Part 1 - Palmerston North City Council
Part 1 - Palmerston North City Council
Part 1 - Palmerston North City Council
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gemconsulting<br />
The extent and nature of the disturbance to the river bed<br />
The proposed works will disturb the bed of the river in the immediate location of the works. As described<br />
in Section 3.2 above, it is intended to excavate a temporary river channel through the beach on the right<br />
bank of the river and to eventually use this gravel to support the rock lining. The proposed works will<br />
disturb the river bed for much of its width in places and for a length of some 550 metres in total.<br />
Some of the proposed works will be permanent, e.g. the rock lining, the snub groyne, bed armouring and<br />
re‐alignment of the River and Mangaotane Stream. Other components will be removed once works are<br />
complete, e.g. cofferdams. It is expected that the beach on the right river bank will reform over time after<br />
completion of the works however, it will not reform to the same extent that it is now due to the river being<br />
closer to the right bank.<br />
Effects on in‐stream habitats, spawning habitats and fish passage<br />
The proposed method of construction will ensure that fish passage is maintained in the Manawatu River at<br />
all times during the construction period. Fish passage will also be maintained for the Mangaotane Stream.<br />
The unnamed tributary only flows during wet weather and it has a small waterfall of about two metres in<br />
height as it joins the river. When the works are complete it is expected that it will continue to have a similar<br />
fall into the river at low flow conditions.<br />
Any habitats in the immediate area of the works will likely be destroyed, however it is expected that such<br />
habitats would quickly reform in this area once the works are complete as has occurred near other rock<br />
linings along the river. Downstream habitats could potentially be affected by sediment released from the<br />
river bed during the works but it is expected that this will be limited to 200 to 300 metre downstream of the<br />
site. Past experience has shown that sediment can be released from such works but that it is minimal and<br />
does not stay in suspension for any great distance. Adherence to the construction methodology will<br />
mitigate the severity of the effects on downstream habitats with the majority of the works taking place out<br />
of the active river channel.<br />
The ‘in river’ construction works are programmed to start in September 2012 and are expected to be<br />
complete by April 2013 thus avoiding the spawning season of trout (May to September).<br />
Effects on flood flows<br />
The works will not restrict or affect flood flows of the river itself, however, the possibility of floods during<br />
the construction phase presents challenges to the project. If a flood were to occur during the construction,<br />
it is possible that stockpiled gravel could be lost downstream. To help mitigate this, the work is to proceed<br />
quickly once commenced. Also, the shape of the stockpile, following the curvature of the river, and the<br />
addition of larger rocks at the upstream end of the stockpile are expected to minimise any losses during a<br />
flood event. Some gravel recovery may be required from downstream if losses are experienced.<br />
Effects on water levels and passage of flows<br />
The passage of flows will be maintained throughout the construction stages of the project. Once the river is<br />
diverted through the temporary channel the works will proceed in the "dry" river bed with no effect on the<br />
river flow. Water levels are not expected to alter as a result of the proposal. The river will still rise and fall<br />
naturally in accordance with rainfall in its catchments.<br />
It is intended to realign the Mangaotane Stream so that it joins the river just upstream of the snub groyne.<br />
Once the initial re‐alignment is completed the only further effects on this stream will be from machinery<br />
crossing it via a ford on the access track to the site. The rock lining will be designed to accommodate the<br />
smaller unnamed tributary so that it will join the river in the approximate location and level that it does<br />
now.<br />
Effects on public access to the river and recreation<br />
During construction, public access to the site, in particular the river, beach and rock stockpile area will need<br />
to be restricted. Currently there are signs restricting access to the river in this location due to the existing<br />
safety concerns in the area so effectively there will be little change from the current situation.<br />
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