This report was originally based on Kiwirail's line closure proposition dated 2012. Since that time more information has come to hand.
Text in italics is quoted from the Kiwirail report from 2012.
6.4. Current and Future Costs associated with the PNGL North of Napier
KiwiRail is currently spending approximately $2.1m annually to maintain the infrastructure to its current condition north of Napier. This is very minimal amount and with the increased frequency of trains over the recent summer season, local staff had observed that increased maintenance would be required in order to hold the existing level of service.
Increasing Drain and Culvert Resilience: Because of the susceptibility and climate conditions in this part of the country, there may be a case to increase the culvert inspections from once every six years to annual inspection. This would add a further $6,000 to the inspection cost per year. The maintenance work found from the extra inspections is estimated to cost between $40,000 and $70,000 per year.
Geotechnical Risks and improve resilience: To decrease line outages due to slips and drop outs, prudent engineering works could be undertaken. A detailed study has not been undertaken so reasonable judgement has been used to generate the estimates used in the table below. The Wairoa to Gisborne section is more vulnerable than the Napier to Wairoa section and this is reflected in the estimates. In this section alone there are at least 13 embankments of similar height to those that have failed recently and may pose a similar risk of embankment collapse due to upstream ponding. Added to this, between Wairoa and Gisborne there are an estimated 30 to 40 cuttings and slopes above track that may pose varying levels of risk to the operation of the line.
River Works: One high risk area that will need a significant amount of work is between 347km and 350km where the rail runs alongside the Kopuawhara River. A combination of river protection, slope stability and drainage work is required along this stretch of the corridor. To remove the risks associated with this area could cost between $0.8m to $1.5m. A nominal amount has been included for the Napier to Wairoa section to acknowledge there are numerous rivers that the railway runs close to.
Bridge 218 PNGL
Ahuriri Estuary Bridge: To maintain bridge 218 PNGL to its existing condition is estimated to cost between $400k and $800k over a 15 year period. This estimate is based on the recent pile repair work completed in 2010. To renew this bridge would cost between $8m to $10m. Other bridges on the line between Wairoa and Gisborne that will require significant work within the next 15 years are bridges 251, 253, 257 and 260 PNGL. These bridges are located within 500m of the coast and the steel plate girders are severely corroded and therefore have reduced section loss in the steel. These spans will need to be renewed with refurbished spare spans and will have a combined cost of approximately $800k and $1.2M.
Other Incidents: Taking into account past history, we could expect a major infrastructure outage due to weather to occur every 2 to 4 years. Given the geography of the line, it makes it difficult to eliminate damage against such events. However, we can certainly limit the damage by improving at risk areas and by maintaining drains, culverts and slopes after severe weather events.
Ahuriri Estuary Bridge: To maintain bridge 218 PNGL to its existing condition is estimated to cost between $400k and $800k over a 15 year period. This estimate is based on the recent pile repair work completed in 2010. To renew this bridge would cost between $8m to $10m. Other bridges on the line between Wairoa and Gisborne that will require significant work within the next 15 years are bridges 251, 253, 257 and 260 PNGL. These bridges are located within 500m of the coast and the steel plate girders are severely corroded and therefore have reduced section loss in the steel. These spans will need to be renewed with refurbished spare spans and will have a combined cost of approximately $800k and $1.2M.
Other Incidents: Taking into account past history, we could expect a major infrastructure outage due to weather to occur every 2 to 4 years. Given the geography of the line, it makes it difficult to eliminate damage against such events. However, we can certainly limit the damage by improving at risk areas and by maintaining drains, culverts and slopes after severe weather events.
Regarding track related derailments, renewals expenditure north of Napier is very low. With large amounts of relatively old track infrastructure, it would not be unexpected to have an occasional derailment on this line.
Maps of the Whareratas section: These maps were drawn months ago and show accurately a number of areas of damage to this section of the line. They are not part of the KRL report quoted above. The aerial footage is from Bing Maps and is more up to date than current GE coverage. The report by KRL was done in 2012 and it is possible further damage has occurred since that time.
This is close to the area 347 to 350 km mentioned above where the line runs alongside the Kopuawhara Stream. A couple of small slips are visible in this section on the aerial imagery that was obtained of the line. These are shown below.
This is close to the area 347 to 350 km mentioned above where the line runs alongside the Kopuawhara Stream. A couple of small slips are visible in this section on the aerial imagery that was obtained of the line. These are shown below.
The left hand aerial is the one at Kopuawhara Viaduct while the right hand one is between No.15 and 16 Tunnels. The slipping is extremely obvious in both those areas along with the debris that has ended up in the river. Clearly major works will be needed here to address the slipping, as Kiwirail has mentioned above.
This is the next map north of the one above and I have marked in three slips which are all clearly visible, as shown below.
The one on the left happened about three years ago and was a bluff above the line which collapsed onto the track. It took about a month to fix. The other two are due to the river undercutting the slope. Again this appears to be the same area mentioned. The one on the right shows the Wharerata Walkway station shelter.
