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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
7. Description of the Proposal
The elements of the proposal that are the subject of this application are the works related to the construction
and operation of the stormwater management system of the TEM and the works related to the construction
and operation of the Mangatawa Flood Bypass. These stormwater control devices are linked by the way they
operate together to manage stormwater in day to day and flood events, but are separate in the issues they
designed to address.
The description below addresses each package of activities separately.
7.1 Mangatawa Flood Bypass Activities
The following is a summary of the key features of the flood bypass proposal and is further detailed in the
technical report attached as Appendix 8.
7.1.1 Increased Drain Storage
On the eastern side of Kairua Road, enclosed between the rail track and the motorway, a storage area will be
created by excavating approximately 21,000m 3 - 28,000m³ over an area of 1.2 ha (this will involve excavating
a set distance from the current railway track allowing for possible track duplication, at a 1V in 5H batter to a
level set above the current drain). The current drain will serve as the main channel with the new storage area
set above normal (dry weather flow) water level. The creation of this additional storage will require excavation
in the drain, conveying of the excavated material and the regrassing of the newly shaped drain and flood plain.
7.1.2 Top Spill Bund
Currently there is an earth bund that starts from the rail track where the catchments of the Papamoa hills drain
under the rail track. This bund will be strengthened and extended and will follow the drain alignment on the
northern side.
The bund will be 3 metres wide at the top with a 1 in 3 slope with a total estimated fill volume of 3,000 m 3 –
4,000m 3 . The bund will follow the rail track alongside the State Highway from chainage 9500 to a highpoint at
chainage 8950. The bund is designed to be overtopped periodically and therefore will require reinforcement
and regular maintenance. An earth bund was chosen as it will be easier to maintain with a 3 metre wide top
and appropriately vegetated batters. The creation of the earth bund will require importing material and
working alongside the current drain.
7.1.3 Bruce Spill Stop Bank
For the Bruce Spill Stop Bank a stop bank will be constructed between the highway and drain from CH7300 to
CH8700. This may take the form of a concrete crash barrier. The barrier will be designed to operate to allow
for over topping.
Status: Final Page 67 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE