The Wind Speeds in Selected Islands During …...The Wind Speeds in Selected Islands During...

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

Structural failures leading to serious damage or collapse, or those severely affecting the use ofthe facility, are important matters, as they weaken the faith of the users in the stability and safetyof the facilities they occupy.

When serious damage or collapse has occurred to a single structure, that occurrence no longerremains an individual issue. All similar structures constructed by the same group or in the samearea, or of the same type, pose nagging questions. Are they safe? Have they assuredserviceability? Are they defective?

It has, therefore, become necessary to understand what the causes of any particular seriousdamage or collapse are in order to make improvements for future design and construction. It hasto be said that it is equally important to understand what are the reasons for success of the manyfacilities that sustained little or no damage during severe natural hazard events.

Though the factors involved in serious damage or collapse are generally technical or proceduralthere 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 andobjective investigation. There are several cases where disagreement about the cause of seriousdamage or collapse exists.

It is important to reduce the inevitable uncertainties by having a better knowledge of the forceswhich may have caused the serious damage or collapse. This information for hurricanes cannotbe obtained from the media or from bulletins describing the overall weather system. For thisreason, and as an essential aid for carrying out forensic surveys of serious damage or collapsescaused by the hurricanes of 2017, the Pan American Health Organisation commissioned a studyof the wind fields in several Caribbean islands during the passages of Irma and Maria. The studywas undertaken by Dr Peter Vickery and the compilation of this document was done by EngTony Gibbs in order to understand better what happened during Hurricanes Irma and Maria.

Dr Peter Vickery of Applied Research Associates was responsible for thepreparation of the maps. His commentary follows:

“Attached are the wind maps we developed providing estimates of wind speeds on variousIslands brought about by Hurricanes Irma and Maria.

“The estimates were developed using a combination of the hurricane track data provided by theNational Hurricane Centre (central pressure and position) coupled with estimates of the radius tomaximum winds (RMW) and the Holland B parameter. B provides information on thepressure-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 inputtingdifferent values into our hurricane wind field model and then comparing time series of bothmodelled and measured wind speeds and surface pressures to the various observations made inthe Caribbean.1

“Observations of wind speeds and pressures came from a combination of airport and Buoy andC-MAN stations. Our estimates of wind speeds are always better if we have good surface levelobservations.”

In another communication Dr Vickery points out: “Two maps are given for Dominica, sort ofupper and lower bounds. I would put more faith in the upper bound estimates but there is greatuncertainty in the wind speeds there.”

Dr Vickery adds: “Wind speeds are presented as estimates of maximum 3-second peak gust windspeeds over land. These gust wind speeds are not necessarily representative of the maximumwind speed in the hurricane, which likely occurred elsewhere. Wind speeds given by theNational Hurricane Center are estimates of the maximum one-minute sustained (average) windspeed 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 afuller explanation of Holland B and RMW in the paper “Statistical Models of Holland Pressure Profile Parameter and Radius toMaximum Winds of Hurricanes from Flight-Level Pressure and H*Wind Data” by Peter J Vickery and Dhiraj Wadhera.

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Anguilla

Saint Kitts and Nevis

St. Martin

Antigua and Barbuda

Sint Maarten

St. Barthelemy

St. Eustatius

Saba

6-9UTC 914,23,1.7

6-6UTC 914,23,1.7

6-12UTC 918,23,1.7

989799

96 959495 9493939293 92

92919192 9191908887878786

86 878687 87

868686858485 85

8383 85858484838282 84848281 838382 83

797979787877777777

175173177171173172169173 177176

169168170169179171172170166171167179 166167169165169172 169167169168165171174 165164167169165173179 167167166169166170 166179 166165169167171 169170 174165167167172

175175 166165166168

166167166172

162161163161172

164164165165164

165170171172

170176167 163164175

163163163

164 165164

166163162162 165176161 162177167161161 165166165161

162168

177166

167167164162167164

166168

172176178169168168168

168169168

169182166170174

177167167166167172

169172181 167167165170

174170

171166169 165167167169167 166 165166167169168 166 166167168 171 168

182170174177 180179174

176179174 188179

178 179167 170170173186 168 171171

107107107104104104105 103103

TNCM

TNCE

BARA9

Legend#* Buoy and C-MAN%, ASOS station

Hurricane Irma (2017): Preliminary Peak Wind Gust (mph)

0 9 18 27 364.5Miles

Estimated 3-second gust wind speeds (mph) at 10 m above ground over flat open terrain from ARA model fit to surface level observationsusing 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.

