Afforestation for flood risk reduction, Glinščica River catchment, Slovenia

The Glinščica River catchment in Central Slovenia is characterised by hill slopes and floodplains suitable for afforestation. The catchment is subject to increasing flood frequency due to climate change impacts. 

The modelling framework was developed specifically for Slovenian hydrological conditions.

To investigate the effects of afforestation on peak river flows and selected ecosystem services, evaluating cost-effectiveness of natural water retention measures versus hard engineering for flood protection.

Challenges addressed

The measure addresses climate change adaptation, specifically the growing risk of river flooding. It offers a nature-based alternative to hard engineering structures, with additional gains for biodiversity and recreation.

Costs of NbS

The study models afforestation of 76.67 ha in the downstream floodplain of the Glinščica River near Ljubljana as a natural water retention measure. The measure is not yet implemented. Almost all of the cost is buying the agricultural land, about EUR 5.5 million, while planting 3,500 trees per hectare costs only about EUR 0.3 million. Total costs amount to about EUR 5.8 million (2023 prices).

More tree cover slows runoff and lowers flood peaks in the river; across the study’s scenarios, peaks fell by 9–13%. Smaller flooded areas mean lower economic losses for homes and infrastructure downstream. Over 100 years at a 4% discount rate, benefits reach about EUR 10.8 million (2023 prices), most of it avoided flood damage, with smaller gains for biodiversity, carbon storage, water quality and recreation.

Benefit–cost ratio

Set against total costs of EUR 5.8 million, the benefits give a BCR of 1.89. It was the only one of three afforestation scenarios with a positive net present value, because it achieves similar flood protection on a much smaller area.

• Integrated modelling using HEC-HMS hydrological model, HEC-RAS hydraulic model, and KRPAN flood damage model. 
• Development of three afforestation scenarios with varying tree cover extent. 
• Calculation of peak flow reductions and flood extension changes. 
• 100-year Cost-Benefit Analysis comparing flood protection benefits against biodiversity and recreational values.

None available

Study overview

The Glinščica River catchment in Central Slovenia implemented a comprehensive assessment of afforestation as a nature-based solution for flood risk reduction. An increasing frequency of flood events due to climate change has heightened community vulnerability, prompting the investigation of natural water retention measures as cost-effective alternatives to hard engineering structures for flood protection.

Three scenarios were evaluated using integrated modelling approaches: hydrological model HEC-HMS, hydraulic model HEC-RAS, and flood damage model KRPAN (developed specifically for Slovenia). The main difference between the scenarios was the extent of afforestation on hill slopes and floodplains, ranging from approximately 15% to 60% increase in tree cover.

Results demonstrated that increasing tree cover reduced flood peaks by 9–13%, with significantly lower flood extensions across most scenarios leading to reduced economic losses. The modelling captured effects on peak river flows and selected ecosystem services, enabling a comprehensive evaluation beyond flood protection alone.

A 100-year Cost-Benefit Analysis revealed that only one scenario showed positive Net Present Value, where afforestation was concentrated in the floodplain areas. In this scenario, benefits from flood protection measures exceeded other ecosystem services such as biodiversity or recreational values. This indicates that for afforestation investments to be economically viable, flood protection benefits must constitute the primary value driver.

Additional ecosystem co-benefits not directly linked to flood protection were considered across all three scenarios, including biodiversity enhancement and recreational opportunities. However, these co-benefits were insufficient alone to justify investment without flood protection benefits.

The study demonstrates that natural water retention measures through afforestation can provide effective flood mitigation while delivering multiple ecosystem services. Economic viability depends on achieving sufficient flood protection benefits, suggesting targeted implementation in flood-prone areas rather than diffuse hill slope afforestation.