Urban transport forms a network of interconnected roads and routes, which means that disruption in one location can quickly generate consequences elsewhere in the city. It only takes flooding at a few critical points for the effects to be felt by residents of areas that have not been directly inundated.
The closure of a single street forces drivers to choose alternative routes, increases traffic volumes on neighbouring roads and can lead to congestion. As a result, travel times increase, fuel consumption rises and, consequently, so do emissions of pollutants into the atmosphere.
The study focused on the indirect consequences of urban flooding that may affect Łódź, a city located on a watershed and therefore not exposed to the direct impacts of river flooding. A team of researchers from the Institute of Environmental Management and Spatial Policy and the Department of Hydrology and Water Management of the Faculty of Geographical Sciences, led by Prof. Szymon Wiśniewski, combined hydrological modelling of flood hazards in Łódź with transport analysis and an assessment of the economic and environmental costs of transport disruption.
When Does a Flood Begin to Affect the Entire City?
The researchers used hydrological modelling to determine where urban flooding could occur under different rainfall scenarios. Information about potential inundation was then integrated with the road network.
In the next stage, changes in the road network were introduced into a transport model. This made it possible to analyse how road closures and restricted access affect the entire system, including travel times, the number of journeys, road loads, public transport use and emissions.
The statement that the transport system’s response to flooding is not linear is one of the study’s most important findings. During moderate disruptions, residents and the transport system are able to adapt to some extent. Some people change their routes, others choose public transport, while certain journeys may be postponed or cancelled altogether.
The situation changes, however, when disruptions become more severe. At that point, the number of vehicles using alternative routes and the time spent travelling begin to increase rapidly. Congestion intensifies, and the costs of operating the entire transport system rise sharply.
The researchers describe these as critical thresholds. As long as disruptions remain below a certain level, the system is able to absorb them to some extent. Once that threshold is exceeded, even a relatively small deterioration in conditions can produce disproportionately large consequences.
Importantly, the effects of flooding do not end at the flooded area. When a major road is closed, traffic is redirected to alternative routes. These routes may then become overloaded even though they lie outside the area directly affected by flooding. In this way, a local event can create problems in other parts of the city.
Dr Przemysław Tomalski explains:
Previous studies indicate that there are locations within Łódź that are particularly prone to urban flooding. These are primarily terrain depressions located in river valleys, for example the Łódka River valley near Zachodnia Street and the Manufaktura complex, as well as road cuttings beneath railway lines, such as the underpasses on Drewnowska Street and Konstantynowska Street beneath the Łódź Kaliska–Zgierz railway line. These are places where it is difficult to drain water during extreme rainfall events, which Dr Adam Bartnik estimated for Łódź at just under 50 mm, corresponding to the maximum 15-minute rainfall event with a 20% probability of occurrence, theoretically occurring once every five years.
He also adds:
For the city’s everyday functioning, however, locations where urban flooding paralyses traffic and causes transport chaos are even more important. When major arterial roads become blocked, these are the points that absorb diverted traffic and help relieve congestion. The analyses identified five such critical locations where changes in traffic volumes are greatest. Four of them are situated along the Aleja Włókniarzy and Jana Pawła II corridor, between Legionów Street and Obywatelska Street.
The analysis showed that travel times, vehicle-hours travelled, emissions and demand for public transport form distinct spatial clusters. This means that assessing risk requires more than simply identifying areas that may be flooded. It is equally important to understand the role these locations play within the wider transport network.
Flood Impacts Come at a Cost
The study also made it possible to estimate the financial costs of selected flood impacts. Among the factors considered were the value of travel time, vehicle operating costs and costs associated with emissions.
The researchers point out that the most significant component of losses is the additional time spent travelling and the decline in the efficiency of the transport network. Emission-related costs change relatively little under moderate disruptions, but increase substantially in the most severe scenarios. In practice, this means that flood impacts can be assessed not only through direct losses but also through time lost by residents, changes in transport system performance and additional costs borne by the city as a whole.
New Research Supports Urban Management
The findings indicate that protecting a city from flooding should not be limited solely to safeguarding locations directly exposed to inundation. It is equally important to identify those parts of the transport network whose failure could have the greatest consequences elsewhere in the city.
Particularly important in this context is the development of blue-green infrastructure, measures that increase water retention and infiltration, and efforts to reduce excessive surface sealing. Alternative routes, well-prepared public transport systems and rapid communication systems informing residents about threats are also essential.
The approach developed by researchers from the Faculty of Geographical Sciences, University of Lodz can be used as a tool for testing a city’s resilience to extreme events. It makes it possible to identify which components of the system are especially vulnerable and where preventive action may have the greatest impact.
Understanding the interdependencies between different elements of urban infrastructure can help improve predictions of the consequences of extreme events and make cities more resilient to them.
Source: Borowska-Stefańska Marta, Bartnik Adam, Kowalski Michał, Tomalski Przemysław, Wiśniewski Szymon, What is the cost of urban flooding? Changes in travel and emission costs of Łódź’s transport system in Poland, Reliability Engineering & System Safety, Elsevier, Vol. 278, Part 1, 2027, Article No. 113458.
