A cyclone hitting Busselton?

Warwick Rowell, Friday 29 November 2013

“Looking over the fence” is an informal expression of one of the fundamental processes of Permaculture design: Sector Analysis. In sector analysis you explore all of the inputs, positive and negative, that are coming at the particular patch you are designing. There is always a history to these inputs, and so it is worth challenging the tendency to see some things happening and to think that they have always been there, and, by implication, that they will continue that way.

One example is explored below.

I first started spending time in Dunsborough in 1992.  Over a number of years, I took every opportunity to talk to long term residents about weather events, and seasonal timing, and the like, to get a feel for the local weather and climate.  Amongst lots of other valuable things, they told me about cyclones hitting the area in 1938, 1955, and 1978.  About how someone nearly two miles inland was catching herring off his verandah, about 1m of water over Commonage Road near Simmos, about a thirty foot boat going over the top of the Busselton jetty without touching it, about a 500m x 500m promontory out into the bay, with 20m high peppermint trees, being washed away in one night…

So, in our patch right on the top of the Naturaliste Ridge, we designed houses, dams, and drainage systems to cope with cyclonic winds, and 250mm of rain in one hour.  But looking after ourselves was not enough, so Bev Clarke, when she was Deputy Shire President, received a copy of the scenario set out below, in about 2008. Then I wrote to the Busselton Times, and they published a brief version in 2012, and recently I sent a copy to Paul Needham – the city’s Director of Planning.

The scenario is this: (I have altered the original sent to Bev and Paul slightly for clarity)

“In the Times on June 10, 2012, Nigel Bancroft was quoted as saying, about the recent The Critical Decade report, ..”a modest sea level rise will make extreme storm events become 100 times more regular.”

I hope that this quote is inaccurate, for several reasons.

Firstly, we have seen the damage caused to beaches along Geographe Bay in the last two or three days by a moderate but persistent Easterly.  If this damage was to be intensified in a real blow, and occur 100 times more frequently, then we face a difficult if not impossible task in protecting houses as well as the town of Busselton.

Secondly, my understanding is that the sea level rise as such will not increase the occurrence of extreme storm events.  The major cause was identified in Professor Bert Main’s report to the Dowding government in 1991:   Increased ocean temperatures on either side of the equator will increase the area of ocean with surface temperatures above 26.5C, which is critical for intensifying low pressure systems into cyclones, and then maintaining or building their strength.  So with longer generative paths, and then tracking further south, more and stronger cyclones could be anticipated in the SW of WA.  We are already well overdue: 1938, 1955, 1978…

Professor Chari Pattiaratchi at a Geocatch seminar in February 2012 was quoted as saying “each centimetre of sea level rise could cause around 1m of beach erosion, which meant that the coastline could be expected to move 100m inland by 2100.”  This simple mathematical extension is not really credible’ what about angles of attack, different desnities and sizes of sand grains, rocks, frequency of events, severity of events..?

Imagine this scenario: a cyclone comes around NW Cape.  It tracks SSE, parallel to the coast.  In front of it, a N-NW swell builds up. As the swells hit Geographe Bay, they are confined between the Cape and say Bunbury, and so become higher still.  Some time before the cyclone itself arrives, the rain starts, and the hills in Busselton’s hinterland start running off, onto the plains.

As the pressure drops with the approach of the cyclone, the ocean level rises.  Air pressure was originally measured in millimetres of mercury – air pressure falls from 1013mm to around 950mm* in a strong cyclone.  But mercury is 13.6 times denser than water – so 60mm of mercury drop in a barometer means water levels will rise about 800mm.   * The lowest pressure ever recorded was 880mm of Mercury, and the highest wind speed ever recorded was 408 km per hour, at Barrow Island.

As the winds pick up we get a wind wave on the top of the swell, on top of the higher water level.  At some time during the 12 – 24 hours passage of a high category cyclone, there is a good chance of a high tide as well.  Say 0.5m tide, 1.0m pressure, 3m swell, 1m wind wave.  A total of 5.5m coming in from the north, with lots of rain coming from the south.   My estimates of swell height possibilities are very conservative – they don’t need to be that accurate to portray the massive problem we face!

Nigel was at the talk by Professor Pranzini in February 2012.  There was a rather dry throw-away line from the prior presenter, Stuart Barr, which I paraphrase here: “We will have to deal with far greater sea level rises from short term events than those we face from possible climate change over this century.”  I am sure he is right.”

I provided further information for the editor:

In 2007, the Intergovernmental Panel on Climate Change‘s Fourth Assessment Report (AR4) predicted that by 2100, global warming will lead to a sea level rise of 180 to 590 mm,[31] depending on which of six possible world scenarios comes to pass, and barring rapid dynamical changes in ice flow. More recent research from 2008 observed rapid declines in ice mass balance from both Greenland and Antarctica, and concluded that sea-level rise by 2100 is likely to be at least twice as large as that presented by IPCC AR4, with an upper limit of about two meters.[32] [33]

In September 2008, the Delta Commission presided by Dutch politician Cees Veerman advised in a report that The Netherlands would need a massive new building program to strengthen the country’s water defenses against the anticipated effects of global warming for the next 190 years. This commission was created in September 2007, after the damage caused by Hurricane Katrina prompted reflection and preparations. Those included drawing up worst-case plans for evacuations. The plan included more than €100 billion, or $144 billion, in new spending through the year 2100 to take measures, such as broadening coastal dunes and strengthening sea and river dikes.
The commission said the country must plan for a rise in the North Sea up to 4.25 feet (1.3 meters) by 2100, rather than the previously projected 30 inches (0.80 meters), and plan for a 6.5–13 foot rise by 2200.[41]

According to the 2004 Arctic Climate Impact Assessment, climate models project that local warming in Greenland will exceed 3° Celsius during this century. Also, ice sheet models project that such a warming would initiate the long-term melting of the ice sheet, leading to a complete melting of the Greenland ice sheet over several millennia, resulting in a global sea level rise of about seven metres.[53]

It seems to me that Busselton is one of the most vulnerable towns in WA to climate change.  Not that any town on the coastal plain would escape from such an event; so this adds another dimension to the problem; all the emergency services would surely be pre-occupied with the situation further north.