Floodplain restoration measures are among the most well-known nature-based solutions for flood risk reduction but practitioners see their limitations in comparison to technical measures when considering both their effectiveness and profitability. The aim of this study is to show the co-benefits (besides flood risk reduction) of floodplain restoration and handle them in terms of monetized ecosystem services (ES). Our work focused on six ES groups for three study areas in the Danube catchment along the Krka, Morava, and Danube rivers. ES mapping through stakeholder engagement is also considered. We applied the methodologies suggested in the Toolkit for Ecosystem Service Site-Based Assessment (TESSA) complemented with alternative methodologies (e.g., questionnaires on social media). Results show annual combined benefits of floodplain restoration in a range from 237,000 USD²⁰¹⁹ at Krka to 3.1 million USD²⁰¹⁹ at Morava, suggesting the utility of ES assessment. The combination of stakeholder workshops and the TESSA guidelines, as well as the newly developed methods, were all central tools to provide decision-makers with arguments to use nature-based solutions for an integrated and holistic riparian land use management.

Floodplains are a fundamental source of multiple functions and services. Despite their various benefits, a dramatic reduction in floodplain areas has occurred in most large river systems over the last few centuries, and is still ongoing. Human modifications (such as river regulation, dam construction, and land use changes) due to economic growth, increasing population size, etc., were and still are drivers of major floodplain losses. Therefore, studies offering solutions for floodplain preservation and restoration are of great importance for sustainable floodplain management. This paper presents methods to identify active, former, and potential floodplains, and their application to the Danube River. We used hydraulic data, historical sources, and recent geospatial data to delineate the three floodplain types. Fifty hydraulically active floodplains larger than 500 ha were identified. According to our results, the extent of Danube floodplains has been reduced by around 79%. With the support of different representatives from the Danube countries, we identified 24 potential floodplains. However, the share of active and potential floodplains in relation to former floodplains ranges between 5% and 49%, demonstrating the huge potential for additional restoration sites. This analysis contributes to an understanding of the current and the past floodplain situation, increases awareness of the dramatic floodplain loss along the Danube, and serves as a basis for future floodplain management.

Ecosystem services in floodplains are manifold. The regulating services regarding hydrological issues (e.g. flood protection, water purification) are of particular importance along rivers, and depend strongly on size and land use of the floodplain. In this paper, we transfer the commonly known land use changes in floodplains over the last 150 years into significant changes of the amount of different regulating ecosystem services. We investigated a floodplain stretch of 17 km along the Danube in Germany (approx. 90 km²). Thus, we mapped the spatial expansion of the active floodplain and the land use distribution for three different times: the earliest (not the pristine) state of 1869 on the basis of a historical map, 1963 after river regulation and 2013 as navigation channel with a hydropower dam on the basis of aerial photographs. The land use types woodland, grassland, arable land, settlements, and water bodies were distinguished. On the basis of land use as a proxy, we calculated the potential of four ecosystem services (flood retention, nitrogen and phosphorous retention, habitat provision) according to the method of Scholz et al. (2012a). The spatial extension of the active floodplain was continuously reduced from 56 km² (1869) to 18 km² (1963) to 11 km² (2013). The amount of grassland and arable land was reduced significantly in the active floodplain, whereas woodland increased. This entails a decrease of f lood retention (-80%), and nutrient retention (nitrogen: -60%, phosphor: -76%). Likewise, habitat provision was significantly reduced. In total, the potential benefits for humans have been negatively affected over the time by land use change and, above all, by the construction of embankments. Therefore, ecosystem services should be regarded by future floodplain management.