Sædmotilitet

Sædmotilitet refers to the capacity of sediment particles to be mobilised and transported by fluid flow, primarily in rivers, streams, and coastal waters. It is a key parameter in sediment dynamics and is influenced by hydraulic, sedimentary, and geomorphic conditions. The mobility of a particle depends on the balance between driving forces, such as shear stress exerted by the fluid, and resisting forces, including gravitational weight, interparticle friction and cohesion. When the bed shear stress exceeds a critical value—often expressed by the Shields parameter—a particle entrains, initiating transport.

Sediment mobility governs the formation and evolution of channel bedforms, such as riffles, bars and dunes, and ultimately shapes the morphology of the aquatic system. In braided rivers, high mobilities produce shifting bars and diverse habitats, whereas low mobilities lead to stable beds with thick organic layers. Coastal applications focus on the transport of sand and fine material, affecting beach nourishment, dune stability and shoreline erosion.

Assessment of sædmotilitet is commonly carried out through laboratory flume tests, field measurements of bed shear stress, sediment sampling, and numerical models. These tools allow estimation of transport rates, prediction of morphological change and evaluation of engineering interventions such as dredging, flow regulation and barrier construction. Understanding sediment mobility is essential for managing water quality, aquatic habitats, navigation channels and flood risk.