Case study: How HS2 managed flood risk while building the first intercity train line in over a century

The HS2, or High Speed 2, is a high-speed railway being constructed to link London and Birmingham.

Originally, in 2010, it was expected to connect London with Birmingham, Manchester, Leeds and York. However, in October 2023 then prime minister Rishi Sunak stated that HS2 would only run from London to the West Midlands.

 

HS2 is the first new intercity railway to be built north of London in over a century.

Part of the reason for the scaling down of the project was escalating costs — HS2 was initially expected to cost £32.7bn when details were confirmed in 2012. By June 2013 this rose to £50bn and by February 2020 this had again spiked to £106bn.

There have been other issues, including public protests, vast project staff turnover and severe construction delays.

Flooding has also been a major issue. Between November 2019 and February 2020 alone, winter flooding was severe in several parts of the UK, including the East and West Midlands.

And HS2 has not just been a victim of natural flooding, but at times has itself been accused of worsening or even causing floods. This January, construction of the HS2 was alleged to be causing flooding in Berkswell, on the outskirts of Coventry.

So what have those involved with the HS2 done to mitigate flood risks?

MITIGATION IN THE DESIGN

The government drew up a detailed plan for flood mitigation from the outset. The first line of defence was route selection for the railway track, with a focus on “areas with the lowest probability of flooding” and avoiding “flood zones wherever reasonably practicable”.

However, the project recognised that it would still have to traverse areas at risk of flooding and so developed the “Exception Test”, which requires it to be demonstrated that the “infrastructure would be safe from flooding over its lifetime, would not increase flood risk elsewhere, and that the wider benefits to society outweigh flood risk”.

The project also focused on hydraulic capacity – in locations where the route will cross watercourses or surface water flow paths, the design aim is for structures to accommodate flood flows up to and including the 1-in-100 (1%) annual probability storm with an appropriate allowance for climate change.

Floodplain storage was another major consideration, with watercourse crossings designed, where reasonably practicable, to avoid encroachment into floodplains. The plan accepted that losses of flood storage may still be unavoidable.

Maintenance access was a key consideration, with an appropriate vertical clearance provided above floodplain ground level to the underside of viaducts to ensure access to riverbanks for inspection and maintenance purposes.

In February 2023, when the project was still expected to continue further north than Birmingham, HS2 Ltd altered its design for the project’s tunnel in Manchester in order to reduce any possible flood impacts.

TECH KEEPING THE PROJECT ON TRACK

The HS2 is a combination of public and private planning and implementation. One such private player in the project is BBV, a joint venture involving Balfour Beatty, VINCI and SYSTRA, which is responsible for delivering the northern section of the HS2.

BBV has had to consider many significant challenges related to potential flooding. These risks include potential damage to construction sites, machinery and materials, as well as safety hazards for personnel.

Traditional flood prediction methods were insufficient in this case for providing the necessary accuracy and timeliness to effectively prepare for and respond to flood events.

To mitigate these risks and enhance operational resilience, BBV needed a technological solution. As a result, BBV implemented the Previsico Flood Intel Platform, which provides real-time flood forecasts and alerts.

“This allowed us to mitigate asset damage during this event and safely demobilise from a flood zone of a site of special scientific interest river.”

The platform combines advanced weather data, hydrodynamic modelling and machine learning to predict flooding. This enabled BBV to receive up-to-date information on potential flood risks and take proactive measures to safeguard its sites and personnel.

On one occasion, construction manager of BBV HS2 Rory Garry says the flood forecasting and sensor technology provided a flood alert specific to its River Blythe HS2 construction site, giving it 36 hours to prepare.

“In this time, we were able to move critical infrastructure, including a crane, excavator, sheets of timer, fuel cubes, etc. This allowed us to mitigate asset damage during this event and safely demobilise from a flood zone of a site of special scientific interest river.”

The system also allowed BBV to customise alert thresholds specific to its project sites, ensuring that the right level of response was activated at the right time. The outcome was improved safety for workers and a reduced risk of accidents, saving on costs as proactive flood risk management minimises the potential damage to equipment and infrastructure, and ensures operational continuity.

It also exemplified the value of data-driven decision-making, as the platform allowed access to detailed, real-time flood data that helped BBV’s project managers to make informed decisions.