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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
Underneath the bridge itself the drain banks will require structural strengthening to maximise the flood flow
capacity. This will require works such as providing a tied back, shot crete (sprayed concrete) wall.
The total volume of earthworks for this activity is 13,000 m 3 – 14,000 m 3
7.1.6 Flood Bypass from Maranui to Mangatawa
It is proposed to install two 2.1m diameter pipe culverts under the highway and railway track that will link the
Maranui swale to the Mangatawa drain. The location of the discharge point in the Mangatawa Drain will be just
upstream of the Mangatawa Lane Bridge, the Maranui swale will potentially spill 6 times a year over a weir into
these pipes and flow to the Mangatawa Drain. This bypass would provide an additional flood outlet for the
Maranui Catchment and a weir will control the flow in such a way as to direct all water up to a third of a 50%
AEP (water quality event) storm event to the existing outlet for treatment in the Maranui treatment pond. Any
storm event bigger than the water quality event will spill over the weir to the Mangatawa Drain and thus
provide additional capacity during flood events for water to drain away. The volume of excavation required at
this location will be approximately 2200m 3 -3100 m 3 .
7.1.7 Sandhurst Culvert
A 1.8 metre diameter culvert under the proposed Sandhurst Interchange will convey Maranui swale flows past
the interchange. The culvert is estimated to be 70m long with concrete inlet and outlet structures.
Constructed in conjunction with interchange construction.
7.1.8 Extension of Maranui Culverts
The current Maranui swale drains to the treatment pond through a 1.8 metre diameter culvert that is 260
metres long and passes under SH2 and the rail track. The culvert discharges 120 metres upstream of the
treatment pond in an open drain. To remove the open drain and release the land for development it is
proposed to change the current open drain to a culvert. The culvert will be extended with the same size
culvert up to the stormwater treatment area with a new concrete outlet structure. The proposed extension is
80 metres long and will require a total amount excavation between 1,500m 3 and 2,000m³. In addition the
existing 1200mm dia culvert will be realigned to discharge directly into the wetland as shown on drawing
3932036 ck431, Works C.
A 750mm dia culvert will be installed to convey stormwater from the TEM alignment to the stormwater
treatment wetland. This will be installed on or about ch_5800 and will connect an existing pipe to the new
pipes leading the wetland area. The volume of excavation required for this area is 1,600m 3 – 2,000m 3 .
7.1.9 Maranui Swale Deepening
Deepening the Maranui swale will create additional storage capacity within the swale. This will provide for the
increased runoff from future urban development and the impact of the climate adjusted rainfall. While the TEM
and the flood management strategy will utilise the swale, additional storage in the swale will only be required
when the Maranui catchment area is developed further. This aspect of the project is not needed at this time
and therefore the earthworks related to this activity will not be applied for in this application. Consent will be
sought at the time of construction.
Status: Final Page 69 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE