SUPERVISOR: Thomas ERTL

PROJECT ASSIGNED TO: Helena GALYS

Throughout history, urban rivers have been progressively transformed from dynamic, natural systems into channelized conduits for water and waste (Khirfan et al., 2020; Sendzimir & Schmutz, 2018). Seen as obstacles, many urban streams were buried, culverted, or redirected into sewer systems (Wantzen et al., 2022). This widespread loss and degradation of urban watercourses have profoundly disrupted hydrological, ecological, and social systems in cities worldwide (Sousa et al., 2025). Without these streams, urban environments lost vital functions such as supporting biodiversity, moderating temperatures, and providing essential resources for both people and ecosystems (Hohensinner et al., 2018; Lehrter et al., 2023). In Vienna, for instance, over two-thirds of historically present streams have disappeared, with many converted into sewers (Stadt Wien, 2025). This trend is not unique to Vienna but reflects a global pattern across urbanized regions (Sendzimir & Schmutz, 2018). As cities now grapple with challenges like rising temperatures, water scarcity, and flooding exacerbated by climate change, reactivating former watercourses presents a compelling opportunity for sustainable urban transformation.

Former watercourses that are currently integrated into sewer systems could serve as valuable water sources for urban applications such as irrigation, street cleaning, or recreational areas (Prenner et al., 2022). Moreover, daylighting – the process of restoring and reintegrating urban rivers into city landscapes – offers not only ecological benefits, such as habitat restoration and improved water quality, but also social benefits, such as enhanced public spaces and community well-being (Wild et al., 2011). These functions are critical for adapting urban areas to a changing climate and align with the ambitious targets of the EU’s Nature Restoration Law (European Parliament & European Council, 2024), which aims to renaturalize 20% of degraded ecosystems by 2030.

This research will thus investigate the potential of urban stream reactivation to contribute to a sustainable urban transformation. To guide these efforts, this doctoral study addresses the following key questions: 

  1. What are the critical success factors and barriers to implementing watercourse daylighting and reactivation projects in urban environments across Europe?

  2. How can water from reactivated watercourses be sustainably and safely integrated into urban applications, such as irrigation, cooling systems, or swimming ponds?

  3. What hydrological and ecological benefits can be achieved by reactivating urban watercourses?

To address these questions, the research will adopt a mixed-methods approach. Expert interviews with practitioners from cities across Europe that have attempted or successfully completed urban watercourse reactivation projects will be conducted to identify key success factors, as well as potential bottlenecks, pitfalls, and challenges. The insights gained will directly inform two ongoing and planned case studies.

The first case study, located in Mannersdorf, Austria, will explore opportunities to utilize water from a buried stream for multifunctional purposes, such as irrigation or urban cooling. The second case study shall extend to an international site in Southeastern Europe, where efforts focus on improving water quality and stormwater management through stream reactivation. By applying the insights from the initial case study to this new setting, the research aims to evaluate the transferability of findings and assess the governance, technical, and ecological dimensions of watercourse reactivation in diverse urban contexts.

The findings of this study shall contribute to the theoretical and practical knowledge of urban river restoration and reactivation. By bridging gaps in governance, policy, and scientific understanding, this research seeks to position reactivated watercourses as vital assets for urban resilience, sustainable water management, and ecological restoration. Ultimately, it aims to provide actionable recommendations for cities to better integrate natural water systems into their urban fabric, addressing local and global challenges alike.

 

References

European Parliament & European Council. (2024). Regulation (EU) 2024/1991 of the European Parliament and of the Council of 24 June 2024 on nature restoration and amending Regulation (EU) 2022/869. eur-lex.europa.eu/eli/reg/2024/1991/oj/eng

Hohensinner, S., Hauer, C., & Muhar, S. (2018). River Morphology, Channelization, and Habitat Restoration. In S. Schmutz & J. Sendzimir (Eds.), Riverine Ecosystem Management (pp. 41–65). Springer International Publishing. doi.org/10.1007/978-3-319-73250-3_3

Khirfan, L., Peck, M. L., & Mohtat, N. (2020). Digging for the truth: A combined method to analyze the literature on stream daylighting. Sustainable Cities and Society, 59, 102225. doi.org/10.1016/j.scs.2020.102225

Lehrter, R., Rutherford, T., Dunham, J., Johnston, A., Wood, D., Haby, T., & Carter, S. (2023). Effects of Culverts on Habitat Connectivity in Streams—A Science Synthesis to Inform National Environmental Policy Act Analyses. pubs.usgs.gov/sir/2023/5132/sir20235132.pdf

Prenner, F., Müller, H., Stern, P., Holzer, M., Rauch, H. P., & Kretschmer, F. (2022). Suitability pre-assessment for decoupling in-sewer captured streams to support urban blue-green climate adaptation measures. Journal of Water and Climate Change, 13(4), 1748–1764. doi.org/10.2166/wcc.2022.458

Sendzimir, J., & Schmutz, S. (2018). Challenges in Riverine Ecosystem Management. In S. Schmutz & J. Sendzimir (Eds.), Riverine Ecosystem Management (pp. 1–16). Springer International Publishing. doi.org/10.1007/978-3-319-73250-3_1

Sousa, M. C., Martins, R., Simões, N. E., & Feio, M. J. (2025). Ecosystem services of urban rivers: A systematic review. Aquatic Sciences, 87(1), 10. doi.org/10.1007/s00027-024-01138-y

Stadt Wien. (2025). Die Wienerwaldbäche. www.wien.gv.at/umwelt/gewaesser/wienerwaldbaeche/

Wantzen, K. M., Piednoir, T., Cao, Y., Vazhayil, A. M., Tan, C., Kari, F. G., Lagerström, M., Gerner, N. V., & Sommerhäuser, M. M. (2022). Back to the surface – Daylighting urban streams in a Global North–South comparison. Frontiers in Ecology and Evolution, 10, 838794. doi.org/10.3389/fevo.2022.838794

Wild, T. C., Bernet, J. F., Westling, E. L., & Lerner, D. N. (2011). Deculverting: Reviewing the evidence on the ‘daylighting’ and restoration of culverted rivers. Water and Environment Journal, 25(3), 412–421. doi.org/10.1111/j.1747-6593.2010.00236.x