RAIB: ‘washout of material’ caused Carmont derailment
The catastrophic derailment at Carmont near Stonehaven was caused by material washed onto the track by heavy rain, according to Network Rail’s safety and engineering director.
Martin Frobisher told RAIL: “The washout of material was driven by very high rainfall. It was certainly the immediate cause of the incident.”
Investigators from the Rail Accident Investigation Branch (RAIB) report that 52mm of rain fell in four hours before the derailment. That’s 75% of the total monthly rainfall for Aberdeenshire in an average August.
RAIB states that above the track a slope rises steeply to a field at that location, and the field then slopes gently upwards.
A drain runs along the edge of the field, then diagonally down the steep slope, reaching an outfall at a track-level ditch. The drain is an 18-inch diameter plastic pipe laid in a trench filled with gravel. Water flowing from land above the railway washed some of this gravel onto the railway, along with some larger pieces of rock.
RAIB states that its investigation includes “the management of earthworks and drainage in this area, including recent inspections and risk assessments”.
Allan Spence, director of regulatory liaison at Network Rail, is leading the industry investigation into Stonehaven.
He told RAIL: “It’s important to state that we don’t yet know the water flows that led to the washing of material onto the track. We are still working to understand all the natural and installed drainage in the area.
RAIB referenced a drain, but a lot of the technical work is about understanding the flows that led to the dramatic event of washing material onto the track.”
Network Rail has carried out an immediate inspection of 500 similar sites in cuttings across the network.
In recent years, the company has increased spending on earthworks and drainage. From 2009-14 it invested £550 million. That figure rose to £952m in 2014-19 and is increasing to £1.3 billion in the 2019-24 period.
Network Rail’s Earthworks Technical Strategy, published in 2018, stated that the current rate of strengthening by renewal or refurbishment is between 0.5% and 1% of the total in each fiveyear period.
“What recent events have told us is about torrential rain leading to debris flows,” said William Powrie, Professor of Geotechnical Engineering at the University of Southampton.
“Rain and run-off is channelled from a large catchment into a smaller area, encountering potentially unstable ground and causing a debris flow, or washout.
“It is rarer than other rotational landslips, which happen more slowly. But I wonder whether these are the really dangerous events, because they are so sudden and because of the nature of the debris.”
Analysis of Network Rail records by the university found that washout was responsible for 25% of railway slope failures between 2012 and 2018.
Powrie added: “This August we had six days when the temperature in southern England was above 30°. Then suddenly, we had monumental thunderstorms. Where earth has dried out, sudden rain does not necessarily infiltrate the ground. There is more run-off. You end up with the water literally washing the surface away.
“Network Rail knows where areas susceptible to the classic landslip lie. It is more difficult to know where the network is susceptible to the challenge of debris flow.
“Washout events are difficult to predict. They also involve conditions and influences well away from the railway and outside Network Rail’s control. As we see increased storminess with climate change, we can expect more of these.”
After Carmont, Network Rail decided it had to improve weather prediction. But calculating where thunderstorms will have the greatest impact is notoriously challenging.
Spence said: “We have a weather provider, MetDesk, which gives us real-time information - radar analysis of what rain is
falling. It’s called nowcasting rather than forecasting, with refresh rates every five minutes.
“We have looked to enhance several aspects of the Rule Book for drivers, trackside staff and operations staff, to make sure we have a consistent way of dealing with adverse rainfall.
“We’ve done a lot of work before about fog, snow, high water levels, flooding over the railhead. Now we are working with RSSB to enhance the modules in relation to high rainfall. We expect those to be published at the next possible opportunity.”
This month, Network Rail launches a new environmental strategy. It will focus on four big themes: a low-emissions, decarbonised railway; resilience to climate change; reducing waste; and biodiversity.
“I think the immediate actions we have taken after Stonehaven provide some reassurance,” said Frobisher.
“Beyond that, we have a significant programme of work on how we adapt to climate change.”
■ For detailed analysis of Network Rail’s actions in the wake of the Carmont derailment, see the next issue of (