop these data sets and maps is provided in the companion document, Methodology for Storm Hazard Mapping. 1.0 Definitions of hazard zones The Terms of Reference given to the contractor by the Caribbean Development Bank (CDB) requested the development of hazard maps designating high, medium, and low zones for wind, wave, and storm surge hazards. Defining such zones is challenging, however, as hazard impacts are strongly determined both by the frequency of occurrence of hazardous conditions ( how often do they occur ) and their intensity or severity ( how bad is it when they occur ). It is important to point out that the use of high, medium and low for hazard zones is practically the reverse of the designations given to the hazards that impact these zones. In the above definition, high hazard zones are designated as such because they are subject to hazard events more frequently than are medium and low hazard zones. Low hazard zones are affected less frequently, and by higher intensity events. A good example is given by the coastal flooding hazard: low lying areas are high coastal flood hazard zones, as they may be inundated frequently by low intensity events, while they also will be inundated by the medium and high intensity events that will also inundate the higher elevation areas. When considering the protection of a given asset, these definitions are similarly reversed. For example, a lifeline infrastructure asset should be built to withstand the low risk phenomena, as the consequences of loss are great. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 1

3 Definition Alternative No 1: The first task is to define hazardous conditions, independent of the frequency with which these may occur. This can be done based on accepted standards of the level of intensity of wind, wave and storm surge at which structures and people that occupy those structures would start experiencing damage. The following conditions are proposed as constituting a hazard: : two-minute average wind of 30 m/s or higher. Wave: crest to trough wave height of 1.0 meters or higher. Storm Surge: water depth 0.3 meters above terrain height or higher. Once a hazardous condition is defined, the frequency with which this condition occurs can be used to determine the hazard level. After analyzing the entire available history of hazardous events for a given study area, a High hazardous zone can be defined as a zone where hazardous conditions are likely to occur during an average lifetime. A Medium hazardous zone can be defined as one where such conditions are less likely to occur in an average lifetime, and such conditions are unlikely to occur in a lifetime in a Low hazardous zone. To translate this subjective definition into a quantitative one, we can use the results of the statistical analysis, as reported in the TAOS Statistical Analysis Package reports for Belize or Saint Lucia. Using this approach, a high hazardous zone is defined to exist where the 50-year MLE 1 event reaches or exceeds the hazardous condition as defined above. In other words, a high wind hazard exists where the 50-year MLE event produces a twominute average wind of 30m/s or greater. Thus, the following definitions for hazardous zones are proposed: High hazard zone: hazardous conditions exist in the 50-year MLE event. Medium hazard zone: hazardous conditions exist in the 100-year MLE event. Low hazard zone: hazardous conditions exist in the 100-year, 90% prediction limit event. Definition Alternative No 2: The definition described under alternative No. 1 is statistically correct, but may be counter-intuitive, and is quite difficult to interpret, since it combines a pre-defined threshold value for the phenomenon (30m/s as the definition of a hazardous wind condition) with a statistically defined frequency of occurrence (areas where the 50- year MLE event reaches hazardous conditions are defined as HIGH HAZARD zones). The primary limitation of alternative 1 is that it only shows where the pre-defined threshold values of wind, wave and surge are (and are not) reached, and how often. 1 Maximum likelihood estimate User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 2

4 Alternative 2 provides a user-friendly way to present hazard information in map form so that users may see the full range of extreme wind, wave and surge values that can occur in the area they are interested in, as well as the frequency with which these values are expected to occur. This approach requires a larger number of maps, but provides more useful information for development planners, emergency managers and structural engineers. For example, the Belize low coastal flood hazard map (bzcc1_flood_100y90pl.pdf) shows the range of values of the phenomenon (water depths) for one of the three return periods (the 100 year 90% event). The three Belize maps (one each for 50- year MLE, 100-year MLE, and 100-year 90% prediction limit) together contain much more information than would a single map showing High, Medium and Low coastal flood hazard zones. The final deliverables have been produced following alternative Study Products Data Sets Nine data sets three each for wind, surge and wave hazards are provided in ESRI Shapefile format. Each data set is provided in two map projections for a total of eighteen Shapefiles for each study area. The primary outputs of the TAOS hazard mapping system are in WGS-84 Geographic coordinates. These outputs were reprojected to the local mapping system used in each study area as discussed in the companion document Mapping and Projection Notes. In addition to the standard files comprising a shapefile (.shp,.shx, and.dbf), a.prj file is provided with each shapefile to document the projection used. data sets Three wind shapefiles are provided. All wind values are expressed as twominute average winds, ASOS 2 compatible. wind_50ymle wind_100ymle wind_100y90pl 50-year Maximum Likelihood Estimate in m/s 100-year Maximum Likelihood Estimate in m/s 100-year 90% Prediction Limit in m/s Each shape file contains three data fields: cat GRASS Category (the unique polygon ID) value The value of the polygon label Text label 2 Compatible with wind any observation made by the Automated Surface Observation System User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 3

5 The following values and labels are used: Value Label m/s m/s m/s m/s m/s m/s m/s m/s 9 over 70 m/s Coastal Flood data sets Three coastal flood shapefiles are provided. The coastal flood level values shown on the maps are measured relative to the terrain height on land and relative to mean sea level in open water. The raised water levels in open water include contributions from storm surge and wave setup. Note that high wave risk areas are flagged in the coastal flood hazard data, as shown in the table below. This recognizes that waves and flooding occur simultaneously, and simplifies the mapping process. It alerts the user that activities in these areas may have to contend with both physical flooding and wave action. flood_50ymle 50-year Maximum Likelihood Estimate Flood (wave flagged) in m flood_100ymle 100-year Maximum Likelihood Estimate (wave flagged) in m flood_100y90pl 100-year 90% Prediction Limit (wave flagged) in m Again, each shape file contains three data fields: cat GRASS Category (the unique polygon ID) value The value of the polygon label Text label The following values and labels are used: Value Label 1 < 0.5m 2 0.5m-1.0m 3 1.0m-2.0m 4 1.0m-2.0m (Wave) 5 2.0m-3.0m (Wave) 6 3.0m-4.0m (Wave) 7 4.0m-5.0m (Wave) 8 5.0m-6.0m (Wave) 9 over 6.0m (Wave) Note: the coastal flood hazard data sets and maps show flood levels and wave information for inland areas and for open water areas up to 300m offshore. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 4

6 Conversions metric-imperial Coastal Flood Metric Imperial Metric Imperial m/s mph < 0.5m < 1.6 ft m/s mph 0.5m 1.0m ft m/s mph 1.0m 2.0m ft m/s mph 2.0m 3.0m ft m/s mph 3.0m 4.0m ft m/s mph 4.0m 5.0m ft m/s mph 5.0m 6.0m ft m/s mph over 6.0m over 19.7 ft over 70 m/s over 156 mph Maps The following tables list the maps produced as part of the storm hazard assessment for Belize and Saint Lucia., wave and surge values were estimated for each of the three levels of frequency of recurrence, as defined in the section on Definitions of Hazard Zones above. Storm surge and wave hazards are presented together in one map labeled coastal flooding (storm surge and wave hazards). The reason for this is that waves can only exist where there is flooding (caused by storm surge), and the wave height is a function of the water depth in the flooded area. The wind hazard is presented in a separate map. All maps are presented in WGS-84 and in the specific projection used by government planning agencies. See Belize and Saint Lucia mapping notes for the definition of the local projections in use. Maps are available in PDF format, and are formatted as 11 x17 scenes, allowing for easy printing on a regular office color printer. Scenes can be mounted together to form larger maps. The maps are also available as shapefiles, which can be integrated into the GIS data base of government and other agencies. Belize maps The study includes the areas of Caye Caulker and Ambergris Caye. Study outputs for both Cayes are presented at 1: scale. To stay within the 11 x17 scene, each Caye is covered by 2 map sheets or files. A higher resolution presentation at a scale of 1:10,000 is made for San Pedro, which spans 4 map sheets. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 5

7 Title Scale File names Hazards Hazard Levels Belize Index map -- Bzidxmap Caye Caulker 1:25K cc1; cc2 Coastal Flood Ambergris Caye 1:25K ac1; ac2 Coastal Flood San Pedro 1:10K Sp1 to sp4 Coastal Flood For all: 50 year MLE 100 year MLE 100 year 90% prediction limit Saint Lucia maps The study includes the entire island of Saint Lucia. Study outputs are presented at 1:25,000 scale. To stay within the 11 x17 scene, Saint Lucia is covered by 31 map sheets or files. An overview map, combining all 31 map sheets, is also presented. A higher resolution presentation at a scale of 1:10,000 is made for the greater Castries area, which spans 9 map sheets. Title Scale File names Hazards Hazard Levels Saint Lucia Index map -- slindexmap Saint Lucia 1:25K slmap02 to slmap32 Coastal Flood Saint Lucia Overview 1:25K slmapovr Coastal Flood Castries Index Map casidxmap Castries 1:10K casmap01 to casmap09 Coastal Flood For all: 50 year MLE 100 year MLE 100 year 90% prediction limit For all: 50 year MLE 100 year MLE 100 year 90% prediction limit Note: the following Saint Lucia map tiles were not produced as they only cover open water areas (see index map): maps 1, 4, 21, 25, 29, 30 User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 6

8 3.0 Application Examples Below are some examples of the use that can be made of the information provided in the statistical report and the maps of wind, wave and storm surge hazards affecting Belize and Saint Lucia. Flood hazard maps Areas where coastal flooding is likely in a 50-year return period should not be used for housing and other activities that can be affected by the flooding. In cases where such areas are used for certain structures, for example tourism infrastructure, these structures should be designed so that they can withstand the expected flooding (water height and wave action) as indicated by the map. Areas where coastal flooding is unlikely to occur in a 50-year return period, but likely in the 100-year return period, can be used for housing and other activities. The owners however would be advised to take precautions and invest in flood proofing to the level indicated on the map. Lifelines and critical infrastructure that need to continue to function under all extreme weather events should be located in areas that a free of flooding according to the 100-year, 90% prediction limit map. Coastal infrastructure, such as sea defences and roads, must be constructed to withstand, at a minimum, the expected flooding and wave action expected within the 100-year return period. Ideally, a higher detail, site-specific analysis should be undertaken as the basis for a risk assessment and determination of design standards. hazard maps With the exception of limited areas with homogenous topography (such as in the Belize cayes) the wind hazard varies according to location and topography (see the Saint Lucia overview wind maps: slmapovr_windhi). Housing should be built to withstand, at a minimum, the wind forces expected to occur at that location as shown on the 50-year return period map. The owner may want to upgrade the wind resistance of his house to withstand the expected 100-year wind. Such an investment would provide additional protection and peace of mind, and it could also be used to negotiate a better insurance rate. Lifelines and critical infrastructure that need to continue to function under all extreme weather events should be designed to withstand the expected wind forces as per the 100-year, 90% prediction limit maps. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 7

9 4.0 Special Notes on the use of this hazard data Notes on TAOS Model Outputs It should be emphasized that the TAOS simulations are designed to report the peak wind speed for the return period in question, independent of wind direction. The degree of protection of a site is highly dependent on the direction of approach of incoming storm events, and the interaction of the site characteristics and storm winds is not necessarily linear. Additionally, the resolution of the simulation can have an influence on the quality of the outputs. The design engineer should evaluate this data with the above factors in mind, and a good knowledge of both the site in question and the proposed use of the structure, then apply sound engineering judgment as to the appropriate wind speed to use to compute load factors. This is especially true when considering TAOS outputs for use in design procedures mandated by building codes. Note on Geographic Datums There are a variety of both vertical and horizontal datums in use in the Caribbean. In constructing the data sets for the numerical modeling, all of the horizontal data were either originally in or converted to WGS-84, while the vertical heights were converted relative to the modeled sea level for 1 September This is an extremely complex subject, due to the changing nature of datums, sea level, and other factors. However, in practice, using the TAOS outputs presented here is straightforward. The values presented are in terms of height above mean sea level (MSL). To use the outputs with any given vertical datum, determine the offset of that datum from mean sea level, and add that value to or subtract it from the TAOS result. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 8

10 Bibliography Johnson, M. E., 1997: Caribbean Storm Surge Return Periods, Organization of American States Caribbean Disaster Mitigation Project Workshop, Kingston, Jamaica, October 31, Johnson, M. E. and C. Watson, Jr., 1999: Hurricane Return Period Estimation, 10th Symposium on Global Change Studies, Dallas, TX, Vermeiren J. (2000). Final Report on the Caribbean Disaster Mitigation Project. Organization of American States: Washington, D.C. Vermeiren J., K. Ford, and C. C. Watson. (1995). Engaging planners and investors in the assessment of storm risk in Jamaica, AMS 9th Conference on Applied Climatology: Dallas, TX. Vermeiren, J. and C. C. Watson. (1994). New technology for improved natural hazard risk assessment in the Caribbean, The Journal of Contingency Planning. 6, Watson, C. C. and M. E. Johnson. (1999). Design, Implementation, and Operation of a Modular Integrated Tropical Cyclone Hazard Model, AMS 23rd Conference on Hurricanes and Tropical Meteorology, Dallas, TX. Watson, C., Jr., 1995: The Arbiter Of Storms: a high resolution, GIS based storm hazard model, Natl. Wea. Dig., 20, 2-9. Watson, C., Jr., 2002a: Using integrated multi-hazard numerical models in coastal storm hazard planning, Solutions for Coastal Disasters 02 Conference Proceedings, American Society of Civil Engineers, Reston, VA. Watson, C., Jr., 2002b: Implications of climate change for modeling coastal hazards, Solutions for Coastal Disaster 02 Conference Proceedings, American Society of Civil Engineers, Reston, VA. User s Guide to Storm Hazard Maps and Data for Belize and St. Lucia Page 9