Wednesday, 23 September 2015

Napier Gisborne Line : Bridge 290, Waipaoa River Bridge [2]

Continuing from the first post in this series here are a series of photos of the bridge extension works in 1958 nearing completion. Published in Gisborne Photo News in August 1958.

The bridge was finally completed two months later with a 56 hour block of line imposed as some of the girders had to be lifted off the temporary deviation and put onto the permanent alignment where the extension joined on at the south end of the bridge. Then the track had to be pushed across at the south end as well.

Gisborne Photo News photos from October 1958.

From the time that this extension was completed for the next 30 years the now 330 metre bridge seems to have had a relatively uneventful life. In March 1988 that all changed. The storm called Cyclone Bola passed up the east coast of New Zealand and caused a flood that peaked at 5360 cumecs (more than 5 million litres per second) in the river. This cut into the riverbank at the south end of the bridge and left more than 100 metres of track suspended in midair. The southern abutment was pushed out of line and the three other piers supporting the 1958 extension spans were all scoured significantly. Along with other damage to the railway, the re opening costs were estimated at around $3.5 million, which is equivalent to about $7 million of today's money. Because the railway at the time was operating uneconomically, NZR stated they wished to close it. However the Labour government had other ideas as Gisborne was a marginal seat and they wished to win it at the next election. Hence, the government provided extra funding to enable the line to be re-opened.

The work carried out in 1988 entailed a further extension of the bridge at the southern end to 475 metres, its present length. The undermining of the four piers supporting the ends of the 1958 extension spans all had to be remedied in some way as part of the works. The former southern abutment pier which had the most serious damage was underpinned with new piling while the scouring of the three others was addressed not by fixing their foundations, but by imposing new load limits on the bridge. Specifically, traffic was given a speed limit over this section and restrictions were also imposed on train running whenever the flood level in the river or wind speeds reached certain limits. The bridge also has a load limit overall of 12 tons in axle loading and in general double headed trains are not allowed to run on the bridge. The reasons the bridge was not fully repaired are due to the economic situation in relation to train operations in the area. Apart from these works another 12 spans were added at the south end to cover the washout of track. The total cost of repairing the bridge was $2.2 million, which is about $4 million in today's money.

These repairs were only a patch up short term solution because all of the pre-1988 bridge foundations have been severely affected by flood-induced scouring in which the pier footings are in a number of places only in the riverbed to a very shallow depth (as little as 1 metre in places) and this could be undermined in a future flood and will need to be addressed in the future. In addition most of these footings are in soft soil, a mixture of sand, silt and clay, rather than gravel or other hard rock material which would normally be the desirable type of foundation material for a large structure such as a bridge. Since the cost of replacing these pier footings would be very high, this represents a significant cost risk for the future operation of the line. The section containing Bridge 290 is still in use being leased to Gisborne City Vintage Railway, but at some future point the accumulated backlog of bridge maintenance will catch up and that section of line will most likely be permanently closed when that happens.

It should be noted that the Waipaoa River is very flood prone and this is exacerbated by the nature of the geography of the East Cape area where a lot of land is of a very soft character, being comprised primarily of weak mudstones and sandstones. The unstable land has been responsible for a great many slips and washouts on the railway in the time since its construction was completed in the early 1940s; and in the case of the river, in heavy rainfall, a lot of back country terrain is washed into the river and becomes part of its flood load, so that the river is liable to rise very quickly in a relatively short space of time. Morever this has also raised the bed level so that the current flood control stopbanks in Gisborne have become less effective over time. Hence the railway bridge is quite vulnerable to large floods in the river and it is only a matter of time before the bridge is too badly damaged to remain open.

References: [1] "Bridge Scour"; Bruce W Melville & Stephen E Coleman; Water Resources Publications (Colorado, USA), 2000. ISBN 1-887201-18-1.