October 2024 floods in the Valencia region, Eastern Spain: One example too many of hazardous debris

Overview of UFDs jams in Catarroja after the October 2024 Valencia floods, Spain. Author: Miquel Albert

The recent devastating flash floods in Valencia region, Eastern Spain, have once more highlighted the role of large debris in enhancing urban flood hazard.

Urban debris – so-called Urban Flood Drifters (UFDs) – are very varied as shown by a recent survey. They include vehicles, deadwood, waste bins, containers of many types, construction materials, and many other objects.

Once these are mobilized by the combined action of flood depth and velocity, they are transformed into powerful drivers of hazard, amplifying risks in the urban environment.

Urban Debris Amplifies Hazards

When objects like vehicles, waste containers and large debris lose their stability, they can further constrict narrow streets, drastically limiting the drainage capacity of the watercourses and exacerbating the possible effects of the floods. In addition to a significant risk to lives, the loss of stability of urban objects during flood events also entails huge economic and environmental losses. According to current statistics, damaged vehicles correspond to 10 to 20% of the total claimed economic flooding losses  in Spain.

Flash floods with abrupt propagation fronts and high flow velocities, like the ones observed this week in the region of Valencia, are particularly hazardous since they are prone to mobilize simultaneously large amounts of UFDs.

A survey of flood-affected areas in Valencia reveals the severe impact of UFDs on overall damage. Citizen-shared images consistently show streets and drainage infrastructure blocked by multistorey piles of vehicles, dumpsters, appliances, and vegetation entangled in dense mud.

Detail of UFDs jam in Catarroja after the October 2024 Valencia floods, Spain. Author: Arnau Bayón

In the extreme flood event in Valencia, large urban flood debris posed a double threat: first, during the flood, where they compromised drainage systems, blocked escape routes, and caused impact damage; and second, in the aftermath, where accumulated debris created a bottleneck in cleanup efforts across vast areas. Limited access for heavy machinery makes this task challenging, leaving much of the work beyond the capacity of the many volunteers on site. This scenario intensifies public helplessness and frustration, leading to distressing situations and severe psychological stress. Furthermore, uncleared streets delay a return to normal life and raise the risk of epidemics from decomposing bodies, wastewater, food debris, and other organic matter.

Another recent example of an extreme flood event in Europe, in which large debris played a role in hazard amplification, was the 2021 floods in German and Belgium. As in Valencia, the combination of deep water, high flow velocities and large amounts of debris proved to be catastrophic, resulting in a substantial increase of the danger to lives, property, and infrastructure. The Valencia floods is yet another paradigmatic case demonstrating the need for urban planners and flood managers to address the mobilization of sediment and debris in their mitigation and emergency planning.

Researching Urban Debris

Standard flood management plans focus solely on hydraulic characteristics of the flow, and mainly on depth of the flood. Large debris are consistently overlooked in flood management practices. Urban flood hazards are more complex since they incorporate the influence of infrastructure, the urban network, sediments and UFDs.

Research into large debris in urban flood contexts is still emerging, but important results were already achieved, by the authors and many other colleagues, regarding the characterization of these and the onset of their movement under flooding conditions. Further efforts are being made by several research groups (1, 2, 3, 4, 5, 6, 7) to incorporate the transport of such debris by flowing waters in hydrodynamics models.

While the knowledge produced so far offers valuable guidance for reducing hazards during such events, an implementation gap remains to bridge between research and practice. The existing research provides results that, although still needing deeper understanding, are already scalable and can be used to build more robust flood management frameworks capable of addressing both direct and such indirect flood hazards, which in turn can inform improved decisions regarding urban planning and flood management.

People retrieving personal belongings from cars in the UFD jam in Catarroja after the October 2024 Valencia floods, Spain. Author: Arnau Bayón.

Mitigation against the effect of UFDs mobilized by extreme floods can imply the redesign of the urban network and redefinition of urban land uses: for instance, computational models, integrating the information about susceptibility to move large debris in cities, help planners to identify vulnerable infrastructure and high-risk zones.

A Call to Action

We have moved from structural solutions for flood protection (e.g. by-pass channels, dam construction) to non-structural solutions (e.g. urban planning and emergency management). We need to advance further and to incorporate debris dynamics (onset of motion and transport) into flood modelling, urban flood hazard assessment will provide a more holistic picture of potential flood scenarios. This will enhance resilience and reduce exposure and vulnerability in flood prone areas of urban communities in case of flash floods.

As the frequency and intensity of extreme weather events continue to increase, urban areas must adapt to address the compound nature of flood hazards.