Heavy rain in Westland, Fiordland and Alps

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tich
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Heavy rain in Westland, Fiordland and Alps

Unread post by tich »

Interesting to see the rakaia mouth flooded yesterday. A conbination of high tide and heavy rinfall in the catchment, wonder if it flooded again today with the NW still persistent about canterbry.
It takes heavy rain in the Alps to flood Canterbury's main rivers in situations where the plains are sheltered by the mountains and get no (or very little) rain, like in the present situation.
I know heavy rain is more normal in Westland, Fiordland and the Alps than elsewhere in NZ, but those areas seem to be in a particularly wet spell at present, so rivers must be running high. This situation brings my mind back to the Civil Defence simulation program on TV1 not too long ago, when the scenario for Chch was the Waimakariri breaking its banks and flooding the city. However, this would obviously need a much more extreme and prolonged northwesterly storm.
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Unread post by NZstorm »

Waimakariri breaking its banks and flooding the city
Is it possible to get such a flood off the Waimak?

Floods in the past were caused by the Avon/Heathcote. I guess they have sorted the problems out with those rivers as CH hasn't had a flood in years.
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Unread post by TonyT »

NZstorm wrote:
Waimakariri breaking its banks and flooding the city
Is it possible to get such a flood off the Waimak?
Very definitely! The 1863 flood was huge, the whole of the area from Belfast to near the Port Hills was under water, about 1m deep through most of the area which is now Chch. it was a result of that which lead to the Mcleans Island area being drained, flood banks installed, and the main channel from around Kainga to the coast re-aligned. Most of that work took place in the late 1800s and early last century, and the fear now is that the levels of thew stopbanks arent high enough for more than about a one in one hundred year event (I think I'm right on that). Moreover, if water does overtop the banks it would have nowhere to drain to, and we could face a scenario like New Orleans where flodwaters hung around for a long time.
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Unread post by tich »

I guess the worst flooding of Canterbury's big rivers would involve heavy northwest rain in the mountains when there's a heavy existing snow cover, which would be melted by the warm rain. So this would most likely be a spring or early summer event. The relative lack of snow on the Alps/foothills at present makes such an extreme flood less likely to occur now.

I see there's been further heavy rain in Westland today, but the warning's been lifted (though rain is still forecast).
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Unread post by Manukau heads obs »

karen on TV1 weather said 2 metres of new snow has meant an extreme avalanhe risk, in the southern alps....
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Unread post by 03Stormchaser »

The most significant flooding threat to the Christchurchurban area is posed by the Waimakariri River, although this has been substantially mitigated by river control works.Breakouts of floodwaters from the Waimakariri River caused damage to urban Christchurch in December 1865, October 1866 and February 1868. The Waimakariri River still threatensmore people and properties than any other New Zealand river.
Source: Christchurch City Council

http://03stormchaser.com/images/flood.jpg
The Waimakariri River in high flood the stop banks have broken and inundated the countryside, the Main North Road is under water.
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Unread post by tich »

karen on TV1 weather said 2 metres of new snow has meant an extreme avalanhe risk, in the southern alps....
I think that snow is about and west of the main divide, and at high altitudes. I haven't seen the mountains from Chch today, but a few days ago there wasn't the normal thick coating of white on them - they looked more like what you'd expect them to look like in November.
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Unread post by Manukau heads obs »

yes i realised it meant on the divide, still interesting to note ;)
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