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#*#*

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Virgin Islands

Virgin Islands, British

6-18UTC 920,23,1.5

6-15UTC 918,23,1.6

918793

8585 87

878784

136 135131130130

130135138134

133133

136134 134

140133131131131131132133 133133132 135134 137

165164161169160 161161157 164

160 158158159165 158

150158158159156

156168157158

156155168

162158156153156155155154 153

158158 162

162155 159155159159162163160156

154155158

157157

156 155159157160157

168157155

154157159155166158

164154157

154157155152152153153156 154154155154 154153168156162 153 160153152152154156

154161

152151156154

157 166153153

166150158 168152156160166152 157158

163160155156

161150 160158151151154

152155 151154

161159150150153

157155154156

151153151152 162155

156156160153

149153158 157156

156154159

155156

161153

154152

153156149

150151

154152

152 158159

161157152156

155158147148149 156

155160

152151153146152 159153153148147148150149158146 154

157150151151150151143144125126

128 158149149150147148121

153151146150119

123117 160150149149155158118 156156

155156155149148149149151

125124151152150

151122126118117116114 166156151 149

156157146

144 154153115114116112111113 148

41052

TIST



0 7.5 15 22.5 303.75Miles


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Turks and Caicos Islands

8-0UTC 919,23,1.6

8-3UTC 920,26,1.56

928888

878788

9090 8887878889

92 8887 88

8687878788 8786868989 889393 87

87868788888992

8787899093 88898891 88878788898989909494 899191 909098 8990889090919192929796 90 929199

90899291919292939399 91999991939292929399 909192929398 909396939393949595 91989394

9698

999196949394949596 93

91919593939495 86938988

888888929495 86

858586868991959494959697 86

868989909198 8886858787888991929596 86869695959697 8789909191 888788888989929797 88879394949898 8788888991919293 898890909496 889192929393959596 89898990909195 899197 89

90939394949697 91899092909091919296 919292 8988

88889191949595999898 899089919293939499 9293969697 90898888929292939395 89899495 89

8990949396 908890888993939494 879088

888889909696 898788909694 8787898995959596 89879091919898 89898990929398 8888919192 888993 908990919695968992929396 92919091 919292949998 919394 9492929293 929394 93 95939494 9596 9798989696 9797 9898

999898999999

999999

100102

104103

111110103111110 105103113 107112111 107115

113 108113112112 110108116 110115114114114 110109119 111116115115116116 115112111

112119119 102112

117116116117117117118120 116115115 115115120118118118119120 122123130124122127 103

102101101127133 104103138102

137136

135100138141 143140137

150141140 101101

145143141148145143 102151147

100 100100 101104101102 103 104104107105107108

110108111112115113112

115112 116112 100116114 104116114 103101122

120124 105104125130129107106105126109109107

110 113112

116126119119119

146 118119 121127125

125125130

127130128

153157152

152133131

134157154166156

155155155

165157159 160155158 156

152151158 151152152158157162 157160

156162163 160167167 159



0 8 16 24 324Miles


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Guadeloupe

Dominica

Martinique

St Lucia

19-9utc 934,13,1.5

19-6utc 942,13,1.5 19-3utc 924,13,1.5

19-0utc 925,13,1.5

18-9utc 977,12,1.5

18-6utc 977,12,1.5

19-15utc 927,12,1.519-12utc 933,13,1.5

18-23utc 950,12,1.518-21utc 950,12,1.5

18-18utc 956,11,1.5

18-15utc 959,11,1.518-12utc 967,12,1.5

19-19utc 920,12,1.25

TLPL

TLPC

TFFR

TFFF

TDPD

TDCF

TAPA

68687067

6266635555 58

545252504950474747454549

444345

3030303029293029

282730282928

2826

5556

5554 5455 55545455 605755565773

5958565772 70 6363 58565664

695669 64

617368616159585858

565664

6169 606971 596667 60606061 6061 63647072646367 616269 636267 6263707275 6971 72 66

6571 667578 7173 7573737881 8076 778184 807879 828588 8384 8280

79799189 859190 84

8583828298 87868687

7878787879 7775 73

737373727070 6968686968686664646464646360 6060606059 57 57575758 54 5454

5251534949 49484846 474746

31323030

30 292829 28272727

2726262726262626

2525252525

104107

104111108

150148148148 152

155152149143161151144143

150150142141140 146145141140138 148144138136 145140137134 136136134135 136132133129128

Legend%, ASOS Station!

