Alarm at more Dunedin flooding

In June 2015 a surprisingly bad storm caused bad flooding in the reclaimed but still low low lying south Dunedin area. In the aftermath there were admissions of poor maintenance of drainage systems (storm water drains, mud tanks, screens and pumping stations). Changes were made with assurances things would work better in the future.

A near record 160.2 mm of rain fell in a day then.

Leading up to yesterday, in January this year, there was low rainfall of 26.4 mm (at the University weather station), and most (16 mm) of that was in one day two weeks ago. Things were very dry.

There was ample warning of impending heavy rain, so there was plenty of time to be properly prepared. Yesterday there was 109.6 mm of rain, significantly less than when the 2015 floods happened.

But there was still serious flooding in south Dunedin, not as bad as in 2015 but bad enough to have a state of emergency to be declared and for homes (and a rest home) to be evacuated.

ODT: Stormwater system overwhelmed

Torrential rain overwhelmed Dunedin’s stormwater system yesterday, flooding parts of Mosgiel and South Dunedin and sparking evacuations and road closures.

History repeated itself for many residents, who faced similar flooding in 2015 and wake this morning facing  another clean-up. A state of emergency was declared at 2.20pm. Dunedin  had received 18mm of rain an hour in the previous two hours.

“That is more than our system is designed to cope with, even though it has operated exactly how it is intended to,” Mayor Dave Cull said.

This is an alarming admission.

South Dunedin’s stormwater system was unable to cope with the 2015 deluge, and struggled with the amount of rain which fell in a short time yesterday.

“It was just the sheer volume of rain in such a short period of time meant that some of the groundwater entered the wastewater system, and that’s what caused the problem,” Civil Defence controller Leanne Mash said.

So two and a half years after major floods in part caused by poorly maintained drainage systems, the improved drainage systems still can’t cope with just two thirds of the rainfall.

This is alarming, not just for those who were directly affected by the flooding, but also by at least one ratepayer, me.

How bad will flooding be if we get a repeat of the 2015 rainfall?

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14 Comments

  1. Sorry for those in Dunedin affected by flooding. It’s a terrible disaster to deal with.

    Here in AKL. The rain is well over, seemed to have stopped around 2.00 am and skys are bright. Hope the sun comes out down south

    Reply
    • The rain mostly stopped by mid afternoon yesterday in Dunedin. Things subsided quite quickly, but the damage was already done in some areas.

      Reply
  2. Reply
  3. Trevors_elbow

     /  February 2, 2018

    Muppets in charge. 2 and a half years since the last event and the DCC is busy building cycleways and not attending to one of the three core water systems (rain water) that should be top of the list….

    Get onto those Green councillors Zedd and get them focused on real issues that need attention right now….

    Reply
    • Zedd

       /  February 2, 2018

      ‘Maybe the rain was not to blame…’

      In 2015 it was mostly put down to badly maintained infrastructure; clogged gutters & filters in mudtanks. After they cleared/cleaned the blockages, the ‘deluge’ receded fairly quickly. Methinks maybe they did not expect it to occur again so suddenly. 2015 was supposedly a once in 50 year event ?? :/

      Reply
      • Griff

         /  February 2, 2018

        King tides means drains backup sooner,that may be why a lessor event resulted in flooding.
        http://www.metservice.com/marine-surf/tides/dunedin
        2015 was supposedly a once in 50 year event.
        It may have been if climate and sea level rise was static.
        Our nz coastal sea level rise has been about 10mm a year for the last decade,probably due to Pacific decadal oscillation
        https://climatedataguide.ucar.edu/climate-data/pacific-decadal-oscillation-pdo-definition-and-indices
        Expect extreme rainfall and flooding to happen at greater frequency.

        Reply
        • Alan Wilkinson

           /  February 2, 2018

          NZ sea level rise has not been 10mm/yr for the last decade, at least not until the US massage the data or cherry pick your start/end points:

          http://archive.stats.govt.nz/browse_for_stats/environment/environmental-reporting-series/environmental-indicators/Home/Marine/coastal-sea-level-rise.aspx

          Reply
          • Griff

             /  February 2, 2018

            NZ sea level rise has not been 10mm/yr for the last decade
            links to data that ends in 2006…………!
            Dates are important when talking about the present Alan
            NZ last 9 Years from a comment at Timinos open mind using data from
            http://www.psmsl.org/data/obtaining/.
            Whitianga +40mm/year, Kawhia + 18mm/year, Devonport +20mm/year. These values were for the rate of rise of the Maximum tides. The rate of rise of the mean SL was not much different but a bit less at +36mm/year in Whitianga, Kawhia + 18mm/year and +16mm/year in Devonport. It seems that the max tides are rising a bit faster than the mean SL does. This is of interest as of course effects of SLR will always be felt at the max tide events.
            Whangarei +13mm/year, Wellington +9.6mm/year,

            Dunedin the town in question
            http://www.psmsl.org/data/obtaining/stations/136.php

            That is 10mm a year for the last decade.

            Note I did not say this extreme rise was wholly due to global warming instead put it down to the SOI.

            Reply
            • Alan Wilkinson

               /  February 2, 2018

              You confuse the baseline specification with the data which ends in 2015. Tamino is a climate warrior so of course is happy to cherry pick his start end points. The long term rise is around 2mm/yr with no evidence of any significant acceleration. This of course is irrelevant to scary reports by MoE and ridiculous council sea level risk maps.

            • Griff

               /  February 2, 2018

              Hover over the end point and tell me the date Alan please
              Then come back and tell me again the data goes to 2015 in your link.

              The long term rise is around 2mm/yr with no evidence of any significant acceleration.

              That is very much incorrect
              Presently sea level is given as 3.4 mm a year over the satellite era

              The trend is a curve not a straight line .
              Here from Steven Nerem

              Recently, my research group been focused on determining if global sea level rise has accelerated over the 25-year satellite altimeter record from TOPEX, and Jason-1, -2, and -3. In doing such an analysis, we must account for phenomena as diverse as the eruption of Mt. Pinatubo in 1991 and the influence of ENSO variability. We must also carefully assess the measurement errors and the influence of decadal variability on a relatively short 25-year record. While we are still completing this research, it appears that long-term sea level rise has accelerated from roughly 2 mm/year in the mid-1990s to 4 mm/year today (2017). The GRACE satellite tells us that most of this acceleration is coming from the loss of ice mass in Greenland and Antarctica. If this acceleration were to continue unchanged into the future, sea level would be rising at a rate of ~10 mm/year in 2100. Our future research is focused on improving this analysis and its errors, and better understanding how sea level might change in the future. We are also looking forward to the launch of the GRACE Follow-On mission in early 2018. Together, satellite gravity and satellite altimeter measurements tells us how much sea level is changing and why it is changing. This work is funded through the NASA Sea Level Change Team (N-SLCT).

            • Alan Wilkinson

               /  February 2, 2018

              Something wrong with your browser or your operation of it, Griff. I see the 2015 data in my link if you don’t. And the heading is quite clear on that: By monitoring site 1891-2015 as is the horizontal scale.

              I was talking about NZ sea level rise as shown by our physical monitoring stations in the data and chart maintained by our Dept of Statistics. NASA can fiddle with its own data but evidently hasn’t managed to get at ours yet. Over the past century there is no significant curvature as you can see from the chart even if you don’t want to.

            • Alan Wilkinson

               /  February 2, 2018

              Looks like Nerem is fitting a curve to an already smoothed/processed data line. That’s potentially a very flawed process especially over a short data set with long cycles.

  4. Ray

     /  February 2, 2018

    It’s not as if NZs weather pattern has changed, long dry spells with heavy rain once in a while despite some bogus claims by the Dunedin City Council.
    The problem is poor infrastructure and poor maintenance.
    As Trevor says, time to stop wasting money and time and do the basics.

    Reply
  5. Labour mayor. What else can you expect?

    Reply

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