Once again, the Manawatu Gorge road is closed by a slip. The latest slip is expected to take until mid-May to be cleared. As people who use the gorge road regularly resign themselves to another bout of waiting for it to open again or take alternative routes, it is time to consider the long term future of this slip prone road.
There is an underlying problem that people need to recognize. The problem is not in the road itself, but in the underlying geological strata, which is soft and easily erodible sedimentary material. Because of that, this is a recurring problem that engineers, road users and planners are simply going to have to learn to work around.
State Highway 3 is a major highway that runs between Woodville in the Wairarapa and Hamilton in Waikato. Probably the largest contributor to State Highway 3 traffic and benefactor of it is Palmerston North, on the Manawatu plain west of the Tararua Range.
But the slips are frequent, often substantial in size and clearing them can take weeks at a time. During this period there is significant financial penalty for the communities at either end of the gorge, and significant costs incurred by transport firms such as trucking companies which have to either delay or divert or even put their cargo on rail. Because of these and other problems, it is necessary to evaluate the options for transport and long term remedial work in the gorge.
Manawatu Gorge also has a railway line running through it, though this has numerous tunnels which protect from the slippage problems associated with the road route. One option would be to significantly increase the railway capacity for freight through the gorge. However this would be a long term solution rather than a short term one and would need input from Kiwi Rail.
Although it does not really suit road freight, a second possibility would be to upgrade the Saddle Road route. However this highlights a second problem as farmers need to stand their stock from feeding four hours before travelling so as to keep the resulting effluent to within the capacity of their rigs. It also raises the question of whether this road is really suitable for carrying large vehicles, given its windy nature and grades.
Slips are going to continue to happen in the gorge. This means that long term consideration needs to be also given to whether or not such measures as roofs need to be considered that enable the smaller landslides to pass over the road and go straight into the Manawatu River. One issue here is that slips are the cause of their toe support failing, which means the slip is likely to happen at or below road level. Terracing the potential slip prone slips is expensive and would involve significant alteration of slopes and not necessarily be guaranteed to work.
It would seem that rail is potentially the best option for freight. However there is no passenger service, due to insufficient demand, which means people needing to use the route either take the Saddle Road alternative or they drive to Porirua, over 100km to the south and back up via Featherston – a trip of nearly 250 kilometres.
But as the problem is in the geology and not necessarily the road, this is going to be a continuing issue.