Election Year. This post is offered in the public interest.
B A C K G R O U N D
On and about 3 June 2015, South Dunedin was severely affected by stormwater flooding – the Council has since discussed why and how council-owned infrastructure failure occurred. The extent of stormwater damage to private property and the upheaval and distress for affected residents, property investors and businesses is undeniable. Council operationals refer to this as the “June 2015 Flood Event” in formal reports.
A public meeting coordinated by Dunedin South MP Clare Curran was held at South Dunedin on Monday, 7 March 2016. At the meeting the South Dunedin Action Group(SDAG) was formed. Group representatives headed by spokesman Ray Macleod have since met with the Mayor and council officials – the first meeting was held on Tuesday, 3 May 2016. A meeting with council staff has followed more recently.
Local media, the Otago Daily Times and Channel 39, are presently covering the anniversary and aftermath of the “flood event”. Noticeably, the city council has yet to formally apologise to all the many people affected by the lack of council-owned infrastructure maintenance and stewardship at South Dunedin during the rain event of June 2015.
Council ‘not liable for flood damage’ (ODT 27/11/15)
“The Dunedin City Council says it is not liable for private property damage caused by the South Dunedin flood, despite admitting problems with its pumping network prolonged the pain for residents …. The issue had been considered by the council’s lawyers and insurers, but the advice from both was the council was not liable, [council infrastructure and networks general manager] Mrs Stokes said.”
Otago Daily Times Published on Jun 4, 2015
Raw aerial video of Dunedin Flooding [Video courtesy One News]
DCC Reports and Responses:
● 30 November 2015 –Council
Agenda – Council – 30/11/2015 (PDF, 39.6 KB)
Report – Council – 30/11/2015 (PDF, 553.9 KB) ‘Infrastructure Performance During June 2015 Flood Event’ (McElhone)
Minutes – Council – 30/11/2015 (PDF, 121.8 KB) | Meeting Video
● 7 March 2016 –Letter, DCC Chief Executive [supplied by DCC]
Sue Bidrose to Neil Johnstone 7.3.16 (PDF, 653 KB)
● 20 April 2016 –DCC Media Release
Report on South Dunedin infrastructure performance during June 2015 flood released
● 26 April 2016 –Infrastructure Services Committee
Agenda – ISC – 26/04/2016 (PDF, 6.3 MB) [agenda and reports]
Item 5 Report, ‘South Dunedin Public Infrastructure Performance during June 2015 Flood Event Follow up’ (Stokes), pp 6-27
Minutes – ISC – 26/04/2016 (PDF, 123.0 KB) | Meeting Video
The following content from consulting engineer Neil Johnstone is provided for your information and convenience. However, the site owner cannot accept any liability for its accuracy or content. Visitors who rely on this information do so at their own risk.
Reviews previously published at this website:
● 8.3.16 Johnstone independent review of DCC report #SouthDunedinFlood
● 19.5.16 Johnstone review of 2nd DCC report #SouthDunedinFlood
█ Third Review | dated 31.5.16
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SOUTH DUNEDIN FLOODING JUNE 2015
A Follow-up Review subsequent to DCC Infrastructure Services Committee meeting 26 April 2016
By N.P. Johnstone, MIPENZ
DCC has produced 2 reports on infrastructure performance during the flooding that reportedly entered approximately 1000 houses and caused in excess of 100 million dollars of damage. I have previously produced 2 independent reviews that are highly critical of DCC’s Stormwater Infrastructure Report (November 2015) and its “follow up” (the “mudtank report”, April 2016).
I also attended DCC’s Infrastructure Services Committee meeting on April 26, and have sighted video recordings of that meeting, which were belatedly posted by DCC close to a month later. I have formed an opinion that DCC has not fully acknowledged its role in failing to prevent much of the flood damage, but (often unreasonably) blames external influences and alleged historical design deficiencies.
I had originally intended to present a blow by blow comment on proceedings of the meeting, but that has proved a bigger task than my time allows, and would probably prove as tedious as the meeting itself. I have preferred not to align statements with the individual staff or councillor that made them, but the reader can view the recording of the meeting at leisure. I can however, not resist the temptation to present the comments of one councillor who stated: I am heartened by how well staff have responded to the challenges…and we can be comforted, I think by the feeling that you are treating our assets like you would your own. South Dunedin residents who had their assets ruined might see things rather differently.
1. THE FLOOD EVENT OF MARCH 8/9 1968
The rainfall event of March 1968 was of the order of 10% greater than that of June 2015, on any comparison. This is incontrovertibly clear from readily available data, and thus demonstrates that the much greater flooding in the recent event should not have happened. DCC has consistently underestimated the significance of the 1968 rainfall.
2. PRE-EXISTING GROUNDWATER LEVELS
DCC has misinterpreted ORC reporting that groundwater levels prior to the onset of rainfall were significantly elevated. DCC has extrapolated this error to claim that no infiltration of rainfall into the ground was possible, thereby explaining away the record surface flooding. ORC’s data is readily available, and demonstrates that groundwater levels only started to increase in concert with the June 3 rainfall, proving that infiltration was both significant and (probably) normal.
3. RAINFALL SIGNIFICANCE
DCC, relying on superficial analyses of other agencies, have claimed that the 24-hour rainfall experienced in June 2015 was a 150-year event, then 100, then 60, then 100 again. The selection of 24-hour duration rainfalls is not entirely appropriate (being rather too long). Nevertheless, the March (or April) 1923 24-hour rainfall was much greater, the 1968 rainfall slightly higher (but ignored in analyses), and at least 2 others between the 1890s and late 1920s were similar, and possibly slightly lower. Analyses of shorter duration events would likely further reduce the assessed recurrence interval of the June flood. I believe it was a 30-year rainfall event at worst.
4. LANDUSE CHANGES
Increased impermeable areas have certainly increased runoff rates; based on DCC-supplied estimates of changes, I have estimated that these changes could have increased flood levels by up to 150mm. Ex-DCC engineers have expressed doubt as to the degree of landuse change. In any case, I would have expected additional stormwater infrastructure to have been installed to compensate.
5. MUDTANK BLOCKAGE
The degree of blocked mudtanks across the catchment was underestimated by DCC, until the release of the mudtank report in April 2016. The extent of the mudtank issue was finally confirmed then, at no great surprise to South Dunedin residents. An unseemly blame game ensued, and may not be over. DCC then took the stance that the plethora of blocked mudtanks had no impact on the depth of flooding across South Dunedin, but may have prolonged the flooding. The claim defies reason, and in part relies on DCC’s adherence to the zero infiltration myth.
6. PORTOBELLO ROAD PUMPING STATION
The partial blockage – and difficulties experienced in the clearing – of the screens at this pumping station has been well publicised, and may have added approximately 200mm to the depth of flooding. Less well publicised are the facts that no emergency staff visited the pump station until about noon on the day of the flood, and (apparently) only attended the station to attempt clearing the screens between visits to other locations during the latter part of the day. Equally poorly understood is that not all of the station’s pumps were operating throughout the flood event due to the manner in which pump cut-ins were programmed. No decision to override the pumps’ programming was made. No information on which pumps remained inoperative has been made available, so it is not possible to attribute the depth of flooding caused by the inability to clear screens, compared with that caused by poor pump management.
7. MUSSELBURGH PUMPING STATION
This pumping station has the ability to bypass the wastewater treatment plant and discharge sewage-contaminated stormwater directly to the ocean at Lawyers Head. The option was not taken in the June 2015 flood. By contrast, in the 1968 event the pumping option at Musselburgh was fully utilised to the extent that approximately 5m3/s was pumped to the ocean for a period of 24 hours. Such pumping would have proved effective as long as individual property gully traps were submerged by stormwater, and may have reduced flood depth by more than half a metre. It is not known (by me) whether a comparable benefit could be achieved under June 2015 conditions, but a very significant opportunity was apparently lost. No information on the Musselburgh station’s pumping operation in the June flood is included in either of DCC infrastructure performance reports.
8. INFRASTRUCTURE DESIGN LIMITATIONS
DCC has relied heavily on the belief that the stormwater system can only handle rainfall intensities of 4.1mm/hr, and therefore regular flooding cannot be avoided. This is contrary to historic reality. The 4.1mm/hr limitation only applies if DCC’s assumption of zero infiltration applies. The assumption has no validity, and the existing infrastructure is far more capable than DCC is stating.
9. EXTERNAL PEER REVIEW
One of the main reasons for the extraordinary delay in the presentation of the “mudtank” report was that a robust external peer review was to be obtained. Reference to such peer review has surfaced occasionally since, but no review has been published. I have sought a copy of any such review – or even confirmation of its existence – from DCC. No such review has yet been confirmed to exist.
10. INFRASTRUCTURE SERVICES COMMITTEE MEETING
Many of the above topics were discussed to a greater or lesser level of detail at the Committee Meeting on April 26. From sitting through the majority of the meeting and viewing video coverage of it later, it became apparent that Councillors’ understanding of the June 2015 flood causes was generally weak, and that many of their questions directed to staff were inadequately or confusingly answered, if they were answered at all. These included:
10.1 Pre-existing Groundwater Levels
Selective reference was made to ORC report text, but no data was referenced. The data disproves the assertion of high groundwater and zero infiltration.
According to staff, mudtank blockage didn’t increase flood levels, yet a raised vegetable patch could.
10.3 Portobello Road Pumping Station
No information on unused pump capacity or its impact on flood levels was given (or sought); and the final word on the subject was that the only issue at the station had been debris blockage, now fixed.
10.4 Musselburgh Pumping Station
In response to a question re pumping rates at this station, the reply was to the effect that “we have no data on this, is there a follow-up question?” There wasn’t a follow-up question on the subject, despite its fundamental importance.
10.5 Infrastructure Design Limitation
The existing infrastructure was repeatedly deemed to be inadequate on the basis that only 4.1mm/hr of rainfall could be accommodated, once pipe storage was exhausted. There was no proviso given that this determination required the impossible condition of zero infiltration of rainwater into the ground.
This review does not exhaust my concerns with the technical presented at the meeting, but I can conclude the following with confidence:
i. Maintenance prior to the flood was inadequate;
ii. Emergency management during the event was poor;
iii. DCC’s understanding and reporting of flood issues remains unconvincing, especially without the benefit of promised external peer review.
31 May 2016
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Posted by Elizabeth Kerr