Improved transboundary flood risk management through better understanding of floodplain connectivity in an impounded, flow regulated river reach.

The LAREDAR project addresses transnational flood risk mitigation in the Danube River Basin by focusing on the roles of lakes and reservoirs and by developing tools and guidance to support coordinated management across countries. Led by the Middle Tisza District Water Directorate, LAREDAR operates under the Danube Region Programme priority on climate adaptation and disaster management and is planned for a 30-month implementation period. Its core intent is to strengthen basin-wide cooperation through an integrated platform built on a joint GIS database and improved understanding of transnational flood effects, enabling sustainable and coordinated action during flood events across borders.

The Austrian–Slovenian Mura River is one of three multinational pilot areas selected to characterize the role of lakes and reservoirs in flood mitigation. The Austrian reach is hydrologically modified by hydropower generation, comprising a series of run-of-river power plants with impounded reaches and narrow embankments raised above the adjacent floodplain. During large floods, overtopping of these embankments enables natural floodplain inundation and creates secondary flowing retention that bypasses the power plants. Yet it remains rather unclear how flow regulation and floodplain flow interact.

Previous studies (Volpi et al., 2018; Cipollini et al., 2022; Stecher and Herrnegger, 2022) show that run-of-river power plants typically exert only minor influence on downstream flood peaks. Within the Austrian reach of the Mura Pilot area the focus is on the interdependencies between main channel and floodplain flows in a hydrologically altered river landscape. The lateral exchange between river and floodplain—its controls, dynamics, and consequences for total flood retention at reach to basin scales—remains insufficiently quantified, potentially limiting effective transnational flood management. We adapt the Floodplain Evaluation Matrix - FEM (Habersack and Schober, 2020) to explicitly account for run-of-river power plants and regulated flow regimes to assess the performance of floodplain-impoundment interrelations.

This work aims to (i) quantify retention effects across multiple spatial and temporal scales, (ii) evaluate the effectiveness of past flood mitigation measures, (iii) provide evidence on when and where floodplain connectivity provides meaningful peak reduction, and (iv) clarify upstream–downstream interactions in a transnational setting. The resulting evidence base will extend current knowledge and support river managers in optimizing flood risk mitigation and targeted prevention measures. It will also foster robust transnational cooperation and data exchange for improved flood risk management.

First findings already underline the important retention effect of existing floodplains, but also indicate the potential of optimizing floodplain connectivity, making better use of impounded river reaches for improved flood management. More detailed, quantified results are expected in 2026.