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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
Receiving Environment
Total Contaminant Loads (kg/yr)
Sediment Copper Zinc
Total
Current 48,778 17.4 23.0
Proposed 23,916 33.8 21.9
Change 24,862
⇓
16.4
⇑
1.1
⇓
It can be seen from the above Table that overall there is a reduction in contaminant discharge from the TEM
and Maranui catchment due to the increased treatment of storm water over the existing system. The current
TCC Maranui urban area is not fully treated to the TP10 standard; however, after the works it will be fully
treated to that standard. MPBI has not yet been developed, so there will be an increase in metals as a result of
industrial development, despite treatment. At present the TEM is partially treated and the proposed treatment
more than offsets the increased area and traffic.
With the appropriate measures in place, adverse environmental effects of the stormwater system on hydrology,
flood risk and water quality will be mitigated and overall will be no more than minor.
Mangatawa Flood Bypass
Water quality was measured by Bioresearches at several locations and there is only one site in the Mangatawa
Flood Bypass area. Water quality information regarding this site is presented in the above sections.
The Mangatawa Drain currently discharges directly into the Tauranga Harbour and spills across State highway 2
at Kairua and Bruce Roads during significant flood events. This proposal will redirect much of this rural runoff
overspill down the Mangatawa drain and is not expected to have any significant effects in relation to water
quality. In addition to this when water overtops the stopbanks the water will travel across the State highway to
the Maranui swale and back to the Mangatawa drain via 2 x 2100 culverts. This will only occur in significant
flood events and will not significantly affect the quality of the water entering the drain.
The effects on water quality arising from the Mangatawa flood bypass structures will have a less than minor
effect on the receiving environments.
9.1.3 Freshwater Ecology
TEM Roadway Stormwater
The main concern with regards to the effects on the ecology of the freshwater habitats is the potential to cause
temporary negative effects on the downstream receiving environments, through sediment inputs from
construction and maintenance operations. Excess sediment loads can affect macro invertebrate communities
and have the potential for direct and secondary effects on fish communities. Such effects can be minimised
Status: Final Page 86 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE