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

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

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: http://www.unc.edu/ims/luettich/jbikman/01_23_2013/literature%20dump/statisticalmodelsofhollandpressureprofileparameter.pdf 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.

Legend #* Buoy and C-MAN ASOS station 172 Anguilla 1 1163 172 171170 176 163 163 1 163 170 163 175 163 177 1 176 117711 1 1681 1 1 168 6-12UTC 8,23,1.7 178 1772 168 168 168 169 174169 168 168 169 172170 182 171174 172 11 1177 181 170 170 11 169 169 1169 11 168169 1 168 1 TNCM 171 168 Sint Maarten St. Martin 177 170182 174 179 180 179 174 176 17 174 178 179 1 173170170 186 168 171 171 St. Barthelemy 6-9UTC 4,23,1.7 Saba 107107107 105 104104104 103103 St. Eustatius TNCE 9998 96 97 9594 95 94 90 Antigua and Barbuda 173175 173 171177 176 1731672 177 17117670168169 179 1 171170172 172166516 174 17116816 169 179 173 17016 1 175179 17111665 170 #* 169 175 17211174 168 1 172 BARA9 6-6UTC 4,23,1.7 86 86 85 86 83 8586 84 868685 83 83 848485 85 84 82 81 82 82 828383 8483 Saint Kitts and Nevis 77777777797979 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 CON17-00029680.Maps are subject to change. Created on: 11/13/2017. 0 4.5 9 18 27 36

Legend #* Buoy and C-MAN ASOS station 1333 136 135130131135 134133134138 130130 134 133132131131131131133 140 133 132133 135 134 137 Virgin Islands, British 160169 1 6-18UTC 0,23,1.5 1 1 159 150 160 159 168 168 155 155155 160 163 155 15555 159 155 155 168160 159 155 155 155159 155 168 1 160 150 160 168 1155 160163 150 160 155 159 150 150 155 1 150 160 155 1155 149 149 159 155 1 146 160 1448147 146 159 155 159 121 1225 1450 148 147 148 119 128 144 143 150 150 147 117123 118 124125 150 148 146 150 149 149 160 155 1449 150 122 149 149 150 148 149 1141117118126 TIST 146 155 155 149 1121114 115 144 113111 148 Virgin Islands 41052 #* 6-15UTC 8,23,1.6 85 85 84 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 CON17-00029680.Maps are subject to change. Created on: 11/13/2017. 0 3.75 7.5 15 22.5 30

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 94 86 111112 115 107 97 95 94 95 86 86 107 96 95 949495 86 86 85 98 90 86 85 112112113 113 108 116 108110 110 96 95 95 85 97 96 9596 86 86 86 114114114115 90 119 97 97 119 111 1010 98 94 11116 117 112 98 94 94 90 119 111112 115 96 96 90 9595 120118117117117116 116 112 102 95 9090 115 116 97 97 96 94 94 1201118118118120 115 96 90 90 90 90 123 122 98 98 99 99 95 95 94 127 103 122124130 94 90 90 979696 95 95 94 133 127 101101102 80 96 94 94 9490 80 103104 96 96 90 90 90 135137 138 94 96 98 96 95 9595 90 98 141138 136 102 100 98 96 90 143 137140 96 95 96 90 90 101 140141150 99 94 94 90 101 141143145 94 94 94 95 94 96 95 143145148 102 99 97 96 97 96 98 9798 147 98 98 98 98 100 9999 99 100 100 102 101 104101 103 104 104 105 107 107 1230 Legend #* Buoy and C-MAN ASOS station 108 108110 112 115 113112 111 99 99 112 115 112116 99 116 122 114 114116 100 104 120 101103 125124 104105 126 105106 107 109 107109 110 113 112 8-3UTC 0,26,1.56 146 155 155155 159 155 160 119 126 1119 127125127 130125125 131128130 133 134 116 119 118 121 160 163 11 159 160 8-0UTC 9,23,1.6 0 4 8 16 24 32 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 CON17-00029680.Maps are subject to change. Created on: 11/13/2017.

12,1.25 19-15utc 7,12,1.5 19-12utc 3,13,1.5 19-9utc 4,13,1.5 TAPA 55 55 5654 55545455 73 5756555760 6972 70 63 63 5756585964 5658 69 63 64 64 56 56 56 58585859 71 56 69 6669 60 59 60 606060 64 7270 64 63 69 63626262 63 63 7572 70 7169 72 66 66 75737171 65 75 8178 73 TFFR 76 73 80 8481 77 7978 80 85 82 8082 8483 85 797984 90 98 82828385 104 104 8686 108 111 107 Guadeloupe Legend ASOS Station Hurricane Maria Track (Version A) 148150TDPD 19-6utc 942,13,1.5 148 148 19-3utc 4,13,1.5 Dominica TDCF 1431455 1431441 140141142150150 138140141145146 1338144 148 134137140 145 135134136 136 133132 136 128129 Martinique TFFF 19-0utc 5,13,1.5 79 78 7578 77 73 70707273737369 6868 666868696868 70 636464646464 62 5960 60606060 63 66 58 57 575757555558 54 54 54 49 525254 53 5251 504950 49 484849 46 474747 46 47 47 494545 454344 18-23utc 950,12,1.5 18-21utc 950,12,1.5 18-18utc 956,11,1.5 18-15utc 959,11,1.5 18-12utc 9,12,1.5 18-9utc 977,12,1.5 18-6utc 977,12,1.5 0 12.5 25 50 75 100 St Lucia TLPC 31 323030 3030 3030 30 29 28 2820 272727 2728 27262627 2830 262626262829 252525 2525 2628 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 CON17-00029680. Maps are subject to change. Created on: 11/16/2017