This map shows the first of the actual washouts at the top. This one is between No. 22 and 23 Tunnels. The Kiwirail report calls this Site 1 and locates it at 353.7 km.
The size of this washout should not be underestimated. It is expected the volume of fill required is around 54,000 cubic metres. The estimated period of repair is around 5 months and the cost around $2 million.
The second major washout is between Tunnel 23 and the site of the daylighted Tunnel 24. KRL calls this “Site 2” at “Big Hut”, 355.6 km.
The second major washout is between Tunnel 23 and the site of the daylighted Tunnel 24. KRL calls this “Site 2” at “Big Hut”, 355.6 km.
The estimate time of repair is around 3 months, requiring 22,000 cubic metres of fill at a cost of around $1 million.
The last map covers the two remaining washouts which are close together.
Firstly you have a slip north of former Tunnel 24, then you have the Beach Loop washout, and then the slip at Wharekakaho. KRL’s report only mentions the last two.
Slip north of Tunnel 24 is obviously developing or is the site of a previous slip that has been cleared along the line. There is obviously a lot of land movement happening here.
This is the current Beach Loop slip. There have been several in recent times but this one has washed out the track at the south end (No.1 Points). It is at “Site 3”, 357.15 km.
This is about a 2 month repair job, will require around 10,000 metres of fill, and is estimated to cost up to half a million dollars.
The last of the four sites is at the Wharekakaho Stream. Historical photos show this embankment was constructed by the classic PWD method of filling in a temporary trestle, as was probably the case all along this section. Kiwirail describes it as Site 4, 358.400 km. Although the track was not washed out at the time, the estimated repair would require a substantial volume of 46,000 cubic metres of fill and would take up to 3 months and cost close to $1 million.
Other Work
Several bridges that would require maintenance are mentioned above. Many of the bridges have wooden piles that will deteriorate and need renewing. Bridge 256 at Nuhaka was found to be susceptible to teredo infestation following its collapse in 2005. Although there are still two sets of wooden piles under this bridge, the piers in the main channel of the river were replaced in concrete when the bridge was rebuilt, lessening the problem. Bridge 297 crossing Gisborne Harbour is susceptible to teredo and has had piles replaced at various times during its life, although the bridge is beyond Gisborne station and has been disused for a number of years.
The major maintenance issue for bridges is Bridge 290, 10 km south of Gisborne, crossing the Waipaoa River. The original 220 metre bridge was twice extended, firstly in 1956-58 and again in 1988 due to flooding problems in the river catchment. The first extension of 110 metres allowed for the development of a new river channel as part of flood control engineering works and was carried out at the same time as the whole bridge was raised by 0.6 - 0.7 metres. The second extension of 145 metres covered a section of track that was washed out by a flood of more than 5000 cumecs (5 million litres per second) due to the Cyclone Bola storm of March 1988. Although the bridge has been safe for use by trains up until 2012 when the line was closed, and is to be brought into use by Gisborne City Vintage Railway later this year when they resume passenger services between Gisborne and Muriwai, since 1988 it has been subject to various operational restrictions. There is an axle loading limit of 14 tons and a low speed restriction, and the bridge is subject to temporary closure whenever certain flood flow or wind speed limits are reached in the riverbed. These are due to the fact that some of the original 1930s and extension 1950s piers are at minimal depth due to flood scour. Although the newest extension has deep founded piers the older foundations are at risk of collapse in future floods and so the bridge remains vulnerable to severe weather events.
Several bridges that would require maintenance are mentioned above. Many of the bridges have wooden piles that will deteriorate and need renewing. Bridge 256 at Nuhaka was found to be susceptible to teredo infestation following its collapse in 2005. Although there are still two sets of wooden piles under this bridge, the piers in the main channel of the river were replaced in concrete when the bridge was rebuilt, lessening the problem. Bridge 297 crossing Gisborne Harbour is susceptible to teredo and has had piles replaced at various times during its life, although the bridge is beyond Gisborne station and has been disused for a number of years.
The major maintenance issue for bridges is Bridge 290, 10 km south of Gisborne, crossing the Waipaoa River. The original 220 metre bridge was twice extended, firstly in 1956-58 and again in 1988 due to flooding problems in the river catchment. The first extension of 110 metres allowed for the development of a new river channel as part of flood control engineering works and was carried out at the same time as the whole bridge was raised by 0.6 - 0.7 metres. The second extension of 145 metres covered a section of track that was washed out by a flood of more than 5000 cumecs (5 million litres per second) due to the Cyclone Bola storm of March 1988. Although the bridge has been safe for use by trains up until 2012 when the line was closed, and is to be brought into use by Gisborne City Vintage Railway later this year when they resume passenger services between Gisborne and Muriwai, since 1988 it has been subject to various operational restrictions. There is an axle loading limit of 14 tons and a low speed restriction, and the bridge is subject to temporary closure whenever certain flood flow or wind speed limits are reached in the riverbed. These are due to the fact that some of the original 1930s and extension 1950s piers are at minimal depth due to flood scour. Although the newest extension has deep founded piers the older foundations are at risk of collapse in future floods and so the bridge remains vulnerable to severe weather events.
