How much damage can a cyclone do?

Feb 20, 2021

The amount of damage to buildings and landscape caused by a cyclone depends on –

  • Cyclone classification (wind speed)
  • Building classification
  • Building condition
  • Surrounding terrain, buildings and vegetation
  • Speed of the cyclone

The cyclone season runs from November to April in the southern hemisphere and cyclones can form when the sea temperatures are 26 degrees or above in favorable conditions.  With my decades of experience in the emergency services the announcement of ‘cyclone’ does a couple things with the general public.  One panic buying in the shops, two they don’t even notice there are warnings, the third just says ‘it’s only Cat 2 what harm can it do?’.  And then there’s the group which have followed directions, have an emergency kit and are prepared.  

The amount of damage depends on-

 Cyclone classification

Cyclones are an area of very low pressure in the atmosphere.  Depending on the region they can go by different names cyclones, hurricanes, tropical storms to name a few.  Cyclones are classed from Cat 1 (lowest) to Cat 5 (highest) based around wind speed. 

Accelerated wind gusts and

Other disasters created by the cyclone

 

Emergency Kit
Tropical Cyclone wind categories and damage.

Building classification

Cyclone wind resistant building codes started to come in after Cyclone Althea and Cyclone Tracey in the 1970’s.   Houses built before then should be upgraded to the new code when they are renovated.  If you have an old house and are not sure you could ask a qualified professional for a building inspection.

There are 4 wind regions (A- normal, B- Intermediate, C- Tropical Cyclones and D- Severe tropical cyclones).  Combined with the 6 different terrain categories the buildings wind risk rating is classed as cyclonic C1-4 (not to be confused with the cyclone categories) and non-cyclonic N1-6.  This building classification will affect the design and construction of a building.  If an old building is moved from one zone to a higher wind zone it will have to be up graded to suit the new wind zone.

 Building condition

The standard a building is built to is only as good as the condition of the building.  A house built in 1985 with an old rusting roof or termites in the walls won’t have the same strength as it was designed for.

Biggest problems in building condition usually come from-

  • Loose, rusty roofs with rotten roof  and building structure
  • Termite damage to walls, roof and floor structure
  • Removal of cyclone building features such as cyclone rods or bracing walls
  • Buildings not built to code
  • Buildings built to code but cyclone exceeds design criteria

Surrounding terrain, buildings and vegetation

The wind affects a building differently depending on the terrain.  Is the area flat and open to the full force of the wind?  On the side of a mountain with wind being compressed as it goes up hill? Surrounded on one or more sides by other houses, vegetation which will break up the wind?  These added buildings and vegetation can also cause problems during a cyclone.

 

 

Emergency Kit
 

Speed of cyclone

The wind rating of a cyclone is based on the speed of the circulation but there is also the speed at which the cyclone is travelling.  A quick moving Cat 3 cyclone may do the same damage as a slow-moving Cat 2 cyclone.  Slow moving cyclones stay in an area longer and cause more damage with continuous wind forces, buildings being sand blasted by debris, sand, dirt etc.  Cyclones also bring a lot of rain so if the system stays in the area for a while flooding is likely.

Accelerated wind gusts

The wind speed in the circulation of the cyclone is not constant.  To start with the wind gradually increases from the outside of the system as you move towards the centre eye of the cyclone.  And then there are random gusts of wind which are faster than the average wind speed that can cause more damage.

The eye of the cyclone

If the cyclone system passes directly overhead the wind will ramp up as the centre Eye gets closer.  Then all of a sudden there is no wind and it seems to be gone.  People are often fooled into going outside at this point.   But don’t do that it’s very dangerous.  For example a Cat 2 cyclone has winds near the centre of 117km/hr with gusts of 164km/hr, after the calm eye has passed you will be hit with 117km/hr immediately coming from the opposite direction. 

 Stay inside listening to the radio until authorities tell you it safe to go outside.  And even then be careful as there could be downed power lines, debris, damaged trees and buildings.

 Other problems created by the cyclone

Disasters don’t come in as a single problem, disasters cause other disasters.   For example heavy rain causes flooding, causing disease spread etc.   Cyclones can have other weather features in and around the system that can cause more damage.

Tornadoes and water spouts can create small areas of extra damage.

  • Storm surge is a rise in sea levels in front of the cyclone that causes inundation along the coastline.
  • Heavy rain can cause localised flooding, flash flooding and the like.
  • Lightning

Cyclone comparison

If you are from overseas and want to understand where does a cyclone category rate I have done up a comparison with a few different models.

 

 

 

Trees can loose branches or fall over in high wind and wet ground.  These branches can land on powerlines, buildings, cars, across roads causing impact damage.

 

Powerlines can be damaged by power poles fall over, tree branches, power lines touching in the wind arcing out blowing pole fuses, transforms etc.

Buildings destroyed by other buildings.  A building in poor condition or not strong enough to withstand the wind can blow apart spearing timbers into surround buildings and structures.  In Cyclone Larry there were areas where a building blew apart and damaged the surrounding buildings.

Boats can break from their moorings and get blown about damaging nearby boats, jetties etc.  A famous photo from Cyclone Yasi was of the Cardwell marina with piles of boats on top of each other pushed against the shore and buildings.

Light structures become missiles, such as garden sheds, caravans, trampolines and the like quickly become airborne causing more damage.

Storm surge rises the sea level of floods water fronts pushing sand and water into buildings.  The surge can also bank up creeks, rivers, drains causing flooding in the area.

Wind damage to buildings.  There is a strong wind force coming from one direction with random wind gusts pushing on the building.  There are pressure gradients between inside the building and outside which can cause more damage.  If a window breaks or a section of wall gives in this creates the wind to enter the building and can cause the lost of the roof or more structural damage to the building.  The wind can also push a building off its stumps or footings if not secured property.  Larger rooms are more likely to get damaged as there are fewer bracing walls supporting the external shell.  That is why you are told to go to a small room such as bathroom during a cyclone.

 

 

 Summary

Cyclones are complicated and dangerous even the small ones.  By preparing your home and family you will be better off if a cyclone should hit your area.  And while cyclones are classed a tropical, the severe lows the non tropical areas get can be more dangerous as the buildings are not designed for the wind speeds and there usually is less notice than with a tropical cyclone.  Doesn’t matter where you live 115km/hr wind will do damage.

Don’t take cyclone warnings lightly as a quiet Cat 1 cyclone today can be a Cat 3 tomorrow and you could be in trouble.

 

 

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