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

The Wairakei catchment was not included in the model as it has already been separately and
comprehensively modelled for TCC by other consultants. This model assumed spills predicted by
the original Opus modelling. The amount of spill assumed in the Wairakei modelling was lower than
predicted by recent modelling, but higher than with proposed Mangatawa mitigation in place, as
shown in the table below. Therefore the Mangatawa mitigation performance will not compromise
the performance of the Wairakei drainage system.
Table 1 - 1% AEP spills to Wairakei Catchment
Wairakei model
Mangatawa model
Peak spill rate (m 3 /s) 21.03 17.13
Spill volume (m 3 ) 151,369 133,103
1.3 Mangatawa Flood Risk and Stormwater Management
The previous section outlines the options considered for mitigation, and the history leading to
selection of options. Later sections of the report describe in some detail the proposed works. This
section sets out in summary the works proposed for flood risk management of Mangatawa spills
and for management of stormwater runoff from urban and highway areas, while the next section
addresses the matter of appropriate outlet for flood flows in particular.
It is proposed that the urban stormwater runoff from Maranui catchment, the adjacent “Arataki” area,
from MPBI industrial zoned land south of the State highway, and from the TEM, be captured and
treated in an expanded wetland treatment area on the site of the current TCC stormwater treatment
ponds. This would capture flows up to at least the stormwater quality storm, and in some parts up
to the pipe system capacity of between 20% and 10% AEP.
To reduce the risk of spill across the TEM and into Papamoa urban area, there are works proposed,
including some as part of the TEM construction, to increase the capacity of the Mangatawa Drain
and to contain flood flows within the drain to a higher level than currently occurs before spill. There
are also works proposed to manage the spills into the Maranui swale area, and to return these
safely to the harbour (as described in the next section).
Spill from Mangatawa will still occur in events of about 10% AEP and above, but at a much lower
volume and flow rate than at present.
1.4 Outlet Options Considered
At present the principal Mangatawa/Maranui flood outlets are the Mangatawa Drain itself, and the
current TCC culvert under SH2 and the railway leading to the stormwater treatment wetland. As
part of the assessment, four outlet options have been considered and the two most suitable ones
are illustrated in Appendix B:
• Pipe the Maranui overflow north to the coast at Papamoa Beach;
• Pipe the Maranui overflow west past the eastern end of Truman Road and out to the harbour
through TCC land;
• Co-locate with the existing outflow from Maranui catchment, with two additional pipes adjacent
to the current Maranui outlet pipe;
• Leave the existing Maranui outlet as it is, and provide the overflow capacity through two pipes
back to the Mangatawa drain immediately west of Mangatawa Lane.
Beca // 17 March 2009 // Page 3
3932036 // NZ1-1604459-15 0.15 Rev A