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

. Sandhurst
The installation of the Sandhurst culvert includes 2,000m 3 of excavation. The Maranui swale
entering the culvert will be blocked off so as to avoid clean water entering the dirty works site.
During rainfall events water will back up in the swale where it will be stored. Clean water diversions
will be in place so no clean water will enter the site. Upon completion of the culvert installation the
Maranui swale will be reconnected and clean water can continue to flow towards the wetland.
c. Extension of Maranui Culverts
The extension to the Maranui culvert includes 2,000m 3 of excavation. During works the drain will
likely continue to convey clean water from the existing highway. To avoid clean water entering the
open site the existing culvert will be dammed; as such clean water will back up the existing swale
where it can adequately be stored until works are complete. Upon the completion of the culvert
extension the dam shall be removed and water will return to flowing through the culvert to the
stormwater treatment wetland.
4.2.4 Lower Mangatawa Drain
Plan 3932036-CK467A
Plan 3932036-CK467A illustrates perimeter controls and sediment treatment devices that may be
used. It shows the different sections of the drain that are to be constructed and gives the
approximate area that will be worked.
Given the area of earthworks required along the drain; works will be undertaken in stages. Stage 1
will include an area of approximately 3ha on the west side of the drain. Clean water diversions will
be placed along the northern boundary to divert clean flows from the working footprint and reduce
the catchment requiring treatment. A stabilised earth bund will be installed along the length of the
northern embankment of the existing drain so as to divert sediment laden flows away from the drain.
A sediment retention pond will be installed in the lower south west corner of the works area to treat
sediment laden flows from the open works area. Treated water will be discharged to the drain. A silt
fence will extend around the boundary of the ecological wetland area so that any water entering the
ecological area from the site will be treated by the fence.
Stage 2 also covers an area of approximately 3ha along the eastern side of the drain. Stage 2
works include batter reshaping. Clean water diversions will be installed to divert clean flows from
the working footprint and reducing the catchment that requires treatment. As per the west side a
stabilised earth bund will be installed along the length of the northern embankment of the existing
drain so as to divert sediment laden flows away from the drain. A sediment retention pond will be
installed in the lower south west corner of the works area so as to treat sediment laden flows from
the open works area. Treated water will be discharged to the drain.
Stabilisation of the batters using hydroseed or other approved methods will be undertaken as works
are completed.
4.2.5 Stormwater Treatment Wetland
Plan 3932036-CK461
Plan 3932036-CK461 gives perimeter areas for stages of works and sediment treatment devices
that may be used. There are three stages of works for Phase 1 of the stormwater treatment
wetland; where as Phase 2 can be undertaken in a single stage of works. NZTA will be responsible
for the works associated with Stage 1 of Phase 1 as outlined below. Tauranga City Council will be
responsible for the Stage 2 and 3 works of Phase 1, with MPBI responsible for Phase 2.
Beca // 1 April 2009 // Page 8
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