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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
The swale bed would need to be deepened by approximately 0.75m.
7.1.10 Lower Mangatawa Drain (from Mangatawa Lane to Tauranga Harbour)
The existing lower Mangatawa drain flows in a south western direction to the Rangataua Bay. The drain is
approximately 800m long, measured downstream from Mangatawa Lane to the Bay. The first 200m of the
drain has a steep embankment on the eastern side that slopes down to a defined channel which flows on
average 2.5m wide. On the western side of the channel is a floodplain of about 3m wide after which the
embankment slopes up to the existing higher ground that is currently being used for farming purposes.
The lower Mangatawa drain (from chainage 6800 to the harbour) will be modified to create more flood storage
on the banks of the drain. The existing spoil by the drain will be removed.
In addition, a stopbank will be created on both sides of the drain to protect the MPBI land from flooding from
the drain. It will involve the removal of non native trees and trimming of the spoil and replanting with native
vegetation.
These stopbanks will be re-vegetated to improve amenity in this area. Maintenance of the drain will be
undertaken by TCC in future and provision has been made for access of maintenance vehicles.
The flood corridor and stop bank will continue as far as the ecological sensitive area by the harbour. After this
point the flow will spread out and flow at a lower level and velocity to the harbour.
The drain improvements include the following:
• Planting native grasses along the drain edge.
• Planting native trees along the drain on both sides outside the 1% AEP storm event flood level.
• Providing flood plains and batters covered in grass.
• Provide access for maintenance.
• Provide a walking track.
Drawings of the lower Mangatawa drain are attached as Appendix 11 drawing number 3932036-ck431 Rev B.
7.1.11 Mangatawa Flood Bypass Flood Levels
The floodwater travelling from Maranui swale to Mangatawa drain via a flood overflow outlet near Mangatawa
lane. will only be used in larger storm events, exceeding the motorway “first flush” stormwater quality storm.
Current modelling forecasts predict that the culverts may be used up to 6 times a year. Up to about the 10 year
storm the flow would only include runoff from the Maranui catchment and the TEM, with no Mangatawa spill
flow included. Above the 10 year flood, there would be Mangatawa spills also included in the discharge.
Status: Final Page 70 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE