Christchurch Flooding

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Master Geek
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#999686 5-Mar-2014 19:08

Fred99:
UncleArk:
Fred99: I see The Press is quoting the CCC now calling it a "100 year event".
I doubt it - the impact might be one in 100 year, but that's because some land had sunk, stormwater systems are damaged etc.

Describing the storm event as a 100yr event is simply a a function of event duration and rainfall intensity measured in mm/h.

In other words, the amount of rain that has fallen in ChCh per hour over the last 24 hours is the equivalent of an event which has an annualised probability of occuring being 1% (or 1 in 100).

In actuality at times the storm event was about a 120yr return period event.

Not according to data I'm looking at.
Just over 70mm of rain in 48 hours. Highest in 1 hour was 4.8mm.
It was nothing out of the ordinary - only slightly above mean annual recorded maximums.
See page 7.
See also page 20, where in 1986 floods, more rainfall was recorded in 24 hours (Chch Gardens) than the 48 hour recording for this latest event.
http://resources.ccc.govt.nz/files/NIWA.pdf

I don't believe it was a "100 year event" at all.

Since you mention the Botanic Gardens site.... I can report that site 325616 (ChCh Botanical Gardens) logged 141.2mm in 1440 minutes (24 hours) for the 24 hour period ending 9:05am 5th March. The data logging interval was 5 minutes.

This is equivalent to 5.88mm/h over 24 hours equivalent to a return period of 119.9 years.

blakamin

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#999721 5-Mar-2014 19:36

sen8or: Meanwhile somewhere in Christchurch......

If I lived down the road from that c*ck, I'd punch his head in...

That wake is really going to help people that might be on the brink of flooding.... :/

mattwnz

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#999722 5-Mar-2014 19:40

blakamin:
sen8or: Meanwhile somewhere in Christchurch......

If I lived down the road from that c*ck, I'd punch his head in...

That wake is really going to help people that might be on the brink of flooding.... :/

I wonder if it is licensed to be on the road too.

Fred99

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#999741 5-Mar-2014 19:59

UncleArk:
Fred99:
UncleArk:
Fred99: I see The Press is quoting the CCC now calling it a "100 year event".
I doubt it - the impact might be one in 100 year, but that's because some land had sunk, stormwater systems are damaged etc.

Describing the storm event as a 100yr event is simply a a function of event duration and rainfall intensity measured in mm/h.

In other words, the amount of rain that has fallen in ChCh per hour over the last 24 hours is the equivalent of an event which has an annualised probability of occuring being 1% (or 1 in 100).

In actuality at times the storm event was about a 120yr return period event.

Not according to data I'm looking at.
Just over 70mm of rain in 48 hours. Highest in 1 hour was 4.8mm.
It was nothing out of the ordinary - only slightly above mean annual recorded maximums.
See page 7.
See also page 20, where in 1986 floods, more rainfall was recorded in 24 hours (Chch Gardens) than the 48 hour recording for this latest event.
http://resources.ccc.govt.nz/files/NIWA.pdf

I don't believe it was a "100 year event" at all.

Since you mention the Botanic Gardens site.... I can report that site 325616 (ChCh Botanical Gardens) logged 141.2mm in 1440 minutes (24 hours) for the 24 hour period ending 9:05am 5th March. The data logging interval was 5 minutes.

This is equivalent to 5.88mm/h over 24 hours equivalent to a return period of 119.9 years.

I'm staggered by that.
Other stations recorded much less than that over a 48 hour period.
What did other stations record?

tdgeek

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#999833 5-Mar-2014 21:23

I live here. I lived in AKL in my childhood.

On the surface, a windy and rainy period, For sure
Clearly, EQ damage has affected areas, no doubt. It is not a 100 year event. Ironically I know our mayor personally, I disagree with Lianne.

tdgeek

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#999834 5-Mar-2014 21:24

UncleArk:
mattwnz:
Fred99: I see The Press is quoting the CCC now calling it a "100 year event".
I doubt it - the impact might be one in 100 year, but that's because some land had sunk, stormwater systems are damaged etc.

It maybe a sign that there is something seriously wrong with both the infrastructure and the land since the EQs. I think it is going to lead to a lot more money needing to be spent somehow on new stormwater drainage. Was not really the smartest place to build a city.

While there are certainly operational difficulties in the post-quake environment with the open and piped stormwater drainage network in Christchurch it must be appreciated that the design capacity of the street side channels ("gutters") and piped systems is typically to serve a 10yr event.

Even a fully operation system will over-top, gorge, and overflow in severe rainfall events as we have just experienced.

