REFLEX: a geomorphological model for flood simulation

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.”

And what about the use of REFLEX in different contexts? From Italy to Africa... Find out more on our website ????