Part 1 - Palmerston North City Council

gemconsulting
3.3 Monitoring and Maintenance
Once the works are completed they will be maintained in perpetuity as a Lower Manawatu Scheme asset.
It will be necessary to monitor the rock lining and groyne on a regular basis, and particularly after any
floods, to ensure that any faults are noted and rectified as required.
The rock lining is designed to be ‘self‐healing’ so that if any rocks are displaced from the lining due to
undermining, the rocks higher up the lining can slip down to take their place and continue to provide
protection of the river bank. The lining can then be topped up from above after the event. For this reason,
vehicular access will be required along the top of the lining and from time to time it is likely that machinery
will be present to carry out repairs and to top up the lining as required.
It is proposed to leave approximately 300 tonnes of rock at Stockpile Site 3 for future maintenance of the
rock lining and groyne. This will reduce the need to transport rock across the Manawatu River. However, it
is likely that machinery will need to cross the river to carry out any maintenance required and as such, it is
proposed to leave the ford in place after the construction. While this structure will likely erode over time, it
is expected to be a simple exercise to bring it back up to useable/crossable state by topping it up with
gravel as or when required.
The ford will also be used to top up the Stockpile Site 3 if required in the future. This will involve the
stockpiling of the rock on the right bank first and then transporting it over the river. It is expected that this
would not occur more than once every five to ten years and would require 15 loads resulting in 30 river
crossings.
4 ALTERNATIVES
4.1 Management of Erosion at ANZAC Cliffs
Several alternatives to manage the erosion at the ANZAC Park cliff area have been considered over recent
years. These include the following:
Realign the river now
This is the option that has been decided upon and for which resource consent is now being sought. Within
this option three proposals have been considered.
The first option considered was to realign the river on a gentler curve by locating it hard against the right
bank. This would involve considerably more excavations and construction of a longer rock lining.
Investigations also proved that there would be issues with an artesian aquifer underneath the site. Also,
the financial cost of undertaking this option was prohibitive.
The second proposal was somewhat less ambitious, and consisted of a rock lining of some 560 metres long
in a similar position to that proposed now but extending upstream an additional 100 metres. Until recently
this was the preferred option. However, there are several reasons why this option has not been adopted.
Surveys of the gravel beach indicated that there would be insufficient gravel available for the back fill
required. Rising costs of transport require that as much material as possible is sourced from the site and
carting in gravel from elsewhere would exceed available budgets. The extra 100 metres of rock lining would
also add significantly to the amount of rock required to be transported to the site. The Regional Council has
been unable to gain access to the river, for construction, from the left bank, meaning that all rock for the
lining now has to be transported through residential areas of town and forwarded across the river. Again
this has added to the overall costs of the project and also causes some additional disturbance to residents
and the river. The Mangaotane Stream is also problematic with it currently joining the Manawatu River
behind the proposed rock lining thereby requiring it to be realigned so that it joins the river upstream of the
lining. The longer lining made this difficult to achieve.
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