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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
9. Assessment of Environmental Effects
This assessment considers the effects of the proposal in its entirety dealing with the works related to
stormwater and earthworks.
9.1 Stormwater Management
Stormwater has the potential to adversely affect receiving watercourses. Flow volume can affect flooding in
areas within a given catchment.
The construction of the TEM and Mangatawa flood bypass has two potential effects on stormwater and
receiving environments:
• Water quantity and associated flood risk, including both drain capacity and ponding in low-lying areas;
• Water quality (non point source pollution) caused by the discharge of contaminants associated with
highway traffic, particularly including sediment, hydrocarbons and heavy metals.
9.1.1 Water Quantity
The existing flood risk in this area is described in the technical report attached to this document (Appendix 8).
In the 18 May 2005 storm extensive ponding occurred in the Papamoa area. The Mangatawa Drain alongside
the motorway overflowed at two locations.
The aim for NZTA is to reduce as much as practicable the frequency of stormwater overtopping and
overflowing the motorway, while TCC’s aim is to reduce the volume of water that would enter the Papamoa
catchment areas (Maranui and the Wairakei). In practice, full containment is not practicable.
The flood risk assessment undertaken for the stormwater management system proposed for the TEM has used
update data for climate change (2055) and as such the effect of flood risk has been addressed
comprehensively.
TEM Roadway Stormwater
The TEM will increase the volume of runoff in this catchment and the road embankment will remove some
existing flood storage, although the latter effect is less than minor. The combination of these has the
potential to have an adverse effect in relation to flood risk.
The stormwater in this area spills across State Highway 2 from Mangatawa to Maranui. From the road, the
stormwater traverses into swales which treat and attenuate the water in day to day events before discharging
to drains. Once the water has spilled into the Mangatawa catchment it will make its way overland (as it does
currently) to the Maranui Swale. At this point 2 x 2100 culverts will be placed underneath the TEM to convey
Status: Final Page 82 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE