Friday, April 7, 2017


The drips under pressure often employ a statistical estimation as a basis of probability and magnitude to label a flood and or a rain event as 1 in 50 year event or similar and in the current disaster in Edgecombe a barely conceivable 1in 500 year event.

Is it time such arrant nonsense is abandoned.
One winter during our 20 years in the Bideford district East of Masterton we endured  3 x 1 in 50 year flood events and 1 x 1 in 100 year flood from the Taureu River. All very academic when a quiet creek rises 40 feet or over thirty meters.

With Earthquakes there is the Richter Scale that has deficiencies  over measuring energy but after 10 000 tremors the logarithmic based Richter did give some relevance.
Yesterday in morning bulletins NZME made a claim that the Mason River, a tributary of the Waiau that drains the southern landscapes of Highway 75, the inland route to the South of Kaikoura, was running at 120 cumecs (a cumec is one cubic meter of water every second, of flow). Now that is real understandable data and to moi, however accurate does indicate a massive flow in a stream that with almost zero extraction has barely visible flow in high summer.

To announce to the world that the Bay of Plenty rural town was enduring a 1in 500 event is so trite and meaningless as who in gods name has data going back  to c1507.
Whereas had the flow data of the Rangataiki at normal April flow, low flow and known previous flood flows been divulged it could have brought into stark focus the magnitude of what actually hit Edgecombe.


Anonymous said...

A one in 500 years doesn't mean something that is only expected to happen once every half century. It is a clumsy way of trying to express a 0.2% chance of a flood of that magnitude happening. With a one in a hundred year event, there is a 1% chance of it occurring in anyone year. With one in ten, the chance of occurrence is 10%. Which is actually quite likely. Every year.

The Wikipedia page on 100 year floods is very interesting.

(The odds of winning lotto are around 4 million to one, and people win (practically) every week)


Anonymous said...

Oops, one in 40 million.


Adolf Fiinkensein said...

Excellent commentary, Michelle!

Paulus said...

I remember the Matuara floods in Southland some years ago.
One in fifty years the experts said but shortly afterwards another equally serious.
One was the last of the fifty year floods and the next was the first of the fifty year flood.

gravedodger said...

Excellent stats comment Michelle, as applauded by Adolf.
My point was that a flow data based analysis might be more help to the unwashed in comprehending the magnitude of a flood event.

ps I wonder how many bottles of water that some want to tax, yes boy wonder Jack, including you, went to waste from the Rangataiki catchment that begins on the plains crossed by the Napier Taupo road in western Hawkes bay region south of lake Taupo, giving Edgecumbe a kick in the goolies on its way to the sea.

Have Family who live on Lake Rotoiti (north) who are being told that lake level is at record Highs.