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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
11. Conclusion
The complexity of stormwater management in this section of the TEM alignment is a consequence of the way
that stormwater behaves across the three catchments in the project baring area, Wairakei, Maranui and
Mangatawa. The May 2005 floods highlighted the need for flood management in the area and Tauranga City
Council has undertaken significant works to address this issue (e.g. the Maranui Swale).
In this assessment of effects it is clear that effects of the earthworks, stormwater management, are no more
than minor providing that the mitigation measures are implemented as design principles and through
avoiding, remedying and mitigating the potential adverse effects of this proposal.
A key part of the management and mitigation of effects is NZTA’s decision to embed the environmental
bottom lines into the contract documentation called the “Principals Requirements”.
11.1 Mangatawa Flood Bypass
A key objective of protecting urban areas from floods is to reduce the volume of water entering these
catchments and to provide a better flow path for flood waters to discharge to the sea.
The spills threaten the safe and efficient operation of the State highway network and the desire by NZTA is to
reduce or restrict completely the number of spills across the State highway. However when allowances for
climate change are made, completely reducing spills is technically constrained by environmental factors, is
costly, as well as potentially creating significant effects on the environment. Therefore a more moderate
approach was chosen.
The proposed option reflects a sustainable management approach by balancing the need to protect the urban
areas in Papamoa and Sate highway infrastructure; and avoiding significant environmental effects on the
Mangatawa drain and lands surrounding the drain.
11.2 TEM Stormwater Management
The methodology will provide a higher level of treatment than existing currently and will reduce the levels of
sediment, copper and zinc to less than is currently experienced providing benefits to water quality in
Rangataua Bay.
The effect from stormwater relates to the management of road way derived stormwater and its impact on the
surrounding catchment and the effects from the flood by pass activities. The effects can be generally described
as flood risk, treatment and conveyance of stormwater. The stormwater system is designed to achieve 100%
attenuation in a 1% AEP event. This level of attenuation is necessary across the entire alignment because of the
sensitivity of the receiving environments in the Special Ecological Site 13 and all three catchments.
Status: Final Page 107 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE