When floodwater enters an urban area, the way risk is typically communicated focuses on depth. Depth maps are intuitive, easy to produce, and well understood by emergency managers and the public. But research from the University of Canterbury is making the case that they are significantly underestimating the actual danger — because velocity, not depth alone, determines whether floodwater can knock a person off their feet or sweep a vehicle off a road.
Dr Lea Dasallas of UC’s Department of Civil and Environmental Engineering led the research, which overlaid flood risk maps incorporating velocity onto Wellington’s transport network. The results were striking: high-risk pedestrian areas increased by more than 80% when velocity was included alongside depth, and medium-risk zones for children and elderly people more than tripled.
Why Shallow Water Can Be Deadly
The intuitive assumption is that shallow floodwater is safer than deep floodwater. Dr Dasallas’s research challenges that directly. When floodwater flows quickly — as it does through constricted urban streets and drainage channels — even 30 centimetres can generate enough force to knock an adult off their feet. The force exerted by moving water is a product of both its depth and its velocity, and conventional depth-only maps capture only half of that equation.
For vulnerable groups — children, elderly people, and individuals with limited mobility — the risk threshold is lower. Moderate-velocity flows at relatively shallow depths can be dangerous for these groups even when they would be manageable for a healthy adult.
Implications for Infrastructure Design
For civil engineers and infrastructure planners, the research underscores the importance of incorporating velocity modelling into flood risk assessments for urban areas. Stormwater systems, drainage networks, and the geometry of streets and open spaces all influence how fast floodwater moves through an urban catchment, and design choices at the infrastructure level directly affect the velocity profile of flood events.
The research is part of the Horizon Europe Minority Report project, which focuses on improving the accuracy of risk communication for underrepresented hazard factors. Its findings have practical implications for emergency management planning, building code requirements in flood-prone areas, and the design of new development in urban catchments.
Explore more on resilient infrastructure and urban flood risk from New Zealand’s civil engineering sector, or connect with specialists in stormwater design and hazard assessment.
