2. Mangatawa catchment consents for earthworks, storm water ...

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1 Project Description 1.1 Introduction The Mangatawa Drain was constructed many years ago, reportedly when the railway was built along the route parallel to what is now SH2. The purpose of the drain was to divert flow that previous flowed north into what was at the time farm land but is now Papamoa urban area. The Mangatawa Drain is about 2,500m long where it is alongside the proposed Tauranga Eastern Motorway (TEM). The drain has a vegetated, earth lined channel that is tightly constrained between the railway (to the south) and the highway (to the north). There is some uncertainty as to who owns the drain and is responsible for its maintenance, although in the past Tauranga City Council (TCC) has undertaken maintenance. It is generally located on either road reserve or railway land. In larger flood events excess flow spills north across the highway at two locations, and into the Papamoa urban area. The two locations are identified in this report as “Top Spill” and Bruce Road (although the spill covers several hundred metres of highway in the general vicinity of Bruce Road). While the discharge can be volumetrically large, it is generally spread as a shallow overflow (a few hundred millimetres deep) or ponding across several hundred metres length of highway, and has resulted in highway closure in the past. The was a closure in May 2005, which lasted for 14 to 18 hours. The latest event where there was a spill across the highway was on 7 March 2009, but this occurred during daylight and the road was not closed. The frequency of spills appears to be about 5 yearly on average at present. TCC has updated their design rainfall figures and these have been applied to this investigation and modelling. These design rainfall figures are based on predicted climate changes to 2055. The TCC design rainfall figures for 2055 are the basis of the TEM design. These 2055 figures were agreed with EBOP and consented. With the climate adjusted rainfall figures the frequency of spills is predicted to increase to about 3 yearly without mitigation measures, and to about 10 yearly with mitigation in place. In addition to closing the highway, the spills discharge into developing urban areas on the northern side of the highway, with potential consequences for TCC for flood risk management. The Top Spill discharge flows to the Wairakei Stream, where there is significant existing urban development, plus plans for growth. This stream has no natural outlet, discharging to ground soakage, so is sensitive to additional flood flow volumes. The Bruce Road spill discharges to TCC’s Maranui Swale, recently completed and designed to collect and convey urban stormwater along the northern side of the highway then under the highway and the railway to the wetland treatment ponds, before discharge to the Tauranga Harbour at Rangataua Bay. The swale was originally designed without accounting for climate change, or for the effects of the Mangatawa spill, and therefore will need significant upgrading to meet design flood level standards in the urban area. 1.2 Background In the 18 May 2005 storm extensive ponding occurred in the Papamoa area. The Mangatawa Drain alongside the highway overflowed at two locations. Opus was initially appointed by TCC to Beca // 17 March 2009 // Page 1 3932036 // NZ1-1604459-15 0.15 Rev A
undertake investigations into the drain performance. They produced three reports 1 and the last was after the May 2005 floods. As part of their work a MIKE11 hydraulic model was built. New Zealand Transport Agency (NZTA) has been working with TCC to find a solution which will avoid or at least reduce overflows. The aim for NZTA was to avoid if practicable the 1% Annual Exceedance Probability (AEP) storm event overtopping and overflowing the highway, while TCC’s aim was to reduce the volume of water that would enter the Papamoa catchment areas (Maranui Swale and the Wairakei Stream) such that the design flood level for Papamoa of 4.5m MD (Moturiki Datum), as defined in the District Plan, was not exceeded in the 2% annual exceedance probability (AEP) storm event. Beca was appointed to adopt the Opus hydraulic model and undertake further modelling and engineering work to investigate mitigation options, as an extension to the work they were carrying out for NZTA on the TEM. Attached in Appendix A is a plan illustrating the various elements within the catchments. Regular meetings were held whereby Beca reported to TCC and NZTA on the results from the modelling, and options for mitigation works. Various mitigation options were considered and used in combinations. These combinations were evaluated based on hydraulic performance and financial costs. The following are the mitigation options that were considered: 1. Improve Top Spill only. 2. Improve Bruce Road Spill only. 3. Balance spill between Top Spill and Bruce Road – low cost option. 