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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
The water levels, peak flow rates and velocity for the preferred option for the 100 year flood are given in the
table below. These have been determined with a hydraulic model of the drainage system.
The following table summarises the 1% AEP results along the Maranui floodway at the stormwater treatment
wetland and lower Mangatawa drain. The current day to day flows along the Mangatawa Drain will remain the
same. The peak rate however can be higher than the present due to the drain capacity improvements.
Table 10: Hydraulic Results 1% AEP
Mangatawa Drain
Maranui floodway
Corridor
Peak flow
Peak Water
Peak
Corridor
Peak flow
Peak water
Peak
width
rate
level (m)
velocity
width
rate
level (m)
velocity
(m)
(m 3 /s)
(m/s)
(m)
(m 3 /s)
(m/s)
Unmitigated
Current situation
Mitigated
Proposed
A - At drain chainage 3,287m
B – At extended culvert outlet
5 17.39 1.82 0.99 24 9.10 1.88 1.45
25 A 46.92 3.03 1.5 12 B 6.51 1.64 1.35
7.1.12 Stormwater Treatment Wetland
The current Stormwater Treatment Wetland is owned by TCC and on land leased from MPBI. It is for the
treatment of stormwater generated from residential areas within the Maranui catchment. This project will
maximise the use of this area to treat and discharge stormwater generated by the TEM. In addition to this the
wetland will be able to accommodate discharges from the MPBI industrial zoned land at Truman lane, and MPBI
industrially zoned land between Truman Lane and the Mangatawa Drain.
The final shape and design of the stormwater treatment wetland will be dictated by the developments that are
proposed to use it. In this instance if the MPBI land is developed to its industrial zoned capacity, the pond
needs to be enlarged beyond its lease area. An area that could be used is between SES13, the existing
wetland, and the proposed MPBI development. If MPBI land remains undeveloped the pond will remain within
the current TCC leased area.
The design will be able to treat and discharge stormwater to accepted regional water quality levels. Detailed
drawings and information of this wetland can be found in Appendix 10.
• The total volume of earthworks required for NZTA TEM works in the proposed stormwater treatment
wetland is approximately 4,500m³ - 5,500m 3 . This is the subject of this application.
• The total volume of earthworks required for TCC works in the proposed stormwater treatment wetland is
16,000m³ - 20,000m 3 . This will be the subject of a future application.
• The total volume of earthworks required for the MPBI portion of the stormwater treatment wetland is
17,000m³ - 24,000m 3 . This will be the subject of a future application.
Status: Final Page 71 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE