Is there strength in numbers? – Assessing cumulative effects of decentralized retention measures in forested catchments

Decentralized retention measures in forested catchments are increasingly discussed as a complementary approach to traditional flood protection. Although forests are generally considered to naturally mitigate flooding, the quantitative contribution of small technical retention structures associated with forest infrastructure, such as ditch modifications, culverts, cross-drains, and small retention elements, remains poorly understood and insufficiently integrated into flood risk management strategies. In particular, a lack of transferable planning methodologies and evidence-based evaluation frameworks limits their systematic application.

The AquaSilva project aims to address this issue by developing a structured, model-based approach for assessing and planning decentralized technical retention measures in forested catchments. The project combines a comprehensive inventory of existing measures with hydrological modeling and targeted monitoring. Retention structures in forested environments, including road-related drainage elements and small-scale retention features, are systematically classified based on their hydrological function, spatial context, and technical design.

Hydrological effectiveness is assessed using modeling approaches that allow the representation of runoff generation, flow routing, and temporary storage at the catchment scale. Scenario-based analyses explore the potential effects of individual and combined retention measures under varying hydrological conditions. When applicable, model-based results may be supplemented with empirical evidence from practice to better understand hydrological responses and refine conceptual assumptions.

The methodology is applied in a pilot area in the Vienna Woods, allowing the derivation of practical planning parameters and transferable recommendations. The main outcome is a practice-oriented framework that links hydrological modeling, monitoring, and decision support. By improving the understanding of how decentralized forest retention measures to runoff attenuation and peak flow reduction, the project supports hydrologically sensitive forest infrastructure planning and provides a scientific basis for integrating forest-based measures into flood risk management at catchment scale.