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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
excess flood waters from the Maranui swale south into the lower Mangatawa drain at Mangatawa Lane. These
will be set so the ‘first flush’ stormwater travels to the TCC wetland via the Maranui Swale and the remaining
diluted flood waters will travel down the Lower Mangatawa Drain. This element of the proposal is vital to this
proposal as it conveys water in large scale flood events away from the Treatment wetland preventing potential
damage to that facility from extreme flood events.
This element of the proposal reduces the flood risk to the Papamoa catchments (Wairakei and Maranui) from
the present situation.
Mangatawa Flood Bypass
i. Upper Mangatawa Drain (Domain Road – Bruce Road)
The primary aim of the Mangatawa bypass is to control the number of motorway closures and the volume of
water spilling into the Maranui and Wairakei catchments without negatively affecting the surrounding
environment.
The frequency and volume of water spilling across the motorway will decrease with the proposal. It is forecast
that the spill occurrence will decrease from approximately 3 yearly events (at present with climate change) to
8. With the proposed mitigation measures the flooding across the motorway is expected about every 10 years.
The effect of diverting this flood volume on the Mangatawa drain is considered to be minor and will have
positive effect on the Maranui and Wairakei catchments.
ii.
Lower Mangatawa Drain (Bruce Road – Rangataua Bay)
While there will be reduced risk of flooding in the Wairakei and Maranui catchments, there is potentially
increased flood risk on the low lying land near the lower Mangatawa Drain (below Mangatawa Lane), south of
the railway.
The mitigation measures required to address this issue is to remove spoil and widen the flood plain and to
provide an earth bund to contain the additional flow particularly the extra peak volume of water and convey it
to the Rangataua Bay. Once past this area the discharge of the Mangatawa Drain to the harbour becomes
diffuse, within the SES 13.
The creation of capacity for flow in the lower Mangatawa Drain will provide the necessary mitigation to reduce
the effects of increased volume and flow rate of water to no more than minor.
9.1.2 Water Quality
The following information has been taken from the Bioresearches Report: Ecological Features of Some Habitats
within Rangataua Bay Catchment (attached as Appendix 5) and the Tauranga Eastern Motorway Resource
Consent application. The following focuses on the ecological effects of the TEM project and in turn the
enlargement of the stormwater treatment pond at Te Maunga.
Status: Final Page 83 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE