Looking below the surface: rethinking flood resilience for a changing climate

The new reality of flood risk and urban resilience

Across the world, cities are confronting an uncomfortable reality: in the last 20 years, extreme flooding disasters have doubled. From coastal storm surges to intense inland rainfall, recent disasters have shown that traditional, surface‑based flood solutions alone can no longer provide the level of protection modern cities require. At Mott MacDonald, we see this as a system‑wide resilience issue that requires a new approach to flood risk management, long-term climate resilience planning, and investment in adaptable urban infrastructure.

Our award-winning design on the Euclid Creek Tunnel provided an innovative, cost-effective solution that improved stormwater management, enhanced water quality, and strengthened urban flood resilience in Cleveland.

Why surface solutions are reaching their limits

Over the past two decades, significant progress has been made in surface flood management. Cities have embraced floodable parks, deployable barriers, smart levees, and nature‑based solutions as part of broader integrated water management and stormwater management strategies, delivering environmental and social value alongside flood mitigation. These interventions play an important role, particularly for frequent, lower‑intensity events.

However, recent events, including Hurricanes Sandy and Ida in the United States, and Storm Daniel in Libya, have highlighted the physical limits of surface infrastructure when faced with extreme rainfall, rapid runoff, or storm surge. Dense urban development leaves little room for large‑scale detention at ground level. Steep topography can overwhelm infiltration‑based systems, and when rainfall intensity exceeds design thresholds, water will always find a path, regardless of surface defenses. 

The conclusion is clear: surface infrastructure remains essential, but it is no longer sufficient on its own.

How underground flood infrastructure strengthens urban resilience

Underground infrastructure fundamentally changes the equation by increasing the capacity and reliability of urban water systems while reducing flood impacts in dense urban environments.

Deep conveyance tunnels and underground storage caverns can be implemented to capture excess flows by diverting floodwaters away from vulnerable areas and temporarily store vast volumes of stormwater until systems recover. Crucially, they do this without competing with buildings, transport corridors, or public spaces.

A hybrid model for urban flood resilience and future‑ready cities

The most resilient cities are not choosing between green infrastructure and underground engineering. Instead, they are adopting integrated water management approaches that combine surface and subsurface assets to improve urban flood resilience and support long-term climate resilience planning. At Mott MacDonald, we advocate a hybrid flood resilience strategy, where each component plays to its strengths:

• Surface and nature‑based solutions that manage everyday rainfall, enhance liveability, and deliver environmental benefits.
• Advanced modeling, forecasting, and early warning systems that improve preparedness and protect life.
• Deep underground conveyance and storage systems that provide capacity and reliability when extreme events exceed surface infrastructure limits.

What has changed is the ability to deliver underground solutions more efficiently and with greater confidence. Advances in geotechnical investigation, three‑dimensional ground modeling, tunnel boring machine (TBM) technology, and tunnel and shaft support systems have significantly improved predictability, reduced risk, and enabled construction in increasingly complex ground conditions. Access to better climate data and models has provided a basis to inform hydraulic design parameters to account for increases in storm intensity, future water surface elevations, and storage volume requirements.

These innovations allow cities to consider underground flood infrastructure not as a last resort, but as a practical and resilient component of long‑term urban planning.

The Thames Tideway Tunnel taking shape.

How we deliver flood resilience underground

Around the world, these principles are already being put into practice to address different flood risks and urban constraints.

In Cleveland, Ohio, the  Euclid Creek Storage Tunnel is part of the wider Project Clean Lake program, a major regional effort to reduce combined sewer overflows (CSO) and improve water quality in Lake Erie. The tunnel plays a key role by capturing and storing excess stormwater during heavy rainfall, helping utilities meet stormwater management objectives while reducing flood risk for surrounding communities.

In London, the Thames Tideway Tunnel – often referred to as the “super sewer” – captures and conveys excess flows that would otherwise overwhelm aging infrastructure, helping to protect the River Thames and the city from pollution and flood-related impacts during storms. 

Meanwhile, Kuala Lumpur’s SMART Tunnel illustrates how multifunctional underground infrastructure can deliver the dual benefits of social outcomes and flood control. By switching between road and stormwater modes, it not only alleviates chronic traffic congestion but also diverts and stores floodwaters during peak events, preventing damage in highly urbanized areas.

Engineering resilience across urban systems

Urban flood resilience is not just about infrastructure – it is about confidence. Confidence for communities that their homes and livelihoods are protected. Confidence for businesses and investors that cities can withstand climate shocks and stresses. And confidence for governments that their investments are delivering lasting value.

From our global experience across water, transport, energy, and underground infrastructure, we understand that successful flood risk management, integrated water management, and climate resilience planning require technical excellence, coordinated investment, and a deep understanding of local context. Whether advising on strategy, designing complex underground infrastructure, or supporting clients through delivery, our role is to help cities make informed, future‑focused decisions.

Looking down to move forward

As climate volatility accelerates, the cost of inaction – or partial solutions – will continue to rise. The question facing city leaders is no longer whether surface infrastructure can be improved, but how cities can build layered and adaptable resilience into the very fabric of the urban environment. 

For many, the answer lies beneath their feet. 

By combining surface innovation with intelligently designed underground infrastructure, cities can strengthen urban flood resilience, improve stormwater management, enhance urban water systems, and deliver the climate adaptation infrastructure needed for long-term infrastructure resilience. At Mott MacDonald, we believe the future of flood management is three‑dimensional – and that the most resilient cities will be those willing to look below the surface to protect what matters most above it.