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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
(c) A maximum flood level in the Maranui catchment of 4.6mRL (Moturiki Datum) in the 1% AEP 4 storm;
(d) Provide for stormwater quality treatment for the runoff from the fully developed Maranui catchment, the
proposed TEM and the development of the MPBI industrial zoned land, to protect the quality of the
Harbour at Rangataua Bay;
(e) Any works on the Mangatawa Drain or any flood ways should be appropriately landscaped to achieve a
multipurpose and aesthetically pleasing outcome, compatible with proposed adjacent development;
(f) Any works on the Mangatawa Drain should be environmentally appropriate, and include consideration of
current and traditional uses, including for whitebaiting;
(g) A cost effective and affordable solution.
We have considered the form that the Mangatawa Drain should take, considering the multiple purposes for
which the corridor would need to serve. The identified objectives and actions are:
• Stable drain banks, reduced risk of erosion;
• Riparian planting adjacent to the channel for cover and bank protection;
• Access for whitebaiting and other stream-related activities;
• Provide floodway capacity, while retaining multi-use capability for most of the time when there is no
flooding (e.g. grassed area for picnicking, specimen trees, etc);
• Access for long term maintenance.
6.1 Further Engineering Options Considered
The “Balanced Spill Option” as described above was further investigated during the concept design stage and
various flood mitigation measures were considered resulting in three options being identified and investigated.
These were:
1. Prevent spill across the State Highway east of Kairua Road, allow spills opposite Bruce Road
2. Prevent spill across the State Highway opposite Bruce Road, allow spills east of Kairua Road
4 The Tauranga District Plan specifies minimum building platform levels for the Papamoa area, based on a 2% AEP design flood level of
4.5mRL established some years ago. In recent work by TCC for the comprehensive catchment consent for Wairakei/Papamoa
Catchment, TCC has also considered the 1% AEP storm, and a design flood level of 4.6mRL. These two values are reasonably
consistent in terms of the design solution to achieve them, with the 1% AEP storm being the more severe event. Because the
Mangatawa spills to Wairakei catchment, we have adopted the higher (1% AEP) storm as the basis for design, consistent with that
catchment, but have also used the 1% AEP storm for the target performance in the Maranui catchment.
Status: Final Page 64 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE