Engineer spotlight: How we’re managing the increasing impact of scour

Q. What is scour? 

A. Put simply: it's moving water that runs past a structure and removes the material that’s there. On a ‘normal’ day, the effects are very minimal; but, when the river that a bridge stands in runs faster (or the bridge is suddenly in flood water), then it can cause a problem.  

The strength of the current becomes faster with severe weather including flash flooding or storms. 

Q. What issues does it cause? 

A. A larger volume of water flows a lot faster, and it carries debris with it. That fast-flowing water can start to ‘eat away’ (i.e. ‘scour’) at the foundation of the bridge’s piers and abutments. It can also cause holes in the riverbed that can destablise the structure.   

The holes in the riverbed can later be filled with more stone from the riverbed, but that hole doesn't necessarily get filled with solid, compact material. It’s very difficult to know if this has happened as it’s underwater.   

Some of the structures weren't originally built in water but have become more exposed to water due to flooding. The effects of scour can sometimes be worse for these structures because they were never built to withstand the flow of water and may have shallower foundations.   

Q. Why is scour a rising problem 

A. Scour is not just an issue that we have to contend with: it’s a problem throughout the country and the world. It’s the number one issue that makes bridges collapse, whether they’re old, like ours, or more modern. We know this through our discussions with other industry experts and members of the bridge community who are experiencing the same issues.   

100 years ago, when our bridges were built, discussions around scour didn’t need to happen. 

A flood event doesn’t automatically mean that scour will occur, but it increases its chances.  

Q. How does it affect how we manage the Historical Railways Estate?  

A. All of our structures are subject to our standard examination process (see more here) plus we have around 100 structures in our estate that have regular, specific scour assessments. These structures have been highlighted because they're carrying a road or are listed, so we need to be quick to spot a potential scour-related issue there. 

They're given a rating that goes from 1 to 6 (1 being the highest). The higher-risk structures are examined every 3 years, while the lower numbers get examined every 6 years. The rating of the structure can go up and down, depending on its condition.  

Part of my role is to educate the team about the effects that scour can have so everyone is aware of the risks associated with it. 

The challenge is that we don’t always know what the foundations of a structure are like because we didn’t build them – but we do all that we can to manage it.  

Q. How can we prevent scour? 

A. If we notice that large pieces of debris have been pushed against a structure, we look to remove it as soon as possible so the debris doesn’t contribute further to the issue and create a weak spot. Water can be incredibly powerful and every case that we come across is different.  

Unfortunately, we can’t easily detect when a hole has been made in the riverbed and prevent the issue. We have to keep a close eye on the structure itself for any damage and protect public safety. We follow a nationally recognised standard and stay up to date through working groups on advancements in technology that could help us to manage the problem even better.  

Case study  

We’re currently working on a two-span structure where a tree trunk hit the middle pier and lodged itself against it during a flood event.   

The blockage quickly became a problem because the water that was passing through that bridge still wants to go through the bridge, but it can’t. This makes the water flow faster and can cause increased damage.   

With this structure, we started off with one proposed solution and, as we learned more about the structure, our plan evolved.  

When the bridge was built, it had some protection around it, but that protection deteriorated with natural wear and tear. It had a ‘stone invert’, which, put simply, is almost like a cobbled street around the based on the structure, which improves the longevity of the riverbed around it.  

Through consultations with environmental organisations, we decided not to replace it like-for-like but, instead, dig out the riverbed and put in a concrete slab covered by natural riverbed material. This will be loved by the creepy crawlies that can be found in the water and help prevent damage.  

Visit our Maintenance and Assessment page to discover more about our approach to caring for the Historical Railways Estate.