By Debby Booth and Claire Cobbett
Highway flooding is a fact of life and always has been; national highway design standards aim to keep water off the trafficked areas of a road for a 1 in 5 year storm (i.e. there’s a 20% chance it will flood in any year). What’s new is the climate crisis, and that means flooding will become more frequent. As engineers, we can recommend different solutions to tackle the issue:
- Realign – to keep the road away from the water
- Resist – build barriers to keep water away from the receptor (road)
- Reduce – lessen the impact of flooding when it occurs
- Recover – make sure things can get back to normal quickly
- Respond – help infrastructure providers plan for when flooding is predicted.
Given the enormous scale of the challenge (Great Britain has 31,800 miles of major roads alone), the more expensive options, realign and resist, are likely to be reserved for the most severe threats – for example where highway flooding could cause a serious incident or inundate nearby properties. The reduce, respond and recover options have potential for widespread and more cost-effective implementation – if our society can accept that highways will flood. The respond option will need to take into account the people affected, as well as the resources that are realistically available to provide proactive interventions when a storm is predicted.
As road users, we may simply need to accept that it’s normal to encounter water on the highway, and change our behaviour to the situation. For example, we need to understand that driving through deep water not only risks wrecking our vehicles but can also create a bow-wave that floods roadside properties. People will need information to help them adapt behaviours – for instance through smart motorways that close lanes during heavy rain, accepting that some lanes are designed to flood during periods of high rainfall.
While preventing water-related collisions remains essential, this doesn’t necessarily mean we need to keep highways completely free from surface water – particularly not on minor roads. A residential road with a 20mph limit could be designed so that during heavy rain a section of the road/footpath stayed dry. This would allow local people to keep moving, albeit slower – whether by car, bus, bike or on foot – and would be cost-effective. We’re now starting to look at different approaches with local authorities, examining how they could split roads into different classes and specify acceptable levels of flooding for each.
Maintenance is arguably a more important consideration than the design itself. What is designed must be maintained, both safely and affordably. Lack of funds /resources are often the reasons for lack of maintenance on existing infrastructure. Do we need to lobby infrastructure authorities and designers to consider the cost benefit of maintenance in more detail and engage with local and expert knowledge to put forward the best long-term solutions as a blend of design, maintenance and resilience? It’s certainly time to consider this question.
The fact is that we need to design not just for flooding but to recover from flooding. At a basic level, this means ensuring that highways and their surrounding assets (e.g. slopes, road surfaces, structures) are not excessively degraded by water. Can we design a slope to remain inherently stable when holding water, rather than relying on draining it into an already saturated landscape? Do infrastructure providers need to consider all benefits (including preventing the road surface and structures from failing early), to justify expenditure on maintenance? Can our clients access funding for walking and cycling infrastructure that will help with maintenance?
These are tough questions and we don’t know all the answers, but we do know that if we keep doing the same things we’ve always done in the face of climate change we can expect more damage and disruption.
It’s time to have a proper conversation about acceptability of flooding and the affordable response to it.