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

Discharge to Papamoa Beach
To discharge at Papamoa Beach would require a pipeline about 1.8km long. The flood level in the
Mangatawa Drain upstream of Mangatawa Lane is of the order of 4.2mRL during the 1% AEP flood.
The existing drain capacity through this area is 17.4m 3 /s, and the additional flow rate that needs to
be discharged to convey the 1% AEP flood with allowance for climate change is a further 29m 3 /s.
To convey this quantity by gravity to the coast at Papamoa Beach would require a box culvert with a
cross-section of approximately 15m 2 (e.g. 10m wide by 1.5 high). This would need an easement of
at least 17m width, in addition to any existing easements, and would need to be on either the “Rifle
Range” block or the Asher block, as all other routes are constrained by existing development. The
outlet to the beach would have significant aesthetic effects, and would affect the morphology of the
coast, potentially putting the coastal dunes (and hence property behind) at risk. The land required
would be about 3.06ha, and the construction cost would be prohibitive. Therefore this option is not
considered practicable.
Discharge along Truman Lane
Discharge to the west along Truman Lane then to the harbour would likely require a route that
followed south of the railway. For this option, some flow would need to be diverted at Mangatawa
Lane, and conveyed alongside the railway in a box culvert. It would then be joined by additional
flow from 2 additional pipes from Maranui swale, and piped along the centre of Truman Lane, with
increased capacity from this point to the harbour. This option would require a 2.6km long box
culvert of a larger size than the Papamoa Beach option.
Alternatively, if the box culvert to the harbour only conveyed the estimated additional 8.8m 3 /s
overflow from Maranui swale, and Mangatawa Drain outlet conveyed the additional flow from
upstream capacity improvements, then the box culvert could be reduced to 1.8km in length, and
approximately 4.8 x 1.0m in size. However, with either option there is minimal corridor available in
places along the route (especially between the TEM, the railway and Truman Lane), and costs
would again be prohibitive. Therefore this option is not considered practicable.
Discharge along upgraded existing flow paths
Of the two remaining options, involving open channel conveyance to the harbour, both follow routes
already serving as drainage paths, with relatively minor changes in the drainage corridor width
required. Even if one of the box culvert routes were to be used, the Mangatawa Drain route would
still need to be able to convey 17.4m 3 /s in the 1% AEP flood, and a stream channel plus flood plain
would need to be retained. The difference would be the size of flood plain required. Further, both
of the open channel options involve much less costly construction works, as they make use of
existing drainage paths, and the principal works required would be earth banks to contain the flow,
and planting of the flood plains.
The Maranui corridor south of the railway is about 200m long adjacent to developable land, while
the Mangatawa corridor is about 800m long adjacent to developable land. Therefore, the total
footprint of these corridors is 1.7ha for Option 1 and 2.3ha for Option 2. If the Mangatawa Papamoa
Blocks Inc (MPBI) land were to be developed with the current drainage system in place, and SH2
and Papamoa were allowed to be flooded, we estimate that these corridors would still require about
1.5 ha to pass existing flood flows, so the additional land required is only about 0.8ha.
It should also be noted that the Mangatawa corridor will include flood plain areas that will normally
be available for public use for passive recreation, so most of that land will have multiple beneficial
uses. By contrast the Maranui floodway is likely to be permanently wet (as is the present bypass),
but with no flow, and unlikely to be suitable for multiple uses. It was not intended for recreational
purposes. Further, we have assumed that the Maranui bypass would be constructed within the land
Beca // 17 March 2009 // Page 4
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