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

The longest section of culvert works is from the existing 1.8 metre Maranui culvert to the wetland.
The Maranui culvert travels beneath SH2 and the rail track conveying stormwater from the existing
swale discharging 120 metres upstream of the wetland area to an open drain. The existing culvert
will be extended to the wetland where a concrete outlet structure will be constructed. The total
extension is 80 metres long and will result in some earthworks to re-grade the drain but is expected
to be no greater than 2,000m 3 .
3.1.4 Lower Mangatawa Drain
The Lower Mangatawa Drain flows in a south-westerly direction towards Rangataua Bay from
Chainage 6800 to Chainage 6100. Works along the Lower Mangatawa Drain include reshaping of
the existing floodplain and the installation of an earth bund along both sides of the drain.
Along the Lower Mangatawa Drain construction will occur in two stages due to the large area of
earthworks required. Due to the low lying nature of the lower reaches of the drain, and the risk of
regular flooding, it is expected that work will progress from upstream down progressively improving
fill to form the bund. As the bund is formed it will be stabilised as soon as practicable to protect it
from storms and flooding. Due to the ecological significance of the drain (see Section 2.2) no works
shall occur within the waterway.
The Lower Mangatawa Drain construction is expected to require approximately 14,00m 3 volume of
earthworks that will primarily be fill. Material excavated will either be reused for the proposed bund
along the western and eastern side of the drain or appropriately removed off site.
3.1.5 Stormwater Treatment Wetland
The existing wetland covers an area of approximately 3.2ha. In order to adequately treat stormwater
from the TEM and surrounding area the wetland will be resized to expand the current treatment
area. The total volume of earthworks proposed in this area is approximately 45,000m 3 . Stormwater
treatment wetland works will be undertaken in two phases. Phase 1 includes the main body of
works on the north-west section of the stormwater treatment wetland; this portion will receive
stormwater primarily from the TEM Maranui swale as well as the Arataki and Papamoa residential
areas. Phase 2 includes the smaller south-east section which will receive stormwater from the
proposed Mangatawa Papamoa Blocks Incorporated (MPBI). The eastern section will eventually be
linked with the main wetland area following Phase 2 works. NZTA is responsible for the initial stage
of stormwater wetland works. The remaining works will be the responsibility of TCC and MPBI (see
Section 4.2.5 for further detail).
Stormwater treatment wetland works include extensive excavation works so as to establish a
greater ponding area across the stormwater treatment wetland. The excavated material not used to
reshape pond margins will be used to establish a bund along the eastern boundary of the Phase 1
area or appropriately removed off site. This material is likely to generally consist of sands, gravels
and silt.
During construction of the TEM there is potential to take advantage of the proposed stormwater
treatment wetland works and utilise some of the area as a temporary sediment retention pond. By
constructing the proposed forebay of the stormwater treatment wetland prior to drainage works the
potential effects of the project on the SES13 (see section 2.2 for further details) beyond the
stormwater treatment wetland can be further mitigated. Diverting runoff from the works area through
the forebay will remove sediment before water enters the main stormwater treatment wetland.
Further treatment within the stormwater treatment wetland will result in relatively clean water
discharging to the SES13.
Beca // 1 April 2009 // Page 4
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