Beyond end of life: rethinking bridge resilience in a changing climate

In recent years, Aotearoa New Zealand has experienced severe weather events that have tested the resilience of our transport networks. The Auckland Anniversary floods and Cyclone Gabrielle exposed vulnerabilities across roads, bridges and supporting infrastructure, causing hundreds of millions of dollars in damage. Insurance claims following recent severe weather events have also highlighted the growing financial consequences of flood-related infrastructure damage and disruption.

These events have prompted a closer look at how we plan, design and invest in transport assets. Recent revisions to flood modelling have reinforced that challenge, with events once considered exceptionally rare now assessed as increasingly likely.

For infrastructure owners, this shifts the focus for ageing assets. The question is no longer simply when a bridge should be replaced. Increasingly, we must also ask whether its replacement can continue to perform under the conditions it is likely to face over the next 50 to 100 years.

Across New Zealand, many bridge owners are preparing for a wave of renewal and replacement decisions. Traditionally, those decisions have been driven by condition assessments, maintenance requirements and structural performance. Climate resilience now sits alongside these considerations.

As rainfall intensity increases and flood models evolve, so does our understanding of network risk due to flooding. Earth Sciences New Zealand estimates that around 26,800 kilometres of road are currently exposed to flooding, with that figure expected to grow as climate impacts become more pronounced. Many ageing bridges now sit within newly identified flood-prone areas, including structures located in some of the country's most remote regions.

That creates a difficult reality. Aotearoa is unlikely to have the financial capacity to replace every bridge vulnerable to future flooding while also upgrading each structure to the highest resilience standards available today. We need a smarter approach that secures priority routes and makes better use of the available bridge replacement budget.

For decades, the preferred response to increasing flood risk has been straightforward: if predicted flood levels increase, raise the bridge. In principle, the engineering logic is clear. Higher bridges reduce exposure to inundation and avoid many flood-related loading challenges. The challenge is that, as design flood levels continue to rise, so does the cost and complexity of achieving that outcome.

Higher bridge levels often require longer bridge structures, more extensive approach embankments, increased scour protection and substantial changes to road approaches over considerable distances. These works may exacerbate flooding elsewhere or simply be impractical. Additional earthworks and land acquisition can quickly become the dominant project costs and, in many rural locations, can be difficult to justify.

As revised floodplains expand both vertically and horizontally, more bridges are exposed to flood risk. This raises the question of whether, rather than designing every bridge to remain above floodwaters during extreme events, some could instead be designed to safely withstand periods of inundation while maintaining their structural integrity.

This approach came into focus on several bridge replacement projects in Auckland and Gisborne, where new structures were required to replace approximately 70-year-old bridges that had been washed away by floods.

Rather than substantially raising the replacement structures, our design team considered options that retained the existing bridge and road alignments and allowed the bridges to be safely overtopped under revised, climate-adjusted flood estimates.

The trade-off is that the crossings would be temporarily unavailable at the peak of a major flood, and their edge barriers might require replacement, particularly where debris is present in the flow. However, retaining the existing alignments enables shorter bridge structures, avoids extensive approach embankments that could alter river flows, and provides a practical and cost-effective solution.

It also offers resilience beyond the design event: once a bridge is fully submerged, the forces acting on it increase only incrementally as flood depth rises, allowing a structure designed for submergence to withstand floods substantially deeper than the design flood.

While retaining bridges on existing alignments can deliver lower costs and improved resilience, the current treatment of flood loading can create varying outcomes depending on designers’ assumptions and interpretations. This is difficult to reconcile with broader network investment priorities.

Designing for inundation sounds relatively straightforward in theory. In reality, it quickly becomes more complex. Existing standards provide a framework for assessing flood loading on submerged bridges, yet much of the current guidance stems from research undertaken in Queensland during the late 1990s. While that work remains highly valuable, a number of areas within current provisions are open to interpretation, leading different designers to adopt different approaches.

It’s found that design outcomes can be heavily influenced by assumptions made early in the process, such as where an approach velocity is taken from a hydraulic model can significantly affect the flood loads applied to a bridge. Factors such as debris loading, concurrent loading scenarios and barrier system design can also have a substantial impact on the final result. I shared these findings at the NZ Bridge and Geotechnical Conference in 2026, focusing on the opportunities for a more consistent and risk-based approach.

Small differences in interpretation can have significant implications for construction requirements, cost and overall project feasibility. It’s an opportunity to refine the design rules for submerged bridges so they provide greater consistency, avoid unnecessary conservatism and reduce the need for departures from the Bridge Manual. 

As New Zealand adapts to a changing climate, resilience should be viewed less as eliminating all risk and more as understanding and managing it effectively.

Not every vulnerability in the transport network can be removed. Instead, investment should be directed to where it delivers the greatest network benefit, while alternative approaches are considered where they provide a better balance of resilience, cost and acceptable risk.

For bridge owners managing ageing assets, changing climate projections and increasing budget constraints, achieving this balance is becoming a defining challenge of infrastructure planning.

The future of bridge design will not always involve building higher. In suitable locations, shorter and more cost-effective replacement bridges can instead be designed to withstand controlled inundation. This approach would allow more bridges to be renewed within available capital budgets, improving resilience across the wider network rather than concentrating investment in a smaller number of highly elevated structures.