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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
b. Wairakei Catchment
The Wairakei Catchment covers most of Papamoa and is a sensitive catchment to stormwater runoff. It is
covered by a comprehensive consent that this application has taken account of.
The Wairakei comprehensive stormwater management consent has included in its calculations an expected
spill over from the Mangatawa catchment as part of the conditions of consent for the 48 hour 50 ARI storm
event.
c. Mangatawa Catchment
The Papamoa hills area is the major catchment contributing to the Mangatawa Drain. The southern part of the
catchment flows directly into the drain and thence without treatment into the harbour, at Rangataua Bay. The
current SH2 runoff in this catchment is partly to the Mangatawa drain, and partly to the Maranui swale. For the
proposed TEM this will change, with both the southern and northern carriageway draining into the Maranui
swale and being treated in the Maranui Pond.
5.12.2 Catchment Hydrology
The Mangatawa Drain is about 2,500m long where it is alongside the proposed Tauranga Eastern Motorway
(TEM). The drain has a vegetated, earth lined channel that is tightly constrained between the railway (to the
south) and the motorway (to the north). There is some uncertainty as to who is responsible for its
maintenance, although in the past Tauranga City Council has undertaken maintenance. It is generally located
on either road reserve or railway land.
In larger flood events excess flow spills north across the motorway at two locations, and into the Papamoa
urban area. The two locations are identified in this report as “Top Spill” and “Bruce Road Spill” (although the
spill covers several hundred metres of highway in the general vicinity of Bruce Road).
While the discharge can be volumetrically large, it is generally spread as a shallow overflow (a few hundred
millimetres deep) or ponding across several hundred metres length of highway, and has resulted in highway
closure in the past. The most recent closure was in May 2005, and lasted for 14 to 18 hours. The frequency of
spills appears to be about 5 yearly on average at present. TCC has updated their design rainfall figures and
these have been applied to this investigation and modelling. These design rainfall figures are based on
predicted climate changes to 2055. With the climate adjusted rainfall figures the frequency of spills is
predicted to increase to about 3 yearly.
In addition to closing the motorway, the spills discharge into developing urban areas on the northern side of
the motorway, with potential consequences for TCC for flood risk management.
The Top Spill discharge flows to the Wairakei Stream, where there is significant existing urban development,
plus plans for growth. This stream predominantly relies on ground soakage, with additional discharge to
Harrisons Cut and the Grant Place discharge, so is sensitive to additional flood flow volumes. The Bruce Road
Spill discharges to TCC’s Maranui Swale (across private land), and conveys urban stormwater along the
northern side of the highway. It then travels under the highway and the railway to the wetland treatment
ponds, before discharge to the Tauranga Harbour at Rangataua Bay. The swale was originally designed without
Status: Final Page 49 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE