Manning's n values ​​and infiltration rates:

Flood modeling requires a detailed understanding of how land cover influences hydrological processes, as vegetation, root systems, soil organic matter content, and surface roughness have a direct impact on runoff generation, infiltration capacity, and resistance to water flow. Implementing Nature-based Solutions (NbS) aims to improve these hydrological processes and reduce flood risk. To achieve this, it is essential to correctly parameterize the effects of NbS by adjusting parameters such as infiltration and surface roughness in the areas where they are implemented.

The parameterization of NbS is based on how land-use interventions modify specific hydrological parameters, particularly infiltration and surface roughness. Parameterization is carried out according to the type of land cover resulting from the implementation of NbS.

1. Active Restoration Interventions: When active restoration is carried out in an area (e.g., restoring a degraded ecosystem), conditions are assumed to be optimal, and the final land cover is considered equivalent to a natural, undisturbed cover of the same type. For example, a restored grassland area is assumed to have a cover equivalent to a forest.


2. Passive Restoration Interventions: In the case of passive restoration, where the ecosystem is left to regenerate naturally, land cover is considered improved by one class. For example, a bare soil area under passive restoration is assumed to become an area with sparse vegetation.

Land Cover Types and Their Hydrological Impact

Land cover directly affects an area’s infiltration capacity and surface roughness. Below is how different land cover types influence these parameters:

1. Forest Areas – High Infiltration Rate:
   
   Forests have the highest infiltration rates due to dense vegetation, deep roots, and thick leaf litter, which improve soil permeability and reduce surface runoff. This type of cover also increases resistance to water flow.


2. Shrublands – Moderate Retention Capacity:
   
   Although shrubs are less dense than forests, they still provide moderate hydrological buffering due to woody vegetation and root systems, reducing surface water flow.


3. Grasslands – Moderate Infiltration:
   
   Well-maintained grasslands have good infiltration capacity, although their hydrological function can be significantly reduced by compaction or overgrazing. Proper management is essential to maintain their hydrological role.


4. Agricultural Lands – Low Infiltration Capacity:
   
   Agricultural lands typically have low infiltration capacity due to intensive tillage, reduced vegetation cover, and soil disturbance factors. These conditions can increase runoff unless mitigated through sustainable farming practices.


5. Bare Soil Areas – Very Low Infiltration Capacity:
   
   Areas with sparse vegetation or bare soil have the lowest infiltration capacities, resulting in higher surface runoff and erosion.

Parameter Assignment for NbS

The approach to parameterizing NbS is based on assigning values for Manning’s n (surface roughness coefficient) and infiltration rates according to the land cover type resulting from the implementation of solutions. The values assigned for different land cover types were adopted from FastFlood and based on WorldCover data and scientific literature.

Example of Parameter Assignment for NbS:

• Active Restoration: If an active restoration intervention converts a grassland area into a forest (e.g., through reforestation), the assigned parameters will reflect the values corresponding to forest cover.


• Passive Restoration: If a bare soil area becomes sparse vegetation due to passive restoration, the Manning’s n and infiltration values will be assigned to those of a denser cover than the original.

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