Thursday, 1 October 2015

Waikokopu Slip Hazard Assessment

A panoramic shot of the area affected by a major landslide at Waikokopu in 1957. This swept away 250 metres of the Napier-Gisborne railway line and the road alongside it and took four weeks work with heavy earthmoving machinery to stabilise and restore transport links. A recent posting to this blog showed detailed photos of the remediation work underway.

Here is a Google Earth 2010 view of the same area, which locally is known as "Blacks Beach". It is just south (railway norm) of Waikokopu, which was the terminus of a branch from Wairoa opened in the 1920s, serving an early small port. The railway was later incorporated into the Napier-Gisborne Line which opened in the early 1940s.

Further examination of the Google Earth photos show that land movement is still occurring along both sides of the road and this is of particular significance at the southern end of this area where the road drops down below the level of the railway while remaining in close proximity to it. This naturally undermines  the railway embankment to some extent and as the face of the embankment alongside the road is still unstable, there must be further risk to the railway line from erosion of this face. The GE photos confirm slipping and crumbling of the face of either the embankment itself or, in places, the cliff face along the top of which the railway runs, is active and continuing at this time, and therefore does represent a real and ongoing natural hazard affecting the railway at this location. In time it will become necessary to stabilise the embankment face above the road either by cutting through the embankment, or spending a lot of money on stabilisation of the face to maintain the embankment in its current form.

In turn the face of the road embankment dropping down into the sea is subject to significant repetitive erosion from wave action and is recognised as a coastal erosion hazard area by Wairoa District Council and Hawkes Bay Regional Council. Certain mitigation measures will be needed in the near future to ensure the road is not undercut by the action of the sea. The railway is another matter altogether but as it is closed, in future it is entirely possible large landslip movements across the railway onto the road may be addressed by permanently removing the railway track to facilitate stabilisation and restoration of the slope.

This then is one of a number of natural hazards impacting on the Wairoa-Gisborne section of the railway and these would require substantial expenditure, tens or hundreds of millions, in years to come should re-opening the railway be contemplated. It is therefore easy to understand that this loss-making railway does not have an economic case for re-opening as there will be no money to expend on the high costs of maintenance and repair.