The potential role of decentralized rainwater infiltration in the Frankfurt Rhein-Main area: A case study

In recent decades, Blue and Green Infrastructure (BGI) has gained prominence in urban planning due to its numerous benefits. Sustainable Drainage Systems (SuDS) like infiltration swales or trenches enhance groundwater recharge while reducing combined sewer discharge. However, in densely populated areas, space is often a limiting factor. Implementing BGI in developed areas is particularly challenging.   

This study aims to investigate the effects and potential locations of the above-mentioned SuDS in the metropolitan area of Frankfurt am Main (Germany), using an analysis of geodata, land use projections and the WABILA water balance model (Henrichs et al., 2016). First, we delimited and classified urban sub-areas based on land use and building composition. Surfaces were segmented into roof, impervious, and green areas using vector files for building and plot perimeters, as well as various raster data (e.g., impervious degree). A SuDS implementation degree was assigned to each sub-area type based on space availability. For example, disperse urban areas could proportionally implement more swales, as more space is available. Else, infiltration trenches were assigned, as they require less space. SuDS were not assigned where a) needed space was unavailable, b) soil permeability was too low, c) a water protection area was present, or d) the groundwater level was too high. Then, we gave the surface types and areas as input for WABILA, a tool for evaluating urban rainwater management measures, integrating also georeferenced climate and geological data. By varying surface configurations, we assessed the effects of increased adoption of SuDS on groundwater recharge, accounting for space limitations within the properties and guidelines for rainwater infiltration.

According to our analysis, a total of 31 million m³ per year could be infiltrated by 2050. This corresponds to a 30% reduction in the total urban rainwater runoff. This potential can roughly be evenly distributed among compact, disperse and industrial settlements or areas. Infiltration swales were assigned the most, followed by combined swale-trench elements and infiltration trenches. The total annual costs of such an implementation range between 15 to 30 million euros. The overall economic benefits were not quantified in this study.

Despite the limitations of the method (e.g., necessary simplification of water quality risks), the results could serve as reference for sustainable urban water management. Many cities in Germany (including Frankfurt) have already begun with intensive programs promoting BGI and SuDS. The presented method can be transferred to other places in Germany, as the used georeferenced data is publicly available and the used software is open source.