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

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
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