The Wind Speeds in Selected Islands During Hurricanes Irma and Maria in 2017 compiled by Tony Gibbs FREng

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Randell King
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1 The Wind Speeds in Selected Islands During Hurricanes Irma and Maria in 2017 compiled by Tony Gibbs FREng Structural failures leading to serious damage or collapse, or those severely affecting the use of the facility, are important matters, as they weaken the faith of the users in the stability and safety of the facilities they occupy. When serious damage or collapse has occurred to a single structure, that occurrence no longer remains an individual issue. All similar structures constructed by the same group or in the same area, or of the same type, pose nagging questions. Are they safe? Have they assured serviceability? Are they defective? It has, therefore, become necessary to understand what the causes of any particular serious damage or collapse are in order to make improvements for future design and construction. It has to be said that it is equally important to understand what are the reasons for success of the many facilities that sustained little or no damage during severe natural hazard events. Though the factors involved in serious damage or collapse are generally technical or procedural there are several others to be considered. These are political, personal, administrative, resource-based and many more. However, this paper will focus mainly on the technical factors. Complete accuracy of findings can never be guaranteed, even in the most competent and objective investigation. There are several cases where disagreement about the cause of serious damage or collapse exists. It is important to reduce the inevitable uncertainties by having a better knowledge of the forces which may have caused the serious damage or collapse. This information for hurricanes cannot be obtained from the media or from bulletins describing the overall weather system. For this reason, and as an essential aid for carrying out forensic surveys of serious damage or collapses caused by the hurricanes of 2017, the Pan American Health Organisation commissioned a study of the wind fields in several Caribbean islands during the passages of Irma and Maria. The study was undertaken by Dr Peter Vickery and the compilation of this document was done by Eng Tony Gibbs in order to understand better what happened during Hurricanes Irma and Maria. Dr Peter Vickery of Applied Research Associates was responsible for the preparation of the maps. His commentary follows: Attached are the wind maps we developed providing estimates of wind speeds on various Islands brought about by Hurricanes Irma and Maria. The estimates were developed using a combination of the hurricane track data provided by the National Hurricane Centre (central pressure and position) coupled with estimates of the radius to maximum winds (RMW) and the Holland B parameter. B provides information on the pressure-wind speed relationship, where, the higher the value of B, the higher the value of the

2 wind speed for the same central pressure. Estimates of RMW and B were obtained by inputting different values into our hurricane wind field model and then comparing time series of both modelled and measured wind speeds and surface pressures to the various observations made in the Caribbean. 1 Observations of wind speeds and pressures came from a combination of airport and Buoy and C-MAN stations. Our estimates of wind speeds are always better if we have good surface level observations. In another communication Dr Vickery points out: Two maps are given for Dominica, sort of upper and lower bounds. I would put more faith in the upper bound estimates but there is great uncertainty in the wind speeds there. Dr Vickery adds: Wind speeds are presented as estimates of maximum 3-second peak gust wind speeds over land. These gust wind speeds are not necessarily representative of the maximum wind speed in the hurricane, which likely occurred elsewhere. Wind speeds given by the National Hurricane Center are estimates of the maximum one-minute sustained (average) wind speed over water anywhere in the hurricane. The Maps The following wind field maps form part of this report: Hurricane Irma Barbuda, Sint Maarten /St Martin, Anguilla and neighbouring islands; Hurricane Irma British Virgin Islands and neighbouring islands; Hurricane Irma Turks & Caicos (Providenciales and Grand Turk) and neighbouring islands; Hurricane Maria Dominica (lower bound) and neighbouring islands; Hurricane Maria Dominica (upper bound) and neighbouring islands; Hurricane Maria Turks & Caicos (Grand Turk) and neighbouring islands. 1 See: for a fuller explanation of Holland B and RMW in the paper Statistical Models of Holland Pressure Profile Parameter and Radius to Maximum Winds of Hurricanes from Flight-Level Pressure and H*Wind Data by Peter J Vickery and Dhiraj Wadhera.

3 Legend #* Buoy and C-MAN ASOS station 172 Anguilla UTC 8,23, TNCM Sint Maarten St. Martin St. Barthelemy 6-9UTC 4,23,1.7 Saba St. Eustatius TNCE Antigua and Barbuda #* BARA9 6-6UTC 4,23, Saint Kitts and Nevis Hurricane Irma (2017): Preliminary Peak Wind Gust (mph) Estimated 3-second gust wind speeds (mph) at 10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Forecast/Advisory 52 at 0300UTC on 9/12/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/13/

4 Legend #* Buoy and C-MAN ASOS station Virgin Islands, British UTC 0,23, TIST Virgin Islands #* 6-15UTC 8,23, Hurricane Irma (2017): Preliminary Peak Wind Gust (mph) Estimated 3-second gust wind speeds (mph) at 10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Forecast/Advisory 52 at 0300UTC on 9/12/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/13/

Turks and Caicos Islands 909082 8686 86 90 86 94 94 90 99 98 90 90 9697 90 90 90 80 99 99 99 90 99 90 110111 104 100 98 9996 90 102 959594 110 111 103 103 103 98 94 9698 96 113 105 96 95 94 94 94 95

5 Turks and Caicos Islands Legend #* Buoy and C-MAN ASOS station UTC 0,26, UTC 9,23, Hurricane Irma (2017): Preliminary Peak Wind Gust (mph) Estimated 3-second gust wind speeds (mph) at 10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Forecast/Advisory 52 at 0300UTC on 9/12/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/13/2017.

6 12, utc 7,12, utc 3,13, utc 4,13,1.5 TAPA TFFR Guadeloupe Legend ASOS Station Hurricane Maria Track (Version A) TDPD 19-6utc 942,13, utc 4,13,1.5 Dominica TDCF Martinique TFFF 19-0utc 5,13, utc 950,12, utc 950,12, utc 956,11, utc 959,11, utc 9,12, utc 977,12, utc 977,12, St Lucia TLPC TLPL Hurricane Maria (2017): Preliminary Peak Wind Gust (mph) - Version A Estimated 3-second gust wind speeds (mph) at10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Intermediate Advisory 41 A at 1200 UTC on 9/26/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/16/2017

7 1 TAPA Legend 19-19utc 0,12, utc 7,12, utc 3,13, utc 4,13, utc 942,13,1.7 Dominica TDCF Guadeloupe TFFR TDPD utc 4,13, utc 5,13, utc 950,12, utc 950,12,1.7 ASOS Station Hurricane Maria Track (Version B) Martinique TFFF 18-18utc 956,11, utc 959,11, utc 9,12, utc 977,12, utc 977,12, TLPC St Lucia TLPL Hurricane Maria (2017): Preliminary Peak Wind Gust (mph) - Version B Estimated 3-second gust wind speeds (mph) at10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Intermediate Advisory 41 A at 1200 UTC on 9/26/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/16/2017

8 Legend 22-15utc 958,36,1 Hurricane Maria Track Turks and Caicos Islands utc 959,35, Hurricane Maria (2017): Preliminary Peak Wind Gust (mph) Estimated 3-second gust wind speeds (mph) at10 m above ground over flat open terrain from ARA model fit to surface level observations using NHC storm track and central pressure data through Intermediate Advisory 41 A at 1200 UTC on 9/26/2017. The maps have been produced for PAHO/WHO under Contract CON Maps are subject to change. Created on: 11/16/2017

9 Interpretation of the Maps The ranges of 3-second gust wind speeds with Saffir-Simpson Categories (over land) are as follows (See the next section for the explanation of S-S Categories): Barbuda -179 mph Cat 4-5 Sint Maarten / St Martin -182 mph Cat 4-5 Anguilla mph Cat 4-5 British Virgin Islands (Tortola and Virgin Gorda) -168 mph Cat 4 Providenciales mph Cat 2-3 Grand Turk (Irma) mph Cat 1-2 Dominica (lower bound) mph Cat 3-4 Dominica (upper bound) 134- mph Cat 3-4 Grand Turk (Maria) mph Tropical storm This note on the correlation of Basic Wind Speeds averaged over 3 seconds with the Saffir-Simpson Scale which uses 1-minute averages is based on the Commentary in the American Society of Civil Engineers (ASCE) document Minimum Design Loads for Buildings and Other Structures ASCE 7. The hurricane reports from the National Hurricane Center include the Saffir-Simpson Hurricane Categories 1 to 5. This scale is relied on by local emergency management agencies in order to warn the populations of the need to prepare for upcoming severe weather systems. The Saffir- Simpson Hurricane Scale has wide acceptance and popularity. Its five Categories are based on wind speed intensity and barometric pressure at the center of the storm. The quoted wind speeds determining the various Categories are sustained wind speeds with a 1-minute averaging time at 33 ft over open water. It is understood that the wind speeds categorising the hurricanes are the most intense in the system typically in the north-east eye wall. Those speeds are not necessarily the ones impacting on any particular island or part of an island. The American Society of Civil Engineers ASCE 7 standard commonly used by engineers for wind-resistant design purposes in the USA and the Caribbean uses a 3-second gust speed at 33 ft above ground in open terrain with scattered obstructions having heights generally less than 30 feet commonly associated with flat open country and grasslands. This is known as Exposure C. The wind speed thus defined is the Basic Wind Speed for use in structural design. It is useful to have approximate relationships between the Basic Wind Speed and the Saffir- Simpson Scale. This is provided by ASCE in a Table in the Commentary section of ASCE 7. The essential parts of that Table are reproduced below:

