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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
3. Similar spill at both Kairua and Bruce Roads
Option 3 provided the best cost and hydraulic performance requiring assessment and inclusion of the
requirements of the Maranui catchment.
Taking into account the effects of the Maranui catchment with the preferred options alone, the following
information is required:
1. Increase the Maranui swale storage capacity
2. Provide additional outlet capacity from the swale to the harbour
3. Utilise natural available storage in the Bruce Road rural area
6.1.1 Outlet and Discharge Points
At present the Mangatawa/Maranui catchment flood discharge points to the Rangataua Bay are at the outlet of
the Mangatawa Drain and via the Maranui wetland term. As part of the assessment, four outlet options were
considered and this is shown in drawings in the attached technical report in Appendix 8.
1. Pipe the overflow north to the coast at Papamoa Beach through undeveloped land. An approximately
1.8km long large box culvert is required with potentially a new outlet to the sea. The construction and
land costs along with the effects on the coast area would be unacceptable and will make this option
prohibitive;
2. Pipe the flow west following the railway down the centre of Truman Lane and beyond and out to the
harbour. This option would require a 2.6 km long large box culvert to the harbour. The same reasons
as for the above point 1 will make this option prohibitive;
3. Maranui flood bypass: Take the overflow down Maranui swale to the culvert crossing the State Highway
and add two additional pipes adjacent to an existing current Maranui culvert;
4. Mangatawa flood bypass: Leave the existing Maranui outlet as it is, and provide the overflow capacity
through two pipes back to the Mangatawa drain immediately west of Mangatawa Lane.
Those four options were narrowed to two principal options based on feasibility and cost, namely Options 3 and
4 above. More detailed rationale for these decisions can be found in the Options report in Appendix 8.
In comparing these outlet options, the Mangatawa flood bypass had more merit overall.
The positives of Option 4 are:
• The required flood corridor becomes a potential area for recreation at the lower Mangatawa drain location.
• Leaves more space for the treatment wetland.
Status: Final Page 65 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE