STATEMENT OF EVIDENCE OF PETER BLACKWOOD

Transcription

1 STATEMENT OF EVIDENCE OF PETER BLACKWOOD PALMERSTON NORTH CITY COUNCIL - PROPOSED PLAN CHANGE 15H: AIRPORT ZONE 1. Thank you for the opportunity to present evidence in support of Horizons Regional Council s (Horizons) submission to the Palmerston North City Council s Proposed Plan Change 15H: Airport Zone. A. QUALIFICATIONS AND EXPERIENCE 2. My full name is Peter Lindsay Blackwood. I am currently employed as the Manager Investigations and Design at Horizons Regional Council ( Council ). I have held this position since 30 October For the period from September 1996 to October 2006 I was employed as the Manager Technical Services for the Bay of Plenty Regional Council. 3. I am qualified with a Bachelor of Engineering (Hons) and gained my Engineer s Registration in I have thirty-nine years experience in civil engineering, including project management, flood control and drainage (policy, asset management, design and supervision), river management and protection works, coastal hazards (storm surge and wave run-up), environmental engineering, water resources (particularly flood frequency), global warming policy and design, civil design (including bridging), financial analysis, irrigation and power station construction. 4. I have authored or supervised the production of numerous designs and reports on river hydrology and hydraulics, floodplain management, global warming impacts, river alignments and erosion protection works. 5. I have managed or supervised detailed floodplain management studies on the Whakatane-Waimana, Waioeka-Otara, Waikanae and Otaki River Schemes. 6. I have prepared numerous design reports under the employ of Council including the following relevant reports: Lower Whanganui River Flood Protection Investigations Stage One: Review of the Current Flood Hazard, Lower Whanganui River Flood Protection Investigations Stage Two: Assessment of Flood Mitigation Options, - 1 -

2 Rangitikei River Scheme Review Number 4, Horizons Regional Council Flood Protection Scheme Standards Summary Report, Non-Scheme Rivers Investigations Prioritising Future Studies and Ohakune Flood Protection Investigations Assessment of Flood Mitigation Options. 7. I confirm that I have read the Code of Conduct for Expert Witnesses and that I agree to comply with it. B. TECHNICAL EVIDENCE IN RELATION TO HORIZONS SUBMISSION Horizons Request for Hydraulic (Hydrologic Neutrality) (Refer to paragraphs to in Michael Duindam Planners Report). 8. In the Horizons Regional Council (Horizons) submission dated 4 March 2015 we advised that Increases to the runway length, new ancillary roads and other developments are likely to substantially increase stormwater runoff. Horizons requests that hydraulic neutrality is achieved in the 0.5% AEP as a minimum. 9. I agree with the amendment of hydraulic neutrality to hydrologic neutrality as advanced in the memorandum by John McCartin Palmerston North City Council (PNCC) Stormwater Asset Engineer, contained in Appendix 6 to the evidence of Michael Duindam. The term referenced by Horizons is one that has been used in the Engineering profession and is not coined by Horizons, though the profession would normally recognise what the meaning is. 10. Horizons staff expressed these concerns to Palmerston North Airport Ltd representatives and PNCC staff at a meeting held at the PNCC Office on Friday 21 March Set out in Horizons submission, we are currently upgrading the Mangaone Stream flood protection, in three locations, to the 0.2% AEP (1 in 500 year) design standard. These locations are: i. Setters Line north of the airport; ii. Flygers Line stopbanks; and iii. Benmore Avenue. 12. It is important to ensure that additionally generated stormwater in the Airport Zone does not materially compromise the standard of flood protection being provided in these locations

3 13. I note that Performance Standard (h) of Rule (Construction, external alteration of, and addition to buildings and structures) requires connection of all sewer, stormwater and water supply services to essential services through a public service corridor. If additional stormwater from the development is connected to the reticulated stormwater network that discharges to the Mangaone Stream upstream of Flygers Line, Horizons flood protection measures could be compromised. 14. A different approach to stormwater management is required by proposed Rule for Accommodation Motels and Residential Centres. Assessment criteria (z) requires the site and building design to mitigate any increase in peak stormwater run-off and peak stormwater flow due to the reduction in permeable surfaces. However, there is no performance standards specified that give plan users clarity on what the critical design storm is that must be mitigated. 15. Horizons concern is where there may be additional hardstand areas, such as the runway extension, where stormwater is proposed to be managed on-site, or discharged from the site, rather than being connected to reticulated stormwater services. If this is the case, there needs to be rules and performance standards included in Section 13 of the Plan to control stormwater discharges to achieve hydraulic neutrality in the 0.5% AEP storm and to ensure that additional stormwater generated does not cause or exacerbate flooding of any other property. 16. Horizons therefore requested that additional rules and performance standards be included in Section 13 of the District Plan to control stormwater discharges from activities, new buildings and new hardstand areas to achieve hydraulic neutrality (replacement of this term with hydrologic neutrality is agreed) in the 0.5% AEP (1 in 200 year) design storm. Please note: i. This is a very similar sized storm to the 1%AEP plus climate change storm, as referenced in the non-mandatory standard NZS4404:2010, Section Design Storms; and ii. Whilst the 0.5% AEP storm and consequent quantum of mitigation is less than that for the 0.2% AEP storm, it appears that this standard of mitigation cannot be exceeded. Michael Duindam Planning Response (refer to paragraphs ) and John McCartin, Stormwater Engineer, Memo (refer Appendix 6)

4 17. Michael Duindam has based his response largely on the expert technical report produced by John McCartin contained in Appendix In paragraph 2 of the memo Mr McCartin advises that he does not consider that the level of stormwater that is likely to be generated for the fully developed Airport Zone scenario would be at a level that would materially compromise flood protection in Settlers Line north of the airport, Flygers Line stopbanks or Benmore Avenue. The volumes of stormwater generated by the changes in this area will always be insignificant compared to the volumes heading down the floodway from the Mangaone s main catchment. 19. In paragraph 3 Mr McCartin advises that the only potential risk of discharge into the Mangaone Stream north of the Flygers Line Floodway would come from the proposed runway extension. Mr McCartin then advises that he considers that any issues related to stormwater management can be dealt with as part of the resource consenting process. Horizons Response to the Michael Duindam Planning Report and John McCartin, Stromwater Engineer, Memo 20. Horizons has very recent experience of flooding complaints from landowners in Fagan Road on the Makowhai Stream located downstream of the Ohakea Airport tributary. This stems from an upgrade of the Ohakea Airport facility resulting in increased impermeable surfaces and more runoff. 21. A Consultant report on this flooding has essentially come to the corresponding conclusion as the McCartin memo paragraph 2; that the volumes of additional water generated by the Ohakea Development are insignificant compared to the floodwater quantum in the Makowhai Stream. In this case a house has flooded and since again come close to flooding. 22. The Makowhai Stream landowner cannot be convinced of the findings. 23. A further difficulty is that in the recent June 2015 storm the Mangaone Stream experienced a 5% AEP (1 in 20 year) flood (154 cumecs at the Milson Line recorder). This resulted in serious flooding of at least two properties immediately downstream of the Flygers Line spillway near Gillespies Line. 24. The other obvious concerns are elevation of flood levels adjacent to the proposed Flygers Line and Benmore Avenue stopbanks and Taonui Basin flood levels. In regard to the latter Horizons went through an exhaustive and very costly exercise when adding - 4 -

5 more water to the Taonui Basin whilst upgrading the Oroua River stopbanks to their 1% AEP (1 in 100 year) level of service. 25. Therefore, the spillway cannot take more water as this will exacerbate the flood levels experienced downstream. Whilst the floodwaters generated may be significantly smaller than those from existing sources, they are an additional component to flooding and will result in an additional water level, regardless of the ambient current flooding sources. 26. In paragraph 2 John McCartin advises that any development within this zone will transfer stormwater into the reticulated stormwater network, which discharges into the Mangaone Stream downstream of the Flygers Line floodway. 27. Horizons design engineers have considered this and accept that the Mangaone Stream downstream of the floodway is expected to contain the quantum of this stormwater. 28. However, we note that stormwater generated by the proposed runway extension is unlikely to reach this reticulated network; instead travelling most likely to the floodway and will require mitigation of the 0.5% AEP flood event. As the floodway operates for up to 24 hours, that is critical duration storm. 29. One observation on the stormwater network that we would like clarification of, is whether the network is indeed designed to carry the 0.5% AEP storm; as most pipe networks are not designed for 0.5% AEP storms. Indeed most stormwater networks are designed for either the 20% AEP (1 in 5 year) or 10% AEP (1 in 10 year) storms. Thus it seems likely that additional flows due to increased impermeable areas would have to flow in the secondary overland flow paths. 30. Section 2.3 of the document entitled Palmerston North City Council Stormwater Design Manual, April 1993 advises that the recommended minimum pipe flow capacity is the peak 5 year return period runoff (20% AEP). Whilst this document has no doubt been updated and the pipe sizes may exceed the 20% AEP standard, it seems likely that a standard similar to this was the prevailing standard when the pipe infrastructure was constructed. 31. Confirmation is therefore required that in the 0.5% AEP storm additional stormwater generated: i. Does not cause or exacerbate flooding of any other property through the overland flow. ii. Will not reach the Flygers Line floodway

6 32. In terms of waiting until a resource consent process considers the proposed airport runway extension, or in fact any other development (such as hypothetically a large maintenance hangar establishment), Horizons is concerned that this should be addressed at the planning stage, being now in at least an appropriate rule. Horizons Response to the Palmerston North Airport Limited further submissions s56/76 and s56/77 (Para Michael Duindam Planning Report) 33. This concern was expressed by the Palmerston North Airport Representatives at the meeting held on 21 March Horizons understand and support the concern expressed. Horizons confirm that the detentions are for very short duration ponding and would not contain standing water for more than a day and occur on very rare occasions. In such occasions there inevitably would be many areas of surface water around and the detained water would not materially add to the numbers of attracted birds. Peter Blackwood MANAGER INVESTIGATIONS AND DESIGN 3 November