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

• Added primary drain cross-sections to reflect updated topographic data
• Adjusted Kairua road culverts to reflect existing situation
• Included the Maranui catchment of which the details are:
- Catchment and natural storage areas
- Swales
- Culverts
- Outlet to harbour
2.1.2 Mitigation
Using the above update, various mitigation options were investigated and the options that provided
satisfactory results and were cost effective are:
• Additional storage within the drains.
• Stop banks along the drain located at the two known spill points.
• Drain improvements, clearing of vegetation and replant with vegetated lining.
• Protection and stabilisation of drain bank in specific locations.
• Decrease secondary losses through the railway bridge crossing.
• An alternative outlet from the Maranui swale to the Mangatawa Drain was included through the
inclusion of the following details:
-
-
Additional culverts below the highway and the railway track.
A weir to allow flood spill but contain and bypass the first flush water to the Maranui swale
and pond.
2.1.3 Inclusion of MPBI land
Further refinement in the model was later carried out to address the effects on the MPBI industrial
land south of the State highway. The aim of the refinement was to optimise the available MPBI
land. This refinement in the model included the following amendments:
• Replace the open drain with an extension of the current 1.8m diameter Maranui Swale outlet
pipe culvert from its current outlet up to the treatment pond.
• Optimise both the Maranui swale and Mangatawa drain floodway corridor options.
• Improve the current pond to a stormwater wetland area that provides treatment for MPBI, TCC
and NZTA.
2.1.4 Summary
The following is a summary of the mitigation work that is included in the modelling and is illustrated
in the drawings in Appendix A:
• Provide stopbank and flood retention “bund”.
• Enlarge the flood storage area within the drain.
• Drain improvements, clearing of vegetation and replant with vegetated lining.
• Protection and stabilisation of drain bank in specific locations.
• Decrease secondary losses through the railway bridge crossing.
• Deepening of Maranui swale.
• Additional culverts to drain from the Maranui swale to the Mangatawa Drain.
• Improve the lower Mangatawa drain conveyance capacity.
• Extend the existing Maranui outlet culvert.
• Extend the Maranui stormwater treatment pond as a wetland.
Beca // 17 March 2009 // Page 7
3932036 // NZ1-1604459-15 0.15 Rev A