I agree

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Fred99

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#999847 5-Mar-2014 21:49

tdgeek: I live here. I lived in AKL in my childhood.

On the surface, a windy and rainy period, For sure
Clearly, EQ damage has affected areas, no doubt. It is not a 100 year event. Ironically I know our mayor personally, I disagree with Lianne.

UncleArk's data above suggests that it was in fact a 100 year event.
But it doesn't tie in with other reports of recorded rainfall I've seen.
I was hoping he'd respond to my post above.

IMO it's important to know this - I'm skeptical (to the point of being cynical) about how the aftermath of the quakes has been politicised.
There are many different parties involved - with many conflicting interests.

Even if this event was worse, flooding seems to be the new "normal" for Chch, ie:
2011 floods
2012 floods
2013 floods

Kim587

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Master Geek
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#999874 5-Mar-2014 22:12

Fred99:
tdgeek: I live here. I lived in AKL in my childhood.

On the surface, a windy and rainy period, For sure
Clearly, EQ damage has affected areas, no doubt. It is not a 100 year event. Ironically I know our mayor personally, I disagree with Lianne.

UncleArk's data above suggests that it was in fact a 100 year event.
But it doesn't tie in with other reports of recorded rainfall I've seen.
I was hoping he'd respond to my post above.

IMO it's important to know this - I'm skeptical (to the point of being cynical) about how the aftermath of the quakes has been politicised.
There are many different parties involved - with many conflicting interests.

Even if this event was worse, flooding seems to be the new "normal" for Chch, ie:
2011 floods
2012 floods
2013 floods

The Metservice website says that their Lyttelton station recorded 96mm of rain yesterday alone-if that's anywhere near accurate I'd say it well and truly meets the threshold for a 100 year event, and backs up the data we've seen from the Botanical Gardens.
Either way, it seems clear that it doesn't require a 100 year event for quite significant flooding to occur in Christchurch these days.

Fred99

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#999879 5-Mar-2014 22:42

Kim587:
Fred99:
tdgeek: I live here. I lived in AKL in my childhood.

On the surface, a windy and rainy period, For sure
Clearly, EQ damage has affected areas, no doubt. It is not a 100 year event. Ironically I know our mayor personally, I disagree with Lianne.

UncleArk's data above suggests that it was in fact a 100 year event.
But it doesn't tie in with other reports of recorded rainfall I've seen.
I was hoping he'd respond to my post above.

IMO it's important to know this - I'm skeptical (to the point of being cynical) about how the aftermath of the quakes has been politicised.
There are many different parties involved - with many conflicting interests.

Even if this event was worse, flooding seems to be the new "normal" for Chch, ie:
2011 floods
2012 floods
2013 floods

The Metservice website says that their Lyttelton station recorded 96mm of rain yesterday alone-if that's anywhere near accurate I'd say it well and truly meets the threshold for a 100 year event, and backs up the data we've seen from the Botanical Gardens.

No I'm sorry - it wouldn't.
100mm / 24 h would be closer to a 1/20 year event for recorded stations in Chch area.

Kim587

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Master Geek
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#999884 5-Mar-2014 23:04

You missed one vital detail-that was only yesterday, and the rain didn't really start falling until about 3pm, so it was more like 90mm in 9 hours. Plus whatever we had today!

Fred99

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#999885 5-Mar-2014 23:23

Kim587: You missed one vital detail-that was only yesterday, and the rain didn't really start falling until about 3pm, so it was more like 90mm in 9 hours. Plus whatever we had today!

No - I didn't miss that. I didn't mention it though, but I did a quick total, and got about 120mm max over a selected 24 hour period.
I still doubt that would be close to a 100 year event. To quote the Niwa document I linked to above:

Heavy rainfalls in Christchurch result from depressions, with centres south and southeast of South Island producing south-westerly frontal rain with preceding
thunderstorms, and with centres north to east of South Island producing north-easterly
to south-easterly rain. Falls of more than 100 mm of rain in 24 hours are not unusual
and up to 170 mm in 18 hours has been recorded.

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Kim587

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Master Geek
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#999886 5-Mar-2014 23:33

Ah, whereas I was going off UncleArk's chart which showed a rate of 10mm/h over 10 hours would suggest close to a 100 year event, although I suppose that is site-specific. Which is the more authoratitive? Either way, we'd need to know what today's readings were for Lyttelton before making final judgements.

Fred99

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#999896 5-Mar-2014 23:59

Kim587: Ah, whereas I was going off UncleArk's chart which showed a rate of 10mm/h over 10 hours would suggest close to a 100 year event, although I suppose that is site-specific. Which is the more authoratitive? Either way, we'd need to know what today's readings were for Lyttelton before making final judgements.

I doubt either is "more authoritative", as the data used in the chart UncleArk's came from is the same data as in the document I linked to.

UncleArk

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Master Geek
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#1000140 6-Mar-2014 13:53

Fred99,

In a previous post you asked what other stations recorded. Sorry that I didn’t respond in a timely fashion. I typically only visit Geekzone once a day...

Here are some other station recordings for your interest.

These are recordings over the 24 hour period ending 9:05am 5th March.

Upper Bowenvale 297mm
Lower Bowenvale 187mm
Sumner 180mm
Lower Styx 78mm
Tyone St (Belfast) 76mm
Firestone Factory (Papanui) 139mm
Horseshoe Lake 134mm
Ocean Outfall 133mm
Akaroa 162mm

To give a complete picture of a storm event requires quite a lot of discussion and some understanding of water hydrology.

Overall, different parts of Christchurch experienced different rainfall intensities giving rise to different “return periods” over the city.

In addition the expected/modelled average rainfall driving expected return periods varies and there are “bands” across the city meaning any given event can be described slightly different, in terms of it’s return period, from one part of the city to another. Expanded, this is akin to understanding that a 100yr rainfall event in Christchurch is a different beast to a 100yr rainfall event somewhere else – say Wellington for example – because the expected statistical average rainfalls for each city are quite different.

When asked for comment it is somewhat pragmatic (for Council) to give a very simplified answer, without a background lesson or explanation, which is picked up by the media and politicians etc.

It is absolutely fair comment that Christchurch city, overall did not experience the same return period event everywhere. Parts of the city experienced a return period well below the 100yr event for that local location… however, and more significantly, parts of the city did experience a return period well in excess of the 100yr event expected for that local location.

Therefore the description of the overall event as a “1 in 100 year” event is somewhat generalised and by no means a complete picture – but enough of a nice rounded number with enough significance to satisfy those (be it media or politicians or Joe Average who doesn’t want to have to understand technicalities) hungry for some nice simple thing they can easily digest and compare to similarly described events of the past.

Fred99

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#1000191 6-Mar-2014 14:43

UncleArk: Fred99,

In a previous post you asked what other stations recorded. Sorry that I didn’t respond in a timely fashion. I typically only visit Geekzone once a day...

Here are some other station recordings for your interest.

These are recordings over the 24 hour period ending 9:05am 5th March.

Upper Bowenvale 297mm
Lower Bowenvale 187mm
Sumner 180mm
Lower Styx 78mm
Tyone St (Belfast) 76mm
Firestone Factory (Papanui) 139mm
Horseshoe Lake 134mm
Ocean Outfall 133mm
Akaroa 162mm

To give a complete picture of a storm event requires quite a lot of discussion and some understanding of water hydrology.

Overall, different parts of Christchurch experienced different rainfall intensities giving rise to different “return periods” over the city.

In addition the expected/modelled average rainfall driving expected return periods varies and there are “bands” across the city meaning any given event can be described slightly different, in terms of it’s return period, from one part of the city to another. Expanded, this is akin to understanding that a 100yr rainfall event in Christchurch is a different beast to a 100yr rainfall event somewhere else – say Wellington for example – because the expected statistical average rainfalls for each city are quite different.

When asked for comment it is somewhat pragmatic (for Council) to give a very simplified answer, without a background lesson or explanation, which is picked up by the media and politicians etc.

It is absolutely fair comment that Christchurch city, overall did not experience the same return period event everywhere. Parts of the city experienced a return period well below the 100yr event for that local location… however, and more significantly, parts of the city did experience a return period well in excess of the 100yr event expected for that local location.

Therefore the description of the overall event as a “1 in 100 year” event is somewhat generalised and by no means a complete picture – but enough of a nice rounded number with enough significance to satisfy those (be it media or politicians or Joe Average who doesn’t want to have to understand technicalities) hungry for some nice simple thing they can easily digest and compare to similarly described events of the past.

Thank you for that - interesting.
Sorry - I wasn't having a go at you for not responding, I was just a little impatient (and lazy - as the data can be d/l from Niwa website).
My concern at the designation of the flooding as 1/100 year is that it may have been politicised by some who'd prefer if there wasn't any immediate need to "do something" - whatever that something might be. Some of the worst flooded areas have been flooded at least once a year since the quakes.
Whatever people may choose to believe about global warming etc, sea level rise is happening regardless. Whatever is done in Chch - because of the immediate need - can set precedent for what is certain to happen elsewhere in NZ.

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