Shallow floodwater can pose a deadly threat if it moves quickly, new research from the University of Canterbury (UC) has found, challenging the common assumption that low water levels are safe to cross.
The study, led by Dr Lea Dasallas from UC’s Department of Civil and Environmental Engineering, revealed that when water velocity is factored into flood modelling, high-risk areas for pedestrians increase by more than 80%, while medium-risk zones triple.
“Floodwater doesn’t just pool – it flows, and when it flows quickly, even relatively shallow water can become extremely dangerous,” Dr Dasallas said.
Most public flood maps currently focus on water depth rather than speed, potentially underestimating danger during extreme weather events. The research, published in the Journal of Flood Risk Management, used central Wellington as a case study to model an extreme rainfall event under future climate conditions.
The findings showed previously “safe” roads and intersections emerged as high-risk zones when velocity was included, particularly where streets act as channels for fast-moving water.
“These are places people still try to drive through or walk across,” Dasallas said. “But once you account for velocity, it becomes clear that those routes are much more dangerous than they appear.”
The team overlaid flood risk maps on to Wellington’s transport network to test access to essential services during peak flooding. Under traditional depth-only modelling, most of the population appeared to retain access to hospitals and public transport hubs. However, when velocity was included, some central city regions were shown to be cut off, especially for pedestrians.
In certain scenarios, nearly all walking routes to key services were deemed unsafe during the flood’s peak. Vehicle access was also significantly reduced where steep terrain and narrow streets created bottlenecks.
The researchers have developed a framework combining flood modelling with transport network analysis to identify streets to avoid and calculate safer alternative routes during emergencies.
“We want to help councils, emergency managers, and the public make more informed decisions before and during flood events,” Dasallas said. “That could mean more targeted road closures, clearer public warnings, and better planning for access to hospitals and emergency services based on how water actually behaves, not just how deep it gets.”
The research was part of the Horizon Europe-funded Minority Report project, which aims to enhance resilience of vulnerable urban populations against climate events.
Dasallas warned that as climate change drives more intense rainfall, relying on outdated flood assessment methods could increase the risk of injury or loss of life, particularly in cities with steep catchments and dense transport networks.
“Understanding flood velocity is essential to keeping people safe, challenging the common perception that shallow floodwater is safe to cross,” she said.
Recent extreme weather in New Zealand has highlighted the urgency of this research. Heavy rains in late January triggered flooding and landslides across the North Island, including a landslide at a Mount Maunganui campsite where emergency services searched for survivors. Several regions, including Whangārei, Thames‑Coromandel, and the Bay of Plenty, were placed under states of emergency due to intense rainfall and ground instability.
Communities such as Te Araroa on the East Coast suffered widespread flood damage, with roads washed out, hundreds of homes without power, and residents rescued from rooftops. Following the deadly January storms, higher tides during early February king tides raised concerns about coastal inundation. Weather services reported that persistent rain and deadly storms affected much of the country, creating unusually wet conditions that disrupted transport and emergency response.
These events reflect a broader pattern: studies show that more than 750,000 people currently live in zones exposed to one-in-100-year flood events, a number set to rise under climate change.
