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

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currently leased by TCC for the Maranui pond, and therefore the land available for treatment would be reduced if the bypass width increased. From our assessment of TCC, NZTA and MPBI development runoff, including the Truman Lane development of MPBI, more land would be required to address stormwater quality treatment and mitigation and the area located to the south east of the current pond outlet is ideally suited as it is not constrained by any proposed land use or known special ecological area. Redirecting the flood overflow back to the Mangatawa Drain at Mangatawa Lane also reduces the flood flow that needs to be taken past the Sandhurst interchange and into the Asher block, thereby reducing the effect on the western end of the Maranui swale. The outlet flow path from the Maranui stormwater treatment pond flows for 400m through the Special Ecological Site 13 (as shown in the District Plan) before reaching the harbour, with the risk of occasional damage to this area during floods if a Maranui flood outlet is used. The Mangatawa flood bypass discharge is directly along the Mangatawa Drain to the harbour, with only a minor impingement on one end of SES 13, reducing this risk. Therefore, the better overall outcome for the MPBI development and the environment is likely to be to minimise the width of the Maranui flood bypass and increase the Mangatawa flood plain width to compensate, as the flood plain can be integrated with the possible future MPBI development. The proposed design involves no change to the size of the Mangatawa main channel. The increased flood flows would be conveyed principally in the flood plain, with the normal (dry-weather) flow contained in the main channel, and therefore the shear stress on the channel bed will not increase by more than a minor amount. This serves to protect the channel from erosion, and helps to preserve its character. This is the same process as occurs in natural river systems, where the main channel usually only conveys small floods, and larger floods spill onto the flood plain. In comparing the two principal options, the Mangatawa flood bypass option appears to have more merit overall. The positives are: • The required flood corridor is more usable in this location. • Provides greater protection and less through flow in the western end of the Maranui swale. • Leaves more space for the Maranui treatment wetland. • Has less effect on Special Ecological Site 13. • Lower cost overall. • Can be integrated with the MPBI development and aesthetics and use of the area. • Appears to fit within the riparian area shown on the MPBI development concept, so there would be no change to the usable area of the development. The aspects where the Mangatawa flood overflow has less merit are: • A slightly larger area is needed for floodway corridor. As a consequence, this report has focussed on the flood bypass and outlet option that returns excess flow to the Mangatawa Drain at Mangatawa Lane, and does not change the current Maranui outlet under SH2 at the Asher Block. However, for comparison purposes, the report presents results for that alternate option as well. 1.5 Floodway Concepts We have considered the form that the Mangatawa Drain should take, considering the multiple purposes which the corridor would need to serve. The identified objectives and actions are: • Stable drain banks, reduced risk of erosion; Beca // 17 March 2009 // Page 5 3932036 // NZ1-1604459-15 0.15 Rev A
• Riparian planting adjacent to the channel for cover and bank protection; • Access for whitebaiting and other stream-related activities; • Provide floodway capacity, while retaining multi-use capability for most of the time when there is no flooding (e.g. grassed area for picnicking, specimen trees, etc); • Reliable access along one bank for long term maintenance. The alternative flood bypass outlet from the Maranui swale to Mangatawa Drain near Mangatawa Lane, upstream of the proposed Sandhurst Drive, is proposed in this report. The following engineering solutions were included: 1. Install 2 x 2.1m diameter pipe culverts below the TEM and the railway track. The inlet to be from the Maranui Swale, with a weir to control the inlet to allow the “first flush” to continue along its current path, and the outlet immediately upstream of the Mangatawa Drain vehicle bridge on Mangatawa Lane; 2. Provide one 1.8m diameter pipe culvert under Sandhurst Interchange for day-to-day drainage and for the “first flush” water to be conveyed for treatment at the stormwater treatment wetland; 3. Utilise the existing 1.8m diameter pipe culvert that discharges from the Maranui Swale to the stormwater treatment wetland to pass this flow south for treatment; 4. Enlarge the wetlands to treat the additional urban and highway area that will reach it (including the development of the MPBI land);and 5. Formalise and protect the flood plain alongside the Mangatawa Drain from Mangatawa Lane to the harbour. 2 Model Studies Opus was initially appointed by TCC to under take investigations into the drain performance, with a specific focus on predicting the frequency and rate of overflow. They produced three reports 2 and the last was after the May 2005 floods. They carried out their analysis using a model developed in MIKE11 software. Beca adapted the Opus model and during the ongoing modelling process three stages were undertaken where the model was adapted to asses specific options and outcomes. Two of these stages included the inclusion of the Maranui Swale and MPBI area, and the assessment of a range of mitigation options. 2.1 Modelling Stage 2.1.1 Preliminary The Opus model was used as a basis for current work, but has been extensively updated to reflect the needs of ongoing analysis. The following lists the amendments to the model: • Included two additional catchment areas (P2 and P3) • Included natural occurring storage areas • Added some secondary drains 2 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 6 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:
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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 and 226: Hydraulic Modelling: Mangatawa Drai
Page 227 and 228: undertake investigations into the d
Page 229: Discharge to Papamoa Beach To disch
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
Page 272: APPENDIX 12 - Drawing 3932036 ck227
Page 275 and 276: Report Tauranga Eastern Motorway -
Page 277 and 278: 1 Introduction This report describe
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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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