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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
The height of the barrier above the drain varies and approximately follows the grade of the road. On average
the barrier is 0.7m high above finish road level.
7.1.4 Maintenance Work in Upper Mangatawa Drain
The current drain is overgrown with shrubs and long grass and weeds. It is proposed to remove the current
vegetation and replant the drain with suitable species of grass and low-growing frangible native plants that
will aid hydraulic conveyance.
The bed of the drain is undulating and it’s proposed to level the bed to a more uniform grade. The excavation
required is in the order of 100 m 3 to 200m 3
It is noted that currently the drain contains parrots feather and reed sweet grass which are invasive weeds that
block waterways. This proposed work will result in the removal of this weed in this locality but there will also
be a requirement for ongoing maintenance to control the weed.
7.1.5 Rail Bridge Embankment Stability
The Mangatawa Drain adjacent to and beneath the rail bridge located just east of Mangatawa Lane will be
modified. The existing drain alignment at the lower rail bridge has to flow through two 90 degree bends when
passing under the rail bridge. Erosion of the embankments is evident especially on the upstream side of the
rail bridge.
The fluctuation of the water levels in the drain, fast flowing water and lack of ground cover can potentially
worsen erosion. The modelling work undertaken has indicated this section of the drain to be a pinch point
resulting in high hydraulic losses through this section of the drain. This causes water to backup and increases
the upstream water level and spills.
To reduce bank erosion and to reduce the hydraulic losses through this section, it is proposed to improve the
alignment of the drain cross-section profile and improve the alignment by having smoother transitions before
and after the railway bridge. The following drain profile is suggested for this purpose:
1. Bottom width, 8 metre wide
2. Side batters, 2v in 1h
3. Top width, 12 metre wide
The drain profile below the rail bridge will be kept the same and engineering works will be carried out on the
sides to improve the stability. A combination of “Gabion” walls and “Reno” mattresses are proposed for this
work.
“Gabion” walls and “Reno” mattresses are proposed to create a stable cross-section profile that would be able
to convey flood flow and improve embankment stability. The improving of the drain alignment would occur
approximately 100m upstream and downstream from the rail bridge and no work is proposed on the bridge
structure.
Status: Final Page 68 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE