Part 1 - Palmerston North City Council

gemconsulting
same as the upstream end of the rock lining) and will be constructed to a height that corresponds with the
annual return flood level.
Temporary Cofferdam
A cofferdam will be constructed from the snub groyne to the temporary river channel to divert the river
into the channel and thereby allowing construction of the rock lining to proceed in a flow free river bed. A
further cofferdam will be constructed at the downstream end of the works to prevent water back‐flowing
into the work area and to minimise sediment release form the works to the river.
Rock Lining
Rock linings generally provide protection against lateral erosion through the placement of rock directly
against the lower sections of the river bank.
In this instance the rock lining will be constructed away from the base of the cliff and will be reinforced by
the gravel previously stockpiled in the middle of the river by placing it behind the rock lining as the work
progresses. The lining will have a similar design to other rock linings already constructed within the City
Reach. It will be approximately 460 metres long. The rock lining will extend one metre below the level of
the river bed when the works are complete with the upper level of the lining at a height corresponding with
the annual return flood level. The lower part of the lining will be constructed with a median rock size of
750kg and be 1.7 metres thick. Above this, the lining will be constructed with a median rock size of 300kg
and will be 1.2 metres thick. The whole lining will have a 2:1 slope (26.6 degrees).
The rock lining will be constructed to allow the unnamed tributary to enter the Manawatu River with the
top part of the wall curving towards the stream as shown on Figure 1 (attached as Appendix B). The stream
will be channelled across the gravel backfill and through this ‘gap’ in the rock lining.
As mentioned, PMB Landco Ltd intends to re‐contour the western cliff. These works will see the cliff
battered back to a slope starting from a point that leaves a seven metre wide bench on the rock lining to
allow access for maintenance of the lining and for a possible future walkway connecting ANZAC Park to Pari
Reserve. The lower part of the re‐contoured cliff will be lined with quarry rock to protect the fine soils from
the cliff being eroded during floods that are higher than the annual return flood level.
Bed Armouring
Bed armouring layers consist of a few widely spaced larger rocks interspersed with smaller rocks. The larger
rocks reduce the flow velocity close to the river bed thereby minimising scouring. The bed armouring
concept has been successfully used in two locations within the City Reach; around the groynes at Fitzroy
Bend and at the downstream end of the Katene Street rock lining.
The new bed of the river will be armoured to prevent any scouring that may undermine the new rock lining.
This armouring layer will be 500mm thick and consist of 80% reject quarry rock and 20% rock with a 300kg
median weight.
Gravel Recovery (if required)
There is a risk that if a flood occurs during the construction period it could potentially wash stockpiled
gravels downstream. If this occurs, recovery of these gravels will likely be required to ensure enough gravel
is available to support the rock lining and complete the works.
Tied Tree Works
Once the rock lining is completed, the river bank upstream of the lining will be protected with tied tree
works. The purpose of this activity is to prevent lateral erosion of the river bank and maintain river
alignment, by providing relatively heavy vegetative protection, developing strong root systems, and
encouraging the deposition of sediment at the toe of the banks. The works will involve anchoring live trees
to the river bed and bank with wire ropes and anchors. Further tree planting will be carried out behind the
tie tree works during the winter following construction.
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