10 Saffir-Simpson Hurricane Category Sustained Wind Speed (1-minute average) Over Water MPH Gust Wind Speed (3-second average) Over Water MPH Gust Wind Speed (3-second average) Over Land MPH > >1 >173 At the coast the over water equivalents should be used. But Dr Peter Vickery states: The winds transition from an over water surface to an over land terrain pretty quickly. About 70% of the transition is complete after 1 km. He adds: As the wind speeds we have produced are on land, although not fully transitioned to open terrain wind speeds, the relationship between the gust wind speeds is closer to those given in the land column of the attached Table from ASCE The actual values vary with distance from the sea-land interface. A fuller explanation of this correlation of Basic Wind Speeds with the Saffir-Simpson Scale can be obtained in the ASCE 7 Commentary. This document is available from: These notes are presented to help users to understand more-clearly wind speeds shown on the maps for Hurricanes Irma and Maria in this document when compared with the wind speeds reported by weather forecasters and the news media, who commonly use the Saffir-Simpson Hurricane Scale. The gust wind speed values given in the Table that are associated with a given sustained wind speed should be used as a guide only. The gust wind speeds associated with a given sustained wind speed may vary with storm size and intensity. Conclusions and Recommendations Hurricanes Irma and Maria were unusually strong wind systems which, nevertheless, were not unprecedented. Nor can we dismiss the possibility of such systems being repeated in the coming years. The Table below shows comparisons of the maximum wind speeds over land for the impacted islands compared with current minimum guidance in published building codes. The numbers do not take into account climate change which would add 13% to Category II buildings and 10% to Categories III and IV buildings.

11 Islands Maximum 3- second gust wind speed over land in 2017 (mph) 50-year return period 3-second gust wind speed for use with ASCE 7-05 Category II buildings (in published codes) 700-year return period 3-second gust wind speed for use with ASCE 7-10 Category II buildings 1700-year return period 3-second gust wind speed for use with ASCE 7-10 Category III and IV buildings Dominica Barbuda St Martin (French) (36 m/s 10-min) Anguilla British Virgin Islands Turks & Caicos Islands (Provo) (not in TCI Code) 170 (not in TCI Code) Category III and IV buildings are critical facilities required for post-severe-natural-hazard-event functions such as referral hospitals. Category II buildings are most buildings in a community. (Building Categories as defined in building codes must not be confused with Hurricane Categories on the Saffir-Simpson Scale.) The above values for the 700-year return period for Category II Buildings and 1700-year return period for Category III and IV Buildings are failure wind speeds 2 therefore a Load Factor greater than 1.0 does not need to be applied. The most recent wind hazard assessment for structural design purposes in the Caribbean was the USAID-funded, PAHO-executed Development of Design Wind Speed Maps for the Caribbean for Application with the Wind Load Provisions of ASCE 7 Prepared by Peter J Vickery and D Wadhera of Applied Research Associates Inc in Dr Vickery was recently asked if he thought the Caribbean design wind speed maps should be updated. He responded: I do think the study should be updated since the model was largely based on ~ 1970 to 2008 data (49 years) and we have added 9 more years (almost a 20% increase in data). 2 Traditionally, building codes state working wind speeds as their basic or reference wind speeds with a fairly short return period such as 50 years. Load factors or factors of safety are then applied to cover a lot of practical inconsistencies and unknowns. For example, ASCE 7-05 would have a load factor of 1.6 for wind (%1.6 for wind speeds). From ASCE 7-10 onwards they decided, in order to have uniform levels of safety across different locations, to eliminate load factors and importance factors and adopt ultimate wind speeds the wind speeds or loads at which facilities should, in theory, fail hence failure wind speeds or loads. The return period was therefore moved to 1700 years for referral hospitals.

Design Wind Speeds for the Caribbean for use with the Wind Load Provisions of ASCE 7

Design Wind Speeds for the Caribbean for use with the Wind Load Provisions of ASCE 7 Prepared by Peter J Vickery and Dhiraj Wadhera Applied Research Associates 854 Colonnade Center Drive, Suite 37 Raleigh,