Getting design right early: Managing flood risk

6 Oct 26

Flood risk is a complex and increasingly important consideration for construction projects, requiring teams to understand not only the sources of flooding but also how activities, temporary works and construction methods can affect vulnerability, Chris Fosbeary, head of engineering at JBA Consulting, told GIRI members.

Flood risk management is a complicated discipline, said Chris. “Projects can involve multiple authorities and a range of potential sources of flooding, from rivers and surface water to sewers, reservoirs, and groundwater. Managing it involves dealing with water authorities, the Environment Agency, local authorities, and wider catchment issues. It’s a jigsaw that we have to piece together to come up with a solution.”

He added that the role of flood risk management is changing, with an increasing emphasis on working with nature and building community resilience. “There’s a growing acceptance that we can’t build to manage the whole impact of climate change. This is causing a mindset shift around how we help communities become more resilient to the impact of flooding.”

The impact of climate change is not simply being felt in terms of rising sea levels, he explained, but also extreme temperatures. The UK’s flood risk assets are also ageing, adding to the pressure at a time when the sector is moving into the digital space and trying to upskill the workforce.

Getting design right early

“At JBA, we are working to embed the GIRI Design Guide within our organisation. We’re doing a lot of work on culture, reinvigorating our culture around sharing information and reporting quality issues. We are revising design stages to add additional hold points, brief revisions, and staged acceptance, and outputs are targeted to provide the right information to manage risks on site. We have also reinforced our information management and our processes around reviewing information.”

He added that early input from specialist contractors is also key, as is working closely with the client. “We've taken a revitalised role in terms of understanding client issues and project constraints earlier on to deliver better outcomes.”

Why construction floods matter

“Flooding is a major cause of construction damages and financial loss. If you have a flood event on site, it will cause programme delays, cost overruns, disruption, damage to assets, material, and plant. Climate change and larger project footprints could further increase these risks. The impact can be significant.”

Good construction flood risk management therefore needs to begin with compliance with Construction (Design and Management) Regulations (CDM), and following the CIREG Guide to Managing Flood Risk During Construction. It also means thinking about temporary works from the outset. “For us, it could be around how we model assets. Are we taking the structure out of line? Are we working around temporary protection?”

Some of this is about site planning and planning for events. “We need to ensure we are planning for emergencies and that the information around managing flood risk is effectively communicated.”

Chris said that much of it links back to the principles in GIRI’s Design Guide, for example, setting clear briefs from the start, understanding risks, roles and responsibilities and identifying triggers and actions. Design gateways should be used throughout project development, while construction methods, temporary works and sequencing should be considered alongside contractual and stakeholder engagement, and key information, particularly live information, should be shared effectively.

Emergency event planning

When it comes to planning for emergency events, Chris explained that understanding the flood risks specific to your site is key. Then identify vulnerabilities and consider triggers and actions. “Flood response plans should sit alongside construction and environmental plans. They should be concise and ready to implement.”

As flooding can occur from a wide variety of sources, including rivers, surface water from reservoirs, coastal sites, or groundwater, sites must identify their sources of risk and how close they are to sources of flooding. Vulnerability will be impacted by location, topography, and land use. 3D models can give a good idea of direction of water flow and help identify who is likely to be affected. Teams also need to understand how much warning they may have, how quickly water levels could change, and how long an event might last, which will be affected by the activities on site.

“Once you understand your project vulnerabilities, consider how you can change the project design to make it more resilient. This may include looking at what materials you are using and whether these can be made more resilient to the kind of risk the site is exposed to.”

He added that it is also important to understand the triggers. “A flood response plan is only effective if you have clear triggers and they need to be based on incoming data. This could be weather forecasts, rainfall thresholds, river warning levels, or groundwater levels observations. The triggers should be specific to the site, the build, and its vulnerabilities.”

Once a trigger has been reached, the plan should outline clear actions and responsibilities. In the case of a warning, these could include stopping high-risk activities in a watercourse, securing temporary works and site facilities, clearing drainage channels and preparing pumps. Depending on the project, responsibility may be shared between the client and contractor, so clear communication and coordination is essential.

Using forecasting to protect construction sites

JBA applied these principles on a project involving the replacement of river control gates and improvements to fish passage downstream. The project faced particularly challenging conditions, with flows ranging from 1m³ per second to 120m³ per second and significant and rapid water level changes.

Chris explained that JBA’s objectives included minimising asset downtime and reducing the flood risk. The design was adapted to increase resilience, including the use of precast units rather than in-situ construction for downstream works, and the use of AI flood modelling to set site-specific triggers. “We used predictive rainfall and forecasting models to understand when peaks were likely to occur and establish triggers for the site team to get clear. We provided 48-hour forecasts, predicted flow rates and email and text alerts, sharing this information with the client and the delivery team.”

Key questions

Chris closed by highlighting the key questions that GIRI members should be thinking about when considering flood risk management on construction sites. “Do you understand the sources of flood risk affecting your site? Do you understand the construction activities taking place and how they could interact with that risk? And can you manage these outputs effectively?”

For construction projects increasingly exposed to changing weather conditions, addressing those questions early could be critical to building greater resilience into both the design and delivery of projects.