Diverting the flow of a major river to stop erosion is no easy task. RICHARD SILCOCK takes a look at an ambitious project that is near completion on the Manawatu River.
THE MANAWATU RIVER, at 180 kilometres in length, is one of the main rivers draining the southern part of the North Island. Originating in the Ruahine Ranges in southern Hawkes Bay it passes through the slip-prone Manawatu Gorge before flowing along the southern flank of Palmerston North and out across fertile plains to the Tasman Sea at Foxton. At times of peak flow it is prone to flooding and gouging its alluvial banks.
In an area known locally as ANZAC Cliffs, which lies on the southern bank of the river upstream from the Fitzherbert Bridge and just south of Palmerston North, significant river erosion had been impacting on the cliffs, the adjacent reserve and proposed housing development land.
A sharp bend in the river was causing gravel to build up on the northern bank, forcing the river to flow along the base of the cliffs’ face. As the channel for the river progressively tightened due to the build-up of gravel, the rate of erosion increased with an estimated 20,000 cubic metres of the soil falling into the river each year.
Acting on concerns from various parties, Horizons Regional Council instigated geotechnical site investigations in 2007-08 with a number of test drillings to ascertain the ground composition of both the river bed and the land surrounding the cliffs.
Consents for the river realignment were granted in 2012 following agreements with the Environment Commission that there would be strict adherence to codes of practice by the contractor that there would be minimal river disturbance.
Having successfully tendered for the project, Goodman Contractors commenced the physical works back in September 2014.
To overcome the problem the river needed to be turned through an angle of 110 degrees to divert it away from the eroding cliffs. The only practicable means of achieving this was to establish a new channel and river flow alignment. This was achieved by firstly creating a substantial, curved gravel embankment upstream to divert the river away from the cliffs and then building a ‘protective wall’ around the cliffs.
This involved shifting 150,000 cubic metres of river gravel from the north bank to create the embankment and then filling the old channel with compacted gravels up to a height of 20 metres to form a bench to protect the cliffs, and then placing around 40,000 tonnes of ‘imported’ armour rock across the face to help protect it.
“It’s been a bit of a stop-go exercise,” says Dave Morgan, project engineer with Goodman Contractors. “Due to the often high and strong river flows over winter and extremes in the weather we have only been able to work on shifting the gravel across the river from late-spring through until mid to late autumn each year. The earthworks on the cliffs required really dry conditions so we were only able to work over the summer periods following the completion of the river work. However, and subject to weather and river conditions, we expect to have the major part of the work completed by the end of this month.
“The project has involved a major diversion of the river, which is approximately 70 metres wide at this section,” says Dave. “We have essentially moved the river gravel that had been deflected and deposited on the north bank across to the south bank and filled what was the previous river channel.
“This was achieved using a number of D8 and D10 bulldozers and excavators and carting the gravel and rock across the river using 40-ton Cat dump trucks, which ‘wade’ across the river at a ford we constructed with ‘imported’ rock early on in the project. The gravel was compacted using a Cat 824 wheeled dozer and ‘loaded’ dump trucks.
“The rock is sourced from the nearby quarry at Linton and trucked to the work site compound on the north bank before being carted across to the south side and placed along the face of the new embankment to form a buttress.”
According to Dave one of the biggest challenges of the project has been associated with working in the river bed itself.
“We have to keep a constant eye on the river level, which can rise quite suddenly,” he says. “We check weather forecasts each day and we are linked up to the Horizons river flow monitoring site so that we can get a heads-up on the state of the river. No equipment is left in the river channel overnight as it could be damaged should the river level rise significantly.”
Allan Cook, group manager operations with Horizons Regional Council, says the key drivers for the project were to protect the integrity of the cliffs, mitigate the risk to public safety as a result of the subsidence, prevent the river encroaching on infrastructure and residential land and help improve the water quality of the river.
Once work is complete, the public recreational walkway along the north bank, which has been temporarily fenced off to the public, will be reinstated. There is also the possibility of creating a new walkway on the south side between Fitzherbert Bridge and residential Vaucluse Heights.
Once the work is completed, the area will undergo a three-year planting programme that will be compatible with the surrounding area and the adjoining Te Motu O Poutoa pa site. This will be maintained by the Palmerston North City Council as a recreational reserve.
The $6.2 million project is a part of the Lower Manawatu City Reach Project which was established to manage flood protection. The project is being managed by a joint team comprising representatives from Horizons Regional Council, Higgins Contracting (now part of the Fletcher Group), and Kevin O’Conner & Associates (engineering consultants) in collaboration with the Palmerston North City Council and PMB Landco (owners of the residential development), which are all contributing to the cost.