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.
Magnitude of the June 2015 Storm
7. The report unquestioningly accepts Otago Regional Council (ORC) data that suggests the event had a 63-year return period. (Previously DCC had been claiming anything up to a 150-year event). The ORC analysis stated that the June 2015 24-hour rainfall (140mm) was the second-highest recorded, well below the 1923 event (229mm). ORC seemingly did not include the March 1968 24-hour rainfall (158mm) in its calculations. That event is known to the writers of the DCC Report (it is described in the 1968 flood report), but is omitted from current consideration without explanation. If the 1968 flood is included in calculations, it is difficult to make a case for the 2015 rainfall being greater than a 30-year event.
8. Also, while the 1929 24-hour rainfall at Musselburgh was lower than the 1923, 1968 and 2015 events, it nevertheless produced a 6-hour intensity of 51mm. This is more than 10% greater than the June 2015 6-hour rainfall (45mm).
My conclusion: The Report’s claim that the June 2015 rainfall had a return period of 63 years remains a substantial over-estimate. A figure of 20-30 years is more defendable, particularly if rainfall durations shorter than 24 hours are considered, as they should be.
Point B: The Impeding Effect of High Groundwater Levels on Infiltration
9. Groundwater level data relied on by the report is sourced solely from ORC bore installations at four South Dunedin locations (Kennedy Street, Culling Park, Tonga Park and Bathgate Park). The data is produced graphically and descriptively in the ORC report (pages 25-30 and figures 21-24).
10. The DCC Report states “During the June event, observations from the Otago Regional Council of the groundwater conditions prior to and during the event show that the groundwater was elevated prior to the event and actually reached the surface at several of the South Dunedin monitoring bores”. In my view the statement is inadequate as it fails to clearly represent the magnitudes, timing and cause of groundwater level rise. The ORC report in fact indicates that:
• GW levels at Kennedy Street were just 0.071 (0.674 minus 0.603)m above average before the flood event, but rose 0.997m during the event, and peaked 0.261m above ground level.
• GW levels at Tonga Park were just 0.087 (0.644 minus 0.557)m above average before the flood event, but rose 0.722m during the event, and peaked 0.346m above ground level.
• GW levels at Culling Park were just 0.123 (0.198 minus 0.075)m above average before the flood event, but rose 0.707m during the event and peaked just above ground level (by 0.075m).
• GW levels at Bathgate Park were just 0.062 (0.751 minus 0.689)m above average before the flood event, but rose 0.238m during the event, and peaked a full 0.532m below ground level.
The small rises in groundwater levels before the main rainfall event are in all four cases attributed by ORC to minor rainfalls that occurred on 12 and 26 May.
11. Figures 21-24 of the ORC report appear to demonstrate that groundwater levels did not reach ground levels until the June 2015 rainfall event was well advanced. I have requested hourly data from ORC to verify this point, and am awaiting that information. In any case, the advent of surface water on playing fields does not necessarily indicate a cessation of infiltration; it simply signifies that rain water has fallen at a rate faster than can be fully absorbed. When the rain reduces or ceases, infiltration rates exceed rainfall and levels drop.
12. The coincident rise in groundwater levels with rainfall demonstrates conclusively that there was substantial infiltration taking place throughout the majority of the rainfall event. The infiltration caused groundwater levels to rise, as should be expected. There was no hindrance to infiltration caused by pre-existing groundwater levels, contrary to DCC’s claims. The most obvious of conclusions is that groundwater levels rose simply and solely as a result of direct infiltration of rainfall. DCC’s oft-repeated claim of impeded (to zero) infiltration is without foundation. Key paragraphs 2 and 5 (both from the “Executive Summary”) of the DCC Report are therefore misleading and/or irrelevant, as are paragraphs 36, 40, 54, 55, 56, 57, 62 and 66. The DCC Report provides no evidence to back its conclusions on this issue.
13. Given that the claim of high groundwater’s impact on flood levels attained in June 2015 can be shown to be entirely without foundation, it is disappointing that ORC has not publicly corrected DCC’s misconceptions. Paragraph 65 of the DCC Report states that “Staff have had initial discussions with ORC …. to assist with improving understanding of the relationships between stormwater and groundwater in South Dunedin”. ORC data clearly show that groundwater levels were driven by rainfall via infiltration; DCC’s unilateral approach is that groundwater prevented infiltration and therefore maximised stormwater quantities. The views are mutually inconsistent. ORC’s conclusions are supported by indisputable data. It is not convincing that the DCC Report should claim zero infiltration as the prime cause of flooding, only to later concede a need to understand the relationships better. It may have been helpful if there had been a summary of the outcome of discussions between the two Councils included in the Report.
Point C: The Partial Blockage of the Portobello Road Pumping Station Screens
14. Note: available pipe network storage is stated in the DCC Report to be a “highly relevant factor”, but the network is in fact demonstrated to contribute a mere 21,000 cubic metres of storage, compared with a total of almost 800,000 cubic meters of rain that fell on the South Dunedin catchment.
15. The performance of the screens is referenced in paragraphs 4, 45, 46, 47, 48, 49, 50, 61 & 63 of the DCC Report. It can be interpreted that maintenance staff chose – or were instructed – to push debris lower on to the screens rather than persevere with attempting the apparently more difficult task of removing it. This approach may have tended to have ensured that the pump system could never operate at full capacity. It is stated that operators attended the site “frequently” throughout the event. Surely there should have been a 100% attendance and effort to maximise screen clearance? The report should have presented a log of man-hours worked at the pump station during the critical period.
16. The report states that a “high level” report has been received to redesign the screens at a cost of $500,000. This seems an unduly significant sum, especially as recent work has recently been undertaken to improve access to the screens for cleaning, according to the Three Waters Strategy of 2012 (page 47). The report should have explained why such expenditure is planned, rather than ensuring that current maintenance staff have adequate training and equipment to keep the screens cleared.
Point D: Increased Impermeability of the South Dunedin Catchment
17. The transition of permeable land to impermeable in recent decades is emphasised as a factor in the report; yet elsewhere the report insists that high ground water levels prevented infiltration anyway. If groundwater levels prevented infiltration, surface conditions would be irrelevant.
18. Paragraph 35 of the DCC Report notes that the drainage network now needs to accommodate 60% of rainfall as runoff, compared with 45.7% half a century or so ago, as a result of changes of land use. The report does not develop the considerable significance of this issue. Considering the June 2015 event: 140mm of rain falling on the South Dunedin catchment (570 ha) produced a total of 798,000 cubic metres of water. Originally, the drainage network would have had to accommodate 365,000 cubic metres (45.7$%) of that water; by June 2015 that requirement would have risen to 479,000 (60%) cubic metres – an additional 114,000 cubic metres. This additional amount of water, distributed over the main identified flooded area (75 ha) would elevate flood levels by 150mm.
Point E: The Pump Flow “Model”
19. The approach utilised of comparing flow volumes is valid, but the assumption of zero infiltration is entirely without foundation, as demonstrated earlier in this review. Figure 4 of the DCC Report appears to assume a maximum continuous flow performance of around 6.3 cubic metres per second. This would assume no pumping station screen blockage and sufficiently clear mudtank grates to allow delivery of that rate of flow. Perfect performance is not usually attainable, and a through-flow of say 5.8 cubic metres per second might not be considered a failure. Figure 6 of the DCC Report suggests however that the volume pumped rarely attained 5.5 cumecs and averaged only about 4.5 cumecs. There was thus a deficit of at least 1.3 cumecs over a 32-hour timeframe. This meant that some 150,000 cubic metres of water was unable to be cleared because of partial screen blockage and (possibly) mudtank blockage. Distributed over the 75 ha flooded area, this would have elevated peak flood levels by approximately 200mm.
20. The spikes in pump output performance shown in the report’s figure 6 almost certainly indicate periodic if limited success in clearing the pump screens. Based on that interpretation, screen clearing does not appear to have been attempted until close to noon on 3 June. Then for approximately 5 hours between 3.30pm and 8.30pm no clearing appears to have been achieved, and possibly not attempted. Belatedly, from about 8.30pm until about noon the following day, fairly intensive efforts appear to have been made with some success.
Point F: Cross-Catchment Flow From St Clair Catchment
21. The DCC Report emphasises that such flow did occur, and appears to portray this as a super-design situation. While the amount of inflow from St Clair is understandably not known, it does need to be appreciated that such flow also occurred in March 1968 (and is referenced in the 1968 flood report). The 3 Waters Report appears to acknowledge that there is a cross-connection that allows St Clair flows to enter the South Dunedin stormwater system in addition to any overland cross-catchment flow that may also eventuate. This would suggest that DCC has allowed South Dunedin to be subject to flood risk for which there has been no compensating/mitigating stormwater infrastructure design.
22. The ORC Report (page 23) notes that comparatively little flooding was experienced in the lowest part of South Dunedin at Tainui relative to the rest of the catchment, and suggests that further work is required to identify the reason for this. To me, at least some reasons (plural) are obvious. Firstly, the Tainui area does not drain to, or rely on, the underperforming Portobello Road Pump Station (its outfall is apparently into the Andersons Bay inlet). Secondly, there has to my knowledge been little or no development over recent decades in the Tainui area of the sort that would reduce the potential for infiltration. Thirdly, there would have been little or [no] impact of St Clair catchment overflow reaching Tainui. That is, the three aggravating causes of the 2015 flood event in South Dunedin as identified by me were all absent in the Shore Street (Tainui) sub-catchment.
23. When questioned on the Tainui “anomaly” at a Council meeting following the release of their report, DCC staff could apparently only suggest that rainfalls in the Tainui area were possibly less than across the rest of South Dunedin. No evidence to support this notion has ever been presented, to my knowledge.
24. The DCC Report has proposed reasons for the extensive flooding of South Dunedin that concentrate on the impact of elevated groundwater levels prior to the event. ORC reporting – possibly misinterpreted by DCC – make it clear that high groundwater did not exist until the rain came. Groundwater levels are a consequence, not a cause. Persistent and unjustified references to high groundwater levels are both misleading and counterproductive given the inclination of some within the wider community to attribute the event to climate change/rising sea level scenarios. The DCC Report – and its apparent internal acceptance – suggests that the organisation may be suffering from a phenomenon known as Scenario Fulfilment Syndrome. This is typically characterised by information that does not conform to prior interpretation (in this case, the impact of claimed high groundwater levels) simply being ignored, prematurely discarded, or put in the “too hard basket”.
25. In my view, the under-performance of the pumping station (certainly) and mudtank* (possibly) systems raised surface flood levels by a combined 200mm plus, and the allowed changes in land permeability contributed a further 150mm. Mudtank* problems could be expected to have caused local flooding issues, at least. The contribution from cross-catchment flows is likely to be significant but is indeterminate, as no data is presented in the DCC Report.
*It is emphasised that no data on mudtank performance is contained in the DCC Report. As a result, mudtank performance is not assessed in this peer review.
26. Had June 2015 flood levels been lower by 350mm (150 caused by land use change, plus 200 caused by infrastructure-related issues) in the June event, it is likely that the impact of flooding would have been substantially reduced, and probably little different from that which occurred during the broadly comparable rain event of 9 March 1968. South Dunedin residents affected by the June 2015 flood event may consider to what extent they were – and presumably still are – disadvantaged by DCC actions, inactions, misinformation and attitude.
N.P. JOHNSTONE MIPENZ
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Posted by Elizabeth Kerr