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

currently leased by TCC for the Maranui pond, and therefore the land available for treatment would
be reduced if the bypass width increased. From our assessment of TCC, NZTA and MPBI
development runoff, including the Truman Lane development of MPBI, more land would be required
to address stormwater quality treatment and mitigation and the area located to the south east of the
current pond outlet is ideally suited as it is not constrained by any proposed land use or known
special ecological area.
Redirecting the flood overflow back to the Mangatawa Drain at Mangatawa Lane also reduces the
flood flow that needs to be taken past the Sandhurst interchange and into the Asher block, thereby
reducing the effect on the western end of the Maranui swale.
The outlet flow path from the Maranui stormwater treatment pond flows for 400m through the
Special Ecological Site 13 (as shown in the District Plan) before reaching the harbour, with the risk
of occasional damage to this area during floods if a Maranui flood outlet is used. The Mangatawa
flood bypass discharge is directly along the Mangatawa Drain to the harbour, with only a minor
impingement on one end of SES 13, reducing this risk.
Therefore, the better overall outcome for the MPBI development and the environment is likely to be
to minimise the width of the Maranui flood bypass and increase the Mangatawa flood plain width to
compensate, as the flood plain can be integrated with the possible future MPBI development.
The proposed design involves no change to the size of the Mangatawa main channel. The
increased flood flows would be conveyed principally in the flood plain, with the normal (dry-weather)
flow contained in the main channel, and therefore the shear stress on the channel bed will not
increase by more than a minor amount. This serves to protect the channel from erosion, and helps
to preserve its character. This is the same process as occurs in natural river systems, where the
main channel usually only conveys small floods, and larger floods spill onto the flood plain.
In comparing the two principal options, the Mangatawa flood bypass option appears to have more
merit overall. The positives are:
• The required flood corridor is more usable in this location.
• Provides greater protection and less through flow in the western end of the Maranui swale.
• Leaves more space for the Maranui treatment wetland.
• Has less effect on Special Ecological Site 13.
• Lower cost overall.
• Can be integrated with the MPBI development and aesthetics and use of the area.
• Appears to fit within the riparian area shown on the MPBI development concept, so there would
be no change to the usable area of the development.
The aspects where the Mangatawa flood overflow has less merit are:
• A slightly larger area is needed for floodway corridor.
As a consequence, this report has focussed on the flood bypass and outlet option that returns
excess flow to the Mangatawa Drain at Mangatawa Lane, and does not change the current Maranui
outlet under SH2 at the Asher Block. However, for comparison purposes, the report presents
results for that alternate option as well.
1.5 Floodway Concepts
We have considered the form that the Mangatawa Drain should take, considering the multiple
purposes which the corridor would need to serve. The identified objectives and actions are:
• Stable drain banks, reduced risk of erosion;
Beca // 17 March 2009 // Page 5
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