Hurricane MariaTrack (Version A)

Hurricane Maria (2017): Preliminary Peak Wind Gust (mph) - Version A

0 25 50 75 10012.5Miles

Estimated 3-second gust wind speeds (mph) at10 m above ground over flat open terrain from ARA model fit to surface level observationsusing 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

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TFFR

TDPD

TDCF

TFFF

TLPC

TLPL

TAPA

Guadeloupe

Dominica

Martinique

St Lucia

20-2utc 909,16,1.1

19-9utc 934,13,1.7

19-6utc 942,13,1.7

19-3utc 924,13,1.7

19-0utc 925,13,1.7

18-9utc 977,12,1.7

18-6utc 977,12,1.7

19-15utc 927,12,1.519-12utc 933,13,1.6

18-23utc 950,12,1.718-21utc 950,12,1.7

18-18utc 956,11,1.7

18-15utc 959,11,1.718-12utc 967,12,1.7

19-19utc 920,12,1.25

66666865

6064615353 55

525050474747454445434246

414143

2726262625252625

242325242523

23

5454

5353 5353 53535353 585654555673

5756555671 69 6262 56545463

685468 62

597267595957575657

545462

5968 596870 586666 58585860 5859 61636972636166 606068 626066 6161707274 6870 71 64

6470 647577 7072 7472727881 7875 758184 787779 808588 8282 8079

78788989 849089 82

8382808199 85858585

7676767577 75 71

7171717069 67666667666665626262626161

5857 585858585555 555555

52 515250485046 46464544 444543

282828262625252524242423

232323232322222222222221

21 21212122 2221

105108

105112110

155153153153 156

160157154148165155148147

154153150150147 159159152148151 157160148 163151153 151150155147 161153152135134

Legend%, ASOS Station!

Hurricane MariaTrack (Version B)

Hurricane Maria (2017): Preliminary Peak Wind Gust (mph) - Version B

0 25 50 75 10012.5Miles


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!

61575656

5758

5857 58

5858 60

5854 6055555454 56535555 60

58585854 555453 565453 595252 5958555554545353 555553 58

56 59635454535553 55535353 58555651 625754545356 60575553 57535255

4545545354514444 596061555453535352 59

51585146

454445 6567636262

636160595454

50 65656665

6665625859555353 5855505146 675444444444 646261616059585855535551505347

646463585146 58594544 63626160595454544444 63626058585848494949

5043 5749484746 61585648

4844444343 636363616059586143 6257575648484747 6756464543 6667686761616060585846444343 656262595744 67

675758604844 67595849 696566636160605943 6262585757 676666574746 66616059 7174696563626060 7373

727168615858 70666560 65646261636143

69686665614242

426969664143

656441 686867646465 666541 686865644041 66646563 6867676340 666465654041 6865 66 66666540 674041 66 6565 68706740 6640 656566 67686864 6565 65 69696668 676866

6666666466

6566 6566 787777

796363 6363 756364 7567 647564 74657464

61657476 74777575 78

777779

747576

64 78757373

737374

766370715858

58585757

575758

615657

5657 60

5855

696757 68

6767555853 5556545155 565357 54

22-15utc 958,36,1

22-12utc 959,35,1

Legend!

Hurricane MariaTrack

Hurricane Maria (2017): Preliminary Peak Wind Gust (mph)

0 10 20 30 405Miles


Turks and Caicos Islands

Interpretation of the Maps

The ranges of 3-second gust wind speeds with Saffir-Simpson Categories (over land) are asfollows (See the next section for the explanation of S-S Categories):

• Barbuda – 164-179 mph – Cat 4-5• Sint Maarten / St Martin – 165-182 mph – Cat 4-5• Anguilla – 161-176 mph – Cat 4-5• British Virgin Islands (Tortola and Virgin Gorda) – 152-168 mph – Cat 4• Providenciales – 108-124 mph – Cat 2-3• Grand Turk (Irma) – 99-109 mph – Cat 1-2• Dominica (lower bound) – 128-161 mph – Cat 3-4• Dominica (upper bound) – 134-165 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 theSaffir-Simpson Scale which uses 1-minute averages is based on the Commentary inthe American Society of Civil Engineers (ASCE) document “Minimum Design Loadsfor Buildings and Other Structures” ASCE 7.

The hurricane reports from the National Hurricane Center include the Saffir-Simpson HurricaneCategories 1 to 5. This scale is relied on by local emergency management agencies in order towarn 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 onwind speed intensity and barometric pressure at the center of the storm. The quoted wind speedsdetermining the various Categories are sustained wind speeds with a 1-minute averaging time at33 ft over open water. It is understood that the wind speeds categorising the hurricanes are themost intense in the system – typically in the north-east eye wall. Those speeds are notnecessarily 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 forwind-resistant design purposes in the USA and the Caribbean uses a 3-second gust speed at 33 ftabove ground in open terrain with scattered obstructions having heights generally less than 30feet – commonly associated with flat open country and grasslands. This is known as ExposureC. 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-SimpsonHurricane Category

Sustained Wind Speed(1-minute average)

Over WaterMPH

Gust Wind Speed(3-second average)

Over WaterMPH

Gust Wind Speed(3-second average)

Over LandMPH

1 74–95 90–116 81–105

2 96–110 117–134 106–121

3 111–129 135–157 122–142

4 130–156 158–190 143–172

5 >157 >191 >173

At the coast the “over water” equivalents should be used. But Dr Peter Vickery states: “Thewinds transition from an over water surface to an over land terrain pretty quickly. About 70% ofthe transition is complete after 1 km.” He adds: “As the wind speeds we have produced are onland, although not fully transitioned to open terrain wind speeds, the relationship between thegust wind speeds is closer to those given in the land column of the attached Table from ASCE7-16. 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 canbe 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 themaps for Hurricanes Irma and Maria in this document when compared with the wind speedsreported by weather forecasters and the news media, who commonly use the Saffir-SimpsonHurricane Scale. The gust wind speed values given in the Table that are associated with a givensustained wind speed should be used as a guide only. The gust wind speeds associated with agiven 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 notunprecedented. Nor can we dismiss the possibility of such systems being repeated in the comingyears.

The Table below shows comparisons of the maximum wind speeds over land for the impactedislands compared with current minimum guidance in published building codes. The numbers donot take into account climate change which would add 13% to Category II buildings and 10% toCategories III and IV buildings.

Islands Maximum 3-second gustwind speedover land in2017 (mph)

50-year returnperiod 3-secondgust wind speedfor use withASCE 7-05Category IIbuildings(in published codes)

700-year returnperiod 3-secondgust wind speedfor use withASCE 7-10Category IIbuildings

1700-yearreturn period3-second gustwind speed foruse with ASCE7-10 CategoryIII and IVbuildings

Dominica 165 126 159 172

Barbuda 179 126 160 168

St Martin(French)

182 115(36 m/s 10-min)

Anguilla 176 130 165 176

British VirginIslands

168 134 169 180

Turks & CaicosIslands (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-eventfunctions 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 HurricaneCategories on the Saffir-Simpson Scale.)

The above values for the 700-year return period for Category II Buildings and 1700-year returnperiod for Category III and IV Buildings are “failure” wind speeds2 therefore a Load Factorgreater than 1.0 does not need to be applied.

The most recent wind hazard assessment for structural design purposes in the Caribbean was theUSAID-funded, PAHO-executed “Development of Design Wind Speed Maps for the Caribbeanfor 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 hethought the Caribbean design wind speed maps should be updated. He responded: “I do think thestudy 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).”

2Traditionally, building codes state “working” wind speeds as their “basic” or “reference” wind speeds with a fairlyshort return period such as 50 years. “Load factors” or “factors of safety” are then applied to cover a lot of practicalinconsistencies and unknowns. For example, ASCE 7-05 would have a load factor of 1.6 for wind (%1.6 for wind speeds). FromASCE 7-10 onwards they decided, in order to have uniform levels of safety across different locations, to eliminate load factorsand 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.

The Wind Speeds in Selected Islands During …...The Wind Speeds in Selected Islands During...

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