1 TAPA Legend 19-19utc 0,12,1.25 19-15utc 7,12,1.5 19-12utc 3,13,1.6 19-9utc 4,13,1.7 19-6utc 942,13,1.7 Dominica TDCF Guadeloupe 5354 5453 53535353 73 5655545658 69 62 62 565556576259 5456 68 72 63 59 62 54 59 54 54 57565757 70 54 68 66 6668 59 58 59 60 58 59585858 63 7269 63 6866 66606060 62 7472 70 7068 71 64 64 7775727070 64 74 TFFR 8178 7572 72 78 8481 75 7977 78 85 80 7980 82 82 84 72 90 99 81808283 105 105 85858585 110 112 108 TDPD 155 148160 147148155 147150150 1481559 148160 163 155150 147 1 134135 19-3utc 4,13,1.7 19-0utc 5,13,1.7 18-23utc 950,12,1.7 18-21utc 950,12,1.7 ASOS Station Hurricane Maria Track (Version B) Martinique 77 76 7576 75 71 6970717171 6666 6566666666 6568 6626262 60 5758 58585858 55 55 5555 64 55535355 52 52 51 505052 50 46 5048 474747 454646 44 454445 43 4544 464243 434141 TFFF 18-18utc 956,11,1.7 18-15utc 959,11,1.7 18-12utc 9,12,1.7 18-9utc 977,12,1.7 18-6utc 977,12,1.7 0 12.5 25 50 75 100 TLPC St Lucia 2828 282627 2626 2626 25 2525 2525 232424242526 23 2323232324 222222232425 212222222325 21 21 21 2123 22 2122 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 CON17-00029680. Maps are subject to change. Created on: 11/16/2017

Legend 22-15utc 958,36,1 Hurricane Maria Track 58 585757 58 57585656 60 5554 58 54 55 58 60 5553 54 55 56 60 54 53 53 54 5455 585858 59 52 52 56 56 55 55 5859 53 53 5354 54 55 55 Turks and Caicos Islands 53 55 58 59 53 53 53 53 5454 55 63 51 55 58 62 4545 52 53 5456 56 53 5354 54 55 57 57 60 44 44 46 51 5155 52 53 53 54 57 50 5353 54 51 55 58 5960 59 6363 65 454445 46 54 54 65 65 47 44 51 50 53 53 55 59 585960 62 6262 65 66 66 5558 65 4444 44 53 46 5051 51 55 53 54 55 58 58 58 59 60 62 45 63 64 44 44 44 5049 54 59 58 59 64 64 43 48 48 49 49 49 48 5454 57 5858 58 60 60 62 6263 63 43 43 44444647 48 56 43 58 58 59 60 62 636363 474748 48 5657 57 43 45 46 56 44 43 43 44 46 58 58 60 60 65 68 66 49 44 48 6058 57 57 59 6262 58 59 43 59 58 60 60 62 63 66 65 69 4647 57 57 57 60 62 6666 7471 60 59 60 62 63 66 65 7373 43 58 58 6869 71 434242 60 64 72 70 4142 63 6 65 65 6566 66 68 69 41 64 64 64 65 66 69 69 41 4041 65 65 64 66 6868 68 40 63 636564 65 66 68 40 65 66 68 41 65 64 65 65 66 66 68 4140 40 66 66 40 6665 65 7068 40 66 65 65 68 65 64 65 65 68 6969 6866 68 66 66 6664 6666 66 65 66 65 7778 63 63 7977 63 64 646363 75 75 65 65 64 74 75 64 74 74 77 74 75 75 78 77 22-12utc 959,35,1 64 63 585757 58 58 56 56 657 58 58 57 55 57 60 58 75 79 74 747373 7373 71 70 77 78 75 57 69 68 5855 5 5456 55 55 5753 54 56 0 5 10 20 30 40 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 CON17-00029680. Maps are subject to change. Created on: 11/16/2017

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 1-176 mph Cat 4-5 British Virgin Islands (Tortola and Virgin Gorda) -168 mph Cat 4 Providenciales 108-124 mph Cat 2-3 Grand Turk (Irma) 99-109 mph Cat 1-2 Dominica (lower bound) 128-1 mph Cat 3-4 Dominica (upper bound) 134- mph Cat 3-4 Grand Turk (Maria) 74-79 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:

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 74 95 90 116 81 105 2 96 110 117 134 106 121 3 111 129 135 122 142 4 130 190 143 172 5 > >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 7-16. 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: https://www.asce.org/templates/publications-book-detail.aspx?id=24136 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.

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 126 159 172 Barbuda 179 126 160 168 St Martin (French) 182 115 (36 m/s 10-min) Anguilla 176 130 176 British Virgin Islands 168 134 169 180 Turks & Caicos Islands (Provo) 124 124 155 (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 2008. 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.