The Wind Speeds in Selected Islands During …...The Wind Speeds in Selected Islands During...
Transcript of 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.
%,
%,
#*
!
!
!
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.
!
!
!
%,
#*#*
#*
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
Legend#* Buoy and C-MAN%, ASOS station
Hurricane Irma (2017): Preliminary Peak Wind Gust (mph)
0 7.5 15 22.5 303.75Miles
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.
!
!
!
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
Legend#* Buoy and C-MAN%, ASOS station
Hurricane Irma (2017): Preliminary Peak Wind Gust (mph)
0 8 16 24 324Miles
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.
!
!!
!
!
!
!!
!
!
!
!
!
!
!
!
%,
%,
%,
%,
%,
%,
%,
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
%,
%,
%,
%,
%,
%,
%,
!
!!
!
!
!
!!
!
!
!
!
!
!
!
!
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
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
!
!
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
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
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.