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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
7.2 TEM Roadway Stormwater
The package of activities this part of the report refers to are between chainage 5250 – to 10850 in the
Mangatawa catchment. This part of the report describes the devices and design related to controlling and
treating the stormwater generated by the road up to the point it leaves the swales to discharge to the
surrounding land/drains or water.
It should be noted that NZTA has received consent (albeit appealed to the Environment Court) for stormwater
resource consents for the remaining three catchments that are affected by the TEM, i.e. Te Maunga, Bell Road
and east of the Kaituna River. The stormwater system that is subject to this application is shown in
Appendix 3.
7.2.1 TEM Stormwater Management Philosophy
The main method for stormwater management is to use swales for stormwater treatment, attenuation and
conveyance, with discharge into the existing drainage network. In this area ALL the stormwater generated by
the road will be treated and attenuated in swales on the northern side of the alignment. Water will be collected
in catch pits from the southern side of the road and piped to the swales which represent a change from the
present situation where the water from the existing Sate Highway drains into the Mangatawa Drain.
The following summary sets out the principal elements of the approach proposed for the TEM component of
this proposal.
1. Treatment and conveyance in Maranui catchment using the TCC drainage system;
2. Swales in the Wairakei catchment providing treatment in accordance with TP10, plus flood storage in
accordance with the TCC Wairakei stormwater discharge consent (currently subject to appeal);
3. Fit in with existing land drainage system as far as practicable;
4. Discharge frequently to existing streams or drains, to avoid the concentration of large flows at fewer
discharge points;
5. Design using Tauranga City Council (TCC) updated rainfall values that account for climate change
(2055);
6. Design consideration up to 1% Annual Exceedance Probability (AEP) storm, but with pipe systems
designed for the 10% AEP storm with the balance as secondary flow.
7.2.2 Swales
The drainage system will generally consist of kerb and channel on the southern side of the proposed TEM
collecting runoff and piping it north at regular intervals under the TEM to swales along the northern side.
Within the Wairakei catchment (from chainage 8,200 to 10,850), these swales will provide treatment and
attenuation before discharge with a pond within the designation North West of the Domain Road interchange.
Within the Maranui catchment, the swales are principally for conveyance, as the treatment will occur in the
Status: Final Page 73 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE