Daily Archives: May 19, 2016

Johnstone review of 2nd DCC report #SouthDunedinFlood

Updated post
Sat, 4 Jun 2016 at 4:11 p.m.

DCC publications:

● 30 Nov 2015 (McElhone)
Infrastructure Performance During June 2015 Flood Event | Meeting Video

● 20 Apr 2016 (Media Release)
Report on South Dunedin infrastructure performance during June 2015 flood released

● 26 Apr 2016 Agenda (and reports) Infrastructure Services Committee
Item 5 (Stokes, pp 6-27) South Dunedin Public Infrastructure Performance during June 2015 Flood Event Follow up | 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.

Received: 18 May 2016 [full text]

An Independent Review of DCC Report : ‘South Dunedin Public Infrastructure Performance during June 2015 Flood Event Follow up’

By N. P. Johnstone, MIPENZ

1. This review complements my peer review of DCC’s first flood report, published in November 2015. This review assesses the content of the second report (described henceforth as “the report”) published in late April 2016, and contextual statements made elsewhere by DCC staff and elected members. The author of the report is Ms R. Stokes. The technical qualifications and relevant experience of the report’s author are not stated.

2. I consider that there is a need for such a review for reasons of historical accuracy and context, the identification of solutions (which can only be achieved be if the problem is understood and acknowledged) and – most importantly – to provide a considered assessment of what South Dunedin’s current flood risk really is, noting that two events (of which only the recent one caused major inundation) in five decades does not suggest a current flood risk much different from that existing in many other established New Zealand communities, despite some landuse changes. It is emphasised that the flood had nothing to do with climate change, nor therefore does this review. The failure to understand the issues may lead to inaction or to inappropriate and expensive actions.

3. This review may be criticised for being repetitive on some issues, but the repetition is at least partly driven by the number of times challengeable information on the flood event and its causes has been circulated by DCC. In many respects, the report under review could be seen as a concentration of such challengeable information. The report is solely based on my research, knowledge and experience; any errors are therefore mine, but hopefully, few.

4. This review has led to the following conclusions:
4.1 Council’s continued insistence that the June 2015 rainfall event was the largest since 1923 remains erroneous;
4.2 Pre-existing groundwater levels were unexceptional, and had no impact on the flooding, contrary to claims made in the report, previously and subsequently;
4.3 South Dunedin does not have a significant imminent exposure to stormwater flooding. This finding is based on the original design parameters, historical performance, an absence of groundwater issues, and provided existing infrastructure is properly maintained, monitored and operated;
4.4 Problems at the Portobello Road Pumping Station caused elevated flood levels and prolonged the period of inundation, but the report acknowledges only the latter;
4.5 Similarly, the now-admitted failures to ensure that mudtanks were properly maintained impacted adversely on flood levels attained in some locations at least, and prolonged the period of inundation in many areas;
4.6 Comparisons with the 1968 flood event can be instructive in assessing the impact of Council failures in 2015 in terms of water level, disruption and cost. The report fails to make such assessments.

5. My review of the first report, written by Ms L. McElhone, was driven by DCC claims that the prime causes of the flood were high sea and ground water levels, a 150-year, then a 100-year, then a 63-year (and incidentally and extraordinarily now again a 100-year*) rainfall event, and confirmed that Portobello Road pumping station issues added not less than 200mm to peak flood levels. That review also demonstrated that the rainfall event of March 1968 was demonstrably larger than that of June 2015, but caused much less damage, and that land use changes added up to 150mm to flood levels (based on DCC’s unconfirmed data on impermeable areas). Any consented landuse changes should, in my opinion, have been compensated for in past years with additional infrastructure to maintain drainage standards and South Dunedin’s protection standards.
(*Ms Stokes to John Campbell on Checkpoint, 21 April 2016).

6. Exaggerated assessments of both the historical significance of the 2015 rainfalls and groundwater levels, and the absence of mudtank information originally helped DCC promote its position of zero liability. The mudtank maintenance failures are at last largely revealed in the new report; significant mudtank maintenance issues were previously reported by Cr Lee Vandervis as early as 2014, but were seemingly largely ignored by DCC. Paragraph 37 of the report which reads: “Mudtank maintenance and performance in general has been the subject of focus for a number of years”, appears vague, and therefore requires elaboration. The statement, if accurate makes the failures more disturbing. There is still some unfortunate reliance on the groundwater myth (paragraphs 2 & 32), and to the underestimation of the 1968 event (paragraph 20). There appear to be newly-entrenched positions at DCC that the existing stormwater system is inadequate, presumably based on the report’s paragraphs 23-27, and reinforced in recent public statements from Ms Stokes and Mayor Cull, that the flood would have occurred (or that a serious flood would have occurred) even if the current system including the mudtanks had operated at optimum. This review strongly disputes such claims, and uses the well-documented event of March 1968 as a very useful “model” for key comparisons between a contained flood (1968) and a disaster (2015).

7. The general understanding was that DCC’s second review was to concentrate in detail on the performance of mudtanks, and was to be peer reviewed. Detailed reporting and peer review processes were understood to be the reason for the extraordinary delays in publication. In reality, only paragraphs 33-61 deal with mudtanks issues and no peer review is included. It is noted that Mayor Cull confirmed to John Campbell on Radio NZ’s Checkpoint programme (22 April 2016) that peer reviews of the report had been produced. The peer review(s) could usefully have been attached to the report; failing that, the report’s author should have explained their absence. Continue reading


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