NbS Parameterization in FastFlood

Nature-based Solutions (NbS) aim to reduce flood risk and improve water regulation by leveraging natural and semi-natural landscapes. Accurate modelling of NbS impacts requires understanding how land cover influences hydrological processes such as runoff generation, infiltration, and flow resistance. In FastFlood, NbS parameterization integrates land cover characteristics to adjust Manning’s n values and infiltration rates, reflecting the hydrological effects of interventions like reforestation, agroforestry, and restoration.

Influence of Land Cover on Hydrology

Different land cover types exhibit varying hydrological performance:

  • Forests: High infiltration and surface roughness due to dense vegetation, deep roots, and organic litter.

  • Shrublands: Moderate infiltration and roughness through woody vegetation and root networks.

  • Grasslands: Moderate infiltration when healthy; compaction or overgrazing reduces hydrological function.

  • Agricultural lands: Lower infiltration and variable roughness, influenced by tillage and reduced vegetative cover.

  • Sparse vegetation / Bare soil: Minimal infiltration and low roughness, leading to high runoff and erosion potential.

These differences form the basis for assigning Manning’s n and infiltration values in NbS scenarios.

Parameter Assignment Methodology

FastFlood applies a land cover-based approach to NbS parameterization:

  1. Intervention Outcome: The hydrological parameters are assigned according to the resulting land cover after the NbS intervention.

    • Active Restoration: Land cover is assumed fully restored to the target type (e.g., bare land → forest).

    • Passive Restoration: Land cover improves one class level (e.g., bare land → sparse vegetation).

     

  2. Land Cover-Based Values: Manning’s n and infiltration values are adopted from FastFlood’s database, derived from WorldCover datasets and literature.

 

Limitations

While the approach simplifies implementation and ensures consistency, it has several constraints:

  • Assumes optimal implementation and ecological maturity of NbS interventions.

  • Ignores transitional states during ecosystem recovery.

  • Static Manning’s n and infiltration values do not account for site-specific factors (soil type, topography, historical land use).

  • Does not capture temporal changes in vegetation or soil properties.

  • Simplifies agricultural systems, ignoring management variability like tillage or crop rotation.