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

embankment settlement rates, and the like. The consent process will set out the environmental
bottom lines for this project that the Contractor will be required to adhere to. Therefore the
methodology outlined below is a simplified summary of what may occur.
As part of this application for the Mangatawa Drain and associated works four situations have been
considered to show that a suitable ESCP can be developed during the detailed design phase to
manage the effects of the works. Due to the varying terrain and contours, the ESCP for the
construction of each of these elements should be considered separately, as the works will either
occur at different times or each would have its own discrete mitigation measures. Erosion and
sediment control measures are discussed in Section 4.2.1.
3.1.1 Mangatawa Drain
The Mangatawa Drain flows in a north-westerly direction alongside the south-western boundary of
the TEM from Chainage 1050 to Chainage 6800.
On the eastern side of Kairua Road, enclosed between the rail track and the motorway, a storage
area will be created by excavating approximately 28,000m 3 from the drain. It is likely that some of
this excavated material will be used for the 4,000m 3 of fill required to establish a 3 metre wide
1 metre high bund at the top of the drain alignment along the northern side. Excess material will
either be re-used elsewhere on site or appropriately removed off site. Further to this approximately
100 – 200m 3 of excavation works will be undertaken to level the drain bed to a more uniform grade.
3.1.2 Rail Bridge Embankment Stability
Erosion of the Upper Mangatawa Drain banks adjacent to and beneath the rail bridge has been
identified as a significant existing issue. Currently the drain alignment flows through two 90 degree
bends when passing beneath the rail bridge. To mitigate the effects and potential for erosion along
this section works will include improving the alignment of the drain as well as installing erosion
control measures along both embankments and the drain invert.
Earthworks associated with this work will mainly consist of excavation works required to realign the
drain, however some fill works will be undertaken on the existing bends. The total volume of
earthworks for this section of the drain is expected to be approximately 14,000m 3 . Gabion baskets,
or similar, will be placed along the drain to protect the banks from erosion with reno mattresses
along the bed of the drain to protect the gabion walls from under scour. Tied back concrete
retaining walls will be constructed under the railway bridge to protect the bridge abutments.
3.1.3 Culvert Extension
a. Flood Bypass from Maranui to Mangatawa
Approximately 3,000m 3 of excavation is required for the installation of two 2.1 metre diameter
culverts under the TEM and railway track, linking the Maranui and Mangatawa Drain. The culverted
bypass will discharge to the Mangatawa drain via a weir structure just upstream of the Mangatawa
Lane Bridge.
b. Sandhurst Culverts
Approximately 2,000m 3 of excavation is required for the installation of a 70 metre long, 1.8 metre
diameter culvert beneath the Sandhurst interchange. The culvert will be installed so as to allow
flows from the Maranui swale beneath the interchange.
c. Extension of Maranui Culverts
Beca // 1 April 2009 // Page 3
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