Part 1 - Palmerston North City Council

gemconsulting
The proposed works have been designed by the MWRC River Engineers. The design has been peer
reviewed by Gary Williams from G & E Williams Consultants Ltd. The peer review is attached to this report
as Appendix C.
It is intended to complete the works over one summer period (preferably the summer of 2012/13).
However it is recognised that this timeframe depends on weather conditions and river flows. If we have
another wet summer the works may need to be put off until more favourable conditions exist. It is also
possible that the works proposed by the applicant occur in one summer and the majority of the works
proposed by PMB Landco Ltd occur the following summer.
3.1 Reasons for the Proposed Works
There are a number of reasons for carrying out the works in the river at the ANZAC Park Cliffs. As noted,
the cliff has started to erode much faster than was anticipated, particularly since 2004, and it is estimated
that some 20,000m 3 of soil is now being eroded from the cliff each year. The rate of erosion can be easily
seen in the aerial photographs attached to this report as Appendix D. This erosion is a significant source of
sediment that is released to the Manawatu River each year, thereby contributing to the rivers already high
sediment loadings. Once the works are completed it is anticipated that the erosion and sedimentation from
the cliff will cease.
The retreating cliff is causing a change in the direction of the river flow to the extent that it will eventually
start to erode the opposite river bank, threatening the stopbank protecting the city. Major and costly
works will be required to protect the integrity of the stopbank to ensure that it continues to protect the city
from floods in the future.
If the cliff is left to erode it will eventually expose the existing residentially zoned land on top of the cliff to
significant risk. At some point works similar to those currently proposed will become necessary. As the cliff
erodes, increased costs can be expected due to the river channel alignment continuing to deteriorate
making it more difficult to rectify. Also if the cliff advances too far towards any residential development it
may not be possible to rectify without purchasing land and or dwellings to complete the works.
There is a significant risk to public safety. The site is a popular recreation site with people using the river for
swimming and kayaking. The cliff is extremely unstable and large slips can occur with little or no warning.
People pursuing recreational activities in that stretch of the river are exposed to a high level of risk. In
addition, risk is also present at the top of the cliff. As residential development continues in the vicinity
people are attracted to the open rural landscape around the top of the cliff. While the cliff has been
fenced, a recent visit to the site revealed that the cliff has now retreated beyond the fence and that it no
longer offers any protection to the public.
3.2 Description of Activities
To achieve the realignment of the river in a way that is physically possible, and to avoid or minimise effects
on the river, certain works will be required to occur in a certain order. The following describes the main
components of the works that will need to be undertaken to achieve the river realignment. A construction
methodology has been developed that describes the works, the sequencing of works and mitigation
methods in further detail. The methodology is attached to this report as Appendix E.
Access and Stockpile Areas
Approximately 30,000 tonnes of rock will be used in the construction of the rock lining, bed armouring and
snub groyne. A stockpile area (Stockpile Site 1) will need to be created on the grassed area between the
river and the Esplanade Walkway to accommodate up to 5,000 tonne of rock at any one time. This
immediate supply of rock is required so that there are no delays once construction of the lining is initiated
and also to smooth out any minor supply delays from the quarry should they arise.
Stockpile Site 1 will be accessed from an existing track leading from the end of Albert Street. Up to 300
tonnes of rock will be transported to the site each day, equating to 15 truck and trailer loads per day. It will
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