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

1 Project Description
1.1 Introduction
The Mangatawa Drain was constructed many years ago, reportedly when the railway was built
along the route parallel to what is now SH2. The purpose of the drain was to divert flow that
previous flowed north into what was at the time farm land but is now Papamoa urban area.
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 highway (to the north). There is some uncertainty as to who owns
the drain and is responsible for its maintenance, although in the past Tauranga City Council (TCC)
has undertaken maintenance. It is generally located on either road reserve or railway land.
In larger flood events excess flow spills north across the highway at two locations, and into the
Papamoa urban area. The two locations are identified in this report as “Top Spill” and Bruce Road
(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 was a closure in May 2005, which lasted for 14 to 18
hours. The latest event where there was a spill across the highway was on 7 March 2009, but this
occurred during daylight and the road was not closed. 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. The TCC
design rainfall figures for 2055 are the basis of the TEM design. These 2055 figures were agreed
with EBOP and consented. With the climate adjusted rainfall figures the frequency of spills is
predicted to increase to about 3 yearly without mitigation measures, and to about 10 yearly with
mitigation in place.
In addition to closing the highway, the spills discharge into developing urban areas on the northern
side of the highway, 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 has no natural outlet, discharging to ground
soakage, so is sensitive to additional flood flow volumes. The Bruce Road spill discharges to TCC’s
Maranui Swale, recently completed and designed to collect and convey urban stormwater along the
northern side of the highway then 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 accounting for climate change, or for the effects of the Mangatawa spill, and
therefore will need significant upgrading to meet design flood level standards in the urban area.
1.2 Background
In the 18 May 2005 storm extensive ponding occurred in the Papamoa area. The Mangatawa Drain
alongside the highway overflowed at two locations. Opus was initially appointed by TCC to
Beca // 17 March 2009 // Page 1
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