Waterfront usage trends across German metropolitan areas: A social-ecological perspective to urban blue-green infrastructure connectivity

The waterfront connects aquatic and terrestrial habitats, and its landscape quality is an indicator of the status of blue-green infrastructure. Especially in urban areas, the consequences of direct human interventions on the waterfront are evident, yet there is a need for more comprehensive and easy approaches to quantify the dynamics of the waterfront landscapes. Herein, we propose a simple, rapid, and effective method, the “Waterfront Development Index” (WDI), for characterizing the relative waterfront usage in metropolitan areas. Based on the Urban Atlas data provided by the Copernicus program of the European Union for German functional urban areas (FUAs), we validate the WDI by comparing waterfront land use and land cover change at federal state, metropolitan, and urban levels (91 FUAs). Our results show remarkable disparities between waterfront land use along running and standing waters. The urban waterfronts covered a wide range of WDI values, highlighting critical urban freshwater waterfronts being relatively more extensively allocated for human land use. Transportation was identified as the most pressing human land use on the waterfronts, especially along the riverbanks. Between 2012 and 2018, 236.5 km of natural, water, or agricultural waterfront have been altered to urban, industrial, and transportation land use. Even though the national level transformation is relatively low (0.4 %), the change ratios of some FUAs are alarming (∼2%). We demonstrate the usefulness and reproducibility of the WDI to monitor and quantify changes in waterfront development over time, posing high relevancy to waterfront landscape conservation, social-ecological systems’ balance, and sustainable urban planning alike.