Semi-retired consulting engineer Neil Johnstone was invited to speak to his independent review of the DCC report, Infrastructure Performance During June 2015 Flood Event (30 Nov 2015), at Monday night’s public meeting held in South Dunedin.
Large numbers of local householders and business people, together with news media, filled Nations Church Auditorium at 334 King Edward Street, to examine why South Dunedin “flooded” on 3 June of last year.
Dunedin City Council personnel who didn’t bother to show up included Mayor Cull, CE Bidrose and members of the Executive Leadership Team (RLT). How many elected council representatives turned up —one, Cr Mike Lord (the question was nearly rhetorical although a couple of councillors had forwarded their apologies).
A fortnight ago Mr Johnstone sent a copy of his review to DCC chief executive Sue Bidrose. Notably, it took until the day of the public meeting for Ms Bidrose to acknowledge receipt and respond to the review by letter —DCC made sure to effect personal delivery to Mr Johnson’s home in Macandrew Bay, followed by an electronic copy some time later.
Copies of the review were circulated at the public meeting —these were in some demand!
Following the close of meeting, the reviewer kindly supplied What if? Dunedin with copy for publication.
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.
An Independent Review if DCC Report
‘Infrastructure Performance during the June 2015 Flood Event’
1. Having lived most of my life in Dunedin and its environs (though never in South Dunedin), and having had a long career in natural hazard identification and mitigation, I am concerned with the standard of understanding and reporting of current natural hazard issues by our local Councils and, to a lesser extent by Government Agencies. I spent many years as Investigations Engineer at the Otago Catchment Board from 1986, and held a similar position at the Otago Regional Council until 2002. During those years I analysed numerous recent and historic flood events; none was more straightforward than the South Dunedin flood event of June 2015, and many were far more complicated. Now semi-retired, I still operate my own small consultancy.
2. In my opinion the DCC Report might best have been produced by independent experts, or – at the very least – have been subject to rigorous expert peer review. Current “victims” of the in-house reporting approach appear to include residents of South Dunedin who were affected by the June 2015 flood, and the wider population of the city and beyond who have been presented with information of questionable validity.
3. I have no personal interest in the South Dunedin area, but do jointly own a property elsewhere in Dunedin City. This paper only peer reviews DCC’s Report Infrastructure Performance During the June 2015 [Flood] Event. Further reviews of other hazard reports are planned. The reader can access online both DCC’s Report on the June 2015 flood event (referred herein to as “the DCC Report”) and ORC’s report Coastal Otago Flood Event, 3 June 2015 (referred herein to as “the ORC Report”). The latter is frequently referenced in the DCC Report.
4. The DCC Report is lacking in detail and thoroughness. It is short, but neither concise nor accurate, in my view. No reason is given why such a simplistic document took virtually six months to produce. By contrast, an earlier DCC report on the South Dunedin flood of 9 March 1968 took about a week to prepare following that event. My review is intended to provide alternative and more plausible explanations for the flooding experienced in June to those given in the DCC Report and accepted and promoted by some Councillors. I have used almost exclusively data provided by ORC and DCC publications. My approach is reasonably “broad-brushed”, but to a level of accuracy I believe limited only by the quality of data available.
5. Specifically, the DCC Report lacks objectivity in that it:
A. exaggerates the historical significance of the June 2015 rainfall,
B. repeatedly (and contrary to very clear evidence) identifies high groundwater levels as a prime cause of the flooding,
C. fails to discuss why staff did not (apparently) continuously attempt clearance of pumping station screens,
D. fails to adequately address the impacts on total runoff volume of reduced ground surface permeability due to land use change,
E. promotes a simplistic flow volume model that contains a key erroneous assumption,
F. fails to quantify ingress of “foreign” water from other sub-catchments, especially St Clair,
G. refers only briefly to the Shore St (Tainui) sub-catchment, and then fails to note that flooding was much less significant there than in the South Dunedin catchment or to explain the reason why,
H. defends the maintenance performance of mudtanks without providing any supporting evidence.
6. With respect to the above lettered points:
Point A: DCC has persistently exaggerated the significance of recent rainfall in the city. Initial claims regarding the June rainfall had it as a 150-year event, and (with respect to a disadvantaged peninsula property owner) reportedly claimed rainfall intensities increasing by 82% as a result of climate change. Now the June [flood] is stated in the Report to be a 63 year event. Such claims are all substantially in error. Rainfall in the March 1968 event is conceded in the DCC Report to be higher than in June 2015, yet the earlier event is omitted from consideration of flood frequency. Inclusion of the 1968 rainfall must substantially reduce the assessed return period of the 2015 rainfall.