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

NZ Transport Agency and Tauranga City Council
Mangatawa Drain Flood Bypass and TEM Stormwater AEE
6. Alternatives Considered
An investigative process was developed to find possible solutions to manage the stormwater derived from the
proposed TEM and to reduce the frequency of spills across the State Highway. The process is outlined in
Section 8 Consultation. For the purposes of this report we refer to the activities related to the flooding issue
as the Mangatawa Flood By-pass and the issues related to the TEM roadway are referred to as the TEM roadway
stormwater.
To address these issues there are three overarching philosophies that could be adopted:
• Leave the situation as it is;
• Prevent spill across SH 2 completely in up to a 100 year storm;
• Balanced spill option – managed spill and mitigate effects.
Retaining the existing situation is not a desirable option, because it involves an unacceptable level of risk to
urban areas of Papamoa, including the proposed developments in the Maranui catchment, e.g. on the Asher
Block. In addition it allows regular spills across the TEM resulting in frequent road closures.
Preventing a spill across SH2 requires that the full design flood would need to be conveyed in the Mangatawa
Drain. Given the tight corridor available between the Railway and SH2, this would require the drain to be either
concrete lined, or at least to have full structural retaining walls both sides over most of its length. This is
unlikely to gain consent because of the adverse aesthetic and environmental effects it would have on the
waterway and discharge point and is not viable because of the significant cost.
It has therefore been concluded that the best approach is the balanced spill option whereby the frequency and
volume of spills is reduced allowing the character of the existing Mangatawa Drain to be retained, combined
with improving its flow capacity with some specific works and improved ongoing maintenance. This will
reduce the risk of flooding across the SH2 and reduce the risk to Papamoa urban area to acceptable levels.
There are a number of outcomes that the project seeks to achieve in respect of the proposed Mangatawa Drain
improvements.
(a) Reduced frequency and volume of flooding across the highway to the north;
(b) An overflow volume discharged to the Wairakei catchment that is no more than was assumed for the TCC
comprehensive stormwater consent for that catchment;
Status: Final Page 63 April 2009
Project Number: 3932036
NZ1-1552092-Mangatawa Drain Flood Bypass and TEM Stormwater AEE