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

The total volume of water spilling across the highway will decrease with the proposed works. The
implication of Graph 2 is that spill occurrence will improve from 15% to 7.5% AEP events. The
difference in prediction of return period between the peak flow and the volume graphs reflects nonlinearity
of the system response, and the difficulties of fitting a suitable curve to the resulting output
data. The precise return periods are of less relevance than the significant reduction in peak flow
and volume that the graphs illustrate.
3 Proposed Flood Risk Mitigation Measures
To be able to mitigate and manage the spills and flood risk, the current structures and natural drains
have to be upgraded. This will involve new structures to be constructed and existing ones
upgraded or expanded. The following are the various mitigation measures that would be required to
enable the mitigation and management of flood risk, as set out in the modelling.
3.1 Increased Drain Storage
On the eastern side of Kairua Road, enclosed between the rail track and the highway, we have
identified a potential storage area that can be used to create more flood storage in the Mangatawa
Drain. The land is owned by Ontrack and is not currently in use. Our understanding is that this area
was used in the past as a station for the moving of cattle. Ontrack does not plan to make use of this
area. They require a 14m wide strip along the track to ensure that the track can be doubled either
side in future.
We are proposing to excavate 25,000m 3 and this will involve excavating a set distance from the
current and possible future additional track at a 1 in 5 batter to a level set above the current drain
invert. The current drain will serve as the main channel with the new floodplain set slightly higher.
Attached in Appendix E are the engineering drawings that illustrate the layout and cross-sections
along the drain. We do not envisage moving the current drain alignment. The creation of this
additional storage will require excavation in the drain, conveying of the excavated material and the
planting of the newly shaped drain and flood plain.
3.2 Top Spill Stop Bank
Currently there is an earth bund that starts from the rail track where the central (P1) and western
(P2) catchments of Papamoa hills drain under the rail track. At present it is not clear who is
responsible for maintenance of this bund. Local residents report that it has failed several times in
the past, apparently due to substandard works having been undertaken, or adjacent works that
compromise its integrity.
We propose to also use an earth bund that will follow the drain alignment on the northern side
between the drain and the TEM, constructed to adequate engineering standards to contain the flow
within the drain, and with a protected face on the TEM side to cope with spills in more extreme
floods. The bund will be set higher (8.1m) and will be 3 metres wide at the top with 1 in 3 batters.
The batters will tie into the existing ground profile. The total estimated fill volume will be 3,500m 3 .
The bund will run from the rail track up to a high point along the proposed TEM where a crash
barrier will continue along to Kairua Road. The bund will be designed to operate in the same
manner as the current where it can be overtopped and thus will require reinforcement and
maintenance. The earth bund 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.
Beca // 17 March 2009 // Page 15
3932036 // NZ1-1604459-15 0.15 Rev A