4. Full mitigation – high cost option. The overall hydraulic performance of each option and the cost to construct was assessed and Option 3 provided the best benefit relative to cost. Further investigation and modelling indicated that the Maranui catchment should be added to the model work, in order to understand the effect of water spilling over the highway and what the effect of climate adjusted rainfall figures would be on the catchment once fully developed. With the inclusion of the Maranui catchment and swale in the model the following objectives were identified: 1. Determine if an increase in capacity of the Maranui swale as a result of the revised TCC design rainfall figures is required; 2. Determine the additional effects of the Mangatawa overflow on the Maranui catchment; 3. Mitigate the effects to acceptable levels that do not compromise the minimum floor level design standard as set by TCC for the Papamoa area; 4. Determine the culvert sizes required at Sandhurst Road interchange; 5. Determine the natural available storage within the Bruce Road catchment area; and 6. Model the effects of a balanced spill approach across the highway at both Top Spill and Bruce Road. 1 Mangatawa Drain Flood Capacity, September 2000; Mangatawa Drain Flood Capacity: Supplementary Report, July 2001; and Mangatawa Drain Flood Assessment, 29 November 2006; all produced by Opus International Consultants Ltd Beca // 17 March 2009 // Page 2 3932036 // NZ1-1604459-15 0.15 Rev A
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Mangatawa Drain Flood Bypass and TE
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NZ Transport Agency and Tauranga Ci
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APPENDIX 1 - Drawing 3932036 ck301R
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APPENDIX 3 - Drawing 3932036 ck119
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APPENDIX 5 - Bioresearches Report:
Page 205 and 206: APPENDIX 7 - Archaeological Assessm
Page 207 and 208: ARCHAEOLOGY B.O.P. HERITAGE CONSULT
Page 209 and 210: 2 INTRODUCTION Project Background T
Page 211 and 212: 4 ARCHAEOLOGICAL LANDSCAPE Previous
Page 213 and 214: 6 U14/1708 & 1709 Figure 3. Cadastr
Page 215 and 216: 8 Figure 6. Part ML 8133 showing Pt
Page 217 and 218: 10 Hungahunga & Toroa A further ton
Page 219 and 220: 12 APRIL 2008 ARCHAEOLOGICAL SURVEY
Page 221 and 222: 14 Historic Places Act In addition
Page 223 and 224: APPENDIX 8 - Hydraulic Modelling: M
Page 225: Hydraulic Modelling: Mangatawa Drai
Page 229 and 230: Discharge to Papamoa Beach To disch
Page 231 and 232: • Riparian planting adjacent to t
Page 233 and 234: 2.2 Hydrological Assessment 2.2.1 C
Page 235 and 236: The Maranui catchment area in the m
Page 237 and 238: Table 7 - 2% AEP Storm Event Top Sp
Page 239 and 240: These results include climate chang
Page 241 and 242: 3.3 Maintenance Work in Mangatawa D
Page 243 and 244: stormwater from the Maranui catchme
Page 245: Appendix A Mangatawa Drain Layout P
Page 249: Appendix C Runoff Hydrographs
Page 255: Appendix E Proposed Lower Mangatawa
Page 263: APPENDIX 10 - Drawings 3932036 ck46
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Page 275 and 276: Report Tauranga Eastern Motorway -
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1 Introduction This report describe
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embankment settlement rates, and th
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3.2 Construction Sequence The typic
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concrete contamination as well as s
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Pahse 1 consists of the western sec
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• Installation of Stage 2 decanti
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Appendix A Erosion and Sediment Con
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Activity Status Mangatawa Drain 1.1
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(b) Natural State (River) Water Qua
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6. Reclamation or drainage of the b
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APPENDIX 15 - Legal Descriptions of
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Path: P:\393\3932036\TGI\55_Project
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32 Harington Street PO Box 903, Tau
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32 Harington Street PO Box 903, Tau
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