The REFLEX model, developed by CIMA Research Foundation, simulates flooding caused by extreme weather events, ensuring rapid calculation times and accuracy even over large river basins. Tested in contexts such as the Zambezi River and the 2022 floods in the Marche region, REFLEX handles complex phenomena like backwater effect and coastal expansion, which are typically included only in physically based hydrodynamic models. Its speed and reliability make it an effective tool for flood risk forecasting and early warning systems.
Imagine needing to predict the impact of a flood: it’s essential to know the water flow rate—how much water is in the river—and whether it will exceed critical levels, overflowing and flooding surrounding areas. But how will the floodwater distribute itself? Which infrastructures are at risk of being damaged?
To answer these questions, researchers in the hydrology and hydraulics fields rely on models that, based on input data such as observations or weather forecasts, perform calculations to provide flood scenarios. Hydrological models are the first tools used to help answer these questions; they focus on river flow rates (i.e., how much water is in a given river section at a specific time) and assess flood risk, but they do not detail how the flooding will occur. Hydraulic models fill this gap, forming part of a chain of tools for evaluating potential flood impacts. However, challenges remain, such as reconciling result accuracy with the need for quick computations, which are crucial for responding to rapidly evolving phenomena.
It is in this context that the new REFLEX model emerges, as recently described in a paper published in the Journal of Flood Risk Management. “REFLEX is a model entirely developed by CIMA Research Foundation. Over the last five years, we have implemented it both in Italy and in other regions worldwide within the framework of international projects, allowing us to evaluate it in various case studies,” explains Lorenzo Alfieri, researcher in Hydrology and Hydraulics at CIMA Research Foundation and co-author of the study.
Introducing REFLEX
What sets REFLEX apart? Like other geomorphological models, REFLEX is a tool that, based on the morphology of the terrain and a given flood volume, aims to represent the maximum flood conditions (i.e., maximum extent and depth) without simulating the entire event’s dynamics. These results, overlaid with maps containing information on infrastructure and population, form the basis for estimating the event’s impact. REFLEX is built on the HAND (Height Above Nearest Drainage) concept. In short, the model classifies the land according to its relative elevation to nearby watercourses, providing an effective representation of local topography. This allows the model to quickly identify areas more prone to flooding based on their elevation relative to the river.
REFLEX introduces two main innovations compared to other geomorphological models. The first is the use of physically based water volumes: many other models perform simulations assuming indefinite water levels, sometimes leading to unrealistic flood volumes in very flat areas. REFLEX, on the other hand, considers realistic flood volumes, calculated based on the time it takes for the flood wave to travel along each section of the river.
The second innovation is the inclusion of two hydraulic phenomena usually only considered in physically based hydrodynamic models. The backwater effect occurs when the river flood, due to terrain morphology, causes flooding not only downstream or in surrounding areas but also upstream, for example, when a gorge narrows the riverbed. Coastal expansion happens at estuaries (in seas or lakes) when flooding affects neighboring basins, creating a delta-like shape.
“We managed to implement these phenomena in the REFLEX model without compromising calculation speed,” Alfieri explains. “This allows us to use it on very large basins and in various flood scenarios, which is essential for climate simulations that account for potential climate change impacts.”
From Italy to Africa
Indeed, as described in the recently published paper, CIMA Research Foundation researchers have employed REFLEX in different contexts. For instance, the model was used during the 2022 floods in the Marche region and in international projects, where it was applied in Fiji, Vanuatu, Laos, and Cambodia. “The largest basin where we used REFLEX was the Zambezi River, the fourth largest in Africa. It was the perfect context to test the model under conditions with minimal data available from ground stations, which are scarce throughout the basin,” Alfieri continues.
“Each case study where we tested REFLEX demonstrated its reliability. We also validated it by comparing it with flood maps produced by hydrodynamic models and satellite data of flood mapping.”
The model’s speed is impressive even for non-specialists: simulations on the Magra River, with a resolution of just 5 meters, took only ten minutes. Simulations on the Zambezi basin, which took just over four hours, were approximately 200 times faster than an equivalent hydrodynamic simulation.
“These features confirm the utility of the REFLEX model within modeling chains that allow for predicting the potential impacts of a flood. Of course, there are still some limitations to work on for improving the model: for instance, in the calculation of flood depth maps, REFLEX assumes a constant water level along each river section, whereas it’s intuitive that water may rise or fall in different sections, influencing flooding in surrounding lands,” concludes Alfieri. “However, this work shows the fundamental role that reliable, comprehensive, and computationally efficient models can play in early flood warning systems.”