flodfåra

Flodfåra, or riverbed, denotes the bottom of a river channel where the bulk of the water flow occurs. It includes the underlying substrate and the materials that form the channel bed, such as bedrock, gravel, sand or silt. The flodfåra defines the cross-sectional area and roughness of the river, and thus influences velocity, discharge capacity and sediment transport.

The flodfåra is formed and continuously modified by erosion, sediment transport and deposition driven by river discharge. In gravel- and sand-bed rivers, bedforms such as ripples and dunes develop on the bed; high flows can scour the bed and deepen the channel, while low flows promote deposition and the formation of bars. In bedrock channels, incision and plucking shape the bed; in braided rivers, multiple channels adapt as sediment supply changes.

Hydraulic properties of the bed affect flow resistance and energy loss. Measurements include depth, cross-sectional area, slope and wetted perimeter; mapping the flodfåra is essential for hydraulic modelling, flood risk assessment and infrastructure design. Ecologically, the riverbed provides substrate for benthic organisms and influences spawning grounds.

In practice, the term is used in hydrology, geomorphology and civil engineering to describe the river's bed, which can range from solid rock to loose sediments, and which is distinct from the riverbank and floodplain.