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

3.3 Maintenance Work in Mangatawa Drain
The current drain is over grown with shrubs and long grass and weeds. We propose to remove the
current plant growth and replant the drain with a grass and appropriate low-growing frangible native
plants that will improve the hydraulic conveyance. The invert of the drain is undulating in places
and we also propose to grade the invert of the drain to a more uniform grade.
We note that currently the drain contains parrots feather and reed sweet grass which are invasive
weeds that block waterways. An active programme will be needed in conjunction with TCC and
EBOP to remove this weed.
3.4 Bruce Road Barrier / Bund
Currently there is no water barrier along this section to prevent over flows. We propose to install a
concrete barrier structure that will follow the road alignment between the drain and the TEM,
constructed to adequate engineering standards to contain the flow within the drain. The structure
will be set to contain and allow spill across the TEM at a controlled section. The height of the
barrier varies and follows the grade of the road. On average the barrier is 0.45m high above finish
road centreline level.
3.5 Lower Rail Bridge Embankment Stability
The current drain alignment at the lower rail bridge has to flow through two 90 degree bends when
passing under the rail bridge upstream of Mangatawa Lane. Erosion of the embankments is evident
especially on the upstream side of the rail bridge. The fluctuation of the water levels in the drain
and fast flowing water can potentially worsen erosion and the fact that there is no well established
natural ground cover along the sides exacerbates the risk. The modelling work we have undertaken
has indicated this section of the drain to be a pinch point resulting in more than expected hydraulic
losses through this section of the drain and thus causing water to back-up, increasing the water
level and spills upstream near Bruce Road.
The work we are proposing to reduce the hydraulic losses through this section is to limit changes in
the drain cross-section profile and improve the alignment by having smoother transitions before and
after the bridge. The work is to tie into the rail bridge abutments as we do not want to alter the
existing structure. We have selected the following drain profile:
• Bottom width, 8 metre wide.
• Side batters, 2V in 1H.
• Top width, 12 metre wide.
We have selected as part of the cost benefit exercise, “Gabion” walls and “Reno” mattresses to
create a stable cross-section profile that would be able to convey flood flow and improve
embankment stability. We have identified the start of the realignment of the drain 120 m upstream
of the rail bridge with a 90 degree long radius curve directly up and downstream of the bridge. The
long radius bends have radii of 25 metres and the final transition back to the natural drain is 70 m
below the bridge.
3.6 Flood Bypass from Maranui to Mangatawa
As discussed in Section 1.4 of this report we propose to install two 2.1 diameter pipe culverts under
the highway and railway track that would link the Maranui to the Mangatawa drain. The Maranui will
potentially spill 6 times a year over the weir. The location of the discharge point in the Mangatawa
Drain will be just upstream of the Mangatawa Lane Bridge. 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
Beca // 17 March 2009 // Page 16
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