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

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water up to a third of a 50% AEP (water quality event) storm event to the existing outlet for treatment. Any storm event bigger will be able to spill over the weir and thus provide additional capacity during flood events for water to drain away. 3.7 Lower Mangatawa Drain Works In line with MPBI development, we propose to create a 25m wide flood corridor that is 800m long and will start downstream of the Mangatawa Lane Bridge and extend to the Special Ecological Site. We propose to improve the drain and create flood plains on either side of the drain to contain flood flows and reduce the potential for scouring of the drain during flood events. The top of the western bank will tie in with MPBI development and provide flood protection. The drain improvements include the following: • Native grasses along the drain edge. • Native trees along the drain on both sides outside the 100% ARI storm event flood level. • Flood plains and batters covered in grass. • Access for maintenance. • Walking track. 3.8 Sandhurst Culvert A 1.8 metre diameter culvert under the proposed Sandhurst interchange has been allowed for and this is to convey Maranui swale flows past the interchange. The culvert is estimated to be 100m long with concrete inlet and outlet structures. 3.9 Extension of Maranui Culvert The current Maranui swale outlet to the treatment pond is through a 1.8m diameter culvert, that is 300m long and drains from the Maranui swale under SH2 and the rail track. The culvert discharges 80m upstream of the treatment pond in an open drain. To remove the open drain and release the land for development, we have identified the potential for changing the current open drain to a culvert. The culvert will be extended at the same size through to the stormwater wetland area, and will have a new concrete outlet structure. The proposed extension is 80m long. 3.10 Maranui Deepening Deepening the Maranui swale will create additional storage capacity within the swale. This will provide for the increased runoff from the future development and the impact of the climate adjusted rainfall. This storage will be required in future when the Maranui catchment is fully developed and for the potential impact of the increase of bigger rain events. The current swale has generally a 1V:5H side batter and we have allowed for side batters of 1V:4H plus deepening of the swale invert by 0.75m. 3.11 Stormwater Treatment Wetland Area The current Maranui stormwater pond has been built and managed by TCC on land that is leased from MPBI, and is for the treatment of the residential areas within the Maranui catchment. There are two culverts that drain to the open drain upstream of the treatment pond. The one culvert is a 1.2 metre diameter culvert from the Arataki residential area and the other is the 1.8 diameter culvert from the Maranui swale. TCC holds a consent (Consent No. 60061) to discharge treated Beca // 17 March 2009 // Page 17 3932036 // NZ1-1604459-15 0.15 Rev A
stormwater from the Maranui catchment to the harbour at Rangataua Bay. TCC has expended considerable effort to improve the natural character of the wetland area and mature trees can be found here. TCC, NZTA and MPBI have agreed to work together and expand the current treatment area to serve the stormwater needs of all parties. A stormwater wetland was identified as the appropriate mitigation device to provide treatment for stormwater. The following principles have been agreed: • Utilise the current lease area. • Retain as much of the higher value established trees and plants as practicable. • Not to encroach on the Special Ecological Site 13 located on the south of the treatment area. • Able to receive and treat stormwater from TCC’s residential areas, MPBI proposed industrial land and the TEM. • Not to encroach on the existing and proposed pipeline easements that TCC has to the west, along the property boundary. • Apply ARC TP10 design standards for the upgrading of the wetland. The following table is the stormwater runoff for the water quality storm events. Party TCC (Maranui and Arataki) Table 12 – Stormwater Treatment Requirement Pervious area (km 2 ) Impervious area (km 2 ) Catchment area (km 2 ) Volume required (m 3 ) 0.79 1.75* 2.54 12,941 MPBI (Industrial) 0.01 0.45 0.46 11,635 TEM (Motorway) 0.04 0.15 0.19 3,247 Total 0.84 2.35 3.19 27,823 * Note this includes roof area, which is disposed to ground and does not contribute to total water quality volume For the wetland upgrading the 2055 design rainfall required an area that could not be accommodated within the area currently available. For the wetland upgrading the 2005 design rainfall figures was used instead and a third of a 50% AEP 24 hour design storm event to determine the runoff. All residential roof runoff for the TCC areas was assumed to go to ground (soakage). This is in line with TCC design approach for the residential areas in the Maranui and Arataki areas. There is a potential 4.47ha area available east of the current wetland area, enclosed between MPBI proposed development, the existing wetland, and Special Ecological Site 13. We have included in our work the additional potential treatment area and attached in Appendix E are the layout and cross-section drawings for the extended treatment area. 4 Conclusions There is currently an unacceptable risk of flooding across SH2 into the Papamoa urban area. Both TCC and NZTA are seeking to reduce that risk, so that: • The frequency and consequences of highway closures are reduced to acceptable levels; Beca // 17 March 2009 // Page 18 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 and 226: Hydraulic Modelling: Mangatawa Drai
Page 227 and 228: undertake investigations into the d
Page 229 and 230: Discharge to Papamoa Beach To disch
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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: 3.3 Maintenance Work in Mangatawa D
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
Page 279 and 280: embankment settlement rates, and th
Page 281 and 282: 3.2 Construction Sequence The typic
Page 283 and 284: concrete contamination as well as s
Page 285 and 286: Pahse 1 consists of the western sec
Page 287 and 288: • Installation of Stage 2 decanti
Page 289: 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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